Study On Catalytic Cracking Of Tar From Sewage Sludge Gasification

Sewage sludge gasification is a new technology of sludge treatments which is efficient,clean and feasible in economy and technology.However,tars are also produced during gasification.Tars can decrease the energy efficiency and cause a threat to pipe and equipment operation.Therefore,tar is a key problem for technology promotion of sludge gasification.Catalytic cracking of tar is recognized as more advanced and effective method,and it gets more attention from researchers.In this paper,a fixed-bed reactor was designed and built to study the characteristics of catalytic cracking of tar.Catalysts of Ni as active component were prepared with some additives,such as Fe,Ce,Cu,et al.The effects of catalytic properties and operation parameters on tar catalytic cracking were obtained.This study can provide a theoretical basis and data support for the development of sewage sludgegasification.Kinds of nickel-based catalysts were prepared by equal volume impregnation,and the palygorskite was used as carrier.The catalytic performance of sludge gasification tar model compound toluene over nickel-based catalysts was analyzed in a fixed-bed reactor.The effects of reaction temperature,steam/carbon molar ratio?S/C?,residence time,additives and Ni content on toluene conversion were investigated.The experimental results showed that with increasing reaction temperature,steam/carbon molar ratio?S/C?,residence time and adding additives,tar conversion increased clearly.H₂ and CO₂ content increased with the increase of water/carbon ratio and residence time,but the content of CO decreased.As reaction temperature increased,CO content increased while H₂ and CO₂ content decreased.The catalytic reactivity of catalysts and tar conversion decreased due to excess of additives.The increase of Fe content led to the improvement of toluene conversion in experimental process.As the amounts of Ce,Cu and Ca were over 3%,the conversion efficiency of tar decreased obviously,even lower than the conversion rate of tar without additives.At the different reaction temperature,Fe loading and steam/carbon molar ratio,the influences of nickel-iron catalysts supported by?-Al₂O₃ and 3A molecular sieve on the catalytic performance of tar were investigated.The results showed that there were the interactions between Ni and supports as well as Ni and Fe to form nickel-iron alloy and NiAl₂O₄ spinel,which can promote catalytic reforming of tar.The tar conversion improved with increasing temperature,while the conversion increased at first and then decreased with the increase in Fe content.As Fe content was 3%,the tar conversion was maximum.At the low steam/carbon molar ratio,the increase of steam can promote tar steam reforming.However,the excess steam kept from interaction between tar and active site and it resulted in low efficiency.The activities of catalysts of the different mixing ratios between?-Al₂O₃ and3A were indicated.The results showed that the catalytic activity and stability improved when Al₂O₃ ratio increased.However,the conversion rate of tar declined significantly due to the more ratio of Al₂O₃,and the best ratio of Al₂O₃ to prepare catalyst was 60%.