Proceedings

The UK Water Industry currently generates approximately 800 GWh p.a. of electrical energy from sewage sludge. With the renewable heat incentive (RHI) the economics are driving the industry to consider replacing Combined Heat Power (CHP) units with gas cleaning and biomethane injection plants, and injecting into the gas network. This study considers the environmental and economic life cycle impact of a number of options to better understand and inform decision makers whether building full scale a Gas to Grid (GtG) plant over CHP is a realistic alternative. The results indicate that RHI incentives are essential in maintaining the economic viability of GtG options, whereas the CHP options are more insulated by the relative real value of electricity to gas. Environmentally the GtG options perform considerably less beneficially and this raises questions to whether the RHI policy is structured correctly in this instance, to drive good environmental practice as such it is argued that the RHI subsidy may be at more risk of change.

The UK Water Industry currently generates approximately 800 GWh pa of electrical energy from sewage sludge, a renewable by-product from wastewater treatment. This recovery of energy has until recently been mainly conducted using two methods: Anaerobic Digestion (AD) and incineration with energy recovery, both developed and deployed with achieving efficiencies in sludge disposal as the main driver (Davis 1996, Barber 2010). Over the past 10 years significant development has been made in anaerobic digestion processes, which improves energy yields from this renewable resource, sewage sludge, which the industry has in abundance. These advanced processes have and are now being implemented at large scale across the UK and within Thames Water (Riches 2010). Typically these new and old installations use combined heat and power (CHP) to convert the bio methane into electricity currently supported under the Renewable Obligation. However, a new UK practice, Gas to Grid (GtG) aims to clean up and inject all of the bio-methane produced in AD into the gas network and is financially supported under the Renewable Heat Incentive (RHI) (DECC 2011). This short study aims to compare these two options to understand the relative technical, economic and environmental performance: in order to inform decision makers a Life Cycle Assessment approach was taken for the short study.

Keywords: Biogas Utilisation, CHP, Gas to Grid, Life Cycle Assessment