Effects Of Heat Treatment On Structure Of Lignite-supported Zinc-based Sorbent And Desulfurization Performance In Mid-temperature Coal Gas

Lignite is an abundant and cheap resource in China. It can be used as thesupport of sorbents because of its certain mechanical strength, porous structure,and plentiful functional groups on the surface. The pyrolysis of coal is a processincluding complicated physical and chemical reactions, during which thestructure of support can be modified and the active component on the supportcan be finely dispersed. The procedure of impregnating the precursor solution ofthe active component directly onto the lignite followed by pyrolysis cancombine the preparation of semi-coke with the thermal decomposition of theprecursor of active component, which can simply the preparation process andsave energy. In our preliminary work, the advantages of high pressureimpregnation in preparing sorbents had been studied, and the optimumimpregnation conditions was obtained: zinc nitrate precursor solution concentration of36wt%, impregnation pressure of20atm and impregnationtime of5h. The change of lignite support structure, the occurrence mode anddistribution behavior of metal active component and the effects of operationparameters of pyrolysis during the process of pyrolysis of impregnated lignitewere the focus in this paper.On this basis, lignite after high temperature impregnating zinc nitrate waspyrolyzed in different conditions to prepare sorbent for coal gas desulfurization.Effects of conditions such as temperature, holding time, heating rate, gas flowrate during temperature programmed pyrolysis and fast pyrolysis process on thestructure and the desulfurization activity of the sorbent have been studied, andthe best preparation conditions were screened out. The impacts of pyrolysisconditions on the crystal form, loading amount and distribution of the activecomponent, and the pore structure of the support which were characterized byXRD, AAS, TEM and N2adsorption had been analyzed and discussedcorrelating with desulfurization performance of the sorbents. The conclusionsare shown as follows:1) The structure of sorbent can be modified significantly by heat treating.The temperature range of temperature programmed pyrolysis is370-600oC,during the pyrolysis of impregnated lignite, the specific surface area increasesgradually from12.043to187.795m2/g as the rise of temperature. Temperaturerange of fast pyrolysis is450-600oC, the specific surface area of sorbent alsoincreases gradually from16.712to133.596m2/g with increasing fast pyrolysis temperatures. The increase of specific surface area might be attributed to theincreased micropores.2) The sorbents prepared by the impregnated lignite through temperatureprogrammed pyrolysis show a decrease of the yield of solid product and agradually increase of active component (from16.97%to22.94%), under thecondition of temperature programmed pyrolysis with the final temperatureincreases. But due to agglomeration of high content active component, theeffective utilization rate of active component reduces with a lowerdesulfurization performance. The optimum condition of preparing the sorbentusing temperature programmed pyrolysis method is: pyrolysis temperature of550oC, holding time of180min, heating rate of10oC/min. The breakthroughtime and sulfur capacity of the sorbent at400oC was720min and25.91g S/100g ZnO, respectively.3) The sorbents prepared by the impregnated lignite through fast pyrolysisshow a worse desulfurization performance with a decrease trend of the activecomponent content (from18.91%to15.49%) as the pyrolysis temperatureincreased during the fast pyrolysis process. The optimum condition is: pyrolysistemperature of450oC, holding time of180min and gas flow of500ml/min.The breakthrough time and sulfur capacity of the sorbent at400oC was1980min and26.92g S/100g ZnO, respectively.4) Pyrolysis of the impregnated lignite in certain condition is an effectivemethod for preparation of the mid-temperature desulfurization sorbent, as it can modify the porous structure of the sorbent and the loading amount anddistribution behavior of the active component on the sorbent effectively.