Renewable and Sustainable Energy Reviews 42 (2015) 712–725

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews

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

Bioenergy and biofuels: History, status, and perspective

Mingxin Guo a,n, Weiping Song b, Jeremy Buhain c
a
Department of Agriculture and Natural Resources, Delaware State University, Dover, DE 19901, USA
b
Department of Chemistry, Delaware State University, Dover, DE 19901, USA
c
Department of Chemical, Biological, and Pharmaceutical Engineering, New Jersey Institute of Technology, Newark, NJ 07102, USA

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

Article history: The recent energy independence and climate change policies encourage development and utilization of
Received 8 May 2014 renewable energy such as bioenergy. Biofuels in solid, liquid, and gaseous forms have been intensively
Received in revised form researched, produced, and used over the past 15 years. This paper reviews the worldwide history,
17 August 2014
current status, and predictable future trend of bioenergy and biofuels. Bioenergy has been utilized for
Accepted 5 October 2014
cooking, heating, and lighting since the dawn of humans. The energy stored in annually produced
biomass by terrestrial plants is 3–4 times greater than the current global energy demand. Solid biofuels
Keywords: include firewood, wood chips, wood pellets, and wood charcoal. The global consumption of firewood and
Biofuel charcoal has been remaining relatively constant, but the use of wood chips and wood pellets for
Firewood electricity (biopower) generation and residential heating doubled in the past decade and will increase
Bioenthanol
steadily into the future. Liquid biofuels cover bioethanol, biodiesel, pyrolysis bio-oil, and drop-in
Biodiesel
transportation fuels. Commercial production of bioethanol from lignocellulosic materials has just
Pyrolysis bio-oil
Biogas
started, supplementing the annual supply of 22 billion gallons predominantly from food crops. Biodiesel
Syngas from oil seeds reached the 5670 million gallons/yr production capacity, with further increases depending
on new feedstock development. Bio-oil and drop-in biofuels are still in the development stage, facing
cost-effective conversion and upgrading challenges. Gaseous biofuels extend to biogas and syngas.
Production of biogas from organic wastes by anaerobic digestion has been rapidly increasing in Europe
and China, with the potential to displace 25% of the current natural gas consumption. In comparison,
production of syngas from gasification of woody biomass is not cost-competitive and therefore, narrowly
practiced. Overall, the global development and utilization of bioenergy and biofuels will continue to
increase, particularly in the biopower, lignocellulosic bioethanol, and biogas sectors. It is expected that
by 2050 bioenergy will provide 30% of the world’s demanded energy.
& 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713
2. Development and utilization of solid biofuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713
2.1. Firewood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713
2.2. Wood chips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714
2.3. Wood pellets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714
2.4. Charcoal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715
3. Development and utilization of liquid biofuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715
3.1. Bioethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715
3.2. Biodiesel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 717
3.3. Pyrolysis bio-oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 719
3.4. Drop-in biofuels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 720
4. Development and utilization of gaseous biofuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 720
4.1. Biogas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 720

Abbreviations: ABE, acetone–butanol–ethanol; FAME, fatty acid methyl esters

n
Correspondence to: Department of Agriculture and Natural Resources, Delaware State University, Dover, DE 19901, USA.
Tel: 1 302 857 6479; fax: 1 302 857 6455.
E-mail address: mguo@desu.edu (M. Guo).

http://dx.doi.org/10.1016/j.rser.2014.10.013
1364-0321/& 2014 Elsevier Ltd. All rights reserved.
 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725 713

4.2. Syngas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 721

5. Biofuel overview and the potential socio-econo-environmental impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 722
6. Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724

1. Introduction development of solid, liquid, and gaseous biofuels, and look into the
future of bionenergy and biofuels. It aims to present a whole picture of
The annual terrestrial primary production adds approximately bioenergy and biofuels and prepare readers to transit into the “after-
120
1015 g of dry vegetative biomass [1], storing 2.2
1021 J of fossil fuel” life.
energy in plant materials [2]. The world energy demand was
5.5
1020 J in 2010. It is predicted to increase to 6.6
1020 J in
2020 and 8.6
1020 J in 2040 [3]. Largely, the bioenergy captured 2. Development and utilization of solid biofuels
each year by land plants is 3–4 times greater than human energy
demands. The bioenergy delivery potential of the world’s total 2.1. Firewood
land area excluding cropland, infrastructure, wilderness and den-
ser forests is estimated at 190
1018 J yr  1, 35% of the current Wood and other plant materials have been directly burned for
global energy demand [4]. heating and cooking since the dawn of modern human beings.
Biofuels refer to plant biomass and the refined products to be Before the discovery of fossil fuels, firewood was the primary fuel
combusted for energy (heat and light). Similar to fossil fuels, for domestic purposes. At 220–300 1C or higher temperature, most
biofuels exist in solid, liquid, and gaseous forms. Human beings dry plant materials ignite in air to cause fire (flame), releasing the
have been utilizing bioenergy and biofuels for domestic purposes inherent bioenergy in heat and light. Combustion starts initially
since pre-recorded history. Wood was burned for heat and light to with exothermic pyrolysis of wood at around 260 1C to generate
cook food, illumine night, warm shelters, and treat clay artifacts. solid char and gaseous fume. Subsequently the char burns to ash
Before the 19th century, wood was the predominant fuel for and the fume to flame. Fire – the combustion of organic matter – is
cooking and heating and plant oil was the chief fuel for lighting prompt oxidation of bio-carbon compounds by oxygen at high
worldwide. Today, however, fossil fuels are the dominant energy temperature and can be simply described as:
sources, meeting 480% of the world’s energy demand [5]. Never-
C6H10O5 þ6O2-6CO2 þ5H2O þheatþ light (1)
theless, fossil fuels are nonrenewable and their reserves are
limited. At the current consumption rates, the supply of petro- It is not clear when human beings started the controlled use of
leum, natural gas, and coal will only be able to last for another 45, fire for heat and light. The initial fire must be from lightening and
60, and 120 years, respectively [6]. The dwindling supply and the volcanic eruption that ignited wood. Nevertheless, human beings had
soaring price of fossil fuels, especially petroleum, compel the grasped the “bow-drill fire making” technique six thousand years ago
world to develop renewable energy alternatives. Moreover, tre- [10]. Archaeologists identified charred animal bones and stone tools
mendous amounts of greenhouse gases have been released from fossil in wood ash in Wonderwerk Cave of Kuruman Hills in South Africa,
fuel consumption, elevating the atmospheric CO2 concentration from providing evidence of controlled fire use by prehistoric “mankind”
the pre-industrial level of 280 ppm to the present nearly 400 ppm and creatures one million years ago [11]. Wood, straw, hay, cattle dung,
causing disastrous climate change effects [7]. The incentives for and peat have been intentionally collected, dried, and burned to
mitigating global climate change further stimulate international com- prepare food and warmth. In the wilderness, bonfires and straw
munities to invest in development and utilization of renewable energy. torches are commonly used for heating and lighting. Fire has also
Of the renewable energy sources, bioenergy draws major and parti- been used to clear land, to attack enemy in war, to smelt ores, and to
cular development endeavors, primarily due to the extensive avail- treat clay artifacts for china, bricks, and tiles.
ability of biomass, already-existence of biomass production Firewood is normally packed in bundles and traded in volume.
technologies and infrastructure, and biomass being the sole feedstock In the U.S., firewood is measured in “cord,” with one cord
for liquid fuels. Currently, commercial production of electricity and equivalent to 128 ft3 (40
80
40 ) or 3.6 m3. The weight of one
transportation fuels from biomass feedstocks is practiced in most cord firewood varies from 1350 to 2600 kg, depending on the
nations. The U.S. bioenergy production reached 4.76
1015 J (4.51 wood type and moisture content. Hardwood is generally “heavier”
quadrillion Btu; 1 Btu¼1054.35 J) in 2011, accounting for 48.8% of than softwood. Green, unseasoned firewood may contain 150%
the renewable energy and 5.8% of the total energy produced in the (commonly 40–100%) moisture on the dry mass basis (The same
year [8]. Approximately 42% of the U.S. corn grains were used to applies in the following if not specified). Seasoned, air-dry fire-
generate 49 billion liters of bioethanol, representing 94% of the liquid wood usually contains 10–25% moisture. “Wetter” firewood is
biofuels produced in 2012 (52.2 billion liters) and replaced 10% of the normally lower in energy content, as certain heat has to be
nation’s demanded gasoline fuel [9]. The U.S. Energy Independence consumed to transform water into steam upon combustion. There-
and Security Act of 2007 mandates to increase annual biofuel addition fore, green firewood should be seasoned for at least 6 months in a
to gasoline from 34 billion liters (9 billion gallons; 1 gal¼3.781 L) in well-ventilated place to bring its moisture content to below 20%
2008 to 136 billion liters by 2022, with 60 billion liters of the biofuel prior to use. The energy content of well-seasoned firewood is
from lignocellulosic materials. A variety of conversion technologies to  15 MJ kg  1, one-third to half of that of fossil fuels [12]. The
produce “drop-in” fuels like butanol and C3–C10 hydrocarbons from energy would be fully released in heat and light upon complete
wood and grasses are under investigation. The extensive production of combustion that produces only CO2 and water vapor emissions.
biomass feedstock and biofuels, however, will generate significant Unfortunately, combustion of wood and other raw plant materials
environmental and socioeconomic impacts on soil, water, land, food, in conventional furnaces is generally incomplete, resulting in signifi-
and civil development. This paper is to summarize the history of cant smokes (a mixture of water vapor, volatile organic compounds,
human utilization of bioenergy, review the present use and and carbon black particulates) and creosote (smoke condensate) that
714 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725

