Experimental Study On Directly And Grade Catalytic Gasification Of Lignite To H₂/CH₄ Rich Gas

At present,oil and natural gas are decreasing,and the whole world is facing the great challenge of energy shortage.The energy structure “rich in coal and short of oil and gas” of China determines that coal will dominate our future energy utilization situation for a long period of time.Lignite is a low-rank coal with high volatile content and good gasification activity and rich in China.Therefore,it is of great strategic significance to alleviate the shortage of oil and gas in China by converting lignite into H2,CH4 and other gases with high heat value,low pollution,and wide application.This paper chooses the lignite as raw material,the catalytic gasification characteristics of lignite were investigated in two ways,direct catalytic gasification and staged catalytic gasification.The main conclusions are as follows.Firstly,Ca/Fe double active component composite catalyst was prepared by using cheap metal compounds as raw materials,the catalytic activity of the lignite and the accumulation of H2 and CH4 was studied.The results showed that the catalytic activity of Ca5Fe7.5 catalyst was the best in different Ca/Fe ratio catalysts.The carbon conversion of Erdos lignite coal was the highest,up to 76.27%,which was increased by a factor of 39.14% compared with that of non-catalytic gasification,which was increased by 105.41%,the yield of H2 was as high as 2.25L/g carbon,which was 1.86 times higher than that of non catalytic gasification,and the volume fraction of H2 increased by 15%.Secondly,the composition,morphology and reducibility of Ca/Fe composite catalysts were characterized.The catalytic reaction mechanism was studied,and the experimental results were fitted by the classical kinetic model.The main active substances in the composite were Ca2Fe2O33,CaFe4O7,CaFe5O7 and other oxides,Ca5Fe7.5 catalyst is the most reductive among them,is also the main reason for good catalytic activity.The mechanism of the catalyst was explained by the mechanism of oxygen transfer redox cycle,and the variation of the carbon conversion with time was best fitted by the homogeneous reaction kinetics model,the kinetic model was used to calculate the activation of Ca5Fe7.5 catalyst for 59.71kJ/mol,the activation energy of lignite steam gasification reaction decreased by 65%.Thirdly,the pyrolysis characteristics of lignite under different pyrolysis conditionswere studied,including the yield and composition of gas and tar and the structure and properties of semi-cokes.The results showed that the total pyrolysis gas accumulation the yield of H2 and CH4 achieved the highest under steam atmosphere while the tar yield is relatively low.That is because H2 O in the atmosphere is active,which can react with tar and carbon easily.The tar yield pyrolysed under nitrogen atmosphere was higher,but its composition was very complex and the utilization value was low.The yield was the highest when pyrolysed under hydrogen atmosphere,and rich in light aromatics,phenols and naphthalene.Although the yield of pyrolysis tar was the lowest pyrolysed under the steam atmosphere,but the composition was the most simple and the light hydrocarbon was the dominant,which took the advantages of easy separation and purification.In addition,the yield of pyrolysis gas increased but the tar and semi-cokes yield decreased with an increase in temperature.Then,the surface chemistry and crystal structure were analysed using FT-IR and XRD.The study found that,compared with nitrogen and hydrogen atmosphere,the oxygen content of semi-cokes pyrolysed under steam atmosphere was high,because H2 O itself contained an oxygen atom,thus it could easily interact with lignite,so reduced cracking reaction of hydroxyl,ether,ketone and other oxygen-containing functional group.As a result,the oxygen containing functional groups wre protected.In addition,steam could hinder the graphitization process and reduced the degree of graphitization.In summary,with the increase of pyrolysis temperature,the amount of oxygen-containing functional groups were fewer and the degree of graphitization were higher.Finally,the catalytic gasification activity of semi-cokes under different pyrolysis conditions were investigated.The results show that the H2 accumulation of three kinds of semi-cokes under different atmosphere ranked as follows: ESC3(H2O)>ESC2(H2)>ESC1(N2),and the rank of CH4 accumulation was ESC3(H2O)>ESC2(H2)>ESC1(N2).The H2 accumulation and selectivity decreased with the increase of pyrolysis temperature.The CH4 accumulation also decreased with the increase of temperature,but CH4 selectivity increased.In addition,when the temperature was lower than 823 K,the semi-cokes activity decreased rapidly,but when higher than 823 K,the activity decreased slowly with the increase of temperature.