The simple energy of biomass
By NATIONAL RURAL ELECTRIC COOPERATIVE ASSOCIATION from October 2013 Issue
How the burning of wood and grass can help make electricity
Biomass has come a long way from putting a log on a fire. But the concept behind this power source is still quite simple: burn plants or waste materials from plants or animals to heat water to create steam that spins a turbine to generate electricity.
Biomass includes fresh plant parts (trees, grasses, crops) or byproducts such as wood chips or corn stubble, plus livestock waste, and even landfill gas. Recent advances in technology have made it possible to use all these items much more efficiently.
In the simplest systems, fresh materials are used with few modifications. In a direct-fired system, the biomass in its original form is burned with nothing else added. Residual heat from the process can be piped off to heat buildings or reused in other ways, increasing power plant efficiencies. In a co-fired system, biomass is mixed with a fossil fuel such as coal. Burning the two fuels together can lower emissions of certain pollutants while maintaining the same amount of electricity production.
More complicated technology changes the biomass into another form to make it more useful. During gasification, superheating (but not burning) the biomass changes it from a solid into a gas. This synthetic gas (syngas) can be used as a substitute for natural gas. During pyrolysis, biomass changes from a solid into a liquid that can be used in place of traditional fuel oil.
Anaerobic digestion is the most time-consuming method for using biomass. As bacteria (anaerobes) break down rotting plant or animal materials over days or weeks, their actions release methane gas, which can be captured and burned to make electricity. The leftover solids can often be used as compost.
–National Rural Electric Cooperative Association