In Situ Catalytic Cracking Of Organic Solid Waste Gasification Tar With Char Based Catalyst

Converting organic solid waste（OSW）into syngas by thermal conversion is an effective way to utilize solid waste,which not only avoids the problem of increased processing difficulty caused by the complexity of OSW raw materials,but also can be combined with downstream processes to its resource utilization.However,due to the high volatile content of OSW,tar is inevitably generated during the gasification process,which affects the element utilization rate of the entire unit,and also causes pipeline blockage and deactivation of catalysts in downstream processes.Catalytic cracking is a commonly used tar purification method,but OSW has high water content and high gasification tar content.Cracking catalysts not only need to maintain activity and stability in an environment with water and oxygen,but also the crude synthesis gas with tar is usually untreated.With only cooled high temperature gas,this harsh scenario presents a great challenge for catalyst design.Considering the above scenario of OSW gas production purification,the cost and source of raw materials shall be considered for the purified catalyst,and regeneration shall be avoided as far as possible after catalyst deactivation.CB-cat（char-based catalyst）is the residual solid product after the thermal decomposition of biomass or coal in inert atmosphere.It is used in the catalytic cracking of tar due to its low cost,easy access,rich surface pore structure and relatively stable physicochemical properties.However,the catalytic activity of CB-cat is not high,and carbon materials will react with water vapor at high temperature and be consumed.To solve these two problems,this paper first improves the catalytic activity of CB-cat by modifying it.In addition,a catalytic reactor that can remove the deactivated catalyst in situ is designed,so that the deactivated CB-cat can be used as raw material into the gasifier while removing the tar in situ from the crude syngas,avoiding the regeneration problem after catalyst deactivation.Four typical agricultural wastes are selected as the raw materials for catalyst preparation in this paper.Taking the carbon content of biomass and the specific surface area of char as the main evaluation criteria,the best raw materials and temperature for CB-cat preparation are selected.On this basis,the structural characteristics of CB-cat obtained by five different preparation methods were compared.Toluene,a typical component in tar that is difficult to dissociate,was used as a model compound to evaluate its performance,and the catalytic cracking mechanism of char to coke oil was explored by comparing the changes of catalyst structure in the reaction process.In addition,the catalyst with good catalytic performance for toluene was used for in-situ catalytic removal of real organic solid waste gasification tar.Finally,the three-dimensional model of the tar catalytic cracking reactor was built by using the multi physical field coupling simulation software,and the structure of the reactor was designed and optimized.The main results and conclusions are as follows:（1）The specific surface area of char prepared from four kinds of biomass:walnut shell>corn straw>rice straw>corn cob.Therefore,walnut shell is selected as the raw material for CB-Cat in this study;The char structures prepared at different temperatures were analyzed by physical adsorption,Fourier transform infrared spectroscopy,and Raman spectroscopy.It was found that the specific surface area and disorder degree of the char at 700°C showed the highest value,and the oxygen-containing functional groups increased with the increase of temperature.The temperature increases gradually and disappears,so the optimal preparation temperature of CB-Cat is 700°C;（2）Five different structures of CB-Cat were prepared by direct pyrolysis,acid-base pretreatment,water vapor activation and KOH post-treatment,and their properties were evaluated with toluene as a model compound of tar.The results showed that:The char prepared by KOH pretreatment of walnut shell raw material has the best cracking activity to toluene,and the toluene conversion rate can reach 92%(700°C,2769.23 h^-1)at the beginning of the reaction;（3）CB-cat with direct pyrolysis,KOH and steam activation is used for in-situ catalytic removal of real organic solid waste gasification tar.The results showed that the activated char pretreated by KOH had the best catalytic performance for tar,which mainly promoted the steam reforming reaction and water carbon reaction of tar.The amount of gas product CO increased from 36%to 41%,the amount of H₂ increased from 23%to 27%,the steam decomposition rate increased from 20%to 62%,and the tar concentration decreased from 40600 mg/Nm³ to 6100mg/Nm³;Compared with no catalyst,under the action of CB-cat,the content of phenols and oxygen-containing substances in tar increased,while tricyclic and hydrocarbon compounds decreased,indicating that tar is mainly pyrolysis reaction on the catalyst surface.（4）Based on the specific surface area of char,element composition,disorder degree and reaction rate,the deactivation kinetic equation of char catalyst was established.It was applied to the multi physical field coupling simulation of tar purification reactor.The results showed that when the reactor height diameter ratio was 1.0 and the catalyst bed height was 0.11 m,the complete removal of 41000 mg/Nm³ tar from crude syngas could be achieved.