The effects of moisture and particle size of feedlot biomass on co-firing burner performance

Abstract

Biomass (conventional and non-conventional) fuels co-fired with coal for power and steam generation are being tested and evaluated at several generation stations in the United States. The co-firing technology is expected to reduce landfill requirements for biomass wastes, and to provide a renewable, low pollution and zero net carbon dioxide fuel. The choice of the biomass depends upon local availability and cost of the transportation. The renewable biomass fuels range from agro to animal waste based fuels. For coal fired power plants located around feedlots where cattle are raised, the renewable biomass is the cattle manure, called feedlot biomass (FB). Thus coal could be mixed with feedlot biomass and then fired in existing boiler burners. A 30 KW (100,000 Btu/hr) boiler burner facility was built at Texas A&M University Boiler Burner Laboratory and the burner was fired with coal or coal-FB blends. Most of the previous data concerned with coal performance results from co-firing of low moisture FB (<10% wet basis). However, FB as collected has high moisture (>25%); so feeding at low flow rate becomes a problem. In order to test the effects of moisture on burner performance, the reactor was modified with external water injection through an atomizer in order to simulate higher moisture. The atomizer uses an airblast to atomize the water into finer droplets. At fixed equivalence ratio and swirl number for the secondary inlet air stream, the test variables selected were simulated moisture contents and particle sizes of feedlot biomass. Measurements of NO[], O₂, CO and CO₂ along the furnace are reported. The summaries of results are as follows. With the atomized air only (i.e. without external water injection), the NO[] concentrations increased from 350 ppm to 650 ppm while CO decreased from 46,000 ppm to 18,000 ppm (data measured at the first probe, 6" from the burner). The external water injection used to simulate high moisture FB decreased the pollutant emissions (NO[]) from 570 ppm (zero external water with atomizing air injection) to 300 ppm (40% water in FB) but increased CO from 2,500 ppm (zero external water with atomizing air injection) to 10,500 ppm (40% water in FB) (data of moisture effect measured at the last probe, 36" from the burner) due to more incomplete burning. The small particles FB produced less NO[] but more CO than those from other sizes.