Catalytic microwave torrefaction and pyrolysis of Douglas fir pellet to improve biofuel quality
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The aims of this dissertation were to understand the effects of torrefaction as pretreatment on biomass pyrolysis and catalytic pyrolysis for improving biofuel quality, and the feasibility of biochar as a cheap catalyst for hydrocarbons production in biomass catalytic pyrolysis and bio-oil upgrading. The process conditions for microwave torrefaction and pyrolysis of Douglas fir sawdust pellets were optimized. Microwave pyrolysis of Douglas fir sawdust pellet produced a comparative bio-oil yield with those from fluidized-bed pyrolysis at the optimization conditions. The phenols and guaiacols accounted for the largest amount of chemicals in the bio-oil. The specific phenolic chemicals are highly related to the reaction temperature. The torrefaction conditions, such as reaction temperature and time, significantly influenced the yields of products. The bio-oils from torrefaction contained valuable chemicals. The energy yields of torrefied biomass ranging 67.03−90.06% implied that most energy was retained in the torrefied biomass. Torrefaction as pretreatment in biomass pyrolysis favored the phenols and sugar production, producing about 3.21 to 7.50 area% hydrocarbons while reducing organic acids and furans in bio-oils. Torrefaction also altered the compositions of syngas by reducing CO2 and increasing H2 and CH4. Torrefaction improved the phenols, hydrocarbons, and hydrogen production in catalytic microwave pyrolysis. The phenols, hydrocarbons, and H2 obtained from torrefied biomass catalytic pyrolysis over biochar were up to 46 area%, 16 area%, and 27.02 vol%, respectively. These results indicated that torrefaction as pretreatment can greatly improve the quality of bio-oil and syngas in biomass pyrolysis and catalytic pyrolysis. Upgraded bio-oil was dominated by phenols (37.23 area%) and hydrocarbons (42.56 area%) at higher biochar catalyst loadings. The biochar catalyst may be as a cheap catalyst in biomass conversion and bio-oil upgrading. The two step-reaction model fits well for Douglas fir sawdust torrefaction with the activation energies of about 112 kJ/mol and 150 kJ/mol for the first and second reaction stages, respectively. Derivative thermogravimetric (DTG) curves showed that the shoulder of hemicelluloses decomposition in torrefied biomass pyrolysis was eliminated. The first-order one-step global model fitted well for the raw and torrefied biomass pyrolysis with the average activation energies in the range of 203.94 −195.13 kJ/mol.