are hazardous to human health, equipment, and the environment. exhibits an average energy efficiency of 85% and reduces the
Other minor emissions from burning firewood include carbon mon- heating cost by $15,000/yr over propane [21]. Co-firing has been
oxide, methane, nitrogen oxides, and sulfur oxide [13]. commercially practiced by numerous coal-fired power plants to
Incomplete combustion of firewood is caused by insufficient generate electricity, in which wood chips are combusted in 15–
oxygen, inadequate mixing, and/or low temperature [14]. The 30 vol% mixtures with pulverized coal to drive steam turbines. The
combustion efficiency of seasoned firewood in a typical burning conversion efficiency of bioenergy to electricity in co-firing ranges
device ranges from 80% to 85% [15]. Considering the 50–80% 33–37% [19]. There are also 222 power plants directly fueled by
thermal transfer efficiency, conventional fireplaces, wood stoves, wood chips in the U.S., producing annually 7.5 billion kW h of
and boilers demonstrate an energy (thermal) efficiency of 40–68% electricity [22]. In 2012, the U.S. generated 57.6 billion kW h of
[14]. The energy efficiency of a furnace is defined as: electricity from wood chips, accounting for 1.42% of the total
thermal energy available in the flue gas produced electricity [3]. The world biopower generation capacity
δcomb ¼ ð2Þ was 70 GW in 2010 and it is projected to increase to 145 GW by
chemical energy in the supplied fuel
2020 [23].
For specially-designed wood stoves that include an additional Compared with coal, wood chips emit much less sulfur oxides
hot-gas burner on the flame top, the combustion efficiency can (SOx) and nitrogen oxides (NOx) upon combustion [24]. Never-
reach 99% and the net energy efficiency 85%. The USEPA-certified theless, wood chips may lose significantly in dry matter weight
wood stoves and fireplaces have an energy efficiency greater than and energy value during storage [25]. Covered storage of pre-dried
75% and fine particulate (PM2.5) emissions less than 0.6 g (MJ)  1 wood chips is economical for bioenergy recovery [26]. To mini-
(1.4 lb/million Btu) [16]. In developing countries, extension of mize wood chip deterioration during storage, torrefaction of the
energy-efficient firewood stoves (or firewood-saving stoves) has material may be employed. Torrefaction is to heat wood at 240–
significantly reduced the consumption of firewood [17]. 300 1C in the absence of air for partial pyrolysis. The treatment can
Firewood was the predominant fuel of the whole world for reduce wood chips by 20% in dry mass and consequently 10% in
cooking and heating before the 19th century. In the U.S., wood- heating value, but results in a dry, hydrophobic, sterile, and
burning fireplace, heater, and cook stove were standard household stabilized product. Screening to remove finer pieces is also
equipment prior to the early 20th century. Even today, nearly 40% necessary prior to use of wood chips in automatically-fed boilers.
(2.6 billion) of the world population (mainly in the rural areas of As wood ash fuses to a greater extent than coal ash at temperature
developing countries in Asia and sub-Sahara Africa) relies on below 750 1C, boiler slagging and fouling can be a problem when
firewood to satisfy its energy requirement, with an annual con- wood chips are used in co-firing at rates higher than 30%. New
sumption of 1730 million m3 [5,18]. In 2012, India, China, United technologies are currently under investigation to improve the
States, and Japan consumed 308, 185, 40, and 0.8 million m3 of performance and energy efficiency of biopower and bioheat using
firewood, respectively [18]. Over the past thirteen years, the world wood chips [27]. Given the present climate change policies,
consumption of firewood has slightly increased by 3%, while the renewable portfolio standards, and environmental regulations,
share in the total energy consumption has been declining [18]. It global production and utilization of biopower and bioheat from
can be projected that in the future the global consumption of wood chips will increase steadily into the future [24].
firewood will remain relatively constant. For a specific nation or
region, the firewood consumption will decrease with industrial 2.3. Wood pellets
advancement and stove renovation. Development of new technol-
ogies for improving combustion efficiency and energy efficiency of Relative to wood chips, wood pellets are a more processed
firewood furnaces will be the main trend. biofuel product. Pellets are made by grinding wood chunks into
sawdust through a hammer mill and subsequently compressing
2.2. Wood chips the sawdust through 6–8 mm holes in the die of a pelletizer. The
high pressure raises the temperature of wood nuggets to plasticize
Direct combustion of firewood instead of any refined or the inherent lignin that glues the pellet as it cools. The pellets are
processed biofuels is the most efficient method to utilize bioe- 2–3 cm in length. The volumetric energy content of pellets can be
nergy. Firewood, however, is bulky and not applicable to small, increased from 9 to 18 MJ m  3 if the wood feedstock is completely
automated heating systems for controlled fuel value. In compar- torrefied before pelletization [28]. Grasses, crop residues, and
ison, wood chips have the advantages. nutshells can also be processed into pellets.
Wood chips are small pieces (1.3–7.6
1.3–7.6
0.3–0.6 cm3) of Wood pellets have a low moisture content in the range of
wood from cutting tree trunks and branches with a wood chipper. 5–10% and a high packing density at around 650 kg m  3. High
The material is commonly used for landscape mulching, play- quality pellets are mechanically durable (o2.5% broken into finer
ground surfacing, and wood pulp producing. Since the beginning particles after each handling), have a light brown color, and
of the 21st century, wood chips have been increasingly used for possess less than 0.7% mineral ash and o1% fine powders [29].
heating (bioheat) and electricity generation (biopower) [19]. Wood The smooth, cylindrical shape and the small size of wood pellets
chips, however, are not used in rural areas of developing countries, allow the biofuel to be fed automatically at fine calibration. The
as wood chippers are not available (affordable) to the residents. In grain-like geometry, high density, and low moisture content of the
the U.S., wood chips are actually less expensive and more available material further permit compact storage, bunker transfer, and
than firewood in that all parts of a tree can be efficiently processed long-distance transport. Wood pellets, however, are more costly
into wood chips with an automated wood chipper and be than wood chips and therefore, their use is limited primarily in
conveniently transported, handled, and stored. To reduce the residential heating of developed countries. In countries like China,
carbon footprint, many universities in the U.S. are shifting to Japan, Germany, U.K., and Netherlands, wood pellets are also used for
automatically-fed wood chip boilers for heat, hot water, and biopower generation. The current U.S. market price of wood pellets is
electricity needs. For example, the woodchip boiler installed in roughly $5 per bag (18 kg or 40 lb) or $250 per ton. Automatically-fed
Colgate University (Hamilton, NY) consumes 20,000 t of locally- pellet stoves are used to burn wood pellets for space heating. The
produced wood chips per year, providing more than 75% of the energy efficiency of such a stove ranges from 70% to 83% [30]. The
heat and hot water that the campus demands [20]. The appliance has a fuel hopper to contain 16–60 kg pellets that are
0.5 MBtu h  1 wood chip boiler at Cayuga Nature Center, NY adequate for one day operation. An auger feeder conveys a few
 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725 715

pellets at a time into the combustion chamber for burning. More fuel to power petrol engine automobiles. One barrel (42 gal)
advanced models are even equipped with a small computer and crude oil can be refined to produce 19 gal of gasoline [37]. Today
thermostat to govern the pellet feed rate. there are over 600 million passenger cars traveling the streets
The global production of wood pellets was 19.1 million tons in and roads of the world, daily consuming 930 million gallons of
2012. The production is projected to increase to 30.2 million tons gasoline. The U.S. daily consumption of gasoline was 366 million
in 2015 and further to 45.2 million tons in 2020 [31]. The present gallons in 2012 [37]. The world’s passenger transportation relies
U.S. wood pelletization capacity is estimated at 6 million tons per on crude oil for adequate supply of gasoline. The heavy depen-
year. In 2012, the U.S produced 4.1 million tons of wood pellets dence on crude oil, however, has generated tremendous envir-
and increased the consumption to 2.8 million tons (from 1.8 mil- onmental and socioeconomic impacts. One example is the 1970s
lion tons in 2008) [18]. The European countries consumed 13 Energy Crisis, during which the economies of major industrial
million tons of wood pellets in 2012, of which 1.3 million tons countries including United States, Canada, Western Europe,
were imported from the U.S., making U.S. the largest wood pellets Japan, and Australia were severely affected by substantial short-
exporter of the year [32]. The rapid expansion of the U.S. wood age and soaring price of crude oil due to OPEC oil export
pellet production has brought severe environmental concerns to embargo (in 1973) and Iranian Revolution (in 1979) as well as
the nation, as more forests and wetlands are being cleared. the growing citizens’ environmental concerns [38]. Further-
more, the supply of crude oil will only be able to last for another
2.4. Charcoal 45 years at the current consumption rate [6]. It is urgent to
develop renewable sources of gasoline substitute that fits the
Compared with fossil fuels, wood fuels (firewood, wood chips, existing liquid fuel supply systems.
and wood pellets) are generally lower in energy content but higher Biomass is currently the only renewable feedstock material to
in combustion emissions. Furthermore, the typical temperature of produce liquid fuel. One commercially-practiced technology is to
wood fire is below 850 1C, unable to melt many metals [33]. To produce ethanol (alcohol) by fermenting plant biomass-derived
overcome these drawbacks, human ancestors developed pyrolysis simple sugars (i.e., glucose, fructose, and other monosaccharides).
techniques to transform wood into charcoal, a carbon-enriched, Ethanol can be used as a gasoline substitute to power petrol
porous, greyish black solid. Heating wood materials in a kiln or engines. The fermentation technology dated back to 4000 B.C., by
retort at around 400 1C in the absence air until no visible volatiles which human beings make alcohol as a drink from berries, grapes,
are emitted yields high quality charcoal. The pyrolysis reaction can honey, and cereals. As a matter of fact, ethanol was tested as an
be illustrated by the equation [34]: engine fuel far before the commercial production of gasoline in
1913. Early in 1826, the American inventor Samuel Morey designed
C6H10O5-3.75CH0.60O0.13 (charcoal) þ2.88H2Oþ 0.5CO2 þ 0.25CO an internal combustion engine that was fueled by ethanol and
þC1.5H1.25O0.38(tar) (3) turpentine to run a boat at 7 to 8 mph (miles per hour). In 1860,
The yield of charcoal is approximately 35% of the original wood the German engineer Nicolaus August Otto developed another
dry mass. Good quality charcoal has an energy content of 28– internal combustion engine that ran on an ethanol fuel blend [39].
33 MJ kg  1, higher than that of coal. It burns without flame and Later the American industrialist Henry Ford constructed tractors
smoke, giving a temperature as high as 2700 1C [34]. As such, that could be powered by ethanol. The obstacle that prevented
wood charcoal is commonly made into briquettes for barbecuing. ethanol from being used as an engine fuel in the U.S. was the
To prepare barbecue charcoal briquettes, small amounts of anthracite alcohol tax enacted in the 1860s to fund the Civil War [40]. Even
coal, mineral charcoal, starch, sodium nitrate, limestone, and borax, when gasoline began to prevail in the late 1910s, many scientists
and sawdust are generally added to sawdust-derived charcoal. The advocated the competence and sustainability of ethanol in the fuel
additives act as binders, improve ignition, promote steady burning, industry. In 1917, Alexander Graham Bell highlighted the abun-
and make manufacturing more efficient [35]. dance of potential feedstocks for the production of ethanol: “any
In the “Bronze Age” back to 3000 B.C., human beings started to vegetable matter capable of fermentation, crop residues, grasses,
use charcoal in metallurgy to smelt ores for copper and iron. farm waste, and city garbage.” An article in a 1918 issue of Scientific
Charcoal was the designated governmental fuel for cooking and American commended the effectiveness of a fuel blend of 25%
heating in China’s Tang Dynasty in 700 A.D. During 1931–1960 in gasoline, 25% benzole, and 50% alcohol, proposing it as a solution
China and the World War II in Europe when gasoline was scarce, to the oil reserve-diminishing problem [39]. A revival of interest in
many automobiles were powered by wood gas, a mixture of ethanol as a fuel was brought forth by the 1973 oil crisis. Substantial
carbon monoxide and hydrogen generated by partially burning research efforts were made to develop biotechnical conversion of
charcoal in a gasifier [36]. Today charcoal is still a valuable product plant biomass to ethanol. These attempts, however, did not finalize
used for cooking, barbecuing, heating, air and water purification, the development of cost-effective, industry-applicable cellulose
art drawing, and steel-making. enzymes that would effectively hydrolyze plant cellulose to simple
The global wood charcoal production amounted 51 million tons in sugars for ethanol fermentation [41]. The first pilot bioethanol plant
2012, increasing by 5% from the 2008 figure. Around 31 million tons with a distillation column was established at South Dakota University
were produced in African countries. In 2012, Brazil, India, China, U.S., (Brookings, SD) in 1979 [39].
and Russia produced 7.6, 2.9, 1.7, 0.85, and 0.053 million tons of Bioethanol is ethanol produced from vegetative biomass
charcoal, respectively [18]. Predicting from the past-five year trend, the through fermentation, in which the following biochemical reac-
world annual production and utilization of wood charcoal will remain tions are involved:
relatively constant at around 50 million tons. (C6H10O5)n (starch, cellulose, sugar) þ nH2O-nC6H12O6 (glucose,
fructose) (4)
3. Development and utilization of liquid biofuels (C5H8O4)n (hemicellulose) þ nH2O-nC5H10O5 (xylose, mannose,
arabinose, etc.) (5)
3.1. Bioethanol
C6H12O6-2CH3CH2OH (ethanol) þ2CO2 (6)
Gasoline, a volatile liquid of C4–C12 hydrocarbon mixture
produced by cracking crude oil (petroleum), is the predominant C5H10O5-5CH3CH2OH (ethanol) þ5CO2 (7)
716 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725

