Study On In-situ Conditioning And Catalytic Combustion Of Biomass Pyrolysis Volatiles

Biomass pyrolysis is the most common used technology for steady production of biochar,combusting pyrolysis volatiles for providing heat in biochar production process willsolve the bottleneck problem of by-products treatment in distributed biomass production.The constituents of pyrolysis volatiles are complex,and the overall combustion performance is poor.Conditioning on pyrolysis volatiles and catalytic combustion are the effective solutions for realizing clean combustion.Considering on the current status of lack of theoretical data due to less studies have been carried out as to conditioning and catalytic combustion of pyrolysis volatiles,targetted at improving the combustion characteristics of biomass pyrolysis volatiles for clean combustion and providing heat in biochar production process,the studies on oxygen lean conditioning and oxygen lean catalytic conditioning on biomass pyrolysis volatiles as well as catalytic and non catalytic combustion of the conditioned and unconditioned biomass pyrolysis volatiles have been conducted,the main study contents as well as the conclusions are as following:TG-DSC is used for thermal kinetic analysis of combustion of the heavy components of biomass pyrolysis volatiles,the results show that:The combustion process is divided into three stages,which are low-boiling components volatilization,tar decomposition and combustion and heavy tar and char combustion.Along with the increase of the temperature,the maximum mass loss rate decreases gradually,and the onset temperature and the final temperature of reactions,the temperature at the maximum mass loss peak and the maximum mass loss rate show the tendency of increasing along with the increase of heating rate.The higher heating rate leads to the shortening of reaction time and the incomplete combustion as well as the unobvious change on the total mass loss at the individual stage.In the stage of co-combustion of heavy tar and char,the overall reaction temperature is higher,the reaction rates show less difference at different heating rates,and the combustion rates are similar in the wider temperature range.The reaction mechanisms for the three stages are all of random nucleation and growth,the apparent activation energy（55.03,85.60,77.22/E/kJ/mol）and pre-exponential factors(3.38,4.54,1.97/lgA/s^-1)calculated by the model at the three stages display the tendency of first increasing and then decreasing.The reaction orders for the three stages are 0.75,0.59 and 0.85 respectively.The prediction on the isothermal combustion process shows that the reaction times for the first and the second stages are short,while the time for the third stage reaction is longer,and at the reaction temperature of 1100℃,it will take around 1 minute for complete combustion.With cordierite as monolithic carrier and by impregnation method,4 kinds of catalysts respectively with the oxides of Fe,Co,Ni and Mnas the active components were prepared.The active components distribution and their status on the catalysts surfaces as well as the stability of the catalysts were analyzed by XRD,SEM and TG-DSC.The TG-DSC experiment on catalytic combustion of heavy components of pyrolysis volatiles shows that:The Manganese based catatlyst has remarkable catalytic effect on the conversion,the combustion temperature at higher temperature is decreased from 550℃to around 450℃,while the oxidation temperature at lower temperature is decreased from 500℃to around250℃.With the stoichiometric air rate ofcombusting charcoal particles in pyrolysis volatiles as the basis,experimental study on in-situ oxygen-lean conditioning on biomass pyrolysis volatiles has been carried out,the results are:The in-situ oxygen-lean conditioning may effectively eliminate the charcoal particles in pyrolysis volatiles.Along with the elevating of conditioning temperature and the increase of the air rate,the contents of alkyl group chemicals in the heavy components of pyrolysis volatiles have shown obvious decreases while the phenols have shown evident increases,a little bit of increases in contents have been found for the lower-content chemicals such as furans,alcohols,indenes,aldehydes and etc.The relative contents of alkyl group,acids,ketones and phenols in light components of pyrolysis volatiles have shown obvious changes,and the changes of phenols’absolute contents are not obvious.After conditioning,the contents of lower carbon compounds are increased while the higher carbon compounds are decreased.In-situ oxygen-lean conditioning has less influences on the gas components of pyrolysis volatiles,the changing rates of H₂,CO₂ are all within±5%.With MnO as the catalyst,experimental study on catalytic conditioning on biomass pyrolysis volatiles has been conducted,the results are:The introduction of catalyst may promote the carbon numbers of the heavy component are transferred to the light components,the position of catalyst have less influence on the transferring of carbon numbers.The catalyst promotes the cracking and aromatization reactions that result in the forming of small molecular incondensable gas and aromaticorganics,and has less catalytic effect on the phenols;oxygen-lean catalytic conditioning has obvious effects,most of the main components in pyrolysis volatiles could have reactions of different extent and their contents were decreased,the phenols may be converted into alkanes due to ring-opening reaction and results in the decrease of their contents and the increase of relative contents of alkanes.The contents of acids and ketones in light components of volatiles have been increased while the relative contents of phenols were decreased.The position of catalyst in the reactor has certain effect on the conditioning of pyrolysis volatiles,at the position where the air and the volatiles are mixed well,the catalyst will have effect on the conversion of more organics.The position,where the alkanes contents are increased and the phenols contents are decreased,is the ideal position.The catalytic conditioning has very weak influence on the gas composition in the pyrolysis volatiles,the change of H₂、CO、CH₄ and CO₂ are all within 2%.The analysis by XRD,FT-IR and SEM shows that:a little bit of oxygen may inhibit char deposit on the surface of catalyst,and less char deposit is formed at the position where the volatiles and air mix well.The long chain alkanes will be decomposed into small molecular organics due to oxidation and cracking,the oxidation reaction is similar to combustion,the cracking reaction follows the free radical theroy ofβbond breaking.With MnO as the catalyst,the cross-over experiments among catalytic and non-catalytic conditioning,catalytic and non catalytic combustion of pyrolysis volatiles have been carried out.The results are that:Along with the increase of the equivalence ratio,the reaction temperature will be increased,the contents of C_mH_n and CO in the flue gas tends to be decreased.The effect of catalyst on combustion is obvious,the overall reaction temperatures for combustion after conditioning are increased and this is helpful for the decreasing of the contents of C_mH_n and CO in the flue gas.Both conditioning before combustion and catalytic combustion of the non-conditioned volatiles could improve the combustion performance of pyrolysis volatilesthat result in the colorfading of the flue gas condensate due to the decrease of organics.Theorganics components in the condensates of flue gases are 4,6-dimethyl-dodecane,eicosane and heneicosane,which sum accounts for40-60%of the total organics.The equivalence ratio of this study is lower than the stoichiometric air fuel ratio,and the residential time is short,trace charcoal particles could be found in the condensate of flue gases.Analysis of XRD,FT-IR and SEM shows that:Char deposit could be found on the surface of the catalyst and conditioning before combustion may reduce the char deposit on the catalyst for combustion.The char on the surface of the catalyst is mainly inorganic,meanwhile,the weak absorption peaks of phenol hydroxyl and C-C bond of aromatics could be found.The catalytic function of MnO in the combustion is realized by the change of valence state of manganese,which results in the decrease of activation energy of combustion,and the increase of conversion rate of medium product to final product,the change of valence state of manganese is actually the transferring process of oxygen atoms.