Turning garbage into gas

Feb 3rd 2011 | from PRINT EDITION

DISPOSING of household rubbish is not, at first glance, a task that looks amenable to high-tech solutions. But Hilburn Hillestad of Geoplasma, a firm based in Atlanta, Georgia, begs to differ. Burying trash—the usual way of disposing of the stuff—is old-fashioned and polluting. Instead, Geoplasma, part of a conglomerate called the Jacoby Group, proposes to tear it into its constituent atoms with electricity. It is clean. It is modern. And, what is more, it might even be profitable.

For years, some particularly toxic types of waste, such as the sludge from oil refineries, have been destroyed with artificial lightning from electric plasma torches—devices that heat matter to a temperature higher than that of the sun’s surface. Until recently this has been an expensive process, costing as much as $2,000 per tonne of waste, according to SRL Plasma, an Australian firm that has manufactured torches for 13 of the roughly two dozen plants around the world that work this way.

Now, though, costs are coming down. Moreover, it has occurred to people such as Dr Hillestad that the process could be used to generate power as well as consuming it. Appropriately tweaked, the destruction of organic materials (including paper and plastics) by plasma torches produces a mixture of carbon monoxide and hydrogen called syngas. That, in turn, can be burned to generate electricity. Add in the value of the tipping fees that do not have to be paid if rubbish is simply vaporised, plus the fact that energy prices in general are rising, and plasma torches start to look like a plausible alternative to burial.

An electric atmosphere

The technology has got better, too. The core of a plasma torch is a pair of electrodes, usually made from a nickel-based alloy. A current arcs between them and turns the surrounding air into a plasma by stripping electrons from their parent atoms. Waste (chopped up into small pieces if it is solid) is fed into this plasma. The heat and electric charges of the plasma break the chemical bonds in the waste, vaporising it. Then, if the mix of waste is correct, the carbon and oxygen atoms involved recombine to form carbon monoxide and the hydrogen atoms link up into diatomic hydrogen molecules. Both of these are fuels (they burn in air to form carbon dioxide and water, respectively). Metals and other inorganic materials that do not turn into gas fall to the bottom of the chamber as molten slag. Once it has cooled, this slag can be used to make bricks or to pave roads.

Electric arcs are a harsh environment to operate in, and early plasma torches were not noted for reliability. These days, though, the quality of the nickel alloys has improved so that the torches work continuously. On top of that, developments in a field called computational fluid dynamics allow the rubbish going into the process to be mixed in a way that produces the most syngas for the least input of electricity.

The first rubbish-to-syngas plants were built almost a decade ago, in Japan—where land scarcity means tipping fees are particularly high. Now the idea is moving elsewhere. This year Geoplasma plans to start constructing a plant costing $120m in St Lucie County, Florida. It will be fed with waste from local households and should create enough syngas to make electricity for more than 20,000 homes. The company reckons it can make enough money from the project to service the debt incurred in constructing the plant and still provide a profit from the beginning.

Nor is Geoplasma alone. More than three dozen other American firms are proposing plasma-torch syngas plants, according to Gershman, Brickner & Bratton, a waste consultancy based in Fairfax, Virginia. Demand is so great that the Westinghouse Plasma Corporation, an American manufacturer of plasma torches, is able to hire out its test facility in Madison, Pennsylvania, for $150,000 a day.

Syngas can also be converted into other things. The “syn” is short for “synthesis” and syngas was once an important industrial raw material. The rise of the petrochemical industry has rather eclipsed it, but it may become important again. One novel proposal, by Coskata, a firm based in Warrenville, Illinois, is to ferment it into ethanol, for use as vehicle fuel. At the moment Coskata uses a plasma torch to make syngas from waste wood and wood-pulp, but modifying the apparatus to take household waste should not be too hard.

Even if efforts to convert such waste into syngas fail, existing plants that use plasma torches to destroy more hazardous material could be modified to take advantage of the idea. The Beijing Victex Environmental Science and Technology Development Company, for example, uses the torches to destroy sludge from Chinese oil refineries. According to Fiona Qian, the firm’s deputy manager, the high cost of doing this means some refineries are still dumping toxic waste in landfills. Stopping that sort of thing by bringing the price down would be a good thing by itself.