Biomass Fuels and Energy
Biomass energy systems utilize solar energy that has been captured and stored in plant material during photosynthesis. While the overall efficiency of conversion of sunlight to stored chemical energy is low, plants have already solved the
two key problems associated with all solar energy technologies that is, how to collect the energy when it is available, and how to store it for use when the sun isn’t shining.
Plants have also very nicely dealt with the greenhouse problem since the carbon released when they use that stored energy for respiration is the same carbon they extracted in the first place during photosynthesis. That is, they get energy with no net carbon emissions.
While there is already a sizable agricultural industry devoted to growing crops specifically for their energy content, it is almost entirely devoted to converting plant material into alcohol fuels for motor vehicles. On the other hand, biomass
for electricity production is essentially all waste residues from agricultural and forestry industries and, to some extent, municipal solid wastes.
Since it is based on wastes that must be disposed of anyway, biomass feed stocks for electricity production may have low-cost, no-cost, or even negative-cost advantages.
Currently there are about 14 GW of installed generation capacity powered by biomass in the world, with about half of that being in the United States. About two-thirds of the biomass power plants in the United States co generate both electricity and useful heat.
Virtually all biomass power plants operate on a conventional steam Rankine cycle. Since transporting their rather disbursed fuel sources over any great distances could be prohibitively expensive, biomass power plants tend to be small and located near their fuel source, so they aren’t able to take advantage of the economies of scale that go with large steam plants.
To offset the higher cost of smaller plants, lower-grade steel and other materials are often used, which requires lower operating temperatures and pressures and hence lower efficiencies. Moreover, biomass fuels tend to have high water content and are often wet when burned, which means that wasted energy goes up the stack as water vapor. The net result is that existing biomass plants tend to have rather low efficiencies—typically less than 20%. Even though the fuel may be very inexpensive, those low efficiencies translate to reasonably expensive electricity, which is currently around 9¢/kWh.
An alternative approach to building small, inefficient plants dedicated to biomass power production is to burn biomass along with coal in slightly modified, conventional steam-cycle power plants. Called co-firing, this method is an economical way to utilize biomass fuels in relatively efficient plants. And, since biomass burns cleaner than coal, overall emissions are correspondingly reduced in co-fired facilities.
Anaerobic digesters are fairly common in municipal wastewater treatment plants where their main purpose is to transform sewage sludge into innocuous, stabilized end products that can be easily disposed of in landfills or, sometimes, recycled as soil conditioners.
Anaerobic digesters can be used with other biomass feedstocks including food processing wastes, various agricultural wastes, municipal solid wastes, bagasse, and aquatic plants such as kelp and water hyacinth. When the biogas is treated to remove its sulfur, the resulting gas can be burned in reciprocating engines to produce electricity and usable waste heat.