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Pyrolysis and Thermal Gasification of Municipal Solid Waste

Gasification and pyrolysis can produce a gas for fuel. The process operates at a high temperature and in the absence of air. Under special conditions, a liquid fuel or chemical feedstock can also be made. The gas may be used in existing boilers or furnaces, or can fire combustion turbines to generate electricity.

Pyrolysis is the thermal decomposition of organic material at elevated temperatures in the absence of air or oxygen. The process, which requires heat, produces a mixture of combustible gases, primarily methane, complex hydrocarbons, hydrogen, and carbon monoxide, and liquids and solid residues.

Gasification of MSW is a special type of pyrolysis where thermal decomposition takes place in the presence of a small amount of oxygen or air. The gas which is generated can then be burned in industrial boilers or cleaned up and used in combustion turbines for electric generators.

The gasification process has been used commercially with coal and wood chips. It was used with MSW in the United States in the 1970s, but those plants have been shut down because of operating and financial problems. Some gasification plants were built and operated in Europe in the early 1980s. Los Angeles had a system designed to use dried sewage sludge. The gas was burned to drive a 10 megawatt steam turbine-generator.

Environmental Permitting Issues for Pyrolysis and Thermal Gasification Facilities

Thermal gasification facilities share some of the same environmental problems associated with mass burn incinerators including:

Meeting air quality requirements

Classification of the ash as a hazardous material

Disposal of ash and other by-products

Possible conflict with adjacent land uses

Disturbances to biological resources

Use of large amounts of water for cooling purposes (if wet cooling towers are used)

Transportation impacts from numerous truck trips from the refuse source to the facility

Health, safety, odor, and traffic impacts

Possible hazardous materials leakage

Conflicts between using MSW for electricity generation and waste reduction techniques, composting, and recycling

According to the California Energy Commission, it is not economical to transport the gas produced by such facilities over long distances, so power generation equipment must be sited near gasification facilities. The gas produced by pyrolysis or thermal gasification can be scrubbed to remove contaminants prior to combustion, so air emissions may be easier to control than with mass burn incinerators. However, scrubbing the gas at high temperature is currently under research and the technology has yet to be demonstrated on a large scale. And the gas from pyrolysis and gasification of MSW may contain hazardous organic compounds that are difficult to remove. Finally, the organic material for gasification which comprises 61% of MSW — paper, cardboard, wood, yard waste, and food scraps — is also the fraction most valuable for composting.

more info:

February 2009: BREDL revises our April 2002 report: Waste Gasification - impacts on the environment and public health

BREDL report: Incineration and Gasification: A Toxic Comparison

BLUE RIDGE ENVIRONMENTAL DEFENSE LEAGUE

www.BREDL.org ~ PO Box 88 Glendale Springs, North Carolina 28629 ~ Phone (336) 982-2691 ~ Fax (336) 982-2954 ~ BREDL@skybest.com