Study On The Structural Evolution Of Biomass High-temperature Pyrolysis Char And The Characteristics Of Its Char Steam Gasification

Biomass gasification is one of the most important methods in thermochemical conversion.However,in the process of biomass pyrolysis and gasification,the clogging of equipment and the problems of coking and slagging caused by biomass tar and ash will have a serious impact safe and stable operation of equipment.The high-temperature and staged pyrolysis gasification can effectively control the generation of ash and tar,and at the same time facilitate the complete reaction and fine control of the conversion of biomass.However,some key issues of biomass high-temperature pyrolysis gasification have not been effectively solved:（1）The energy and exergy distribution and contribution of each pyrolysis product during the high temperature pyrolysis of biomass;（2）The effect of pyrolysis temperature and time on the evolution of biomass char physical and chemical structure,such as the surface microstructure,pore structure,carbon structure,and functional group structure during the high-temperature pyrolysis of biomass;（3）The influence of biomass coke physicochemical structure on steam gasification characteristics and the key process parameters of biomass high temperature steam gasification system.Based on the above key issues.The research contents are as follows:Based on the thermodynamic analysis of biomass high-temperature pyrolysis,the energy,exergy,energy efficiency,exergy efficiency and exergy loss efficiency of biomass high-temperature pyrolysis products are studied on experimental data to produce of biomass and its products.The biomass raw pyrolysis process at different temperatures are analyzed,the distribution of energy and exergy of each pyrolysis gas,the distribution of total energy and exergy of pyrolysis gas,and the influence of pyrolysis temperature on the energy,exergy,energy efficiency and exergy efficiency of pyrolysis tar.And the effect of biomass pyrolysis temperature on the energy and exergy,energy efficiency and exergy efficiency and exergy loss efficiency of biomass char.It is found that each pyrolysis gas component contributes differently to energy and exergy.The contribution of energy and exergy value of each component in the pyrolysis gas at800℃and 900℃is CO>CH₄>H₂>CO₂,and the pyrolysis temperature increases from1000℃to 1200℃,the contribution of energy and exergy of each component is CH₄>CO>H₂>CO₂.According to the characteristics of high-temperature pyrolysis utilization of biomass particles and pellets,a high-temperature pyrolysis system for biomass particles and pellets was designed and built.Based on the different pyrolysis conditions of biomass particles and pellets,the biomass char physical structure,such as surface microstructure,char pores structure,the surface molten ash structure and distribution were study.Investigate the influence factors of biomass types,pyrolysis temperature and time on the physical structure of biomass particles and pellets high-temperature pyrolysis char.The results show that the physical structure of biomass particles and pellets char during the high-temperature pyrolysis has great changes.Among them,the pore structure of biomass particles char is less affected by molten ash than the pore structure generated by pyrolysis.The specific surface area of corn straw and rice husk particles char increased,the maximum specific surface area was 1300℃for 13 s,and the specific surface area was 588.51 m²/g and 141.17 m²/g,respectively.During the biomass pellets pyrolysis,the corn straw ash melt on the surface of the char at1200℃-1400°C,and the molten ash gathers into a larger volume of molten ball during a long residence times,and there are cracks inside.The cracks in the char at 1400℃are larger and fewer than the cracks at 1200℃.There is no molten ash ball on the surface of rice husk pellets char during pyrolysis temperature at 1200-1400°C.The influence factors of pyrolysis conditions on the chemical structure of biomass particles and pellets high-temperature pyrolysis char are explored.Pyrolysis temperature and residence-time of high-temperature pyrolysis process of biomass particles char have a significant impact on the chemical structure.As the pyrolysis temperature increases,the biomass particles char undergoes dehydrogenation condensation and graphitization.Significant increase,C=O,C=C and CO intensity decrease.As the residence time increases,the content of functional groups in corn straw char is further reduced to disappear,and the disappearance time is advanced from 9 s at 900°C to 3 s at 1300°C.The C-O intensity in rice husk char decreased,but it did not disappear.The number of methylene groups on the surface of straw char and rice husk char decreased from 0.35and 0.42 to 0.28 and 0.24,respectively.During the high-temperature pyrolysis of the biomass pellets,molten balls appeared on the surface of corn straw and rice husk char,and the main components of the molten ash balls were Ca Al₂Si₂O₈ and Si O₂,respectively.The stretching of the C-O bond and C-H are important components of the biomass pellets char,and the carbon structure is more orderly.Finally,researches on the steam characteristics of biomass particles and pellets char were carried out on the biomass particles and pellets char steam gasification experimental,respectively,and the gas online analysis system derives the gas product release characteristics.According to the characteristics of biomass high-temperature pyrolysis char,a biomass high-temperature pyrolysis and its char gasification system is proposed,and the key process parameters of the system are carried out.The effects of pyrolysis temperature,O/C ratio of air and biomass char mixture and the amount of steam in the gasification agent on the gasification of biomass char were studied in detail.The results show that the char conversion rate of biomass particles char is more affected by the chemical structure of biomass char than its physical structure,while the char conversion rate of biomass pellets char is affected by the combination of physical and chemical structures.When the biomass pellets char gasification temperature is 1300℃（slightly higher than the characteristic temperature range of straw ash melting and lower than the melting temperature of rice husk ash）,the carbon conversion rate of corn straw pellets char is inhibited,but the carbon conversion rate of rice husk char is not inhibited.Through system process simulation,it is found that 43%-46%and 42%-44%of the pyrolysis gas of corn straw and rice husk that need to be burned can make the whole system energy self-sufficiency and reach the required temperature for pyrolysis.When the gasification agent is composed of air and steam and the O/C is set to 1,it is recommended that the steam fraction in the gasification agent is 30%-40%.When the O/C is set to 1.5,it is recommended that the steam fraction in the gasification agent input is 60%.