The Experimental Study And Numerical Simulation Of Nitrogen Oxides Reduction By Gaseous Fuel Reburning

Nitrogen oxides (NOx) have been recognized as acid rain precursors that impose a significant threat to the environment. Coal combustion is a major anthropogenic source of NOx. Recently, China has specified more rigorous limits for the NOx emissions. Among the most recent developments for reducing NOx emissions, reburning technology is considered to be one of the most promising and cost-effective NOx reduction strategies for coal combustion systems. Therefore, it has great scientific and practical significances to investigate the reburning process.This thesis provided an overview of the present researches on the reburning, especially the reburning process by gaseous fuel. The overview revealed the key influencing factor of the gaseous fuel reburning process. Simultaneously, it made the object of this study clear. The following investigations are made in this thesis:Firstly, the GRI-Mech3.0 mechanism was applied to calculate the chemical kinetic reactions in the gaseous fuel (CH4) reburning process. The results revealed the critical reactions for the NOx reduction in the reburning process. The influences of reburn fuel quality, reburn fuel percentage of total heat input, reburn zone temperature, residence time and excess air on gaseous fuel reburning process were also analyzed by the chemical kinetic calculation.Subsequently,experimental studies on gaseous fuel reburning process have been performed in two one-dimensional coal combustion furnaces and a single-burner furnace. Detailed profiles of temperature, O2 concentration and NO concentration in these furnaces were measured. The influences of primary fuel (coal) quality, reburn fuel quality, reburn fuel percentage of total heat input, reburn zone temperature, residence time, excess air on gaseous fuel reburning process were also quantitatively analyzed. The results of the experiments showed that above 50% reduction of NOx emissions can be achieved under the optimizational reburn configurations. The appropriate amount of gaseous reburn fuel is about 10%~15% of the total thermal input. The reasonable residence time of reburn zone is about 0.6s~0.9s. The optimized average excess air coefficient for reburn zone is about 0.8~0.9. The reasonable temperature of reburn zone should not exceed 1500K.According to the experiments, a nitric oxide model was incorporated into a comprehensive coal combustion model for predicting NO reduction in the gaseous fuel reburning process. Profile comparisons showed that the modelling methodology deployed in this study was adequate to predict the overall combustion behaviour in the furnace. The reburning-NO submodel depicted quite well the observed behaviour of NO annihilation and was capable to properly predict the NO reduction levels in the reburning process, nevertheless it must state that the present model has somewhat under-predicted the NO concentration in the reburning process. The computationally economic reburning-NO submodel, on the basis of the"partial equilibrium"approach, requires the solution of only a few transport equations to simulate the complicated physical and chemical process inherent in the reburning technology. It is expected that this model represents a useful technique to simulate, design and optimize the practical gaseous fuel reburning process. It also provides a basis for further studies.Finally, the gaseous fuel reburning technology was successfully applied on the 1# 350MW boiler of Baoshan Power Plant. The operating results showed that the NOx exhaust emission was about 196mg/Nm3 (O2=6%), which has reached the NOx emission standard in developed country. The boiler after modified worked well and the equipments operated normally. The combustion efficiency and security of the boiler have not been changed. The investment and operating cost of the whole demonstration project was very low. It had good benefits in economy and environment. This demonstration project adopted the gaseous fuel reburning technology on 300MW grade unit boiler for the first time and filled the domestic gaps in the large-scale reburning technique applications. It provided a successful demonstration for the popularization and commercial application of reburning technology.