Co-firing of coal and cattle feedlot biomass (FB) fuels. Part II. Performance results from 30 kW t (100,000) BTU/h laboratory scale boiler burner ☆

Abstract

The use of cattle manure (referred to as feedlot biomass, FB) as a fuel source has the potential to both solve waste disposal problems and reduce fossil fuel based CO 2 emissions. A co-firing technology is proposed where FB is ground, mixed with coal, and then fired in existing, pulverized coal-fired boiler burner facilities. A research program was undertaken in order to determine (i) fuel characteristics, (ii) combustion characteristics when fired along with coal in a small scale 30-kW t (100,000 BTU/h) boiler burner facility, and (iii) combustion and fouling characteristics when fired along with coal in a large pilot scale 150-kW t (500,000 BTU/h) DOE-NETL boiler–burner facility. Part I presented a methodology for fuel collection, fuel characteristics of the FB, its relation to ration fed, and the change in fuel characteristics and volatile oxides due to composting. Part II addresses the pyrolysis characteristics of coal, FB, and blend and presents results on the performance of 90:10 coal:FB (PC) blend as fired in a 30-kW t boiler–burner unit. The boiler–burner unit is made of steel and lined with a cast ceramic liner for long duration operation and a commercial feeding system is used for firing the coal and the blend. Thermogravimetric analyses (TGA) performed on coal, FB, and 90:10 coal:FB blend reveal that biomass will start releasing gases at 273 °C (523 °F) which is about 100 °C (212 °F) lower than that of coal. The maximum rate of volatile release is about 0.000669 kg/s kg for FB while that of coal is 0.000425 kg/s kg. The experiments revealed that the 90:10 blend burns more completely in the boiler, due to the earlier release of biomass volatiles and higher amount of volatile matter in FB. The NO x emission for coal was 290 ppm, 0.162 kg/GJ (0.3768 lb/mm BTU) and 260 ppm, 0.1475 kg/GJ (0.343 lb/mm BTU) for the 90:10 blend at 10% excess air. Even though the effective N content of the blend increased by 18%, compared to coal the NO x emission decreased which is attributed to the higher VM of FB and more N in the form of NH 3. However, due to limited residence time and higher VM, the CO emission increased from 15,582 ppm, 5.29 kg/GJ (12.305 lb/mm BTU) to 22,669 ppm, 7.81 kg/GJ (18.16 lb/mm BTU) when fuel was switched from coal to 90:10 blend. Large scale pilot plant tests performed at the 150-kW t facility (DOE-NETL) reveal increased falling potential for the blend compared to coal (Part III), emissions were negligible.