Energy can be generated from the materials we throw away, which are often high in energy content. This can occur as part of a number of processes, which are detailed more below.
Municipal waste or commercial and industrial waste can be burnt directly or treated into a more refined product prior to being burnt. Materials recovery processes extract the elements of a waste stream that can be re-used or recycled (e.g. plastics, paper, cardboard), with the residual product treated to form a fuel. Solid recovered fuel ('SRF') is an example of this, and can have a calorific value of between 17 and 22 mega joules ('MJ') per kilo, compared with coal at 29 MJ/kg. SRF is used as a replacement for coal as an energy source in cement production as well as in waste to energy plants.
Advanced conversion technologies (or 'ACT') is a form of waste treatment that involves the transformation of a solid fuel (such as SRF) into a gas (referred to as a 'syngas'), which is combusted in a boiler or gas engine to generate electricity. The transformation from a solid to a gas involves heating the solid fuel in a limited supply of oxygen, a process also referred to as gasification. There are a number of these facilities around the world, however the technology (in particular the combustion of syngas in a gas engine) is still relatively unproven.
Biogenic wastes, such as food waste or slurries, can be biologically treated in order to extract the energy in those wastes as a gas (which is primarily methane, referred to as 'biogas'), which can be combusted to produce electricity or directly injected to the gas network. Anaerobic digestion is a process where biogenic material is broken down by bacteria in the absence of oxygen, to produce methane.
