| Description |
Stoker coal-fired systems are recognized as the current field technology for medium capacity steam generation facilities. Very little emphasis has been placed on the control of pollutant emissions, particularly nitrogen oxides (N0X), from these systems. The purpose of this research was to identify and investigate those factors which influence N0X formation in fixed-bed combustion. The research concentrated on four combustion parameters found to affect NOx emissions from the coal bed: overall furnace stoichiometry; distribution of the combustion air (to provide staged and oxidant-rich conditions); superficial heat-release rate; and residence times of oxidizable species in the fuel-rich zone. The approach was experimental, utilizing a six inch by 10 inch fuel bed fixed in a removable reactor-drawer assembly. The furnace was operated in a batch mode to provide a Lagrangian simulation of the time/temperature/environmental history of a small bed segment in a larger combustion facility. The effluent gases were sampled with a water-cooled, stainless steel probe and continuously monitored for NO, CO, C02, and 02. A southern Utah bituminous coal, sized at one-half inch through one inch, was fired in all of the experiments. Typically, as a coal run proceeded, the superficial burning rate increased, reached a maximum and then decreased toward the run termination. Exhaust gas concentrations of NO displayed a similar trend with respect to time. However, NO emissions passed through a maximum prior to the peak combustion rate. Staging the combustion air resulted in a substantial decrease in NOx emissions. Exhaust concentrations decreased approximately linearly with decreasing bed stoichiometry. Overall stoichiometry had wery little effect on NO emissions under staged conditions. Overfire-air height (first-stage-residence time) was not of first order importance under the conditions of this work. Furthermore, the results showed a strong effect of burning rate on N0X formation. Increasing the bed heat-release rate increased the N0X emissions under both staged and excess-air conditions. |
