Experimental Investigation On Char Nitrogen Conversion During Coal Char Combustion

China is the largest coal consumption country in the world. For our country, thecoal-dominated energy structure will not change in the short term. A large numberof NOx and other pollutants produced during the combustion of coal have causedserious environmental problems. In order to use coal resources more environmentalfriendly and meet the increasingly stringent emission standards, it is necessary tocarry out in-depth experimental study of fuel nitrogen conversion during coalcombustion.Three typical kinds of coal (lean coal, bituminous coal and lignite), which areat different coal rank, have been selected. The coal char samples were preparedunder three different pyrolysis temperatures (900℃,1000℃,1100℃) in an inertatmosphere in the one-dimensional tube furnace. At the same time, physicalproperties of parent coal or coal char samples such as porosity analysis, XPSanalysis and coal char secondary pyrolysis characteristics have been studied. Mostof the coal volatile has fully released during coal pyrolysis process, and coalparticles undergo obvious volume swelling. The pore structures of coal charparticles are more complex than parent coal particles, and the higher pyrolysistemperature or lower parent coal rank, the more complex pore structure of coal charparticles. Nitrogen-containing functional groups in coal char are mainly N-6and N-5. As the pyrolysis temperature increased, N-6levels tend to increase, N-5levels arerelatively stable, while N-Q levels tend to decrease. The char secondary pyrolysisprocess is more difficult than that of coal pyrolysis. The major nitrogen-containinggasses released during slow secondary pyrolysis of coal char are N2O, HCN, andNO, while it is mainly HCN during fast secondary pyrolysis. After the release of thesmall amount of residual volatiles in coal char during secondary pyrolysis process,the coal char surface area increases significantly, and it is depended on the way ofsecondary pyrolysis and coal char categories.A horizontal tube furnace and a vertical fixed bed reactor have been used tostudy the conversion law of char N to NO under different fixed-bed combustionconditions. Char N is gradually released accompanied the combustion processduring coal char combustion. The coal char particles are at batch condition duringthe experiment, and the more coal char loading, the stronger the secondaryreduction reaction of NO, and the final NO production amount is less. The mixtureof quartz sand could reduce the interaction between char particles, which wouldweaken the secondary reduction reaction of NO. The interaction between charparticles in fixed bed reactor is much stronger than that in the horizontal tubefurnace, so the conversion rate of char N to NO is lower. In the temperature range of 850℃~1050℃, as the reaction temperature increased the conversion rate of char Nto NO increases firstly and then decreases. In the oxygen concentration range of2%~20%, the conversion rate of char N to NO increases at first and then decreasesas the oxygen concentration increased; and the higher the reaction temperature, thegreater the oxygen concentration corresponds to the maximum conversion rate ofchar N. With the increase of parent coal rank, the increase of coal char nitrogencontent and the decrease of ash content, the conversion rate of char N to NO showsincreasing tendency.