There are only two manmade structures on Earth that are large enough to be seen from the far reaches of the universe: the Great Wall of China and the Fresh Kills landfill. [1] In the United States, about 4.5 pounds of trash per day and up to 56 tons of trash per year are created by the average person. [1] Roughly seventy percent of this municipal refuse, or solid waste, is buried in landfills. There is no surprise, then, when one learns that landfills, which also emit harmful greenhouse gases, are closing at the rate of one per day in the U.S. simply due to being full. [1] Municipal refuse gasification, a waste-to-energy process built on plasma arc technology, tries to resolve these issues through an innovative approach to garbage disposal which results in reduced landfill use, while producing renewable energy.
Currently, incinerators are used to reduce both the solid mass and volume of waste, before disposing of the garbage into a landfill. Depending on the composition of the refuse, incineration may reduce mass of the original waste by 80-85% and reduce the volume by 95%. [2] While this does not completely eliminate the need for landfills, incineration provides a way to significantly reduce the storage space required to dispose of waste. Why, then, is there a need for the plasma arc gasification technology mentioned above? The cancer-causing, hormone altering chemical dioxin was first discovered in an old incinerator in the United States in 1978 and later at other incinerators throughout the country. [3] Although modern incinerators curb up to 99% of dioxin production, some argue if there is a “safe” limit for a hazardous substance. [3]
The foundation of the plasma arc based gasification process is the process of gasification itself. Gasification is a process that converts carbonaceous materials, such as coal, petroleum, biofuel, or biomass, into mainly carbon monoxide and hydrogen. Generally, this chemical conversion is initiated by reacting raw material at high temperatures (greater than 700 degrees celsius) with small controlled amounts of oxygen. [4] The resulting gas mixture is called synthesis gas, or syngas, which has a calorific value, and therefore can be used as fuel. [4] Since syngas is composed of mainly hydrogen and carbon monoxide, the process of gasification can be thought of as a method for extracting energy from many different types of organic materials. This makes gasification a very vital process, as it raises the value of low value raw materials (carbonaceous materials) by transforming them to marketable products and fuels. Now that we have an understanding of the foundation of plasma arc gasification, we can take a deeper look at how municipal refuse gasification research has progressed.
Municipal refuse gasification, a process for converting garbage into fuel and electricity without incinerating it, grew out of the perpetual need for increased waste disposal and more energy. Using plasma arc technology, this gasification process works a little like the big bang, only backward (you get nothing from something). [5] Plasma arc gasification (PAG) is the gasification of matter, but in an oxygen-starved environment and at temperatures reaching that of the surface of the sun (~6,000 degrees Celsius). [6] Plasma gasification, however, does not combust the waste whereas incinerators do. PAG breaks down organic materials into syngas, much like the gasification process described above. Syngas can either be directly burned in gas turbines to produce electricity, or it can be converted into other fuels, including gasoline and ethanol. [6] These high temperatures combined with oxygen-poor environments prevent the refuse from catching fire and eliminates the production of dioxins and furans, two toxic chemicals which are produced during incineration. Organic materials are resolved into syngas, but inorganic materials are also given value as part of the municipal refuse gasification process.
One way to deal with metals and other inorganic materials in garbage is to isolate and recycle them, but there is also an alternative when using plasma arc technology for gasification. The materials that do not convert to syngas shift into a liquid state. When this liquid cools, it turns into a stable rock-like substance called slag. Since slag is stable substance, this ensures that its contents do not leach out into their surroundings, and may be used as construction material. [7] It is easy to see the benefit of plasma arc gasification: in theory, there should be no remaining waste to deal with, and landfills would become obsolete. As a result, the plasma arc gasification approach cuts out the landfill, which is a victory for crowded urban areas that are constantly surrounded by landfills. In addition, this type of approach prevents trash from decomposing and producing methane, which has more than 20 times the heat-trapping potential of carbon dioxide. [7] As with any new technology, however, there are hurdles that PAG has to overcome.
Critics of PAG state that this technology is still unproven on a large scale. [5] Brad Van Guilder, a scientist at the Ecology Center in Ann Arbor, Michigan, states that obsidian-like slag contains toxic heavy metals which break down when exposed to water. [5] His claim implies that one day, if the slag is dumped into the landfill, then it could contaminate local groundwater. Monica Wilson, the international coordinator for the Global Alliance of Incinerator Alternatives, maintains that in the cool-down phases of PAG, the components in syngas could re-form into toxins. To rebute these claims, Startech, a PAG start-up company, states that when the syngas is produced from the PAG process, it is maintained at a temperature high enough to make it impossible for syngas to re-form into toxins. [5] No claims have yet been made to refute the critical view of slag mentioned above, however. Still, there are other critics that wonder how these multi-million dollar PAG plants will sustain themselves in this age of economic recession.
