In the design of waste-to-energy incinerators, the size of the plant is a critical factor. Planners need accurate infor mation about the amount and type of waste the plant is to burn (see Sections 10.1 to 10.5) as well as projections for future solid waste management practices in the community.
Next, planners must determine what to burn. In keeping with the hierarchy of the Pollution Prevention Act (PPA), a state-of-the-art strategy provides for the maximum amount of source reduction and recycling, including composting, before incineration. Furthermore, materials that are not recyclable and are unsuitable for burning because they are noncombustible, explosive, or contain toxic substances or pollutant precursors, should be separated from the waste to be burned. These activities preserve natural resources, improve incinerator efficiency, and minimize pollutant emissions and ash quantity and toxicity.
A general, overriding principle in the design of a solid waste incinerator is to use the correct size incinerator for the amount of anticipated waste. Combustion is most efficient when an incinerator consistently burns the quantity and quality of MSW it was constructed to burn, as follows:
If the plant is oversized (i.e., if the amount of MSW available for burning is less than the plant was designed to take), it may operate less than full time. Each start up and shutdown causes unsteady burning conditions, resulting in reduced overall efficiency. Such unsteady state conditions increase the generation of incomplete combustion and particulates. More importantly, a plant that is oversized for the amount of waste available to burn has higher per ton disposal costs. If an incinerator is undersized (that is, more MSW is available to be burned than originally planned), too much MSW may be loaded into the furnace. Overloading an incinerator can result in increased generation of incomplete combustion as well as an increased volume of unburned matter and ash. Also, an undersized incinerator that is not overloaded requires additional expenditures of alternative methods of waste disposal and recycling.
In determining the amount of MSW being generated, planners should collect actual waste data just prior to design and sizing. Waste composition studies should ideally sample waste from different neighborhoods at different times of the week and year, as shown in Figure 10.3.1. Some communities use average waste composition from other towns or cities to estimate their own waste composition. However, this method can be misleading since the composition of MSW changes not only from place to place but also over time.
Information about projected population growth and future trends in the volume and composition of waste is just as critical as current waste data, especially since waste management methods are changing. Incinerators are typically designed for at least a twenty-year lifetime, and incinerator arrangements often include long-term (fifteen- to thirty-
TABLE 10.9.1 KEY FEATURES OF NEW FEDERAL MSW INCINERATOR REGULATIONS (NSPS), COMPARED TO INFORM STATE-OF-THE-ART STANDARDS
New Source Performance Standards INFORM State-of-the-Art Standard
None Recyclables, noncombustibles, and wastes containing toxic materials or pollutant precursors removed
Good Combustion Practices
Carbon monoxide emissions:
50-150 ppm (depending on furnace type) 50 parts per million
Plant-specific maximum load level Plant-specific maximum flue gas temperature at inlet to final particulate control device
Pollutant Emissions Levels
(7% O2, dry basis) PARTICULATES
0.015 g per dry standard cu ft DIOXINS/FURANS
30 nanograms per dry standard cu m— total dioxins and furans
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