Theoretically all plant materials can be used to generate organic refuses can be the feedstock for bioethanol. Dedicated
bioethanol, as cellulose, hemicellulose, and lignin are the three biomass crops such as switchgrass, miscanthus, giant reed, energy
major construction compounds interwoven with one another to cane, napier grass, grain sorghum, shrub willow, and hybrid poplar
form cell walls of all plants (Fig. 1). Nevertheless, it is fairly can also be grown on marginal land for the purpose [45].
challenging to “unbraid” these three compound fibrils and depo- Lignocellulosic bioethanol production involves three categories
lymerize cellulose and hemicellulose into simple sugars (Eqs. of costs: the costs of feedstock, the costs of sugar preparation,
(4) and (5)). Research has been intensively conducted to develop and the costs of ethanol production. Among these three categories,
effective “pretreatment” methods for obtaining simple sugars from conversion of cellulosic components into fermentable sugars is the
lignocellulosic materials. major technological and economical bottleneck. Research has been
Commercial bioethanol is currently produced from starch/ focusing on development of cost-effective techniques for extract-
sugar-based crops including sugar cane, sugar beet, sweet ing simple sugars from lignocellulosic biomass. Two feedstock
sorghum, corn, wheat, barley, potato, yam, and cassava. The treatment technologies have been proposed: acid hydrolysis and
global commercial bioethanol production was 4.0 billion gallons enzymatic hydrolysis [46]. Acid hydrolysis is to treat pulverized
in 1990. It increased slightly to 4.5 billion gallons in 2000 but plant mass with an acid solution (e.g., sulfuric acid) to help sugar
rapidly to 23.3 billion gallons in 2010. In 2013 the figure was release. If a dilute acid (e.g., 1–10% H2SO4) is used, a high
23.4 billion gallons, with 56.8%, 26.7%, 5.9%, 3.0%, and 2.1% of the temperature (e.g., 237 1C) and high pressure (e.g., 13 atm) envir-
production in the U.S., Brazil, Europe, China, and Canada, onment is necessary. The treatment is rapid (e.g., less than 15 s)
respectively [42]. During 2010–2013, the annual U.S. bioethanol and can be in a continuous flow, but the sugar yield is around 50%
consumption amounted roughly 13 billion gallons. In fact, the U. (of the cellulose-based figure). The low sugar recovery is due to
S. invested 42% of its harvested corn grains (114 million tons/yr) rapid degradation of the produced sugars to furfural and other
in bioethanol production [43], attempting to replace 10% of its chemicals under high temperature and pressure conditions (Fig. 1).
gasoline demand with bioethanol. Sugar cane is the predomi- Considering that 5-carbon sugars (pentoses) are degraded more
nant feedstock for bioethanol in Brazil. The European countries rapidly than 6-carbon sugars (hexoses), a two-stage acid hydro-
use primarily wheat and sugar beet to produce bioethanol. In lysis process can be employed to decrease sugar degradation:
China, the principal bioethanol feedstocks are corn, wheat, and plant biomass is initially subject to mild conditions (e.g., 135 1C) to
cassava, while in Canada, they are corn and wheat. recover pentoses and then experiences harsher conditions to
The typical procedure for manufacturing bioethanol from food release hexoses [47]. The ethanol yield per dry ton wood by this
crops is outlined in Fig. 2. It involves a number of steps such as
milling, liquefication, saccharification, fermentation, distillation,
drying, and denaturing. Given corn as the feedstock, corn kernels
are ground to 3–4 mm flour, wetted with water into slurry, and
“cooked” (heated by steam at 110–120 1C for 2 h) into corn mash.
The enzymes α-amylase and glucoamylase are added during the
following liquefication (80–90 1C) and saccharification (30 1C)
stages, respectively. The saccharified corn mash is then fermented
at 32 1C for 50 h with addition of yeast, yielding 8–12% ethanol by
weight in the slurry. Ethanol is recovered from the slurry by
distillation, de-watered by passing through molecular sieves, and
denatured by mixing with 2–5% gasoline [44]. Typically, 2.5–
2.9 gal of bioethanol can be generated from 1 bushel (25.4 kg)
corn grains.
Bioethanol made from food crops is viewed as “first-genera-
tion” biofuel, which competes with animal feed and human food
for the source materials. To minimize the adverse impacts,
manufacturing “second-generation” bioethanol from non-food
lignocellulosic plant materials has been explored. Indeed, ligno-
cellulosic materials are widely available: forest slashes, crop Fig. 2. The technical flow chart of bioethanol production from corn in the U.S.
residues, yard trimmings, food processing waste, and municipal (Source: Renewable Fuels Association)

Fig. 1. Composition of lignocellulosic materials and their potential hydrolysis products and further degradation compounds (chemicals behind the dashed line) [102].
 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725 717

method is approximately 55 gal [46]. If a concentrated acid (e.g., bioethanol from lignocellulosic materials has not started in the
30–40% H2SO4) is used, lignocellulosic materials are initially nation, largely due to the low profitability with the current
treated with a dilute acid under mild conditions (e.g., 100 1C) for conversion technology and feedstock supply system. The Energy
2–6 h to recover pentoses. The solid residue is then rinsed by Independence and Security Act of 2007 regulates that the U.S.
water, press-drained, and soaked in the concentrated acid solution reduces its gasoline consumption by 20% within 10 years and
at 100–150 1C for 2–4 h. The reactor contents are then filtered to increases biofuel addition to gasoline from 4.7 billion gallons in
remove lignin and may be recycled to the dilute acid hydrolysis 2007 to 36 billion gallons in 2022, with 21 billion gallons of
reactor to reuse the acid. The overall sugar recovery by this biofuels (bioethanol 18.15 billion gallons) from non-corn pro-
method can reach by 80%. The process is slow, however, and the ducts. Accordingly, commercial production of bioethanol from
acid is difficult to separate from the sugar solution. Without acid lignocellulosic feedstocks should start in 2010, with targets of
recovery, lime must be used to neutralize the sugar solution prior 0.5 billion gallons in 2012 and 1.7 billion gallons in 2013. In fact,
to fermentation, creating additional costs and calcium sulfate the nation produced merely 20,000 gal of cellulosic ethanol in
waste. By this method 60–70 gal of ethanol can be produced per 2012 and 218,000 gal in 2013 through pilot plant operations
dry ton of corn stover [46]. In enzymatic hydrolysis, active [51]. Worldwide, the first commercial-scale cellulosic ethanol
enzymes are added to decompose lignocellulosic materials into plant (The Crescentino Bio-refinery, Crescentino, Vercelli, Italy)
simple sugars. As cellulose in plant materials exists in well- entered into full operation on October 9, 2013. The plant is
organized crystalline structure and is protected by hemicellulose running by the Italian company Beta Renewables to annually
and lignin layers, it is highly resistant to enzymatic attack. produce 20 million gallons of ethanol from wheat straw and
Pretreatment, such as freezing, radiation, steam explosion, or giant reed using the patented “Proesa” technology in which
autohydrolytic hydrothermal deconstruction, is necessary to initi- plant biomass is pre-treated with steam (high temperature and
ally degrade hemicellulose and solubilize lignin [48]. Glycoside pressure), followed by enzymatic hydrolysis. Beta Renewables is
hydrolase and carbohydrate esterase enzymes can readily liberate going to build another 80-million-gallon cellulosic ethanol plant
fermentable sugars from pre-treated lignocellulosic substrate. in Fuyang, Anhui, China. The first U.S. commercial cellulosic
Research has been intensively conducted to complement the ethanol plant (Indian River BioEnergy Center, Vero Beach, FL.
commercial preparation of these cellulases. It was discovered that Owned by the Swiss firm Ineos Bio) with the designed 8 million
the fungus Trichoderma reesei could produce cellobiohydrolases gallon annual capacity is at the testing stage to produce
(exo- β-1,4-glucanases) and Cx-cellulases (endo-β-1,4-gluca- bioethanol from vegetative residues using gasification and
nases) that are effective in depolymerizing cellulose. This anaerobic fermentation. Official operation is expected in 2014
fungus contains 200 glycoside hydrolase genes that control the [52]. A few more commercial scale cellulosic ethanol plants are
production of at least 10 different enzymes necessary for under construction in the U.S., including Abengoa Cellulosic
complete hydrolysis of lignocelluloses. The protein structure of Ethanol Biorefinery (Hugoton, KS. 25 Mgal/yr), Bluefire Biore-
these enzymes was well studied. Substantial efforts have been finery (Fulton, MS. 18 Mgal/yr), and Poet Liberty (Emmetsburg,
carried out to improve the major proteins in the enzyme IA. 25 Mgal/yr). It is clear that commercial production of
preparations [41]. Thermostable cellulases that retain high cellulosic ethanol will speedily expand in the near future,
cellulose-degrading activity at 4 70 1C were also identified from boosting the global production and utilization of bioethanol as
several mesophilic and thermophilic fungal strains Talaromyces a gasoline alternative.
emersonii, Thermoascus aurantiacus, Chaetomium thermophilum,
Myceliophthora thermophila and Tribulus terrestris. Application
of these enzymes in cellulosic bioethanol production is under
experiment. In general, the costs for preparing celluloytic 3.2. Biodiesel
enzymes have been significantly reduced during the past dec-
ades, yet it is still the major obstacle for commercial-scale Diesel is a C8–C25 hydrocarbon liquid fuel produced from
enzymatic hydrolysis processes. In the meanwhile, new yeast petroleum by fractional distillation at 200–350 1C. One barrel
strains for more effective ethanol fermentation have been (42 gal) of crude oil can generate 10 gal of diesel [37]. The fuel
developed [49]. Using advanced technologies in biomass pre- has an energy content of 38.3 MJ L  1, higher than that of gasoline
treatment and acid/enzymatic hydrolysis, pilot cellulosic etha- (34.7 MJ L  1) [53]. Diesel is an essential fuel for diesel engines and
nol plants operated by a number of companies (e.g., Abengoa an important fuel for home heating in developed countries. Most
Bioenergy, Salamanca, Spain; Iogen Energy, Ottawa, Canada; and heavy-duty vehicles and machines are powered by diesel engines,
Poet, LLC., Sioux Falls, SD, USA) have achieved the ethanol such as tractors, trucks, construction machineries, mining equip-
productivity of 68–83 gal t  1 biomass. ment, and military carriers. Due to different ignition mechanisms,
Thermochemical transformation was also studied for produ- diesel engines cannot use gasoline as the fuel, and versus versa.
cing bioethanol from lignocellulosic materials. Plant biomass is The average daily consumption of distillate fuel oil (diesel and fuel
first gasified to produce syngas (a mixture of H2 and CO). The oils) in the U.S. was 107 million gallons in 2012. In the same year,
syngas is then introduced into a specially-designed fermenter the world daily production of distillate fuel oil amounted 1057
where particular microorganisms utilize the syngas for energy million gallons [3]. Given the world’s dwindling petroleum
and generate ethanol. Alternatively, the syngas is passed reserves, renewable fuel sources alternative to petrol diesel are
through a reactor and is transformed to ethanol in the presence warranted.
of chemical catalysts (3CO þ3H2-CH3CH2OH þCO2). Ethanol Biodiesel, a yellowish liquid derived from vegetable oil,
yields up to 50% of the syngas mass have been obtained using animal fats, algal lipids, or waste grease through “transester-
the thermochemical transformation methods [46]. Commercial ification” in the presence of alcohol and alkaline catalyst (Eq.
scale plants to produce ethanol from syngas have not entered (8)), has been commercially produced and used a petrol diesel
into practice, likely due to the high technological complexity substitute. In chemical structure, biodiesel is mono-alkyl esters
and low economic viability. of fatty acids. Though specific properties vary with the feedstock
Gasoline blended with 10 volume% bioethanol (E10) is now lipids, biodiesel demonstrates a specific gravity 0.873–0.884,
ubiquitous on the U.S. fuel market [50]. The bioethanol, how- kinematic viscosity 3.8–4.8 mm s  2, cetane number 50–62,
ever, is predominantly from corn. Commercial production of cloud point  4–14 1C, and flash point 110–190 1C. Its energy
718 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725

