Microwave-assisted catalytic thermochemical conversion of organic solid wastes for biofuels production

Abstract

Organic solid wastes (e.g. agricultural residues, industrial residues, municipal solid waste, and others) as a renewable and abundant energy resource have received a tremendous amount of interest from all over the world to be used as a feedstock for the production of biofuels and chemicals. Using microwave-assisted thermochemical conversion technology, organic solid waste resources with low energy density can be converted into clean and high energy density biofuel including bio-oil, combustible gases, and other high-quality fuel products. In this dissertation, co-pyrolysis of corn stover and scum was studied with an aim to improve the bio-oil quality. In addition, sequential two-step catalytic fast microwave-assisted pyrolysis of biomass on a packed-bed HZSM-5 catalyst was investigated to determine the optimal catalytic upgrading temperature and catalyst stability. Furthermore, a benchtop continuous fast microwave pyrolysis system was designed and fabricated. Sewage sludge was used as the feedstock in the testing of the continuous microwave-assisted pyrolysis system. The potential energy recovery from each product was investigated. Moreover, a continuous microwave-assisted pyrolytic gasification mini-plant (6kg/h) was designed, developed, and fabricated. Performance of the continuous microwave-assisted pyrolytic gasification mini-plant was investigated. The main achievements of this research are as follows: In Chapter 3, co-pyrolysis of corn stover and scum was investigated to improve the bio-oil quality. Experiments were conducted based on the batch microwave-assisted pyrolysis reactor. Scum was used as a hydrogen-donor to increase the effective hydrogen index (H/Ceff). A significant synergistic effect between corn stover and scum was achieved to maximize the production of bio-oil and aromatic hydrocarbons when the H/Ceff value exceeded 1. CaO and HZSM-5 were used as the catalysts to improve the bio-oil quality. The addition of CaO catalyst increased both the bio-oil yields and aromatic yields. The possible reaction mechanism can be surmised as follows: when pyrolysis vapor passed through the mesoporous CaO catalyst, the heavy compounds such as large phenols and anhydrosugars were cracked into light compounds, followed by conversion of the light compounds into hydrocarbons over the microporous HZSM-5 catalyst. The optimal co-pyrolysis temperature, CaO to HZSM-5 ratio, and corn stover to scum ratio were 550 ℃, 1:4, and 1:2, respectively. In Chapter 4, sequential two-step catalytic fast microwave-assisted pyrolysis of biomass was developed to find out the optimal catalytic upgrading temperature and the stability of catalyst. A newly developed sequential two-step catalytic fast microwave-assisted pyrolysis of biomass through a packed-bed HZSM-5 catalyst for high quality bio-oil production was developed and investigated. Through the sequential two-step fMAP set up, the pyrolysis and catalytic cracking and upgrading processes can be flexibly and independently controlled, and the catalyst recovery and regeneration process is simplified. In addition, the catalyst bed temperature and catalyst loading significantly affected the product distribution. The optimal pyrolysis and catalyst bed temperature were 550 ℃ and 425 ℃, respectively. The highest bio-oil yield (33.38 wt. %) and maximum proportion of aromatic compounds (26.20 %) were achieved. Furthermore, increasing the catalyst loading reduced the bio-oil yield but improved the bio-oil quality. Moreover, X-ray Diffraction (XRD) analysis was conducted on the fresh, spent and regenerated HZSM-5 catalyst samples, and the catalyst characterization indicated a good stability of HZSM-5 catalyst during the upgrading process. In Chapter 5, a continuous fast microwave pyrolysis system was developed based on Chapter 4 and was used for sewage sludge utilization. A benchtop continuous fMAP system for ex-situ catalytic upgrading of the pyrolytic vapors was designed, fabricated and tested. After examining the effects of pyrolysis temperature on the physical, chemical, and energetic properties of the products, it is concluded that the optimal pyrolysis temperature for maximum overall energy recovery is 500 °C, at which the total Qrecovery is 18.42 MJ/kg with energy values of bio-oil, gas and biochar are 7.16 MJ/kg, 8.69 MJ/kg and 2.57 MJ/kg, respectively. In addition, SEM micrographs of sewage sludge and biochar samples showed that the raw sewage sludge had relatively smooth surface while the biochar had rough surface and porous structure. The formation of the pores of biochar may be due to the evaporation of water and emission of volatile matter during the pyrolysis process. Furthermore, high HHV of bio-oil (20.61 MJ/kg) and gas (22.5 MJ/Nm3) imply that sewage sludge can be used as a promising biomass resource in the future. In Chapter 6, a continuous microwave-assisted pyrolytic gasification mini-plant was designed and developed based on Chapter 3, 4, 5 and performance of the system was investigated using pinewood pellets as the feedstock. The product distribution and energy balance were investigated. Produced gases were analyzed by Micro-GC and tar was analyzed using GC/MS. The LHV of produced gases from wood pellets pyrolytic gasification can reach 14.18 MJ/Nm3. In summary, the research work presented in this dissertation aims to provide scientific reference and technical support for the research and further development of microwave-assisted catalytic thermochemical conversion technology for organic solid waste processing. Using microwave-assisted thermochemical conversion technology, organic solid waste resources with low energy density can be converted into clean and high energy density biofuel including bio-oil, combustible gases, biochar and other high-quality fuel products. Microwave-assisted catalytic thermochemical conversion of organic solid waste is a promising technology for valuable biofuels production.