Experimental Investigation on NOₓ Generation Characteristic and Burnout Performance of Co-Combustion of Carbon-Based Solid Fuels under Deep-Staged Combustion

With the development of low-rank coal chemical industry, the production of low-volatile carbon-based solid fuels, such as pyrolyzed and gasified semi-cokes, is ever-increasing. However, it is difficult to use such fuels efficiently because of their poor burnout performance and high NOₓ generation during the combustion process. Co-combustion of semi-cokes with bituminous coal is a promising approach for large-scale utilization of semi-cokes with ultra-low volatile content. Nevertheless, the NOₓ generation and burnout characteristics of co-combustion of multiple carbon-based solid fuels are yet to be fully understood. In this paper, the co-combustion characteristics of semicokes and bituminous coal were investigated using a deep-staged co-combustion system. The experimental results indicate that NOₓ generation displays an increasing trend with the mass fraction of gasified semi-coke. The unburned carbon (UBC) generation in fly ash increases with the proportions of gasified and/or pyrolyzed semi-cokes. The NOₓ generated from char-N of semi-cokes is reduced by CO and CHᵢ generated from volatile-N of bituminous coal. Compared to air combustion, the increase of oxygen concentration of the O₂/CO₂ atmosphere is useful for the blended fuels to reduce NOₓ generation and improve burnout performance because of the prolonged residence time under oxygen enrichment conditions. The conversion of fuel nitrogen to NOₓ (N-NOₓ) and the nitrogen left in ash (N-ash) in O₂/CO₂ and O₂/Ar atmospheres decrease with increasing oxygen concentration, with more fuel nitrogen converted into N₂. When the oxygen concentration of over-fire air varies from 21 to 100%, the NOₓ generation is increased by 23% and the UBC is decreased by 45%. The present study can offer better understanding on large-scale clean utilization of pyrolyzed and gasified semi-cokes in the utility coal-fired power plants.