Supercritical water gasification vs. biomethanation:Process simulation under Norwegian conditions

Abstracts Biomass as an energy source currently generates a growing interest worldwide for three main reasons that are worth mentioning, although they are usually well-known : environmental friendly aspect, as biomass is in a large extend carbon neutral, security of supply, and potential for smaller scale, decentralized energy production. However, it has been highlighted that energy production from biomass must not compete with food production. In this regard, non edible feedstocks are of interest, and among them wet biomass. That name encompasses macro algae and microalgae, sludge from waste water treatment plants, cattle manures, food industry wastes and any other biomass stock or waste with a very high moisture content, around 70% or more. Due to that high moisture content, conventional methods such as direct combustion, pyrolysis or gasification are not adapted. In this paper, two conversion methods are studied and compared: biomethanation and supercritical water gasification (SCWG). The aforementioned SCWG process is modeled under Aspen Plus, while the biomethanation process is modeled with Superpro. The final product is methane-enriched gas for the SCWG process (mixture of CH4, H2, CO, CO2 and some other compounds), and a mixture of CO2 and CH4 for the biomethanation process. Different raw materials are used as input, particularly macro algae, which are an interesting feedstock in Norway. Energy efficiency obtained from the results of the simulation with Aspen Plus for the SCWG process, whereas experimental yields on different substrates are used for the biomethanation process. The energy efficiencies of both processes running on different feedstocks are compared. Eventually, an economic analysis is conducted to compare the two processes with another criterion in the Norwegian context. Results show that SCWG coupled to an efficient heat integration pattern leads to energy efficiency significantly higher than biomethanation, depending on the feedstock. For water Hyacinth, the overall energy efficiency ranges between 70% and 80% according to the temperature difference at the Pinch point with SCWG, whereas it is 55% for biomethanation. The difference is even higher for feedstock’s containing large share of lignin, which is not treated by conventional biomethanation plants. Those advantages should lead to the development of SCWG technology in the future, despite current larger investment costs. Keywords: Hydrothermal gasification, supercritical water, wet biomass, biomethanation, sludge, energy efficiency, combined heat and power, heat integration, Pinch analysis, process simulation, Aspen Plus, Superpro, macro algae, techno-economic analysis.