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Slow Pyrolysis of Rice Straw: Analysis of Biochar, Bio-Oil and Gas

Article in Southern Brazilian Journal of Chemistry · June 2018

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SOUTHERN BRAZILIAN JOURNAL OF CHEMISTRY
SLOW PYROLYSIS OF RICE STRAW: ANALYSIS OF BIOCHAR, BIO-OIL AND GAS

KLUNK, Marcos Antônio1*; DASGUPTA, Sudipta2; DAS, Mohuli2

1 University of Vale do Rio dos Sinos, Graduation Program in Mechanical Engineering
2 Department of Earth Sciences, IIT Bombay, India Federation

* Correspondence author
e-mail: marcosak@edu.unisinos.br

Received 12 May 2018; received in revised form 30 June 2018; accepted 14 August 2018

ABSTRACT

Biomass is the term attributed to any renewable resource derived from organic matter that can be used
in energy production. Agricultural production generates residues that are of great importance for their energy use,
of which sugar cane, eucalyptus, and rice. Various residues are generated from rice cultivation, among which the
rice husk and rice straw are the most important. Several thermal conversion technologies have been developed
for the use of biomass in industry. Pyrolysis has been notable for its ability to produce biofuels at different stages
of aggregation. The slow pyrolysis of biomass has been proposed as a pretreatment method to improve the
physical-chemical characteristics of rice straw. In this process is produced, mainly, a solid called biochar, which
has a higher energy content when compared to the biomass of origin. This study investigated the slow pyrolysis
of rice straw at 300 - 700°C for the purpose of obtaining biochar, bio-oil, and gases for energy purposes. The
experimental results show that pyrolysis temperature has important roles in yield product. The highest biochar
yield was observed at a temperature of 300°C with 49.91 wt%. This represents 47% more when compared to
yield at 700°C (33.87 wt.%). This behavior is linked to the proximate analysis results for fixed carbon 26.01 wt.%
at 300°C. The high pH of the biochar was attributed to the presence of alkali metals, according to XRF. Thermal
decomposition of the biomass resulting in a gradual increase of bio-oil (16.81 - 34.70 wt.%) and gas (6.53 - 18.05
wt.%) on a wet basis. Thus, in the dry base parameter, the bio-oil increases from 19.22 - 30.6 wt.% and the gases
at 9.42-20.19 wt.%. Drying of the raw material showed, by the results, a significant increase in the co-products
formed. As a consequence, we have a more efficient energy process.

