Advanced thermal treatment (ATT) turns combustible solid waste – from households and businesses – into heat and energy using pyrolysis and/or gasification. This is not a new concept and has traditionally been used to produce fuels such as charcoal, coke and town gas. However, it is only with the sharp escalation in landfill costs that it has become commercially feasible to use the technology to treat household and commercial waste, and generate renewable energy.
ATT should not be confused with incineration, where a naked flame is used to destroy organic material, resulting in the release of chemical emissions and particulates.
For pyrolysis/gasification no naked flame is used. Instead, the procedure takes place in a sealed chamber heated by an external source. The heat causes organic materials to decompose gently over time, releasing a carbon-rich gas that can be used to generate electricity.
The waste used as fuel is material that is not easily recyclable and might otherwise go to landfill. The residual ash can be processed and used by the construction industry.
ATT plants come in a variety of sizes, from onboard units servicing large ships to more traditional plants handling anything up to 500,000 tonnes of waste per annum. There are also specialist units designed to treat particular waste streams, for example old tyres.
Waste-to-energy developments that combine both ATT and anaerobic digestion elements can be particularly efficient as the former is suited to dry materials while the latter is more appropriate for wet matter (food waste, for example).
There is no shortage of material to fuel the plants. Worldwide, thermal waste-to-energy plants treated just 11 per cent of municipal solid waste (222 million tons) in 2011 compared to the 70 per cent that was landfilled, with the remainder recycled.
According to Pike Research, waste-to-energy will grow rapidly over the next decade, generating an estimated 283TWh of electricity and heat by 2022, up 28 per cent.
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