Municipal Solid Waste Mass Burn
Converting refuse or MSW to energy can be accomplished by a variety of technologies. The degree of refuse processing determines the method used to convert municipal solid waste to energy. Unprocessed refuse is typically combusted in a water wall furnace (mass burning). After only limited processing to remove non-combustible and oversized items, the MSW is fed on to a reciprocating grate in the boiler. The combustion generates steam in the walls of the furnace, which is converted to electrical energy via a steam turbine generator system. This is similar to coal and biomass furnaces. Other furnaces used in mass burning applications include refractory furnaces and rotary kiln furnaces, which use other means to transfer the heat to the steam cycle or add a mixing process to the combustion. For smaller modular units, controlled air furnaces, which utilize two-stage burning for more efficient combustion, can be used in mass burning applications.
Applications
The avoided cost of disposal is a primary component in determining the economic viability of a waste to energy facility. For this reason, areas where land costs are high and landfills must be sited far from waste sources are the most likely locations for WTE plants. Large MSW facilities typically process 500 to 3,000 tons of MSW per day, although there are a number of facilities in the 200 to 500 ton per day size range.
Resource Availability
MSW plants are high capital cost projects that require a cheap and abundant fuel source to operate profitably. For this reason, they are typically cited near large population centers or in areas where land is valued at a premium. The average American generates about 4.5 pounds of garbage per day, most of which would otherwise be sent to landfill. Similar to biomass, the cost of fuel transportation is a primary factor in the economics of an MSW plant. New plants are usually not economically viable unless a high tipping fee can be secured.
Environmental Impacts
The products of combustion of MSW are similar to those of most organic combustion materials. Particulate matter must be abated and nitrogen oxides can form if the combustion temperature is too high. Unlike coal, the sulfur emissions from MSW are low. One possible emission that is atypical of other fuels is dioxin. The US EPA has ruled that some types of dioxins are carcinogenic. This issue is debated intensely in the scientific community, but MSW plant construction faces opposition in many communities because of it.
An obvious benefit of burning MSW is that it reduces landfill deposits. The bottom ash recovered from a MSW furnace is reduced to one-tenth of its original volume.
Refuse Derived Fuel
Refuse derived fuel (RDF) is an evolution of MSW technology. Instead of burning the trash in its bulky native form, trash is processed and converted to fluff or pellets for ease of handling and improved combustibility.
To ensure a proper mix of fuel, trash is typically sorted to remove metals, “heavies” and other undesirable materials. The remaining “clean” trash is conveyed to a mulching facility that shreds the material into small pieces. These pieces are delivered as fuel to a combustor. Due to the extensive pre-processing and sorting of the material, RDF facilities are often considered to be more compatible with local recycling efforts than mass burn facilities.
Applications
RDF is preferred in many refuse to energy applications because it can be combusted with technology traditionally used for coal. Spreader stoker fired boilers, suspension fired boilers, fluidized bed boilers, and cyclone furnace units have all been utilized to generate steam from RDF. Fluidized bed combustors are often preferred for RDF energy applications due to their high combustion efficiency, capability to handle RDF with minimal processing, and inherent ability to effectively reduce nitrous oxide and sulfur dioxide emissions. In all MSW or RDF boiler types, the boiler tube metal temperature must be kept at a temperature less than 800ºF to minimize boiler tube degradation due to chlorine compounds in the flue gas.
Typical RDF facilities process 500 to 2,000 tons of RDF per day (the average amount produced by 200,000 to 800,000 residents).
Environmental Impacts
RDF faces the same environmental opposition as MSW while providing the same environmental benefits. RDF plants are generally viewed as being more compatible with recycling efforts. RDF plants using fluidized bed technology can potentially achieve lower emissions than mass burn plants.
Economics
Economic feasibility of WTE facilities is generally difficult to assess. Costs are highly dependent on transportation, processing, and tipping fees associated with a particular location. Values given in this section should be considered representative of the technology at a generic site.