Fundamental Study On Dusty Tar Pyrolysis

The staged conversion technology based on coal pyrolysis/combustion process can take full advantage of low rank coal for co-production of gas,liquid fuel,and power,realizing the comprehensive utilization of low rank coal resources.Dusty tar,which is an undesired product from quench process of coal pyrolysis/combustion system,contains substantial dust particles and heavy tar.The high content of dust and heavy molecular weight components in dusty tar increases the difficulty in further refining and makes the post treatment very expensive.A new process,the coal pyrolysis/combustion coupled with recirculating pyrolysis of dusty tar is proposed.To be more specifically,dusty tar was recirculated back to the pyrolysis reactor of coal pyrolysis/combustion system and converted into lighter tar,fuel gas and char.This new process will only generates light tar,pyrolysis gas and char,significantly increasing the utilization efficiency of coal.Based on the new process,dusty tar pyrolysis was conducted to improve the yield of light tar product.The factors that have influences on light tar formation were investigated.The results suggest that(1):Decomposition of aliphatic chains in heavy tar which can be completed at 500? can generate light tar product.Fast pyrolysis can favor light tar formation compared with slow pyrolysis.(2):High heating rate can decrease the content of 3,4 ring aromatics and O/N/S element,and increase the content of 1,2 ring aromatics than slow heating rate,improving the quality of tar product.(3):Compared with N2 atmosphere,CH4 has little effect on tar pyrolysis.The H2 atmosphere can increase the yield of light tar product and improve the quality of tar product.Compared with high heating rate,the ability of H2 atmosphere to increase the yield of light tar product is more obvious at slow heating rate.(4):Dust can inhibit conversion of heavy tar in feed into tar product while favors gas and char formation.Fuel gas production from dusty tar pyrolysis is also studied.The effect of temperature,residence time of volatiles,steam and solid particles on pyrolysis gas yield,volumetric heating value of gas and the gas efficiency of dusty tar pyrolysis were discussed.(1):Below 600?,the pyrolysis gas mainly derives from decomposition of chemicals with boiling point higher than 360?.On the contrary,most of the pyrolysis gas is derived from decomposition of components with boiling point lower than 360? when temperature exceeds 600?.(2):As temperature elevates from 360? to 950?,yields of gas and gas efficiency of dusty tar pyrolysis simply increase while the volumetric Low Heating Value(LHV)of gas obtains a peak value of 29.98 MJ/m3 at 400?.(3):Pyrolysis with steam can increase the gas yield and gas efficiency of dusty tar pyrolysis while the LHV of gas is reduced.(4):Solid particles favor conversion of dusty tar into pyrolysis gas and decrease the LHV.Ash,which is rich in Fe oxides,can decrease gas efficiency of dusty tar pyrolysis while dust,CaO and Al2O3 can increase it.Pyrolysis of heavy fraction(HF)of dusty tar was performed.The role of light fraction(LF)of dusty tar,anthracene(ANT)and hydrogenated anthracene(HAN)during fast pyrolysis of HF were evaluated.It is found that:(1):During pyrolysis of HF without LF,23%-23.6%HF can be converted into light tar product.During co-pyrolysis of HF with LF,33.1%-38.9%HF was converted into light tar product.LF play vital roles in converting HF into light tar product.(2):ANT and HAN both favor light tar product formation.HAN can provide H atom to fragments from HF pyrolysis,increasing yield of light tar product while ANT can capture H atom from HF pyrolysis and be transformed into HAN,and then take part into HF pyrolysis.HAN has higher H donation ability than ANT.(3):During co-pyrolysis of LF with HF,LF can provide H atom to cracked species from HF pyrolysis,favoring conversion of HF into light tar product.The molecular structure of HF was constructed based on the elemental analysis and NMR analysis of HF.The pyrolysis process of HF was simulated by the reactive forces field molecular dynamics based on GPU(GMD-Reax).The energy barrier for conversion of cracked species was analyzed.The evolution of pyrolysis product was obtained.(1):Radicals are formed during HF pyrolysis.Temperature increase will facilitate radical formation.(2):The evolution of gas yield coincides with experimental results.(3):There is a peak yield of light tar product during simulation of HF pyrolysis.The further polymerization of cracked species derived from HF pyrolysis can reduce the yield of light tar product obtained at the end of simulation.(4):The key step to increase the yield of light tar product is to suppress polymerization reactions of 1-3 ring fragments and enhance the hydrogenation and ring-opening reactions of large cracked species(larger than 4-ring).Finally,the mass and heat balance of the proposed process is analyzed.The results suggest that re-pyrolysis of dusty tar in the downer reactor can increase the yield of light tar product and pyrolysis gas,improve heating value of gas and enhance gas efficiency of coal pyrolysis while slightly reduce the pyrolysis efficiency.