Keywords: Biomass, Pyrolysis Process, Renewable Energy, Biofuel, Tar

The pyrolysis of rice straw involves the

1. INTRODUCTION thermal decomposition of polymer compounds,
releasing organic vapors, and leaving carbon-rich
Thermochemical conversion technologies solid residues (char) (Jahirul et al., 2012). Pyrolytic
(pyrolysis, gasification, and combustion) have vapors can be separated into condensable
been developed for the use of biomass and hydrocarbon compounds (oil, also known as tar)
applied in industry to produce different types of and non-condensable gases (Hosokai et al.,
energy (Brown et al., 2013; Bridgwater, 2012). 2014).
Biomass is considered a potential for renewable The pyrolytic process is influenced by
energy sources in the future. Biomass can be thermodynamic and kinetic parameters that alter
categorized as agricultural waste being the only the products. These parameters involve
one among renewable energy resources that can temperature, heating rate, particle size, reaction
be converted into an energy source (Jiang and atmosphere, and residence time of the volatiles
Ellis, 2010). (Antal and Grønli, 2003).
The advantage of biomass in relation to Recent studies, published in the literature,
fossil fuels is the low emission of sulfur and bring the pyrolytic processes using rice straw
nitrogen (Carpenter et al., 2014; Demirbas, 2011; (Zhang et al., 2013, Chatterjee et al., 2013, Huang
Mohan et al., 2006). The use of biomass as a et al., 2013, Pattiya and Suttibak 2012; Fu et al.,
source of renewable energy contributes to the 2012, Huang et al., 2012, Wu et al., 2012).
mitigation of environmental impacts caused by In this work, the use of pyrolysis is a process of
greenhouse gas emissions (Jamilatun et al., 2017, thermal decomposition in the absence of oxygen
Lai et al., 2013, Zhang et al., 2012). or when the oxygen content is at an incomplete
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combustion level (Klunk and Ponomarev, 2017; coming from a farm without pre-treatment.
Vitali et al., 2013; Zhang et al., 2010). In this Proximate and ultimate analyses are listed in
thermal conversion process, up to 70% of the Table 1. The analysis has been carried out in
biomass energy can be converted into a liquid LECO elemental analyzers (CHNS-O)
product (Anex et al., 2010). Fast pyrolysis is a well- (FlashSmart - Thermo ScientificTM) and
studied technology that is reaching an early stage thermogravimetric analyzer (TGA-1000 - NAVAS
marketing (Jiang and Ellis, 2010; Effendi et al., INSTRUMENTSTM), respectively. The calorific
2008). Fast pyrolysis technology has been value was measured in an isoperibolic calorimeter
investigated mainly for the production of bio-oil. It (C 6000 ISOPERIBOL PACKAGE 1/10 - IKATM).
is typically performed using a fluidized bed to The chemical composition of ash (Table 2),
increase the heating rate (100°C / m) and at including silica and major metal compounds, was
temperatures around 500°C to maximize the bio- determined by X-ray fluorescence (S6 JAGUAR
oil yield. Higher temperatures and increased vapor EasyLoad – BrukerTM). In this study, the ash
residence time cause thermal cracking of content was measured by ASTM D1102-84. This
hydrocarbon compounds, decreasing bio-oil yield requires gradual heating to 580–600°C repeated
(Klunk and Ponomarev, 2017, Zhang et al., 2013, by 30 min periods until the sample weight does not
Eom et al., 2013, Pattiya and Suttibak 2012). change (<0.2 mg) (Jamilatun et al., 2017;
Unlike rapid pyrolysis, slow pyrolysis is carried out Surahmanto et al., 2017; Huang et al., 2013; Wu
at a heating rate of about 5°C/min and temperature et al., 2012; Peng et al., 2011).
range of 300-700°C (Jamilatun et al., 2017;
Surahmanto et al., 2017; Huang et al. , Wu et al., 2.2. Low pyrolysis
2012, Peng et al., 2011).
The objective of slow pyrolysis in rice straw Low pyrolysis experiment of rice straw was
is the production of biochar and applicability in the carried out at temperatures of 300, 350, 400, 450,
soil with the purpose of increasing fertility. Biochar 500, 550, 600, 650, 700°C. The details of the
is responsible for increasing the retention of reactor were described elsewhere (Lee et al.,
nutrients and water in the soil, providing the growth 2013a). Three hundred grams sample of rice straw
of microorganisms (Hossain et al., 2011; Brockhoff was placed in the reactor, and the experiments
et al., 2010; Gaskin et al., 2010). The amount of were carried out with a heating rate of 5°C/min.
bio-oil and non-condensable gas products in terms Nitrogen was continuously supplied at a flow rate
of mass and energy is considerably larger than of 2.0 L/min to purge pyrolysis vapors from the
that for biochar (Kim et al., 2012; Turns, 2011; reactor. Once the reactor attained the target
Phan et al., 2008). temperature, it was maintained for 1 h for complete
Bio-oil is a renewable fuel or chemical pyrolysis.
feedstock, but its chemical properties are not as Pyrolytic vapors containing condensable
good as biochar due to its high water content and gases (bio-oil) and non-condensable gases
a large number of compounds resulting in acidity passed through the condensers and the gas
and toxicity (Agar and Wihersaari, 2012; Gómez et analysis system. The connection tube from the
al. 2012). Pyrolytic gases are composed largely of reactor to the bio-oil condensers was heated to
CO and CO2, leading to poor fuel quality. 400°C. The biochar and bio-oil collected after the
This study presents the slow pyrolysis test was collected and weighed to determine the
characteristics of rice straw to provide mass yields. The composition of gases was
comprehensive information for the chemical continuously analyzed by an on-line gas analyzer
properties and yields of the three pyrolysis for CO and CO2. The gas yield was calculated by
products (biochar, bio-oil, and gases). The mass difference. The mass yields on a wet basis were
yield, elemental composition, and other key converted into a dry, ash-free basis to evaluate the
properties of the products were analyzed for product distribution from the organic fraction in
pyrolysis temperatures of 300-700°C. Based on biomass. The biochar yield was also converted to
these results, considerations required for an ash-free basis.
application of the slow pyrolysis technology to rice
straw were discussed. 3. RESULTS AND DISCUSSION

