Plasma Arc operates at a much higher temperature (between 3000°C and 15000°C) creating a thermal plasma field by directing an electric current through a low pressure gas stream. The intense high temperature zone can be used to dissociate the waste into its atomic elements by injecting the waste into the plasma, or by using the plasma arc as a heat source for combustion or pyrolysis.
Typically plasma arc has been proposed in Australia for the destruction of hazardous waste rather than the generation of energy from municipal waste. However, Nufarm Australia has been operating a Plascon plasma arc unit at Laverton in Victoria to destroy chlorinated pesticide waste since 1992 while generating electricity. Dioxin emissions have been detected in the emissions.
Traditional mass combustion incinerators tend to operate at much lower
temperatures (typically 750°C -1000°C) and burn waste in the presence of uncontrolled levels of oxygen with no pre-treatment of municipal waste (although some facilities remove a percentage of the recyclables from the waste stream ). Those incinerators that generate energy use the heat from combusting waste to generate steam for turbines to generate power
Gasification and pyrolysis use thermal treatment to break down waste at high temperatures. The major difference between these incinerators and ‘old’ incinerators (sometimes referred to as ‘combustors’) is that these technologies break down thewaste in a low oxygen environment. These technologies are not new as gasification systems have been in use since the mid19th century and pyrolysis since the 1950s.
While these processes have been subject to incremental changes over time, there have been no fundamental process changes for decades. Both of these technologies have the same pollution control devices available to them as combustion incinerators and experience similar problems in controlling their emissions.
The high temperature, low-oxygen process breaks the waste down into solid, liquid and gas residues. The gas component is a combination of hydrogen (around 85%) carbon monoxide, and low levels of carbon dioxide, nitrogen, methane and some hydrocarbon gases. The combination of gas is referred to as ‘syngas’ which is combusted in a secondary process to generate electricity.
In order to generate syngas the waste used in these processes must be rich in carbon and includes paper, plastics and organic matter such as kitchen and garden waste. The syngas can be used to generate energy or as a feedstock in the petrochemical industry.
Gasification allows the use of low levels of oxygen but not enough to cause combustion of the waste. Pyrolysis heats and degrades the waste in the absence of oxygen. Both processes usually operate at or above 750°C. Some pyrolysis units may also engage a secondary gasification system to extract higher levels of syngas.
Virtually all gasifiers and pyrolysis plants have four stages of operation:
- Waste Feedstock preparation: The plant may take mixed waste that has had low calorific value materials removed (sand and concrete) and some recyclables such as glass extracted by a Materials Recovery Facility (MRF). Alternately the feedstock may be a form of Refuse Derived Fuel (RdF) from a Mechanical and Biological Treatment (MBT) plant.
- Heating the waste: Thermal treatment of the waste in a low oxygen(gasification) or nil oxygen (pyrolysis) environment to generate syngas, oils and char or ash.
- Gas filtering: to remove some (but not all) of the hydrocarbons, dioxin and particulate.
- Use syngas for energy generation: Electricity can be generated via a steam turbine or gas engine or potentially used for combined heat and power (CHP).
Track record of gasification, pyrolysis and plasma arc.
Despite the claim that these technologies are proven and reliable, they are not widely used in the waste management industry and have experienced serious problems with pyrolysis in particular, found to create considerable amounts of dioxin and furans when burning waste.
A 2008 US study surveyed a large range of gasification and pyrolysis technologies and reported that:
- they are unproven on a commercial scale for treating MSW in the United
- the residuals from the process can be hazardous,
- they require pre-treatment of waste, and
- are more expensive than other technologies.
Of the few facilities that have been operational in the US and Europe, many have been plagued with operational problems, serious emissions breaches or financial failures.
 Foth Infrastructure & Environment, LLC (2008) Updated Research Study Gasification, Plasma Ethanol and Anaerobic Digestion Waste Processing Technologies. Prepared for Ramsey/Washington County Resource Recovery Project. p. viii