Experimental Study Of The Heterogeneous Tar Removal Over Biomass Chars Produced By Oxidative Pyrolysis

The depletion of fossil fuels has caused a huge increase in emissions of both the greenhouse gases and pollutants,which are key environmental problems and has made the search and R&D activities for alternative energy sources more urgent than ever.The utilizable biomass resources are abundant in China,with annual production of about 4.6 hundred million tons?standard coal?.As one of the promising biomass thermochemical conversion technologies,gasification has attracted more attentions in recent few decades.The tar problems are the major obstacle to developing the biomass gasification technology.The two staged downdraft gasifier has been proven to be an effective gasification technology,which produces syngas with low tar content by combining homogeneous tar conversions in the throat part and heterogeneous tar removals in the char bed.However the pyrolysis section of the gasifier is externally heated by exhaust gases,which could bring some problems including inefficient pyrolysis,limited scale-up,low systematic energy efficiency,etc.Air-staged two stage downdraft gasifier has received much attentions in recent years,it brings a stream of air to its pyrolysis section and realizes energy self-sustaining process by oxidizing part of the fuels to provide heat needed for an endothermic pyrolysis process of the rest of the fuels,and it has higher systematic energy efficiency theoretically.The present study is with the context of this type of gasifier,it focus on the heterogeneous tar removals in char bed section and sets its goal of decreasing the tar content of the output syngas.First,the effects of different conditions in the oxidative pyrolysis process on the microstructures of chars are studied,and the results are a theoretical basis for the char modification process.Second,systematic factors analysis is crucially investigated on the heterogeneous removal process of model tar compounds,naphthalene and toluene,over oxy chars?short for oxidative pyrolysis chars?,a regulation method is obtained to reduce the tar content of the process and a highly probable mechanism is concluded for the process.Third,the removal process of mixture tars over oxy chars is investigated and the aforementioned regulation method is verified.The results of present study can be used as the theoretical basis for the R&D works of the actual air staged gasification technology.For the microstructure modification process of chars by oxidative pyrolysis,the macro-TG reactor combined with test instruments including gas adsorption analyzer,SEM and FTIR etc.,is used to investigate effects of temperature,oxygen concentration,heating rate,residence time and biomass types on chars'physical pore structures,surface chemical functional groups and AAEM contents.Oxidative pyrolysis process promotes an increase in pore structures of chars,mainly for micro pores.The oxidative pyrolysis increases the specific surface area of chars by oxidation reactions between oxygen and raw material,chars.With higher temperatures,the improvement extent increases.The surface area of the pinewood oxy char at 900°C is 642.05 m²/g,which is much higher than that of inert char,i.e.197.63 m²/g.With an increase in the oxygen concentration,the surface area of biomass char is enlarged,mainly for micro pores and small meso pores.The oxidative pyrolysis promotes the changes in the surface functional groups and AAEM contents of chars primarily by the in-situ exothermic effect.An increase in C=O stretching vibration intensity is found in the pinewood oxy char at 300°C,which indicates that the increase in the amount of oxygenated functional groups is caused by the chemisorption of oxygen,however this trend can only be found in relative lower temperature.A micro reactor is constructed and combined with quantitative analysis of GC-MS,to study effects of temperature,char activation extent,time stream,type of chars,reaction atmosphere?inert,CO₂,H₂O?and inlet tar concentrations on the heterogeneous removal of naphthalene over oxy chars.Compared with inert chars,oxy chars have higher initial reactivity,a period of naphthalene breakthrough time and higher naphthalene conversion efficiency during the heterogeneous conversion process.The presence of CO₂ delays the naphthalene breakthrough time of oxy chars and increases the conversion efficiency.Chars are reactivated and their reactivity are sustained by the CO₂ gasification of deposited soot and chars.The breakthrough time of oxy chars in the steam atmosphere condition is longer than that of CO₂ atmosphere condition.In the condition of steam atmosphere with high inlet tar concentration at 900°C,the naphthalene is completely removed by oxy chars during the 3 h tests.The hydrogen concentration is a good indicator to the catalytic reactivity of chars in heterogeneous tar removals.The micro reactor is combined with quantitative analysis of GC-MS to investigate effects of temperature,char activation extent,time stream,type of chars,reaction atmosphere,CO₂ concentration and residence time in char bed on the heterogeneous removal of toluene over oxy chars.Compared with inert char,the oxy char condition has higher toluene conversion and lower benzene yielding rate,especially at early stage.With temperature increasing from 700-900°C,both the toluene conversion and overall tar conversion increase in the oxy char condition.The pine wood oxy char condition has higher toluene and overall tar conversions than the rice straw condition.An increase in the residence time of tars in the char bed promotes toluene and overall tar conversions.With a rise in the CO₂concentration from 10-20 vol.%,the tar conversion of 800°C condition slightly decreases,however the 900°C condition shows a complete removal of tars in the whole 3 h test.The oxy char condition with steam atmosphere shows higher toluene and overall tar conversions than the inert atmosphere condition,however slightly lower than the CO₂ atmosphere condition.A probable mechanism for the heterogeneous tar removal over biomass oxy char is concluded in present study,based on experimental results and relevant literatures.The carbon active sites are the main catalytic sites for the heterogeneous tar removal process,primarily including the basal defect carbon sites and edge carbon sites.The presence of biomass char promotes the nucleation of soot particles and also the surface growth of them by greatly“increase”the radical sites on the soot particles;Based on aforementioned experimental results,a regulation method is concluded for the heterogeneous tar removal process in the gasifier char bed section with low tar content,which mainly includes the use of oxy chars,addition of oxidizing gases?CO₂ and steam?,rise of the char bed temperature,maintaining at 900°C and above.The research for the mixture tars removal over oxy chars are divided into two steps.First,a two stage hot-rod reactor is used to study the primary tar components and their amounts for tars after the homogeneous conversion of biomass pyrolysis tars,then the composition of mixture tars is determined.Second,the micro reactor is used to investigate effects of temperature,char activation extent,reaction atmosphere,CO₂concentration and residence time in char bed on the target process.The mixture tars include four individual tar components,which are benzene,toluene,styrene and naphthalene,with weight ratio of 4:2:1:3 respectively.The aforementioned regulation method is effective for the heterogeneous removal process of mixture tars.For the aspect of the increase in the residence time of tars in char bed,which is one of the regulation sub-methods,it could be realized by extending the height of char beds in practical processes.