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2.5 Topic d: Incineration

Most of the MSW combustion currently practiced in North America incorporates energy recovery in the form of steam, which is used either to drive a turbine to generate electricity or directly for heating or cooling. In the process, the volume of solid waste is reduced by up to 90% and its weight by up to 75%. In past years it was common to simply burn MSW in incinerators to reduce its volume and weight, but energy recovery has become more prevalent since the 1980s. While about 30% of the MSW stream was incinerated without energy recovery in 1960, this has decreased to about 1% today. Currently, waste-to-energy (WTE) incineration is used to manage about 10-15% of the MSW stream in North America.

It should be noted, however, that the recent development of regional landfills providing relatively inexpensive disposal capacity has made it more difficult for capital-intensive WTE plants to compete. Additionally, court decisions restricting communities' ability to control the flow of their wastes has made it more difficult for local authorities to meet contractual obligations to WTE plants.

The amount of solid waste processed in WTE facilities varies significantly by region. The northeastern US currently incinerates and recovers energy from over 40% of its solid waste, while many states incinerate less than 2% of the solid waste they generate. There are currently about 160 WTE facilities in the US. In Canada, the number is much lower. In fact, the province of Ontario was only operating three incinerator facilities in 1991 and placed a ban on any new facilities being constructed.

Specific technologies

The three most widely used and technically proven WTE technologies used in North America are (a) mass-burn combustion, (b) modular combustion, and (c) refuse-derived-fuel production and combustion. Over the last several years, local governments have largely favored mass-burn systems that recover electricity over other WTE technologies, such as modular units and steam-only processes. Several other emerging WTE technologies have been pilot-tested, but are not yet commercially proven. These include fluidized-bed combustion, anaerobic digestion, gasification, pyrolysis, and other related processes that convert solid waste to gaseous, liquid, or solid fuel through thermal processing.

Mass-burn systems are the predominant form of WTE in North America. Operating mass-burn facilities process about 60% by weight of the solid waste from which energy is recovered. Mass-burn systems generally consist of either two or three combustion units ranging in capacity from 50 to 1,000 tons per day; thus, facility capacity ranges from about 100 to 3,000 tons per day. About 90% of operating mass-burn facilities generate electricity. These facilities can accept refuse that has undergone little preprocessing other than the removal of oversized items, such as refrigerators and sofas. Although this versatility makes mass-burn facilities convenient and flexible, local programs to separate household hazardous wastes (e.g., cleaners and pesticides) and recover certain recyclables are necessary to help ensure environmentally responsible incineration and resource conservation.

Modular combustors are usually prefabricated units with relatively small capacities of between 5 and 120 tons of solid waste per day. Typical facilities have between one and four units for a total plant capacity of about 15 to 400 tons per day. Because of their small size, only about 7% of solid waste that undergoes energy recovery in North America is processed at modular WTE facilities. The majority of modular units produce steam as the sole energy product. Because of their small capacity, modular combustors are generally used in smaller communities or for commercial and industrial operations. Their prefabricated design gives modular facilities the advantage of shorter construction time frames. On average, capital costs per ton of capacity are lower for modular units than for mass- burn and refuse-derived fuel plants.

"Refuse-derived fuel" (RDF) commonly refers to solid waste that has been mechanically processed to produce a storable, transportable, and more homogeneous fuel for combustion. RDF production and combustion plants are described in the Sound Practices section. As of 1991, 14 facilities in the US processed RDF for off-site combustion; 12 facilities combusted RDF in dedicated boilers on-site; and 7 facilities combusted RDF with other fuel (i.e., co-fire RDF). The vast majority of RDF combustion facilities generate electricity. On average, capital costs per ton of capacity are higher for RDF combustion units than for mass-burn and modular WTE units.

Several emerging WTE technologies are at varying stages of research and development, and could become available commercially in the future. These processes include fluidized-bed combustion, pyrolysis and gasification, and anaerobic digestion. Pyrolysis and gasification, and anaerobic digestion are discussed in the section on composting. In a fluidized-bed combustor, instead of a grate supporting a layer of solid fuel, the furnace contains a bed of sand or limestone supported by an air distribution system. Several facilities in the US use fluidized beds to co-fire RDF with other fuels (e.g., sewage sludge) and at least two facilities dedicated to fluidized-bed solid waste combustion are under development. They are large-scale plants that incorporate front-end processing with materials recovery.

Environmental concerns

In North America the major public concerns about the environmental risks of WTE facilities are the potential emission of contaminants into the air through exhaust stacks (i.e., particulates, nitrogen oxides, sulfur dioxide, carbon monoxide, metals, acid gases, and dioxins) and into water through ash leachate. US and most state and provincial air pollution control laws and regulations, however, have been strengthened in recent years to specifically address potential impacts from WTE air emissions. To meet these standards, modern pollution control equipment effectively removes the vast majority of the emissions of concern. The major air emission control technologies employed in North American WTE facilities are fabric filters or baghouses, electrostatic precipitators, and scrubbers. The Sound Practices section contains more detail on these issues. Integration of WTE with the other elements of the solid waste management system, such as recycling and landfilling, is another important issue in North America.

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