Is it really possible to generate clean energy by incinerating garbage?

Illuminating answers to environmental questions.
Jan. 2 2008 8:05 AM

Can We Turn Garbage Into Energy?

The pros and cons of plasma incineration.

A moving grate incinerator. Click image to expand.
A municipal solid waste incinerator

My town council is considering a proposal to build a plasma incinerator. The company behind the project says the facility will convert solid waste into energy, without producing any harmful emissions. Call me a cynic, but their pitch sounds way too good to be true. Am I right to be suspicious?

As proponents of this waste-disposal method always hasten to point out, "plasma incineration" is actually a misnomer—well, at least the "incineration" part. There is no combustion required, and thus no flames or acrid smoke. A more accurate moniker is "plasma gasification," since the end products of the process are syngas and an inorganic solid that can be used to make asphalt or concrete. This peculiar transformation is made possible by a device long cherished by steel cutters: the humble plasma torch.

Advertisement

Since these torches aren't on fire, you can banish from your mind the image of irate villagers storming Dr. Frankenstein's castle. Instead they work by shooting an electric current across an electrode assembly, thereby ionizing an inert gas—sometimes nitrogen, sometimes just plain air. That ionized gas, or plasma, in turn becomes scorchingly hot, with temperatures that can range upward of 27,000 degrees Fahrenheit—hotter than the surface of the sun. Garbage that passes through that sizzling stream doesn't stand a chance: Its molecular bonds are torn asunder, leaving behind  syngas consisting mostly of hydrogen and carbon monoxide, and slag that, when cooled, resembles obsidian.

Once the garbage has been zapped, the syngas is cleansed of harmful traces; it's particularly important to get rid of any hydrogen chloride, which can be done by adding calcium oxide. Heavy metals, meanwhile, must be removed from the slag—no one wants their asphalt to contain lots of mercury and cadmium, both of which are highly toxic.

Then the decontaminated syngas is burned like natural gas, producing enough electricity to power the plant itself, and for resale to the electrical grid. According to Sun Energy Group, which has proposed building a massive plasma gasification facility in New Orleans, disposing of a ton's worth of trash will yield 55.2 kilowatts of power. On top of that, companies claim that plasma gasification plants emit relatively small amounts of carbon dixoide—about on par with that of comparably sized natural gas plants. (Though a nonrenewable fossil fuel, natural gas emits less CO2 than either coal or oil when burned.)

So, why doesn't every hamlet in America do away with its landfills and build one of these wondrous plants? The plasma gasification industry claims it's mostly a matter of economics: Burying garbage has long been a lot cheaper than zapping it, even if you factor in the money to be made selling electricity. Landfills charge (PDF) municipalities an average of $35 per ton of trash; according to a recent study in Hamilton, Ont., dropping off a ton of garbage at a plasma gasification plant would run $172 per ton.

Plasma gasification companies dispute this figure, contending that their method has become more affordable because of increasing efficiency in electricity generation: Canada's Plasco Energy Group, for example, says that 46 percent of zapped waste now becomes energy, compared with 18 percent with earlier plant designs.

The cost gap could be even smaller if plasma gasification plants labeled their electricity as "green" and sold it at a premium to eco-minded customers. But many environmentalists bristle at this prospect, claiming that plasma disposal technologies are merely updated versions of mass-burn incinerators, which have fallen out of vogue in the United States because of problems with dioxin emissions. The activists' chief gripes, summarized here (PDF), are that syngas emissions contain toxic acids and other pollutants, and that the slag retains dangerous levels of heavy metals even after being cleaned. They also note that it's prudent to doubt a technique that's historically been used to get rid of chemical weapons, PCBs, and other nasty remnants of an earlier, less eco-conscious age. (At facilities that handle such dangerous materials, the syngas isn't burned to produce electricity.)

Maybe the environmentalists are right, and maybe they're overreacting—unfortunately, nobody really knows. There is a noticeable dearth of impartial studies assessing the emissions of existing plasma gasification plants that handle municipal solid waste. The hope is that someone will closely monitor the operation of Plasco's pilot project in Ottawa, which aims to process a somewhat piddling 75 tons of garbage per day. (The proposed New Orleans plant, by contrast, is designed to handle 2,500 tons a day.)

Given how little we know about plasma gasification's environmental impact at this point, the Lantern advises caution. While the process certainly holds promise, beware of any company that touts it as a zero-emissions miracle that will quickly pay for itself. And no matter how many millions your town pours into plasma, it's not going to change the fact that we should focus first on reducing waste, rather than figuring out ways to perpetuate the more reckless aspects of our consumption.

Is there an environmental quandary that's been keeping you up at night? Send it to ask.the.lantern@gmail.com, and check this space every week.

Brendan I. Koerner is a contributing editor at Wired and a columnist for Gizmodo. His first book, Now the Hell Will Start, is out now.

  Slate Plus
Political Gabfest
Oct. 31 2014 1:29 PM You, The Gabfest, and a Hotel Room Win tickets to attend a taping of the Political Gabfest, live from David’s Chicago hotel room.