COAL COMBUSTION EFFICIENCY IN CFB BOILER

CHINA PARTICUOLOGY Vol 3, Nos 1-2, 2005COAL COMBUSTION EFFICIENCY IN CFB BOILERHairui Yang and Guangxi YuAuthor to whom correspondence should be addressed. E-mail: yhr@mail. tsinghua eoAbstractThe carbon content in the fly ash from most Chinese circulating fluidized bed( cFB )boilers is much higher than expected, thus directly influencing the combustion efficiency. In the present paper, carbon burnout was investigated both infield tests and laboratory experiments. the effect of coal property, operation condition, gas-solid mixing, char deactivation.residence time and cyclone performance are analyzed seriatim based on large amount of experimental resultsA coal index is proposed to describe the coal rank, defined by the ratio of the volatile content to the coal heat value, is auseful parameter to analyze the char burnout. The carbon content in the fly ash depends on the coal rank strongly CFBboilers burning anthracite, which has low coal index, usually have high carbon content in the fly ash On the contrary, theCFB boilers burning brown coal, which has high coal index, normally have low carbon contentPoor gas-solid mixing in the furnace is another important reason of the higher carbon content in the fly ash. Increasingthe velocity and rigidity of the secondary air could extend the penetration depth and induce more oxygen into the furnacecenter. Better gas solid mixing will decrease the lean oxygen core area and increase char combustion efficiencyThe fine char particles could be divided into two groups according to their reactivity. One group is"fresh "char particleswith high reactivity and certain amount of volatile content the other group of char particles has experienced sufficientcombustion time both in the furnace and in the cyclone, with nearly no volatile. These" old"chars in the fly ash will bedeactivated during combustion of large coal particles and have very low carbon reactivity. The generated fine inert charparticles by attrition of large coal particles could not easily burn out even with the fly ash recirculation. The fraction of largecoal particles in coal feed should be reduced during fuel preparation processThe cyclone efficiency controls the particle residence time in CFB loop, especially for that of the fine particles. So thecyclone efficiency, especially the cut size, will greatly influence the carbon content in the fly ashKeywords CFB boiler, combustion efficiency, char, fly ash中國煤化工CNMHG