Concerns about diminishing fossil fuels and increasing greenhouse gas emissions are driving many countries to develop renewable energy sources. In this respect, biomass provides a carbon-neutral and sustainable solution. Pyrolysis and gasification belong to thermochemical processes which are currently the most appropriate and widely used among all the biomass utilization technologies. Microwave irradiation can provide heating for biomass pyrolysis and gasification, and has many advantages over conventional heating methods. In this dissertation, microwave heating was used in biomass pyrolysis and gasification for the production of bio-oil and syngas, respectively. In addition, in order to utilize the syngas produced, a single-step process was investigated for converting syngas to dimethyl ether (DME) on various bifunctional catalysts. In Chapter 2, the microwave heating characteristics of various biomass feedstocks and microwave absorbents were examined and compared. Experimental results show that microwave absorbents absorbed the microwave irradiation more effectively than biomass. The addition of these microwave absorbents to biomass feedstock during microwave-assisted thermochemical conversion significantly improved the heating characteristics. Among the three microwave absorbents studied, silicon carbide (SiC) exhibited higher microwave absorbing ability than activated carbon (AC) and graphite (GE), which was mainly attributed to a higher dielectric loss tangent (tan ) value of silicon carbide. In addition, higher microwave absorbing ability and heating rates were achieved when more microwave absorbents were used. Finally, a fast microwave-assisted biomass conversion system was developed. In Chapter 3, fast microwave-assisted catalytic co-pyrolysis of microalgae and scum on HZSM-5 catalyst for bio-oil production was investigated. The effects of co-pyrolysis temperature, catalyst to feed ratio, and microalgae to scum ratio on bio-oil yield and composition were examined. Experimental results show that temperature had great influence on the co-pyrolysis process. The optimal temperature was 550 ºC since the maximum bio-oil yield and highest proportion of aromatic hydrocarbons in the bio-oil were obtained at this temperature. The bio-oil yield decreased when catalyst was used, but the production of aromatic hydrocarbons was significantly promoted when the catalyst to feed ratio increased from 1:1 to 2:1. Co-feeding of scum improved the bio-oil and aromatics production, with the optimal microalgae to scum ratio being 1:2 from the perspective of bio-oil quality. The synergistic effect between microalgae and scum during the co-pyrolysis process became significant only when the effective hydrogen index (EHI) of feedstock was larger than about 0.7. In addition, to better understand the fMAP of microalgae, the different roles of three major components, i.e., carbohydrates, proteins, and lipids, were investigated. Cellulose, egg whites, and canola oil were employed as the model compounds of the three components, respectively. Non-catalytic and catalytic fMAP were carried out to identify and quantify some major products, and several reaction pathways were proposed for the pyrolysis of each model compound based on the data obtained. Moreover, a two-step process of microalgae pyrolysis and downstream catalytic reforming was conducted and compared with the one-step process for bio-oil production. The results show that a lower bio-oil yield and higher bio-oil quality were achieved for the two-step process than the one-step process at the same catalyst to feed ratio. The main advantages of the two-step process lie in catalyst saving and reuse. Furthermore, fast microwave-assisted catalytic pyrolysis of sewage sludge was investigated for bio-oil production, with HZSM-5 as the catalyst. Pyrolysis temperature and catalyst to feed ratio were examined for their effects on bio-oil yield and composition. Experimental results show that microwave is an effective heating method for sewage sludge pyrolysis. Temperature has great influence on the pyrolysis process. The maximum bio-oil yield and the lowest proportions of oxygen- and nitrogen-containing compounds in the bio-oil were obtained at 550 oC. The oil yield decreased when catalyst was used, but the proportions of oxygen- and nitrogen-containing compounds were significantly reduced when the catalyst to feed ratio increased from 1:1 to 2:1. Essential mineral elements were concentrated in the biochar after pyrolysis, which could be used as a soil amendment in place of fertilizer. Results of XRD analyses demonstrated that HZSM-5 catalyst exhibited good stability during the microwave-assisted pyrolysis of sewage sludge. In Chapter 4, the microwave-assisted biomass conversion system developed in Chapter 2 was used in corn stover gasification for syngas production. Three catalysts including Fe, Co and Ni with Al2O3 support were examined and compared for their effects on syngas production and tar removal. Experimental results show that microwave is an effective heating method for biomass gasification. Ni/Al2O3 was found to be the most effective catalyst for syngas production and tar removal. The gas yield reached above 80% and the composition of tar was the simplest when Ni/Al2O3 catalyst was used. The optimal catalyst to biomass ratio was determined to be 1:5–1:3. The addition of steam was found to be able to improve the gas production and syngas quality. Results of XRD analyses demonstrate that Ni/Al2O3 catalyst had good stability during gasification process. Finally, a new concept of microwave-assisted dual fluidized bed gasifier was put forward for the first time in all studies in the literature. To further utilize the syngas produced from biomass gasification, single-step synthesis of DME from syngas on bifunctional catalysts containing Cu-ZnO-Al2O3 and seven different zeolites was investigated in Chapter 5. Various characterization techniques were used to determine the structure, reducibility, and surface acidity of the catalysts. Experimental results show that the zeolite type had great influence on the activity, selectivity and stability of the bifunctional catalyst during the syngas-to-DME process. Zeolite properties including density of weak and strong acid sites, pore structure, and Si/Al distribution were found to affect the CO conversion and DME selectivity of the bifunctional catalyst. In addition, the deactivation of the bifunctional catalyst could be attributed to the sintering of metallic Cu and a loss of the zeolite dehydration activity. In summary, microwave irradiation is an effective heating method for biomass thermochemical conversion for biofuel production. Fast microwave-assisted biomass pyrolysis and gasification, using silicon carbide as the microwave absorbent, were carried out for the production of bio-oil and syngas, respectively. In addition, single-step synthesis of DME from syngas on various bifunctional catalysts was conducted with the aim of fully utilizing the syngas produced from biomass gasification. Although there are still many challenges associated with the production of biofuels via fast microwave-assisted thermochemical conversion, this dissertation offers a valuable insight into the potential of and some basic mechanisms of the technology.
University of Minnesota Ph.D. dissertation. December 2015. Major: Bioproducts/Biosystems Science Engineering and Management. Advisor: Roger Ruan. 1 computer file (PDF); xi, 118 pages.
Fast microwave-assisted thermochemical conversion of biomass for biofuel production.
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