Experiments and Simulation on Co-Combustion of Semicoke and Coal in a Full-Scale Tangentially Fired Utility Boiler

Abstract

Co-firing semicoke and coal in utility boilers is a promising approach to cleanly and efficiently utilize semicoke, while the co-combustion characteristics have yet to be fully understood, including the effects of the blending method and secondary air distribution mode on NOx emission and combustion efficiency. In the present work, both full-scale experiments and simulation were carried out to investigate the co-combustion characteristics of semicoke and coal in a 300 MW coal-fired utility boiler with a focus on the blending method and air distribution. The results indicated that co-firing semicoke would reduce the furnace temperature and that both NOx emission and incomplete combustion heat loss increased with the blending amount of semicoke. Hence, to limit the excessive deterioration of boiler performance, the blending amount of semicoke was recommended to be below 50% for the in-furnace blending method and below 33% for the out-furnace blending method. When the in-furnace blending method was employed, the semicoke and coal were recommended to be fed from burners in alternate layers, in particular, without semicoke fed from the top and/or bottom burners. The NOx emission was proved to positively correlate with the average height of semicoke burners, while the burnout ratio increased at first and then decreased with the increasing average height. The in-furnace blending method with optimal burner allocation was proved to have advantage over the out-furnace blending method. The secondary air distribution of the pagoda down type could reduce NOx generation and unburned carbon due to the formation of a high-temperature and strong reducing atmosphere within the furnace.