feedstocks and fuels exhibit a wide range of physical, chemical,
and agricultural process engineering properties. Despite
range of possible sources, biomass feedstocks are remarkably uniform
in many of their fuel properties, compared with competing feedstocks
such as coal or petroleum. For example, there are many kinds of coals
whose gross heating value ranges from 20 to 30 GJ/tonne (gigajoules
per metric tonne; 8600-12900 Btu/lb). However, nearly all kinds of
biomass feedstocks destined for combustion fall in the range 15-19
GJ/tonne (6450-8200 Btu/lb). For most agricultural residues, the
heating values are even more uniform – about 15-17 GJ/tonne
(6450-7300 Btu/lb); the values for most woody materials are 18-19
GJ/tonne (7750-8200 Btu/lb). Moisture content is probably the most
important determinant of heating value. Air-dried biomass typically
has about 15-20% moisture, whereas the moisture content for oven-dried
biomass is around 0%. Moisture content is also an important characteristic
of coals, varying in the range 2-30%. However, the bulk density (and
hence energy density) of most biomass feedstocks is generally low,
even after densification – between about 10 and 40% of the
bulk density of most fossil fuels – although liquid biofuels
have comparable bulk densities.
Most biomass materials
are easier to gasify than coal, because they are more reactive,
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 sulphur content is much lower than for many
fossil 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 herbaceous 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 uniform 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
to petroleum-derived diesel fuel,
apart from its negligible sulfur and ash content. Bioethanol has
only about 70% 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 – 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 following spreadsheet contains
one table (Continued on 3 separate sheets or tabs) that shows
some "typical" values or a range of values for
selected compositional, chemical and physical properties of biomass
feedstocks and liquid biofuels.
Figures for fossil fuels are provided for comparison.
Sources for further information:
DOE Biomass Feedstock Composition and Property Database.
PHYLLIS - database on composition
of biomass and waste.
Nordin, A. (1994) Chemical elemental characteristics of biomass fuels.
Biomass and Bioenergy 6, 339-347.
Source: All information in Appendix B was taken from a fact sheet
by Jonathan Scurlock, Oak Ridge National Laboratory, Bioenergy Feedstock
Development Programs. P.O. Box 2008, Oak Ridge, TN 37831-6407