Abstract
Biomass shares around 10 to 14% of the world energy supply. Biomass utilization for energy production is expected to increase in the future because of the foreseen decrease in fossil fuel supply. Moreover, the energy coming from biomass is considered to be CO2 neutral, therefore not contributing to the increase in greenhouse gas emission and global warming. Gasification is a peculiar way to produce energy from biomass, as well as added-value products. Syngas is the desired product of gasification, which can be converted into electricity, heat, chemicals and fuel through Fischer-Tropsch. Other than syngas, tar and char are the main by-products of the gasification process. Tar is a mixture of organic compounds with molecular weights higher than benzene and is one the main hurdles in gasification commercialization on an industrial scale. Char is the carbonaceous by-product of biomass pyrolysis and gasification. Nowadays, it is considered as a waste and disposed of in landfills. Consequently, it represents a loss for plant owners. Nonetheless, char’s physiochemical properties, e.g., high carbon content, large surface area and porosity, make it attractive for alternative and innovative applications. Moreover, the large surface area and the highly porous structure influence the catalytic activity of char. In the first part of the thesis, the focus is on how the different gasification technologies affect the char properties. Char can be used as a support in catalyst preparation. For this purpose, three char samples were collected from commercial small-scale gasification plants, based on three different technologies, i.e., downdraft gasification, updraft gasification and dual stage gasification. The char samples were characterized by using elemental composition, surface morphology and functional group identification. From these char samples, acidic catalysts were prepared by mixing a calculated amount of char with sulfuric acid in a closed cup autoclave at 150°C for 12 hours. The synthesized catalysts were characterized by various analytical techniques to explore their physicochemical properties that were correlated with their catalytic activities and stabilities. The results showed that the physicochemical properties of the synthesized catalysts strongly depend on the char production technique. The catalytic activity of the synthesized char-based acidic catalysts was evaluated by employing the methyl ester production from waste cooking oil with methanol. The results demonstrated that the dual stage gasification char catalyst showed higher ester conversion and better reusability than the updraft and downdraft gasification char catalysts. The optimization of esterification was studied by varying the different reaction variables, such as catalyst loading, oil to methanol ratio, reaction time and reaction temperature. The results showed that the catalyst loading and oil to methanol ratio were the most influential factors. All the catalysts were shown to have substantial regeneration and reusability and could be used up to six times without any significant loss in the ester conversion activity. The produced biodiesel fuel properties were within the range of the prescribed ASTM and EN standards. In order to understand which are the main factors affecting the char properties, especially its carbon content and surface area, char was produced under different operating conditions (i.e., carrier gasesnitrogen or carbon dioxide, process temperature set at 500°C, 700°C and 900°C, and initial heating rates of 10°C min-1 , 30°C min-1 or 50°C min-1 ). The results showed that temperature and carrier gas are the most influential factors affecting the char properties. Char produced under an inert nitrogen atmosphere has a higher yield as compared to those obtained under the carbon dioxide atmosphere; whereas, the reverse trend was observed for the surface area. These outcomes suggest that the desired properties of char can be produced by tuning the process conditions with existing pyrolysis and gasification technologies depending on the specific foreseen application.