Plasma gasification is a thermal process that converts organic matter into a syngas (synthesis gas) which is primarily made up of hydrogen and carbon monoxide. A plasma torch powered by an electric arc ionizes gas and transforms organic matter into syngas, producing slag as a byproduct. It is used commercially as a form of waste treatment. It has been tested for the gasification of refuse-derived fuel, biomass, industrial waste, hazardous waste, and solid hydrocarbons, such as coal, oil sands, petcoke, and oil shale.
A plasma torch passes strong electric current under high voltage between two electrodes as an electric arc. Pressurized gas is ionized passing through the plasma created by the arc. The torch's temperature ranges from . The temperature determines the structure of the plasma and forming gas.
The waste is heated, melted and finally vaporized. At these conditions molecular dissociation occurs by breaking apart molecular bonds. Complex molecules are separated into individual atoms. The resulting elemental components are in a gaseous phase (syngas). Molecular dissociation using plasma is referred to as "plasma pyrolysis."
Smaller torches typically use an inert gas such as argon, while larger sizes require nitrogen. Electrodes vary from copper or tungsten to hafnium or zirconium, along with other alloys.
Feedstocks are most often refuse-derived fuel, biomass waste, both or biomedical waste and hazardous materials. The content and consistency of the waste directly impacts performance. Extracting treatable material improves consistency. Too much inorganic material such as metal and construction waste increases slag production, while decreasing syngas production. However, the slag is chemically inert and safe to handle. Shredding waste to create uniform particles is generally required. This creates an efficient transfer of energy which breaks down the material.
Added steam supports steam reforming.
Pure synthesis gas consists predominantly of carbon monoxide (CO) and hydrogen (H<sub>2</sub>). Inorganic compounds in the waste stream melt, including glass, ceramics, and metals.
The temperature and lack of oxygen prevents the formation of many toxic compounds such as furans, dioxins, nitrogen oxides, or sulfur dioxide in the flame. However, dioxins form during cooling.
Metals can be recovered from the slag and sold. Inert slag produced from some processes is granulated and can be used in construction. A portion of the syngas feeds on-site turbines, which powers the plasma torches and thus supports the feed system.
Some plasma gasification reactors operate at negative pressure, although most attempt to recover gaseous and/or solid resources.
The main advantages of plasma torch technologies for waste treatment are:
Main disadvantages of plasma torch technologies for waste treatment are:
Plasma torch gasification is used commercially for waste disposal at five sites worldwide with a combined design capacity of 200 tonnes of waste per day, half of which is biomass waste.
Energy recovery from waste streams using plasma gasification is used in one or two installations treating 25âÂÂ30 tonnes per day.
The US Navy employs the Plasma Arc Waste Destruction System (PAWDS) on its latest generation Gerald R. Ford-class aircraft carrier. The compact system treats all combustible solid waste.