Piggery waste to sustainable fuels via indirect supercritical water gasification and membrane reforming at 600 °C: a techno-economic assessment

Abstract

Efficient management of pork effluent can substantially reduce emissions from Australia's pork industry by up to two-thirds, and contribute to the important global challenge of decarbonising the agriculture sector. In contrast to anaerobic digestion, which faces operational challenges and low yield, supercritical water gasification (SCWG) offers a high-efficiency conversion of biomass with no residue disposal concerns. This study presents an environmentally-sustainable process for hydrogen production through the SCWG of piggery manure coupled with steam methane reforming (SMR) via a hydrogen-selective Pd-based membrane. The proposed process enables the endothermic SCWG-SMR processes to operate at a temperature below 600 °C, making it compatible with emerging molten nitrate salt thermal energy storage systems. The ‘green heat’ supply options namely concentrating solar thermal, photovoltaic (PV), wind turbines, and hybrid PV/wind, are optimised for a design-point process thermal input of 2.3 MW_th, considering levelised cost of heat (LCOH) and capacity factor (CF) as the objective functions. A detailed steady-state physical model of the plant is developed in Aspen Plus software. The proposed plant produces 14.4 kg H₂ per h using 150 kg h⁻¹ of dry piggery waste with a 73% methane conversion rate. The lowest levelised cost of hydrogen—obtained from the LCOH-CF Pareto front curves—is 13.8 USD per kg achieving a capacity factor of 80% at a renewable multiple of 2 and a storage capacity of 18 hours. Given the high cost of the produced hydrogen, the future potential of this technology would appear to lie in the production of gasoline/diesel, methanol, ammonia or bioplastics.