Renewable and Sustainable Energy Reviews 66 (2016) 775–801

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews

journal homepage: www.elsevier.com/locate/rser

Pre-processing of sugarcane bagasse for gasification in a downdraft

biomass gasifier system: A comprehensive review
Anthony Anukam a,b,n, Sampson Mamphweli a, Prashant Reddy c, Edson Meyer a,
Omobola Okoh b
a
Fort Hare Institute of Technology, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa
b
Department of Chemistry, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa
c
Department of Chemistry, Durban University of Technology, P.O. Box 1334, Durban 4000, South Africa

ar t ic l e i nf o a b s t r a c t

Article history: The processing of sugarcane bagasse as a potential feedstock for efficient energy production has attracted
Received 21 April 2015 a great deal of attention in the sugarcane industry, which has traditionally inefficiently burned bagasse in
Received in revised form boilers for steam and electricity generation. Alternative technologies for more efficient utilisation of
15 January 2016
bagasse for energy production within the industry has also been hindered by the high degree of com-
Accepted 23 August 2016
Available online 1 September 2016
plexity involved in bagasse handling and pre-processing before it can be utilised as an energy feedstock.
This can be attributed to unfavourable characteristics of mill-run bagasse, which includes low bulk and
Keywords: energy densities, a wide range of particle sizes and shapes as well as high moisture content. Gasification
Sugarcane bagasse is regarded as one of the most promising energy recovery technologies for the widespread use of biomass
Biomass because of its higher efficiency when compared to the combustion technology commonly used by the
Gasification
sugarcane industry. There has been a strong drive to identify efficient pre-processing methods that can
Pre-processing
be applied to bagasse to make it a suitable feedstock for energy production in thermochemical con-
Downdraft gasifier
Process efficiency
version systems. This work provides a comprehensive review on the pre-processing of bagasse for ga-
sification, and the gasification technology options for its conversion into energy, with a particular em-
phasis on the downdraft gasification technology.
& 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776
2. Sugarcane bagasse generation and handling in South Africa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777
3. Sugarcane bagasse as an alternative energy resource . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777
4. The composition and properties of sugarcane bagasse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778
4.1. Proximate and ultimate analysis of sugarcane bagasse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778
4.2. The heating value of sugarcane bagasse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778
4.3. The bulk density of sugarcane bagasse. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 779
4.4. Microstructure, macrostructure and chemical composition of sugarcane bagasse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 779
5. Issues related to sugarcane bagasse utilisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780
6. The pre-processing of sugarcane bagasse. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 781
7. The various pre-processing methods to improve bagasse quality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 781
7.1. Densification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 781
7.1.1. Pelleting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 781
7.1.2. Briquetting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782
7.2. Comparison of different technologies used for densification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 785
7.3. Densification systems process variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 785

Abbreviations: SCB, sugarcane bagasse; GHG, greenhouse gas; EOR, enhanced oil recovery; USDE, United States Department of Energy; USEPA, United States Environmental
Protection Agency; SERI, Solar Energy Research Institute; NETL, National Energy Technology Laboratory; EBIA, European Biomass Industry Association
n
Corresponding author at: Fort Hare Institute of Technology, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa.
E-mail address: aanukam@ufh.ac.za (A. Anukam).

http://dx.doi.org/10.1016/j.rser.2016.08.046
1364-0321/& 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
776 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

7.3.1. Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 786

7.3.2. Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 786
7.3.3. Die geometry and speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 786
7.3.4. System retention time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 786
7.4. Torrefaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 787
7.5. Drying and demoisturising . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 787
7.6. Grinding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 788
8. The gasification process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 788
8.1. Drying zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 789
8.2. Pyrolysis zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790
8.3. Oxidation zone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790
8.4. Reduction zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790
9. Benefits of the gasification technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790
9.1. Efficiency benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790
9.2. Environmental benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790
9.3. Feedstock flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 791
9.4. Product flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 791
9.5. Carbon capture, storage and utilisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 791
10. Types of gasification technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 791
10.1. The fixed-bed updraft or counter-current gasifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 791
10.2. The fixed-bed downdraft or co-current gasifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 792
10.3. The fixed-bed crossdraft gasifier. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 792
10.4. The fluidised bed gasifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793
10.5. The entrained flow gasifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793
10.6. The plasma gasifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794
11. The choice of gasifier for sugarcane bagasse gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 796
12. Influence of gasifier design on the gasification process of sugarcane bagasse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 796
13. The chemistry of sugarcane bagasse gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 796
14. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 797
15. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 797
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 799
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 799

1. Introduction pathway which is most appropriate for all types of biomass in-
cluding SCB. There are a number of required characteristics for a
Sugarcane bagasse (SCB) is the fibrous residue obtained after pre-processing method to be considered industrially viable. These
the extraction of the sucrose-rich juice from sugarcane stalks. The characteristics include the requirement that the pre-processing
various uses of SCB have been widely reported in the literature, method should result in minimum degradation with maximum
and these include the manufacture of pulp and paper, animal feed, component recovery; it should have a low energy demand or be
furfural, and other value added products [1,2]. However, these are conducted in a way that the energy can be re-used in other process
limited markets which are also highly competitive. SCB has been steps as secondary heat; and, it should have low capital and op-
considered as an agricultural biomass residue of great importance erational costs [8,9]. The use of physical methods for pre-proces-
as a fuel for the sustainable production of electricity [1,3]. In the sing can be considered as meeting all of these requirements. Pre-
past, excess SCB was burned as a means of solid waste disposal, processing of SCB is intended to overcome inherent issues related
but as the cost of auxiliary fuels increased, the need to derive to the disperse nature of bagasse, its high moisture and inorganic
greater energy from all the SCB available to the factory became contents as well as its low energy and bulk densities. These
imperative. Currently in countries like South Africa, bagasse is shortcomings limit the widespread deployment of the thermo-
used as a convenient fuel for the sugarcane industry but through chemical conversion systems using bagasse as feedstock for en-
inefficient combustion processes. This inefficient usage necessi- ergy production purposes, rendering these systems unattractive.
tates that in some instances supplementary fuels such as coal be However, the complexity of SCB (in chemical composition and
used in significant amounts during factory operations. Conse- heterogeneity) is so high that its use as an energy feedstock re-
quently, for the industry to produce more energy from available quires further research and development to better understand the
SCB and offset the use of costly or non-renewable energy sources, exact pre-processing and thermal conversion system parameters
more cost-effective and efficient technologies are required. with respect to the polymeric structure and mineral composition
The application of SCB for optimal energy production requires of the material. One of the thermochemical conversion pathways
an understanding of its composition, for which many studies have by which SCB can be converted into energy is through gasification,
been performed. Mill-run bagasse contains approximately 50% fi- which is a thermal devolatilisation process that breaks down any
bre, 48% moisture and about 2% sugar [4–7]. This composition carbon-based material into its basic chemical constituents [10].
makes SCB an ideal material for energy production, however, its The process is based on a series of complex reactions that are in-
efficient utilisation for energy production in thermochemical fluenced by many factors including the composition of the feed
conversion systems has been impeded by a number of factors, one material to be converted, the pre-processing conditions of the feed
of which is its handling and pre-processing which must be con- and the operating conditions of the gasifier [11]. The feedstocks
sistent with the energy conversion system that it is used in. There required for gasification, the advantages and disadvantages of the
are various pre-processing methods that are available for biomass; gasification technology as an energy production process are de-
however, there seem not to be a universal method or technology tailed later in Section 9 of this review.
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 777

This work therefore provides a comprehensive review of the purposes to undertake efficient pre-processing of the material to
pre-processing methods (with emphasis on the physical method of make it suitable for the energy conversion systems. As excess
pre-processing) that can be applied on SCB to increase its value as bagasse is generated during sugar processing, the sugar industry
a feedstock for the purpose of gasification in a downdraft biomass boilers are basically operated inefficiently to also dispose of the
gasifier system. The idea is to identify and recommend the best excess bagasse since there must be a balance between bagasse
pre-processing method for sugarcane bagasse after careful review
production and utilisation [17]. Fig. 1 shows a simplified SCB
of the various pre-processing methods. A description of the gen-
generation process diagram.
eration and current use of SCB and its handling including issues
related to its efficient utilisation and the quality attributes of a pre-
processed SCB are painstakingly described. The composition and
properties of bagasse in relation to gasification are also discussed
3. Sugarcane bagasse as an alternative energy resource
together with the possible effects of pre-processing not just on
chemical and structural transformation of SCB, but also on gasifi-
The selection of feedstock for energy production purposes is
cation process efficiency. The pre-processing methods that are
dependent upon certain criteria such as potential yield per hec-
better suited for the purpose of gasification are also highlighted
tare, feedstock properties and the potential uses [18]. SCB was
and described together with a review of the various gasification
technologies including a comparison of the advantages and dis- chosen for this review because of its potential availability in excess
advantages of each gasification technology as well as the most of its usage as well as because of the fact that its use as a potential
suitable gasification technology for SCB conversion based on low feedstock for efficient energy production has not been fully ex-
cost, simplicity, ease of operation and efficiency. plored. However, the value of SCB as a fuel for energy production
largely depends on its calorific value, which in turn depends on its
composition, especially with regard to its moisture content and to
2. Sugarcane bagasse generation and handling in South Africa the calorific value of the sugarcane plant, which mainly depends
on its content of sucrose [19]. In sugar mills, bagasse is usually
A surplus amount of SCB is generated in South Africa (about combusted in furnaces for steam production, and the steam in turn
3.3 million tons of raw bagasse is generated per annum) [12]. On
is used for power generation; but the challenge of this process is
average, about 30 t of wet (about 50% moisture) SCB is produced
related to the net electrical efficiency, which is extremely low
per 100 t of cane crushed per annum, and approximately 150 t of
(between 10–20%) when compared with the gasification process,
dry bagasse per 100 t of cane crushed is also produced per annum;
so for every 3 kg of cane crushed, 1 kg of bagasse is produced, an which can have an efficiency as high as 67–80% [20,21]. Another
amount could generate about 124 t/h of heat in the form of process limitation of the use of the boiler technology for bagasse com-
steam to the mill, and about 56 MWe of electricity (10 MWe to be bustion is the duration of startup, which is usually up to 8 h as
used by the sugar mill itself and 46 MWe by the rest of the com- well as the use of auxiliary fuels as startup fuels, which results in
munity where the mill is located) [13–15]. Notwithstanding, SCB SO2 and NOX emissions including particulate emissions due to
handling begins with the harvesting of the sugarcane crop from poor conditions of combustion in the boiler while it is cold during
the growing fields before it is being transported to the sugar fac- the startup period [21]. There are several pathways by which SCB
tory where it is crushed to extract the sucrose-rich juice [16]. After can be converted into energy and some of those pathways include
sucrose extraction, the resultant fibre (bagasse) is either im- gasification, pyrolysis, liquefaction, fractionation, fermentation
mediately used as fuel in boilers or is stored for future use. Storage and hydrolysis [8]. However, the main focus of this review is on
could be either in open heaps or in the form of bales. The process
the pre-processing of SCB for the purpose of gasification, taking
of handling the sugarcane fibre before and after processing the
into account the quality of the pre-processed bagasse and its
cane could result in the introduction of extraneous substances
possible effects on gasification output parameters such as effi-
(foreign debris) that could appear as impurities, which could fur-
ther reduce the value of SCB as a fuel for energy production as well ciency of the process and product gas yield.
as damage energy conversion systems including releasing harmful Knowing the basic characteristics of biomass is key to suc-
chemicals when bagasse is combusted [3]. Therefore, handling cessful operation of the energy conversion system using the bio-
methods of SCB coupled with its low energy and bulk densities as mass as feedstock [3]. The following section therefore details the
well as its heterogeneous size and shape make it necessary for composition and properties of SCB and their impact on
systems using bagasse as feedstock for energy production gasification.

Bagasse

Sugar cane
Cleaning Juice Raw
& Washing Extraction Juice

White sugar
Crystallization Multiple Clarified
& Juice Juice
Effect
Centrifugation Preheating Purification
Evaporatio

Fig. 1. A simplified process diagram for the generation of sugarcane bagasse [16].
778 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

4. The composition and properties of sugarcane bagasse Table 2

Typical ultimate analysis data of sugarcane bagasse (dry ash free basis).
The composition and the inherent properties of the source of
Ultimate analysis (wt%)
biomass determine both the choice of the conversion process and
any subsequent processing challenges that may arise, as the bio- C H O N S Reference
mass choice is equally influenced by the form in which the energy
44.1 5.7 47.7 0.20 2.30 [23]
is required, with the interplay between these two aspects enabling
43.77 6.83 47.46 Not reported Not reported [24]
the introduction of flexibility into the application of biomass as an 56.32 7.82 27.54 0.89 Not reported [25]
energy source [5]. The following sub-sections also presents and 44.1 5.26 44.4 0.19 Not reported [26]
focuses on relevant properties and composition of SCB and their
effect on gasification process efficiency based on studies con-
ducted by previous authors. Table 1 by the first two authors.
Pre-processing of SCB has significant effects on all downstream
4.1. Proximate and ultimate analysis of sugarcane bagasse processes and would ultimately influence the overall yield of the
gasification process and cost [27]. As opposed to proximate ana-
The composition and properties of biomass can be described in lysis, ultimate analysis provides the elemental composition of
terms of proximate and ultimate analyses, which are normally the biomass. It is performed by the complete combustion of a fuel,
first steps taken to evaluate the suitability of any biomass material with the composition of the final products of combustion analysed
for conversion into energy [22]. Proximate analysis provides the and the main elements of the solid biomass determined. The ul-
fuel properties in terms of the weight percentages of moisture, timate analysis results of SCB from previous studies are also pre-
volatile matter and fixed carbon as well as ash content of the sented in Table 2.
material. It is performed by heating the material to a set tem- It is evident from Table 2 that the main elemental constituents
perature, resulting in the decomposition of the material at that of SCB are C and O2 with a negligible amount of H2. During gasi-
temperature to generate volatile substances. The volatile sub- fication, H2 plays a role in the final product gas composition due to
stances released from the decomposition reactions contain a series its impact in the water-gas shift reaction discussed later in Section
of gaseous molecules of CO, H2 and CO2 together with other hy- 13 (Table 6) of this review [28]. The high O2 composition is due to
drocarbons. The rate of decomposition and the released gas the alcohol (OH) and carboxylic acid (COOH) groups in the main
composition is dependent upon temperature and the heating rate constituents of bagasse which are cellulose, hemicellulose and
of the decomposition reaction [22]. Pyrolysis or devolatilisation lignin, and which also accounts for the high reactivity and high
are terms used to describe these decomposition reactions. The ignition stability of SCB when used as fuel in thermochemical
moisture content of the biomass is the amount of water molecules conversion systems such as the gasification systems [29]. However,
that bond physio-chemically to the material, and can be removed the difficulties in using biomass materials such as SCB as fuel for
by heating without the occurrence of chemical reactions in the energy production relates to its content of inorganic constituents,
process. Char is obtained as the left-over from the devolatilisation as some types of biomass may contain significant amounts of Cl, K
process of the biomass, and consists of fixed carbon and ash [22]. and S in the salts of these elements (KCl and K2SO4) which are
The proximate analysis data of SCB are listed in Table 1. quite volatile. Their release may lead to large amounts of deposi-
The data in Table 1 shows the extent of variation in the com- tion on heat transfer surfaces, resulting in decreased heat transfer
position of SCB, which can be attributed to a number of factors and enhanced rates of corrosion [30]. Unscheduled plant shut-
including the source of the bagasse, the variety of the sugarcane down may be experienced due to severe deposits, and significant
crop and the growing conditions of the sugarcane including soil amounts of aerosols as well as relatively high emissions of HCl and
texture and composition where the cane was grown as well as SO2, which may also be generated due to the release of Cl, alkali
weather and other conditions [3]. Although there is no significant metals and S in the gas-phase [30]. The metal/inorganic elements
variation in the composition of bagasse from Table 1, but it can of bagasse are not given in Table 2 but can be determined from
also be clearly seen that SCB generally contain more volatile other analyses such as ash analysis.
matter, which results in a lower char yield when bagasse is com-
busted or gasified. The moisture content varies depending on the 4.2. The heating value of sugarcane bagasse
source of the bagasse and specific handling conditions; however it
is generally low because it is often air-dried from initial moisture The conversion process of SCB or any biomass material begins
content that is normally close to or above 50% as received before
with the knowledge of the energy content of the biomass, mea-
utilisation [23]. Generally, the proximate analysis of biomass pro-
sured in the units of MJ/kg [31]. The thermal conversion of bio-
vides a measure of the ease with which the biomass can be ignited
mass in the presence of excess amounts of air to release energy in
and subsequently gasified or oxidised, depending on how the
the form of heat is termed its heating value or calorific value. This
material is to be utilised as a source of energy [23]. However, pre-
is usually measured using a bomb calorimeter; however in the
processing before conversion is intended to lower moisture con-
absence of equipment for measuring the heating value of biomass,
tent by drying, hence the low moisture contents reported in
two common equations are used to estimate this value. These are
the Dulong equation and the Boie equation [32,33]. The Dulong
Table 1
Typical proximate analysis data of sugarcane bagasse (dry basis).
equation can be written as follows [31,32]:
⎛ H − O⎞
Proximate analysis (wt%) HV ( MJ/kg)=33, 823×C +144, 250⎜ ⎟+9419×S
⎝ 8 ⎠ (1)
Moisture Volatile matter Fixed carbon Ash Reference
where HV is the heating value of the material in MJ/kg, and C, H, O
1.14 69.99 16.39 1.42 [23] and S are the elemental mass fractions of the material. However,
9.51 74.98 13.57 1.94 [24] Eq. (1) is only valid when the O2 content of the biomass is less than
Not reported 84.83 13.28 1.89 [25]
Not reported 75.80 20.11 4.21 [26]
10% [31].
The Boie equation is given by the following [31]:
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 779

