What are municipal waste combustors?

Also known as incinerators or waste-to-energy plants, these are facilities that burn municipal solid waste (commonly known as trash or garbage) at a very high temperature (approximately 2,500°F) to generate electricity or steam power. The combustion process reduces this material about 90 percent by volume and 75 percent by weight, so significantly less waste needs to be buried in landfills.

There are more than 100 municipal waste combustors in operation across the country. Seven of these facilities are located in Massachusetts and, together, they burn roughly 38 percent of the municipal solid waste generated across the state. Of the remainder, about 34 percent is recycled and 28 percent is either buried in modern lined landfills or exported to out-of-state disposal facilities.

Additional information about solid waste management trends and goals in Massachusetts can be found in the Solid Waste Master Plan.

What do these facilities burn?

Municipal waste combustors burn a wide range of household and commercial solid wastes. These include empty product packaging and containers; used clothing, consumer goods and office supplies; food scraps and other organic materials; and numerous other items.

Specifically banned from disposal in Massachusetts combustion facilities are bottles and cans, single-resin plastic containers, most grades of cardboard and paper, leaves and grass clippings, hazardous and medical wastes, cathode ray tube devices (i.e. televisions and computer monitors), large home appliances ("white goods") and lead acid car batteries.

Facility operators monitor incoming waste shipments for materials that are recyclable or toxic and may turn away loads that contain significant quantities of banned materials as well as wastes that are bulky or otherwise difficult to manage. Everyone in Massachusetts can play a role in reducing the volume and toxicity of waste going into municipal waste combustors by participating in local recycling programs and hazardous household product collections. To learn about opportunities in your town or city, see: How & Where to Recycle

How do municipal waste combustors work?

There are several proprietary technologies for generating power by burning trash, but all of them work in essentially similar ways.

Incoming trucks dump waste in an enclosed receiving area. From there, the trash is either fed directly into a furnace (a process known as "mass burn") or chopped into smaller pieces and sifted for recoverable metals (to create "refuse derived fuel" or RDF) before entering the combustion chamber.

An intense combustion process produces large quantities of super-heated steam in a boiler. Steam is sometimes a facility's main energy byproduct, but more often it is used to drive large turbine generators that produce electricity. A small fraction of this energy is used to power the facility, while the bulk of it is sold to energy customers, generally power distribution companies.

During combustion, ash drops into water-filled troughs that cool it off and wet it down, making it easier to handle. The ash residue is sifted to retrieve recoverable metals, which are typically recycled, and then is transported to a landfill that is approved for ash disposal.

Once cooled, gases from the combustion process travel through a series of air pollution control systems designed to remove acid gases, heavy metals, organic chemicals and particulate matter. The remaining gases and pollutants are released to the atmosphere through a stack and are known collectively as the facility's "emissions."

What wastes do these facilities generate?

Municipal waste combustors generate several forms of waste: ash residue, which must be buried in landfills that are specifically approved to accept ash; wastewater from cooling and ash dewatering operations; and air emissions, which are the primary focus of this Web site.

Air emissions from a well operated and maintained combustion facility are generally much lower than the limits established by law to prevent significant risk to public health and the environment. Pollution controls, monitoring, and government inspections ensure that facilities are operating as cleanly as possible. At the same time, their emissions may still contain levels of pollutants that, when combined with air emissions from many other sources, may cause health and environmental impacts. These include:

  • Acid gases, such as hydrogen chloride and sulfur dioxide, that contribute to acid rain and can cause or aggravate breathing problems;
  • Chlorine-containing organic chemicals, including dioxins and furans, that are known or suspected to cause cancer and birth defects;
  • Fly ash and soot particles, seen as smoke, that reduce visibility and can make it harder to breathe;
  • Heavy metals, such as mercury and lead, that can affect the human brain, kidneys, liver and nervous system, as well as child development; and
  • Nitrogen oxides (NOx), a chief ingredient of ground-level ozone ("smog"), that can cause or aggravate breathing problems.

The Massachusetts Department of Environmental Protection (MassDEP) regulates municipal waste combustors to ensure that their emissions remain below levels and concentrations that could pose significant risks to public health or the environment.

For additional information about the types of pollution MassDEP regulates and measures, their environmental and health effects, MassDEP's risk-based emission standards for various pollutants and the agency's ambient air monitoring network, see: Annual Air Quality Report

How are emissions from the combustion process controlled?

While the specific pollution controls used vary from plant to plant, all facilities are equipped with systems for reducing levels of nitrogen oxides (NOx), sulfur dioxide (SO2) and various chemicals.

At all facilities, a scrubber neutralizes acid gases. Electrostatic precipitators, fabric filters or a combination of these systems trap fly ash and other particles. Carbon is also injected into combustion gases to further reduce concentrations of mercury and organic chemicals.

These control technologies combine to significantly reduce emissions of gases, heavy metals and soot so they will not exceed the limits established by MassDEP regulations and individual facility permits.

With state-of-the-art controls in place, combustion facilities emit considerably less toxic and smog-causing pollution than they did even a few years ago. It is important to note, however, that no combination of technologies available today can completely eliminate emissions from combustion