Emission and transformation of various pollutants during coal-ammonia combustion in fluidized bed: CO/CO2/CxHy/NH3/NOx/SO2/Hg

Abstract

Coal-ammonia combustion is a hot coal combustion technology for achieving carbon emission reduction, which lacks systematic studies on the combustion in fluidized beds. In this work, the effects of combustion temperature, ammonia doping amount, and excess air ratio on the emission and transformation of various pollutants (CO, CO₂, C_xH_y, NH₃, NOx, SO₂, Hg) of Guizhou anthracite with ammonia combustion were studied on a bubbling fluidized bed. It shows 850 °C is the optimal combustion temperature for the Guizhou anthracite, which achieving the complete combustion of carbon in coal and combustible carbon compounds in flue gas with the highest CO₂ concentration. Increasing the ammonia doping amount benefits for reducing the concentrations of CO₂, C_xH_y, SO₂, NOx in flue gas and mercury content in fly ash, but increasing that of NH₃, H₂O, Hg⁰ in the flue gas. 20 % is the optimal ammonia doping amount. Increasing the excess air ratio is beneficial to the co-combustion of coal and ammonia, where the concentrations of CO, C_xH_y, H₂O, SO₂, and NO₂ in the flue gas gradually decrease with opposite pattern to CO₂. The concentrations of NOx, NO, Hg⁰ in flue gas and the mercury content in the fly ash firstly increase and then decrease. The 1.2 is the optimal excess air ratio. Combustion temperature, ammonia doping amount, and excess air ratio affect the migration and transformation of various pollutants in the flue gas mainly through influencing the combustion state of coal and ammonia, the carbon content on fly ash surface, and the redox atmosphere of gas-gas/gas-solid reactions. Combustion temperature of 850 °C, ammonia doping content of 20 %, and excess air ratio of 1.2 are the optimal combustion conditions for coal-ammonia in a bubbling fluidized bed.