HV ( MJ/kg)=35, 160×C +116, 225×H−11, 090×O+6, 280×N shape factor (ɸ) for the particle is defined by length (l) and dia-
meter (d) of the particle. Bulk density is also affected by other
+10, 465×S (2) factors such as surface characteristics of the biomass [43]. For
where, C, H, O, N and S are the elemental mass fractions of the biomass materials, bulk density is commonly expressed on an
biomass material. oven-dry-weight basis (with moisture content of the biomass
SCB is known to have a low heating value between 17 and nearing zero percent), or as received basis where the biomass
20 MJ/kg [23] because of its high O2 composition, which is typical moisture content (MCw) is indicated. Most biomass feedstocks
of biomass materials. Feedstocks with high heating values are al- generally exhibit low bulk densities, including SCB which has a
ways better for gasification, and the conversion efficiency of a relatively low bulk density (75–200 kg/m3), compared to other
gasification process is based purely on energy in the feedstock biomass materials; the bulk density of biomass can be determined
[34,35]. A high heating value material is beneficial as it leads to experimentally from the following equation [3,24,29,43]:
improved functionality and reduced energy use of feedstock con- ⎛ w2 − w1 ⎞
ρb = ⎜ ⎟
veyor at power plants [36]. The heating value of biomass is actu- ⎝ v ⎠ (4)
ally dictated by the amount of C and O2 in the biomass, with the
material having a higher amount of O2 than C, which as a result where ρb is the bulk density of the biomass material in g/cm3, w2 is
lowers the calorific value per unit volume of the material [37]. Low the weight of the container and biomass in grams (g), w1 is the
energy density of biomass implies that cost of transportation weight of the container in grams (g), while v is the volume of the
would be high per unit energy, with more space required for container in cm3.
storage, thereby, making material logistics expensive. System ef- Fibrous materials with larger particles have low bulk density
ficiency is also affected by low energy dense materials; and low because they have more pore volume than smaller particles [44].
energy density also means that more fuel would be required to Low bulk density materials create feeding difficulties in gasifica-
obtain the same amount of energy [3,38]. tion systems as materials with low bulk density do not allow for
gravity feed inside the gasifier, a condition that leads to poor
4.3. The bulk density of sugarcane bagasse combustion conditions within gasification systems, resulting also
in reduced process efficiency [45].
Efficient and economic conversion of biomass to energy rests
on consistent and economic transportation of the biomass from 4.4. Microstructure, macrostructure and chemical composition of
the field to the bio-refinery. One major factor that affects the de- sugarcane bagasse
livery cost of biomass is its bulk density during collection and
transportation as well as during storage [39]. Bulk density is a SCB is highly complex in structure as well as in chemical ma-
biomass property that not only determines the cost of feedstock keup, so to better understand and describe thermochemical con-
delivered to a bio-refinery, but also affects the design and opera- version processes using bagasse as feedstock for energy produc-
tion of energy conversion systems and heat transfer equipment tion, examination and analysis of the microstructure and macro-
[40]. The bulk density of a material, denoted рb, is defined as the structure as well as the chemical makeup of bagasse are necessary
weight per unit volume of that material, expressed in kilograms [3]. The microstructure of bagasse is linked to its low molecular
per cubic metre (kg/m3), and depends on certain factors such as weight substances which include the organic and inorganic sub-
composition, particle size (l, d) and shape (ɸ) as well as particle stances present in its structure, while its macrostructure are re-
orientation (s) and specific density of individual particles (рp) in- lated to the cellulose, hemicellulose, and lignin present [46]. A
cluding particle size distribution (PSD) and moisture content (ω) of diagrammatic representation of the composition of SCB based on
the biomass material [41,42]. The relationship between these the micro and macromolecular substances present is shown in
parameters is given in Eq. (3) [43]. Fig. 2.
Cellulose and hemicellulose have the formulae (C6H10O5)m and
(
ρb = f c , ρp , l, d, φ , s , PSD, ω, p ) (3) (C5H8O4)n respectively. These two compounds are polysaccharides
with degree of polymerisation represented by m and n, and with
where l and d are particle length and diameter respectively. degrees of polymerisation that are less than 10,000 and 50–300 for
The variables on the right hand side of Eq. (3) are not all in- cellulose and hemicellulose respectively [22]. Lignin is highly ir-
dependent of each other. For example, moisture content has an regular in structure and consists of aromatic, phenolic and various
impact on particle density. Similarly, particle size and distribution hydrocarbon groups. However, the fraction of these three main
depends on the type of pre-processing method applied during the components of SCB varies among the species of sugarcane as well
preparation of the biomass such as drying or grinding, while a as its origin; therefore, the composition of SCB is usually written in

Bagasse

Micro-molecular Substances Macro-molecular Substances

Organic Matter Inorganic Matter Polysaccharides Lignin

Extractives Ash Cellulose Polyoses

Fig. 2. A schematic representation of the composition of sugarcane bagasse. Copyright American Chemical Society. Reproduced with permission from [46].
780 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

Cellulose

Lignin

Hemicellulose

Cellulose
bundles
Fig. 3. Structural arrangement of the plant cell wall showing the location of cellulose, hemicellulose and lignin in the plant matrix. Reproduced with permission by Elsevier
from [50,51].

empirical form as CHxOy, which is based on the molar fraction of content and high metallic ion content as well as heterogeneous
the elements present in bagasse [3]. Lignin provides many struc- size, weight and shape, including storage related problems [3].
tural functions in plants including acting as glue to the cellulose These shortcomings, as previously highlighted, limits the use of
and hemicellulose fibres, and as intrinsic resin. It also helps to bagasse as feedstock for energy production due to certain opera-
form pellets or briquettes without binders because of its ther- tional challenges experienced during the conversion process of the
mosetting properties at working temperatures of 4140 °C [47]. material, which include low process efficiency [23]. In addition to
Adhesion in the structure of lignocellulosic plant material is per- the problems earlier highlighted for materials with low bulk
mitted by the lignin content of that material, acting as a bulking density, handling and transportation challenges including storage
and rigidifying agent; and the strength characteristics of briquettes and combustion process challenges with regard to gravity feeding
made from lignocellulosic biomass materials are attributed to the in the conversion system are also issues related to materials with
adhesive properties of thermally-softened lignin [48,49]. The ar- low bulk density. This would remain prone to caking in the pyr-
rangement of cellulose, hemicellulose and lignin in the plant olysis zone of the conversion system, a condition which increases
matrix is shown in Fig. 3. This is presented to better understand the possibility of bridging in the system; therefore, for bridging to
the arrangement of the main constituents of plant biomass and the be avoided, the ratio of the throat diameter to fuel diameter
structural functions of lignin. should be at least 6.8:1 [53]. Since gasification involves a series of
The chemical composition of SCB or any biomass material for concurrent and parallel reactions, the thermochemical processes
that matter is highly dependent on the material species and its taking place in the reactor is influenced by devolatilisation, which
source [22]. Studies have shown that C, H2 and O2 are the pre- in turn affects the yield and composition of the product gas, while
dominant chemical elements contained in SCB. In addition, a heating at an optimum rate for rapid devolatilisation of the major
number of other inorganic elements are also present which re- components may also be hindered with feedstock having low bulk
presents the major source of metallic ions and other acidic sub- density [54,55]. High moisture content also indicates that the al-
stances that are formed when bagasse is combusted or gasified, ready low energy density of bagasse would be further reduced as it
including trace elements which act, even at very low concentra- leads to a reduction in energy value which could result in the
tions [3]. However, the behaviour of cellulose during gasification is production of less heat per unit mass of material during thermo-
important in understanding the gasification of biomass materials chemical conversion of bagasse, subsequently leading to reduced
as gasification rate becomes faster with increased biomass cellu- process efficiency. The high metallic ion content of bagasse leads
lose content, with tar and gas yields increasing as the content of to the production of acidic substances that may corrode system
cellulose increase, while char yield decreases; the main gas pro- components, which may require intermittent replacement be-
ducts from cellulose are CO2 and CO due to monosaccharides cause of the production of highly corrosive ingredients, and may
compounds such as glucose from cellulose, which are decomposed also result in plant shutdown; heterogeneous size, weight and
through a decarboxylation process. The reaction behaviour of shape also create problems related to decrease in specific area and
hemicellulose is similar to that of cellulose, while lignin decom- surface energy per unit mass of bagasse, which may not facilitate
poses at a much wider temperature range forming CO2 and CH4 faster rates of heat transfer [3]. Feeding biomass into the conver-
during gasification, due largely to the dealkylation of the side sion system also becomes a challenge with irregular feedstock size,
chain of the alkylphenols in the lignin structure [52]. weight and shape as these may lead to some sort of blockages
within the conversion system, resulting in poor combustion con-
ditions, usually necessitating that some conversion systems are
5. Issues related to sugarcane bagasse utilisation strict in terms of feedstock size requirements [3,45]. SCB, by its
very nature consists of degradable or fermentable materials and
There are various technical issues related to the use of su- represents a bulk material that presents a media through which
garcane bagasse (SCB) as a fuel for energy production purposes. moisture can migrate or be absorbed, a factor that add to storage
These are low energy density, low bulk density, high moisture related problems such as compounded mass loss due to microbial
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 781

contamination and wind erosion as a result of exposure to the 7.1. Densification

environment leading to under-usage of the feedstock [16,23]. This
factor greatly affect thermochemical conversion systems using The most serious biomass size or shape-related problems when
bagasse as feedstock for energy production purposes due to the using improperly sized or shaped materials are issues linked to the
high level of impurities and particulate matter as well as high chemistry of the biomass and its conversion process. Biomass
amount of tar produced along with soot formation and agglom- thermochemical conversion to energy takes place at the interface
eration, making the technologies unattractive [3]. of heat, O2, and the biomass as well as other reactive components
as an interface reaction on the surface of the biomass material.
Densification is one pre-processing method usually applied to
achieve uniform properties and to increase the densities of bio-
6. The pre-processing of sugarcane bagasse
mass materials [27]. With densification, the problem of hetero-
geneous size and shape are addressed. A densified material is easy
There are many challenges of efficiently converting raw SCB
to handle, transport and store and densification can be achieved
into usable and affordable customised bioenergy feed material.
with most biomass materials including SCB, provided that they
The pre-processing of SCB is intended to overcome technical issues
attain the correct moisture content and particle size [56]. Methods
of using bagasse as feedstock for energy production. Some of these
commonly used to achieve densification are pelleting or briquet-
issues have been previously highlighted, and the quality of a pre-
ting. These methods are detailed in the following sub-sections. A
processed material depends on the pre-processing methods ap-
comparison of the different technologies for densification is also
plied [27]. As earlier mentioned, large quantities of bagasse is described including the advantages and disadvantages of each
generated for every ton of cane crushed [13]. While most of this is technology as well as their process variables.
consumed internally as fuel in boilers, there are still huge excess
quantities to be handled. This excess quantity and the hetero- 7.1.1. Pelleting
geneous form in which it is present pose a serious challenge re- The issues stated earlier of using bagasse for energy production
lated to fire hazards to the sugar mill where it is mostly used. In makes the material difficult and expensive to store, handle and
addition, microbial reaction can lead to spontaneous combustion transport. Bagasse pelleting could potentially reduce storage and
due to the burning properties of SCB [16]. Therefore, one of the handling costs. It could also reduce the consumption of fuel oil
measures that would help mitigate these hazardous conditions is (residue obtained from petroleum distillation) at the mill, result-
to resort to efficient methods of disposing the excess bagasse ing in viable use of these alternatives. However, the use of biomass
generated, but yet maximizing this disposal process by thermo- pellets or briquettes for energy production purposes depends
chemically converting it to useful energy. This would, however largely on the type of conversion system employed [27]. Energy
require efficient pre-processing steps due to the disperse nature of conversion systems such as the fixed bed downdraft gasification
bagasse and other inherent technical issues previously described. systems are relatively strict in terms of size requirements. There-
Apart from these challenges, pre-processing of SCB also allows the fore, feedstock for conversion in these systems must be uniformly
removal of foreign materials and dirt, eliminating significant sized from 4 to 10 cm in length, and about 30–50 mm in diameter
downstream conversion issues related to the formation of slag and so as to avoid blockage of the throat of the gasifier, as blockage
other foreign material issues. may lead to poor gasification conditions. As a result, the use of
pellets in this type of gasifier is highly discouraged due to the size
of the pellets, as blockages may occur within the combustion zone
of the gasifier, resulting in combustion (a condition that leads to
7. The various pre-processing methods to improve bagasse
poor process efficiency) instead of gasification [45].
quality
Even though biomass pellets have specifications, there are
various sizes, densities and composition of pellets produced de-
Biomass, including SCB is difficult to work with when com-
pending on how the biomass material is to be utilised [56]. Pellet
pared with conventional fuels like coal, which are used by a
shape is determined by the equipment used to make the pellet.
variety of energy conversion systems, with their benefits making
Fig. 4 shows sugarcane bagasse pellets obtained from a pellet mill.
them almost the exclusive source of energy for most industrial
systems [56]. This has significantly limited the deployment of the
conversion systems using biomass as feedstock for energy pro-
duction purposes. SCB in its original form is difficult to successfully
use as a feedstock in conversion systems for energy production
purposes under the current circumstances in which it is generated.
This has been largely due to its wet and dispersed nature as well as
other issues previously pointed out. However, the process of ga-
sification is the most demanding among all thermal conversion
processes in terms of product end-use, which can be affected by
pre-processing of the feedstock to be converted [57]. As earlier
stated, pre-processing of biomass is usually applied to reduce
technical challenges associated with low bulk and energy densities
as well as heterogeneous size. Among the pre-processing methods
available to overcome the technical issues of using SCB as an en-
ergy resource, and to make it a valuable feedstock are drying,
grinding, densification, torrefaction and steam explosion. A de-
tailed description of these methods and a comparison of their
advantages and disadvantages are given in the following sub- Fig. 4. Sugarcane bagasse pellets. Used with the permission of Hang Xanh Inter-
sections. national from [56].
782 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

III
I

II
I

IV II
V
III II I

VI

Fig. 5. A schematic diagram of a pellet mill. From [58]. Used with permission. III

