SUB-PROBLEMS INCLUDING LEADING TO IMPAIRMENT AND BEST PRACTICES FOR SOLID WASTE DISPOSAL USING MUNICIPAL INCINERATORS
(See endnotes 80, 81, 82, 83)
Incinerators reduce the volume of materials in a controlled manner and reduce toxicity and potential sources of infection by burning waste at extremely high temperatures. Major problems relate to noise at the facility, dust and dirt, contamination of the ambient air, land, and water, and potential for injuries and disease to the people working with the solid waste. Energy can be created as a byproduct of the incinerator process.
There are three different types of technologies used in the burning of municipal solid waste. They are mass burn facilities, modular systems, and refuse-derived fuel systems. Mass burn facilities are the most common ones used in the United States, even though presorting and recycling are highly recommended to reduce waste quantities and reuse materials to save the use of raw materials and protect the environment. The burning occurs in a single combustion chamber with excess air which helps mix the materials and the turbulence allows the air and burning to reach all parts of the waste. The grate vibrates to help create further turbulence. The heat within the process converts water to steam and the steam is used to help generate electricity. Flue gases and ash are produced. Flue gas may contain carbon dioxide, nitrogen oxides, sulfur oxides, and a variety of gases which may be produced from reactions with chlorine as well as organic compounds and inorganic compounds.
The ash is taken to a landfill. The air leaving the system carries numerous particulates and must go through a high-efficiency baghouse filtering system. Some 99% of the particulates are captured there. The fly ash particles fall into hoppers and are transported through closed conveyor systems, wetted down and mixed with the bottom ash from the grate. Scrap metals are removed at this end of the process also. The ash is then hauled in leak-proof trucks to a landfill which is designed to protect the groundwater from the landfill contents.
The ash is about 15-25% of the weight of the municipal solid waste that was burnt and 5-15% of the volume. The fly ash, which is the fine particles, is about 10-20% by weight (the numbers vary with the source) of the total ash and may contain numerous contaminants. The bottom ash, which is 80-90% of the weight, contains basically, silica, calcium, iron oxide, and aluminum oxide.
Modular systems burn unprocessed, mixed municipal solid waste in the same manner as the mass burn facilities but are much smaller and can be moved from site to site.
Refuse-derived fuel systems best operate through an air-fed gasification system which has been identified by the US EPA and Department of Energy. In this process, the materials that contain organic carbons are broken up into compounds by their constituent parts. This occurs under very high temperatures of 600-800°C and in an oxygen-starved environment. It can also be carried out under air-fed gasification systems at 800-1800°C, or in plasma or plasma arc systems at 2000-2800°C. Syngas is produced and it is composed mainly of carbon monoxide, carbon dioxide, methane, and water vapor. Where an air-fed system is used, nitrogen gas will also be produced. The syngas is then burnt in a heat recovery boiler which makes steam and then turns a turbine to make electricity. (See endnote 84.)