Energy Recovery from the Combustion of Municipal Solid Waste (MSW)
Energy recovery from waste is the conversion of non-recyclable waste materials into usable heat, electricity, or fuel through a variety of processes, including combustion, gasification, pyrolization, anaerobic digestion and landfill gas recovery. This process is often called waste to energy.
On this page:
- Energy Recovery from Combustion
- The History of Energy Recovery from Combustion
- Frequent Questions on Energy Recovery from Combustion
- How much waste does America combust for energy recovery?
- Why are MSW combustion facilities not more common in the United States?
- What is the ash generated by combustion and what happens to it?
- Which regulations apply to energy recovery from waste?
- Does EPA consider burning for energy recovery to be waste minimization?
Energy recovery from the combustion of municipal solid waste is a key part of the non-hazardous waste management hierarchy, which ranks various management strategies from most to least environmentally preferred. Energy recovery ranks below source reduction and recycling/reuse but above treatment and disposal. Confined and controlled burning, known as combustion, can not only decrease the volume of solid waste destined for landfills, but can also recover energy from the waste burning process. This generates an energy source and reduces carbon emissions by offsetting the need for energy from fossil sources and reduces methane generation from landfills.
At an MSW combustion facility, MSW is unloaded from collection trucks and placed in a trash storage bunker. An overhead crane sorts the waste and then lifts it into a combustion chamber to be burned. The heat released from burning converts water to steam, which is then sent to a turbine generator to produce electricity.
The remaining ash is collected and taken to a landfill where a high-efficiency baghouse filtering system captures particulates. As the gas stream travels through these filters, more than 99 percent of particulate matter is removed. Captured fly ash particles fall into hoppers (funnel-shaped receptacles) and are transported by an enclosed conveyor system to the ash discharger. They are then wetted to prevent dust and mixed with the bottom ash from the grate. The facility transports the ash residue to an enclosed building where it is loaded into covered, leak-proof trucks and taken to a landfill designed to protect against groundwater contamination. Ash residue from the furnace can be processed for removal of recyclable scrap metals.
Common technologies for the combustion of MSW include mass burn facilities, modular systems and refuse derived fuel systems.
Mass Burn Facilities
Mass burn facilities are the most common type of combustion facility in the United States. The waste used to fuel the mass burn facility may or may not be sorted before it enters the combustion chamber. Many advanced municipalities separate the waste on the front end to save recyclable products.
Mass burn units burn MSW in a single combustion chamber under conditions of excess air. In combustion systems, excess air promotes mixing and turbulence to ensure that air can reach all parts of the waste. This is necessary because of the inconsistent nature of solid waste. Most mass-burn facilities burn MSW on a sloping, moving grate that vibrates or otherwise moves to agitate the waste and mix it with air.
Modular Systems burn unprocessed, mixed MSW. They differ from mass burn facilities in that they are much smaller and are portable. They can be moved from site to site.
Refuse Derived Fuel Systems
Refuse derived fuel systems use mechanical methods to shred incoming MSW, separate out non-combustible materials, and produce a combustible mixture that is suitable as a fuel in a dedicated furnace or as a supplemental fuel in a conventional boiler system.
The first incinerator in the United States was built in 1885 on Governors Island in New York, NY. By the mid-20th Century hundreds of incinerators were in operation in the United States, but little was known about the environmental impacts of the water discharges and air emissions from these incinerators until the 1960s. When the Clean Air Act (CAA) came into effect in 1970, existing incineration facilities faced new standards that banned the uncontrolled burning of MSW and placed restrictions on particulate emissions. The facilities that did not install the technology needed to meet the CAA requirements closed.
Combustion of MSW grew in the 1980s. By the early 1990s, the United States combusted more than 15 percent of all MSW. The majority of non-hazardous waste incinerators were recovering energy by this time and had installed pollution control equipment. With the newly recognized threats posed by mercury and dioxin emissions, EPA enacted the Maximum Achievable Control Technology (MACT) regulations in the 1990s. As a result, most existing facilities had to be retrofitted with air pollution control systems or shut down.
