Conversion of Biomass to Renewable Liquid Feedstocks in Pyrolysis-based Applications
Time: Fri 2019-10-18 10.00
Subject area: Materials Science and Engineering
Doctoral student: Henry Persson , Materialvetenskap
Opponent: Professor Louise Olsson,
Supervisor: Docent Weihong Yang, Materialvetenskap
The production of chemicals and fuels currently rely on fossil resources which are associated with global warming as well as economic and political instabilities. The demand for renewable alternatives is increasing as legislations on greenhouse gas emissions are becoming more stringent. Biomass is the main renewable carbonaceous feedstock that can be converted into chemicals and fuels. Pyrolysis allows the conversion of biomass to liquid feedstocks that could substitute the reliance on fossil crude. However, liquids originating from pyrolysis of biomass have unfavourable characteristics with respect to technical requirements from end-users. Therefore, further research and development of the biomass-to-liquid conversion is needed to enhance the quality of derived liquids. By developing liquefaction processes based on the influence of the biomass characteristics on the derived products, the conversion routes can be optimized to produce targeted precursors. In this dissertation, the selective conversion of lignocellulosic biomass into renewable liquid feedstocks by combining pyrolysis with upstream and downstream process modifications was experimentally investigated. Pre-treatment of biomass in aqueous solutions of organic acids found in the pyrolysis liquid was investigated to reduce the ash content in biomass, known for catalyzing the cracking of pyrolysis vapors. Results show that the major fraction of ash can be removed without affecting the volatile matter of biomass. The composition of pyrolysis liquids derived from pre-treated biomass is significantly different to raw biomass, with increased selectivity of anhydrosugars and suppression of low molecular weight compounds such as carbonyls. Stepwise pyrolysis was investigated to produce multiple fractionated liquids with compositions based on the thermal stability of the biomass polymers. A process concept of two pyrolysis units connected in-series, operating at 200 to 300 C and 550C, respectively, was investigated. The results show that stepwise pyrolysis can be used to concentrate chemical compounds into fractionated liquids without reducing the total amount of liquid derived from biomass. However, complete separation of chemicals in a two-step pyrolysis setup faces technical difficulties due to the overlap in thermal decomposition temperatures of the biomass polymers. Catalytic pyrolysis was studied for the production of aromatic hydrocarbons from biomass. Metal-doped zeolitic catalysts were prepared and studied based on the catalyst activity and the deactivation characteristics. Fe and Ni were impregnated into HZSM-5 followed by catalytic pyrolysis and investigation of the liquid and coke properties. The coke composition reflects the catalytic activity observed for upgraded liquids. Metal-doping promotes the conversion of vapors into aromatic hydrocarbons, increases the catalyst deactivation rate, and alters the catalyst regeneration conditions. The influence of the vapor composition fed for catalytic upgrading was studied by comparing the differences when using the pre-treated demineralized biomass mentioned above and raw biomass in catalytic pyrolysis. For ex-bed catalytic pyrolysis at 600C using HZSM-5, pre-treated biomass results in higher conversion of biomass to aromatic hydrocarbons compared to raw biomass. This could be explained by a favorable composition of secondary pyrolysis vapors from pre-treated biomass for catalytic upgrading over HZSM-5. Lastly, a continuous ex-situ catalytic fast pyrolysis process was experimentally investigated. The performance of HZSM-5 for continuous upgrading of pyrolysis vapors was evaluated by varying the biomass feeding rate to the pyrolyzer followed by upgrading over a fixed amount of catalyst. The results indicate the significance of the biomass-to-catalyst ratio in the design of large-scale processes in terms of the magnitude of catalytic conversion of pyrolysis vapors.