density (high heating value) is 38–45 MJ kg  1, approximately 3363 kg ha  1 yield of seeds with 20% oil content. For canola, the
90% of that of petrol diesel [54]. oil productivity is around 350 gal ha  1. In recent years, selected

(8)

Here R1, R2, and R3 are different or the same aliphatic hydro- algae species were also studied for cultivating as a biodiesel
carbon groups. feedstock. The strains with high oil contents (e.g., 420%) and
Vegetable oil and animal fats were widely used in oil lamps for satisfactory biomass potential (e.g., 4 20 dry t ha  1 yr  1) include
lighting before the invention of gas lights and electric lights in late Chaetoceros calcitrans, Skeletonema costatum, Phaeodactylum tricor-
18th century, but for making biodiesel it did not occur until 1930s nutum, Chlamydomonas reinhardtii, Calluna vulgaris, Dunaliella
[55]. When the German engineer Rudolf Diesel invented the salina, Dunaliella teriolecta, Scenedesmus obliquus, and Neochloris
compression-ignited diesel engine in 1893, he envisioned that oleabundans. Critical challenges still exist in cost-effective cultiva-
pure vegetable oil could power the machine for agriculture in tion of algae, in energy-efficient harvest of algal biomass, and in
remote areas of the world. He carried out extensive tests on effectual algal oil extraction.
various vegetable oil fuels, believing that farmers could benefit The common technical procedure of biodiesel production is
from producing their own fuel. At the 1900 World’s Fair in Paris, presented in Fig. 3. Moisture-free vegetable oil is pre-heated to
the French Otto Company demonstrated a diesel engine that was 50–60 1C. A “methoxide” mixture of methanol (20% volume of the
fueled by peanut oil. The attention for developing vegetable oil- oil) and sodium hydroxide (5 g L  1 oil) is added to the oil in a
based fuels, however, was quenched by the later widespread closed reactor to start transesterification (Eq. (8)). If yellow grease
availability and low price of petrol diesel. Despite this, interest in is the feedstock, the material has to be pre-treated to remove any
vegetable oil to power internal combustion engines still existed in water and particulates, and undergo “acid-catalyzed esterification”
a number of nations. Experiments in Belgium, Germany, Italy, with methanol to eliminate free fatty acids prior to the base-
France, Japan, China, Argentina, and other countries recognized catalyzed transesterification (Fig. 3). After 2 h of transesterification
the high viscosity of vegetable oils as the key problem when used at 50–60 1C under mechanical stirring, the mixture is allowed to
to fuel diesel engines. Various methods were tested to overcome settle at room temperature for 2–12 h. The bottom crude glycerin
this drawback, such as pre-heating the vegetable oil, blending it layer is then separated from the top crude biodiesel layer by
with petrol diesel or ethanol, and thermochemically cracking the funnel drain. The crude biodiesel contains small amounts of
oil. In 1937, the Belgian scientist George Chavanne patented the methanol, soap, and mono/di/triglycerides. These impurities need
“Procedure for the transformation of vegetable oils for their uses to be removed prior to fuel use of biodiesel. Purification is
as fuels,” presenting the transesterification (alcholysis) method for generally achieved by water washing and drying or by membrane
breaking down triglyceride molecules (oil and fat) by replacing the refining [58]. The biodiesel yield from the transesterification
glycerin moiety with ethanol or methanol (Eq. (8)). This is the reaction is typically the same volume as the feedstock oil. The
earliest description of biodiesel generation [56]. In 1977, the reaction conditions and purification operations, however, greatly
Brazilian scientist Expedito Parente developed the first industrial
process for the production of biodiesel. In 1989, the world’s first
industrial-scale biodiesel plant was operated in Asperhofen, Aus-
tria to produce biodiesel from rapeseed. The U.S. started its
commercial operation (Pacific Biodiesel, Maui, Hawaii) in 1996 to
process waste grease into biodiesel. The historically high petro-
leum prices after 2001 and the increased awareness of energy
security promoted biodiesel to a popular fuel on the global fuel
market. In 2013, the world biodiesel production reached 6289
million gallons [57].
Vegetable oils, algal lipids, animal fats (beef tallow, pork lard,
chicken fat, etc.), and yellow grease (used vegetable oil) can all be
used as feedstock to produce biodiesel. Common oil seed plants
include camelina, canola, castor bean, coconut, jatropha, palm,
peanut, rapeseed, soybean, sunflower, and tung. The oil produc-
tivity of soybean is approximately 194 gal ha  1 given the typically Fig. 3. The technical flow chart of biodiesel production from (used) vegetable oil
 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725 719

influence the yield and quality of the final biodiesel products. In Fast pyrolysis is to rapidly raise the temperature of dried and
the U.S., the quality of biodiesel is regulated by the ASTM D6751 ground plant biomass (o2 mm) to approximately 500 1C within
(Standard Specification for Biodiesel Fuel Blend Stock (B100) for 2 s in a continuous flow system in the absence of oxygen. Typical
Middle Distillate Fuels). In European countries, it is by the EN- yields of biochar, bio-oil, and syngas in fast pyrolysis are 10-30%,
14214 (Automotive fuels – Fatty acid methyl esters (FAME) for 50–70%, and 15–20% of feed biomass, respectively [61]. Most
diesel engines – Requirements and test methods). organic residues can be used to produce pyrolysis bio-oil, such
Over the past decade, signification research progress has been as wood, switchgrass, crop straw, sawdust, sugar cane bagasse,
achieved to optimize the biodiesel production procedure (transes- peanut hulls, and poultry litter. However, materials low in nitrogen
terification), to determine the feedstock oil characteristics that and ash contents are preferred; wood is the common feedstock for
control the biodiesel quality, to identify plant and algal species quality bio-oil [59]. By manipulating the temperature, duration,
that have high oil yield potential, and to develop value-added and oxygen availability of pyrolysis, it is possible to optimize for
utilization of the crude glycerin. By reviewing the published one or more of the three products. Up to 75% of the feedstock
literature, Hoekman et al. [54] concluded general guidelines of biomass (and its energy) can be converted to bio-oil by optimized
feedstock selection for producing biodiesel that meets the two fast pyrolysis [62].
critical fuel quality desires “(1) low temperature performance and Crude pyrolysis bio-oil contains significant contents of colloidal
(2) oxidative stability. For good low temperature performance, char particles and water and differs significantly from petrol
biodiesel should have low concentrations of long-chain saturated distillate fuels in physical, chemical, and energy properties
FAME. For good oxidative stability, biodiesel should have high (Table 1). More than 300 compounds were identified in pyrolysis
concentrations of saturated and monounsaturated FAME, but low bio-oil as acids, alcohols, aldehydes, esters, ketones, sugars, phe-
concentrations of multi-unsaturated FAME.” nols, sugars, furans, guaiacols, syringols, alkenes, aromatics and
The world production of biodiesel has increased steadily from nitrogen compounds [59,63]. Due to its high moisture content and
213 million gallons in 2000 to 6289 million gallons in 2013 [57]. acid content, crude pyrolysis bio-oil is instable, corrosive, viscous,
The top five biodiesel production countries are the European low in energy density, immiscible with hydrocarbon fuels, and
Union (mainly Germany, France, Spain, Italy, and Poland), Brazil, difficult to ignite [64,65]. Crude bio-oil can be directly burned
Argentina, the U.S., and China, using oils from soybean, rapeseed, using standard atomization techniques in industrial scale combus-
canola seed, sunflower seed, castor bean, animal fats, and yellow tion systems, provided that the burner is set up to accommodate
grease. The U.S. produced 1339 million gallons of biodiesel in 2013, the unique characteristics of the liquid [59]. Nevertheless, sig-
solely from soybean oil [58]. Currently B20 (20% biodiesel, 80% nificant upgrading is needed before the liquid can be utilized as a
petroleum diesel) is the most common biodiesel blend in the U.S. petrol distillate fuel alternative. The upgrading is to reduce the
With the world’s growing demand for biofuels, global production moisture content and acidity of bio-oil and improve its heating
and consumption of biodiesel will maintain consistent increase. value and storage stability. Over the past decades, a number of
The advantages of biodiesel also include a cleaner emission profile, upgrading techniques have been investigated, including hydro-
production simplicity, ease of use, and cost-competitiveness. The genation, hydrodeoxygenation, esterification, catalytic cracking,
promising future of biodiesel, however, also lies in the world’s molecular distillation, supercritical fluidization, emulsification,
ability to economically produce renewable feedstocks such as steam reforming, and blending [66,67]. Moderately upgraded
vegetable oils and algal lipids in sustainable manners, without bio-oil has been applied to substitute for heavy fuel oils (e.g.,
supplanting land for food production. diesel and No. 2 heating oil) to power static appliances including
boilers, furnaces, engines, and electric generators. Economically
3.3. Pyrolysis bio-oil feasible, industry-applicable techniques for upgrading bio-oil into
transportation fuels, however, are still under development. Pyr-
Fossil fuels were formed from remains of plants and animals olysis bio-oil also serves as a potential feedstock for valuable
millions of years ago under the work of geothermal heat and chemicals, preservatives, lubricants, binders, paints, thickeners,
pressure. Attempting to produce similar fuels, human beings stabilizers, etc. Yet with the present technologies, recovery of pure
treated vegetative biomass in a simulated high temperature, high compounds from bio-oil is not economically unattractive [59].
pressure, and oxygen-free environment. Yet the products were Globally, a number of pilot plants have been operating at up to
little like fossil fuels. 200 kg h  1 feedstock processing capacity to convert woody bio-
Pyrolysis is to heat plant biomass at 300–900 1C in the absence mass into bio-oil. These plants include DynaMotive, PyroVac Inc.,
of air. The technique was used in ancient China to prepare charcoal Canada; ENEL, Italy; Ensyn, USA; Bioware Technologia, Brazil; BTG,
and by indigenous Amazonians to generate biochar three to five
thousand years ago. Pyrolysis of plant materials results in three Table 1
products: biochar (the black, solid residue), bio-oil (the brown Properties of wood-derived pyrolysis bio-oil as compared with petroleum distillate
vapor condensate), and syngas (the uncondensable vapor) (Fig. 4). fuels [64].
There are generally two types of pyrolysis techniques: slow
pyrolysis and fast pyrolysis, referring to the heating rate [59]. Properties Pyrolysis bio-oil Petroleum distillate fuel

