Research on enhanced denitrification technology in pulverized coal combustion process

In order to further promote the denitrification efficiency in the combustion process,especially to solve the problem of high NO_x emission in the furnace when burning low volatile meagre coal or anthracite,a technology with ammonia injection denitrification in the high temperature intensified reduction zone-multi-staged enhanced reduction pulverized coal combustion technology( referred to as MERC)was proposed based on the reducing NO mechanism by ammonia.By means of a 50 k W down-fired furnace facility,the effects of key technical parameters,i.e.,ammonia-nitrogen ratio( RNSR),mixing degree of reductant and flue gas,and carrier gas type of reductant on denitrification efficiency were studied.The comparative experiments of MERC and SNCR were carried out,and the engineering application test of MERC was performed.The results show that the mixing degree of reducing agent and flue gas in the furnace can be improved by using double-spray guns,and thus,the denitrification efficiency can be increased from 45% to 70%.The NO concentration is between 500-700 mg/Nm~3 when air acts as a carrier gas and the ammonia-nitrogen ratio exceeds 1.2.The NO_x concentration rises firstly and then falls as the ammonia-nitrogen ratio increases.When the flue gas is used as the carrier gas,the NO_x concentration is greatly reduced to be only ca.100 mg/Nm~3.The results show that the lower oxygen content in the carrier gas of the reducing agent,the more beneficial it is to enhance the effect of ammonia reduction in high temperature.The reducing agent carrier gas needs to reduce oxygen content as much as possible or use oxygen-free medium. By comparing the results of SNCR and MERC experiments,it is found that the NH3 consumption of conventional SNCR is higher than that of MERC when the ammonia-nitrogen ratio is less than 2,then the reaction of NH3 with NO tends to be saturated after the ammonia-nitrogen ratio exceeds 2.5,and the excess NH3 reacts with SO_2 to form( NH4)2 SO3 and NH4 HSO3,which is the reason of clogging of the air preheater caused by excessive SNCR injection in the project.Laboratory research results are further practiced in engineering.Low nitrogen technological reform was applied to 125 MW coal-fired boiler in a power plant.Since the No.1 and No.2 boilers in this power plant were fed anthracite+lean and coal+bituminous coal mixture,the concentration of NO_x at the outlet of the boiler was unstable since the retrofit of the low nitrogen combustion system in 2015.The inlet of the tail denitrification unit is below 500 mg/m~3 and the maximum is 1 200 mg/m~3.In order to ensure that the retrofit achieves the target value of ultra-low NO_x emission,the urea spray gun before exhaust air was added on the basis of the original design scheme of combustion optimization test+SNCR retrofit+SCR optimization.The results show that the denitrification efficiency in the furnace is higher than 65%.Combining with SCR device at the tail of the boiler,the ultra-low emission of NO_x concentration at the chimney position is achieved,which is not higher than 30 mg/m~3.Eventually,the MERC application was realized,and the field test of retrofit project proved the MERC application achieved the desired effect.