The influence of air-stage method on flameless combustion of coal gasification fly ash with coal self-preheating technology

Abstract

As the by-product of circulating fluidized bed (CFB) coal gasification, coal gasification fly ash (CGFA) contains low volatile matter (lower than 6 wt%) and high ash matter (higher than 40 wt%), which cannot be burnt steadily and effectively with conventional combustion technologies. However, the annual output of CGFA is enormous in China and the available energy is considerable. Therefore it is necessary to achieve a highly efficient and clean combustion of CGFA. As a novel combustion technology, coal self-preheating combustion has proven its potential to utilize semi-coke, anthracite and other flammable solid fuels, which can also be applied for CGFA. In this paper, flameless mode of CGFA was achieved in a 30 kW coal self-preheating combustion test rig and the influence of air-stage method on image luminosity, temperature profiles and NOX emissions was studied. The results show that stable flameless combustion of CGFA can be easily established, and in the combustion zone, the image luminosity is uniform and no obvious flame fronts can be observed. Compared with flame mode, the combustion zone is enlarged in flameless mode and the temperature profile is more uniform. The effect of the tertiary air nozzle position on temperature profiles is greater than that of the secondary air equivalence ratio. And with the increase of λ2, the temperatures of zone close to the preheated fuel nozzle decrease. The oxygen concentration along the axis of the down-fired combustor is uniform with the peak value less than 6 vol% for all cases. The exit NOX emissions can be lowered down a lot for flameless combustion, and the general emissions are less than 100 mg/Nm3 (@ 6% O2) with the lowest value even less than 50 mg/Nm3 (@ 6% O2).