Appendix 1

Calculating energy recovered from waste

It is commonplace to extract energy from waste as part of the waste management process, while reducing the waste bulk in the process.

The energy potential of waste depends on the mix of materials and their moisture content. The higher the calorific value of the waste the more energy can be extracted. The Digest of United Kingdom Energy Statistics (DTI, 1997) reports the gross (upper) calorific value of unsorted municipal solid waste (MSW) at 9 – 9.5 MJ/kg. If MSW is sorted and dried to provide a refuse-derived fuel (RDF) its calorific value increases to 18.5-18.7 MJ/kg (DTI, 1997 & DTI, 2004). Calculations by BFF, based on the South West waste composition and calorific values for each product or material ( Nolan-ITU and TBU Environmental Engineering Consultants, 2001 and ASSURRE, 2001), gives a slightly higher gross calorific value of 14.9 MJ/kg for unsorted MSW. The net (lower) calorific value of unsorted MSW has been calculated as 6.2 MJ/kg. Removing materials with a low calorific value could increase the calorific value of MSW. An example of this could be the use of mechanical biological treatment (MBT) (see Strategy Unit, 2002 for further information on MBT) as proposed in some of the Stepping Forward Waste Scenarios.

Taking these net and gross figures for unsorted MSW (6.2 MJ/kg & 14.9 MJ/kg) and assuming that conventional electricity production, using waste as fuel, operates at 35% efficiency with 2.5% network losses ( Minett, 2003), this equates to a useful electricity supply of between 0.59 kWh/kg and 1.42 kWh/kg (2.1 MJ/kg – 5.1 MJ/kg). No allowance has been made for electricity consumed within the plant, production 'down time', or useful heat extraction (as might occur in a CHP waste to energy plant).

It was not possible to ascertain exact details of the technologies proposed for energy from waste, so all the Stepping Forward scenarios use an average figure of useful energy recovery from waste of 1.0 kWh/kg (3.5 MJ/kg), which is equivalent to about 55% of the calculated net calorific value of MSW – an optimistic figure.

The calorific value of waste should not be confused with its embodied energy, which is the energy required to create the products and materials that eventually end up in the waste stream. On a material-by-material basis these are often very different. For example, timber has a high calorific value but, typically, a relatively low embodied energy as it requires little energy to cultivate, harvest, process and transport. One estimate of the embodied energy of UK MSW is 35 MJ/kg (based on BFF & Imperial College of Science & Technology, 2000). Therefore, the useful energy produced from waste represents about 10% of the embodied energy of MSW.