Page 73 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

52 Biomass Gasification, Pyrolysis and Torrefaction

Compared to lignocellulosic compounds, carbohydrates are easier to dis-
solve, so it is relatively easy to derive liquid fuels from them through fer-
mentation or other processes. For this reason, most commercial ethanol
plants use crops as feedstock.
Natural crops and vegetables are a good source of starch and sugars and,
therefore can be hydrated. Some vegetables and crops (e.g., coconut, sun-
flower, mustard, and canola) contain fat, providing a good source of
vegetable oil. Animal waste (from land and marine mammals) also provides
fat that can be transformed into bio-oil. If carbohydrate is desired for the
production of biogas, whole crops, such as maize, Sudan grass, millet, and
white sweet clover, can be made into silage and then converted into biogas.
There are two types of crop biomass: the regularly harvested agricultural
crops for food production and the energy crops for energy production.

3.2.2.3 Waste Biomass
Waste biomasses are secondary biomass, as they are derived from primary
biomass like trees, vegetables, meat during the different stages of their pro-
duction or use. Municipal solid waste (MSW) is an important source of
waste biomass, and much of it comes from renewables like food scraps,
lawn clippings, leaves, and papers. Nonrenewable components of MSW like
plastics, glass, and metals are not considered biomass. The combustible part
of MSW is at times separated and sold as refuse derived fuel (RDF). Sewage
sludge that contains human excreta, fat, grease, and food wastes is an impor-
tant biomass source. Another waste is produced in sawmills during the pro-
duction of lumber from wood. Table 3.3 lists the composition and heating
values of some waste biomass products.
Landfills have traditionally been an important means of disposing of gar-
bage. A designated area is filled with waste, which decomposes, producing
methane gas. Modern landfilling involves careful lining of the containment

TABLE 3.3 Typical Composition of Some Waste Biomass
Moisture Organic Matter HHV
Biomass (wt.%) (dry wt.%) Ash (dry wt.%) (MJ/dry kg)
Cattle manure 20 70 76.5 23.5 13.4
Sewage 90 98 73.5 26.5 19.9
RDF 15 30 86.1 13.9 12.7
Sawdust 15 60 99.0 1.0 20.5
Source: Adapted from Klass (1998, p. 73).