Distributed and micro-generation from biogas and agricultural application of sewage sludge: Comparative environmental performance analysis using life cycle approaches

1 m3 biogas can save 0.92 m3 natural gas. An average UK household (with a demand of 2 kWe) requires 180,000 MJ or 5000 Nm3 or 4.76 t biogas per year, from 15.87 t sewage sludge processed through AD. The proton exchange membrane fuel cell (PEM FC) is suitable for building micro-generations; micro gas turbine (Micro GT), solid oxide fuel cell (SOFC) and SOFC-GT hybrid are suitable for distributed generations upto 500 kWe and occasionally over 500 kWe; engine and ignition engine above 1 MWe. These CHP technologies can be ranked from the lowest to the highest impacts per unit energy production: PEM FC is the environmentally most benign option, followed by SOFC, SOFC-GT, Engine or Micro GT and Ignition engine (with the highest impact potential), respectively. In terms of avoided global warming, acidification and photochemical ozone creation potentials, compared to equivalent natural gas based systems, the biogas based PEM FC micro-generation and Micro GT distributed systems achieve the greatest avoided emissions with the most cost-effectiveness. Application of digested sludge as fertiliser has more toxicity impacts, however, has greater avoided emissions in acidification and photochemical ozone creation potentials on the basis of inorganic N:P:K fertiliser, compared to the biogas production for the natural gas grid system.