Study On Co-combustion Performance Of Cobalt Based Catalyst For Ventilation Air Methane And Boiler Coal Combustion

Methane is 25 times more potent as a greenhouse gas than carbon dioxide.As the main source of methane emissions,the ventilation air methane（VAM）is difficult to directly enrich or utilize because of low methane concentration（generally less than 1%）,large fluctuation of methane and oxygen concentration,unstable flow,and coal dust impurities.The VAM is mostly directly discharged into the atmosphere,which not only causes energy waste,but also leads to environmental pollution and exacerbates atmospheric warming.Based on the high heating capacity of coal-fired boiler and the high efficiency of methane catalytic combustion,considering the VAM into coal-fired boiler,under the catalytic action of catalyst to realize the efficient utilization of low concentration methane in VAM.The basic theory and application of VAM and boiler coal catalytic co-combustion are still in the initial stage.There are many kinds of catalysts used for methane catalytic combustion and coal catalytic combustion.Among them,the transition metal catalyst shows good catalytic activity of methane and coal due to its changeable metal valence states,and has a good application prospect.In-depth understanding of VAM and boiler coal co-combustion mechanism,and the design and development of high activity and high stability catalysts are the core issues that need to be solved in the VAM and boiler coal catalytic co-combustion system.This paper intends to prepare supported cobalt catalyst with SiO₂as the carrier.The catalysts with good catalytic performance for low concentration methane combustion were obtained by optimizing the preparation conditions,and the structure-activity relationship between the catalyst structure and the catalytic performance of low concentration methane was established by using a variety of characterization methods.The best catalyst was selected by comparison;The combustion performance of coal fired boiler catalyzed by cobalt-based catalyst was investigated,the influence mechanism of catalyst on combustion performance of coal fired boiler was clarified,and the best catalyst was selected;The combustion performance of the best catalyst for VAM and boiler coal co-combustion was investigated,and the influence mechanism between methane catalytic combustion and boiler coal catalytic combustion was clarified.The main contents and conclusions are as follows:（1）Preparation of Co₃O₄/SiO₂-x series catalysts with controllable size of Co₃O₄nanoparticles and study on catalytic combustion performance of low concentration methaneCo₃O₄/SiO₂-x catalyst with controllable size and uniform distribution of Co₃O₄nanoparticles was prepared by coordination polymer method and by adjusting calcination temperature（x is calcination temperature,respectively 500,600,700,800）.Combined with the combustion performance and physicochemical characterization results of catalysts for low concentration methane,it was found that with the increase of calcination temperature,Co₃O₄ nano particle size gradually increased,and the catalytic activity of Co₃O₄/SiO₂-x series catalysts gradually decreased.When the calcination temperature was 500℃,Co₃O₄/SiO₂-500 with the smallest particle size has good catalytic activity and stability due to its rich Co³⁺species content,excellent oxygen transfer capacity and methane adsorption capacity.In order to investigate the influence of methane and oxygen concentration fluctuations on the catalytic performance of Co₃O₄/SiO₂-500 catalysts under different concentrations of methane and oxygen in VAM,the catalytic performance of Co₃O₄/SiO₂-500 catalysts was evaluated.The results show that the catalyst can adapt to different methane concentrations in VAM,but oxygen concentration has a great influence on its catalytic performance,and increasing the oxygen transfer performance of the catalyst is one of the effective ways to improve the activity of the catalyst.（2）Study on combustion performance of Co₃O₄/SiO₂-x series catalysts for coal-fired boilersThe combustion performance of Co₃O₄/SiO₂-x series catalysts for pulverized coal was investigated by TG-DSC and fixed bed reactor-gas chromatography.It was found that the addition of cobalt catalyst effectively promoted the full combustion of coal,which was attributed to the catalytic combustion of volatile matter in coal.By comparison,the catalytic performance of Co₃O₄/SiO₂-x series catalysts decreases with the increase of Co₃O₄ nano particle size.When the roasting temperature is 500℃,the catalytic performance of Co₃O₄/SiO₂-500 with the smallest particle size is the best for lignite.In addition,in order to study the universality of Co₃O₄/SiO₂-500 catalyst,the combustion performance of Co₃O₄/SiO₂-500 catalyst was investigated.The results show that lignite with the largest volatile content has the best combustion effect under the catalytic action of catalyst,which further proves the catalytic combustion effect of catalyst on the volatile content of coal pyrolysis,and also indicates that Co₃O₄/SiO₂-500 catalyst is more suitable for coal with high volatile content.（3）Catalytic performance of Co₃O₄/SiO₂-500 for low concentration methane co-combustion with coalThe catalytic performance of Co₃O₄/SiO₂-500 for low concentration methane and lignite co-combustion was investigated by using a fixed-bed reactor and gas chromatography.It was found that the complete combustion temperature of low concentration methane decreased from 900℃to 700℃in this co-combustion system,which may be due to the heat released from the combustion of volatilizes in coal burning providing the necessary high temperature conditions for methane combustion and promoting its free radical reaction.In addition,by investigating the influence of fluctuation of methane concentration and oxygen concentration on the catalytic co-combustion performance of lignite and VAM,it was found that the change of oxygen concentration had no significant effect on the decomposition combustion of lignite in the co-combustion system,which was attributed to the catalytic combustion-promoting effect.However,the reduction of oxygen concentration delays the catalytic combustion of low concentration methane to a certain extent.Methane concentration changes have significant impact on the performance of coal combustion,coal methane concentrations increase will reduce the degree of contact with oxygen,thus reduce the coal combustion performance,this is because for coal CH₄ adsorption ability stronger than the O₂,increase the temperature of reaction,can effectively reduce the coal to methane adsorption capacity and adsorption capacity,and realize the full combustion of coal.