Waste-to-energy Introduction

Californians create nearly 2,900 pounds of household garbage and industrial waste every second; a total of 45 million tons of waste per year (according to the California Integrated Waste Management Board)!

Until recently, the only place to put that trash was in landfills. Today, however, waste and its by-products are being recycled into more useful products. Some waste materials can also be used as a fuel in power plants to create electricity or other forms of energy.

These WASTE-TO-ENERGY power plants are defined by the type of fuel source they use: biomass, digester gas, industrial waste, landfill gas, and municipal solid waste. All together there are 103 waste-to-energy plants in California with a total installed capacity of 1,070 megawatts, about 2 percent of the state's total electrical capacity. These plants produced 5,701 million kilowatt-hours of electricity in 1997.

Municipal Solid Waste Power Plants

Municipal solid waste, MSW, can be directly combusted in waste-to-energy facilities as a fuel with minimal processing, known as "mass burn"; it can undergo moderate to extensive processing before being directly combusted as refuse-derived fuel; or it can be gasified using pyrolysis or thermal gasification techniques.

Each of these technologies can produce electricity as well as an alternative to landfilling or composting the MSW. In contrast with many other energy technologies that require fuel to be purchased, MSW facilities are paid by the fuel suppliers to take the fuel (known as a "tipping fee"). The tipping fee is comparable to the fee charged to dispose of garbage at a landfill.

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Mass Burn

Incoming trucks deposit the refuse into pits, where cranes then mix the refuse and remove any bulky or large non-combustible items (such as large appliances). The refuse storage area can be maintained under lower-than-atmospheric pressure to prevent odors from escaping. The cranes move the refuse to the combustor charging hopper to feed the boiler.

Heat from the combustion process is used to turn water into steam, with the steam then routed to a steam turbine-generator for power generation. The steam is then condensed via traditional methods (such as wet cooling towers or once-through cooling) and routed back to the boiler. Residues produced include bottom ash (which falls to the bottom of the combustion chamber), fly ash (which exits the combustion chamber with the flue gas [hot combustion products]), and residue (including fly ash) from the flue gas cleaning system.

The combined ash and air pollution control residue typically ranges from 20 percent to 25 percent by weight of the incoming refuse processed. This ash residue may or may not be considered a hazardous material, depending on the makeup of the municipal waste.

It may be possible to avoid the production of hazardous ash by preventing the sources that create hazardous waste from entering the system. It is also possible to treat the ash. Both of these methods avoid the costs of disposal at a limited number of landfills classified as able to handle hazardous materials. Non-hazardous ash can be mixed with soils for use as landfill cover, or can be sold (or given away) for such beneficial uses as pavement aggregate or for cinder block production.

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Refuse-Derived Fuel

Refuse-derived fuel (RDF) typically consists of pelletized or "fluff" MSW that is the by-product of a resource recovery operation. Processing removes iron materials, glass, grit, and other materials that are not combustible. The remaining material is then sold as RDF. Both the RDF processing facility and the RDF combustion facility are usually located near each other, if not on the same site.

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Pyrolysis/Thermal Gasification

Pyrolysis and thermal gasification are related technologies. Pyrolysis heats organic material to high temperatures in the absence of gases such as air or oxygen. The process produces a mixture of combustible gases (primarily methane, complex hydrocarbons, hydrogen, and carbon monoxide), liquids and solid residues. Thermal gasification of MSW is different from pyrolysis in that the thermal decomposition takes place in the presence of a limited amount of oxygen or air.

The producer gas that is generated in either process can then be used in boilers or cleaned up and used in combustion turbine/generators. The primary area of research for this technology is the scrubbing of the producer gas of tars and particulates at high temperatures in order to protect combustion equipment downstream of the gasifier and still maintain high thermal efficiency.

Both of these technologies are in the development stage with a limited number of units in operation. The Hyperion Energy Recovery System operated by the city of Los Angeles had a system designed to fire dried sewage sludge in a staged fluidized bed combustor. The resulting gas was then combusted in stages, and the heat was used to turn water into steam, driving a 10-MW steam turbine-generator.