7.1.1.1. The pellet mill. Basically, pellet mills are designed in such a Fig. 6. The schematic diagram of a pellet mill die and its working principle. From
[58]. Used with permission.
way that a large roller pushes the biomass through a small hole in
a thick metal die that gets narrower around the centre of the die as
the material gets compressed, binding together in the process the generation of dust during feeding into the conversion systems
under high temperatures. Pressure and friction between the die because the pellets are easily disintegrated causing difficulties in
and the biomass cause significant heat up of the pellets as the handling [27]. In addition, the pellet mill is unable to handle large-
material is compressed. Cutters are used to chop the pellets into sized feedstocks with high moisture contents [58].
length as the material begins to cool in its new shape of pellets on
the other side of the die which is approximately 30 mm in dia- 7.1.2. Briquetting
meter. The size of most pellets produced is in the range of ¼” and Briquetting is a high-pressure compaction technology used to
9/19″ [56]. A schematic diagram of a typical pellet mill is shown in increase the densities of biomass materials and remains a viable
Fig. 5, where I, II, III, IV, V and VI represents the feeder, the con- and attractive solution to biomass utilisation as a potential feed-
ditioner, the pelleter, the speed reducer, the motor and base of the stock for energy production. The process of briquetting is usually
mill respectively. carried out with a hydraulic, mechanical or a roller press type of
The working principle of the pellet mill is such that the ma- briquetting machine. After briquetting, the densities of the bio-
terial from the feeder is uniformly delivered to the conditioner mass are increased between 900 and 1500 kg/m3, which can
under controlled addition of steam or a binder to improve the conveniently be used in conversion systems or even in open fires;
process of pelletisation. A permanent magnet is used to discharge and larger sizes of materials with higher moisture contents can be
the feed from the conditioner into a feed spout which leads to the handled by a briquetting machine unlike the pellet mill. Briquet-
die of the mill. The rollers of the mill are driven by friction as the ting increases biomass densities and address the problem of het-
die revolves, forcing the feed through holes in the pelleting die. erogeneous size and shape of the biomass, resulting in uniform
Fig. 6 presents a schematic of a typical pellet mill die with holes in and improved combustion characteristics as well as low particu-
between the die. The pellets extrude through the holes in the die. late emissions [27]. The technologies for briquetting are classified
I, II and III represent unpelleted material, the pellets extruded according to the method used to compress the material. These
through the die plate, and pellet knives respectively. The un- include the piston press, the screw extruder and the pellet mill
pelleted material is forced through the holes of the die and is which has earlier been described. The piston press densification
made to extrude via a plate on the die of the mill as represented by technologies include the hydraulic piston press, the mechanical
the schematic diagram in Fig. 6. The pellet knives chop the pellets piston press and the roller press [60]. The following section elu-
into size. cidates the types of briquetting technologies that can be used for
Pelleting may be a viable option for most biomass conversion the densification of biomass. Their merits and demerits are also
systems because of the ease of handling of pellets, however, a described.
major limitation in using biomass pellets is the energy require-
ments and associated cost of producing them. Compressing the 7.1.2.1. The hydraulic piston press. The most common type of bri-
biomass material through the openings of the die requires the use quetting machine used for biomass densification is the hydraulic
of large motors, which in turn requires large amount of energy for piston press, due to the ease of charging its furnace, its compact
compression to be achieved [56]. Furthermore, the biomass ma- and light nature and its low output levels [27]. This type of bri-
terial must be fairly small so as to be forced through the holes of quetting machine works on a principle of fluid-pressure trans-
the die, which may require the use of hammer mills to get the mission that is based on Pascal's law, where the pressure exerted
material into sawdust form. Therefore, the costs of energy and anywhere in a confined incompressible fluid is transmitted equally
equipment for pelletisation are significant limitations of using this in all directions throughout the fluid in such a way that variations
form of densified biomass feedstock. The power consumption of in pressure remain the same. A schematic representation of the
the pellet mill is in the range of 15–40 kWh/t of biomass [59]. hydraulic press briquetting machine is presented in Fig. 7.
Another disadvantage associated with the use of biomass pellets is The material for briquetting is fed through the feeding cylinder
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 783

Feeding
cylinder

Main
Piston
cylinder

Die Flywheel

Briquettes out Electric

motor

Compression
chamber
Base

Fig. 7. Schematic of a hydraulic piston press showing its components. From [58]. Used with permission.

production of biomass briquettes ranging from 200 to 2500 kg/h

[27]. It is designed as an eccentric press due to the mechanism of
compaction, which transforms rotational force from the motor into
a force that performs the compression; hence it is primarily a
mechanical process. In this type of briquetting however, the piston
reciprocates after mounting it eccentrically on a crank-shaft with a
rotating flywheel. An oil-bath is used to hold the shaft and the
piston rod as well as the guide for the rod. Fig. 9 shows a typical
mechanical piston press used for biomass densification.
The top of the piston of a mechanical piston press is normally
shaped with a half-spherical section that protrudes and functions
to get adherence of the compressed material from the one pre-
viously formed in the stroke. The die has a diameter in the range of
40,125 mm and remains a key factor that determines briquette
quality [61]. The high forces acting during compression are ab-
sorbed by the moving parts which are mounted within a sturdy
frame. Compression pressure in the mechanical briquetting press
is in the range of 110–140 MPa, and a combination of this pressure
Fig. 8. Sugarcane bagasse briquettes from a hydraulic press. From [62]. Used with with the heat produced as a result of friction from the walls of the
permission.
die raises the temperature of the material to a level where the
lignin content of the biomass begins to melt, acting as a binder in
which often works by pre-compacting the material before the the process to produce a stable briquette [61]. Unlike the hydraulic
main cylinder is pressurised. Energy is transmitted from the piston press which is driven by a hydraulic motor, the mechanical
electric motor as it rotates, to the piston which pushes the material piston press is driven by an electric motor. Energy loss in the
into the compaction chamber where material compaction takes machine is minimal with optimal output in relation to power
place. After compaction, the material is further pushed from the consumption [27]. Fig. 10 presents a process flow diagram for
compaction chamber to the die section which allows for controlled ceaseless briquetting using a mechanical piston press.
expansion and cooling of the briquettes. The briquetted material After grinding, the material for briquetting is fed from a feeding
then finally extrudes through the die, assuming the size as well as system that is designed with a drilling device inside and which
the shape of the die. The pellets are released from the die rela- functions by rotation to push the material up by centrifugal force
tively warm and fragile, and would therefore need to be cooled to the cyclone where separation of air and the raw material takes
further before it can be cut to the desired size [61]. Fig. 8 shows place through gravity and vortex. The feeding system is mounted
bagasse briquettes made from a hydraulic piston press type of some distance away from the briquetting press as shown in Fig. 10.
briquetting machine. The rotary valve also functions by rotation, regulating the passage
The bulk density of briquettes is lower than 1000 kg/m3 due to of the material from the cyclone into the dosing silo which acts as
limited pressure which is usually about 30 MPa; however, the a storage tank and designed with a protruding feeding screw
hydraulic type of briquetting machine can tolerate material somewhat underneath. The protruding screw system pushes the
moisture content above 15% [61]. Another advantage of this type of material from the silo to the briquetting machine where the ma-
briquetting machine is its limited daily service due to its long terial is compressed under high pressure (between 110–140 MPa).
technical life. The die, piston and cylinder of the hydraulic press The briquettes are formed through the die section of the machine
are the main wear parts, and the service lives given by manu- and remains relatively warm and fragile until cooled further be-
facturers of this type of machine for these parts are between 500 fore been cut into pieces of the desired size.
and 1000 h [60].
7.1.2.3. The roller press. Roller press briquetting machines have
7.1.2.2. The mechanical piston press. The mechanical piston press is been in use since 1870 and operate on the principle of pressure
a type of briquetting machine typically used for large-scale and agglomeration such that pressure is applied between two
784 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

Feed
material
Piston
guide

Control
panel

Crank-
shaft
Piston rod

Electric
Briquettes
motor
out
compartmen
Die

Fig. 9. Main features of a mechanical briquetting press. Used with permission from [63].

Fig. 10. Flow diagram for ceaseless briquetting using a mechanical piston press. Used with permission from [63].

Feed In roller presses, the material for briquetting is compressed

between two counter-rotating rollers such that initial densification
Feed material occurs through compression by the screw feeder in the feed me-
chanism. Removal of air from the material is the primary purpose
Screw feeder of the initial densification process. The rollers are designed and
arranged in such a way that a small gap exists between them to
Rotating roller allow for compression; and the distance between the two rollers
Rotating roller
depends on a number of factors such as particle size, type of
biomass and moisture content of the biomass [24]. High pressure
Die is created as the material flows between the two rollers, resulting
in final compaction of the material. The two rotating rollers
function by drawing the feed material on one side, and the bri-
quetted material discharged on the opposite side of the rollers
with die openings to form the briquettes of the desired shape and
Briquettes size as the material passes in between the rollers. The shape of the
briquetted biomass is dependent on the type of die [64].

Fig. 11. Schematic of a roller press briquetting machine. From [58]. Used with
7.1.2.4. The screw compaction technology. Screw presses were ori-
permission. ginally developed and used for sawdust briquetting. The aim of
using the screw press for briquetting is for smaller particles to be
brought closer to each other so that they are made stronger due to
counter rotating rollers. Briquetting using roller presses happens the forces acting on them, and in effect, providing more strength
to be another approach readily adapted to densifying biomass to the briquetted material [27]. During briquetting using the screw
materials to use for energy applications [58]. A schematic diagram press, a rotating screw helps to move the material from the feed
of the roller press briquetting machine is shown in Fig. 11. port through the barrel and against the die of the press. This
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 785

Electric motor

Hopper

V – Belt
Die

Feeder Screw

Fig. 12. A schematic diagram of a screw extruder. Reproduced with permission

from [61].

results in significant pressure and friction between the walls of the

Fig. 13. Briquette with a concentric hole produced from a screw extruder/press.
barrel and the material due to shearing of the material [27]. Fig. 12 Used with permission by New-Air Technical Services Ltd. Leicester, England from
shows a schematic of a typical screw extrusion press for biomass [27].
briquetting.
The screw extruder basically works by continuously forcing the
Table 3
material into a die with the help of the feeder screw. The biomass General briquette specification made
is heated under increased temperature due to the combined im- from a screw extruder. Used with
pact of the high rotational speed of the feeder screw and the barrel permission by New-Air Technical Ser-
wall friction as well as the internal friction in the material. The vices Ltd. Leicester, England from [27].
heated material is forced through the die of the extruder to form
Material prior to extrusion
briquettes with the required shape or size. The main demerits of
the screw press include the severe wear issue of the die and its Moisture content 8%
head which results in high maintenance cost as well as its power Average particle size 2–6 mm
Bulk density 200 kg/m3
requirements, which is also high when compared to the piston
Calorific value 17.8 MJ/kg
press such as the hydraulic, the mechanical or the roller press;
however there are various advantages associated with the use of After extrusion
Moisture content 4%
this technology, one of which is its continuous output with more
Bulk density 1400 kg/m3
uniformly sized briquettes as well as partially carbonised outer Calorific value 19.53 MJ/kg
surface of the briquettes, which helps facilitate ignition and Ash content 0.3–0.5%
combustion and protects the briquettes from ambient moisture
including the fact that the briquettes form a concentric hole that
helps for better combustion due to air circulation during briquette the feedstock, the energy consumption of each technology and the
combustion; the screw extruder also runs smoothly without any suitability of the densified material for different end-use
load-attributed shock [59]. Fig. 13 shows a heat log with a con- applications.
centric hole made from a screw extruder. It is evident from Table 4 that the screw extruder has a higher
The specification of a typical briquette made from a screw ex- wear rate compared to other densification technologies (even
truder is given in Table 3. This is presented to better understand though the briquettes made from it may be suitable for gasifica-
the features of biomass after screw press method of pre- tion), which results in high maintenance cost and this wear rate
processing. supports its demerits previously mentioned [58]. It is also clear
It is evident from Table 3 that the bulk density of briquettes from Table 4 that the energy requirement of the screw extruder is
increased significantly from 200 kg/m3 to 1400 kg/m3 after screw higher than the other densification technologies with 36.8 kW h,
extrusion, which is typical of a briquetted biomass material made being the minimum energy that can be consumed per ton of bri-
from a screw extruder. The high bulk density results in improved quettes [58]. For the piston press, the maximum consumable en-
combustion characteristics of the briquettes and allows for gravity ergy is about 77 kW h/t, which is significantly less than that con-
feeding with reduced particulate emission when combusted in sumed by the screw extruder. The roller press and the pellet mill
thermochemical conversion systems [27,45]. have low maintenance costs even though they are characterised by
high wear rates, which is most probably due to their low energy
7.2. Comparison of different technologies used for densification consumption rate [58].

There are various technologies available for densification of 7.3. Densification systems process variables
SCB. These technologies are classified according to the type of
equipment and mode of operation including the operating con- Studies have shown that process variables such as pressure, die
ditions of the equipment as well as the applied pressure and temperature and die geometry play a major role in densification of
temperature [65]. Table 4 summarizes the comparison between biomass. Other process variables that also play major roles include
the various densification technologies in terms of the properties of material variables such as moisture content, particle size and
786 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

Table 4
Comparison of the performance of different densification technologies [58,59,66,67].

Piston press Screw press Roller press Pellet mill

Optimum moisture content of material 10–15% 8–9% 10–15% 10–15%

Particle size of material Larger Smaller Larger Smaller
Wear of contact parts Low High High High
Output from machine In strokes Continuous Continuous Continuous
Specific energy consumption (kW h/t) 37.4–77 36.8–150 29.91–83.1 16.4–74.5
Throughputs (t/h) 2.5 0.5 5  10 5
Density of briquettes/pellets 1000–1200 kg/m3 1000–1400 kg/m3 600–700 kg/m3 700–800 kg/m3
Maintenance Low High Low Low
Combustion performance of briquettes Moderate Very good Moderate Very good

shape; and material composition such as cellulose, hemicellulose transition temperature and the durability values of the material
and lignin [51]. System variables for densification are important if were lower compared to the ones within the temperature range.
the desired densities, durability and quality of the densified ma-
terial are to be achieved, with proper process conditions ensuring 7.3.3. Die geometry and speed
improved briquette quality [27,68]. The following sections high- The geometry (shape and size) of the die greatly affect both the
lights the process variables that play important roles in attaining amount of material that can be densified and the energy required
the desired product quality during briquetting of biomass, and for densification, and influences material properties such as bulk
these quality attributes are usually measured in terms of density density and durability as well as moisture content, with degree of
and durability as well as in terms of heating value [27,58]. compression determined by die length to diameter (L:D) ratio
during densification [27]. The pressure needed to press materials
7.3.1. Pressure through the press channels in the matrix and through the matrix
Pressure is one important densification process variable that itself is actually determined by the die dimensions of the densi-
play a major role in the quality of a briquetted biomass material as fication system, where the density of the briquette is greater for a
the density of the material is proportional to the pressure applied constant mass of material when smaller die diameters are used at
since an increase in pressure ensures a significant increase in the a given pressure [76]. The impact of die geometry and die speed as
density of the material [69]. Fractures may occur in biomass bri- well as particle size of biomass was studied by Tabil and So-
quettes due to pressure applied beyond the optimum pressure khansanj [73]. They found that using a smaller die with higher
required for briquetting, which may also lead to uneven briquette length to diameter (L:D) ratio significantly increased the durability
combustion during gasification of the briquettes. As a result, bri- of briquettes at a die speed rotation of about 250 rpm. Barrel
quetting pressure should be kept at a value that is optimum to temperature of the die and screw speed also affects, to a great
increase deformation due to influence on mechanical strength extent, quality attributes such as density and hardness of the
[69]. Diffusion of molecules from one particle to another at contact biomass material during densification using a screw extruder [77].
points may form solid bridges due to the application of high It has also been found that the flow rate of biomass feed is sig-
temperatures and pressure during briquetting, leading to a rise in nificantly affected by length to diameter ratio (L: D) of the die and
briquettes density [27]. When a biomass material is compressed at screw speed during densification using a screw extruder, and the
a pressure rate of about 0.24–5 MPa, there seem to be a significant quality of the final briquetted material is also greatly affected by
effect on density when moisture content of the biomass remains at this flow behaviour [78]. The effect of three die sizes on durability
about 10.3% [70]. A pressure increase from 300 to 800 MPa with of briquetted biomass was also studied by Heffiner and Pfost in
material moisture content of 7% on a wet basis would sharply 1973 [79]. They used die sizes of 4.8
44.5, 6.4
57.2, and
increase the density of the material by 78.6% during compaction 9.5
76.2 mm respectively. Their results showed that the best
[71]. durability values were briquettes produced on the smallest die
(4.8
44.5).
7.3.2. Temperature
The importance of temperature during densification of biomass 7.3.4. System retention time
cannot be overstated because it greatly affects quality attributes Briquette quality is also greatly affected by the time interval
such as bulk density and durability, and high temperature con- between the point of feeding into the densification system and the
ditioning during densification results in increased durability [72]. time required for compaction [73]. A study by Al-Widyan et al. [78]
Past studies [71] showed that the rate of compaction and dimen- found that retention time between 5 and 20 s had no significant
sional stability increased at temperatures between 60 and 140 °C impact on the quality and durability of biomass briquettes, while
during briquetting of biomass, and that briquette expansion was Li and Liu [68] also concluded that at low pressure, retention time
reduced when system die temperature was between 90 and had more impact on the durability and stability of briquettes than
140 °C. Due to chemical degradation, a charred surface and slight at high pressure, concluding that at the highest pressure of
discoloration could be noticed on briquettes at temperatures 138 MPa, the impact of retention time became negligible. The ef-
above 110 °C, while significant improvement on durability could fect on density of biomass briquettes is also negligible for a re-
be achieved at temperatures less than 90 °C [73,74]. The glass tention time greater than 40 s; and a 10 s time of retention could
transition temperature (75 and 100 °C) behaviour of lignin can be result in a 5% increase in briquette density whereas the effect
used to understand biomass behaviour during densification be- could significantly be reduced at retention times longer than 20 s
cause temperature is inversely proportional to the moisture con- [27]. Generally, retention time has a significant impact on the
tent of the biomass material [75]. Another study by Kaliyan and density of biomass materials and depends on a number of factors
Morey [75] using three different temperatures, which included such as temperature, pressure, flow rate, and so on. The final
two within the glass transition temperature range and one outside briquette obtained during densification would also depend on
it (150 °C), suggests that the temperature outside the glass system die geometry, the magnitude and mode of compression as
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 787

