Clearing the Air

Features - Emissions Control

Air pollution from waste-to-energy facilities has decreased over the years and continues to show improvement over other energy sources.

  • September 7, 2012
  • Max Lee, Ph.D., P.E.

Major waste companies are putting more of their focus on using waste to produce a broad range of energy products from syngas and oils to arc plasma in addition to conventional waste to energy (WTE). In fact, the Environmental Protection Agency (EPA) in a joint study with European counterpart agencies, conducted research comparing the comparative benefit of WTE to landfilling (with landfill gas capture).

The results were clear that air pollutant emissions from WTE are far less than landfilling with energy recovery. For the purposes of this article, we will focus on the conventional concept of WTE, also deemed a municipal waste combustor (MWC), as a thermal combustion unit capable of combusting 30 or greater tons of MSW per day (EPA regulations apply at 30 tons per day). In today’s world, the old means of combustion of waste without energy capture, simply for volume reduction, is no longer practiced. In fact, modern WTE facilities not only recover energy but capture and provide steam as an additional product, maximizing combustion efficiency and services.

In the United states, 86 conventional WTE facilities can produce 14,000 megawatt hours of electricity (9 percent of renewable electricity),  enough to power about 1.2 million homes or save 30 million barrels of oil per year. The United States used 2.5 million megawatt hours of electricity in 2011 and the per capita rate has steadily grown over the years, in part due to our ever increasing demand for living space heating and cooling, and energy-hungry electronics.
At the same time, the volume of solid waste generated has grown from 81 million to 243 million tons of municipal solid waste (MSW) per year from 1960 to 2009, with per person generation rates increasing from 2.7 to 4.3 pound per person per day in this same period. (See table below)

With the onset of the Resource Conservation and Recovery Act (RCRA) (starting with the Solid Waste Act of 1976), municipalities needed to develop high capital investment projects for dealing with solid waste under modern regulations. These investments can be a serious burden for even the largest municipalities, requiring long-term bonding or other investiture. As such, the days of “mom-n-pop” waste disposal operations disappeared, and our country’s modern solid waste disposal infrastructure began. These factors of increasing power demand, increasing volumes of solid waste and new RCRA regulations spurred larger capital intensive WTE projects to handle solid waste disposal. A number of communities across the country initiated WTE for waste disposal. These communities were typically in highly populated areas with little nearby land for the traditional landfill. And these same urbanized areas typically had growing power demands.

Florida is an example of one area where WTE  has grown and now provides 3.25 million megawatts per year through combusting 12 percent of the state’s MSW. The potential value of WTE is obvious when you consider the shrinking available land for disposal in Florida and that its population has grown from 5 million in 1960 to near 20 million today. The Miami-Dade County Resource Recovery Facility is a great example. Its WTE facility (startup in 1982) today provides up to 77 MW of baseload electricity to the grid.

WTE plants require initial high capital investment, which is typically implemented through bonding and local ordinances that are designed to restrict and control the flow of waste to the WTE. This control of waste flow can have a negative impact on the free flow of waste and impede commerce (waste is a commodity) that would otherwise allow a broader range of waste material usages to develop. Nevertheless, WTE facilities can provide many benefits including a clean-emitting steady local source of power that reduces landfilling.


EPA has developed programs under the Clean Air Act (CAA) to control air pollution from stationary sources of air pollution. These programs include:

  1. Section 129 Maximum Control Technology Standards (MACT)
  2. New Source Review (NSR)

These two programs have complex requirements that come into play at different times in the life of a WTE facility and affect different air pollutant emissions.

NSR and MACT programs are EPA’s primary tools to control air pollutant emissions. NSR is applied for a “new” source, which is either a new construction or a re-construction or modification of a facility. Depending on the amount of each pollutant being emitted from the project, the emissions resulting from the new source will be limited on a case-by-case basis.

The NSR program continues to ratchet down on new sources and evolve based on upgrades in air pollution control technology and other regulatory factors. The MACT program for Municipal Waste Combustor (MWC) is issued through Section 129 (specifically addresses waste combustion), applies to new sources depending on the date of constructing the combustion unit and the operating capacity of the unit. EPA issued the MACT standards for MWCs in the early 1990s with the rules being applied not only to new units but required retrofitting of older units by 2000 and 2005, depending on various factors of size and unit age.

After the MACT standards were in force, related air pollution emissions from MSW combustion facilities decreased by a factor of twenty.  (See the table above.) The EPA claimed the results to be “outstanding.” The total emissions of hazardous air pollutants have dropped more than 94 percent in this time period, from nearly 58,000 tons in 1990 to about 3,300 tons in 2005.

The air pollution controls deployed to reach these significant reductions include:

  • activated carbon injection for control of metals control and dioxin/furans;
  • sorbent injection (via ductwork or direct to furnace) for control of metals and acid gases (primarily HCl and SO2);
  • electrostatic precipitators or baghouses for control of particulate matter;
  • flue gas recirculation for improved combustion (control of NOx, VOC and CO);
  • spray dryer adsorber for control of acid gases; and
  • selective non-catalytic reduction for control of nitrogen oxides.

EPA estimates greenhouse gas (GHG) emissions range from 10 to 20 million metric tons, depending on the different methods used to estimate the biogenic fraction of MSW. EPA’s eGrid (a database of information on electrical generators in the United States) indicates about 53 percent of the energy generated by MSW combustion facilities is from biogenic sources and 47 percent is fossil-derived power.

If we presume 53 percent of the GHG emissions are from biogenic sources, the MSW combustion facilities generate less than the GHGs of coal or oil or natural gas. (See table at the bottom of pg. 31.) Related to the CAA program of NSR and New Source Performance Standards (NSPS), the biogenic portion of GHG emissions can severely affect how the facility must comply with these programs. This biogenic fraction may not need to be included when determining the applicability of the GHG emissions for these programs. For example, the amount of CO2 emissions for combustion of biogenic materials has been exempted for purposes of the NSR program for state programs either under the federal EPA NSR program or adopting that federal criteria. Also note that EPA has recently implemented a GHG reporting rule for which EPA estimates the biogenic fraction in MSW to be 60 percent.

A more comprehensive approach to evaluate the overall pros and cons of WTE is through life-cycle emission analysis, which includes a broader range of quantification factors to GHG emissions, including:

  • avoided methane emissions from landfills;
  • energy generation potential that offsets fossil fuel use;
  • metals recovery; and
  • emission savings from the avoidance of long-distance transport to landfills.

EPA models show that MSW combustors actually reduce the amount of GHGs in the atmosphere compared to landfilling. EPA estimates the GHG savings from WTE to be about 1.0 ton of GHGs saved per ton of MSW combusted.

The modern WTE facility controls air pollutant emissions to meet and exceed regulatory standards. The emissions also are comparable or lower than that from conventional fossil fuels. Required air pollution control devices have dramatically reduced emissions in the last 20 years.

Given that these facilities require initial high capital investment for both initial construction and upgrades, WTE facilities have a competitive disadvantage compared to low-cost conventional disposal options.

Nevertheless, WTE facilities can provide many benefits, including a clean-emitting steady local source of power that reduces landfilling. Indirect reductions of emissions from WTE can also be considered beneficial, including reduction of extraction, processing and transportation of fossil fuels, metals recovery from the waste and reduction of landfilled materials.

The author is president of Koogler and Associates Inc., Gainesville, Fla. More information is available at Angela Morrison of Hopping Green & Sams P.A., Tallahassee, Fla., contributed to the article and can be contacted at