Study On Low No_X Combustion Coupled With SNCR And SCR To Control No_X Emissions

Nitrogen oxides in the atmosphere primarily come from industrial pollution, domestic pollution and traffic pollution. In China, about65%of NOx is produced by pulverized coal combustion, and the amount of NOx released from pulverized coal boilers accounts for about80%of NOx produced by pulverized coal combustion. From1998to2007, the total NOx emissions of coal-fired power plants in China is growing year by year, therefore, it is a very urgent task to reduce the NOx emissions of coal-fired boilers.This article,whose theme is to reduce the NOx emissions of coal-fired boilers, investigates the low-NOx reform of pulverized coal burner, the split-air arrangement in furnace and the flue gas treatment technology. Experimental and numerical methods are conducted to study the features these low NOx projects, denitrification efficiency and the impacets on steam parameters.Fistly, experiments were conducted on a300MW boiler to investigate the effects of using low NOx burners and the split-air technology on NOx emissions. Three layers of SOFA wind were arranged in the upper part of the furnace, and the amount of SOFA wind accounted for about20%of the amount of secondary ai. As a result, the main combustion region in the furnace was in an oxygen lacking condition and the combustion temperature was lower, making the NOx was reduced. This transformation project could significantly reduce NOx emissions without reducing the boiler efficiency. NOx emissions were reduced from850mg/Nm3to425mg/Nm3,and the denitrification efficiency was up to50%.Secondly, the effects of the SOFA wind swing on the furnace temperature, NOx emissions and boiler efficiency were studied under different boiler loads:(1) the swing of the SOFA winds had little effect on the furnace temperature field.(2) when the boiler load was300MW, the up swing of the SOFA winds could reduce the concentration of NOx in flue gas by20ppm or so. Howerer when boiler load was 270MW or240MW, the lowest NO content in the flue gas was detected when SOFA wind had no swing.(3) under the condition of300MW, the boiler efficiency reached the top value when the SOFA wind swinged down, and the efficiency increased as the SOFA wind swinged up. On the contrary, the efficiency of240MW increased as the SOFA wind swinged up.Thirdly, it is introduced that the SNCR denitrification system was first installed on the300MW unit. Using urea as a reducing agent, the effects of different gun combinations and installation locations of guns on the NOx emissions were investigated.The optimal spray gun combinations and optimal spray gun positions were found under different loads. According to above research and analysis, the denitrification efficiency of the300MW reached the highest when the upper two long guns and the upper short guns were put into operation. CFD simulation were conducted to calculate the distributions of temperature, important components and urea pyrolysis process, and especially the effects of different NSRs and different pulverized coal concentrations of the third wind on denitrification efficiency and ammonia escape.(1) With the increase of the NSR, the NOx removal rate increase meanwhile, but at the same time the ammonia slip would also increase. Consenquently, to chose a proper NSR not only the NOx removal rate but also the ammonia slip should be considered.(2) When the powder rate in tertiary air was less than10%, denitrification efficiency decreased with the increase of the powder rate in tertiary air. When the powder rate in tertiary air was more than10%, denitrification efficiency would be increased. But the amount of ammonia slip would also increased with the increase of the powder rate in tertiary air.(3) urea was pyrolysised completely soon after injection into the furnace, which was a common phenomenon in different conditions, for that the heat of urea pyrolysis was very small compared to the amount of heat contained in flue gas. So despite the different temperature distributions in the furnace of different conditions, the flue could provide sufficient energy to ensure that urea be completely pyrolysised.Finally, numerical simulation methods were conducted on a600MW boiler to study the denitrification effect of SNCR+SCR hybrid denitrification system. Results revealed that SNCR denitrification efficiency was40.6%, SCR denitration efficiency was43%, and the integration denitrification efficiency was66%. After the SNCR+SCR denitration, the NOx concentration in the flue gas is83.6mg/Nm3and the ammonia slip is about3.2ppm, which meeted the requirements of the national coal-fired power plant boiler NO emission.