well as the type and properties of the biomass including storage to the elimination of hydroxyl groups (OH), without the support of
conditions [27]. hydrogen bond formation [91,92]. Non-polar unsaturated com-
pounds that result from the rearrangement reaction process of
7.4. Torrefaction torrefaction help preserve the biomass by reducing biological de-
gradation, which may render it less useful for energy production
In torrefaction, biomass materials are heated between the purposes [82,93]. More O2 and H2 are driven off as compared to C,
temperatures of 200–300 °C to change their properties to obtain a increasing the calorific value of the material in the process [94].
much better feedstock quality for energy conversion purposes The net calorific value of a biomass material that can be obtained
[38]. Torrefaction involves heating the biomass to a target tem- after torrefaction is in the range of 18–23 MJ/kg (LHV, dry), or 20–
perature in the complete absence of O2 to drive off H2O and vo- 24 MJ/kg (HHV, dry), depending on analysis conditions [89,95].
latiles in order to increase the energy density of the material [78]. The proximate and ultimate compositions of the biomass are also
This process is described as a mild form of pyrolysis since volatiles significantly changed after torrefaction, making the material sui-
are removed, resulting in a product with about 80–90% of the table for fuel applications [38].
original calorific value of the material, but with only 70% of the In general, increasing the torrefaction temperature results in an
initial weight [80]. The torrefaction process results in a very dry increase in the C content of the torrefied material, with O2 and H2
product with essentially no biological activity leading to microbial contents decreasing due to the formation of H2O, CO and CO2 [27].
spoilage [80–82]. The decrease in the ratio of H2 to C (H:C) and that of O2 to C (O:C)
There are many advantages associated with torrefaction of as torrefaction temperature and time increase results in less
biomass which include reduction in feedstock variability caused by smoke and reduced formation of water vapour as well as reduced
differences in types and species of the biomass, climatic and sea- energy losses during the process of combustion or gasification
sonal variations as well as storage related conditions [83]. Torre- [27]. A wide range of torrefaction processes were carried out by
faction helps develop a feedstock with uniform properties and Sadaka and Negi [96]. Their results showed a significant reduction
improves the physical characteristics of the material [84]. The in the moisture content of the torrefied material from an initial of
feedstock properties affected are hydrophobicity, grindability and 70.5% to a final of 49.4% and 48.6% respectively, with a corre-
the ability of the material to form briquettes. The lignin content of sponding increase in calorific value from an initial of 15.3 MJ/kg to
biomass is considered the basic binding agent in the material, and a final of 16.9 MJ/kg. Increasing the torrefaction temperature from
the ability of any biomass material to form briquettes is evaluated 230 to 280 °C and time from 1 to 3 h results in an increase in the C
on the basis of the amount of lignin contained in the material [27]. content of the material, with a reduction in H2, O2 and N2 contents
It is therefore, generally believed that the higher the amount of respectively due to H2O, CO and CO2 formation [92]. Another study
lignin in a biomass material, the better the binding ability of that conducted by Bridgeman et al. [97] on the torrefaction of reed
material and the milder the process conditions. A number of lig- canary grass and wheat straw with composition and properties
nin-active sites are opened up during the process of torrefaction, similar to those of SCB showed a reduction in moisture content
breaking down the hemicellulose matrix to form unsaturated from an initial value of 4.7% to a final value of 0.8%, with the
compounds with better binding properties [85]. A flow diagram content of C increasing from a value of 48.6% to a final value of
for the production of torrefied biomass briquettes as proposed by 54.3%, and H2 decreasing from 6.8% to 6.1% as well as a decrease in
Bergman in 2005 [86] which could make the material suitable for N2 content from 0.3% to 0.1%.
energy conversion purpose is presented in Fig. 14.
During torrefaction, most of the water contained in the biomass 7.5. Drying and demoisturising
is evaporated as the material undergoes drying and heating. This
happens due to chemical reactions via a thermo-condensation Moisture content of biomass remains one of the major factors
process at a temperature above 160 °C resulting in the formation that affect the performance of densification processes and that of
of CO2 [87]. The hemicellulose content of the biomass is more energy conversion systems because the quality of a densified
affected by the decomposition reactions than the cellulose and material as well as successful operation of densification systems
lignin content, with the material retaining most of its energy and are highly moisture sensitive, which preferably should not exceed
losing its hygroscopic properties [88,89]. The production of gas 15% [65]. For biomass materials such as SCB which is excessively
increases, resulting in the formation of CO and other heavier moist at the source where it is generated, drying is essential if it is
compounds including hydrocarbons such as phenol at reaction to be efficiently used as an energy resource. Drying and de-
temperature of approximately 280 °C. At this temperature, the moisturising is one pre-processing method required during ther-
reaction is considered entirely exothermic. Temperatures beyond mochemical conversion of biomass such as SCB to energy and
300 °C are not recommended as these temperatures result in results in higher quality products. Drying and de-moisturising
pyrolysis instead of torrefaction [90]. During torrefaction, the have been used to form more stable and dense briquettes because
biomass loses its recalcitrant nature due to the breakdown of its it significantly increases the throughput of the briquetting ma-
hemicellulose matrix and cellulose depolymerisation, resulting in chine, reducing the energy requirement per kg of briquettes
a reduced fibre length [82,89,90]. The process of torrefaction also formed [98–100]. Following size reduction, moisture content has
results in shrinkage of the material, making it fragile, flaky and to be reduced to a considerable level for which a dryer maybe used
light-weight, while improving grinding and pulverising properties, depending on the conversion technique employed for the biomass
and in the process, making the material hydrophobic mainly due material [101]. Drying may also be achieved using the heat

Drying Torrefaction Size reduction Densification Cooling

Briquettes

Fig. 14. A flow diagram of the production of torrefied biomass briquettes. Reproduced with permission from [86].
788 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

Moist air out

Buffer Drying ducting

Hot air
generator
Cyclone Hopper
(Material In)
Fan

Dried
material
out
Fig. 15. A pneumatic dryer. Reproduced with permission from [102].

generated from the plant where the biomass is to be converted relatively strict in terms of size requirements [110]. Prior to den-
into energy. Fig. 15 shows a pneumatic-type of dryer that can be sification, the biomass is ground to a certain particle size so as to
used for biomass drying. partially break down the lignin content of the material, increasing
Energy is required during moisture removal, which therefore its specific area and improving binding properties [111]. The total
increases the energy needed for pre-processing; however, heat surface area and the pore size of the material including the
recovery from a waste heat source from the dryer could improve number of contact points for inter-particle bonding increases with
energy efficiency as drying is an essential pre-processing method particle size reduction during compression [112]. The process of
required in the conversion of any biomass material into energy size reduction is energy intensive, and for this reason, it cannot be
because this process provides a measure of easy ignition of the met through combustion of the material. However, the energy
biomass during gasification [101]. There are however, different demand for size reduction can be reduced when the material is
drying methods that can be applied during biomass drying de- first torrefied, and the reduction in energy can be as high as 80%
pending on how the biomass material is to be utilised as a source [113].
of energy. A classification of these methods is presented in Table 5.
During gasification, excess moisture in bagasse may reduce
gasification system thermal efficiency; however, steam generated 8. The gasification process
from moisture evaporation reacts with volatiles and char, con-
verting them into product gas, and playing a role in the water-gas Gasification is one of the most flexible technologies that can be
shift reaction (Table 6) which enhances H2 production [103–105]. used to produce clean energy. It is a thermo-chemical process that
This observation is true because of moisture removal due to breaks down virtually any carbon-containing material into its basic
heating from room temperature to a temperature of approximately chemical constituents, collectively known as synthetic gas (syn-
100 °C, and the latent heat of vaporisation as well as steam heating gas). This process consists of a number of physical and chemical
to gasification temperature lost from the system increases thermal processes including rate-determining steps, and takes place under
cost [106]. limited supply of O2 so that partial oxidation can increase the ef-
ficiency of the entire process [114]. The location of the chemical
7.6. Grinding processes depends on the type of gasification technology, and the
three major types are the fixed bed, fluidised bed and the en-
Size reduction of biomass materials are quite demanding due to trained flow gasification systems [115]. A detailed description of
the fibrous and tenacious nature of their structure. It is an im- these types of gasifiers is given in Section 11. Of these various
portant energy-intensive process that is essential for energy con- types of gasifiers, the fixed bed is the most commonly used, and
version purpose of biomass materials; however, the energy con- since the downdraft gasifier (which a fixed bed type of gasifier) is
sumption of size reduction depends on certain factors including the focus of this review, the fundamental chemical kinetics of each
the initial particle size of the biomass, moisture content, biomass gasification technology based on the operation of the downdraft
feed rate, biomass properties and the machine variables [107–109]. gasifier are described, with emphasis on the four main processes
For thermochemical conversion of bagasse or any other biomass (drying, pyrolysis, oxidation and reduction) occurring in the gasi-
material, size reduction is necessary because most energy con- fier. Each of this processes is characterised by its own energy re-
version systems cannot process feedstocks in their raw form; as quirements and can be endothermic or exothermic, incorporating
previously mentioned, conversion systems are designed to ac- heat and mass transfers as well as the chemical kinetics of the
commodate specific feedstock sizes; hence most systems are reactions and pore diffusion as the main rate- controlling me-
chanisms involved in the processes. Other types of gasifiers are
Table 5 also reviewed, including their merits and demerits as well as their
A classification of the various methods of drying [101].
mode of operation so as to establish a clear justification for the
Active dryers Passive dryers selection of the downdraft gasifier for the gasification of SCB.
Fig. 16 shows the heat and mass flows characterised by the four
Boilers (flue gas or steam) Solar dryer main gasification processes based on the operation of a downdraft
Dryer burners Open sun
Waste heat recovered from facility processes Natural ventilation
gasification system.
The mechanisms of heat and mass flows vary in magnitude
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 789

Table 6
Chemical reactions involved in the gasification of biomass [144,147,148].

Reaction name Chemical equation °

∆H923 (kJ/mol)a

Material reforming - (endothermic)

C nHmOp + ( 2n − p )H2O = nCO2 + ( m/2 +2n − p)H2 (5)

Water-gas shift -35.6 (exothermic)

CO + H2O → CO2 + H2 (6)

Methane reforming þ 224.8 (enodthermic)

CH4 + H2O → CO + 3H2 (7)

Water-gas (i) þ 135.8 (endothermic)

C + H2O → CO + H2 (8)

Water-gas (ii) þ 100.3 (endothermic)

C + 2H2O → CO2 + 2H2 (9)

Oxidation (i) -394.5 (exothermic)

C + O2 → CO2 (10)

Oxidation (ii) -111.5 (exothermic)

C + 0. 5O2 → CO (11)

Boudouard þ 171.4 (endothermic)

C + CO2→2CO (12)

Methanation -88.9 (exothermic)

C + 2H2 → CH4 (13)

according to the physical and chemical processes characterised by 8.1. Drying zone
each zone, which include temperature, air moisture, heat losses,
mass flow rate of air and gas, solid phases, feed rate, feed size, and This zone lies at the top of the gasifier, and the material is fed
moisture content [115]. The following sub-section details the into the reactor at this point. As the material descends down into
processes in each zone of a gasification system based on the op- the gasifier, particles are consumed in this zone. The main function
eration of the fixed bed systems. of the drying zone is to drive off moisture in the material in the

Process: Drying Primary Secondary Char Char

Pyrolysis Pyrolysis Combustion Gasificatio

Temp (°C) <120 200 600 300~800 800~110 1100~600

Primary Secondary
char cha

Biomass Oil

Primary Reformed Combustion Char

gas gas gas gas

Heat Flow
Fig. 16. Heat and mass flows in a gasification process [116]. With kind permission from Springer Science and Business Media.
790 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

form of water vapour. The heat necessary for this process is drawn The gasification process occurs as char reacts with carbon and
from the pyrolysis and combustion zones of the gasifier [115]. The steam to produce carbon monoxide and hydrogen. During this
drying rate depends on the surface area of the material, tem- process only a small percentage of methane is formed due to the
perature, velocity and relative humidity of the surrounding air as very high temperature in this zone which is not favourable for its
well as particles internal diffusivity, and this rate is governed by formation. In this zone, hot gas temperature is also substantially
internal and external mass transfers [117]. The drying zone is reduced. An important consideration here is ash removal. If the
characterised by a low temperature that is less than 120 °C, and ash were not removed continuously through the ash grate, ash
heat transfer from this zone takes place by conduction from the would then build up inside the reduction zone and contaminate
material. The ability of the heat generated for drying to completely the reduction charcoal [120]. This would quickly lead to over-
remove all the water from the material in this zone depends on heating, which if not stopped in time could destroy the hearth. The
the thermal conductivity of the material as some materials have automatic variable-speed ash grate prevents over-heating, pro-
low thermal conductivity [118]. vided it operates at the correct speed for a certain fuel; and the
products of partial combustion (water, carbon dioxide and in-
8.2. Pyrolysis zone combustible partially cracked pyrolysis products) now pass
through a red-hot charcoal bed where reduction reactions take
This zone lies just below the drying zone with no air allowed in place [121]. The reactions here are endothermic, causing the
the zone during gasification. It draws heat from the surrounding temperature in the zone to decline from 1500 °C to about 600 °C
oxidation zone. The material is initially broken down to tar, char [115].
and volatiles when heated in the absence of air between the
temperature ranges of 200 °C and 600 °C. At a temperature of
250 °C, volatiles are released in a process known as primary pyr- 9. Benefits of the gasification technology
olysis where tar is generated near the surface. In this process, the
tar generated is cracked at high temperatures (above 600 °C) into Gasification technology has many great benefits, some of which
secondary char after escaping into a gas phase to form hydro- have been highlighted in the following sub-sections.
carbons such as methane [116,119]. This process is referred to as
the secondary pyrolysis. In the pyrolysis zone, about 80–95% of the 9.1. Efficiency benefits
mass of the feed material for gasification is converted into liquid-
phase products such as tar, oil, water, and gaseous phase products Gasification process efficiency is a major factor that influences
such as CO, H2 and CO2, including hydrocarbons, with about 5–20% the technical and economic viability of using the entire gasifica-
of reactive char remaining [120]. The composition and distribution tion technology for energy production; It is defined as the energy
of the products obtained at the pyrolysis stage of a gasification content of the product gas divided by the energy content of the
process depends on certain factors which includes feed composi- gasification feedstock [45]. More economical electric power is an
tion, particle size, temperature and heating rate as well as the important benefit of higher process efficiencies [115]. In a typical
residence time of the gaseous components [117]. Char production biomass gasification power plant, heat from the plant can be
also depends on the heating rate of the gasification process, and converted into other forms of energy such as steam that drives a
the lower the heating rate, the higher the production of char; at a steam turbine/generator in addition to the syngas produced dur-
low heating rate of 50 °C/min the reaction slowly breakdown the ing gasification, which can also be used as fuel in stationary gas
material, driving off carbon dioxide and water vapour in the pro- turbines for electricity generation. In gasification power plants,
cess, and making the carbon content in the solid product higher only about a third of the energy value of the feed material is ac-
than that in the feedstock [115]. The char and CO2 produced in the tually converted; biomass gasification typically gets dual applica-
pyrolysis zone then drifts down the oxidation zone for further tion from its product gases. The first application involves firing the
reaction. product gases in a gas turbine, after being cleaned of impurities to
generate one source of electricity. Secondly, some of the heat
8.3. Oxidation zone generated in the gasification process and the hot exhaust of the
gas turbine are then used to generate steam for use in steam
The oxidation zone, also known as the combustion zone, lies turbines/generators. This dual source of electricity generation is
below the pyrolysis zone. It represents the zone through which called a ‘’combined cycle’’ which is considered a much more effi-
oxygen is fed into the system to aid combustion. Oxygen in the cient way of converting the energy in biomass into usable energy
form of input air reacts with the char produced in the pyrolysis [122].
zone, thus producing combustion gases such as CO2 and H2O
(water vapour). Char combustion is very rapid in this zone and 9.2. Environmental benefits
results in a steep rise in temperature due to the exothermic nature
of the reaction. The temperature in this zone is between 800– The environmental benefits of biomass gasification stem from
1100 °C, and the heat produced from the combustion of char is the the ability of the technology to achieve extremely low con-
main source of heat to other regions of the gasifier, while the hot centrations of SOX and NOX together with reduced emission of
combustion gas and water vapour produced in the combustion particulates from combusting biomass-derived gases. Biomass
zone are drawn into the reduction zone [122]. Apart from heat materials including SCB generally exhibit low sulphur concentra-
generation, the combustion zone also serves to oxidise all con- tions, which can be converted to hydrogen sulphide that can be
densable products from the pyrolysis zone. For these to be effec- captured by processes presently used in the chemical industry. In
tively achieved, temperature distribution must be uniform and biomass gasification, the syngas produced is almost free of fuel-
cold spots avoided in the zone, which owes it to the gasifier geo- bound nitrogen with NOX from the gas turbine limited to thermal
metry and the air inlet velocity [3,45]. NOX, and advanced emission control processes to reduce NOX to as
low as 2 ppm are being developed. These include multi-con-
8.4. Reduction zone taminant control processes to reduce pollutants to parts per billion
levels which would be effective to clean up mercury and other
At the bottom of the downdraft gasifier lays the reduction zone. trace elements, including metals [122]. Biomass gasification also
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 791

