Mechanical Biological Treatment (MBT), Gasification, Pyrolysis and the Covanta Solution

Covanta believes that there is no "one size fits all" answer to waste management and energy generation and works with local authorities to find bespoke solutions to their waste management and energy needs while guaranteeing to meet landfill diversion targets. The Covanta solution is compatible with a range of complementary technologies including:

Mechanical Biological Treatment (MBT)
MBT processes incorporate mechanical sorting and separation of the waste stream to separate the biodegradable materials, which are sent to a biological process, from the non-biodegradeable materials. MBT is often used as an umbrella term for all types of mechanical biological treatment including BMT (Biological Mechanical Treatment) systems. The difference depends on the order of the process steps e.g. mechanical sorting followed by biological digestion or vice versa. It is important to recognise that MBT systems are PRE - processes and not a means of disposal in themselves. Even after MBT has recovered recyclables and compostable fractions , there remains a role for EFW to process around 50% of inputs remaining as "Residues". This is sometimes referred to as refuse derived fuel (RDF) or secondary recovered fuel (SRF) which can be burnt on or off site.

MBT divides the waste mechanically into recyclables, residues and an organic fraction that can be used for composting or anaerobic digestion. BMT differs whereby biological technology is used to dry and sterilise the waste before removing non combustibles

Gasification and Pyrolysis

Both pyrolysis and gasification are thermal processes that turn waste into fuel by heating them under controlled conditions.

Gasification processes occur under conditions where the quantity of air/ oxygen input is controlled so that combustion of the feedstock does not occur. As a result gasification produces a gaseous output known as 'syngas' (short for synthesis gas) and ash or slag product.

While there are several gasification processes being promoted for treating Municipal Solid Waste (MSW) in the UK and many Local Authorities have shown interest in gasification technology as a standalone option, so far no plant utilising gasification technology has operated on a sustained commercial basis in the UK to treat MSW. It remains a promising technology at relatively small scale and Covanta continues to monitor deployment of these facilities.

Pyrolysis occurs under conditions where oxygen is absent or there is insufficient oxygen for gasification reactions to take place resulting in outputs that can be a mixture of solid (char), liquid (oil) and gases.

There are currently no UK pyrolysis plants in operation processing Municipal Solid Waste.

How do Gasification and Pyrolysis differ from incineration?

Whereas incineration fully converts waste into energy to be used immediately for electricity or district heating, both pyrolysis and gasification convert the waste into intermediates which can be combusted to use the heat energy immediately or be transported for use elsewhere. It is important to note however that most commercial processes use combinations of these technologies and nearly all commercial gasification systems actually follow gasification with combustion.