David Ciplet, the U.S. coordinator of the Global Alliance for Incinerator Alternatives, suggests that reusing, recycling and composting materials are the most economical and sustainable ways to save energy. [8] Ciplet argues that while the PAG industry is limiting the debate to landfills and incinerators, the fact is that there are more than just two options on the table. He asserts that the PAG facilities would prove to be an economic disaster due to the high cost of construction and in the face of cheaper alternatives for waste removal, such as recycling. [8] Eric Lombardi, the director of Eco-Cycle, a proponent of zero waste communities, affirms that reusing and recycling materials would be more economically viable. [8] To further assert this, Lombardi points out that the value of recyclable materials has doubled in recent years. In addition, Lombardi notes that there is a market for 85 percent of the material that Americans are throwing away. [8] PAG proponents, however, see the economic battle as very manageable.
Startech emphatically claims that perhaps the most amazing part of the PAG process is that it is self-sustaining. [5] Startech’s plasma converter draws its power from the electrical grid to get started, where the initial voltage is roughly equal to that of a zap from a police stun gun. Once the cycle is under way, however, the produced syngas is fed back into a cooling system, generating steam that drives turbines to produce electricity. [5] About two thirds of the power is utilized to run the plasma converter, but the rest can be sold back to the utility grid. In fact, Startech claims that even a blackout would not stop the operation of the facility, due to this self-sustaining mechanism. The question remains: how exactly can cities actually start making money from their own garbage?
Louis Circeo, the director of plasma research at the Georgia Institute of Technology, states that it will prove cheaper to take garbage to a plasma plant than it is to dump it on a landfill. [5] A Startech machine that costs roughly $250 million can handle 2,000 tons of waste daily, which is approximately what a city of a million people amasses in that time span. [5] Usually, municipalities haul garbage to landfills, where the landfill operator charges a "tipping" fee to dump the waste (around $35 per ton.) A $250 million machine could start to pay for itself in less than a decade, and that is without taking into account the money made from selling excess electricity and syngas; after the break-even point, PAG would result in pure profit. [5] Profit aside, critics continue to question the adverse affect of PAG on current re-use, recycle, and compost programs.
David Ciplet puts the spotlight on the fact that gasification facilities are disincentives to recycling. Since gasification facilities require a guaranteed waste stream for decades in order to cover their high costs, they serve as a deterrent for recycling. [8] Ciplet emphasizes that the materials that are best in terms of energy extraction are also the materials that people readily recycle, like plastic bottles. [8] Danny May, chief financial officer of Alter NRG, a company that sells the technology involved with a PAG plant's operation, counters that while recycling is always the best option, plasma gasification is an active part of a good recycling program. [8] Since not all materials can be recycled, the idea of "zero-waste" may not be realistic, while being a valiant goal. For now, industry leaders lie in wait to see how PAG development projects pan out and if the plasma gasification process can really turn garbage into gold.
© Soham Chowdhury. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Municipal Solid Waste in the United States," US Environmental Protection Agency, 23 Nov 09.
[2] P. M. Lemieux, "Evaluation of Emissions from the Open Burning of Household waste in Barrels," U.S. Environmental Protection Agency, EPA-600/R-97-134a, November 1997.
[3] H. Huang and A. Buekens, "On the Mechanisms of Dioxin Formation in Combustion Processes," Chemosphere 31 4099 (1995).
[4] C.-Z. Li, "Gasification: a Route to Clean Energy," Process Safety and Environmental Protection, IChemeE 84, 407 (2006).
[5] M. Behar, "The Prophet of Garbage," Popular Science, 1 Mar 07.
[6] M. Pourali, "Application of Plasma Gasification Technology in Waste to Energy—Challenges and Opportunities," Sustainable Energy, IEEE Transaction on 1, 125 (2010).
[7] S. Malone, "Modern-Day Alchemy: Turning Trash into Power," Reuters, 20 May 08.
[8] L. Haidostian, "Plasma Gasification Projects Fire up Amid Controversy," GAIA, 10 Jun 08.