CO2 SOURCE CAPTURE TRANSPORT

Gasification plant CO2 separation from Pipelines, tanker,

other gaseous effluents ship
by chemical or physical
techniques

INJECTION
CO2 SOURCE
Injection of CO2
Long-term storage into geological
security and closure long-term storage

Fig. 17. The process of carbon capture, storage and utilisation. Used with permission from [124].

offers further environmental advantages in addressing the concern product syngas. About 45% of the syngas produced from biomass
about atmospheric buildup of greenhouse gases (GHGs), such as gasification worldwide is used in the production of chemicals and
carbon dioxide. If oxygen is used as the gasifying agent instead of 28% for the production of liquid fuels, while 19% and 8% of world
air, the emission of CO2 appears in the form of a concentrated gas syngas production power are produced from power and gaseous
stream in the syngas at high pressure, which can be captured and fuels respectively [122].
sequestrated more easily at reduced costs. CO2 capture, storage
and utilisation is described in Section 10.5. If air is used as the 9.5. Carbon capture, storage and utilisation
gasifying agent, the CO2 that results in the process appears diluted
and as such, more expensive to separate [122]. Carbon capture, storage and utilisation are a series of techno-
logical processes intended to mitigate GHG emissions. The process
9.3. Feedstock flexibility relies on the production of a concentrated stream of CO2 that can
be transported to a storage site [124]. Gasification of biomass lends
Gasification systems have been developed to accommodate itself to efficient CO2 removal because of the high temperature and
various types of feedstock, however, there is a need to understand pressure associated with the product gas. CO2 from a gasification
gasifier operation to optimise the control of syngas properties plant can be captured and prevented from escaping into the at-
based on feedstock variability. The feedstock flexibility of gasifi- mosphere through either utilisation or storage. Two ways to
cation technology arises from the ability of the process to thermo- achieve these are through sequestration, which involves injecting
chemically break down any carbon and hydrogen containing ma- the CO2 into deep geological formations for permanent storage;
terials to a gas containing simple compounds that can be further and through CO2 enhanced oil recovery (CO2 EOR), which relies on
processed into several products. In the case of the sugarcane in- underground injection of CO2 into mature oilfields to sweep re-
dustry in South Africa which generates large amounts of SCB as by- sidual oil, storing it in the process [124]. It occurs via four stages
products, during sugarcane off crop season, the industry could that are applicable in large centralised sources including gasifica-
potentially rely on the co-gasification of bagasse with coal as a tion power plants [122,124]. Fig. 17 shows a simplified flow dia-
means of overcoming the challenges of season limitation of ba- gram of the stages involved in this process.
gasse if energy requirements of the industry are to be met. Both The first stage involves CO2 capture from the gasification plant
feedstocks (SCB and coal) are solid fuels and the equipment de- before it is transported to a suitable storage site for injection into
signed for bagasse combustion is also assumed to be able to use deep geological formations where it is physically trapped below
coal as well, and CO2 emission should decline proportionally to the impermeable rocks [125]. It is of importance to monitor the CO2
amount of coal offset by SCB [123]. after injection to ensure it is permanently stored and remains safe
Continuing research and development on feedstock pre-pro- for human health and the environment [124].
cessing technologies and gasifier design remains the key to in-
creasing adaptability of the gasification technology for any kind of
feedstock. Research in this area is intended to minimise fuel costs 10. Types of gasification technologies
for the gasification technology. Using more than one feedstock in a
single facility reduces project risk and extends its lifespan [122]. Since there must be interaction between the gasifying agent in
However, there may be a need for the pre-processing of both a gasification process, in this case, air or oxygen, and the biomass
feedstocks to make them suitable for gasification operations. material, gasifiers are classified according to the way the gasifying
agent is introduced into the system [115,126]. Each type of gasifier
9.4. Product flexibility is characterised by its own unique operational merits at a parti-
cular set of circumstances. The most common types are described
The ability of the syngas produced to be further processed into in the following sub-sections.
high-energy dense products is a major advantage of the gasifica-
tion technology. Liquid fuels such as diesel, gasoline and jet fuel as 10.1. The fixed-bed updraft or counter-current gasifier
well as synthetic natural gas including hydrogen are some of the
value-added products that can be obtained after further proces- In the updraft gasifier, the material is fed at the top of the ga-
sing the syngas from the gasification of biomass [122]. A variety of sifier and moves downward as it gets converted, i.e. the material
fertilisers and other high-value chemicals which include naphtha, for gasification move counter-current to the flow of the gasifying
sulphur, phenol, anhydrous ammonia and ammonium sulphate agent as it gets converted into syngas, hence this type of gasifier is
can also be produced from further processing of the gasification also known as the counter-current type of gasifier. In moving
792 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

Feed Feed

Gas out Drying zone

Drying zone

Pyrolysis zone
Oxidation
Pyrolysis zone

Reduction zone Air Air

Oxidation zone Reduction zone

Ash grate Ash grate

Gas out
Gasifying
agent Ash
Ash
Fig. 19. Fixed bed downdraft or co-current gasifier. Reproduced with permission
Fig. 18. Fixed bed updraft gasifier or counter-current gasifier. Reproduced with from [127].
permission from [127].

counter-current to the gas flow, the material passes through the Dehydration, as a consequence of moisture evaporation occurs
drying, pyrolysis, reduction and the oxidation zones. Fig. 18 shows in the drying zone of the gasifier, and the evaporated moisture
a schematic representation of a typical updraft gasifier. serves as a reaction agent during gasification. The product gas
The fundamental chemical kinetics, with emphasis on the four exits from the bottom of the gasifier, and contains significantly less
main processes (drying, pyrolysis, oxidation and reduction) oc- amount of tar, compared to the updraft gasifier with high quan-
curring in a gasification system have been given for each gasifi- tities of tar in the product gas. As a result of this the need of gas
cation technology based on the operation of the downdraft gasifier cleaning reduces, and therefore leaves the gas suitable for a wide
in Section 9, however, not much was mentioned about heat gen- variety of applications. Depending on the temperature of the
eration within the system for a typical updraft gasifier. In this type oxidation zone, tar and pyrolysis products from the feed pass
of gasifier, the heat for pyrolysis and drying is mainly supplied by through a glowing bed of charcoal and are converted into a gas
the syngas which flows upward and partly by radiation from the containing CO, H2 and CO2 CH4 [115].
oxidation zone. The oxidation zone lies just below the reduction The major advantages of the downdraft gasifier stem from its
zone in a typical updraft gasifier, where combustion reactions low tar production rate (most of the tar produced is disintegrated
occur, followed by reduction reactions and the production of tar in the high-temperature oxidation zone of the gasifier, low en-
and volatiles which are carried in the gas stream [127]. The ash trainment of particulate matter, low capital, operational cost and
grate serves to allow ash generated in the process to be collected its simplicity and ease of operation. The downdraft gasifier design
at the bottom of the gasifier, just below the oxidation zone. High has simple and easy control systems when compared to other
charcoal burn-out, low gas exit temperatures as a result of internal types of gasifiers with expensive and complicated control systems
heat exchange and high efficiency, as well as flexibility in feed- such as the fluidised bed, the entrained flow and the plasma ga-
stock and ease of operation are among the advantages of the up- sification systems described in Sections 10.4, 10.5 and 10.6 re-
draft gasifier [128]. Another advantage of this type of gasifier is the spectively. The wear rate of the downdraft gasifier is minimal; as a
ability of the technology to tolerate feedstock with high moisture result, the maintenance cost is low [129]. Its major disadvantage
content, since the gasifying agent is introduced from the bottom of lies in its difficulty to handle feed with high moisture and ash
the gasifier and the gas exit is at the top of the gasifier. This allows contents, and its inability to operate on a number of unprocessed
for high rate of heat transfer as the hot gas from the oxidation fuels. Lack of internal heat exchange and lower heating value of
zone of the gasifier interacts with the biomass material, drying the the product gas from a downdraft gasifier are also some of the
material on its way out [45]. However, the major limitations of minor drawbacks of the system when compared with the updraft
using the updraft gasifier are the excessive amount of tar produced system [130]. As with most gasification systems, another major
in the syngas and poor capability of loading as well as issues re- drawback of the downdraft gasifier is the inability of scale-up. The
lated to channelling in the equipment which can lead to explosive downdraft gasifier cannot be developed on a large scale (max-
situations as a result of breakthrough in oxygen [127]. imum thermal and power outputs are approximately 1300 kWth
and 400 kVA respectively) due to non-uniform heat distribution
10.2. The fixed-bed downdraft or co-current gasifier within its oxidation zone, which also has been attributed to its
design characteristics, however the time required to ignite and
A solution to the problem of tar entrainment experienced by bring the plant to a working temperature is approximately 20–
other types of gasifiers was addressed through the design of the 30 min, which is quite shorter than that required for the updraft
downdraft gasifier in which the gasifying agent is introduced at or gasifier [45].
above the oxidation zone of the gasifier. Downdraft gasifiers are
similar to updraft gasifiers, except that the zones are located in 10.3. The fixed-bed crossdraft gasifier
reverse order, where pyrolysis products pass through the high
temperature oxidation zone and undergo further decomposition The fixed bed crossdraft gasifier exhibits many of the operating
into combustion products [127]. A schematic of a typical down- features of the downdraft gasifier. The gasifying agent is in-
draft gasifier is presented in Fig. 19. troduced from the side near the bottom, while the product gas
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 793

Feed CO + H2 + CH4

Drying zone
Fluidized
bed
Pyrolysis zone

Oxidation
Biomass
Air Gas out

Reduction zone
Fluidizing Gases

Ash
Oxygen

Fig. 20. A schematic diagram of a crossdraft gasifier. Reproduced with permission

Ash
from [127].

Fig. 21. Schematic diagram of a fluidised bed gasifier. Reproduced with permission
from [132].
exits from the opposite side of the system. The ash bin and the
oxidation zone as well as the reduction zone in a crossdraft gasifier
are separated from each other, unlike the updraft and the down- fluidised bed systems [127]. Char blow-out are minimised as much
draft type of gasifiers. Due to this design characteristic, the type of as possible because the fluidised bed gasifier, as with most gasi-
feedstock for gasification is limited to low ash materials such as fication systems, is equipped with a cyclone.
wood and charcoal [127]. A schematic representation of this type A major advantage of the fluidised bed gasifier stems from its
of gasifier is shown in Fig. 20. high rate of heat and mass transfer as well as excellent gas to solid
Concentrated partial zones operating at a temperature as high contact including excellent heat transfer characteristics. Other
as 2000 °C makes the load following abilities of the crossdraft advantages include good temperature control, flexibility in feed-
gasifier suitable for gasification of low-ash feed materials [131]. stock, co-feeding tendencies, large heat storage capacity, and a
Startup time is about 5–10 min, which is much faster when com- good degree of mixing [132]. In the light of these advantages,
pared to other types of gasifiers such as the updraft or the several shortcomings can also be encountered when fluidised bed
downdraft gasifiers. However, this type of gasifier (crossdraft) has gasification systems are used. These includes high tar content of
disadvantages in terms of its exit gas temperature and gas velocity, the syngas produced (500 mg/m3 gas), poor response to changes
which are usually high, and also its poor reduction in CO2. These in load and incomplete carbon burn-out as well as the possibility
shortcomings are a consequence of its design, however the choice of scale-up which is very minimal [127]. High capital and opera-
of one gasifier type over the other is dictated by a number of tional costs as well as complicated and expensive control systems
factors including the feedstock for gasification and the final are also some of the limitations of using this type of gasifier.
available form of the feedstock, i.e. its size, moisture and ash Fluidised bed gasifiers operate at pressures slightly above the at-
contents among other factors [127]. mospheric pressure, which requires that leaks of any sort be
avoided. These drawbacks have limited the widespread deploy-
10.4. The fluidised bed gasifier ment of these systems for energy production purposes.

Fluidised bed gasifiers were originally developed to overcome 10.5. The entrained flow gasifier
operational challenges associated with fixed bed gasification sys-
tems. This type of gasifiers have no distinct reaction zones com- The entrained flow gasification systems are characterised by
pared to the fixed bed systems as drying, pyrolysis and gasification fuel particles that are dragged along with the gas stream at high
occur simultaneously, with a relatively low gasification tempera- temperatures typically between 1300 and 1500 °C; they are also
ture which is approximately 750–900 °C [127]. They are made of characterised by short residence times and small fuel particles
chemically unreactive materials such as ash, sand or charcoal that (typically o100 mm), and operate under pressure with pure oxy-
acts as a heat transfer medium. A schematic representation of the gen, with capacity often in the order of several hundreds of
fluidised bed gasification system is shown in Fig. 21. megawatts (MW) [133]. Fig. 22 shows a sketch of the entrained
In the fluidised bed gasification system, the gasifying agent is flow gasifier.
blown through a bed of solid particles which has been pre-heated Materials for gasification using the entrained flow systems are
to a set temperature. The feedstock for gasification is introduced at generally introduced into the gasifier by pneumatic feeding sys-
the set bed temperature which is high enough for ignition. The tems after pressurising the system. The gasifying agent mostly
gasifying agent is blown through a distributor plate at a controlled employed is oxygen, which is co-currently fed into the gasifier
rate, allowing the material to undergo fast pyrolysis, giving rise to with the feedstock, resulting in the oxidant entraining the material
a component mixture of gaseous materials. Further reactions re- as it flows through the gasifier. High temperature and pressure as
lated to tar-conversion reactions occur in this gaseous mixture. well as extremely unstable flow are characteristics of entrained
The high quantities of tar generated in this process are not com- flow gasification systems. These result in high throughput and
pletely disintegrated because of the low temperatures of the rapid feed conversion, with gasification reactions occurring at a
794 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

Fuel Oxygen

Pressurised
water
outlet

Cooling
screen

External cooling
Pressurised jacket
water inlet Gas outlet
Quench
water

Water
overflow

Ash

Fig. 22. Schematic diagram of an entrained flow gasifier [134].

