Mechanism Of In-situ Catalytic Cracking Of Biomass Tar Over Biochar With Multiple Active Sites

Biomass tar is the bottleneck in the development of efficient utilization of biomass syngas.Catalytic cracking is an effective way to eliminate tar.As the tar cracking catalyst,biomass char has attracted extensive attention all over the world.Based on the characterization of active sites on the biochar surface,this paper studied the selectivity of different active sites,the reaction pathway of tar molecules,and the effect of biochar active structure and AAEM species on the catalytic cracking of biomass tar.It mainly focuses on the removal of biomass tar during the biomass gasification and utilization processes.This scientific key is studied from the pretreatment analysis of biomass raw materials（rice husks and sawdust）,effect of AAEM species on the pyrolysis reaction kinetics of biomass,formation characteristics of pyrolysis biochar,characteristics of tar formation and its homogeneous conversion,in-situ/ex-situ CO₂ and H₂ O activation of biochar,catalytic cracking of model tar compounds with biochar,and the catalytic cracking mechanism of biochar with multiple active sites on real tars.（1）The mechamism of formation and homogeneous transformation of biomass tar is studied by the Py-GC/MS and a two-stage fluidized bed/fixed bed reactor.It can be seen that during fast pyrolysis of biomass,the removal of AAEM species reduces its weight loss rate.With the pyrolysis temperature increasing in the range of 500⁹00 oC,the tar composition of high molecular weight decreases gradually.The polymerization of tar precursor is limited by the AAEM species and the formation of the heterocyclic carbon ring is further inhibited.The molecular weight of rice husk pyrolysis tar is mainly distributed in 110¹29.The yield of sawdust fast pyrolysis tar is higher than that of rice husk.The molecular weight distribution of sawdust pyrolysis tar is widely,which contains larger moleculars（150²09）.Temperatures of 700⁹00 oC are required for the homogeneous transformation of biomass tar in the H₂O/CO₂ atmosphere,which especially affect the polycyclic aromatic hydrocarbons（PAHs）.The effect of 15 vol.% H₂ O on the tar homogeneous cracking is significantly higher than that of 29 vol.% CO₂.These H/O/OH free radicals can react with the active free tar fragments generated from the first stage of thermal decomposition demonstrating the importance of H₂O/CO₂ cracking agents in the homogeneous conversion of biomass tar.The effect of temperature is mainly reflected in the promotion of tar thermal decomposition reaction caused by the corresponding free radicals,among which its effect on the conversion of PAHs in biomass tar is particularly significant.（2）Characteristics of the formation and H₂O/CO₂ activation of biochar with multiple active sites are analysied by thermogravimetry/Fourier-transform infrared spectroscopy（TG-FTIR）and a laboratory-scale fixed-bed reactor.The results indicate that the catalytic effects of ion-exchanged K⁺ and Ca²⁺ on the pyrolysis gas products vary in terms of their amounts,but not their species.During pyrolysis,K⁺ and Ca²⁺ increase the contents of carboxyl-carbonate or aromatic C=C structures and surface C=O functional groups.During H₂O/CO₂ gasification of biochar,volatilization of K and Ca linked in the whole biochar matrix by relatively stable organic bonds in pre-loaded biochar is less than that in the form of chemical adsorption（or surface adhesion）on post-loaded biochar interface.The effect of K is mainly on the formation of O-containing functional groups and the transformation from small ring systems to larger ones,while the catalytic effect of Ca is only to increase the proportion of large aromatic ring structures（≥ 6 fused benzene rings）.The biochar-CO₂ reaction takes place mainly at the gas-solid interface of biochar,while the biochar-H₂ O one is throughout the whole biochar particle.CO₂ produces more micropores in the biochar,whereas H₂ O favored the formation of mesopores,which are more important for the tar cracking.A better distribution/availability of active sites（i.e.surface K/Ca species and O-containing groups）is obtained on AAEM pre-loaded biochar surface,resulting in high biochar specific reactivity and char conversion than AAEM post-loaded biochar.（3）A first-order fixed-bed reactor is used to investigate the removal of model tar compounds（toluene,naphthalene and phenol）from the biochar with multi-active sites.It can be seen that during the cracking of model tar compounds in 15% H₂ O or pure CO₂ at 800 oC,the release of K from biochar samples is nearly twice as that of Ca.More O-containing functional groups are formed on K-loaded biochar than Ca-loaded and H-form biochars.H₂ O or CO₂ activation increases the lattice defects and surface functional groups in biochars to promote the com bination between biochar and model tar compounds.Tars containing heteroatoms are converted faster than those containing aliphatic chains and pure aromatic rings.On catalytic cracking of naphthalene and toluene with biochars,in 15% H₂ O atmosphere,the effect of K is about 10% greater than that of Ca,while in pure CO₂ atomsphere the effect of K is approximately 5% greater than that of Ca.The pathways for tar reformed in H 2O or CO₂ by K and Ca in biochar include direct homogeneous cracking and consumed by H₂ O or CO₂ gasification on biochar surface.（4）The reaction mechanism of in-situ catalytic cracking biomass tar with multi-active biochar is investigated in a two-stage fluidized bed/fixed bed reactor.The results indicate that adding H₂ O or CO₂ is found to improve the homogeneous and heterogeneous cracking of biomass tar,the latter of which involved first forming an intermediary coke product that was subsequently gasified by H₂O/CO₂.Activation of biochar by H₂O/CO₂ impacted the morphology of biochar surface and distribution of metal species.During tar cracking,the presence of H₂O/CO₂ also affects the creation and regeneration of pore structures,influencing the biochar structure and dynamical distribution of AAEM species,which ensure enough surface active sites to maintain the catalytic activity of biochar.The addition of H₂O/CO₂ is found to notably enhance in-situ cracking of both large and small aromatic ring systems in biomass tar over biochar.The mechanisms of in-situ tar H₂ O cracking by K and Ca species were different: tar cracking into low-quality tar or small-molecule gas may be catalyzed by K,while the combination of tar with biochar would be promoted by Ca.The volatilizations of K and Ca with the presence of volatiles are to a large extent in accordance with their valences(monovalent K⁺ and divalent Ca²⁺)and their boiling points.The subsequent transformation from the small aromatic ring systems to the larger ones occur due to the volatile-biochar interaction.During the in-situ tar H₂ O cracking over biochar,K and Ca act as the active sites on biochar surface to promote the increase of active intermediates（C-O bonds and C-O-K/Ca）,which promotes the tar-biochar interactions.In summary,it establishes the theories of acid/alkali multiple active sites formed by surface oxygen-containing functional groups and AAEM species in biochar,and obtains the selectivities of different active sites and reaction path of tar molecules.It reveals the coupling mechanism of tar cracking over biochar with surface active groups and catalytic species,which provides a theoretical basis for the modified methods to apply the catalytic cracking of biochar catalysts on biomass tar.