Study On The Thermal Conversion Characteristics Of Biomass Based On Heat Carrier Heating

Biomass is an ideal "zero carbon emission" energy source,the clean and efficient use of biomass is an important measure to achieve the goal of "carbon neutrality and emission peak",alleviate energy supply and promote sustainable development.Biomass gasification is a technology that converts biomass into syngas.The traditional gasification process requires self-heating combustion to provide the heat required for the reaction,which will result in energy waste and pollution.Therefore,the traditional way of heat source for gasification is gradually being replaced.New gasification technology adopts more environmentally friendly and efficient ways to obtain heat.This approach achieves comprehensive energy utilization.Among them,biomass gasification by using heat carrier has a good development prospect.This paper studied the reaction characteristics of biomass under heat carrier heating,providing a relevant theoretical basis for designing and improving solar-thermal-coupled biomass gasification equipment.The contents are as follows:(1)The effect of the heat carrier on the morphological changes and the projected area shrinkage rate of biomass during the temperature-rise period was investigated using a high-temperature stage coupled with optical microscope system(HTSOM).The gasification reaction characteristics of biomass at different heat carrier temperatures were analyzed and compared with conventional gasification.The results showed that the projected area increased due to the high elastic state of ellulose during the temperature-rise period,and then the projection area decreased and the area shrinkage rate increased because the large amount of volatile fraction released.Under the heat carrier heating,biomass gasification showed two modes:combined gasification-pyrolysis mode and conventional gasification mode.The combined gasification-pyrolysis mode was observed at the heat carrier temperature of 600～700℃.The gasification reaction rate decreased due to the simultaneous occurrence of lignin pyrolysis and coke gasification,and the escape of lignin pyrolysis gas reduced the concentration of gasification agent in the pores.When the temperature of the heat carrier was 750℃～850℃,the trend of area shrinkage was same as conventional biomass gasification,with the difference being that biomass does not have residual carbon that was difficult to react.(2)A high speed camera equipment combined with a visual dropper furnace was used to capture the reaction process of biomass at different heat carrier temperatures and oxygen concentrations.In the study of biomass gasification reaction characteristics,the gray value change was applied to divide the boundary between volatile matter release and coke reaction stages.Under the influence of bed resistance,the position and temperature of biomass in the heat carrier bed had a significant impact on biomass gasification,and the release time of biomass volatiles in the bed voidage accounted for a shorter proportion of the total gasification time.No obvious volatile flame was found during the biomass combustion process,which was related to factors such as oxygen concentration,gas-solid contact and furnace airflow.The heat carrier temperature and oxygen concentration had a significant impact on the flame area and flame brightness,and its thermal storage performance made the flame brightness of biomass combustion relatively stable.(3)A EMCCD hyperspectral camera imaging system combined with a visual dropper furnace was used to study the release characteristics of Na*and K*during biomass reaction process The results showed that oxygen concentration affected the reaction behavior of biomass.At low oxygen concentration,no volatile combustion occurred,while at high oxygen concentration,a small amount of volatile combustion occurred.Meanwhile,the curve of alkali metal radiant intensity on the time axis presented two peaks,corresponding to the reverse end of volatiles combustion and the early stage of coke reaction.The biomass burnout time was shortened with the increase of heat carrier temperature,and the peak value of alkali metal radiation intensity was increased with the increase of heat carrier temperature,which had a highly fitting relationship.