Plasma gasification or plasma-assisted gasification can be used to convert carbon-containing materials to synthesis gas that can be used to generate power and other useful products, such as transportation fuels. In an effort to reduce both the economic and environmental costs of managing municipal solid waste, (which includes construction and demolition wastes) a number of cities are working with plasma gasification companies to send their wastes to these facilities. One city in Japan gasifies its wastes to produce power. In addition, various industries that generate hazardous wastes as part of their manufacturing processes (such as the chemical and refining industries) are examining plasma gasification as a cost-effective means of managing those wastes streams.
Plasma is an ionized gas that is formed when an electrical discharge passes through a gas. The resultant flash from lightning is an example of plasma found in nature. Plasma torches and arcs convert electrical energy into intense thermal (heat) energy. Plasma torches and arcs can generate temperatures up to 10,000 degrees Fahrenheit. When used in a gasification plant, plasma torches and arcs generate this intense heat, which initiates and supplements the gasification reactions, and can even increase the rate of those reactions, making gasification more efficient.
Courtesy of Westinghouse Plasma Corporation (WPC), a division of Alter NRG Corp.
Inside the gasifier, the hot gases from the plasma torch or arc contact the feedstock, such as municipal solid waste, auto shredder wastes, medical waste, biomass or hazardous waste, heating it to more than 3,000 degrees Fahrenheit. This extreme heat maintains the gasification reactions, which break apart the chemical bonds of the feedstock and converts them to a synthesis gas (syngas). The syngas consists primarily of carbon monoxide and hydrogen—the basic building blocks for chemicals, fertilizers, substitute natural gas, and liquid transportation fuels. The syngas can also be sent to gas turbines or reciprocating engines to produce electricity, or combusted to produce steam for a steam turbine-generator.
Because the feedstocks reacting within the gasifier are converted into their basic elements, even hazardous waste becomes a useful syngas. Inorganic materials in the feedstock are melted and fused into a glassy-like slag, which is nonhazardous and can be used in a variety of applications, such as roadbed construction and roofing materials.
Plasma Gasifier, Courtesy of Westinghouse Plasma Corporation (WPC), a division of Alter NRG Corp.
Plasma technologies have been used for over 30 years in a variety of industries, including the chemical and metals industries. Historically, the primary use of this technology has been to decompose and destroy hazardous wastes, as well as to melt ash from mass-burn incinerators into a safe, non-leachable slag. Use of the technology as part of the waste-to-energy industry is much newer.
There are currently plasma gasification plants operating in Japan, Canada and India. For example, a facility in Utashinai, Japan has been in commercial operation since 2001, gasifying municipal solid waste and auto shredder waste to produce electricity. There are a number of proposed plasma gasification plants in the United States.
Benefits of Plasma Gasification
Plasma gasification provides a number of key benefits:
- It unlocks the greatest amount of energy from waste
- Feedstocks can be mixed, such as municipal solid waste, biomass, tires, hazardous waste, and auto shredder waste
- It does not generate methane, a potent greenhouse gas
- It is not incineration and therefore doesn’t produce leachable bottom ash or fly ash
- It reduces the need for landfilling of waste
- It produces syngas, which can be combusted in a gas turbine or reciprocating to produce electricity or further processed into chemicals, fertilizers, or transportation fuels—thereby reducing the need for virgin materials to produce these products
- It has low environmental emissions