Slow pyrolysis is to heat organic residues in batch reactors at 300– Water content (wt%) 15–30 0.1
600 1C in the absence of air for a number of hours or days. The pH 2–3 –
typical yields of biochar, bio-oil, and syngas in slow pyrolysis are Density (kg m  3) 1.2 0.94
35%, 30%, and 35% of the dry feedstock biomass, respectively [60]. Elemental composition (wt%)
C 54–58 85
H 5.5–7.0 11
O 35–40 1.0
N 0–0.2 0.3
Ash 0–0.2 0.1
High heating value (MJ kg  1) 16–19 40
Viscosity (50 1C) (cSt) 33–83 190
Solid particulates (wt%) 0.2–1 1
Distillation residue (wt%) Up to 50 1
Fig. 4. Pyrolysis of plant biomass to generate char, bio-oil, and syngas.
720 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725

Ecosun, The Netherlands; PYTEC, PKA, Chemviron Carbon, Ger- and animals that experienced great heat and pressure over
many; DynaMotive, Compact Power, UK; Lambiotte, Belgium; thousands of years. The energy content of natural gas is
Novasen, Senegal; and BEST Energies, Australia [59]. The resulting 38.2 MJ m  3 (1027 Btu ft  3) or 53.5 MJ kg  1 (The density of
bio-oil is used as heating fuels and as industrial feedstock natural gas is 0.717 kg m  3 at standard temperature (0 1C) and
materials. Driven by the increasing interest in renewable fuels, pressure (1 atm)). It was in 1821 that natural gas was initially
pyrolysis bio-oil has a promising future. Nevertheless, commercial exploited at commercial scale in Fredonia, NY for lighting [77].
production and utilization of pyrolysis bio-oil as a petrol fuel Today natural gas is extensively used for cooking, heating, trans-
alternative is still facing major technological challenges [68]. portation, electricity generation, and industrial manufacturing. In
2011 the global natural gas production reached 4096 billion N m3
3.4. Drop-in biofuels (N: standard conditions with 0 1C temperature and 1 atm pres-
sure), with the U.S. as the largest producer accounting for nearly
On the volume basis, bioethanol has an energy density 67% of 20% of the production [78].
that of gasoline, while biodiesel 90% of that of petrol diesel. More Biogas is a renewable gaseous fuel alternative to natural gas. It
important, bioethanol and biodiesel demonstrate higher oxygen is generated by anaerobic digestion of organic wastes. Raw biogas
content and greater dissolution capability than petrol fuels and consists of 60–65% of methane (CH4), 30–35% of CO2, and small
therefore, are more corrosive to engines and fuel storage and percentages of water vapor, H2, and H2S [79]. After purification to
distribution equipment [69]. At blending rates above 20%, biofuel remove CO2, H2S, and other impurities, the upgraded, pipeline-
blends can damage elastomer and metallic parts of engines, quality biogas (now named biomethane) is used as a natural gas
storage tanks, and dispensers, threatening the infrastructure substitute.
compatibility of the currently existing fuel supply system. Organic wastes like plant residues are gradually decomposed
Drop-in biofuels are biomass-derived liquid hydrocarbons that by microorganisms to smaller molecules in the natural environ-
meet the existing petrol distillate fuel specifications and be ready ment. Under conditions with sufficient air supply, the final
to “drop in” to the existing fuel supply and use infrastructure. decomposition products are generally CO2 and H2O. In circum-
Presently at the research and development stage, drop-in biofuels stances like landfill sites and manure lagoons where little oxygen
have the features to minimize the infrastructure and engine is available, an anaerobic environment forms, and certain micro-
compatibility issue. Drop-in biofuel candidates include butanol, organisms break down organic residues to CH4 and CO2. Microbial
liquefied biomass, sugar hydrocarbons, syngas complexes, and decomposition of biomass materials in the absence of oxygen is
others [70]. These biofuels follow different production pathways, termed anaerobic digestion. Four basic biological steps are involved
such as fermentation of lignocellulosic sugars, catalysis of ligno- in anaerobic digestion [79]:
cellulosic sugars, hydropyrolysis of biomass, hydrothermal bio-
mass liquefaction, bioethanol transformation, and syngas (1) Hydrolysis. Anaerobic bacteria such as Bacteriocides, Clostridia,
upgrading. Butanol can be produced by the acetone–butanol– Bifidobacteria, Streptococci, and Enterobacteriaceae hydrolyze
ethanol (ABE) fermentation process using the solventogenic clos- large organic molecules into smaller, simple ones, e.g., carbo-
tridial bacteria strains (e.g., Clostridium acetobutylicum EA2018 and hydrates to sugars, proteins to amino acids, and lipids to
Clostridium beijerinckii BA101) that convert biomass-derived fatty acids.
sugars to ABE [71]. Commercialization of this technology is in (2) Acidogenesis. Acidogenic bacteria convert simple organic mole-
progress, where a bioethanol plant can be readily modified to cules to carbon dioxide, hydrogen, ammonia, and organic acids.
produce biobutanol. Lignocellulosic sugars can be transformed (3) Acetogenesis. Acetogenic bacteria convert organic acids to
into petrol fuel-like hydrocarbon fuels via catalysts-assisted dehy- acetic acid, along with additional hydrogen, ammonia, and carbon
drogenation, deoxygenation, hydrogenolysis, and cyclization reac- dioxide.
tions [72]. In the presence of hydrogen gas, pyrolysis of algal (4) Methanogenesis. Methanogenic bacteria decompose acetic
biomass at 300–400 1C results in predominantly (i.e., 85%) hydro- acid to methane and carbon dioxide.
pyrolysis oil that can be refined to quality biofuels [73]. If water-
based slurry containing 20% oil-rich algal biomass is hydrother- The overall reaction of anaerobic digestion is as:
mally treated at 300–350 1C under 150–200 atm pressure, an
C6 H12 O6 -3CO2 þ 3CH4 ð9Þ
organic liquid will be generated for further upgrading to drop-in
biofuels [74]. Using particular catalysts such as ruthenium dipho- Anaerobic digestion has been utilized by animal farms, waste-
sphine, ethanol can be efficiently upgraded to n-butanol [75]. water treatment plants, and even individual households to man-
Another emerging biofuel technology is Syngas to Gasoline Plus age waste and/or to produce energy (Fig. 5). Many organic wastes
(STGþ), through which biomass-derived syngas is transformed
into gasoline via a series of catalysts-assisted reactions: COþ
2H2-CH3OH (methanol); 2CH3OH-CH3OCH3 (dimethyl ether) þ
H2O; nCH3OCH3-C6–C10 hydrocarbons (gasoline) [76]. Overall,
production and utilization of drop-in biofuels is a future renewable
fuel development direction. Intensive research is needed to
optimize these technologies for cost-effectiveness and economic
viability.

4. Development and utilization of gaseous biofuels

4.1. Biogas

Natural gas, consisting of 95% methane (CH4) and 5% ethane

(C2H6), propane (C3H8), butane (C4H10), nitrogen (N2), and carbon
dioxide (CO2), is a gaseous fossil fuel formed from buried plants Fig. 5. The structure of a typical anaerobic digester in a rural household in China.
 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725 721

