Synthetic natural gas production using CO2-rich waste stream from hydrothermal carbonization of biomass: Effect of impurities on the catalytic activity

The utilization of biomass and bio-waste, particularly through hydrothermal processes, has shown promise as a technology for converting these materials into valuable products. While most research has traditionally focused on the solid and liquid byproducts of these hydrothermal treatments, the gaseous phase has often been over-looked. This study specifically investigates the conversion of off-gases produced during hydrothermal carbonation (HTC) into synthetic natural gas, offering a readily marketable product with economic potential. Although the methanation of conventional flue gases has been extensively studied, dealing with non-standard off-gases from processes like HTC presents challenges due to the presence of minor impurities like CO and CH4. This novel research seeks to experimentally evaluate the methanation of HTC off-gases using nickel-based catalysts and analyze how these impurities affect the catalytic performance. The studied catalysts include nickel supported by ceria and alumina, as well as alumina supported nickel-cobalt systems. The results demonstrate that these catalysts exhibit high CO2 conversion and CH4 selectivity under ideal gas conditions. However, when real gas compositions with impurities are considered, CO2 conversion decreases at lower temperatures (ca. 20% lower conversion for real gas vs. ideal), probably due to side reactions such as CH4 cracking. This difference becomes less pronounced at higher temperatures. Nevertheless, the catalysts perform satisfactorily, especially at temperatures exceeding 350 degrees C. In conclusion, this study sheds light on the methanation of HTC off-gases and underscores the significance of understanding how impurities in real gases impact the process, providing potential directions for future research.