Experimental Study On Gasification Of Coal Char With Steam And CO₂

Coal gasification is the key and main technology of clean coal transformation and utilization technology. Generally, coal gasification technology practically use steam together with a certain ratio of air or oxygen as gasifying agent. If a certain amount of CO₂ was added in the gasifying agent to replace a part of steam, then higher-concentration CO gas or gas with different composition will be produced, which can meet needs of different gas utilization. At the same time, steam consumption can be reduced, that is, the treated volume of sewage can be reduced, so this technology has very important practical significance. Therefore, a project fund of youth innovation (2006QN38) supported by China Coal Research Institute has been approved, which priority research on the gasification characteristic of CO₂ substitute part of steam as gasifying agent.Three coal char samples made from three different rank of raw coals—Baoyi lignite, Shendong bituminite and Wangpo anthracite, were selected as study objects in this paper combined with the project. Besides basic coal quality analysis of raw coal and coal chars, specific surface area and pore diameter distribution test of coal chars also have been done. On this basis, coal char reactivity at normal pressure, with temperature 1173～1323K and four different mixture ratio of steam and CO₂ were studied systematically by using thermal analysis technology. Mixture reaction model was used to calculate kinetic parameters. Compensation effect between kinetic parameters was analyzed and synergistic effect of co-gasification of coal char with steam and CO₂ were researched in this paper. At last, tube type furnace experiments for one kind coal char (Baoyi coal char) have been done on normal pressure; coal gas composition and gas productivity were analyzed.Based on thermobalance experiment date carbon conversion, reactivity exponent (R) and mean specific rate were calculated, the result show that:(1) The reactivity of three kind of coal chars from high to low are: Shendong coal char>Baoyi coal char>Wangpo coal char. This result did not agree with the conclusion of predecessors, which is the reactivity of coal char currently reduce along with the increase of coal rank; this may be caused by different content of minerals and then different catalysis in raw coal. (2) The carbon conversion of three coal chars, at the same reaction time with all experiment mixture ratio of gasifying agent all increased, along with the increase of temperature, and the time needed to reach the lagest carbon conversion were shorten.(3) Except one or two experiment, the carbon conversion of three coal chars, at the same reaction time under all experiment conditions all increased, along with the increase of steam content and the decrease of CO₂ content in gasifying agent. The reaction rates of char-steam were lager than that of char-CO₂ at all conditions.(4) The reaction exponent and the mean specific reaction rate with the same mixture ratio of gasifying agent all increased along with the increase of temperature. Except one or two experiment, the reaction exponent and the mean specific reaction rate of three coal chars at the same temperature all increased along with the increase of steam content and the decrease of CO₂ content in gasifying agent.(5) Mixture reaction model were used to deal with experimental data, and the kinetic parameters n, k, Ea, lnk₀ were gained. The total reaction order of three coal chars were 0.7197～1.7846; the activity energy of three coal chars with 100% steam were 134.8～187.5kJ/mol, with 60% steam and 40% CO₂ were 119.4～180.8 kJ/mol, with 30% steam and 70% CO₂ were 136.9～199.7kJ/mol , with 100%CO₂ werel26.6～230.9kJ/mol. The higger the activity the lager the pre-exponential, kinetic compensation effect were observed between them, which can be explained by transition state theory.(6) When the mixture gas of steam and CO₂ were used as gasifying agent, synergistic effect on gasification reaction rate was observed for Baoyi coal char gasification with 60% steam and 40% CO₂ at 1323K, no synergistic effect on gasification reaction rate were found for other conditions. Obvious synergistic effects on gasification reaction rate were found for Shendong coal char and Wangpo coal char on all experimental conditions.Based on the coal gas composition of Baoyi coal char gasificatin in a tube type furnace at 1273K and normal pressure, the follwing result were gained:(1) Both steam and CO₂ participated in the co-gasification process of coal char with steam and CO₂.(2) Along with the increase of steam content and the decrease of CO₂ content in gasifying agent, at the same reaction time, the content of H₂ in the mixed gas decreased, the content of CO approximately increased, the content of CO₂ approximately increased and the content of CH₄ approximately decresed. (3) At the same reaction time, the value of H₂/CO reduced and the content of H₂+ CO₂ in the mixed gase decreased, along with the decrease of steam content and the increase of CO₂ content in gasifying agent.(4) Synergistic effects on gas productivity were observed during Baoyi coal char gasification with the mixture gas of steam and CO₂ in tube type furnace, and gas productivity is increased. The gas productivity of Baoyi coal char gasification with 100% H₂O is higher than with 100% CO₂, moreover the gas productivity of Baoyi coal char gasification with 100% CO₂ is not low.