high rate, allowing for high carbon conversion efficiencies in the process) into a fuel gas that still contains the entire chemical and
range 98–99.5%. The residence time of the entrained flow gasifier heat energy from the waste; this process employs a plasma torch
is relatively low (in the order of few s), hence it operates at high that is powered by an electric arc to ionise gas and catalyse the
temperature to achieve high carbon conversion efficiencies. The organic material into syngas. Plasma gasifiers are commercially
polyaromatic hydrocarbon compounds such as tar, phenols and used as a form of waste treatment, and have, however also been
other liquid compounds produced from the devolatilisation of the tested for the gasification of biomass and solid hydrocarbons such
feedstock are decomposed into a mixture of gases containing CO, as coal [134]. Fig. 23 presents a schematic of a typical plasma ga-
H2 and trace amounts of light gases with similar properties to the sification system showing its main components.
hydrocarbons. The major advantage of the entrained flow gasifier Plasma gasification takes place under high temperatures, ty-
lies in its ability to handle a variety of feedstock that can be fed pically above 6000 °C, and the process is driven by a plasma torch
into the gasifier in dry or slurry form, producing a clean, tar-free system which is located at the bottom of the gasifier. The feedstock
syngas. A simpler operation of the gasifier involves the slurry feed, is broken down into its constituent elements and dramatically
but this introduces water into the gasifier, which needs to be increasing the kinetics of the various reactions occurring in the
evaporated to ensure optimum efficiency; the addition of water gasification zone, converting all organic materials into carbon
results in a product gas with higher ratio of H2 to CO, and a re- monoxide (CO) and hydrogen (H2) in the process. Any residual
duced thermal efficiency. This remains one of the limitations of material of inorganic and heavy metals will be melted and pro-
using the entrained flow gasifiers for the gasification of biomass duced as a vitrified slag which is highly resistant to leaching [134].
materials such as SCB. Another issue with the entrained flow ga- Fig. 24 shows the components of a plasma gasifier torch and il-
sifier is the high temperatures involved, which tend to increase lustrates how the torch operates during gasification.
wear rate by shortening the life span of the components of the The plasma gasifier torch has two electrodes which creates an
gasifier, including refractory lines because at high gasification arc as electricity is fed to the touch. The process gas is heated to an
temperatures, ash changes to the three states of matter (liquid, gas internal temperature as high as approximately 14,000 °C as inert
and solid states), wearing down refractory materials and causing gas is passed through the arc-forming electrodes. The temperature
fouling which can also lead to unplanned plant shutdown for re- close to the torch can be as high as 2760 °C to approximately
pairs [135]. 4500 °C, and due to these high temperatures, the waste is com-
pletely broken down into its basic elemental components [134].
10.6. The plasma gasifier There are no polyaromatic hydrocarbons (such as tar), and het-
erocyclic organic compounds (such as furan) formed during plas-
The plasma gasification technology uses a plasma technology to ma gasification because of high temperatures associated with the
convert any organic matter into syngas in oxygen-deficient en- process. All metals become molten and flow out through the
vironment. In this type of gasifier, wastes are not combusted as bottom of the reactor. Inorganic materials such as silica, soil,
they are in incinerators; instead, they are converted through an concrete, gravel, glass, etc. are converted into glass; no ash is
ionised gas (a gas containing free flowing electrons that give po- formed at these high temperatures [136]. A process flow diagram
sitive and negative charges to atoms, thus becoming a highly ef- for plasma gasification is presented in Fig. 25.
ficient conductors of electricity and generator of heat in the The feedstock (usually organic waste) is delivered into the feed
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 795

Syngas
outlet

Freeboard
zone
Feed

Air Feed
Plasma
torch Gasification
zone

Removable
bottom
Metal and slag
output

Fig. 23. A schematic diagram of a plasma gasifier [134].

Heated
Electrodes process

Entering
Plasma
process
column
gas

Fig. 24. A plasma gasifier torch [129].

Feed
m
material
Air or
Oxygen
Syngas

Plasma Particulate
torches removal

Slag & Quench

Recovered
Metals

Fig. 25. The plasma gasification process [137]. Reproduced with permission from Phoenix Energy Australia Pty Ltd.

system and made to pass through an air-tight system which pre- to accommodate potentially highly variable nature of the feed
vents gases escaping into the atmosphere. The plasma torches material [137]. The plasma arc is embedded in the plasma torch
function to provide part of the heat needed to drive the en- and as a result, the waste material is not directly exposed to the
dothermic gasification reactions. The torches are powered by an plasma arc, hence the classification of the process as plasma in-
automatic control system that adjusts the gasification conditions duced gasification. The main advantage of plasma gasification
796 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

includes non-discrimination between types of wastes for gasifi- because of the highly corrosive nature of the plasma flame which
cation. It can take virtually any type of waste, enabling the waste are a source of toxic compounds [136].
material to be used as fuel without the need for pulverising; no Based on the aforementioned technical and economic draw-
prior sorting of the waste is necessary [134]. However, the major backs on the use of other types of gasifier for the conversion of SCB
drawbacks hindering the widespread use of this type of gasifica- into energy, the downdraft gasifier appears to be the most suitable
tion technology at both laboratory-and pilot-scale include its high gasification system due to its simplicity and ease of operation.
capital and operational costs together with the highly corrosive However, as stated earlier, its only difficulty lies in its inability to
nature of the plasma flame. The latter arises as a result of bom- handle feed with relatively high moisture and ash contents, and its
bardment by hot species from the flame, which may lead to fre- inability to operate on a number of unprocessed fuels, which
quent maintenance and replacement of components, resulting in simply requires that pre-processing of the feedstock for gasifica-
intermittent shutdowns, including the production of toxic com- tion be undertaken to achieve the needed gasification process
pounds due to embedded filters and gas treatment systems which efficiency.
are sources of toxic materials [136]. There is also limited in-
formation in the literature on studies dealing with modelling and
simulation of gasification processes involving plasma gasifiers. 12. Influence of gasifier design on the gasification process of
Overall, plasma gasification is not well suited for the gasification of sugarcane bagasse
SCB because of the extremely high temperature of the process
[134]. Gasifiers are designed in accordance with feedstock properties
requirements, with each having its own unique operational ad-
vantages at a particular set of circumstances. This indicates large
11. The choice of gasifier for sugarcane bagasse gasification differences in the way gasifiers are designed including dimensions,
material feeding point, reactor geometry, throat angle and throat
All types of gasifiers described in Section 11 can produce syngas diameter (in the case of downdraft gasifiers), etc. The influence of
for combustion purposes, with each designed for specific types of these design parameters on both input and output parameters
feedstock. However, gasifier efficiency remains an important factor during gasification of biomass cannot be overstated. They ulti-
that determines the technical operation as well as the economic mately influence parameters not limited to syngas composition,
viability of using a gasifier system [138]. Among other factors, the calorific value and process efficiency. These parameters favour the
efficiency and effectiveness of these gasifiers is dependent upon gasification process of biomass including SCB under standard de-
the type and design of the gasifier [139]. Fluidised bed systems sign parameters as well as under various gasifier operating con-
have been known to achieve higher efficiencies than other types of ditions, regardless of the type of biomass material used as feed-
gasifiers but are not preferred for bagasse conversion for the stock; these design parameters were tested when Hanaoka et al.
purpose of electricity generation because of their high capital and [141] worked on 12 different types of biomass and found that
operational costs as well as their high tar production rate; the syngas production increased with increasing feed volatile matter
production of tar creates major operational challenges such as content under standard gasifier design parameters, regardless of
clogging in engine valves resulting in high maintenance costs due feedstock variety. A study also conducted by Anukam et al. [3] on
to processes involved in tar removal [140]. Fluidised bed gasifiers the simulated gasification process of SCB in a downdraft system
are also known to have complex and expensive control systems proved that process efficiency, gasification rate and syngas heating
when compared to other types of gasifiers, which limits their la- value were all affected by certain gasifier design parameters such
boratory or large scale applications. Among the fixed bed systems, as throat angle and throat diameter. They concluded that efficiency
updraft gasifiers are more efficient but suffer from excess amount improved upon employing gasification systems with smaller
of tar in the product syngas which is not suitable for gas engines or throat angles and throat diameters. Akay and Jordan [54] also
turbines. Poor loading capabilities and breakthroughs in O2 due to performed an experimental study on the gasification of fuel cane
poor channeling (a situation prone to explosion) are also issues bagasse (FCB) in a downdraft gasifier and reported that optimum
related to the use of this type of reactors; however downdraft efficiency obtained for the gasification process was as a con-
gasifiers are preferred for the gasification of SCB or any other type sequence of free fuel flow down the combustion zone through the
of biomass material for the purpose of electricity generation be- narrow throat of the gasifier due to sized fuel particle, which fa-
cause of their low entrainment of tar and particulates, which are cilitated faster rates of heat transfer and rapid gasification reac-
characteristics of their design [136]. They are the most commonly tions. According to Kaupp and Goss [53], to avoid bridging during
used type of gasifier due to their low capital and operational costs, gasification of biomass and to improve efficiency, the ratio of the
and due also to their simplicity and ease of operation combined throat diameter to the maximum diameter of the fuel should be at
with high process efficiency, with minimal wear and tear rate that least 6.8:1. In another study conducted by Corella et al. [142] on
results in low maintenance cost in comparison to other types of the gasification of biomass using a fluidised bed reactor with an
gasifiers [45]. Crossdraft gasifiers are also a type of fixed bed ga- internal diameter of 60 mm, the effect of feeding point location
sifiers that are efficient and effective. As previously stated, al- was reported and it was concluded that the fluidised bed system
though they exhibit many of the operating characteristics of the used in their study suffered from poor bed material mixing under
downdraft system, they are not well suited for the conversion of tested fluidisation conditions because of high char yield when the
biomass materials such as SCB for electricity and/or heat genera- biomass was fed at the top of the gasifier, compared to feeding at
tion purposes because of their high sensitivity to the formation of the bottom. They further concluded that high char yield was as a
slag and high pressure drops [128]. Entrained flow gasification result of segregation of the conversion products in the bed, which
systems are not common like the other types of gasifiers, but are acted to restrict further chemical reactions between the char and
also known to be efficient. A major limitation in the use of this volatiles from the material, adding that gasifier design also influ-
type of gasifier, as previously mentioned, lies in the high tem- enced the residence time of the product syngas.
peratures involved, which tends to increase wear rate, making
intermittent repairs unavoidable as a consequence of fouling. 13. The chemistry of sugarcane bagasse gasification
Plasma gasifiers are also not in widespread use because of their
expensive nature and extremely high temperature as well as This section describes the reaction mechanism involved in the
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 797

ii. Cracking &

reforming
Volatile
Product gas:
i. Pyrolysis
Biomass Tar H2, CO, CH4,
CO2, C2H4,
C2H6.
Char
iii. Char gasification

Fig. 26. General reaction mechanism and product distribution of the gasification process of biomass. Reproduced with permission by Elsevier from [143].

conversion of SCB to facilitate an understanding of the chemistry 14. Summary

involved in its gasification process.
Clearly, as described in Section 8.1, there are no chemical re- A summary of the comparison of the advantages and dis-
actions taking place in the drying zone of a gasification process advantages of various pre-processing methods for sugarcane ba-
employing the downdraft system due to the low temperature as- gasse for the purpose of gasification are shown in Table 7. This is
sociated with this zone. However, material degradation occurs in presented to have a clearer view of the best pre-processing
the pyrolysis zone due to higher temperatures in this zone. A method for sugarcane bagasse intended for gasification purposes.
simplified general reaction mechanism for the gasification of The efficiency and effectiveness of a particular pre-processing
biomass is given in Fig. 26. method are usually linked to the advantages and disadvantages
The process in Fig. 26 involves three main steps that are dis- associated with each method, which in turn are also used as key
tinguished by reaction temperature. The first step involves devo- indicators to determine the technical and economic viability of the
latilisation which occurs in the temperature range of 300 and pre-processing method [156]. However, it is quite evident from
500 °C. At this temperature, 70–90% of the material is converted to Table 7 that, of all the pre-processing methods described in this
volatile matter and solid char, while the second step incorporates review, torrefaction appears to be a more reliable method than
tar cracking and reforming reactions, which occur at a tempera- other pre-processing methods because of its limited disadvantages
ture above 600 °C; this dominates reaction processes by influen- such as low volumetric energy density of the torrefied material,
cing the final composition of the product syngas during gasifica- which can be compensated for by densifying the torrefied mate-
tion [144]. The tar is typically the condensable organic matter with rial, hence most studies dealing with biomass pre-processing for
molecular weights greater than benzene [145,146]. The last step of gasification purposes recommends a combination of torrefaction
the gasification process incorporates char gasification reactions and densification since both methods address each other's draw-
backs. The reduced power consumption of torrefaction also makes
which occurs at high temperatures greater than 800 °C. Table 6
it a better choice for sugarcane bagasse pre-processing compared
provides a series of complex reactions that includes gas-solid re-
to other pre-processing methods since the process does not ne-
actions between particles of the biomass material and the gasifi-
gatively affect material composition and improve its grinding
cation medium (gasifying agent), which are a result of the reaction
properties for better gasification.
mechanism presented in Fig. 26.
A summary of the comparison of the various types of gasifiers
Tar and volatile matter cracking are due to low temperature
in terms of the advantages and disadvantages associated with each
regions of the gasification process which are usually o700 °C, and
of them are also presented in Table 8. This is presented for the
are therefore represented by Eq. (5) in Table 6, while Eq. (6), the
same reason previously adduced for the pre-processing methods.
prominent water-gas shift reaction in the gasification process of
biomass remains the most dominant of the reactions due to high
temperatures under which the reaction occurs ( 4700 °C), and
15. Conclusions
which, to a large extent, determines the final composition of the
product gas [149]. The final product composition of a gasification
The pre-processing of SCB for gasification has been compre-
process can be manipulated by controlling the reaction para- hensively reviewed together with various gasification systems for
meters, including the temperature of gasification and the ratio of its conversion into energy. It can be noted that pre-processing
the gasifying agent to that of the material [150]. The impact of plays a very significant role in the gasification process of SCB when
these reaction parameters is to, amongst other things, enhance employing the downdraft gasification system, and remains a key
biomass conversion to the desired product gas. Tar concentration bottleneck in the efficient utilisation of bagasse for energy pro-
greater than 3 g/Nm3 in the product gas can cause significant duction. Pre-processing is intended to overcome the limitations of
problems in gasification systems as well as in gas engines because efficiently using SCB for the purpose of energy production and also
it leads to blockage of valves and other engine components, re- decrease the release of soil and water pollutants which may be
sulting in shutdowns; however, tar destruction leads to a higher produced due to elemental composition of bagasse. The pre-pro-
gas yield [151–154]. A solution to the problem of tar entrainment cessing methods reviewed showed that the shortcomings of one
during gasification is addressed when the biomass material for method can be compensated for by other methods, since the
conversion is taken through efficient and effective pre-processing methods seem to be complementary in their limitations and ad-
steps to lower the concentration of tar forming elements such as vantages. This also applies to the gasification systems compre-
carbon, hydrogen and oxygen as well as nitrogen and sulphur; hensively reviewed. However, among all pre-processing methods
these elements react at different stages of the gasification process reviewed, torrefaction was identified as the most efficient and
depending on the reaction conditions of the process [140]. The effective method considering its advantages over the other
properties of gasification by-products such as tar depend strongly methods. Process variables such as temperature, pressure, time of
on the feedstock properties and the type of gasifier used [155]. retention play a major role in the quality attributes of a pre-
798 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

Table 7
A comparison of the advantages and disadvantages of various pre-processing methods. Reproduced with permission from [35].

Pre-processing Advantages Disadvantages

method

Size reduction  Basic method that brings the feedstock to the size required by the  Storage of the sized material can increase microbial activity and dry
gasification technology specification matter losses
 Storage of the material can be a source of significant emissions of GHG
(CH4, N2O), due to microbial activities
 Non-friable character of bagasse can cause problems during sizing
Drying  Reduces loss of dry matter during storage of bagasse  Natural drying is commonly applied, however, it has a disadvantage of
 Reduces the risk of self-ignition and material decomposition unforeseeable weather conditions
 Reduces fungal development and activity during storage  Drying using dryers require sized material which can constitute a
 Increases material's ignition ability and potential energy input problem due to non-friable character of bagasse
during gasification
Pelletising  Higher energy density with transportation cost benefits  Despite benefits, pellets can be sensitive to mechanical damaging and
 Permits automatic handling and feeding during thermal can absorb moisture, swell, loose shape and consistency
conversion  Specific storage environment is required for safe and efficient storage
 Require less storage space
 Dry feedstock with good storage properties, reduced health risk,
reduced energy losses and higher calorific value
Briquetting  Higher energy density with possibility for more efficient  Easy moisture uptake which may lead to biological degradation and loss
transportation of structure
 Require less storage space  Briquettes require special storage conditions
 Reduces the possibility of spontaneous combustion during storage  Hydrophobic agents can be added to the process of briquetting, but this
 Allows for gravity feeding in the gasifier during gasification leads to significant increase in cost
 Combustion rate can be compared with coal
Torrefaction  Improved hydrophobic nature-easy and safe storage with reduced  Results in low volumetric energy density
biological degradation
 Power consumption during sizing is reduced due to improved
grinding properties of the torrefied material
 Increased uniformity and durability

processed SCB. Therefore, it would be recommended to study the upon the concentration of trace elements in bagasse and how the
influence of pre-processing on other gasification process condi- gasification process is also affected by these elements as some
tions (such as syngas composition and yield). More fundamental trace elements can negatively influence the process even at very
studies need to be undertaken to better understand how pre- low concentrations. In addition, considering the fact that most
processing impacts on SCB both on a molecular and structural renewable energy resources lack the ability to deploy the power
level, and how these affect its gasification process. There is also a generated to meet demand at any given time and need to be
need for studies to be conducted on the impact of pre-processing supported by storage facilities or facilities that use fossilised

Table 8
A comparison of the advantages and disadvantages of various gasifiers [127–129,137].

