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Waste-to-Energy

Waste-to-energy (WTE) is when municipal solid waste is burned in a controlled environment to create steam or electricity.  Through this process the volume of solid waste is reduced by about 90%.  In 2004, there were 89 WTE plants operating in 27 states.  WTE was used to manage 33.1 million tons, or 14%, of trash in the U.S in 2003. 

Energy is sold to electric generating utilities which distribute it to local homes and businesses.  WTE plants in the U.S. generate enough electricity to power nearly 2.3 million homes.  Energy created in a WTE facility has about the same environmental impact as energy produced from natural gas, and less impact than from oil or coal plants.

Before combustion, recyclables can be removed.  Each year nearly 700,000 tons of ferrous metals and more than 3 million tons of glass, metal, plastics, batteries, ash, and yard waste are separated out for recycling. 

Primary WTE Technology Options

Ash Management

Waste-to-energy generation produces two types of ash:

  • Bottom Ash includes both large and moderate-sized unburned and unburnable matter which remains after the municipal solid waste has passed through the furnace, or combustion chamber.  This ash comprises 75-90% of all ash produced through WTE, depending on the technology employed. 
  • Fly Ash is a powdery material suspended in the gas stream which is collected in the pollution control equipment.  It tends to have higher concentrations of metals and organic materials.

Bottom ash and fly ash are usually combined when collected to facilitate storage, handling, and transportation.  A benefit of combining them is that it binds metal particles to other materials, reducing the potential that these metals will leach into ground water once they are disposed.

By law, all ash is tested in accordance with U.S. EPA rules before leaving the WTE facility to ensure it is safe for disposal or beneficial reuse.  According to Integrated Waste Services Association, ash from WTE facilities can be used safely and beneficially.  WTE ash is sometimes used as daily and final cover for landfills.  It can also provide a substitute for the aggregate in road base materials, and be used in building construction and as part of artificial offshore reefs.

Air Emissions

New U.S. EPA rules, effective 1995, are among the most stringent environmental standards for WTE facilities in the world.  These rules mandate that all facilities use the most modern air pollution control equipment available to ensure that WTE smokestack emissions are as clean as possible, and are safe for human health and the environment.

Burning any fuel, including municipal solid waste, can produce a number of pollutants, such as carbon monoxide, sulfur dioxide, and fine particles containing heavy metals.  Other toxic organic compounds, such as dioxins, are also potential emissions from any combustive activity where certain chemical compounds are present, a situation that could take place in the WTE process.  The issue is whether concentration levels of these compounds released into the air are high enough to be harmful to human health or the environment.

According to the U.S. EPA and the International Waste Services Association, WTE processes reduce potential air pollutants to "benign" levels.  In many cases, the extremely high temperatures in WTE boilers prevent harmful pollutants entirely.  In addition, other air pollution "scrubbing" devices capture gasses leaving the boilers.  This prevents any potentially harmful emissions which may have emerged even after the high temperatures in the boiler. 

Air emission control devices in a WTE facility usually include:

  • Dry Scrubbers "wash" the air emissions from the WTE process (called the gas stream) and remove any acidic gases by passing the gas stream through a liquid.
  • Electrostatic Precipitators (ESP) use high voltage electricity to remove up to 98% of all particles remaining in the gas stream after passing through the scrubbers, including any heavy metal particles.
  • Fabric Filters (baghouses) consist of a series of nearly two thousand fabric bags made of heat-resistant material which filter remaining particles from the gas stream.  This includes any large concentrations of condensed toxic organic compounds (such as dioxins) and heavy metal compounds.

In a properly operated facility, the air which makes it through the pollution controls described above and into the smokestack is "clean" by U.S. EPA standards.  Smokestack height is an extra, final precaution taken to assure that remaining pollutants in the gas stream do not reach the ground in a concentration which could possibly be harmful.

Primary WTE Technology Options

Mass Burn

Mass burn is combusting municipal solid waste without any pre-processing or separation.  The resulting steam is employed for industrial uses or for generating electricity.  Mass burn facilities are sized according to the daily amount of solid waste they expect to receive.  Most mass burn plants can remove non-combustible steel and iron for recycling before combustion using magnetic separation processes.  Other non-ferrous metals can be recovered from the leftover ash.

Modular Incinerators

Modular incinerators are small mass burn plants, with a capacity of 15 to 100 tons per day.  The boilers for modular incinerators are built in a factory and shipped to the WTE site, rather than being built on the WTE site itself.  The advantage of a modular WTE incinerator is flexibility.  For example, if more capacity is needed, modular WTE units can be added.  These facilities are used primarily by small communities and industrial sites.  Costs limit the use of this technology because the return on investment in terms of energy produced over time is much lower than in mass burn plants.

Refuse-Derived Fuel (RDF)

RDF plants process solid waste before it is burned.  A typical plant will remove non-combustible items, such as glass, metals and other recyclable materials.  The remaining solid waste is then shredded into smaller pieces for burning.  RDF plants require significantly more sorting and handling than mass burn, but can recover recyclables and remove potentially environmentally harmful materials prior to combustion.  RDF can be burned in power boilers at factories or even at large housing complexes. 

Sometimes RDF materials are "densified" (compacted at high pressure) to make fuel pellets.  The "pellet fuel" may also include various sludges, by-products of municipal or industrial sewage treatment plants.  A major advantage of pellet fuel as an RDF is that it can be burned along with other kinds of fuel in existing power boilers.  This means RDF pellet fuels can compete with traditional fuels, such as coal, on the open market.