Renewable Energy through Co-Firing
Switchgrass in a Coal-Fired Unit
Switchgrass is a native prairie grass that is highly productive on marginal farmland. It is harvested with traditional farm equipment, requires little care, and grows well in the US southeast. Pilot-scale testing has shown the potential for switchgrass as a renewable energy source capable of reducing carbon dioxide (CO2) emissions. When burned with coal in coal-fired plants, studies have also indicated that sulfur dioxide (SO2), mercury, and perhaps nitrogen oxides (NOx) could be reduced.
To study the potential for switchgrass-based renewable energy, Alabama Power, and Southern Company, with its partners Southern Research Institute and Auburn University, designed and constructed a unique handling, processing, and combustion system for co-firing switchgrass with coal at Alabama Power’s Plant Gadsden. Extensive tests with this system coupled with the results of pilot-scale testing at Southern Company’s combustion test facility were used to understand and optimize the handling, combustion, and efficiency of this renewable energy system.
For the project, fields of switchgrass were harvested into large round bales. During six weeks of detailed testing starting in March of 2001, these bales were chopped and pneumatically conveyed by the new delivery system and into the Plant Gadsden furnace through specially designed burners. The boiler and unit were instrumented to measure or calculate many parameters including emissions and boiler efficiency. Switchgrass loadings of up to 10% by energy displaced were achieved, with full load grass input at typically 7 to 8 per cent of the input energy. Switchgrass co-firing was then continued through the summer.
Transport and handling of the grass caused considerable problems, in large part because the testing was designed to push the unique system to its physical limits. As a result, much experience and data were obtained on requirements to reduce risk of future pipe plugging, as well as optimizing energy costs for tubgrinding and handling the bales.
Environmental advantages with switchgrass co-firing were clearly documented in the tests. CO2, SO2 and mercury emissions decreased with the introduction of switchgrass and there was no adverse effect on NOx emissions. Switchgrass co-firing reduced boiler efficiency by 0.3% to 1.0%, which was considered to be acceptable considering the higher moisture content of the grass. Modifying switchgrass storage procedures to decrease absorbed moisture should improve efficiency results during a three-year, extended evaluation beginning in 2002.
In summary, switchgrass co-firing was tested successfully, and has potential as a renewable energy source. Future testing will focus on fuel procurement issues, improved system efficiencies, and long-term impact on slagging and fouling. Other sources of similar biomass such as local hay, agricultural waste, and sawdust will also be explored at the facility.