Study On Production Of Biochar From Fluidized Bed With Internal And External Heating

Biomass pyrolysis for carbon production is an important technology of biomass thermochemical conversion.However,the problems existing,such as low productivity,dependence on external energy and secondary pollution,are not addressed.In the thesis,the characteristics of biomass pyrolysis was studied on a lab-scale reactor.A biochar production system by fluidized bed with internal and external heating,processing capacity reaching1000kg/h,was designed and constructed.And optimization of operation parameters and utilization of biochar were reaserched to provide guidance for development of the technology.The experiment of biomass pyrolysis for carbon production on a lab-scale fluidized bed reactor,using rice straw,rice husk and sawdust as raw materials,was carried out to study the effects of feedstock,temperature and atmosphere on the yield and property of biochar.The results showed that the carbon yield of rice husk was the highest,followed by that of rice straw and sawdust.A steady decline of carbon yield was found with the increase of pyrolysis temperature,but the lower temperature was not conductive to devolatilization and fixed carbon enrichment.Different from the weak oxidativity of CO₂,the import of O₂ could cause the violent oxidation of biochar and tar,resulting in the transfer of solid and liquid phase products to gas phase products.The carbon yield of the 3 raw materials decreased respectively by 3.34%,4.28%and 6.72%as the O₂ concentration increasing from 0%to 8%,and excessive O₂ concentration led to consumption of fixed carbon and accumulation of ash.Based on the results from lab-scale reactor,the guidance of parameters for engineering platform was as follows:450⁵50℃temperature for rice straw and rice husk,400⁵00℃temperature for sawdust;2⁶%O₂ concentration;10²0%CO₂ concentration.Gas phase products are sent to the combustion furnance in the biochar production system by fluidized bed with internal and external heating,to provide energy for the whole system and avoiding secondary pollution.The mode of internal and external combined heating is formed by heat exchange and reaction exotherm of high-temperature flue gas in the heating jacket and middle-temperature flue gas in the fluidized-bed reactor,realizing the cascade utilization of energy.The cold-state experiment was conducted to acquire the characteristics of feeding,air distributor resistance and bed pressure drop.Rice husk was used in the hot-state experiment,the carbon yield reached 30.99%in stabilization condition,and the energy recover rate of fluidized-bed reactor achieved 90.16%.The biochars produced from the lab-scale reator and engineering platform were used as adsorption materials to study the property and mechanism of Cd²⁺adsorption by biochar.The adsorption process could be preferably described by the pseudo-second-order kinetic equation and the Langmuir equation,indicating that the adsorption reaction of Cd²⁺by biochar was a monolayer chemical adsorption on the homogeneous surface.By characterization of biochars before and after adsorption,such as SEM-EDS,FTIR and XPS,the main mechanisms of adsorption could be summarized as:ion exchange and precipitation between Cd²⁺and inorganic minerals,complexation between Cd²⁺and oxygen-containing functional groups,and Cd²⁺-πinteraction.The contribution of each function was ranked as follows:inorganic minerals>πbonds>oxygen-containing functional groups.The inorganic minerals contributed12.84mg/g（85.41%）in the process of adsorption by biochar derived from rice husk under500℃,playing a dominant role in it.The higher pyrolysis temperature could enhance Cd²⁺-πinteraction,while had negative effects on the contribution of oxygen-containing functional groups and inorganic minerals.The biochar produced from the engineering platform shows a great adsorption property,with an adsorption capacity of 17.68mg/g,implying the competitive advantages of the system on production technology and product performance.