such as livestock manure, food processing waste, kitchen waste, rather slowly due to challenges in capital investment, technical
yard trimmings, sewage, and sludge can be used in anaerobic maintenance, and economic viability. The number of operating
digestion for biogas production. Woody waste should be excluded, farm-biogas plants increased from 25 in 2000 to 176 in 2011, with
as most anaerobic microorganisms cannot degrade lignin [79]. 15 new installations a year at the present trend. Biogas from the
Generally, organic waste in water slurry is placed in an air-tight farm digesters provided sufficient heat to the farms and generated
container called “anaerobic digester” (Fig. 5). After 7–14 days 541 million kW h of electricity in 2011 [89]. In addition, 594 U.S.
biogas will start to generate. Depending on the temperature and municipal solid waste disposal facilities and 1238 municipal
waste types, 30–60 days are generally needed for anaerobic wastewater treatment plants are collecting biogas from the sites,
digestion to be complete such that little biogas is further produced recovering 155 billion MJ of energy for heat and electricity. In
[80]. The black, odorous liquid and solids left in the digester need 2012, approximately 9.1 billion kW h of electricity were generated
to be cleaned out and replaced by fresh organic wastes. Biogas by biogas in the U.S. [88].
potential of a particular waste feedstock can be estimated by the It is estimated that worldwide 47–95 billion kW h of electricity
Buswell Equation [81]: were generated from biogas in 2012 [90]. China is presently the
largest biogas producer, harvesting 6 billion N m3 of biomethane
CcHhOoNnSs þ1/4(4c  h  2o þ3n þ2s)H2O-1/8
per annum. Globally, biogas is still in its infant stage, playing a
(4c þh  2o  3n 2s)CH4 þ1/8(4c  hþ2o þ3nþ 2s)
minor role in the overall bioenergy sector. The global potential for
CO2 þnNH3 þ sH2S (10)
biogas, however, is rather impressive. If majority of the biowaste is
Typically, one ton biowaste (dry weight) would yield 120 m3 of anaerobically digested to produce biogas, the yield would displace
biomethane that can produce net 200 kW h of electricity [82]. one quarter of the current natural gas consumption and cover 6%
of the global primary energy demand. More than 1000 billion
Many factors influence anaerobic digestion and biogas genera- N m3 of biomethane could be harvested annually assuming the
tion. In the digester organic waste should exist in water slurry that available portion of the world agricultural byproducts (manures,
contains 15–40% solids. The C/N ratio of the slurry should be less crop residues) and domestic wastes (municipal organic solid
than 24:1 so microorganisms can obtain enough nitrogen to grow. refuse, food processing waste, and sewage sludge) were processed
The pH of the slurry should be within 6.5–8.0. Anaerobic micro- by anaerobic digestion [91]. With the significant increases in
organisms are active at temperature 25–60 1C (77–140 1F). Tem- public awareness of anaerobic digestion and biogas and the
perature should be maintained at a mesophilic range availability of supporting structure and technology, biomethane
(35–40 1C) or a thermophilic range (50–55 1C) to maximize the is showing a steady growth into the future, particularly in China
microbial activity [82]. In winter time with ambient temperature and Germany.
less than 10 1C or a hot environment at above 70 1C, anaerobic
digestion nearly stops [83]. It also takes time for anaerobic
4.2. Syngas
microorganisms to establish a fully effective population in an
anaerobic digester. A common practice is to introduce anaerobic
Syngas is another gaseous biofuel produced from gasification or
microorganisms by “seeding” digesters with sewage sludge or
pyrolysis of plant materials. Chemically syngas consists of 30–60%
cattle slurry that already contain anaerobic microorganism popu-
CO, 25–30% H2, 5–15% CO2, 0–5% CH4, and lesser portions of water
lations [84]. Modern steel-plastic digesters usually have self-
vapor, H2S, COS, NH3, and others, depending on the feedstock
heating and self-mixing functions, able to maintain high biogas
types and production conditions [92].
productivity during the winter time [79].
Gasification is commercially practiced to produce syngas. In the
Biogas was used by human beings to prepare warm bath water
operation, carbon-rich materials such as coal, petroleum, natural
as early as in the l0th century B.C. in ancient Assyria. Back to more
gas, and dry plant biomass is rapidly heated to above 700 1C in the
than 2000 years ago, people in China and India already practiced
high temperature (e.g., 1200 1C) combustion chamber of a gasifier
anaerobic digestion of animal manure for a flammable gas [85]. In
and partially burned in the presence of controlled air flow to yield
1808, the English chemist Sir Humphry Davy determined that the
syngas (Fig. 6). In the gasifier, wood biomass experiences three
flammable gas from cattle manure ponds was methane. The first-
thermal transformation phases: dehydration, pyrolysis, and partial
recorded anaerobic digestion plant was constructed in Bombay,
oxidation. In the initial dehydration phase, air-dry biomass swiftly
India in 1859. Biogas was collected from a sewage treatment
loses its moisture before its temperature reaches 200 1C. Pyrolysis
facility to light street lamps in Exeter, England in 1895. The U.S.
begins as the temperature increases, and biomass is converted to
scientist A.M. Buswell and other colleagues studied anaerobic
char and vapor. In the presence of O2, char is partially oxidized to
digestion as a science in the 1930s to select best anaerobic bacteria
generate CO and CO2, while the vapor is combusted to CO2 and
and digestion conditions for promoting methane production [86].
H2O. As the hot char particulates, CO, CO2, H2O rise in the
In 1950s, China built 3.5 million family-sized, low-technology
combustion chamber, further reactions occur that char is oxidized
anaerobic digesters in the rural area to provide biogas for cooking
by CO2 to yield CO, or by H2O to yield CO and H2, and CO reacts
and lighting (Fig. 5). In 2012, the total number increased to 45
with H2O to produce CO2 and H2. The mixture of CO, H2, and CO2 is
million, of which roughly 65% are in operation [87]. India has more
then recovered as syngas (Fig. 6). The major reactions can be
than 4.5 million small-scale anaerobic digesters to produce biogas
simplified as follows [92]:
from manures. There is a trend in these two countries toward
using larger, more sophisticated digestion systems with improved Wood-char þvapor (11)
biogas productivity and digester cleansing convenience. Europe
had 8960 biowaste digesters (predominantly farm-based; 171 Char þO2-COþCO2 (12)
industrial-scale ones) in 2012 to convert agricultural, industrial,
Vapor þO2-CO2 þH2O (13)
and municipal wastes to biogas. Germany was the leading country,
operating 6800 biogas plants to meet the nation’s natural gas
Char þCO2-CO (14)
demand and generate 18.2 billion kW h of electricity [88]. The U.S.
started to install manure-based digester systems on livestock Char þH2O-COþH2 (15)
farms to produce biogas in late 1970s, with financial incentives
from the federal government. The sector has been developed COþH2O-CO2 þH2 (16)
722 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725

Fig. 6. The structure of a Mitsubishi Heavy Industries gasifier (left) and a Lurgi Dry-Ash gasifier (right) for syngas [92].

The typical gasification of wood to syngas has a carbon conver- 5. Biofuel overview and the potential socio-econo-
sion rate of 92% (wood to CO, CO2, CH4), a hydrogen conversion environmental impacts
rate of 71% (wood to H2, CH4), and an energy conversion rate of
62% (wood bioenergy to syngas energy). The syngas yield is Global production and utilization of bioenergy is in various
around 1.2 N m3 kg  1 wood (2.3 N m3 kg  1 if with 48% N2) [93]. solid, liquid, and gaseous forms of biofuels. Advantages and
Woody biomass low in nitrogen and ash content is usually used disadvantages of these biofuels are briefed in Table 2. In general,
for syngas production. Herbaceous biomass is generally high in solid biofuels are most available in source materials, most efficient
ash, especially silica and potassium and tends to cause gasifier in feedstock energy recovery, and most effective in conversion
slagging. technology and production cost, but the products are bulky,
inconvenient to handle, low in energy density, and only applicable
Crude syngas contains 48% of N2 and minor amounts of tar, to solid fuel burners. Liquid biofuels are energy-dense, convenient
particulates, CO2, and H2S. Its energy density (lower heating value) to transport, and can substitute for gasoline and petrol diesel;
is approximately 5.3 MJ N m  3 [93]. The fuel can be directly however, they are low in net energy efficiency, have stringent
burned to generate electricity. More commonly the gas is purified requirements for feedstocks, and involve complicated conversion
and used as a source material for synthesizing transportation fuel, technology and high production cost. Gaseous biofuels can be
methanol, ethanol, methane, dimethyl ether, and other products. produced from organic waste materials and residues using well-
Through the Fischer–Tropsch process, purified syngas has been practiced techniques, yet there are fuel upgrading and byproduct
successfully converted to diesel. Hydrogen separated from syngas disposal challenges.
was tested to power hydrogen fuel cells for electricity generation The global biofuel utilization reached 0.55
1020 J yr  1 in 2012,
and fuel cell electric vehicle propulsion [94]. However, the pur- accounting for 10% of the world energy consumption and 80% of
ification can be costly and energy-consuming. Conventional pur- the total renewable energy production [97]. Bioethanol and
ification methods include physical and chemical adsorption (e.g., biodiesel in particular, are being widely produced to complement
water quenching, amine scrubbing) [95]. Considering that the the rapidly depleting petroleum reserves. It is predictable that by
purified gas has to be reheated prior to use in a gas turbine or 2050, bioethanol and biodiesel would be the dominant fuels to
chemical synthesis, approaches that can be integrated in gasifiers power passenger cars and heavy vehicles. Automobiles powered
such as catalytic tar cracking/reforming, CO2 elimination, and H2 by electricity, natural gas, and hydrogen are emerging, but the
separation have been researched [92,96]. currently existing liquid fuel supply systems will restrict them
Due to the stringent requirements for feedstock in moisture from becoming the main stream.
content, ash content, size, homogeneity, bulk density, and energy Intensive production and utilization of biofuels has generated
content, worldwide biomass-based gasification is currently at the great social, economic, and environmental impacts. The socio-
early demonstration stage [94]. Further research and development economic impacts include land rights, food security, energy
is needed to better understand the economics of biomass, improve security, economic viability, local prosperity, labor conditions,
the cost-effectiveness of feedstock preparation, and achieve the and policies. The environmental impacts extend to greenhouse
economic viability of bio-syngas production. In 2010, there were gas emissions, biodiversity, land uses, soil conservation, and water
144 commercial gasification plants operating globally with a total resources. The impacts vary with characteristics of the biofuel
capacity of 71,205 MWth (mega watt, thermal). China, Europe, and supply chains, the production system management, and the
the U.S. had 56, 42, and 17 plants, respectively. However, only 0.5% conditions of production regions [98]. In general, production and
of the production was from biomass; the rest was from coal (51%), utilization of biofuels enhance a nation’s energy security and
petroleum (25%), natural gas (22%), and petcoke (1%) [92]. Of the independence, reduce conventional pollutant and greenhouse
produced syngas, 45% was used to synthesize chemicals, 38% to gas emissions, promote research and development, create job
make transportation fuels, 11% to generate electricity, and 6% as a opportunities, and increase farm income. On the other hand,
gaseous fuel [92]. biomass feedstock production requires land, water, fertilizers,
 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725 723

and other resources and may cause land use change, engender

Pilot production

 25
109 m3
additional pressure on water resources (i.e., pollution and over-

development
o 5
106 gal

4.5
1011 m3
o 1
106 gal
23
109 gal

63
108 gal
17
108 m3
production

3.5
108 t
use), and increase food prices. In certain scenarios, biofuel produc-

20
106 t

51
106 t
Annual

tion may emit more greenhouse gases and consume more fossil

stage
Early

CH4
energy on an energy-equivalent basis [99]. These negative impacts,
how-
ever, can be minimized through careful planning and technological

Low net energy efficiency; corrosive to existing gasoline fueling devices; competing with food and

Competes with food production; feedstock is limited to lipids; corrosive to existing diesel fueling
Bulky, low in energy density; high hazardous emissions from incomplete combustion; unsuitable

Involves chipping cost; tends to decay during storage; bulkier and lower in energy density than

advances.

High production cost; bulk, inconvenient for transport; cannot be used in liquid fuel and gas
Higher processing cost; lower energy content than coal; only be used in solid fuel burners

6. Summary and conclusions

Usually in rural areas; requires intensive feedstock collection and waste disposal
Bioenergy in firewood, charcoal, and plant oils was the pre-
dominant (i.e., 495%) energy source for human activities such as

Upgrading is needed prior to fuel uses; immature upgrading techniques

coal; ash slagging and boiler fouling; unsuitable for precise combustion

cooking, heating, lighting, metallurgy, and clay baking before 1840.

Char and bio-oil as byproducts; stringent requirements for feedstock

It was until 1905 that the fossil energy overweighed bioenergy in
supporting human life. Over the past one hundred years with the
world’s population growth and industrial development, the annual
global primary energy consumption has increased nearly 10 times,
Immature, complicated conversion technology; high cost

reaching 5.5
1020 J in 2010. The current energy demand was met
by fossil fuels at 80%, biofuels at 11.3%, uranium at 5.5%, and other
renewable source energy at 2.2% [100]. In the past two decades
Low net energy efficiency; not cost-effective

from 1991 to 2010, the global consumption of bioenergy increased

steadily from 46.55
1018 J to 62.74
1018 J. Of all the renewable
energy sectors in 2010, bioenergy accounted more than 80% [100].
devices; substantial processing cost

Biofuels have been developed and utilized in three forms: solid

biofuels – firewood, wood chips, wood pellets, and wood charcoal;
liquid biofuels – bioethanol, biodiesel, pyrolysis bio-oil, and drop-
feed for source materials

in transportation fuels; and gaseous biofuels—biogas and syngas.

for automated burners

Direct combustion is the most efficient method to utilize bioe-

nergy. As such, solid biofuels, especially firewood, have consis-
Disadvantages

tently been the top consumed biofuels. Nearly 40% of the world
population relies on firewood for cooking and heating. The global
burners

production of firewood has remained relatively constant at around

1900 million m³ per annum, though regional flocculation with 4–
12% decreases in North America, Asia-Pacific, and Europe and 3–5%
increases in Africa, Latin America, and the Caribbean occurred in
Renewable substitute for gasoline; low combustion emissions; existing
Convenient to transport, handle, and store; low SO2 and NOx emissions;
More convenient to transport, handle, and store than firewood; lower

