Fundamentals Of A Two-stage Coal Gasification With Low Tar Content

In order to effectively utilize powder coal and inhibit the formation of tar during gasification process, a novel two-stage gasification process, including a fluidized bed pyrolyzer and a downdraft fixed bed gasifier was proposed in this paper. This process is fit to treat powder coal and is easy to enlarge scale. It has the advantages of gas-solid mass transfer, heat transfer for the fluidized bed and effective tar removal in situ char for the downdraft fixed bed. Pyrolysis gas containing tar, goes through the char bed and then the tar is adsorbed and catalyzed reforming in situ by char produced from coal pyrolysis. Low tar gasification in a simple two-stage gasification process is realized and a clear fuel gas can be obtained.In order to remove the tar in pyrolysis gas effectively and reach the goal of low tar gasification, a lab-scale experimental apparatus was built. The thermochemistry fundamentals of coal pyrolysis in fluidized bed and tar removal in downdraft fixed bed were investigated in this paper. Firstly, this article investigated the characters of coal pyrolysis in a fluidized bed reactor. The effects of temperatrue, equivalence ratio (ER) and the mass ratio of steam to coal(S/C) on product distribution and reactivity of char and tar were studied. Then, the effects of removing tar of pyrolysis gas was studied at hight temperature in downdraft fixed bed.The influences of reaction temperature (1000-1200℃), second air/coal ratio (ER 0-0.06) and the gas residence time in char bed on tar content were investigated.With regard to fluidized bed pyrolysis, the gas yield increased with the increases of the temperature, ER and S/C, whereas the yields of char and tar correspondingly decreased. The simultaneous presence of O2 and steam in the atmosphere increased the specific surface area and reactivity of the produced char. However, the gasification reactivity of char produced at 900℃in N2 and/or in an atmosphere with an ER 0.22 at 850℃was low because of occurrence of carbon graphitization. The inclusion of O2 and steam into the atmosphere had a great affect on the tar properties. Compared with the pyrolysis in pure N2, the tar produced in such atmospheres contained fewer single-ring aromatics and phenolic, ketonic and aliphatic hydrocarbons, which makes the tar more reactive for cracking and reforming. The optimal operating conditions were 850℃, ER=0.15 and S/C=0.15.The tar content of fuel gas was decreasing, with the increase of temperature, senond air and gas residence time. The tar content reduced to 0.513 g/Nm3, We can found that polymerization reacton, cyclization reaction of tar happened at high decomposition temperature and the tar components were decreasing from the GC-MS ananlysis of tar produced from different temperature. The contents of H2 and CO were increased distinctly, but the CH4 and C2H4 was reduced with the increased of reaction temperature in the second stage.When the ER was 0.06, the content of tar reduced to 0.233 g/Nm3. As the GC-MS analysis suggested, the oxo heterocyclic compounds were decomposed, but polycyclic and high C/H ratio hydrocarbons were increasing; The contents of H2, CH4 and C2H4 decreased with ER increase, but CO and CO2 were increased.The catalysis of the char is more effective, with the increase of gas residence time. When the residence time was 6.48s, the tar decrease to 86 mg/Nm3. The GC-MS analysis shown that the tar components were some stable and refractory heavy hydrocarbons at longer residence time. Lengthening residence time remove these materials incompletely, but it can reduce the contents of the tar.