The gasification of biomass is potentially an efficient and economically viable technology to assist in reducing the global dependence on fossil fuels and carbon dioxide emission. CE-CERT steam hydrogasification technology combines hydrogen with water to convert carbonaceous matters to a high energetic synthesis gas which offers several advantages for biomass gasification compared to other mainstream gasification technologies. Although steam hydrogasification appears to be an advantageous gasification technology, it still needs to be improved for optimum efficiencies to assess the overall commercial viability. A particular challenge for the scaling-up steam hydrogasification process is the low reactivity of carbon with hydrogen. The gas residence time in the large-scale gasifier needs to be substantially long to account for the slow reaction rate in the presence of hydrogen. The consequence is that much larger size and more expensive gasifier is required to implement the large-scale CE-CERT process which leads to a considerably high initial investment cost. The main objective of this thesis is to investigate the effect of biosolids serving on the steam hydrogasification of biomass for improving the steam hydrogasification efficiency. The use of biosolids is motivated by its low cost, integrated in the steam gasification as the feedstock and the potential for catalytic effect due to its typically high concentration in iron and calcium. This research will not only contribute to technological analysis for the scaling-up CE-CERT steam hydrogasification process but also provide an alternative pathway for biosolids energy recovery in a cost-efficient manner.
Characteristics and kinetics of steam hydrogasification of biomass was investigated initially in chapter two with the objectives to optimize the reaction conditions and to establish a simplified approach for kinetic measurements in the steam hydrogasification. A newly designed laboratory-scale reactor, referred to as the inverted batch reactor is used in this thesis. This reactor succeeded performing the desired realistic thermal conditions to allow for fundamental laboratory studies of the steam hydrogasification. The heating rate and temperature were found to have significant influence on the steam hydrogasification. Kinetic parameters were obtained by using a first-order kinetic model based on product gas formation.
Steam hydrogasification of the co-mingled biosolids with biomass was investigated in chapter three to determine the effect of biosolids on the steam hydrogasification of biomass. It is experimentally demonstrated that the biosolids integrated in the feed dramatically improved the steam hydrogasification efficiency not only in carbon conversion but also in gasification reaction rate. The steam hydrogasification with 50wt% biosolids in the feed yielded more than 70% carbon conversion at 700oC. There was an approximately 6% increase in the carbon conversion compared to the steam hydrogasification of biomass. The gasification rates of CH4 and CO formation were enhanced by 18% and 12%, respectively. The characterization of metal elemental compositions in the biosolids revealed that the biosolids was particular rich in Fe, Ca.
Catalytic steam hydrogasification of biomass with different catalysts based on metal compounds of Fe, Ca, Mg, Na and K is investigated in chapter three. Steam hydrogasification of biomass with non-catalysts, with the in-situ biosolids and with the added catalysts were evaluated to identify the catalytic activity of biosolids. The catalytic activity of catalysts in the steam hydrogasification for CO formation followed the order of iron catalysts > biosolids > other tested catalysts. And the catalytic activity for CH4 formation was in the sequence of calcined dolomite > biosolids > other tested catalysts. Biosolids in-situ integrated in biomass feedstock was found to be highly active and effective to catalyze the steam hydrogasification and improve the steam hydrogasification efficiency. The catalytic action of biosolids on the steam hydrogasification may be attributed to the synergistic effect of iron, calcium and alkali metals in the biosolids.