Each of these technologies presents the opportunity for both electricity production as well as an alternative to landfilling or composting the MSW. In contrast with many other energy technologies that require fuel to be purchased, MSW facilities are paid by the fuel suppliers to take the fuel (known as a “tipping fee"). The tipping fee is comparable to the fee charged to dispose of garbage at a landfill. Another MSW-to-electricity technology, landfill gas recovery, permits electricity production from existing landfills via the natural degradation of MSW by anaerobic fermentation (digestion) into landfill gas. Anaerobic digestion can also be used on municipal sewage sludge; it is discussed separately.
Mass burn technology, the most common MSW-to-electricity technology, involves the combustion of unprocessed or minimally processed refuse. The major components of a mass burn facility include:
- Refuse receiving, handling, and storage systems
- The combustion and steam generation system (a boiler)
- A flue gas cleaning system
- The power generation equipment (steam turbine and generator)
- A condenser cooling water system
- A residue hauling and storage system
Incoming trucks deposit the refuse into pits, where cranes then mix the refuse and remove any bulky or large non-combustible items (such as large appliances). The refuse storage area is maintained under pressure less than atmospheric in order to prevent odors from escaping. The cranes move the refuse to the combustor charging hopper to feed the boiler.
Heat from the combustion process is used to turn water into steam, with the steam then routed to a steam turbine-generator for power generation. The steam is then condensed via traditional methods (such as wet cooling towers or once-through cooling) and routed back to the boiler. Residues produced include bottom ash (which falls to the bottom of the combustion chamber), fly ash (which exits the combustion chamber with the flue gas [hot combustion products]), and residue (including fly ash) from the flue gas cleaning system.
The combined ash and air pollution control residue typically ranges from 20 percent to 25 percent by weight of the incoming refuse processed. This ash residue may or may not be considered a hazardous material, depending on the makeup of the municipal waste.
It may be possible to avoid the production of hazardous ash by preventing the sources which create hazardous waste from entering the system. It is also possible to treat the ash. Both of these methods avoid the costs of disposal at a limited number of landfills classified as able to handle hazardous materials. Non-hazardous ash can be mixed with soils for use as land-fill cover, or can be sold (or given away) for such beneficial uses as pavement aggregate.
While the gross resource potential of MSW in California is estimated to be as high as 2,000 MW, no new MSW-to-electricity facilities are planned within California. At present, the tipping fees in California are generally insufficient to make MSW-to-electricity facilities cost-competitive with other forms of electric generation.
Permitting Issues for Mass Burn Facilities
Some of the major issues associated with mass burn facilities include:
- Ability to meet air quality requirements
- Possible classification of the ash as a hazardous material
- Disposal of ash and other by-products
- Possible conflict with adjacent land uses
- Disturbances to biological resources
- Use of large amounts of water for cooling purposes (if wet cooling towers are used)
- Changes to visual quality due to power plant structures and traffic patterns
- Transportation impacts from numerous truck trips from the refuse source to the mass burn facility (note that collection and transportation would already be occurring, so the mass burn facility would only cause a change in traffic patterns)
- Likely public opposition because of uncertainties over health, safety, odor, and traffic impacts (since it is most economical for the facility to be located near urban centers where the waste is generated)
- Possible conflicts between using MSW for electricity generation and programs/goals for waste reduction techniques and recycling
- Possible hazardous materials leakage that may necessitate site cleanup after facility closure
Refuse-derived fuel (RDF) typically consists of pelletized or fluff MSW that is the by-product of a resource recovery operation. Processing removes ferrous materials, glass, grit, and other materials that are not combustible. The remaining material is then sold as RDF. Both the RDF processing facility and the RDF combustion facility are located near each other, if not on the same site.
The RDF can then be used in one of several configurations:
- Dedicated RDF boilers designed with traveling grate spreader-stokers
- Co-firing of RDF with coal or oil in a multi-fuel boiler
- Dedicated RDF fluidized-bed boiler
As of 2008, there in only one commercial RDF facility in operation in California, a 10.5 MW facility in the City of Commerce, operated by the Los Angeles County Sanitation District.
Permitting Issues for Refuse-Derived Fuel Combustion Facilities
The permitting issues discussed above for mass burn facilities also apply to RDF combustion facilities.
Pyrolysis and thermal gasification are related technologies. Pyrolysis is the thermal decomposition of organic material at elevated temperatures in the absence of gases such as air or oxygen. The process, which requires heat, produces a mixture of combustible gases (primarily methane, complex hydrocarbons, hydrogen and carbon monoxide), liquids and solid residues.
Thermal gasification of MSW is different from pyrolysis in that the thermal decomposition takes place in the presence of a limited amount of oxygen or air. The producer gas which is generated can then be used in either boilers or cleaned up and used in combustion turbine/generators. The primary area of research for this technology is the scrubbing of the producer gas of tars and particulates at high temperatures in order to protect combustion equipment downstream of the gasifier and still maintain high thermal efficiency.
Both of these technologies are in the development stage with a limited number of units in operation. The Hyperion Energy Recovery System operated by the City of Los Angeles had a system designed to fire dried sewage sludge in a staged fluidized bed combustor. The resulting gas was then combusted in stages, and the heat was used to turn water into steam, driving a 10 MW steam turbine-generator.
Permitting Issues for Pyrolysis/Thermal Gasification Facilities
Most of the permitting issues discussed above for mass burn facilities also apply to pyrolysis and thermal gasification facilities. It is not economical to transport the gas produced by such facilities over long distances, so the power generation equipment must be sited with the gasification facilities. As with most refuse-to-energy facilities, it is typically only economical to site gasification facilities near urban centers.
Air emissions may be easier to control than with mass burn technology because the gas produced by the pyrolysis or thermal gasification facility can be scrubbed to remove contaminants prior to combustion. However, scrubbing the producer gas at high temperature is currently under research and the technology has yet to be demonstrated on a large scale. In addition, the pyrolysis and gasifier streams may contain organic compounds of concern that are difficult to remove.