Effects of the Secondary Air Excess Ratio on the Self-Preheating Combustion Characteristics and NOx Emission of Semi-Coke

The effects of the secondary air excess ratio (lambda(2)) on the self-preheating combustion characteristics and NOx emission of semi-coke have been experimentally studied on a bench-scale test rig. Flameless combustion of the high-temperature preheated fuel (coal gas & coal char) has been achieved in all experimental cases. Through fire-observation windows, the combustion zone was transparent and no flame fronts were seen. Additionally, different lambda(2) in this study were basically within the common range of self-preheating combustion technology. The results manifested that the rapid ignition of coal gas and the recirculation of high-temperature flue gas resulted in relatively high combustion temperatures close to the secondary air nozzle exit. With the increase of lambda(2), the unburned carbon content of fly ash increased slightly and combustion efficiencies of all experiments exceeded 98%. Moreover, the exit NOx emission was not reduced linearly, which was dissimilar from conventional air staging combustion. The main reason was that the volatile-N was released in the self-preheating device, and the air staging method could not effectively reduce the char-NOx. In the course of the experiments, the lowest NOx emission was 64.35 mg/m(3) (@6%O-2) when lambda(2) was 0.40.