When the U.S. Environmental Protection Agency (EPA) issued the MACT (Maximum Achievable Control Technology) standards for emission guidelines for municipal solid waste (MSW) combustors in the early 1990s, waste-to-energy (WTE) facilities responded constructively.

According to the EPA, emissions from MSW combustion facilities decreased by a factor of 20 after the MACT controls were put in place. From 1990-2005, total emissions regulated by the Clean Air Act—mercury, cadmium, lead, particulate matter, hydrogen chloride, sulfur dioxide, and nitrogen oxides (NOx)— dropped more than 94 percent, from nearly 58,000 tons in 1990 to approximately 3,300 tons in 2005 (See table below).

Under the Clean Air Act, WTE facilities are required to have the most state-of-the-art air pollution control equipment and technology available, says Harvey Gershman, president of Gershman, Brickner & Bratton (GBB), an international solid waste management consulting firm based in Fairfax, Va.

Pat Sullivan, director of Long Beach, Calif.-based SCS Engineers’ western region consulting and engineering operations, adds that all WTE facilities must operate in accordance with some of the most stringent environmental standards in the world that are designed to protect public health and the environment.
 

Sophisticated Systems

To adhere to these standards, WTE plants nationwide invested more than $1 billion in upgrades to their air quality control systems, according to the Energy Recovery Council (ERC, www.wte.org), Washington, D.C. The national trade association, which represents the companies and communities engaged in the WTE sector, says on its website that the results were “so dramatic that the [EPA] wrote that the ‘upgrading of the emissions control systems of large combustors to exceed the requirements of the Clean Air Act Section 129 standards is an impressive accomplishment.’”

“The performance of the MACT retrofits has been outstanding,” the EPA said, according to the ERC’s website.

ERC President Ted Michaels agrees with the EPA that WTE facilities’ advancements for emissions control have been nothing short of noteworthy. “What the facilities have done since 1990 has been remarkable,” Michaels says. “More than a billion dollars was spent by local governments and companies … to comply with these standards.”

Michaels says much of the improvements are based on four key technologies, what he calls a “suite of technology” that “many WTE plants today will have.”

While the 85 WTE facilities located in 23 U.S. states are not required to incorporate all four emission control technologies into their systems, “it’s a pretty routine lineup,” Michaels adds.

The four technologies, according to Michaels and the ERC, are:

• Fabric filter or bag house—works like a giant vacuum cleaner with hundreds of fabric filter bags that filter the air of soot, smoke and metals.
• Scrubber—sprays a mixture of lime and water into the hot exhaust gases. The lime/water mixture cools the hot exhaust gases and neutralizes acid gases such as sulfur dioxide. Cooling the exhaust gases also improves the capture of heavy metals such as mercury and trace organics such as dioxins.
• Carbon injection system—blows powdered-activated carbon into the exhaust gas to adsorb mercury and further reduces emissions of trace organics such as dioxins.
• Selective non-catalytic reduction (SNCR)—converts nitrogen oxides—a contributor to urban smog—to harmless nitrogen by spraying ammonia or urea into the hot furnace.
  
   

Low Levels

Sullivan says these air pollution control technologies have allowed WTE facilities to maintain low emission levels. “Over time, as regulations have gotten more complex and stringent, and emission control technology has advanced, WTE plants have been able to achieve very low emissions levels,” he says.

Sullivan continues, “The WTE plants in operation today are well controlled from an emissions standpoint.”

Gershman agrees. “WTE facilities currently perform well below permit requirement limitations for emissions, so in the U.S., emissions are never left untreated,” he says.

The EPA, which considers WTE a “clean, reliable, renewable source of energy,” according to the ERC’s website, has concluded, as a result of the controls employed at WTE plants, that WTE generates electricity with “less environmental impact than almost any other source of electricity.”

Gershman highlights the Onondaga County Resource Recovery Facility, Syracuse, N.Y., which incorporates all four of the aforementioned technologies in its WTE facility.

