Study Of Biochar-catalyzed Biomass Tar Coke Accumulation Characteristics And Reforming Regulation

Biomass is the fourth largest type of energy besides coal,oil and natural gas,and seeking effective utilization of biomass has significant implications for energy supply and the achievement of carbon neutrality.The thermal conversion of biomass can convert biomass into high quality syngas,but this inevitably produces tar,a harmful substance.Catalytic reforming is an effective way to remove tar in practical applications,and biochar is gaining more and more attention as an efficient and low-cost catalyst for tar reforming.The application of biochar in tar reforming suffers from coke deposition and deactivation.Coke accumulation shortens the service life of biochar which is not conducive to the long-term stable operation of biomass thermal conversion plants.Based on the process of biochar-catalyzed biomass tar reforming and removal,focusing on the key scientific issue of coke deposition on the biochar surface in tar reforming,this thesis provides a theoretical basis and data support for the use of biochar in biomass thermal conversion.The evolution of biochar/tar properties in a biomass gasifier is studied using a one-stage fixed-bed experimental system.The gasification process of biomass（800°C,15 vol%H₂O）mainly shows the mutual transformation between gas-solid phases,and the tar yield is basically stable at 1.5%.The biochar produced from feeding corn straw under a steam atmosphere for 30 min has a higher content of O-containing functional groups,no significant increase in aromatization,and is considered to have the best catalytic effect on tar reforming conversion.A two-stage fixed-bed experimental system is used to investigate the coke generation characteristics of biochar-catalyzed tar reforming at the gasifier throat（700 °C）,where the coke accumulation level of the biochar reaches saturation at 40 min of feeding.The coke accumulation blocks the microporous pores and adheres to the external surface of the biochar,resulting in an increase in the external surface area and a decrease in the microporous area.In addition,coke deposition inhibits reactions such as biochar gasification,water-gas shift and methane steam reforming.The heterogeneous reforming of toluene,styrene,phenol and indene components of tar on the biochar surface is inhibited.The pathway for the removal of naphthalene is dominated by homogeneous conversion in steam.Acid washed biomass,potassium loaded biomass and calcium loaded biomass are prepared by acid washing and impregnation loading.Biochar catalysts with different surface AAEMs contents are prepared by pyrolysis-activation method,and the effects of steam addition and biochar surface AAEMs on coke accumulation characteristics are investigated using a one-stage fixed bed experimental system.In the 650 °C biochar-catalyzed tar reforming,coke deposition on the biochar surface is not saturated during the feeding time（10～50 min）of biomass in inert atmosphere,while coke accumulation is saturated at 30 min of feeding after the addition of steam.The addition of steam to tar reforming depletes the active coke on the biochar surface,resulting in an increase in aromatization and a decrease in the number of O-containing functional group structures on the surface.AAEMs in biochar are essential in tar reforming.Potassium loaded biochar has a maximum tar reforming capacity of 94.9%,while the tar removal efficiency of acid washed biochar is only27.8%.The reformed potassium loaded biochar maintains the highest microporous area.The loaded K/Ca elements affect the structure of coke,leading to an increase in the number of O-containing structures in it.AAEMs have a catalytic effect on methane cracking and the effect of K element is stronger than that of Ca element.The regenerative effect of partial oxidation on the catalytic activity of deactivated biochar and the regulation of partial oxidation in biochar-catalyzed biomass gasification tar reforming are investigated using a one-stage fixed-bed experimental system.After regeneration by partial oxidation,the microporous area and external surface area of the deactivated biochar surface are significantly increased,and the O-containing functional group structure on the surface becomes more abundant.During the partial oxidation regeneration of the deactivated biochar,the aromatic components of the biochar are consumed preferentially by reaction,while the graphitized structure in the biochar starts to be destroyed as the intensity of oxygen action increases.At a regeneration time of 7 min with an oxygen concentration of 0.4%,the recovery of catalytic activity of the biochar for tar reforming is 108.1%and its overall efficiency reaches a maximum of 79.1%.Partial oxidation regulation maintains the pore channel structure of the biochar,but there is a conversion of micro-pore to meso-macropore pores.The loss of O-containing structures on the biochar surface is effectively suppressed,but also contributes to increased graphitization.When the oxygen concentration is increased to 0.4%,the complete conversion amount of tar is increased by 27%and the hydrogen yield is increased by 16%.Further increasing the oxygen concentration results in more hydrogen being consumed,the hydrogen yield decreases.The effects of pyrolysis temperature（500～800 °C）,catalytic temperature（600～750 °C）and steam concentration（10 vol%～40 vol%）on biochar-catalyzed tar reforming from biomass pyrolysis are investigated sequentially using a two-stage fixed-bed experimental system.As the pyrolysis temperature increases,the tar yield decreases continuously and the yields of CH₄,CO and H₂all increase significantly.The catalytic temperature range of 600～700°C is suitable for biochar,and 650°C is the most suitable catalytic temperature for biochar in this study,at which the tar yield is reduced to 0.15%.The addition of steam reduces the tar yield to an even lower level and the actual value is no longer available by laboratory testing methods.Styrene,indene and naphthalene are completely converted and removed from the tar.The GC-MS peak area of toluene in tar decreases by more than 70%when 30～40vol%steam is added.The addition of steam also promotes the methane steam reforming reaction and the water-gas shift reaction,and the H₂yield increases significantly.