Renewable Natural Gas (RNG) has been identified as an important alternative fuel that can help to achieve a number of national goals related to the reduction of fossil fuels and to the reduction in carbon dioxide emission. RNG can be produced from various carbonaceous materials such as biomass and organic wastes via a gasification process. The CE-CERT steam hydrogasification technology combines hydrogen with steam under pressurized conditions to convert a wet feedstock to a methane enriched syngas which can be upgraded to RNG and used for electric power and as an alternative transportation fuel.
The main objective of this thesis is to develop a new process configuration of the steam hydrogasifier (SHR) using a water gas shift (WGS) reactor for the increased production of RNG in an economical manner. The producer gas from the SHR consists of H2, CH4, CO, CO2 and steam. The WGS process converts CO using the existing steam in the product stream from the SHR to H2 and CO2. This results in a self-sustainable internal source of hydrogen that can be generated internally and of sufficient quantity to be recycled back to the SHR. In this study, a mixture gas comparable to the composition of the producer gas from the SHR when using biosolids comingled with green waste was used as the input gas to the WGS reactor. A lab-scale WGS reaction system using a commercially available high temperature shift (HTS) catalyst was designed and built. The influence of temperature, space velocity, gas composition and particle size of catalysts on the conversion of CO using this system was investigated. It was found that at optimum conditions approximately 65% of the CO can be converted using the HTS catalyst operating at 350oC and atmospheric pressure. The hydrogen produced at this condition was sufficient to be recycled back to the SHR. Also, the overall change of CH4 production was negligible.
A mixture gas containing a wide range of H2S was carried out in the shift reactor using a Sour Gas Shift (SGS) catalyst to determine the influence of sulfur impurities. The conversion of the mixture gas with 350ppm of sulfur was 55% at the reaction temperature of 450oC and a space velocity of 2500h-1, compared to 65% with no sulfur. The percent conversion of CO could be improved if the composition of CO2 was decreased.
A larger scale HTS reactor was designed and built using the data from the experiments mentioned above. This was integrated into a bench scale Process Demonstration Unit (PDU) of the SHR with a capacity of 0.1TPD of dry feed. This process configuration demonstrated the production of high levels of RNG based for the CE-CERT technology using co-mingled biosolids and woody biomass as the feedstock. The reaction temperature of the SHR and WGS reactor were set at 750oC and 350oC, respectively, with an operating pressure at 150psi. A carbon conversion of 42% for the SHR and CO conversion of 60% of WGS were achieved at those conditions. The final product gas contained over 85% of CH4 after gas cleanup in which 85MJ/day or 32.6MJ/m3 of energy was produced. This result clearly shows the viability of the process and provides critical design information to upscale to a pilot facility in the near future.
