Performance of Natural Gas Cofiring on a Coal-Fired Spreader Stoker at Dover Light & Power

Gas cofiring is an efficient way to enhance performance and treat operational problems with coal-fired stoker boilers. Firing from 5 to 30 percent of total heat input through side-wall gas burners offers a spectrum of benefits including faster and cleaner warmup and light-off, reduced opacity, recovered derate, coal flexibility, and improved turndown and load following. In this project, the 165,000 Ib/hr Babcock & Wilcox spreader stoker boiler at Dover Light & Power was retrofit with dual Coen gas-fired burners rated at 37 MMBtu/hr each. The burners were positioned on opposite sidewalls 6 feet above the grate and offset 8 feet. A high pressure drop burner design with a small throat opening was used to increase penetration into the combustion gases and minimize pressure part modifications. The gas burners in the staggered configuration promote better mixing and heat distribution and reduce the load per unit area on the grate. The unit has been operated since August 4 1995, cofiring gas a majority of the time. Test results with cofiring show recovery of lost capacity derate and significant improvement in efficiency, emissions and operability. Firing from 10 to 15 percent of total heat input from gas increased boiler efficiency 2 to 3 percent due to improved carbon utilization and reduced excess air. Carbon in fly ash was reduced by 35 to 40 percent by cofiring. Excess oxygen was a function of coal quality and was reduced up to 2 to 3 percent with gas cofiring. Particulate emissions at the electrostatic precipitator inlet were reduced by an average of 24 percent with 10 percent gas cofire.