Page 287 - Synthetic Fuels Handbook

FUELS FROM CROPS 273

will be consumed over the whole cross section, so that the solid bed temperature will not
reach the local extremes (hot spots) observed in the oxidation zone of conventional gasifiers
due to poor heat transfer. Moreover, the air nozzles in conventional gasifiers generate caves
and create obstacles that may obstruct solid flow especially for solids of low bulk, like rice
husk. On the other hand, the entry of air through the top of the bed creates a downward flow
of the pyrolysis gases, and transports the tars products to the combustion zone. Thus, flow
problems due to the caking of rice husk caused by back mixing of tar are avoided.
The gasification process is amenable to a variety of biomass feedstocks such as waste
rice hulls, wood waste, grass, and the dedicated energy crops. Gasification is a clean process
with few air emissions and, when crops are used as the feedstock, little or no ash results.
In the gasifier, biomass is converted into a gaseous mixture of hydrogen, carbon monox-
ide, carbon dioxide, and other compounds by applying heat under pressure in the presence of
steam and a controlled amount of oxygen. The biomass produces synthesis gas (Chap. 7).

C H O + O + H O → CO + CO + H + other products
2
6
2
6
12
2
2
The above reaction uses glucose as a surrogate for cellulose. Biomass has highly vari-
able composition and complexity, with cellulose as one major component.
The gasification process could play a significant role in meeting the goal of greenhouse
gas mitigation. It is likely that both in the transition phase to a hydrogen economy and in
the steady state, a significant fraction of hydrogen might be derived from domestically
abundant crops. In addition, the co-firing applications of crops biomass (and other biomass)
with coal (Chap. 5), biomass can provide up to 15 percent of the total energy input of
the fuel mixture. Such concepts address greenhouse gas mitigation by co-firing biomass
and coal to offset the losses of carbon dioxide to the atmosphere that are inherent in coal
combustion processes (even with the best-engineered capture and storage of carbon). Since
growth of biomass fixes atmospheric carbon, its combustion leads to no net addition of
atmospheric carbon dioxide even if vented. Thus, co-firing of crops or crop residues (or
other biomass with coal) in an efficient coal gasification process, affording the opportunity
for capture and storage of carbon dioxide, could lead to a net reduction of atmospheric
carbon dioxide. Cheaper, though less plentiful, biomass residue could supplant crops as
gasifier feedstock leading to a less significant impact on the environment than would farm-
ing bioenergy crops.
Along similar lines, willow biomass crops have been shown to be a good fuel for farm-
based power production using advanced gasification technology (Pian et al., 2006). The
fuel gas can be used for generating electricity, using micro-turbines modified to operate
on low-Btu gas, or for other farm energy needs. Willow biomass was found to make a fuel
for ash-rejection gasifiers with a predicted net gasification efficiency of about 85 percent.
Analysis showed that developing a method to co-gasify willow with various amounts of
low-cost wastes, such as dairy farm animal waste, can be an excellent way to reduce the
fuel cost, to increase the overall fuel availability and help work around problems resulting
from seasonal availability of bioenergy crops. Co-gasification of dairy farm wastes along
with willow offers an economical way to dispose the wastes and manage nutrient flows on
a dairy farm.
Agricultural residues can be divided into two groups: (a) crop residues and (b) agro-
industrial residues. Crop residues are plant materials left behind in the farm after removal of
the main crop produce. The remaining materials could be of different sizes, shapes, forms,
and densities like straw, stalks, sticks, leaves, haulms, fibrous materials, roots, branches,
and twigs.
Due to high-energy content, straw is one of the best crop residues for solid biofuels.
However, straw has several disadvantages—it has a higher-ash content, which results in
lower-calorific value. In order to improve its bulk density, the straw is generally baled