Steam Catalytic Cracking Of Coal Pyrolysis Tar Over Modified Iron-Based Oxides

Coal tar is an important product of coal pyrolysis,and clean fuel oil or valuable chemicals can be produced from coal tar.Steam catalytic cracking is considered as one of promising methods for the coal tar upgrading.The utilization of Fe2O3 has been attracted much attention because of its catalytic properties,low price and widely source.It has been studied extensively in the catalytic cracking of heavy oil.In this paper,Fe2O3 was modified by doping different metal oxides and the process parameters were optimized to improve the performance of iron-based oxides in steam catalytic cracking.By characterization of iron-based oxides,the effect of metal oxides doping on the structure and properties of Fe2O3,and the mechanism of metal oxides in steam catalytic cracking were clarified.The activity of steam catalytic cracking of coal tar was evaluated.Moreover,the maximum light tar yield was obtained by optimizing the process parameters.The main research contents and results are as follows:(1)A serial of iron-based oxides including ceria,zirconia,alumina,cobalt,nickel and barium oxides were prepared and used for the steam catalytic cracking of coal tar.The effect of different metal oxides doping on the products distribution was investigated.It was found that the specific surface area and pore volume increase while the crystal size decreases when Fe2O3 is doped with the investigated metal oxides.FeAl shows the largest specific surface area and pore volume.Doping of CeO2 or ZrO2 improves the steam catalytic cracking performance.FeZr shows the highest water conversion being 32.6 wt.%.FeCe can be reduced more easily with its lower reduction temperature and smaller reduction peak area as compared to other iron-based mixed metal oxides.FeCe shows the highest light tar content with 68.5 wt.%,and yield of light tar is 51.6 wt.%.CeO2 was further used as dopant,and several Fe1-xCex(x=0.05/0.10/0.20)was prepared and applied for the steam catalytic cracking.The role of Ce doping on Fe2O3 was recognized through several characterization techniques.With doping of Ce,the activity increased.Among these Fe1-xCex,Fe0.sCe0.2 shows best catalytic performance with the light tar content and yield up to 71.5 wt.%and 61.6 wt.%,respectively.Some Fe in Fe2O3 was replaced by Ce to form a solid solution,resulting in increase of specific surface area and oxygen vacancy concentration.Therefore,the steam catalytic cracking performance increase with Ce doping.(2)Al2O3/CeO2 and Al2O3/ZrO2 co-doped Fe2O3 were prepared,and the resulting modified catalysts including FeAlCe and FeAlZr and used for steam catalytic cracking of coal tar.It is shown that the prepared FeAlCe and FeAlZr could avoid excessive cracking of coal tar and improve the light tar yield.The light tar yield over FeAlZr1,FeAlZr2,FeAlCel and FeAlCe2 were 63.2,58.1,60.2 and 55.1 wt.%,respectively,higher than that on Fe2O3 being 49.7 wt.%.Co-doped Fe2O3 catalysts can increase the specific surface area and provide more active sites,which is beneficial for the cracking of coal tar.It was found that steam catalytic cracking of coal tar follows a partial oxidation mechanism and active oxygen species from steam dissociation and Fe2O3 catalysts could take part in the upgrading of coal tar.It is revealed that the specific surface area,pore volume,lattice oxygen and the O-on the Fe2O3 catalysts are the main factors in the determining the steam catalytic cracking performance.(3)The effect of process paramenters(including:reaction temperature,reaction time,oil/catalyst ratio,etc.)on the steam catalytic cracking performance of coal tar were studied.The results showed that when the reaction temperature,reaction time and oil/catalyst was 550?,60 min and 1.5 mL/g,respectively,the highest light tar yield is 69.2 wt.%.The regeneration performance of FeAlZrl was evaluated.The effect of regeneration times of FeAlZrl on the light tar yield and products distribution showed that the FeAlZrl keeps stable activity after used for 7 times,which indicated a good regeneration performance.The light tar yield was about 60 wt.%during 7 runs.