Mature production technology; as feedstock for industrial chemicals

Renewable substitute for petro diesel; existing feedstock production

Renewable feedstock; gasoline substitute; compatible with existing

the past decades [18]. Wood chips have been increasingly used in
From organic waste and residues, wide feedstock sources; fits the

co-firing for electricity generation since the beginning of the 21st

century. The world electricity generation from wood chips is
Renewable, readily available, cheap, most energy efficient

expected to double from 70 GW in 2010 to 145 GW in 2020. Due

to the handling convenience and lower moisture content, wood
Renewable feedstock; simple conversion technology
SO2 and NOx emissions than coal upon combustion

pellets become more and more popular in residential house

heating in developed countries. The global production of wood
A gasoline alternative from non-food biomass
Advantages, disadvantages, and 2013 production status of different biofuels.

pellets is projected to expand from 15.4 million tons in 2010 to

Stable, high energy content, clean burning

45.2 million tons in 2020. Biomass is also the sole source for renewable
transportation fuels. The global consumption of bioethanol increased
from 4.5 billion gallons in 2000 to 21.8 billion gallons in 2012.
suitable for precise combustion

feedstock production systems

However, the bioethanol was predominantly made from starch/

sugar-rich food crops such as corn and sugar cane. The tremendous
existing natural gas grid

technological advances in converting lignocellulosic biomass into

simple sugars, specifically in feedstock pre-treatment and in industrial
enzyme preparation, eventually realized the commercial-scale produc-
fueling systems

tion of second-generation bioethanol from crop residues in 2013. It is

Advantages

expected that the global bioethanol production and utilization will

systems

increase significantly in the soon future. In the meanwhile, the world

production of biodiesel increased from 213 million gallons in 2000 to
5670 million gallons in 2012. The further increase, however, depends
on development of new oil sources, such as cost-effective algal
Corn/sugar

Cellulosic
Firewood

Biodiesel

Pyrolysis
Charcoal

Drop-in

cultivation and oil extraction. Substantial attempts were also

ethanol

ethanol

Syngas
pellets

bio-oil

Biogas
Wood

Wood
chips

fuels

made to produce liquid hydrocarbon fuels from plant biomass by

pyrolysis. More research is needed to develop practical techniques for
upgrading crude pyrolysis bio-oil to quality transportation fuels. Other
Gaseous
Biofuels

drop-in fuels such as biobutanol and sugar hydrocarbons are being

Liquid
Table 2

Solid

developed to overcome the infrastructure-incompatibility drawbacks

of bioethanol and biodiesel. The technologies are yet in the infant and
724 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725

early demonstration stages. Renewable bioenergy has also been [19] DOE.. Biomass co-firing: a renewable alternative for utilities. (DOE/GO-
recovered from organic wastes by anaerobic digestion of animal 102000-1055). Washington, DC: U.S. Department of Energy; 2000.
[20] Colgate. Wood-fired boiler. Hamilton, NY: Colgate University; 2013. ( 〈http://
manures, municipal organic refuse, sewage sludge, and food- www.colgate.edu/distinctly-colgate/sustainability/climate-action-planning/
processing waste for biogas. With the technology development, renewable-energy〉).
biogas may displace one quarter of the current global natural gas [21] BTEC. Wood chip boiler reduces heating cost. Washington, DC: Biomass
consumption in the future. In addition, wood has been tested via Thermal Energy Council; 2013. ( 〈http://www.biomassthermal.org/resource/
CaseStudies/ Containerized-High-efficiency-wood-chip-boiler.pdf〉).
gasification to produce syngas that can be used for synthesizing [22] BBI. Biomass plants. Grand Forks, ND: BBI International; 2014. ( 〈http://
industrial chemicals or for electricity generation. Nevertheless, the biomassmagazine.com/plants/listplants/biomass/US/〉).
technology is not economically advantageous at the current fossil [23] GlobalData. Biopower—global market size, feedstock analysis, regulations
and investment analysis to 2020. London, UK: GlobalData; 2011.
fuel prices.
[24] DOE. Biopower technical strategy workshop summary report. Washington,
It is evident that the global consumption of bioenergy will DC: U.S. Department of Energy; 2010.
continue to increase under the influence of present renewable [25] White MS, Curtis M L, Sarles R L, Green DW. Effects of outside storage on the
energy and climate change policies. Biopower (by co-firing), energy potential of hardwood particulate fuels: Part II higher and net
heating values. For Prod J 1983;33:61–5.
lignocellulosic bioethanol, and biogas will be the most prosperous [26] Purdue Extension 2011. Properties of wood waste stored for energy produc-
bioenergy and renewable sectors with highest growing potential tion. ID-421-W. West Lafayette, IN: Purdue University; 2011.
in the soon future. To achieve the pre-set goal of carbon emission [27] OECD.. Bioheat, biopower and biogas: developments and implications for
agriculture. Paris, France: OECD Publishing; 2010.
reductions in the energy sector, the World Energy Council pre-
[28] IEA. Global wood pellet industry market and trade study. IEA bioenergy task
dicted that the bioenergy consumption would increase three-fold 40. Paris, France: International Energy Agency; 2011.
to 2050 relative to the 2010 level [101]. There is strong indication [29] Kofman PD. Simple ways to check wood pellet quality. Bioenergy News
that bioenergy will meet 30% of the whole world’s energy demand 2007;2006-2007:8–9 (Winter).
[30] DOE. Wood and pellet heating. Washington, DC: U.S. Department of Energy;
by 2050. 2013. (〈http://energy.gov/energysaver/articles/wood-and-pellet-heating〉).
[31] van Tilburg M. More pellets, please. Peachtree Corners, GA: Site Selection
Magazine; 2013. (〈http://www.siteselection.com/issues/2013/jul/world-re
ports.cfm〉).
Acknowledgements
[32] IER. Europe’s renewable fuel of choice: wood. Washington, DC: Institute for
Energy Research; 2013. (〈http://www.instituteforenergyresearch.org/2013/
Financial support to compiling the information was from the 04/24/europes-renewable-fuel-of-choice-wood/〉).
[33] Urbas J, Parker WJ. Surface temperature measurements on burning wood
USDA-AFRI competitive grant no. 2011-67009-30055.
specimens in the cone calorimeter and the effect of grain orientation. Fire
Mater 1993;17:205–8.
[34] Antal MJ, Gronli M. The art, science, and technology of charcoal production.
References Ind Eng Chem Res 2003;42:1619–40.
[35] Goldwyn M. Then zen of charcoal: how charcoal is made and how charcoal
[1] Broadmeadow M, Matthews R. Forests, carbon and climate change: the UK works. Pampano Beach, FL: AmazingRibs, Inc; 2013. 〈http://www.amazin
contribution. Information note. . Edinburgh: Forestry Commission; 2003. gribs.com/tips_and_technique/zen_of_charcoal.html〉.
[2] Ashton S, Cassidy P. Energy basics. In: Hubbard W, Biles L, Mayfield C, Ashton [36] De Decker K. Wood gas vehicles: firewood in the fuel tank. Low-Tech Mag
S, editors. Sustainable forestry for bioenergy and bio-based products: 2010 (January 18, 2010.).
trainers pages curriculum notebook. Athens, GA: Southern Forest Research [37] EIA. Frequently asked questions. Washington, DC: U.S. Energy Information
Partnership; 2007. p. 189–92. Administration; 2013. (〈http://www.eia.gov/tools/faqs/〉).
[3] EIA. International energy outlook 2013. Washington, DC: U.S. Energy Infor- [38] Lifset R. American energy policy in the 1970s. Norman, OK: University of
mation Administration; 2013. Oklahoma Press; 2014.
[4] Haberl H, Erb KH, Krausmann F, et al. Bioenergy: how much can we expect [39] Songstad DD, Lakshmanan P, Chen J, Gibbons W, Hughes S, Nelson R.
for 2050? Environ Res Lett 2013;8:031004. http://dx.doi.org/10.1088/1748- Historical perspective of biofuels: learning from the past to rediscover the
9326/8/3/031004. future. In Vitro Cell Dev Biol—Plant 2009;45:189–92.
[5] IEA.. World energy outlook 2013. Paris, France: International Energy Agency; [40] Carolan MS. A sociological kook at biofuels: ethanol in the early decades of
2013. the twentieth century and lessons for today. Rural Sociol 2009;74:86–112.
[6] IEA. Resources to reserves 2013. Paris, France: International Energy Agency; [41] Viikari L, Vehmaanpera J, Koivula A. Lignocellulosic ethanol: from science to
2013. industry. Biomass Bioenergy 2012;46:13–24.
[7] IPCC.. Climate change 2013—the physical science basis. Geneva, Switzerland: [42] RFA. Ethanol industry statistics. Washington, DC: Renewable Fuel Associa-
Intergovernmental Panel on Climate Change; 2013. tion; 2014. (〈http://www.ethanolrfa.org/ pages/statistics〉).
[8] EIA.. Annual energy review 2011. Washington, DC: U.S. Energy Information [43] EIA. 2012 brief: U.S. ethanol prices and production lower compared to 2011.
Administration; 2012. Washington, DC: U.S. Energy Information Administration; 2013. ( 〈http://
[9] USDA. US bioenergy statistics. Washington, DC: United States Department of www.eia.gov/todayinenergy/ detail.cfm?id=9791〉).
Agriculture Economic Research Service; 2013. [44] Mosier NS, Ileleji K. How fuel ethanol is made from corn. Extension ID-328.
[10] Hermann K, Dietmar R. A history of India. 5th ed.. New York, NY: Routledge; West Lafayette, IN: Purdue University; 2006.
2010. [45] Bomgardner MM. Seeking biomass feedstocks that can compete. Chem Eng
[11] Berna F, Goldberg P, Horwitz LK, Brink J, Holt S, Bamford M, Chazan M. News 2013;91(32):11–5.
Microstratigraphic evidence of in situ fire in the Acheulean strata of Wonder- [46] Badger PC. Ethanol from cellulose: a general review. In: Janick J, Whipkey A,
werk Cave, Northern Cape province, South Africa. PNAS 2012;109(20):
editors. Trends in new crops and new uses. Alexandria, VA: ASHS Press.
E1215–20.
2002, p. 17–21.
[12] ORNL. Bioenergy conversion factors. Oak Ridge, TN: Oak Ridge National
[47] Lenihan P, Orozco A, O’Neill E, Ahmad MNM, Rooney DW, Walke GM. Dilute
Laboratory; 2013. (〈http://bioenergy.ornl.gov/papers/misc/energy_conv.html〉).
acid hydrolysis of lignocellulosic biomass. Chem Eng J 2010;156:395–403.
[13] CalEPA. Wood burning handbook. No. 0911-019. Sacramento, CA: California
[48] Kumar P, Barrett DM, Delwiche MJ, Stroeve P. Methods for pretreatment of
Environmental Protection Agency; 2009.
lignocellulosic biomass for efficient hydrolysis and biofuel production. Ind
[14] Quaak P, Knoef H, Stassen H. Energy from biomass—a review of combustion
and gasification technologies. World Bank technical paper no. 422. Washing- Eng Chem Res 2009;48:3713–29.
ton, DC: The World Bank; 2009. [49] Bratis A. Review of recent pilot scale cellulosic ethanol demonstration. Golden,
[15] FAO. The potential use of wood residues for energy generation. In: Energy CO: National Renewable Energy Laboratory, Department of Energy; 2013.
conservation in the mechanical forest industries. FAO forestry paper 93. [50] EPA. E15—frequently asked questions. United States Environmental Protection
Geneva, Switzerland: Food and Agriculture Organization of the United Agency; 2013. (〈http://www.epa.gov/otaq/regs/fuels/additive/e15/e15-faq.htm〉).
Nations; 1990. [51] IER. EPA ignores reality with 2014 ethanol mandates. Washington, DC:
[16] EPA. Consumers—energy efficiency and wood-burning stoves and fireplaces. Institute for Energy Research; 2013. (〈http://instituteforenergyresearch.org/
Washington, DC: United States Environmental Protection Agency; 2013. analysis/epa-ignores-reality-with-2014-ethanol-mandate/〉).
(〈http://www.epa.gov/burnwise/ energyefficiency.html〉). [52] Ineos Bio. Ineos Bio announces operational progress at Florida plant. Grand
[17] Schueftan A, Gonzalez A. Reduction of firewood consumption by households Forks, ND: Ethanol Producer Magazine; 2013. (〈http://www.ethanolproducer.
in south-central Chile associated with energy efficiency programs. Energy com/articles/10528/
Policy 2013;63:823–32. ineos-bio-announces-operational-progress-at-florida-plant〉).
[18] FAO. FAOStat—forestry database. Geneva, Switzerland: Food and Agriculture [53] AFDC. Alternative Fuels Data Center—fuel properties comparison. Washing-
Organization of the United Nations; 2013. ton, DC: Alternative Fuel Data Center, Department of Energy; 2013.
 M. Guo et al. / Renewable and Sustainable Energy Reviews 42 (2015) 712–725 725

