DESIGN AND BALANCE-OF-PLANT OF A DEMONSTRATION PLANT WITH A SOLID OXIDE FUEL CELL FED BY BIOGAS FROM WASTE-WATER AND EXHAUST CARBON RECYCLING FOR ALGAE GROWTH

摘要

The design and balance-of-plant of an integrated anaerobic digestion (AD) biogas solid oxide fuel cell (SOFC) demonstration plant is presented. A notable feature of the plant is the CO_2 capture from the SOFC anode exhaust via an oxy-combustion reactor. The captured CO_2 is fed to a photo-bioreactor installation downstream of the SOFC where C is fixed in an algae bio-fuel. The main plant sections are described in detail including the gas cleaning unit, fuel processing, SOFC 'hot-box', oxy-combustor, CO_2/H_2O condensation unit and finally algae bio-reactor. The demonstration plant is fed with biogas from AD of the by-product sludge of the greatest waste-water treatment plant in Italy, serving over 2 million population equivalents in the Torino metropolitan area. In this work, the main BoP components and engineering issues concerning the design of the SOFC plant are detailed. The as-produced biogas is firstly treated to remove moisture and then filtered to remove sulfur, halogens and siloxanes. Dry clean biogas (roughly 60-65% CH_4, 35-40% CO_2) is sent to a steam-reformer. The reformate gas is thus used to feed a 2 kWe SOFC module (operated at ～ 800 ℃). The cathode off-gas is kept separated from the anode and is used to pre-heat inlet fresh air; the anode outlet stream is sent first to an oxy-combustor to yield an almost pure H_2O-CO)_2 mixture that is eventually cooled down to 300-400 ℃. Steam is condensed and separated in a dedicated condenser unit. The resulting pure CO_2 is thus pressurized (8 bar) and available for sequestration or other uses. Due to the limited size of the demo plant, the choice was to feed it to bioreactors with algae, where the latter are grown with sunlight and CO_2 indeed. A tubular photo-bioreactor has been chosen with a productivity of 20 g/day/m~2 of dry algae. The outlet stream will be an algae purge that, due to its low mass flow, could be re-sent to the biogas digesters. A system analysis of a scaled-up version of the biogas fed SOFC power plant, with heat integration included, is also carried out with a calculated overall electrical efficiency exceeding 55% (LHV basis).