Type of gasifier Advantages Disadvantages

Updraft  Small pressure drops  High sensitivity to tar and fuel moisture content
 Good thermal efficiency  Startup of internal combustion (IC) engine require relatively long
 Little tendency towards the formation of slag time
 Poor reaction capability with heavy gas loads
Downdraft  Flexible adaptation of gas production to loads  Design tends to be tall
 Low sensitivity to charcoal dust and tar content of fuel  Not feasible for very small particle size of fuel (size requirement
 Low entrainment of particulates relatively strict)
 Simple and easy control systems  Only limited to small-scale applications (scale-up not feasible)
 Simplicity and ease of operation  Unable to handle feedstock with high moisture and ash contents
 Wear rate is minimal, as a result, maintenance cost is reduced  Inability to operate on a number of unprocessed feedstocks
Crossdraft  Short design height  Highly sensitive to slag formation
 Very fast response time to load  High pressure drops
 Flexible gas production  Poor reduction in CO2
 Limited to low ash materials such as wood and charcoal
Fluidised bed  Excellent gas to solid contact  Increased tar production rate
 Excellent heat transfer characteristics  Poor response to changes in load
 High rates of heat and mass transfer  Incomplete carbon burn-out
 Good temperature control  Possibility of scale-up is minimal
 Large heat storage capacity  Complicated and expensive control systems
 Good degree of mixing  High capital and operational costs
Entrained flow  High throughput and rapid feed conversion as a consequence of the high  Water is introduced into the gasifier due to slurry feedstock, which
temperature and pressure characterised by the process results in reduced thermal efficiency
 Feedstock can be fed in dry or slurry form  High temperature increases wear rate
Plasma  Does not discriminate between types of waste for gasification  Highly corrosive nature of the plasma flame
 Does not require prior sorting of the waste for gasification  Extremely high temperature of the process leads to high wear
 No need for pulverising rates
 Production of toxic compounds due to embedded filters and gas
treatment systems
 High capital and operational costs
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 799

resources, there is a need for an investigation to be undertaken on [22] Xu Q. Investigation of co-gasification characteristics of biomass and coal in
where the energy required for pre-processing would emanate fluidized bed gasifiers [Ph.D. thesis]. University of Canterbury; 2013.
[23] Anukam A, Mamphweli S, Meyer E, Okoh O. Gasification of sugarcane ba-
from, and how this energy would balance with that produced gasse as an efficient conversion technology for the purpose of electricity
during gasification. generation. Fort Hare Papers. Multidiscip J Univ Fort Hare 2013;20(1)
Pre-processing difficulties for a dedicated heat and power plant ISSN:0015-8054.
[24] Islam MR, Islam N and Islam MN. Fixed bed pyrolysis of sugarcane bagasse
using SCB as feedstock can be addressed when the pre-processing for liquid fuel production. In: Proceedings of the international conference on
methods and their effects are well-understood. The technical and mechanical engineering. (paper ID ICME03)-TH-16, Dhaka, Bangladesh;
economic viability as a consequence of its design characteristics 2003.
[25] Das P, Ganesh A, Wangikar P. Influence of pretreatment for deashing of su-
makes the downdraft gasifier well suited for the gasification of
garcane bagasse on pyrolysis products. Biomass Bioenergy 2004;27(5):445–
SCB. 57.
[26] Jorapur R, Rajvanshi AK. Sugarcane leaf-bagasse gasifiers for industrial
heating applications. Biomass Bioenergy 1997;13(3):141–6.
[27] Tumuluru JS, Wright CT, Kenny KL, Hess JR. A review on biomass densifica-
Acknowledgements tion technologies for energy application. A technical report prepared for the
U.S Department of Energy; 2010. Contract DE-AC07-05ID14517.
[28] Kumar A, Jones DD, Hanna MA. Thermochemical biomass gasification: a
The authors wish to thank the National Research Foundation
review of the current status of the technology. Energies 2009;2(3):556–81.
(NRF)-Sasol Inzalo Foundation (SIF) (Grant number 95584), the [29] Nordin A. Chemical elemental characteristics of biomass fuels. Biomass
Fort Hare Institute of Technology (FHIT) (Grant number P822) and Bioenergy 1994;6:339–47.
[30] European Biomass Industry Association. Operational problems in biomass
the Govan Mbeki Research and Development Centre (GMRDC)
combustion. 〈http://www.eubia.org/index.php/about-biomass/combustion/
(Grant number P755) as well as the Chemistry Department of the operational-problems-in-biomass-combustion〉; 2012 [last accessed, De-
University of Fort Hare( Grant number D154) for their financial cember 2014].
supports. [31] Capareda SC. Biomass energy conversion, Sustainable Growth and Applica-
tions in Renewable Energy Sources. In: Dr. Majid Nayeripour editors. ISBN:
978-953-307-408-5, InTech. 〈http://www.intechopen.com/books/sustain
able-growth-and-applications-in-renewable-energy-sources/biomasse
References nergy-conversion〉; 2011 [last accessed January 2015].
[32] Gupta SC, Manhas P. Percentage generation and estimated energy content of
municipal solid waste at commercial area of Janipur, Jammu. Environ Con-
[1] Pandey A, Soccol CR, Nigam P, Soccol VT. Biotechnological potential of agro- serv J 2008;9(1):27–31.
industrial residues. I: sugarcane bagasse. Bioresour Technol 2000;74:69–80. [33] Annamalai K, Sweeten JM, Ramalingam SC. Estimation of Gross heating
[2] Reddy MR, Chandrasekharaiah M, Govindaiah T, Reddy GVN. Effect of phy- Values of Biomass Fuels. Transactions of the ASAE. Am Soc Agric Eng 1987;30
sical processing on the nutritive value of sugarcane bagasse in goats and (4):1205–8.
sheep. Small Rumin Res 1993;10:25–31. [34] Chandrakant T. Biomass Gasification-Technology and Utilisation. Humanity
[3] Anukam A, Mamphweli S, Meyer E, Okoh O. Computer simulation of the Development Library. Artes Institute, Glucksburg, Germany. 〈www.pssurvi
mass and energy balance during gasification of sugarcane bagasse. J Energy val.com〉; 2002 [Last accessed November 2012].
2014 [Article ID 713054]. [35] Maciejewska A, Veringa H, Sanders J, Peteves S. Co-firing of biomass with
[4] Walford S. Sugarcane bagasse: how easy is it to measure its constituents? In: coal: Constraints and role of biomass pre-treatment. Luxembourg: Office for
Proceedings of the South African Sugar Technologists Association. Vol. 81; Official Publications of the European Communities; 2006. p. 113.
2008. p. 266–73. [36] O’Donovan A, et al. Acid pre-treatment technologies and SEM Analysis of
[5] McKendry P. Energy production from biomass (part 1): overview of biomass. treated grass biomass in biofuel processing. Biofuel Technologies. Berlin
Bio-Resour Technol 2002;83:37–46. Heidelberg: Springer; 2013. p. 97–118.
[6] Ramjeawon T. Life cycle assessment of electricity generation from bagasse in [37] Unger PW. Managing agricultural residues. Florida: CRC Press, Inc.; 1994.
Mauritius. J Clean Prod 2008;16:1727–34. [38] Nunes LJR, Matias JCO, Catalão JPS. A review on torrefied biomass pellets as a
[7] Deepchand K. The role of regulators in a reforming power sector in sub- sustainable alternative to coal in power generation. Renew Sustain Energy
saharan Africa. ESI Afr J 2004(Issue 2):. Rev 2014;40:153–60.
[8] Demirbas FM. Biorefineries for biofuel upgrading: a critical review. Appl [39] Sokhansanj S. Cost benefit of biomass supply and pre-processing, BIOCAP
Energy 2009;86:S151–61. research integration program synthesis paper. Ottawa, Canada: BIOCAP Ca-
[9] Agbor VB, Cicek N, Sparling R, Berlin A, Levin DB. Biomass pretreatment: nada Foundation; 2006.
fundamentals toward application. Biotechnol Adv 2011;29:675–85. [40] Woodcock CR, Mason JS. Bulk solids handling: An introduction to the prac-
[10] United States Department of Energy. Gasification technology research and tice and technology. Glasgow, Scotland: Blackie and Son Ltd.; 1987.
development. 〈http://energy.gov/fe/science-innovation/clean-coal-research/ [41] S Clarke and F. Preto Biomass densification for energy production. Factsheet
gasification〉. [last accessed, January 2014]. of the Ontarion Minitry of Agriculture, Food and Rural Affairs; 2011.
[11] Slopiecka K, Bartocci P, Fantozzi F. Thermogravimetric analysis and kinetic [42] Peleg M. Physical characteristics of food powders. In: Peleg M, Bagley EB,
study of poplar wood pyrolysis. Appl Energy 2012;97:491–7. editors. Physical properties of food. Westport, Connecticut: AVI Publishing
[12] Johannes T. Pyrolysis of sugarcane bagasse. Masters dissertation. University Company, Inc.; 1983. p. 293–321.
of Stellenbosch; 2010. [43] Lam PS, Sokhansanj S, Bi X, Mani S, Lim CJ, Womac AR, Hoque M, Peng J,
[13] South African Sugar Association. Sugarcane crushed by mills. Facts and Fig- JayaShankar T, Naimi LJ, Nayaran S. Physical characterization of wet and dry
ures. 〈http://www.sasa.org.za/sugar_industry/FactsandFigures.aspx〉; 2014 wheat straw and switchgrass-bulk and specific density. In: Presented at the
[last accessed February 2015]. American Society of Agricultural and Biological Engineers International
[14] Rocha GJM, Gonçalves AR, Oliveira BR, Olivares EG, Rossell CEV. Steam ex- meeting. Minneapolis, Minnesota: Minneapolis Convention Center; 2007.
plosion pretreatment reproduction and alkaline delignification reactions [Paper no. 076058].
performed on a pilot scale with sugarcane bagasse for bioethanol production. [44] Mani S, Tabil LG, Sokhansanj S. Grinding performance and physical proper-
Ind Crops Prod 2012;35(1):274–9. ties of wheat and barley straws, corn stover and switchgrass. Biomass
[15] Lewis C. The South African Sugar Industry. Geography Department, Rhodes Bioenergy 2004;27(4):339–52.
University; 2004. [45] Mamphweli S. Implementation of a 150 KVA biomass gasifier system for
[16] AXA Bharti General Insurance Company Limited. Lead article: Loss preven- community economic empowerment in South Africa [Ph.D. thesis]. University
tion in sugar industries. A quarterly loss prevention digest. 10th ed.; May of Fort Hare, South East Academic Libraries System (SEALS); 2010 with
2013. website: http://contentpro.seals.ac.za/iii/cpro/app?id¼ 0191823284620280
[17] Kinoshita CM. Co-generation in the Hawaiian sugar industry. Bioresour &itemId...def.
Technol 1991;35(3):231–7. [46] Mohan D, Pittman CU, Steele PH. Pyrolysis of wood/biomass for bio-oil: A
[18] Kurian JK, Raveendran GN, Hussain A, Raghavan GSV. Feedstocks, logistics critical review. Energy Fuels 2006;20:848–89.
and pre-treatment processes for sustainable lignocellulosic biorefineries: a [47] van Dam JEG, van den Oever MJA, Teunissen W, Keijsers ERP, Peralta AG.
comprehensive review. Renew Sustain Energy Rev 2013;25:205–19. Process for production of high density/high performance binderless boards
[19] Zafar S. Energy potential of bagasse. Bioenergy Consult. 〈http://www.bioe from whole coconut husk—Part 1: Lignin as intrinsic thermosetting binder
nergyconsult.com/energy-potential-bagasse/〉; 2014 [last accessed, January resin. Ind. Crops Prod. 2004;19(3):207–16.
2015]. [48] Anglès MN, Ferrando F, Farriol X, alvadó J. Suitability of steam exploded
[20] Asadullah M. Barriers of commercial power generation using biomass gasi- residual softwood for the production of binderless panels: Effect of the pre-
fication gas: a review. Renew Sustain Energy Rev 2014;29:201–15. treatment severity and lignin addition. Biomass Bioenergy 2001;21:211–24.
[21] Aul E. & Associates. Bagasse combustion in sugar mills. Emission factor [49] Granada E, González LML, Míguez JL, Moran J. Fuel lignocellulosic briquettes,
documentation for AP-42, section 1.8.. 〈http://134.67.104.12/html/chief/ die design, and products study. Renew Energy 2002;27:561–73.
fbgdocs.htm〉; 1993 [last accessed October 2014]. [50] Murphy JD, McCarthy K. Ethanol production from energy crops and wastes
800 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801