2. MATERIALS AND METHODS 3.1. Product yields

2.1. Raw material Figure 1 represents the yields of the slow

pyrolysis of rice straw in the temperature range of
Rice straw was used as raw material, 300 to 700°C, on a wet basis (Fig. 1A) and dry
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basis (Fig. 1B). al., 2010-1). The water content in the bio-oil is
On a wet basis, the biochar yield slightly affected by the pyrolysis temperature since
decreased with increasing temperature (Fig. 1A) its yield remains in the range of 30 to 23 wt.%.
ranging from 49.91 - 33.87 wt.%. This is due to Ketones are the second largest organic group in
thermal decomposition of the biomass, resulting in the bio-oil. Ketones are formed by condensation
a gradual increase of bio-oil (16.81 - 34.70 wt.%) reactions of the fraction derived from
and gas (6.53 - 18.05 wt.%). The biochar yields carbohydrates and decomposition of the various
include ash content that remained in the solid oxygenates and furans. When the pyrolysis
residue. In turn, the yield of bio-oil and gas have temperature is increased, cracking reactions are
the contribution of moisture. more severe, and lighter compounds are formed.
After the removal of moisture and ash Acid functional groups have their highest yield,
content, the yield of the products had another and the main compound of this group is acetic
behavior. Fig. 1B the biochar had a variation of acid. They can cause corrosion in subsequent
44.02 - 34.50 wt.%. Therefore, the bio-oil and processing when the equipment is made of poor
gases yield varied from 19.22 - 30.6 wt.% and quality material. In addition, organic acids are
9.42-20.19 wt.%, respectively. This can be results valuable by-products after separation. Therefore,
of cracking (secondary pyrolysis) of primary tar the removal of acids is important for the use of bio-
compounds by temperature and by hot char oil as an intermediary in the production of fuels and
surface. Considering the significant influence of chemicals.
ash and moisture content in the raw material, it The gas fraction is composed mainly of
was desirable to convert the biochar yield to a dry, carbon dioxide and carbon monoxide (Table 4).
ash-free basis. The yields increase with temperature due to the
increase in decarboxylation and decarbonylation
3.2. Biochar composition reactions. In addition, this fraction is also
composed of small amounts of hydrocarbons (C2-
Table 3 shows the properties of biochar. As C4). The concentration of CO increases with
the temperature of the pyrolytic process increases, temperature, while that of CO2 decreases. This is
the amount of volatile matter is expelled from the because at temperatures below 400°C the
biomass (32.20 - 7.07 wt.%), making the material decarboxylation reactions prevail, but at
more carbonaceous (as a function of fixed carbon temperatures above 400°C the main secondary
26.01 wt.% at 300°C). In consequence, there is a reactions are those of decarbonylation, and
decrease in the higher heating value from 15.80 therefore the release of CO is greater (Tripathi et
MJ/kg to 8.77 MJ/kg, representing a reduction of al., 2016; Tripathi, Sahu and Ganesan 2016). The
55%. The surface area (BET) increased from 3.91 yield of light hydrocarbons (C2-C4) and H2 also
to 43.39. This value is indicative of good increases with temperature due to cracking
adsorption capacity when applied to the soil. The reactions. These non-condensable gases are of
surface area of the biochar from the 'rice straw' the low energy value in a pyrolysis process due to
was relatively low when compared to the biochar dilution with the entrainment gas (N2). In addition,
of other lignocellulosic biomasses. For example, the low CO2 yield is a favorable environmental
the biomass of wood and sugarcane bagasse is feature involving the RS pyrolysis process
above 100 m2/g (Lee et al., 2013a, Lee et al., (Stefanidis et al., 2011).
2013b). The pH of the bio-oil increased from 8.75
to 11.92. The alkalinity in rice straw is due to the 4. CONCLUSIONS
presence of alkaline compounds (MgO, CaO,
Na2O, K2O) according to the X-ray fluorescence Rice straw is considered a high added
(Table 2). value agricultural residue. Slow pyrolysis is the
process responsible for this energy
3.3. Bio-oil and gas composition
transformation. Co-products such as biochar, bio-
oil, and gases are obtained with different yields at
Table 3 summarizes the bio-oil properties
temperatures of 300 to 700 ° C due to the pyrolytic
for the mean of light organic and heavy organic
process. The experimental results show that
phases. The bio-oil compounds were in Table ,
pyrolysis temperature has important roles in the
according to major individual compounds.
biochar, bio-oil, and gas yield.
Oxygenated compounds make bio-oil unstable
The highest biochar yield was observed at a
and reduce miscibility with hydrocarbons and
temperature of 300 ° C with 49.91 wt%. This
calorific value (Bridgwater, 2012; Heo et al., 2010).
represents 47% more when compared to yield at
In addition, the heterocyclic compounds of the bio-
700 ° C (33.87 wt.%). This behavior is linked to the
oil make it viscous and easy to polymerize (Heo et
SOUTHERN BRAZILIAN JOURNAL OF CHEMISTRY.
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Established in 1993.
19
results of "Proximate analysis" for fixed carbon of the impacts of feedstock and
26.01 wt.% At 300 ° C). The high pH of the biochar pretreatment on the yield and product
was attributed to the presence of alkali metals, distribution of fast pyrolysis bio-oils and
according to XRF. vapors. Green Chemistry, 2014, 16,
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A)

B)

Figure 1. Product yield From slow pyrolysis of rice straw, A) on wet basis and B) dry basis

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Table 1. Properties of the rice straw

Proximate analysis Ultimate analysis

Weight (%) on dry Weight (%) on wet
Parameters Elements
basis basis
Volatile matter 65.71 Carbon 50.59
Fixed carbon 15.13 Hydrogen 7.02
Ash 14.29 Nitrogen 2.55
Moisture 6.41 Oxygen 48.10
HHV* (MJ/kg) 20.88
*HHV: Higher Heating Value

