Thomas Stringfellow and Robert Witherell, CH2M HILL Engineers, Inc.
January 13, 2014 | 13 Comments
Waste-to-Energy (WTE) or energy-from-waste is the process of generating energy in the form of electricity and/or heat from the incineration of waste. In the U.S., some cities primarily in the northeastern and mid-Atlantic, burn part of their municipal solid wastes. Hemmed in by major population centers, landfill space in these areas is at a premium, so burning wastes to reduce their volume and weight makes sense. Combustion reduces the volume of material by about 90 percent and its weight by 75 percent. The heat generated by burning wastes has other uses, as well, as it can be used directly for heating, to produce steam or to generate electricity.
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In 1885, the U.S. Army built the nation’s first garbage incinerator on Governor’s Island in New York City harbor. Also in 1885, Allegheny, Pennsylvania built the first municipal incinerator. As their populations increased, many cities turned to incinerators as a convenient way to dispose of wastes.
These incineration facilities usually were located within city limits because transporting garbage to distant locations was impractical. By the end of the 1930s, an estimated 700 incinerators were in use across the nation. This number declined to about 265 by 1966, due to air emissions problems and other limitations of the technology. In addition, the popularity of landfills increased.
In the early 20th century, some U.S. cities began generating electricity or steam from burning wastes. In the 1920s, Atlanta sold steam from its incinerators to the Atlanta Gas Light Company and Georgia Power Company.
Europe, however, developed waste-to-energy technologies more thoroughly, in part because these countries had less land available for landfills. After World War II, European cities further developed such facilities as they rebuilt areas ravaged by war.
The use of municipal waste combustion for energy in the U.S. is not common; the nation had only 87 such facilities in 2007 and has added several more today, while Europe has more than 430 such facilities. By the 1990s, after the tax credits extension of 1986 finally ended, fewer waste-to-energy plants were built. Figure 1 shows the generic process of converting waste to energy.
Recently in the U.S. WTE has been deemed a Renewable Energy source. According to the EPA the definition of Renewable Energy - “Renewable Energy is energy obtained from sources that are essentially inexhaustible, unlike natural gas, coal and oil, of which there is a finite supply.” According to the Department of Energy (DOE) – “Renewable energy sources include: wood and other biomass, solar (Photovoltaic and Thermal), wind, geothermal, wastes [Municipal Solid Waste (MSW), Refuse-Derived Fuel (RDF), Landfill Gas (LFG)] and any other sources that are naturally or continually replenished.” By definition, the DOE describes renewable energy as a “non-deplete-able source of energy.”
The technologies described in this paper all produce energy, we will not address pure incineration or other means of reducing municipal solid waste that does not produce energy. We will also not address the Non-Thermal Technologies (Anaerobic Digestion, Landfill Gas, or Hydrolysis and Mechanical Biological Treatment.
The purpose of this paper is to provide a technical evaluation of the available technologies and provide an indicative cost estimate ranges associated with each.
The technologies we reviewed are as follows:
As mentioned earlier we did not evaluate the Non-Thermal Technologies.
Direct Combustion Mass Burn and Refuse Derived Fuel
As mentioned above Mass Burn facilities have been in existence for decades and as the technology reflects it literally burns/combusts everything, leaving only noncombustible material. There are over 100 of these facilities operating in the U.S. and considerably more in Europe and Asia. Refuse Derived Fuel (RDF) is the process of removing the recyclable and noncombustible from the municipal solid waste (MSW) and producing a combustible material, by shredding or pelletizing the remaining waste. There are only 19 RDF facilities in the U.S., but as energy prices climb and landfill permitting gets more difficult there may be an increase in the number of these facilities. Figure 2 and 3 are B&W’s rendition of typical Mass Burn and RDF technologies.
Pyrolysis is the thermo-chemical decomposition of organic material, at elevated temperatures without the participation of oxygen. The process involves the simultaneous change of chemical composition and physical phase that is irreversible. Pyrolysis occurs at temperatures >750°F (400°C) in a complete lack of oxygen atmosphere. The syn-gas that is produced during the reaction is generally converted to liquid hydrocarbons, such as biodiesel. Byproducts from the process are generally unconverted carbon and/or charcoal and ash.
There are various types of Pyrolysis technologies ranging from carbonization to rapid or flash type systems. Table 1 below shows the different types and comparisons of the process conditions and major products.
Figures 4 and 5 show the process flows for the fast and rapid pyrolysis processes that are being offered commercially. We are aware of small modules operating throughout the world, but to our knowledge there are no systems operating at large industrial sized.