2.2 The Nature of Biofuels, Technologies, and Production Pathways
25
indoor smoke from biomass combustion is a “dangerous cocktail” of pollutants.
They include carbon monoxide, nitrogen oxides, formaldehyde, and more than
a hundred others, responsible for acute respiratory infections, chronic pulmonary
disease, asthma, lung cancer, cataracts, and tuberculosis (WHO 2006).
1
There have been significant efforts to replace traditional with “modern” biofuels,
which can be produced on large industrial scales (Goldemberg and Coelho 2004;
WHO 2006; Agoramoorthy and Hsu 2008). Modern biofuels are produced from a
range of sources and technologies. There are several fuel types, and sometimes the
same one can be produced using different feedstocks and processing methods. The
next sections provide an overview of some of the most important biofuels and their
production pathways.
2.2.2 Modern Fuel Uses of Solid Biomass: Pyrolysis
and Electricity Cogeneration
Modern technologies have allowed for the use of solid biomass as a fuel in cleaner
and much more varied and efficient ways. They include, for instance, pyrolysis for
heat production and technologies for electricity generation from biomass.
Pyrolysis, in contrast to combustion, consists of burning biomass in the absence of
oxygen. There are traditional pyrolysis pathways (by burning biomass and covering
it with soil to avoid reaction with oxygen) that have been used by Amazonian indigenous communities for centuries (Lehmann et al. 2006). However, technology has
allowed for more efficient processes such as “fast pyrolysis,” in which biomass is
heated to around 500 °C for only a few seconds or even less (Bridgwater 2003;
Kim 2015). The result usually consists of bio-oil, vapors, and a solid product called
biochar. While the vapors can be used in heating, bio-oil can be used as a replacement for liquid fossil fuels, though it has the disadvantage of being incompatible
with petroleum products (Bridgwater 2003). The cost-effectiveness of the process
also needs further improvement (Bridgwater 2012; Kim 2015). Finally, biochar is
a cleaner form of charcoal, as it avoids the harmful emissions from conventional
combustion. It has been regarded as a useful soil additive that stores carbon, improves
water retention, ensures the bioavailability of nutrients, and reduces the need for
fertilizer applications (see Lehmann et al. 2006; Lee et al. 2017).
Another significant utilization of solid biomass is in cogeneration, i.e., generation
of both heat and electricity. It consists of bringing agricultural residues such as
corn stalks, crushed sugarcane, or other biomass into a boiler at high temperatures
to produce steam, which can flow into a turbine generator to produce electricity
(Purohit and Michaelowa 2007). This power can be used either locally or exported
into a grid, or both. The most prominent example of this pathway is the so-called
1 In contrast, however, there are anecdotal suggestions that indoor smoke may in some cases help
prevent malaria and other insect-borne diseases, though the evidence for this remains weak (see
Biran et al. 2008).
25
indoor smoke from biomass combustion is a “dangerous cocktail” of pollutants.
They include carbon monoxide, nitrogen oxides, formaldehyde, and more than
a hundred others, responsible for acute respiratory infections, chronic pulmonary
disease, asthma, lung cancer, cataracts, and tuberculosis (WHO 2006).
1
There have been significant efforts to replace traditional with “modern” biofuels,
which can be produced on large industrial scales (Goldemberg and Coelho 2004;
WHO 2006; Agoramoorthy and Hsu 2008). Modern biofuels are produced from a
range of sources and technologies. There are several fuel types, and sometimes the
same one can be produced using different feedstocks and processing methods. The
next sections provide an overview of some of the most important biofuels and their
production pathways.
2.2.2 Modern Fuel Uses of Solid Biomass: Pyrolysis
and Electricity Cogeneration
Modern technologies have allowed for the use of solid biomass as a fuel in cleaner
and much more varied and efficient ways. They include, for instance, pyrolysis for
heat production and technologies for electricity generation from biomass.
Pyrolysis, in contrast to combustion, consists of burning biomass in the absence of
oxygen. There are traditional pyrolysis pathways (by burning biomass and covering
it with soil to avoid reaction with oxygen) that have been used by Amazonian indigenous communities for centuries (Lehmann et al. 2006). However, technology has
allowed for more efficient processes such as “fast pyrolysis,” in which biomass is
heated to around 500 °C for only a few seconds or even less (Bridgwater 2003;
Kim 2015). The result usually consists of bio-oil, vapors, and a solid product called
biochar. While the vapors can be used in heating, bio-oil can be used as a replacement for liquid fossil fuels, though it has the disadvantage of being incompatible
with petroleum products (Bridgwater 2003). The cost-effectiveness of the process
also needs further improvement (Bridgwater 2012; Kim 2015). Finally, biochar is
a cleaner form of charcoal, as it avoids the harmful emissions from conventional
combustion. It has been regarded as a useful soil additive that stores carbon, improves
water retention, ensures the bioavailability of nutrients, and reduces the need for
fertilizer applications (see Lehmann et al. 2006; Lee et al. 2017).
Another significant utilization of solid biomass is in cogeneration, i.e., generation
of both heat and electricity. It consists of bringing agricultural residues such as
corn stalks, crushed sugarcane, or other biomass into a boiler at high temperatures
to produce steam, which can flow into a turbine generator to produce electricity
(Purohit and Michaelowa 2007). This power can be used either locally or exported
into a grid, or both. The most prominent example of this pathway is the so-called
1 In contrast, however, there are anecdotal suggestions that indoor smoke may in some cases help
prevent malaria and other insect-borne diseases, though the evidence for this remains weak (see
Biran et al. 2008).