[54] Hoekman SK, Broch A, Robbins C, Ceniceros E, Natarajan M. Review of [77] DOE. The history of natural gas. Washington, DC: U.S. Department of Energy;
biodiesel composition, properties, and specifications. Renewable Sustainable 2013. (〈http://fossil.energy.gov/education/energylessons/gas/gas_history.
Energy Rev 2012;16:143–69. html〉).
[55] Thomson J. The Scot Who Lit the World—the Story of William Murdoch [78] EIA. International energy statistics [Online]. Washington, DC: U.S. Energy
inventor of gas lighting. Dennistoun, Glasgow G31 2JJ, UK: Janet Thomson; Information Administration; 2013. (〈http://www.eia.gov/cfapps/ipdbproject/
2003. 〉).
[56] Knothe G. Historical perspectives on vegetable oil-based diesel fuels. [79] Weiland P. Biogas production: current state and perspectives. Appl Microbiol
Information 2001;12:1103–7. Biotechnol 2010;85:849–60.
[57] EIA. Monthly biodiesel production report. Washington, DC: U.S. Energy [80] Mata-Alvarez J, Mace S, Llabres P. Anaerobic digestion of organic solid
Information Administration; 2014. (〈http://www.eia.gov/biofuels/biodiesel/ wastes: an overview of research achievements and perspectives. Bioresour
production/〉). Technol 2000;74:3–16.
[58] Atadashi IM, Aroua MK, Abdul Aziz AR, Sulaiman NMN. Refining technolo- [81] Buswell AM, Mueller HF. Mechanism of methane fermentation. Ind Eng
gies for the purification of crude biodiesel. Appl Energy 2011;88:4239–51. Chem 1952;44:550–2.
[59] Vamvuka D. Bio-oil, solid and gaseous biofuels from biomass pyrolysis [82] European Bioplastics. Factsheet Mar 2010. Anaerobic digestion. Berlin,
processes—an overview. Int J Energy Res 2011;35:853–62. Germany: European Bioplastics; 2011.
[60] Guo M, Shen Y, He Z. Poultry litter-based biochar: preparation, characteriza- [83] Kardos L, Juhasz A, Palko GY, Olah J, Barkacs K, Zaray GY. Comparing of
tion, and utilization. In: He Z, editor. Applied research in animal manure mesophilic and thermophilic Anaerobic fermented sewage sludge based on
management: challenges and opportunities beyond the adverse environ- chemical and biochemical tests. Appl Ecol Environ Res 2011;9(3):293–302.
mental impacts. Hauppauge, NY: Nova Science Publishers; 2012. p. 171–202. [84] Gunaseelan VN. Anaerobic digestion of biomass for methane production: a
[61] Laird D A, Brown RC, Amonette JE, Lehmann J. Review of the pyrolysis review. Biomass Bioenergy 1997;13:83–114.
platform for coproducing bio-oil and biochar. Biofuels, Bioprod Biorefin [85] Lusk P. Methane recovery from animal manures: the current opportunities
2009;3:547–62. casebook. Golden, CO: National Renewable Energy Laboratory; 1998 (NREL/
[62] Carpenter D, Westover TL, Czernik S, Jablonski W. Biomass feedstocks for SR-580-25145).
renewable fuel production: a review of the impacts of feedstock and [86] Nayono SE. Anaerobic digestion of organic solid waste for energy production.
pretreatment on the yield and product distribution of fast pyrolysis bio- Karlsruhe, Germany: KIT Scientific Publishing; 2009.
oils and vapors. Green Chem 2014;16:384–406. [87] Xia Z. Domestic biogas in a changing China: can biogas still meet the energy
[63] Ringer M, Putsche V, Scahill J. Large-scale pyrolysis oil production: a needs of China’s rural households? London, UK: International Institute for
technology assessment and economic analysis. Technical report NREL/TP-
Environment and Development; 2013.
510-37779. Golden, CO: National Renewable Energy Laboratory, Department
[88] Stephan D. Germany remains the world’s leading biogas energy producer.
of Energy; 2006.
Würzburg, Germany: Vogel Business Media; 2013. (〈http://www.process-
[64] Czernik S, Bridgwater AV. Overview of applications of biomass fast pyrolysis
worldwide.com/management/markets_industries/ articles/391244/〉).
oil. Energy Fuels 2004;18:590–8.
[89] Costa A, Voell C. Farm digester industry in America. BioCycle 2012;53(2):
[65] Xu J, Jiang J, Sun Y, Lu Y. Bio-oil upgrading by means of ethyl ester
38–40.
production in reactive distillation to remove water and to improve storage
[90] Rajgor G. Renewable power generation—2012 figures. Part six: Biomass.
and fuel characteristics. Biomass Bioenergy 2008;32:1056–61.
Renewable Energy Focus 2013; 14(4):24-25.
[66] Zhang L, Liu R, Yin R, Mei Y. Upgrading of bio-oil from biomass fast pyrolysis
[91] World Bioenergy Association. Fact sheet on biogas—an important renewable
in China: a review. Renewable Sustainable Energy Rev 2013;24:66–72.
energy source. Stockholm, Sweden: World Bioenergy Association; 2013.
[67] Ruddy DA, Schaidle JA, Ferrell JR, Wang J, Moens L, Hensley JE. Recent
[92] NETL.. Syngas composition. Albany, OR: National Energy Technology Labora-
advances in heterogeneous catalysts for bio-oil upgrading via ex-situ
tory; 2013.
catalytic fast pyrolysis: catalyst development through the study of model
[93] Wan C, Yu F, Zhang Y, Li Q, Wooten J. Material balance and energy balance
compounds. Green Chem 2014;16:454–90.
analysis for syngas generation by a pilot-plant scale downdraft gasifier.
[68] Lehto J. Known challenges associated with the production, transportation,
storage and usage of pyrolysis oil in residential and industrial settings. J Biobased Mater Bioenergy 2013;7(6):690–5.
Washington, DC: Office of Energy Efficiency and Renewable Energy, Depart- [94] E4tech. Review of technologies for gasification of biomass and wastes.
ment of Energy; 2012. (〈http://www1.eere.energy.gov/bioenergy/pdfs/pyro NNFCC project 09/008 final report. London, UK: E4tech Ltd.; 2009.
lysis_lehto.pdf〉). [95] Talmadge M, Biddy M, Dutta A, Jones S, Meyer A. Syngas upgrading to
[69] Kass MD, Theiss TJ, Janke CJ, Pawel SJ, Lewis SA. Intermediate ethanol blends hydrocarbon fuels technology pathway. Golden, CO: National Renewable
infrastructure materials compatibility study. (ORNL TM-2010/326). Oak Energy Laboratory; 2013 (Technical report NREL/TP-5100-58052).
Ridge, TN: Oak Ridge National Laboratory; 2011. [96] Richardson Y, Blin J, Julbe A. A short overview on purification and con-
[70] AFDC. Drop-in biofuels. Washington, DC: Alternative Fuel Data Center, ditioning of syngas produced by biomass gasification: catalytic strategies,
Department of Energy; 2012. (〈http://www.afdc.energy.gov/fuels/emerging_ process intensification and new concepts. Progress Energy Combust Sci
dropin_biofuels.html〉). 2012;38:765–81.
[71] Schiel-Bengelsdorf B, Montoya J, Linder S, Dürre P. Butanol fermentation. [97] Haberl H, Erb KH, Krausmann F, et al. Bioenergy: how much can we expect
Environ Technol 2013;34:1691–710. for 2050? Environ Res Lett 2013;8:031004. http://dx.doi.org/10.1088/1748-
[72] Biddy M, Jones S. Catalytic upgrading of sugars to hydrocarbons technology 9326/8/3/031004.
pathway. Golden, CO: National Renewable Energy Laboratory; 2013 (Tech- [98] van der Hilst F, van Eijck J, Verstegen J et al. Impacts of biofuel production—
nical report NREL/TP-5100-58055). case studies: Mozambique, Argentina and Ukraine. Final report. Heidelber-
[73] Duan P, Bai X, Xu Y, Zhang A, Wang F, Zhang L, Miao J. Non-catalytic glaan, The Netherlands: Utrecht University; 2013.
hydropyrolysis of microalgae to produce liquid biofuels. Bioresour Technol [99] EPA. Economics of biofuels. Washington, DC: Environmental Protection
2013;136:626–34. Agency; 2014. (〈http://yosemite.epa.gov/EE%5Cepa%5Ceed.nsf/webpages/Bio
[74] Biddy M, Davis R, Jones S, Zhu Y. Whole algae hydrothermal liquefaction fuels.html〉).
technology pathway. Golden, CO: National Renewable Energy Laboratory; [100] Koppelaar R. Word energy consumption beyond 500 exajoules. Santa Rosa,
2013 (Technical report NREL/TP-5100-58051). CA: Post Carbon Institute; 2012. 〈http://www.resilience.org/stories/
[75] Dowson GRM, Haddow MF, Lee J, Wingad RL, Wass DF. Catalytic conversion 2012-02-16/world-energy-consumption-beyond-500-exajoules〉.
of ethanol into an advanced biofuel: unprecedented selectivity for n-butanol. [101] WEC.. World energy resources: bioenergy. London, UK: World Energy
Angew Chem 2013;52:9005–8. Council; 2013.
[76] Primus. Introduction to Primus’ STGþ Technology. Hillsborough, NJ: Primus [102] Taherzadeh MJ, Karimi K. Acid-based hydrolysis processes for ethanol from
Green Energy; 2013. (〈http://www.primusge.com/〉). lignocellulosic materials: a review. BioResources 2007;2:472–99.