How Things Work: Plasma converters
Imagine a waste disposal system that takes virtually anything in and produces pure energy in return. For those who have seen Back to the Future, a Mr. Fusion-like system of power would seem like an object of dreams. Today, however, plasma converter plants are promising nearly that. They provide a way of waste disposal that is not only environmentally friendly, but also produces a net gain of power in return.
The concept of a plasma converter is simple: destruction of waste by contact with superheated plasma, at a temperature of around 10,000°F.
First, the waste is ground up in giant grinders to ensure maximum surface area for efficient conversion. In a separate chamber, plasma, which is basically a mixture of positively and negatively charged particles, is created by passing an inert gas between two electrodes with an extremely high voltage difference.
This high voltage rips the electrons from the molecules of the gas, turning it into a “soup” of dissociated electrons and ions. Under these conditions, the pressurized gas can reach temperatures as high as 25,000°F. When the gas reaches the chamber holding the waste material, its temperature drops down to about 7000°F.
The waste is then poured into a furnace, where it is exposed to jets of plasma until it has been completely broken down into slag and gaseous components. The gaseous components are drained at the top while the slag is collected in pools below.
Heat from the drained gas is used to boil water, powering steam turbines that generate electricity. About two-thirds of this electricity is sufficient to power the plant itself; the remaining third is sold to the power grid. This makes plasma converters essentially self-sufficient. In fact, some plasma converter plants are marketed as power plants rather than waste disposal facilities to make them seem more appealing to investors.
Contrary to what one might think, plasma converters are not simply large incinerators; they do not destroy waste by burning or oxidization. Rather, the incredibly high temperatures of the plasma physically dissociate molecules, essentially breaking them down into their constituent atoms. Denser atoms eventually aggregate in liquid pools, forming slag, while lighter atoms form superheated gas, which helps maintain the heat in the furnace.
Depending on how the slag is cooled, it can take on various forms. Slag cooled in air forms a material which is strong and black, and can be used to pave roads. Slag cooled in water forms a sandy substance that can be used in construction.
One of the most useful end-products of slag, however, is formed by cooling it in jets of compressed air. Slag cooled in this way forms long strands of molten rock before solidifying. The result is a product, called rock wool, that resembles cotton candy. Rock wool has several interesting properties, including excellent insulation, extremely low density, and high absorbency. Experts speculate that these traits could be useful in fields like hydroponic agriculture, in which plants are grown without soil, and also in containing oil spills.
Another major byproduct of the plasma converters is syngas, a mixture of hydrogen, carbon monoxide, and several other gases in varying ratios depending on the waste processed. Syngas can be refined into a fuel source, which can then be used to generate power. Syngas produces a large amount of power, though it is slightly less efficient than natural gas. The components of syngas, when purified, can also be put to use within the chemical industry.
One of the great advantages of plasma reactors is their ability to greatly reduce the mass and volume of waste. A study published in 1997 in the Journal of Environmental Management states that the weight of the slag is about 20 percent of the weight of the original waste, and the volume of the slag is about 5 percent that of the original waste’s volume. Even if the slag eventually ended up in a landfill, it would greatly reduce the amount of land required for the same amount of waste — not to mention that most of the waste would be put to better use.
In these days when land is becoming scarce, landfills are beginning to cost more and more per cubic ton. For instance, New York City spends an average of $90 dealing with every cubic ton of waste it produces, according to an article in Popular Science. In the same article, Startech, a plasma converter firm, states that a few of their plants could do it for under $36 a ton, and selling the byproducts would actually net up to a $15 profit per ton.
Seen in this light, plasma converters seem like a promise too good to be true. They provide a scheme for waste processing that is environmentally friendly, produces no hazardous byproducts, and creates one of today’s most valuable commodities: pure energy.