Table 2. Chemical composition of the rice straw ash

Compound Weight (%)

SiO2 71.82
Al2O3 1.55
Fe2O3 0.28
MnO 0.01
MgO 4.99
CaO 3.03
Na2O 0.20
K2O 0.15
TiO2 0.02
P2O5 0.09
Loss on ignition 10.99

Table 3. Properties of the biochar

300 350 400 450 500 550 600 650 700

P.T a
(°C) (°C) (°C) (°C) (°C) (°C) (°C) (°C) (°C)
P.A b VM 32.20 30.17 22.39 20.71 15.40 13.44 9.92 8.41 7.07
FC 26.01 25.64 23.88 21.08 20.99 19.85 18.13 20.48 22.01
Ash 35.55 36.87 39.90 41.07 46.10 47.53 49.60 50.39 52.97
U.A c C 63.34 65.08 69.72 70.83 77.53 78,66 80,61 82.07 86.62
H 3.79 3.00 2.44 2.20 1.97 1.21 1.01 0.92 0.82
N 2.99 2.81 2.32 2.09 1.83 1.68 1.40 1.09 0.79
O 20.81 18.90 16.73 15.05 13.92 12.62 8.83 7.53 6.98
HHV d 15.80 14.77 13,92 12.54 11,96 10.66 9.77 9.00 8.77
BET e 3.91 11.88 19,70 36.63 40.81 44.49 59.58 50.72 43.39
APD f 100.4 99.03 85.42 83.74 81.79 79.92 78.00 76.64 75.07
PV g 0.027 0.038 0.041 0.044 0.048 0.050 0.052 0.054 0.055
pH 8.75 9.10 9.55 9.93 10.10 10.99 11.15 11.30 11.92
a Pyrolysis temperature; b Proximate analysis; c Ultimate analysis; d Higher Heating Value

(MJ/kg-dry); e Surface area (m2/g); f Average pore diameter (Å); Pore volume (cm3/g); 1 Volatile
matter; 2 Fixed carbon; 3 Carbon; 4 Hydrogen; 5 Nitrogen; 6 Oxygen

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 Table 4. Yields of the main chemical compounds in the bio-oil

300 350 400 450 500 550 600 650 700

Compounds
(°C) (°C) (°C) (°C) (°C) (°C) (°C) (°C) (°C)
Acetic acid 2.30 2.33 2.38 3.40 2.32 2.42 2.53 2.85 2.92
Propanoic acid 1.26 1.39 1.45 0.92 0.29 0.29 0.30 0.28 0.20
Formaldehyde 0.28 0.30 0.30 0.47 0.49 0.50 0.57 0.68 0.74
1-Hydroxy-2-propanone 1.98 1.77 1.58 1.32 0.89 0.72 0.43 0.31 0.22
1,3-Cyclopentanedione 3.88 3.52 3.14 2.67 2.59 1.90 0.81 0.77 0.65
2-3-methyl-2-cyclopentenone 1.34 1.30 1.19 1.18 1.38 1.31 0.42 0.36 0.29
Alkyl-phenols 2.99 3.09 3.07 3.72 3.77 3.96 6.62 6.99 7.03
Guaiacols 4.79 4.50 3.91 3.32 2.84 2.80 2.74 2.38 2.00
Catechols 1.44 2.00 2.90 3.34 3.42 3.50 3.76 4.01 4.17
2,2,4-Trimethyl-1,3-dioxalane 3.06 2.89 2.41 2.30 1.65 1.52 0.97 0.88 0.54
Carboxylic Anhydrides 1.83 1.44 1.19 1.09 1.01 1.08 1.10 1.33 1.45
Furans 4.03 4.37 4.39 4.65 5.12 4.85 4.24 4.00 3.83
Nitrogenated compounds 0.90 1.00 1.08 1.36 1.61 1.34 1.11 1.09 0.97
Water 30.92 27.99 26.32 24.27 24.52 24.61 25.21 24.54 23.12

Table 5. Influence of temperature on gas composition

300 350 400 450 500 550 600 650 700

Compounds
(°C) (°C) (°C) (°C) (°C) (°C) (°C) (°C) (°C)
H2 0.73 0.99 1.45 1.55 1.95 2.85 3.52 3.99 4.39
CH4 2.56 2.74 3.44 3.73 5.30 5.99 7.92 10.55 10.69
CO 49.31 51.35 54.41 55.49 57.20 58.86 59.30 60.51 61.88
CO2 55.12 54.07 52.93 50.38 47.62 45.06 40.87 38.92 35.57
C2-C4 1.55 2.98 3.73 4.55 4.97 5.67 5.99 6.48 6.93

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