Study On Catalytic Reforming Of Biomass Pyrolysis Volatile Matter Under Microwave-assisted Heating

Biomass pyrolysis is a thermochemical process for conversion of biomass into a series of highly value-added products in the absence of oxygen or in an oxygen-deficient atmosphere.Volatile matter contains many condensable fine chemicals such as phenol,and non-condensable high-calorific gas products such as syngas.However,biomass pyrolysis can be a complex and difficult process to control,resulting in a low selectivity of the desired product and undesirable product properties,significantly limiting the widespread practical application of biomass pyrolysis products.Thermal cracking and catalytic reforming are the two main methods used for upgrading volatile matter.Thermal cracking method generally demands a high energy input to maintain a high temperature system to decompose the macromolecular bio-oil compounds into small molecular compounds and gas products.In contrast,catalytic reforming,when the proper catalyst is used,has been proven to be a promising technique for converting bio-oils into fine chemicals and fuel gases at low temperatures.Catalyst is the core of catalytic reforming technology.Among the numerous catalysts,carbon-based catalysts,specifically activated carbon,exhibit excellent catalytic activity for volatile matter reforming due to its well-developed pore structures and high surface areas.Another important factor affecting the catalytic reforming process is the heating method.Microwave-assisted heating method with the advantages of uniform heating,fast heating rate,and volumetric heating can greatly reduce the occurrence of secondary side reactions during the reforming process,which is beneficial to the improvement of reforming efficiency.In this study,a two-stage experimental bench was used to conduct thermal cracking and activated carbon catalytic reforming of palm kernel shell pyrolysis volatiles.The effects of reaction temperature and heating method on product distribution and composition were investigated.The changes of phenol concentration in bio-oil and hydrogen-rich syngas content in gas products were analyzed.Meanwhile,guaiacol and syringol were selected as the model compounds,the effects of thermal cracking and catalytic reforming processes on the product distribution of model compounds under different heating methods were studied,and the role of microwave in the thermal cracking and catalytic reforming processes was discussed.Thermal cracking and catalytic reforming of palm kernel shell pyrolysis volatiles under electric heating method showed that both thermal cracking and catalytic reforming promoted the generation of gas products.Compared with thermal cracking,catalytic reforming shows higher characteristics of promoting the conversion of bio-oil to gas product.Moreover,the activated carbon catalytic reforming process promotes the formation of phenol and hydrogen-rich syngas.Thermal cracking and catalytic reforming of palm kernel shell pyrolysis volatiles under microwave-assisted heating method showed that higher bio-oil conversion was obtained in the microwave-assisted catalytic reforming than that in the electric-heating catalytic reforming,in addition,the selectivity of phenol in bio-oil and hydrogen-rich syngas in gas products was significantly enhanced.By comparing the yields of phenol and hydrogen-rich syngas after catalytic reforming under the two heating methods,it was found that activated carbon catalytic reforming of palm kernel shell pyrolysis volatiles under microwave-assisted heating has a great potential for poly-generation of phenol and hydrogen-rich syngas.The optimum experimental conditions for the poly-generation of phenol and hydrogen-rich syngas were as follows: pyrolysis temperature of PKS was600°C,and volatiles reforming temperature was 700°C.Under these conditions,the yields of phenol and hydrogen-rich syngas were 5.56mg/g and 426.67 m L/g,respectively.Moreover,activated carbon catalyst exhibited better reusable performance under microwave-assisted heating.In view of the phenomenon that the thermal cracking and catalytic reforming process under microwave-assisted heating has high selectivity to phenol and hydrogen,guaiacol and syringol were selected as bio-oil model compounds.The role of microwave was studied systematically by comparing the product yields and distribution during the catalytic reforming under microwave-assisted heating with those under the electric heating mode.Results showed that the microwave-assisted heating method and higher reaction temperature are beneficial to the conversion of model compounds.Due to the polarization effect of the microwave on the polar groups in the structure of the model compound,the reaction in the thermal cracking process under microwave-assisted heating mainly around the methoxy group because of its poor stability,which increase the phenolics concentration in bio-oil.In the process of catalytic reforming of model compounds,an evident dielectric loss phenomenon induced by microwave radiation,resulted in the generation of electron-hole pairs on the surface of activated carbon.The electron-hole pair is the main active site of the reforming reaction and easily adsorbs strong electron-donating groups,particularly methoxy,in the chemical structure of the model compound,which subsequently promoted the demethoxylation and methane reforming reactions to selectively generate phenol and hydrogen.