Of the many problems created by the conveniences of modern life, garbage is the smelliest. Even before the days of plastics and mass consumerism—all the way back to the discovery of fire—humans have fallen back on two reliable ways of dealing with their refuse: tossing it in a hole in the ground or burning it. Little has changed in the 21st century, but technology has given us the means to convert the energy released by burning trash into electricity, a practice known as waste-to-energy (WTE) incineration. As landfills begin to overflow and WTE incineration technology becomes cleaner and more reliable, this option—which boasts the two-for-one bonus of creating something we need while reducing the mass of something we don’t—is becoming more and more attractive to cities and countries.
That trend is especially true in Europe, where there are over 450 such facilities processing municipal solid waste into kilowatts, with Denmark, Germany, and the Netherlands taking especially aggressive strides in building new ones. While the sharp growth in WTE facilities is in part a result of Europe’s relative scarcity of land and high-cost of energy, it is also due to the fact that the European Union imposes steep taxes on landfills and some EU countries have harsh restrictions against opening new dumps. But in contrast to the American approach of placing power plants as far away as possible from where people live, the Europeans are building their WTE plants in the middle of population centers, right where the garbage is generated. This helps to reduce these nations’ carbon footprints by not trucking trash long distances, but it also speaks to Europeans’ acceptance of, and trust in, the technology.
Here in the U.S., the situation is quite different. There are only 86 WTE incineration plants in the country today, and the majority are at least 15 years old. The most recent WTE facility was completed in 2008 and—in an relative boom—two others are under construction in Palm Beach, Fla., and Honolulu. The reason for the country’s slow adoption of the technology is partly due to strong opposition from communities, who don’t want such dirty-sounding places in their backyards, and from environmentalist groups that feel we should be striving toward the goal of recycling everything we can and composting the rest. Building a WTE facility means committing to a steady supply of waste, a fact that seems to stand in the face of promoting recycling. That fear is mitigated, however, by the fact that the nations in Europe that have most embraced WTE, such as Denmark and Germany, have very comprehensive recycling programs.
Then there is the issue of emissions. Incinerating municipal solid waste results in a number of potentially dangerous pollutants. Before cleaning, flue gasses can contain significant amounts of heavy metals, dioxins, furans, sulfur dioxide, and hydrochloric acid. And there’s the solid output—flyash and bottom ash—which represents 10 to 15 percent of the mass of the waste that went into the incinerator. Today’s WTE facilities are equipped with a sophisticated array of scrubbing devices, such as particle filters, electrostatic precipitators (which remove particulate by way of electrostatic charge), and baghouse filters (not much different from the bag in your vacuum cleaner), which reliably remove somewhere on the order of 99.8 percent of atmospheric pollutants. Furthermore, those metals and chemicals can be recovered for resale to industry. Flyash and bottom ash can also be recycled as components in concrete or asphalt. Since the 1990 amendment to the Clean Air Act, all U.S. power plants, WTE or otherwise, have been retrofitted with these emissions-cleaning technologies. According to Nickolas Themelis, a Columbia University engineering professor and head of the Waste-to-Energy Research and Technology Council (WTERT), the total weight of dioxins released by all WTE energy plants in the U.S. per year is less than 10 grams. “It’s less than a cigarette butt,” he says.
How it Works
While there are differences from WTE plant to WTE plant, the typical modern variety—such as those being built in Europe—is a moving grate incinerator. Nonrecyclable garbage arrives at the facility in trucks, where it is weighed and then dumped into a collection area. Most European municipalities impose additional fees for people who send recyclable materials to WTE facilities, encouraging waste carters to be meticulous in their sorting. What then winds up at the incinerator is primarily organic matter. In the collection area, a big claw crane tosses the garbage to create a homogenous mix to promote even burning. Once mixed, the crane lifts the fuel and drops it into the “throat,” where a system of moving grates begin to move it gradually down toward the incinerator. The grates move independently in a staggered fashion, slowly sifting the garbage down from grate to grate until it reaches the lowest level of grates in a furnace at the bottom. On the way, heat rising from below evaporates moisture in the garbage to ensure that it combusts with the highest level of potential energy output. Once in the extreme heat of the furnace, the trash combusts.
This process is aided by outside air that flows through vents in the grates (the air also cools the mechanisms to maintain mechanical integrity) and also through high-speed nozzles that fan the flames. To make sure that toxic organic compounds are properly broken down, the flue gasses must be maintained at a temperature of 850 C (1,560 F) for two seconds. Auxiliary backup burners are often used to maintain this temperature. The grates then proceed to the ash dump, where the ash is washed off of the grates with water.
The flue gasses rise from the furnace and through a boiler where their heat is transferred to steam. The steam is heated up to 400 C (752 F) and then, at a pressure of about 580 pounds per square inch, passes through a turbine. The turning of the turbine generates electricity in a process similar to that of a coal or natural-gas power plant. The steam can also be used to feed municipal steam-heating systems.
Once the flue gasses leave the boiler they are less than 200 C (392 F). At that point they pass into the cleaning system. Particle filters remove fine particulates and acid-gas scrubbers remove hydrochloric, nitric, and hydrofluoric acids, as well as mercury, lead, and other heavy metals. Activated carbon particles are then injected to collect volatile metals and organic molecules. The flue gas then passes through a fabric bag filter where are particles are removed. Basic scrubbers remove sulfur dioxide (which turns into synthetic gypsum by way of a reaction with lime).
Nearly all of the carbon content of the waste is released into the atmosphere as carbon dioxide. While that may sound alarming at first, it is actually an improvement over dumping the garbage into a landfill, where the biodegradable parts of the waste would be converted to methane through anaerobic decomposition—and methane has a much higher heat-trapping potential than carbon dioxide. According to Themelis, 1 ton of municipal solid waste sent to a WTE plant instead of to a landfill reduces greenhouse gas emissions by the equivalent of 0.5 to 1 ton of carbon dioxide; he gives a range to account for the varying degrees of efficiency of capturing gas at landfills. It is also recognized that most biodegradable matter in municipal solid waste comes from plants that used atmospheric carbon dioxide to grow in the first place. If the dead plants that go to WTE facilities are replaced with new plantings, then the carbon dioxide released by their incineration will be absorbed from the atmosphere and reconstituted into new plant matter. So, go plant a tree.