Solid waste incineration

Incineration is the burning of waste in a specially designed combustion chamber. The idea of burning garbage is not new, but with the increase in knowledge about toxic chemicals known to be released during burning, and with the increase in the amount of garbage to be burned, incineration now is done under controlled conditions. It has become the method of choice of many waste management companies and municipalities.

According to the Environmental Protection Agency (EPA), there are 135 operational waste combustion facilities in the United States. About 120 of them recover energy, and in all, the facilities process about 14.5% of the nation's 232 million tons (210.5 million metric tons) of municipal solid waste produced each year.

There are several types of combustion facilities in operation. At incinerators, mixed trash goes in one end unsorted, and it is all burned together. The resulting ash is typically placed in a landfill .

At mass burn incinerators, also known as mass burn combustors, the heat generated from the burning material is turned into useable electricity. Mixed garbage burns in a special chamber where temperatures reach at least 2000°F (1093 °C). The byproducts are ash, which is landfilled, and combustion gases. As the hot gases rise from the burning waste, they heat water held in special tubes around the combustion chamber. The boiling water generates steam and/or electricity. The gases are filtered for contaminants before being released into the air.

Modular combustion systems typically have two combustion chambers: one to burn mixed trash and another to heat gases. Energy is recovered with a heat recovery steam generator. Refuse-derived fuel combustors burn presorted waste and convert the resulting heat into energy. In all, incinerators in this country in 1992 typically consumed 80,100 tons (72,730 metric tons) of waste each day, and generated the annual equivalent of 16.4 million megawatts of usable power, equal to roughly 30 million barrels of oil.

One major problem with incineration is air pollution . Even when equipped with scrubbers , many substances, some of them toxic, are released into the atmosphere . In the United States, emissions from incinerators are among targets of the Clean Air Act , and research continues on ways to improve the efficiency of incinerators. For now, however, many environmentalists protest the use of incinerators.

Incinerators burning municipal solid waste produce the pollutants carbon monoxide , sulfur dioxide , and particulates containing heavy metals . The generation of pollutants can be controlled by proper operation and by the proper use of air emission control devices, including dry scrubbers, electrostatic precipitators, fabric filters , and proper stack height.

Dry scrubbers wash particulate matter and gases from the air by passing them through a liquid. The scrubber removes acid gases by injecting a lime slurry into a reaction tower through which the gases flow. A salt powder is produced and collected along with the fly ash . The lime also causes small particles to stick together, forming larger particles that are more easily removed.

Electrostatic precipitators use high voltage to negatively charge dust particles, then the charged particulates are collected on positively charged plates. This device has been documented as removing 98% of particulates, including heavy metals; nearly 43% of all existing facilities use this method to control air pollution .

Fabric filters or baghouses consist of hundreds of long fabric bags made of heat-resistant material suspended in an enclosed housing which filters particles from the gas stream. Fabric filters are able to trap fine, inhalable particles, up to 99% of the particulates in the gas flow coming out of the scrubber, including condensed toxic organic and heavy metal compounds. Stack height is an extra precaution taken to assure that any remaining pollutants will not reach the ground in a concentrated area.

Using the Best Available Control Technology (BAT), the National Solid Wastes Management Association states more than 95% of gases and fly ash are captured and removed. However, such state-of-the-art facilities are more the exception than the rule. In a study of 15 mass burn and refuse-derived fuel plants of varying age, size, design, and control systems, the environmental research group INFORM found that only one of the 15 achieved the emission levels set by the group for six primary air pollutants (dioxins and furans , particulates, carbon monoxide, sulfur dioxide, hydrogen chloride, and nitrogen oxides ). Six plants did not meet any. Only two employed the safest ash management techniques, and the EPA is still trying to define standards for air emissions from municipal incinerators.

Ash is the solid material left over after combustion in the incinerator. It is composed of noncombustible inorganic materials that are present in cans, bottles, rocks and stones, and complex organic materials formed primarily from carbon atoms that escape combustion. Municipal solid waste ash also can contain lead and cadmium from such sources as old appliances and car batteries.

Bottom ash, the unburned and unburnable matter left over, comprises 75–90% of all ash produced in incineration. Fly ash is a powdery material suspended in the flue gas stream and is collected in the air pollution control equipment. Fly ash tends to have higher concentrations of certain metals and organic materials, and comprises 10–25% of total ash generated.

The greatest concern with ash is proper disposal and the potential for harmful substances to be released into the groundwater . Federal regulations governing ash are in transition because it is not known whether ash should be regulated as a hazardous or nonhazardous waste as specified in the Resource Conservation and Recovery Act (RCRA) of 1976.

EPA draft guidelines for handling ash include: ash containers and transport vehicles must be leak-tight; groundwater monitoring must be performed at disposal sites; and liners must be used at all ash disposal landfills.

While waste-to-energy plants can decrease the volume of solid waste by 60–90% and at the same time recover energy from discarded products, the cost of building such facilities is too high for most municipalities. The $400 million price tag for a large plant is prohibitive, even if the revenue from the sale of energy helps offset the cost. But without a strong market for produced energy, the plant may not be economically feasible for many areas.

The Public Utility Regulatory Policy Act (PURPA), enacted in 1979, helps to ensure that small power generators, including waste-to-energy facilities, will have a market for produced energy. PURPA requires utilities to purchase such energy from qualifying facilities at avoided costs, that is, the cost avoided by not generating the energy themselves. Some waste-to-energy plants are developing markets themselves for steam produced. Some facilities supply steam to industrial plants or district heating systems.

Another concern over the use of incinerators to solve the garbage dilemma in the United States is that materials incinerated are resources lost—resources that must be recreated with a considerable effort, high expense, and potential environmental damage. From this point of view, incineration represents a failure of waste disposal policy. Waste disposal, many believe, requires an integrated approach that includes reduction, recycling , composting , and landfilling, along with incineration.

[Linda Rehkopf ]