for use as a transport fuel in Ireland. Appl Energy 2005;82:148–66. [82] Bergman PCA, Kiel JHA. Torrefaction for Biomass Upgrading. In: Published at
[51] Shaw M. Feedstock and process variables influencing biomass densification the 14th European biomass conference and exhibition. Paris, France; 2005.
[Ph.D. thesis]. Saskatoon, Saskatchewan, Canada: University of Saskatch- [83] Bergman PCA. Combined torrefaction and pelletisation: The TOP process
ewan; 2008. 2005; ECN-C-05-073.
[52] Lv D, Xu M, Liu X, Zhan Z, Li Z, Yao H. Effect of cellulose, lignin, alkali and [84] Arcate JR. Global markets and technologies for torrefied wood in 2002. Wood
alkaline earth metallic species on biomass pyrolysis and gasification. Fuel Energy 2002;5:26–8.
Process Technol 2010;91:903–9. [85] Zanzi R, Ferro DT, Torres A, Soler PB and Bjornbom E. Biomass torrefaction.
[53] Kaupp A, Goss JR. Technical and economical problems in the gasification of In: Proceedings of the 6th Asia-Pacific international symposium on com-
rice hulls. Physical and chemical properties. Energy Agric 1981;1(201):1981– bustion and energy utilization. Kuala Lumpur; 2002.
3. [86] Shafizedeh F. Pyrolytic reactions and products of biomass. In: Overend RP,
[54] Akay G, Jordan CA. Gasification of fuel cane bagasse in a downdraft gasifier: Milne TA, Mudge LK, editors. Fundamentals of Biomass Thermochemical
influence of lignocellulosic composition and fuel particle size on syngas Conversion. London: Elsevier; 1985. p. 183–217.
composition and yield. Energy Fuels 2011;25:2274–83. [87] Williams PT, Besler S. The influence of temperature and heating rate on the
[55] Di Blasi C, Signorelli G, Portoricco G. Countercurrent fixed-bed gasification of slow pyrolysis of biomass. Renew Energy 1996;7:233–50.
biomass at laboratory scale. Ind Eng Chem Res 1999;38:2571–81. [88] Bourgeois J, Doat J. Proceedings of Conference on Bioenergy. Vol. 3. Göte-
[56] Tallaksen J. Biomass gasification: A comprehensive demonstration of a borg; 1985, p. 153.
community-scale biomass energy system. A case study in biomass pre-pro- [89] Bergman PCA, Boersma AR, Kiel JHA, Zwart RWH. Development of torre-
cessing. Final report submitted to the USDA Rural Development Grant 68- faction for biomass co-firing in existing coal-fired power stations. Biocoal
3A75-5-232; 2011. Concept Version, ECN Report; 2005..
[57] Tchapda HA, Pisupati SV. A review of thermal co-conversion of coal and [90] Hakkou M, Pétrissans M, Gérargin P, Zoulalian A. Investigation of the reasons
biomass/waste. Energies 2014;2014(7):1098–148. http://dx.doi.org/10.3390/ for fungal durability of heat-treated beech wood. Polym Degrad Stab
en7031098 ISSN 1996-1073. 2006;91:393–7.
[58] Kaliyan N, Morey RV, White MD, Doering A. Roller press briquetting and [91] Arias B, Pedida C, Fermoso J, Plaza MG, Rubiera F, Pis JJ. Influence of torre-
pelleting of corn stover and switchgrass. Am Soc Agric Biol Eng 2009;52 faction on the grindability and reactivity of woody biomass. Fuel Process
(2):543–55. Technol 2008;89:169–75.
[59] Grover PD, Mishra SK. Biomass briquetting: Technology and practices. Re- [92] Pastorova I, Arisz PW, Boon JJ. Preservation of D-glucose oligosaccharides in
gional Wood Energy Development Programme in Asia. Bangkok: Food and cellulose chars. Carbohydr Res 1993;248:151–65.
Agricultural Organization of the United Nations; 1996. [93] Wooten JB, Crosby B, Hajaligol MR. Evaluation of cellulose char structure
[60] Visviva Renewable Energy. Various briquetting and pelletizing technologies. monitored by 13C CPMAS NMR. Fuel Chem Div 2000;46:191–3.
〈http://www.vvenergy.com/various_briquetting_and_pelletizing_technolo [94] Uslu A, Andrè PC, Faaij AB, Bergman PCA. Pre-treatment technologies, and
gies.html〉; 2011 [last accessed, December 2014]. their effect on international bioenergy supply chain logistics. Techno-eco-
[61] FAO Corporate document repository. The briquetting of agricultural wastes nomic evaluation of torrefaction, fast pyrolysis, and pelletization. Energy
for fuel. 〈http://www.fao.org/docrep/t0275e/T0275E05.htm#Chapter%208. 2008;33:1206–23.
Hydraulic%20piston%20presses〉; 2013 [last accessed, December 2014]. [95] Prins MJ. Thermodynamic analysis of biomass gasification and torrefaction
[62] Agro Energy. Bagasse biomass briquettes. 〈http://www.indiamart.com/rk- [Ph.D. thesis]. The Netherlands: Eindhoven Technical University; 2005.
agro/bagasse-briquettes.html〉; 2014 [last accessed December 2014]. [96] Sadaka S, Negi S. Improvements of biomass physical and thermochemical
[63] Nielsen CF. Basic Briquetting Control System-A real briquette maker. 〈http:// characteristics via torrefaction process. Environ Prog Sustain Energy 2009;28
www.cfnielsen.com/producten/22〉; 2014 [last accessed December 2014]. (3):427–34.
[64] Yehia KA. Estimation of roll press design parameters based on the assess- [97] Bridgeman TG, Jones JM, Shiel I, Williams PT. Torrefaction of reed canary
ment of a particular nip region. Powder Technol 2007;177:148–53. grass, wheat straw and willow to enhance solid fuel qualities and combus-
[65] Chandak SP. Physical conversion technologies. A trainning on technologies tion properties. Fuel 2008;87:844–56.
for converting waste agricultural biomass into energy. San Jose, Costa Rica: [98] Bhattacharya SC, Sett S, Shrestha RM. State-of-the-Art for biomass densifi-
Organized by United Nations Environment Programme (UNEP, DTIE, IETC); cation. Energy Sources 1989;11:161–82.
2013. [99] Bhattacharya SC. State-of-the-Art of utilizing residues and other types of
[66] MacMahon MJ. Additives for physical quality of animal feed. In: Beaven DA, biomass as an energy source. RERIC Int Energy J 1993;15(1):1–21.
editor. Manufacturing of Animal Feed, Herts. England: Turret-Wheatland [100] Aqa S, Bhattacharya SC. Densification of preheated sawdust for energy con-
Ltd.; 1984. p. 69–70. servation. Energy 1992;17(6):575–8.
[67] Yaman S, Şahan M, Haykiri-açma H, Şeşen K, Küçükbayrak S. Production of [101] FAO. Biomass briquetting: Technology and Practices. In regional wood energy
fuel briquettes from olive refuse and paper mill waste. Fuel Process Technol development program in Asia 1996; Report prepared by P.D. Grover and S. K.
2000;2000(68):23–31. Mishra, GCP/RAS/154/NET,f40_97.html; October 29, 2007.
[68] Li Y, Liu H. High pressure densification of wood residues to form an up- [102] Apted D. Dryers. Alaska Pellet Mill. Anchorage Alaska. 〈http://www.alaska
graded fuel. Biomass Bioenergy 2000;19:177–86. pelletmill.com/dryers〉; 2014 [last accessed January 2015. Last accessed
[69] Demirbas A, Şahin-Demirbaş A, Demirbaş AH. Briquetting properties of March 2015].
biomass waste materials. Energy Sources 2004;26:83–91. [103] Lv P, Yuan Z, Ma L, Wu C, Chen Y, Zhu J. Hydrogen-rich gas production from
[70] Hill B and Pulkinen DA. A study of factors affecting pellet durability and biomass air and oxygen/steam gasification in a downdraft gasifier. Renew
pelleting efficiency in the production of dehydrated alfalfa pellets 1998; A Energy 2007;32:2173–85.
special Report. Vol. 25. Tisdale, SK, Canada: Saskatchewan Dehydrators [104] Yan F, Luo S-Y, Hu Z-Q, Xiao B, Cheng G. Hydrogen-rich gas production by
Association; 1988. steam gasification of char from biomass fast pyrolysis in a fixed-bed reactor:
[71] Smith IE, Probert SD, Stokes RE, Hansford RJ. The briquetting of wheat straw. influence of temperature and steam on hydrogen yield and syngas compo-
J Agric Eng Res 1977;22:105–11. sition. Bioresour Technol 2010;101:5633–7.
[72] Butler JL, McColly HF. Factors affecting the pelleting of hay. Agric Eng [105] Hosseini M, Dincer I, Rosen MA. Steam and air fed biomass gasification:
1959;40:442–6. comparisons based on energy and exergy. Int J Hydrog Energy
[73] Tabil LG, Sokhansanj S. Process conditions affecting the physical quality of 2012;37:16446–52.
alfalfa pellets. Am Soc Agric Eng 1996;12(3):345–50. [106] Singh RN. Equilibrium moisture content of biomass briquettes. Biomass
[74] Shankar TJ, Sokhansanj S, Bandyopadhyay S, Bawa AS. A case study on op- Bioenergy 2004;26:251–3.
timization of biomass flow during single screw extrusion cooking using ge- [107] Bitra VSP, Womac AR, Chevanan N, Miu PI, Igathinathane C, Sokhansanj S,
netic algorithm (GA) and response surface method (RSM). Food Bioprocess et al. Direct mechanical energy measured of hammer mill comminution of
Techno. 2008;3(4):498–510. switchgrass, wheat straw, and corn stover and analysis of their particle size
[75] Kaliyan N, Morey R. Densification characteristics of corn stover and distributions. Powder Technol 2009;193:32–45.
Switchgrass. In: Presented at the ASABE annual international meeting. [108] Soucek J, Hanzlikova I, Hulta P. A fine disintegration of plants suitable for
Portland, OR, ASABE Paper No. 066174, ASAE, St. Joseph, MI, USA;2006. composite biofuels production. Res Agric Eng 2003;49(1):7–11.
[76] Heffiner LE, Pfost HB. Gelatinization during pelleting. Feedstuff 1973;45 [109] Lopo P. The right grinding solution for you: roll, horizontal or vertical. Feed
(23):33. Manag 2002;53(3):203–6.
[77] Shankar TJ, Bandyopadhyay S. Process variables during single screw extru- [110] Tumuluru JS, Tabil LG, Song Y, Iroba KL, Meda V. Grinding energy and physical
sion of fish and rice flour blends. J Food Process Preserv 2005;29:151–64. properties of chopped and hammer-milled barley, wheat, oat, and canola
[78] Al-Widyan MI, Al-Jalil HF, Abu-Zreig MM, Abu-Handeh NH. Physical dur- straws. Biomass Bioenergy 2014;60:58–67.
ability and stability of olive cake briquettes. Can Biosyst Eng 2002;44:341–5. [111] Peleg M, Mannheim CH. Effect of conditioners on the flow properties of
[79] Van der Stelt MJC, Gerhauser H, Kiel JHA, Ptasinski KJ. Biomass upgrading by powdered sucrose. Powder Technol 1973;7:45–50.
torrefaction for the production of biofuels: a review. Biomass Bioenergy [112] Drzymala Z. Industrial briquetting-fundamentals and methods. Studies in
2011;35:3748–62. mechanical engineering. 13 Warszawa: PWN-Polish Scientific Publishers;
[80] Felfli FF, Luengo CA and Beaton PA. Bench unit for biomass residues torre- 1993.
faction, In: Kopetz H. editor. Biomass for Energy and Industry, Proceedings in [113] Agar D, Wihersaari M. Torrefaction technology for solid fuel production. GCB
international conference. Wurzburg, Germany; 1998. p. 1593–95. Bioenergy 2012;4(5):475–8.
[81] Lehtikangas P. Quality properties of fuel pellets from forest biomass [Li- [114] Gustafsson E. Characterization of particulate matter from atmospheric flui-
centiate Thesis]. Department of Forest Management and Products, Report 4, dized bed biomass gasifiers [Ph.D. thesis]. Linnaeus University; 2011 https://
Uppsala; 1999. www.divaportal.org/smash/get/diva2:412937/FULLTEXT01.pdf.
 A. Anukam et al. / Renewable and Sustainable Energy Reviews 66 (2016) 775–801 801

[115] Jayah TH. Evaluation of a downdraft wood gasifier for tea manufacturing in edenergy.com/d_plasma.html〉; 2014 [last accessed, December 2014].
Sri Lanka. Master's dissertation. The University of Melbourne; 2002. [137] Phoenix Energy. Plasma gasification. 〈http://www.phoenixenergy.com.au/
[116] Reed TB. Types of gasifiers and gasifier design considerations. In: Reed TB plasma-gasification/〉; [last accessed, December 2014].
editor. Biomass gasification, Principles and Technology. New Jersey: Noyes [138] Schapfer P, Tobler J. Theoretical and practical investigations upon the driving
Data Corp.; 1981. p. 184–99. of motor vehicles with wood gas. Bern; 1937.
[117] Buekens AG, Schoeters JG. In: Overend RP, Milne TA, Mudge LK, editors. [139] Mamphweli NS, Meyer LE. Evaluation of the conversion efficiency of a 180
Modelling of biomass gasification. Fundamentals of Thermochemical Bio- Nm3/h Johannson biomass gasifier. Int J Energy Environ 2010;1(1):113–20.
mass. London: Elsevier; 1985. [140] Milne T, Evans R, Abatzoglou N. Biomass gasifiers "Tars": Their nature, for-
[118] Graboski M, Bain R. Properties of biomass relevant to gasification. In: Reed TB mation, and conversion. National Renewable Energy Laboratory. 〈http://
editor. Biomass Gasification, Principles and Technology. New Jersey: Noyes www.nrel.gov/docs/fy99osti/25357.pdf〉; 1998 [last accessed December
Data Corp; 1981. p. 41–69. 2015].
[119] Reed TB, Das A. Handbook of biomass downdraft gasifier engine system. [141] Hanaoka T, Minowa T, Miyamoto A, Edashige Y. Effect of chemical property of
Golden, Colorado: SERI; 1988. waste biomass on air-steam gasification. J Jpn Inst Energy 2005;84:1012–8.
[120] Reed TB, Markson M. A predictive model for stratified downdraft gasification [142] Corella J, Aznar MP, Delgado J, Aldea E. Steam gasification of cellulosic wastes
of biomass. In: Tillman DA, Jahn EC, editors. Progress in biomass conversion, in a fluidized bed with downstream vessels. Ind Eng Chem Res
4. New York: Academic Press; 1983. p. 219–54. 1991;30:2252–62.
[121] Solar Energy Research Institute (SERI). Generator Gas-The Swedish Experi- [143] Higman C, van der Burgt M. Gasification. United States of America: Gulf
ence from 1939–1945. Golden, Colorado: SERI,; 1979 [Chapter 2]. Professional Publishing; 2003.
[122] National Energy Technology Laboratory. Gasification Technology R&D. World [144] Antal Jr. JM, Edwards WE, Friedman HL, Rogers RE. A study of the steam
Gasification Database. 〈http://energy.gov/fe/science-innovation/clean-coal- gasification of organic wastes. Project report to United States Environmental
research/gasification〉; 2010 [last accessed, December 2014]. Protection Agency: Liberick, W.W. Project Officer; 1984.
[123] Sami M, Annamalai K, Wooldridge M. Co-firing of coal and biomass fuel [145] Abatzoglou N, Barker N, Hasler P, Knoef H. The development of a draft pro-
blends. Prog Energy Combust Sci 2001;27:171–214.
tocol for the sampling and analysis of particulate and organic contaminants
[124] Surridge AD. Carbon sequestration leadership forum: South Africa status
in the gas from small biomass gasifiers. Biomass Bioenergy 2000;18:5–17.
report. Report to the National Committee on Climate Change. Pretoria; 2004.
[146] Maniatis K, Beenackers AACM. Introduction: tar protocols. IEA Bioenergy
[125] Hitchon B. Aquifer disposal of carbon dioxide: Hydrodynamic and mineral
gasification task. Biomass Bioenergy 2000;18:1–4.
trapping-proof of concept. Alberta, Canada: Geoscience Publishing; 1996.
[147] Franco C, Pinto F, Gulyurtlu I, Cabrita I. The study of reactions influencing the
[126] National Energy Technology Laboratory. An overview. NETL gasification
biomass steam gasification process. Fuel 2003;82:835–42.
technologies Training; 2004. [Reprinted from M. Ramezan, Coal-based gasi-
[148] Turn S, Kinoshita C, Zhang Z, Ishimura D, Zhou J. An experimental in-
fication technologies course].
vestigation of hydrogen production from biomass gasification. Int J Hydrog
[127] Food and Agriculture Organization of the United Nations. Wood gas as engine
Energy 1988;23:641–8.
fuel 1986; 〈www.fao.org〉; 1986 [last accessed, April 2012].
[149] Walawender WP, Hoveland DA, Fan LT. Steam gasification of pure cellulose. 1.
[128] Rajvanshi AK. Biomass gasification. In: Yogi Goswami D, editor. Alternative
Uniform temperature profile. Ind Eng Chem Process Des Dev 1985;24:813–7.
energy in agriculture, vol. 2. CRC Press; 1986. p. 83–102 [Chapter 4].
[150] Florin NH, Harris AT. Enhanced hydrogen production from biomass with
[129] Westinghouse Plasma Corporation. Waste to Energy. 〈http://www.westing
house-plasma.com/waste_to_energy/〉; 2015 [last accessed January 2015]. in situ carbon dioxide capture using calcium oxide sorbents. A review. Chem
[130] Eggen A, Kraatz R. Gasification of solid waste in fixed beds. Mech Eng Eng Sci 2008;63:287–316.
1976;98:24–9. [151] Dayton D. A review of the literature on catalytic biomass tar destruction;
[131] Panwar NL, Kothari R, Tyagi VV. Thermo chemical conversion of biomass-Eco 2002. [NREL/TP-510-32815].
friendly energy routes. Renew Sustain Energy Rev 2012;16:1801–16. [152] Han J, Kim H. The reduction and control technology of tar during biomass
[132] Lalou C. Distributed power with advanced clean coal gasification technology. gasification/pyrolysis: an overview. Renew Sustain Energy Rev 2006. http:
Cornerstone. The official Journal of The World Coal Industry. 〈http://corner //dx.doi.org/10.1016/j.rser.2006.07.015.
stonemag.net/distributed-power-with-advanced-clean-coal-gasification- [153] Sutton D, Kelleher B, Ross JRH. Review of literature on catalysts for biomass
technology/〉; 2014 [last accessed, January 2015]. gasification. Fuel Process Technol 2001;72:155–73.
[133] Van der Drift A, Boerrigter H, Coda B, Cieplik M, Hemmes K. Entrained flow [154] Sutton D, Kelleher B, Doyle A, Ross JRH. Investigation of nickel supported
gasification of biomass. Ash behaviour, feeding issues, and system analyses. catalysts for the upgrading of brown peat derived gasification products.
ECN Energy Innov 2004 ECN-C-04-039. Biosource Technol 2001;80:111–6.
[134] National Energy Technology Laboratory. 〈http://www.netl.doe.gov〉; 2012 [155] The Global Carbon Capture and Storage Institute. Gasification by-products.
[last accessed, May 2012]. 〈http://decarboni.se/publications/34-gasification-products〉; 2014 [last ac-
[135] National Energy Technology Laboratory. Commercial gasifier: entrianed flow cessed February 2015].
gasifiers. 〈http:/www.netl.doe.gov/research/coal/energysystems/gasification/ [156] Koppejan J, Sokhansanj S, Melin S, Madrali S. Status overview of torrefaction
gasifipedia/entrainedflow〉; 2006 [last accessed, December 2014]. technologies. IEA Bioenergy Task 32 report. 〈http://www.ieabcc.nl/publica
[136] Lewis R. The recovered energy system. Extracting energy from waste without tions/IEA_Bioenergy_T32_Torrefaction_review.pdf〉; 2012. p.1–54 [last ac-
combustion. A discussion on plasma gasification. 〈http://www.recover cessed December 2015].