Experimental Study And Numerical Simulation On Waste Pyrolysis Combined With Coal Combustion

Due to the dual drive of urgent capacity requirement to depose the increasing municipal solid waste output and energy structure optimization to reduce the coal consumption,the co-combustion of coal and refuse derived fuel?RDF?for power generation has drawn great attention,and the state has issued relevant support policies.However,on account of the uneven particles,complex components and high moisture content,the fuel quality of raw RDF is poor and it is not suitable for directly co-combustion in coal-fired boilers.Therefore,a dual-bed poly-generation system coupling RDF pyrolysis and coal combustor is designed in this study.In this system,RDF is converted into char for co-firing with coal by means of pyrolytic gas cycle and ash heat carrying,and the volatiles are converted into combustible syngas.In this study,the pyrolysis experiment of simulated RDF under the atmosphere of simulated pyrolytic gas?PG?,the co-pyrolysis experiment of real RDF mixed with fly ash under PG atmosphere,and the numerical simulation on the co-combustion of RDF pyrolysis char with coal in a small fluidized bed were carried out.First,the effect of pyrolysis gas atmosphere?20%CH₄,10%CO₂,70%N₂?on simulated RDF pyrolysis was investigated in a laboratory-scale fixed bed reactor,compared to N₂atmosphere.It was found that under the same temperature and reaction time:the char yield produced in PG atmosphere decreased slightly and the liquid yield increased.The PG atmosphere contributed to the lower VM/FC mass ratio,as well as the H/C and O/C atomic ratios.In addition,the PG atmosphere was found to promote the production of CO and H₂,as well as the H₂/CO mole ratio,while inhibit he production of CH₄and CO₂.PG-char can obtain superior fuel quality than lignite coal under all experimental conditions in this study,and better than bituminous coal except at?600?,3minutes?.Under?700?,6min?or higher temperature,the PG-char approached anthracite coal.Then,the real RDF was co-pyrolysis with coal ash under the pyrolytic gas atmosphere?50%CO₂,25%CO,10%CH₄,10%H₂,5%N₂?.Compared to value in N₂atmosphere,char yield decreased by 2.06-1.50wt%,while tar yield increased by1.50-2.67wt%when the temperature rose from 400?to 700?,and the syngas yield was inhibited under PG atmosphere.Based on the chemical composition analysis and combustion index,PG atmosphere improved the comprehensive fuel characteristics of char,while reduce the char energy output slightly.The char produced under the condition of 600?and PG atmosphere was most similar as the designed coal.The syngas composition was slightly influenced by different pyrolysis atmospheres.The heat value of syngas produced under PG atmosphere was higher than that under N₂atmosphere,but gas energy output decreased.The metal oxides contained in coal ash promote the pyrolysis products to switch to the gas phase,which reduces the yield of tar and coke,and this phenomenon was more obvious with the increase of ash mass ratio at 700?.Fe₂O₃in coal ash provided lattice oxygen to oxidize part of tar,and consumed a small amount of H₂and CO through gas-solid reaction,promoting the generation of CO₂consequently.Finally,a two-dimensional CFD model was established based on a laboratory-scaled fluidized bed,to simulate the co-combustion of coal and RDF char.The accuracy of coal combustion model in fluidized bed was verified in terms of flow field,temperature field and concentration field.After that,char produced by the pyrolysis of real RDF under PG atmosphere was added into the model and co-firing simulation was conducted with the mass ratio of 10%.It was found that when the coal was burnt with the char produced at different temperature?from 400?to 700??:the O₂concentration of the flue gas at furnace outlet slightly decreased,and the CO₂concentration increased first and then decreased.The concentration of CO decreased significantly.Chemical incomplete combustion heat loss q₃and mechanical incomplete combustion heat loss q₄decreased,the combustion efficiency?increased from 96.05%to 96.32%;The overall heat flux of combustion decreased;The wall heat dissipation and the heat generation of heterogeneous combustion changed greatly,while the heat generation of heterogeneous combustion did not decrease significantly.Then,the PG-600 char was selected for co-firing simulation with coal according to the mass ratio of 10-30%.It was found that with the increase of the mixture firing ratio from 10%to 30%,the O₂and CO₂concentration increased gradually,and the CO emissions increased significantly.Both the heat loss q₃and q4 promoted,and the combustion efficiency?decreased consistently;The overall heat flux of the furnace decreased;Consequently,it is suggested that the mixing ratio of char and coal should be controlled in a reasonable range according to the calorific value of char.