Experimental Study on Co-Firing of Syngas as a Reburn/Alternative Fuel in a Commercial Water-Tube Boiler and a Pilot-Scale Vertical Furnace

Abstract

Co-firing synthesis gas (syngas) with conventional fuels within a conventional boiler is an effective method that partially replaces the use of fossil fuels with low-grade fuels or renewable energy sources such as waste or biomass. This study investigates the characteristics of syngas cofiring and reburning in a commercial oil-firing boiler and a pilot-scale vertical combustion chamber with refractory. Syngas cofiring was tested in two separate apparatus. The first was a commercial water-tube boiler containing one heavy oil burner of 0.7 MWth (for 1 ton steam/h). The second was a vertical furnace with 4 heavy oil burners, for various heat replacements by syngas cofiring at various heating values. Temperature distributions and the gas composition at the exit of the combustion chamber were measured for all cases and the thermal efficiencies under various cofiring conditions were evaluated through heat and mass balance calculations. Combustion stability remained unaffected if less than 20% heat was replaced by syngas cofiring; this was true for syngas of low calorific values (3.7 MJ/Nm(3)), and especially in vertical furnaces with horizontally installed multiburners. The reburning effects induced by syngas cofiring were confirmed by testing two reburning conditions in the vertical chamber: reburning only and reburning in combination with air-staged combustion of which the total thermal input was 2 MWth. These conditions reduced NOx emission by 30-50%, indicating that high temperatures of the radical production are essential for reducing NOx emissions. Meanwhile, syngases containing low-calorific values yielded heat efficiency losses. Conversely, heat efficiency increased when syngases possessing higher heating values were applied to the boiler.