Gershman explains that the plant’s SNCR system injects ammonia into the boiler to control nitrogen oxide emissions; an activated carbon injection in the scrubber controls mercury and dioxin emissions; a dry scrubber injects lime slurry to control acid gas emissions; and a fabric filter bag house includes numerous cylindrical bags that filter out and remove particulate matter.

Emissions from the Onondaga County Resource Recovery Facility are monitored through continuous emissions monitors (CEMs) and annual stack testing. According to the facility’s website, the Onondaga County plant recovers about 9,000 tons of ferrous and nonferrous metals each year, provides enough electricity to power nearly 30,000 residential homes and claims to have one of the highest recycling rates in New York and possibly even the nation.

Regulated Retrofit

Michaels points to the success of the Montgomery County Resource Recovery Facility, Dickerson, Md., which, like the Syracuse facility, monitors its emission levels through CEMs every 10 seconds. The real-time data compiled from the emissions monitors is updated hourly on its website.

“That’s an effort of total transparency. They are effectively communicating the successes of the facility,” Michaels says regarding the Maryland facility’s emissions reports posted online.

The Montgomery County Resource Recovery Facility also completed a retrofitting project four years ago.

The facility installed Covanta Energy’s LN (Low NOx) technology, which reduced NOx concentrations to about half of the amount designed by regulatory standards, according to the facility’s website. Emissions monitoring data indicates that the NOx levels, on average, have been controlled to 90 parts per million (ppm), compared to the permitted emission limit of 180 ppm. The facility had previously recorded concentrations of about 170 ppm prior to the new technology installation.

Before the resource recovery facility integrated the LN technology, an SNCR process would inject aqueous ammonia into the flue gas through the furnace walls. In this process, ammonia chemically combines with NOx to form nitrogen and water, which is released through the stack into the outside air.
 

The facility still incorporates this SNCR process, while the LN technology modifies the balance of air and temperature distribution in the furnace to reduce NOx generation.

“The Low NOx technology is not a machine you put at the end of the stack, but it is a method of using different air flow to reduce the formation of NOx,” Michaels explains. “It helps a lot, and that is within the last five years.”

Gershman says an EPA case study on the Montgomery County facility shows that the overall cancer risk to human health has “decreased over time, even with incorporating new risk methods and more exposure pathways.”
 

Proactive Strategies

Wheelabrator Technologies, Hampton, N.H., has introduced innovative net greenhouse gas reduction projects at its WTE facilities in Westchester, N.Y., and Baltimore, Md., offsetting the use of fossil fuels.

For example, after nearly 30 years of converting postrecycled waste into clean electricity for New York homes and businesses, a newly constructed steam line project puts steam from the Wheelabrator Westchester WTE plant to productive use by a large commercial laundry operation and major employer in the city of Peekskill, N.Y. This partnership is expected to reduce greenhouse gas emissions from the laundry facility and expand the use of renewable power sources.

The White Plains Linen laundry operation will significantly reduce the amount of natural gas it uses to make steam and hot water, from 1 million therms per year to less than 90,000 therms per year, the company says. This steep reduction in natural gas usage will eliminate 4,775 metric tons of greenhouse gas emissions annually, equivalent to taking 995 passenger vehicles off the road.

Emission limits for WTE facilities could decrease even further as the EPA is required every five years to evaluate whether lower limits can be achieved, and if they are required to further protect public health and the environment. New WTE facilities also will be required to further evaluate new emissions control technologies as they become available in the future.

Michaels says the required air pollution control devices have improved since tighter standards were implemented two decades ago, and there will continue to be “modifications to the system.

“The facilities that are operating in these communities are proud of the work they are doing. They want people to be happy and comfortable with this very large investment these communities have made,” Michaels says.

The author is associate editor of Renewable Energy from Waste magazine and can be reached at [email protected].