Operational Characteristics Of NO_x Emission Control System Of A Large-scale Pulverized Coal-fired Utility Boiler And The Optimization

Nowdays,it is of common practice for large-scale pulverized coal-fired power plants in China to be frequently operated at partial loads for a long time,which may significantly affect the NOx emission control and economic performance.The work presented in this thesis was addressed to the problems of high NOx emissions during lower load operation and co-firing lean coal in a 1000 MW ultra-supercritical coal-fired utility boiler.Based on the historical operation data analysis,combustion tuning tests,computational fluid dynamics(CFD)numerical tests,and the economic analysis using Integrated Environment Control Model(IECM),the NOx emission characteristics of the unit boiler,the operational optimization for controling NOx production,and the influence on the economic performance of the NOx emission control systems are systematically studied.Based on the historical operation data,the operating characteristics of low NOx combustion system and SCR system are analyzed and assessed.The results show that NOx emission from the furnace increases with the decrease of the operation load,and the NOx emission concentrations can be as high as 600-700 mg/m3 at low load operation with lean coal co-firing.Combustion operating conditions especially at low load operation,including the boiler operating excess oxygen level,mills in service,the distribution of coal feeding rates,and SOFA ratio,all have the influence on the NOx emission concentration to certain degree.The effect of coal quality on NOx formation is significant,and co-firing lean coal leads to a significant increase in NOx concentration over the whole operating load range.Low-load operation and co-firing lean coal lead to a high NOx emission concentration,which consequently result in a significant increase in the ammonia injection and the operating cost of the SCR system.Analysis on the operating characteristics indicates that the tuning and optimization of the main operating parameters of the combustion system during co-firing lean coal at low-load operation have the potential to reduce boiler NOx emissions and thereby to reduce SCR operating costs.The in-situ combustion adjustment tests have also proved that co-firing lean coal at low load operation can lead to high NOx formation.However,whether operating at high load or low load,regardless of burning sole bituminous coal or co-firng lean coal with bituminous coals,the combustion tuning and optimization are able to significantly reduce NOx emission concentration,and maintain the amount of ammonia injected into the SCR system at the lowest levels of daily operation.It demonstrates the possibility of improving the economic performance of the whole NOx emission control system especially at the low load operation through combustion optimization with considering the operation of SCR system.The NOx emission charactreristics of the boiler and high temperature corrosion tendency of waterwalls were analyzed by CFD simulations.The results show that using a large quantity of the overfire air is beneficial to reducing NOx emssions,but is likely to lead to an increase in high temperature corrosion of waterwalls because of the increasingly enhanced reducing atmosphere.Changing the mill in service has a significant effect on NOx emission concentration.The main reason is that it changes the position of the main combustion zone,which in turn changes the residence time of the flue gas in the reduction zone.The mid four mills operation mode is the preferred for low load operation because it not only achieves a lower NOx concentration but also reduces the risk of high temperature corrosion.Econimy evaluation model of the IECM control is used to analyze the influence of the boiler combustion operation on the economic performance of the low NOx control systems of the unit.The results show that unit load and coal quality are the key factors affecting the operation costs of the NOx control systems.The lower the operation load,the higher the costs of the power generation and NOx reduction.With the decrease of the unit load,the impact of coal quaity on the cost of power generation is more significant.Whether the high or low load,regardless of burning sole bituminous coal or co-fimg lean coal,both the annual total operation cost and the operation cost of unit power generated are reduced through the combustion optimization despite the cost of unit NOx removal increasing.These demonstrate that the combustion optimization not only reduces NOx emission from furnace but also is able to improve the economic performance of the operation of the NOx emission control systems,and it is a reasonable and effective technical measure.