Transformation Of Oxygen During Chemical-looping Pyrolysis Of Microalgae

Biomass chemical looping pyrolysis（CLPy）is a new type of pyrolysis technology that couples the chemical looping pyrolysis process with the chemical looping gasification process.The simultaneous production of high-quality bio-oil and syngas in the pyrolysis process and the regeneration of oxygen carriers,which has significant advantages compared to the direct pyrolysis process.By analyzing the pyrolysis characteristics and oxygen elements migration behavior of biomass in the process of chemical looping pyrolysis,it can provide a theoretical basis for the preparation of high-quality bio-oil or chemicals by chemical looping pyrolysis.Therefore,in this paper,Chlorella was used as raw material to study the effect of reduced Ca₂Fe₂O₅（Re-CF）oxygen carrier on the pyrolysis characteristics and oxygen migration path of Chlorella during chemical looping pyrolysis.The main research contents are as follows:Firstly,the pyrolysis characteristics of the chemical looping pyrolysis of Chlorella were studied,and the pyrolysis kinetics of each component of Chlorella was analyzed by a multi-component pyrolysis kinetic model based on Gaussian fitting of differential thermogravimetric（dw/d T）curves.Compared with the direct pyrolysis process,Re-CF oxygen carrier promoted the decomposition of lipids.At the same time,the Re-CF oxygen carrier acts as an oxygen acceptor in the pyrolysis stage and reacts with oxygen-containing species and functional groups in the pyrolysis product to promote the transfer of oxygen elements in the pyrolysis product to the solid-phase product,resulting in a reduction in weight loss during the chemical looping pyrolysis process.In the gasification stage,the Re-CF oxygen carrier participates in the gasification reaction of pyrolysis coke as an oxygen donor,which promotes the transfer of oxygen elements in solid-phase products to gas-phase products,resulting in an obvious thermal weight loss peak in the chemical looping pyrolysis curve.The calculation results of the reaction kinetics of each component of Chlorella show that the addition of Re-CF oxygen carrier reduces the pyrolysis temperature of carbohydrates and proteins,and significantly increases the pyrolysis activation energy of proteins and lipids.The kinetic model of is proteins also transformed from a 1.5-order reaction series model to a 2-order reaction series model,while the pyrolysis temperature of lipids is slightly increased.The pre-exponential factors of the pyrolysis process of the three components were all enhanced to a great extent.Secondly,the Chlorella chemical looping pyrolysis experiments were carried out in a fixed bed reactor,the pyrolysis mechanism and oxygen transfer behavior of Chlorella during the chemical looping pyrolysis were analyzed by GC-MS,FTIR and XRD characterization methods.The Re-CF oxygen carrier significantly reduced the oxygen content of the bio-oil and improved the quality of biomass oil.There are two main ways of oxygen migration in the process of chemical looping pyrolysis:（1）The ketone reaction of pyrolysis products.The pyrolysis products are converted into ketones by the bulk ketonization of Ca O and the surface ketonization of the metal oxide under the action of Re-CF oxygen carrier,which promotes the transfer of oxygen elements from liquid products to solid products and gas products.（2）Ketones and carboxylic acids undergo decarboxylation and hydrodeoxygenation reactions to generate hydrocarbons and alcohols under the action of Re-CF oxygen carrier,and alcohols can further generate hydrocarbons,to achieve the transfer of oxygen elements from liquid-phase products to gas-phase products.During the pyrolysis process,the gas-phase products were mainly CO₂ and H₂,while the Re-CF oxygen carrier was oxidized to Ca₂Fe₂O₅ and Ca CO₃.In the process of gasification,the oxidized Re-CF oxygen carrier reacts with pyrolysis char to generate high-quality syngas,which realizes the transformation of oxygen elements from solid-phase products to gas-phase products,and realize the cyclic regeneration of oxygen carrier.