Fly ash is captured in the hoppers at the waste-to-energy plant in Lee County, Fla., and then returned via screw conveyors and mixed with the bottom ash. From there it is sent through the dischargers to the residue house. Photo: Lee County Solid Waste Division
Fly ash is captured in the hoppers at the waste-to-energy plant in Lee County, Fla., and then returned via screw conveyors and mixed with the bottom ash. From there it is sent through the dischargers to the residue house. Photo: Lee County Solid Waste Division

Although it devolved from an EPA-touted panacea in the 1970s to a nearly defunct model in the 1990s, recent developments indicate incineration is returning to the mix as an acceptable form of solid waste management.

Less than a year ago Ft. Myers, Fla., opened a 600-ton/day (TPD) facility that converts municipal solid waste into energy through combustion. Tampa, Fla., is building a 600-TPD incinerator that's expected to open next summer. Rochester, Minn., has a 200-TPD incinerator under construction at its existing 200-TPD facility. And York, Pa., is adding a 600-TPD incinerator to its existing solid waste program.

Modern incineration with energy recovery has become the choice for solid waste management throughout the modern world, except for the United States, where misconceptions still linger. Most cities undergoing expansions to their combustion programs already have proven to their communities that incineration can be both clean and effective, despite critics who argue that incineration undermines recycling efforts.

The 86 communities nationwide that have been home to incinerators for the last 15 years commonly achieve recycling rates that exceed the average, according to studies by the Earth Science Center at Columbia University in New York City. Those communities are simply burning the leftovers after very intensive recycling efforts, supporters say.

The advanced technology in solid waste burning is the mandatory air pollution control system: Waste-to-energy (WTE) plants do not use an open fire. Modern incineration must use a combustion process in a carefully controlled furnace that includes an automatic combustion control system for maintaining consistent high temperatures and pressures that generate low levels of air emissions. All flue gasses are directed through a series of air pollution control devices for thorough cleaning.

Those special emission controls respond quickly and automatically to changes in the fuel and conditions in the furnace. Computer-controlled monitors sample the air continuously at several places in the furnace, the air pollution control system, and in the stack. The computers adjust the rate at which the waste is burned, the combustion air for the fire, and even auxiliary fuel. The monitors also adjust the addition of lime, activated carbon, and other chemicals used to remove pollutants.

Another common element among those communities is that they all recover the energy released by the combustion process.

If all the solid waste in America were to be burned in modern plants, it could produce approximately 4% of the nation's electric energy while reducing highway traffic and eliminating the consumption of millions of gallons of diesel fuel used to transport millions of tons of solid waste — much of it not recyclable — to distant landfills. In San Francisco, for example, recycling leftovers are hauled at least 70 miles, the equivalent of more than 2,000 TPD produced by 100 semitrailers.

Incineration was banned for a short time in Denmark and Sweden in the 1980s, until university studies concluded that modern incineration with energy recovery was the only good choice for solid waste management. The European Union has adopted statutes that encourage WTE and ban the burial of any waste containing more than 2% combustible materials.

Burning of almost any material can be done so that emissions are as clean as the burners on a gas stove. Special furnaces with modern computerized controls burn matter cleanly, and pollution control devices such as scrubbers, baghouses, and electrostatic precipitators neutralize or remove various components of the flue gases before they reach the smoke stack.

The emissions from a modern central waste combustion plant are as clean as a natural gas-fired home furnace, based on a comparison of published EPA emission factors for sources of equivalent energy input.

In many plants today, the computer monitors send their automatically observed information about the operating incinerators over data lines directly to local environmental protection authorities. Every plant must report its pollution control data to environmental regulators, and waste-to-energy plants are among the most closely regulated power plants in the country.

Like coal, oil, and natural gas, burning trash produces various gases that must be controlled to protect human health and the environment (such as carbon monoxide, nitrogen oxides, acid gases, solid particulates, and other trace matter such as mercury and dioxins), which are all controlled in a modern incinerator. Waste-to-energy plants meet or exceed the strictest federal standards set by the EPA, which were further tightened May 2006.

As is also required by their operating permits, the 86 U.S. WTE plants also have demonstrated that recycling is compatible with modern WTE and central incineration plants. Solid waste combustion programs take what is left after thorough curbside and drop-off recycling has been accomplished. Peer-reviewed papers, such as those presented at various American Society of Mechanical Engineers Solid Waste Processing Division conferences and at North American Waste to Energy Conferences, show that the waste left over after very thorough recycling has a very similar energy content to trash without recycling, roughly half the energy pound-for-pound as coal.

Further, computer modeling of modern solid waste management choices by the EPA and recent studies in Hawaii have shown that recycling is not always the best choice for the environment or for human health: Trucking causes accidents; trucking and shipping have environmental costs, too.

State and federal transportation departments and the insurance industry thoroughly document highway accidents and the millions of miles driven by trucks. Trucking solid waste long distances to landfills causes accidents and burns up immense quantities of diesel fuel. Semitrailers hauling solid waste average about 2 miles/gallon.

Advocates of incineration argue that no clean energy source should be excluded from the total mix of energy available in the United States. The EPA has stated that modern incineration with energy recovery is one of the cleanest sources of new electricity, second only, perhaps, to wind turbines.

— John Norton, PE, DEE, is chair of the Solid Waste Processing Division of the American Society of Mechanical Engineers, a member of the Board of Directors of Green Energy Ohio, and the owner of the Norton Engineering Co. in Dayton, Ohio.