In-situ Upgrading Of Coal Pyrolysis Tar With Steam Catalytic Cracking

Low-rank coals reserve account for a large proportion of total coal resources in China,and are characterized by high content of volatiles,moisture and oxygen,but low heating value.Pyrolysis is an important technology for achieving clean and efficient utilization of low-rank coals.However,owing to the low H/C molar ratio of coal,the tar produced by coal pyrolysis usually contains high pitch component?boiling point above 360 ??,which reduces the quality of tar and blocks subsequent transportation pipeline.Therefore,improving the coal pyrolysis tar quality is an urgent problem to be solved.Hydrogenation pyrolysis have been used to improve tar quality.Since hydrogen is expensive and the production of H₂ requires extra equipment,water can be use as an inexpensive hydrogen substitute.In this paper,steam catalytic cracking and in-situ catalytic cracking of pyrolysis tar were combined to improve the quality of tar by optimizing the catalyst and reaction conditions.In this paper,Pingshuo coal was used as coal sample,and Ni/Al₂O₃ catalysts with different nickel contents were prepared by impregnation method to investigate the effects of nickel content on light tar yield,water conversion and upgrading effect.The results showed that Ni/Al₂O₃ catalysts could enhance the water conversion and light tar yield.At the pyrolysis temperature of 650 ?,the catalyst with Ni content of 4 wt.%?4Ni/Al₂O₃?exhibits the best performance,contributing to the highest water conversion of 4.74 wt.%and light tar yield of11.90 wt.%,respectively.Correspondingly,the content of light fraction?boiling point less than360 ??in tar sample increases from 48.0 wt.%to 67.0 wt.%,and the average molecular weight decreases from 353 amu to 253 amu over 4Ni/Al₂O₃.This is consistent with the increase of benzenes,phenols and naphthalenes,and the decrease of long-chain aliphatic hydrocarbons,3-and 4-ring aromatics in upgraded tar.In addition,the higher ratio of uncondensed aromatic protons to condensed aromatic protons in upgraded tar reveals the fact that 4Ni/Al₂O₃ can promote ring-opening reaction of PAHs,which can well interpret the tar upgrading process.On the basis of 4Ni/Al₂O₃ catalyst,ZrO₂ was added as an auxiliary agent to enhance the effect on improving coal pyrolysis tar quality.4Ni/ZrO₂-Al₂O₃ catalysts with different ZrO₂contents were prepared by two-step impregnation method to investigate its effect on the quality improvement of pyrolysis tar.The results showed that 4Ni/ZrO₂-Al₂O₃ catalysts could increase the light tar yield as compared with both original tar and tar over 4Ni/Al₂O₃.Among these prepared catalysts,the catalyst with ZrO₂ mass content of 5 wt.%?4Ni/5ZrO₂-Al₂O₃?shows the best performance,contributing to the highest tar and light tar yields,being 18.68 wt.%and12.61 wt.%,respectively.In the upgraded tar,the light tar content increases from 48.0 wt.%to67.5 wt.%,while pitch content decreased from 52.0 wt.%to 32.5 wt.%.The mechanism of in-situ steam catalytic cracking upgrading of coal pyrolysis tar was explored by isotope tracer method.Using D₂O as a tracer,²H NMR and GC/MS analysis of original and upgraded tar was performed.The significant deuterium peak appeared on the upgraded tar spectrum,indicating that D₂O was involved in the catalytic upgrading of tar.According to GC/MS analysis,the presence of·D in coal tar further proves that steam can participate in the tar catalytic upgrading process.The free radicals generated by tar catalytic cracking were stabilized by the active hydrogen radicals obtained by steam catalytic cracking,thereby improving light tar amount.