The Experimental Study On Nitrogen Releasing Regularity And NO_x Reduction During The Coal Combustion

The potential coal and economy of coal-fired power generation lead to the condition that the coal consumption will be dominate among all the energy sources in the future in China. The coal smoke becomes the main source of air pollution, during which the SO_x, NO_x is the primary noxious substance. In order to actualize cleanliness and high efficiency combustion, it is urgent to control the pollutant produced during the coal combustion at the present time. Although studies on the rules of nitrogen releasing from the pulverized coal have been carried out widely, large capacity units now usually use blended coals, besides new combustion technique emerged, under these conditions the characteristics of NO_x emission still don't bring enough attention. All these result in the low efficiency of combustion and increasing of pollutant. This paper studied the rules of nitrogen releasing and the reduction of the nitrogen oxide comprehensively.First of all, the nitrogen releasing characteristics of different coals, which include NO and NO₂, were studied by test-bed of one-dimensional flame of pulverized coal, and tube furnace of static combustion. The two main nitrogen oxide products, NO and NO₂, were detected and analyzed. According to those, effect of coal quality, burning property and nitrogen form on the nitrogen releasing characteristic during combustion is obtained. The distribution* and existing form were founded out. The study emphasized on the factors that affect the concentration of NOx including coal property, structure and operation parameters. First, Research under different atmosphere on the releasing characteristic of NO_x emission was carried out. The results show that the concentration of NOx change evidently with the α if α <1 in one-dimensional flame of pulverized coal, Otherwise it changes indistinct. It was shown that the time of NO_x emission bring forward, under atmosphere mixed with CO, The releasing time of nitrogen in volatile and char approach, and the amount of NO_x reduced, In high temperature air combustion and oxyboosted combustion, the releasing time of nitrogen moved advance, and NOx congregate in the beginner of combustion, on the contrary nitrogen released separately and the amount of NO_x reduced greatly during the test of tube furnace of static combustion; Second, the thermal analysis indicates: the nitrogen oxides produce abundantly between the temperature of 600℃ and 800℃, the formation of NOx will bring forward in the condition of high heating rate, on the contrary the release will defer, the effect of heating rate to the NO_x emission is not distinct . The rules of NO_x emission along with the rate of burning out are as follows: the peak of NO_x come after the highest ratio of losing weight, thelower the rank of coal the earlier appear of NOx; Third, stage-air combustion has great impact on the emission of nitrogen oxides, in this paper, we find out that it is appropriate when the proportion of first and second air; Fourth, The element of nitrogen always form the intermediate, more nitrogen turn into N2 and the concentration of NOx is low if the coal of pulverized coal is more fine.Next, the characteristics of coal combustion were studied through the thermogravimetric analyzer, furthermore, the nitrogen releasing rules of blended coals were investigated. The time of ignition and burning out, the occurrence mode of nitrogen in coal and the difference of degree and process of single-phase or multiphase oxidation/deoxidize result in the complexity of the rules of nitrogen releasing characteristics. First, to the nitrogen releasing rate curves of blends, there are two peaks always, different components of the blends have their own igniting temperature and burnout time, so the components combust separately and the burnout time increase, the width of the peak widen, the main component plays a dominant action during the combustion of blend. Second, the concentration of NOx released by blend shows plus trait of component coals, but has small fluctuate, it is lower than the plus of components. When the proportion of two coals is 1:1, the releasing characteristic of nitrogen become worse because of tacking. Third, proper ratio of mixture can reduce the concentration of NOX, when HuangTai coal mixed with different coal type, the best scales are: HT:LC=1:3, HT:OC=1:3, HT:HZ=3:1.Finally, the effects to the efficiency of SNCR including type of reducing agent, temperature, scale of NH3/NO, residence time were studied comprehensively based on the research of nitrogen releasing characteristics. Ammonia and urea are in common use in SNCR, it indicated that the ammonia is the best to the efficiency of denitration, there exist a best temperature window, in our study it is between 900°C¹000"C, the best scale of NH3/NOx come between 1.0—1.6, the best residence time is about Is.This study explores the nitrogen release regularity combined with the practical engineering problems, it is hoped that there is some basal action on nitrogen capture during combustion, preventing high temperature corrosion of water-wall tubes and optimizing coal blending technology.