26
2 The Contested Sustainability of Biofuels …
“bagasse cogeneration,” which utilizes the fibrous remains of sugarcane crushed for
sugar or ethanol. This utilization has allowed many sugar mills in Brazil to become
not only self-sufficient in energy but also to sell it into the power grid and improve
the sector’s economic performance (Goldemberg et al. 2008).
2.2.3 Gaseous Biofuels: Methane and Synthesis Gas
The use of gaseous biofuels dates back to the early 1900s, with biogas as a replacement for kerosene in rural areas (Buswell and Boruff 1933). Biogas is a methane-rich
gas produced from the anaerobic decomposition of biomass such as animal manure,
agriculture residues, or organic household wastes. This process naturally occurs in
landfills, but it can also be utilized for energy production using biogas digesters—
sealed airless containers where anaerobic bacteria ferment the organic matter and
produce methane gas (Sagar and Kartha 2007; Agoramoorthy and Hsu 2008). The gas
is then used to generate heat or electricity, thus working as a fossil fuel replacement.
Another way biomass can replace fossil energy is through syngas production (for
“synthesis gas”). This gas consists of a mixture mostly of hydrogen and carbon
monoxide, resulting from the combustion of feedstock at temperatures of about
500 °C (though newer methods using plasma reactors at 1700 °C have been sought)
(Mozaffarian et al. 2004; Van Oost et al. 2008). The syngas can be used as a replacement for natural gas, for the same purposes (e.g., heating) and utilizing the same
infrastructure (Mozaffarian et al. 2004). This process, called thermo-chemical gasification, is not new—it has been used for decades with coal as a feedstock (Beychok
1975). The novelty, however, is the utilization of biomass feedstocks as a substitute. As the conversion technology is well established, there is potential for existing
infrastructure to be adapted (Worldwatch Institute 2006; Koh and Ghazoul 2008),
though the utilization of different biomass feedstocks still poses challenges (Dayton
et al. 2019).
2.2.4 Liquid Biofuels: Ethanol
Ethanol is an alcohol conventionally made by the fermentation of carbohydrates
extracted from plants rich in either starch (e.g., corn, cassava) or sugar (e.g., sugarbeet, sugarcane). The production from sugar feedstocks includes milling, pressing,
fermentation, and distillation of the crop, while starchy plants require first converting
starch into sugar (Sagar and Kartha 2007). More advanced pathways can utilize
any cellulosic biomass, such as grass, wood, straw, or other agricultural residues.
Since cellulose (a primary constituent of plant biomass) is, in fact, an insoluble
carbohydrate, it can be broken down through enzymatic hydrolysis and utilized in
the same way as sugar crops (Field et al. 2008). This cellulose processing allows
for utilizing all parts of the plant, thus making better use of the feedstocks. Most
2 The Contested Sustainability of Biofuels …
“bagasse cogeneration,” which utilizes the fibrous remains of sugarcane crushed for
sugar or ethanol. This utilization has allowed many sugar mills in Brazil to become
not only self-sufficient in energy but also to sell it into the power grid and improve
the sector’s economic performance (Goldemberg et al. 2008).
2.2.3 Gaseous Biofuels: Methane and Synthesis Gas
The use of gaseous biofuels dates back to the early 1900s, with biogas as a replacement for kerosene in rural areas (Buswell and Boruff 1933). Biogas is a methane-rich
gas produced from the anaerobic decomposition of biomass such as animal manure,
agriculture residues, or organic household wastes. This process naturally occurs in
landfills, but it can also be utilized for energy production using biogas digesters—
sealed airless containers where anaerobic bacteria ferment the organic matter and
produce methane gas (Sagar and Kartha 2007; Agoramoorthy and Hsu 2008). The gas
is then used to generate heat or electricity, thus working as a fossil fuel replacement.
Another way biomass can replace fossil energy is through syngas production (for
“synthesis gas”). This gas consists of a mixture mostly of hydrogen and carbon
monoxide, resulting from the combustion of feedstock at temperatures of about
500 °C (though newer methods using plasma reactors at 1700 °C have been sought)
(Mozaffarian et al. 2004; Van Oost et al. 2008). The syngas can be used as a replacement for natural gas, for the same purposes (e.g., heating) and utilizing the same
infrastructure (Mozaffarian et al. 2004). This process, called thermo-chemical gasification, is not new—it has been used for decades with coal as a feedstock (Beychok
1975). The novelty, however, is the utilization of biomass feedstocks as a substitute. As the conversion technology is well established, there is potential for existing
infrastructure to be adapted (Worldwatch Institute 2006; Koh and Ghazoul 2008),
though the utilization of different biomass feedstocks still poses challenges (Dayton
et al. 2019).
2.2.4 Liquid Biofuels: Ethanol
Ethanol is an alcohol conventionally made by the fermentation of carbohydrates
extracted from plants rich in either starch (e.g., corn, cassava) or sugar (e.g., sugarbeet, sugarcane). The production from sugar feedstocks includes milling, pressing,
fermentation, and distillation of the crop, while starchy plants require first converting
starch into sugar (Sagar and Kartha 2007). More advanced pathways can utilize
any cellulosic biomass, such as grass, wood, straw, or other agricultural residues.
Since cellulose (a primary constituent of plant biomass) is, in fact, an insoluble
carbohydrate, it can be broken down through enzymatic hydrolysis and utilized in
the same way as sugar crops (Field et al. 2008). This cellulose processing allows
for utilizing all parts of the plant, thus making better use of the feedstocks. Most