Research On Tar Formation During Fluidized Bed Coal Pyrolysis And Gasification Process

As one of the important research directions on Clean Coal Technology, Coal gasification technology is significant in highly utilization of energy in China. However, because of not working under the condition of high temperature and high pressure, the production of tar is inevitable. Tar may erode and jam the pipelines; on the other hand, it may aggrandize the difficulty of raw gas cleaning, and the soiling solution is hard to dispose. Not as the target product, tar produced in thermal cracking of coal can be cracked into gas as soon as possible, then not only the problems referred can be solved, but also the yield of gas can be increased, developed the clean and highly efficient utilization of coal, so it is significant to investigate the effect factors and fundamental removal method of tar.In this paper we give a detailed review on various factors affecting tar content. The factors in the gasifier include three parts: proper selection of operating parameters, use of bed additive/catalyst, gasifier modifications. The influncing parameters include temperature, pressure, residence time, heat speed, gasifying agents, coal, etc. The bed additives include dolomite, limestone, olivine and Ni-based catalysts. The proper gasifier design include secondary air injection to the gasifier, two-stage gasifier, cocurrent moving bed gasifier with internal recycle and separate combustion zone of pyrolysis gas, etc. The factors outside the gasifier include thermal cracking, catalyst cracking, wet method, dry method, etc. Each method has its advantage and disadvantage, a combination of both tar removal inside and ouside gasifer should be applied some time. The clean coarse gas flow chat was introduced.Cold trapping was used for tar sampling and gravimetric analysis of tar samples was performed to achieve tar content in flue gas. An experiment has been made on a 1 MW test facility, using limestone as the catalyst and the air and the recycled gas as the gas agent. On the multi-function test, gas from YanZhou and ZaoZhuang bituminite gasification was used as the source of tar, Tar content in the gas was influenced by the gasification conditions, such as the temperature, bed additive, gas agent(air gasification, N₂ pyrolysis and different amount of steam and air), which can help us to investigate the mechanism of tar production and conversion in gasification process.For the complex composition and large molecular weight, one set of composition analysis methods was erected for coal tar, in which tar samples are separated into Aliphatics, Arommatics, Easter, Polar and Aspheltenes fractions, and GC/MS is used for the characterization of each fraction except Aspheltenes. Much information of tar composition can be got and avail to analysis the mechanical of tar conversation.The tar cracking model was constructed. And we found that the activation energy of tar cracking in 900℃ coal gasification added the CaO is twice that of 800, Which can explain the...