Page 242 - Synthetic Fuels Handbook

228 CHAPTER EIGHT

Some advantages of biomass over conventional fossil fuels are the low sulfur content
and highly reactive char. In addition, biomass materials do not cake and can therefore be
easily handled in both fluidized and moving bed reactors. Finally, catalyst poisons are not
present in biomass in significant concentrations. This can be important for the initial ther-
mal processing as well as for subsequent upgrading operations.
Generally, biomass feedstocks and fuels exhibit a wide range of physical, chemical, and
agricultural/process engineering properties. However, despite their wide range of possible
sources, biomass feedstocks are remarkably uniform in many of their fuel properties, com-
pared with competing feedstocks such as coal or petroleum.
For example, there are many kinds of coals whose gross heating value ranges from 8600
to 12,900 Btu/lb. However, nearly all kinds of biomass feedstocks destined for combustion
fall in the range 6450 to 8200 Btu/lb. For most agricultural residues, the heating values are
even more uniform (6450 to 7300 Btu/lb); the values for most woody materials are 7750 to
8200 Btu/lb. Moisture content is probably the most important determinant of heating value.
Air-dried biomass typically has about 15 to 20 percent moisture, whereas the moisture
content for oven-dried biomass is around 0 percent. Moisture content is also an important
characteristic of coals, varying in the range of 2 to 30 percent. However, the bulk density
(and hence energy density) of most biomass feedstocks is generally low, even after densifi-
cation, about 10 to 40 percent of the bulk density of most fossil fuels. Liquid biofuels have
comparable bulk densities to fossil fuels.
Most biomass materials are easier to gasify than coal because they are more reac-
tive with higher ignition stability. This characteristic also makes them easier to process
thermochemically into higher-value fuels such as methanol or hydrogen. Ash content is
typically lower than for most coals, and sulfur content is much lower than for many fos-
sil fuels. Unlike coal ash, which may contain toxic metals and other trace contaminants,
biomass ash may be used as a soil amendment to help replenish nutrients removed by
harvest. A few biomass feedstocks stand out for their peculiar properties, such as high
silicon or alkali metal contents; these may require special precautions for harvesting,
processing, and combustion equipment. Note also that mineral content can vary as a
function of soil type and the timing of feedstock harvest. In contrast to their fairly uni-
form physical properties, biomass fuels are rather heterogeneous with respect to their
chemical elemental composition.
Among the liquid biomass fuels, biodiesel (vegetable oil ester) is noteworthy for its
similarity to petroleum-derived diesel fuel, apart from its negligible sulfur and ash content.
Bioethanol has only about 70 percent the heating value of petroleum distillates such as
gasoline, but its sulfur and ash contents are also very low. Both of these liquid fuels have
lower vapor pressure and flammability than their petroleum-based competitors and this is
an advantage in some cases (e.g., use in confined spaces such as mines) but a disadvantage
in others (e.g., engine starting at cold temperatures).
The most suitable biomass resources for thermal conversion are wood and the organic
portion of municipal solid waste. Crop residues and grasses are of intermediate value,
although thermal conversion might prove more effective than fermentation. If the versatility
of a liquid fuel is desired, gasification may be combined with methanol or Fischer-Tropsch
synthesis. Very wet resources such as aquatic biomass and animal wastes are not suited to
thermal conversion, and are best reserved for anaerobic digestion (Chap. 9).

8.2 THE CHEMISTRY OF BIOMASS

Biomass is typically composed 75 to 90 percent by weight of sugar species, the other 10 to
25 weight percent being mainly lignin.