Catalytic Upgrading Of High Temperature Coal Tar Over Activated Carbon Catalysts

High temperature coal tar has the characteristics of high content for heavy components and poor quality,which is not conducive to its industrial application.Catalytic cracking is an important way to improve the quality of tar,which convert the heavy components into light tar.Activated carbon（AC）is used as catalyst for tar catalytic upgrading because of its rich specific surface area and developed pore structure.In this dissertation,commercial activated carbon was chosen as catalyst for coal tar upgrading,and further modified by physical activation,chemical activation and metal loading to improve its catalytic performance.To improve the tar yield,catalytic upgrading of coal tar coupled with methane dry reforming was proposed.By this way,the yield and quality of light tar were improved via the suppression of excessive tar cracking by the combination of cracking fragments of tar with small molecule free radicals from methane dry reforming.The main work and results are as follows:Commercial activated carbon（AC）was chosen as catalyst and modified by steam activation and KOH activation.The effects of reaction conditions,KOH/AC mass ratio,steam dosage and ash removing on the structure of activated carbon and the catalytic upgrading effect of heavy coal tar were investigated.The results showed that the optimum reaction temperature is 550 ^oC and the optimum amount of catalyst is 1.0 g.Both KOH activation and steam activation can improve the structural properties of activated carbon.When KOH as activator,with the increase of KOH/AC mass ratio,the specific surface area first increased and then decreased.When the KOH/AC mass ratio was 2,the resultant AC-2AT reached the largest specific surface area of 799 m²/g.In comparison,the specific surface area increased with the increase of steam dosage.When the steam dosage was 0.25 m L/min,the specific surface area of AC-0.25ST reached the maximum.AC-2AT and AC-0.25ST had a large number of mesopores to provide more active sites,thereby showing excellent catalytic performances,which promoted the tar upgrading and the generation of gases such as H₂ and CH₄.AC-0.25ST has more abundant pore structure,which makes the content of light tar as high as 90.5%,but the yield of light tar is reduced;while AC-2AT improved not only the content of light tar,but also the yield of light tar.AC-0.25ST and AC-2AT showed high selectivity to naphthalenes.The content of naphthalenes in the upgraded tar reached 50.3%and 47.2%,respectively.It was first revealed that there exists a good linear relationship between the mesoporous surface area and the content of light tar.Additionally,the removal of ash from activated carbon was not conducive to the tar upgrading,resulting in the reduce of light tar content.Fe/AC,Co/AC and Ni/AC were prepared by impregnation method,and their performances in catalytic tar upgrading were investigated.The results showed that the loading of metal led to the reduction of specific surface area.The supported catalyst was conducive to the conversion of heavy components into light tar,which was related with its rich porous structure and high dispersion of metals.The contents of 4-rings and 5-rings heavy aromatics in tar were decreased.And the content of light aromatics was increased,especially naphthalene,biphenyl and anthracene compared with thermal cracking,which increased from 32.0%,2.5%and 0.3%to39.2%,6.9%and 16.1%over Co/AC,respectively.With the increase of Ni loading,the yield of light tar first increased and then decreased,and the maximum yield of light tar was 49.8 wt.%and the content of light tar was 84.0%over 10Ni/AC.The regeneration of 20Ni/AC in steam atmosphere found that its catalytic activity gradually decreased with increasing regeneration times.Ni/AC-ST were prepared by first impregnation and the subsequent steam activation,and its performance on catalytic upgrading of coal tar in N₂ and CH₄/CO₂ atmosphere was studied.The results showed that the catalysts had higher specific surface area and pore volume.Ni/AC-ST catalyst was conducive to tar upgrading because of Ni loading and rich specific surface of the catalysts,which provided more active sites for tar cracking.With the increase of Ni loading,the specific surface area and pore volume gradually decreased,and the performance for catalytic upgrading was weakened.The 5Ni/AC-ST exhibited best catalytic effect,and the content of light components increased from 65.0%to 90.5%in N₂ atmosphere,while the yield of light tar decreased from 46.6 wt.%to 41.1 wt.%.In CH₄/CO₂ atmosphere,Ni/AC-ST catalyst significantly improved the content and yield of light tar.The content of light components was further increased to 94.5%,and the yield of light tar was increased from 41.1 to 61.7 wt.%compared with N₂ atmosphere,which was attributed to the fact that the free radical fragments generated in the process of methane dry reforming could stabilize the tar cracking molecules,inhibiting the occurrence of excessive cracking reaction and polycondensation reaction.Therefore,under the CH₄/CO₂ atmosphere,the yield of light tar could further improve.