Economic analysis of sewage sludge gasification in supercritical water for hydrogen production

Supercritical water gasification (SCWG) is a process that is currently being developed for the gasification of biomass in supercritical water. The process principle is to make use of the changes of specific physical and chemical properties of water above the critical point. These properties allow for a nearly complete conversion of biomass into energy-rich fuel gases, namely hydrogen, methane, and carbon dioxide. The present article deals with the economic analysis of SCWG with sewage sludge as feedstock. SCWG is aimed at generating hydrogen as secondary fuel and, in addition, at serving as a method for disposing of bio-based sewage sludge waste. Based on the results of this economic analysis, the competitiveness of the process can be assessed in comparison to conventional technologies of hydrogen production. The costs of hydrogen production and revenues obtained from the disposal of sewage sludge are determined using the method of the total annual revenue requirement. Under the assumptions made, carrying charges have by far the largest share in the revenue requirement. In contrast, fuel costs are comparably small. It is found that supercritical gasification may well be competitive due to the revenues associated with the disposal of sewage sludge as a waste product. To study the influence of the assumptions made in the economic analysis on the final results, a sensitivity analysis is made.