Fugitive Emissions

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As a method of waste management, incineration refers to the burning of waste. It helps reduce the volume of landfill material and can render toxic substances non-hazardous, provided certain strict guidelines are followed. There are two basic types of incineration: municipal and hazardous waste incineration.

The process of incineration involves the combination of organic compounds in solid wastes with oxygen at high temperature to convert them to ash and gaseous products. A municipal incinerator consists of a series of unit operations which include a loading area under slightly negative pressure to avoid the escape of odors, a refuse bin which is loaded by a grappling bucket, a charging hopper leading to an inclined feeder and a furnace of varying type—usually of a horizontal burning grate type—a combustion chamber equipped with a bottom ash and clinker discharge, followed by a gas flue system to an expansion chamber. If byproduct stream is to be produced either for heating or power generation purposes, then the downstream flue system includes heat exchanger tubing as well. After the heat has been exchanged, the flue gas proceeds to a series of gas cleanupsystems which neutralizes the acid gases (sulfur dioxide and hydrochloric acid, the latter resulting from burning chlorinated plastic products), followed by gas scrubbers and then solid/gas separation systems such as baghouses before dischargement to tall stacks. The stack system contains a variety of sensing and control devices to enable the furnace to operate at maximum efficiency consistent with minimal particulate emissions. A continuous log of monitoring systems is also required for compliance with county and state environmental quality regulations.

There are several products from a municipal incinerator system: items which are removed before combustion such as large metal pieces; grate or bottom ash (which is usually water-sprayed after removal from the furnace for safe storage); fly (or top ash) which is removed from the flue system generally mixed with products from the acid neutralization process; and finally the flue gases which are expelled to the environment. If the system is operating optimally, the flue gases will meet emission requirements, and the heavy metals from the wastes will be concentrated in the fly ash. (Typically these heavy metals, which originate from volatile metallic constituents, are lead and arsenic.) The fly ash typically is then stored in a suitable landfill to avoid future problems of leaching of heavy metals. Some municipal systems blend the bottom ash with the top ash in the plant in order to reduce the level of heavy metals by dilution. This practice is undesirable from an ultimate environmental viewpoint.

There are many advantages and disadvantages to municipal waste incineration. Some of the advantages are as follows: 1) The waste volume is reduced to a small fraction of the original. 2) Reduction is rapid and does not require semi-infinite residence times in a landfill. 3) For a large metropolitan area, waste can be incinerated on site, minimizing transportation costs. 4) The ash residue is generally sterile, although it may require special disposal methods. 5) By use of gas clean-up equipment, discharges of flue gases to the environment can meet stringent requirements and be readily monitored. 6) Incinerators are much more compact than landfills and can have minimal odor and vermin problems if properly designed. 7) Some of the costs of operation can be reduced by heat-recovery techniques such as the sale of steam to municipalities or electrical energy generation.

There are disadvantages to municipal waste incineration as well. For example: 1) Generally the capital cost is high and is escalating as emission standards change. 2) Permitting requirements are becoming increasingly more difficult to obtain. 3) Supplemental fuel may be required to burn municipal wastes, especially if yard waste is not removed prior to collection. 4) Certain items such as mercury-containing batteries can produce emissions of mercury which the gas cleanup system may not be designed to remove. 5) Continuous skilled operation and close maintenance of process control is required, especially since stack monitoring equipment reports any failure of the equipment which could result in mandated shut down. 6) Certain materials are not burnable and must be removed at the source. 7) Traffic to and from the incinerator can be a problem unless timing and routing are carefully managed. 8) The incinerator, like a landfill, also has a limited life, although its lifetime can be increased by capital expenditures. 9) Incinerators also require landfills for the ash. The ash usually contains heavy metals and must be placed in a specially-designed landfill to avoid leaching.

For the incineration of hazardous waste, a greater degree of control, higher temperatures, and a more rigorous monitoring system are required. An incinerator burning hazardous waste must be designed, constructed, and maintained to meet Resource Conservation and Recovery Act (RCRA) standards. An incinerator burning hazardous waste must achieve a destruction and removal efficiency of at least 99.99 percent for each principal organic hazardous constituent. For certain listed constituents such as polychlorinated biphenyl (PCB), mass air emissions from an incinerator are required to be greater than 99.9999%. The Toxic Substances Control Act requires certain standards for the incineration of PCBs. For example, the flow of PCB to the incinerator must stop automatically whenever the combustion temperature drops below the specified value; there must be continuous monitoring of the stack for a list of emissions; scrubbers must be used for hydrochloric acid control; among others.

Recently medical wastes have been treated by steam sterilization, followed by incineration with treatment of the flue gases with activated carbon for maximum absorption of organic constituents. The latter system is being installed at the Mayo Clinic in Rochester, Minnesota, as a model medical disposal system.

Fugitive Emissions; Solid Waste Incineration; Solid Waste Volume Reduction; Stack Emissions

Brunner, C. R. Handbook of Incineration Systems. New York: McGraw-Hill, 1991.

Edwards, B. H., et al. Emerging Technologies for the Control of Hazardous Wastes. Park Ridge, NJ: Noyes Data Corporation, 1983.

Hickman Jr., H. L., et al. Thermal Conversion Systems for Municipal Solid Waste. Park Ridge, NJ: Noyes Publications, 1984.

Vesilind, R. A., and A. E. Rimer. Unit Operations in Resource Recovery Engineering. Englewood Cliffs, NJ: Prentice-Hall, 1981.

Wentz, C. A. Hazardous Waste Management. New York: McGraw-Hill, 1989.

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