1. How much waste does America combust for energy recovery?
Currently, there are 75 facilities in the United States that recover energy from the combustion of municipal solid waste. These facilities exist in 25 states, mainly in the Northeast. A new facility was built in Palm Beach County, Florida in 2015.
A typical waste to energy plant generates about 550 kilowatt hours (kWh) of energy per ton of waste. At an average price of four cents per kWh, revenues per ton of solid waste are often 20 to 30 dollars. For more information, read Is It Better to Burn or Bury Waste for Clean Energy?
2. Why are MSW combustion facilities not more common in the United States?
According to the Advancing Sustainable Materials Management: Facts and Figures Report, the United States combusted over 34 million tons of MSW with energy recovery in 2017.
MSW combustion accounts for a small portion of American waste management for multiple reasons. Generally speaking, regions of the world where populations are dense and land is limited (e.g. many European countries, Japan), have greater adoption of combustion with energy recovery due to space constraints. As the United States encompasses a large amount of land, space limitations have not been as important a factor in the adoption of combustion with energy recovery. Landfilling in the United States is often considered a more viable option, especially in the short term, due to the low economic cost of building an MSW landfill verses an MSW combustion facility.
Another factor in the slow growth rate of MSW combustion in the United States is public opposition to the facilities. These facilities have not always had air emission control equipment, thus gaining a reputation as high polluting. In addition, many communities do not want the increased traffic from trucks or to be adjacent to any facility handling municipal waste.
Additionally, the upfront money needed to build an MSW combustion facility can be significant and economic benefits may take several years to be fully realized. A new plant typically requires at least 100 million dollars to finance the construction; larger plants may require double to triple that amount. MSW Combustion facilities typically collect a tipping fee from the independent contractors that drop the waste off on a daily basis to recover costs. The facilities also receive income from utilities after the electricity generated from the waste is sold to the grid. A possible third stream of revenue for the facilities comes from the sale of both ferrous (iron) and non-ferrous scrap metal collected from the post-combusted ash stream.
3. What is the ash generated by combustion and what happens to it?
The amount of ash generated ranges from 15-25 percent (by weight) and from 5-15 percent (by volume) of the MSW processed. Generally, MSW combustion residues consist of two types of material: fly ash and bottom ash. Fly ash refers to the fine particles that are removed from the flue gas and includes residues from other air pollution control devices, such as scrubbers. Fly ash typically amounts to 10-20 percent by weight of the total ash. The rest of the MSW combustion ash is called bottom ash (80-90 percent by weight). The main chemical components of bottom ash are silica (sand and quartz), calcium, iron oxide, and aluminum oxide. Bottom ash usually has a moisture content of 22-62 percent by dry weight. The chemical composition of the ash varies depending on the original MSW feedstock and the combustion process. The ash that remains from the MSW combustion process is sent to landfills. Visit EPA's Landfill Methane Outreach Program for additional information on how facilities recover energy from landfills.
4. Which regulations apply to energy recovery from waste?
Energy recovery from waste is important in the development of sustainable energy policies. EPA continues to develop regulations that encourage energy recovery from materials that might otherwise be disposed of as solid waste.
Identification of Non-Hazardous Materials that are Solid Waste
The 2011 non-hazardous secondary material (NHSM) final rule under the Resource Conservation and Recovery Act (RCRA) identifies which non-hazardous secondary materials are, or are not, solid wastes when burned in combustion units. This determines which Clean Air Act emission standards a combustion unit is required to meet.
5. Does EPA consider burning for energy recovery to be waste minimization?
Waste minimization, the term employed in the RCRA statute, is defined to include both source reduction and certain types of environmentally sound recycling. EPA's highest priority is to achieve reductions through source reduction. However, if this is not achievable, then environmentally sound recycling is also an Agency priority.
Recycling activities closely resembling conventional waste treatment activities (such as burning for energy recovery) do not constitute waste minimization. Also, treatment for the purposes of destruction or disposal is not part of waste minimization, but is, rather, an activity that occurs after the opportunities for waste minimization have been pursued.