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"Why don't they just fill a rocket full of garbage and shoot it at the sun?" Many who want to reduce pollution but have a flimsy grasp on rocket science and economics have asked this question. But what if such a feat could be accomplished without a rocket? That's essentially what plasma gasification is designed to do.
Plasma gasification is the process of breaking down matter at the atomic level by exposing it to high temperatures. Gasification isn't combustion or incineration. Those processes result in unpleasant, toxic byproducts. Gasification uses extreme heat, in the form of plasma arcs, to reduce matter to its basic elements.
Superheated plasma is nothing new. For decades, plasma arcs, which reach temperatures of up to 8,000 degrees Fahrenheit, have been used in foundries to melt metals. However, a few years ago several waste disposal startups in the United States and Japan took notice of a plasma arc technology developed by NASA that produced an arc at temperatures exceeding those on the sun's surface. Any solid matter that came into contact with such an arc would be reduced to nothing more than synthesis gas - or syngas, which is generally carbon monoxide and hydrogen - and an inert slag.
Video: City Manager Ray Kerridge describes Sacramento, Calif.'s plan to vaporize solid waste.
It turns out the syngas can be used to produce useful products like diesel and methanol or separated and sold to industries that use hydrogen and carbon monoxide in manufacturing. The slag also has several applications, such as asphalt and tile production. In addition, syngas must be cooled before it's stored. The cooling process results in considerable amounts of steam, which can run the turbine that powers the plasma arc to begin with.
Sacramento, Calif., is considering constructing a plasma gasification plant in which to dump garbage. In addition to its own landfills, Sacramento currently sends tens of thousands of tons of waste to landfills in Nevada, a costly process that's neither sustainable nor environmentally friendly. In February 2008, the city began negotiations with Sacramento-based U.S. Science and Technology (USST) to possibly make Sacramento the first American city to build a plasma gasification plant to treat waste.
"Sacramento has long been on record as being a sustainable and green city," said Jim Rinehart, Sacramento's economic development manager. "So one of the notions to support that premise of making Sacramento, the most sustainable city in the nation was to look at our municipal solid waste activities and see if we can improve upon them."
Rinehart said in late 2007 the city issued an RFP for alternatives to the city's existing waste-management policy. There were 11 respondents. One of them was USST, which proposed a plasma arc gasification procedure that would convert municipal solid waste to energy and products the U.S. Environmental Protection Agency has deemed to be safe roadbed material - all with no or few emissions.
Several gasification plants are already operating in Japan, gasifying up to 280 tons of solid waste every day. Of course, all this sounds too good to be true. Why aren't we already doing it?
David Prinzing, vice president and chief engineer of USST, said the holdup is due to the usual suspects: politics and economics.
"There are a lot of vested interests or special interest groups around municipal solid waste, and so introducing change can be difficult," Prinzing said. "It takes time. Then, on the economics side, I
think you have to have the right set of factors; the shipping fees, the purchase agreements must have the right rate (cents/kilowatt hour); and then also I think people have sort of ignored the opportunities around solid byproducts. There's a tremendous amount of opportunity there, so when we look at the economics for our project [in Sacramento] ... all of the factors come into play and it really looks like the right time for the right technology in the right place."
Still, like all cities, Sacramento has its fair share of naysayers. Whether it's failed attempts to build a new basketball arena or a downtown skyscraper that blocks the City Council's view of the state Capitol building, Sacramento has a knack for nipping progress in the bud.
In fact, The Sacramento Bee recently ran an editorial that reflects the city's tendency to forever consider itself San Francisco's inferior sibling. "If the city were to embark on this plan, it would be moving from a decidedly low-tech approach to handling waste to an approach on the cutting edge of technology," the editorialist wrote. "At the moment, Sacramento trucks its trash over the Sierra [Nevada] to a landfill in Nevada. It's one thing to say that the city needs to stop doing that. It's quite another to say that Sacramento's ready to lead the world in the technology of trash."
Rinehart and Prinzing believe the city is ready. They say the unease some feel about plasma gasification could be soothed if people better understood how it works.
"In high school, you learn about the three states of matter: solids, liquids and gas," Rinehart explained. "This plasma arc gasification is a representation of what is called the fourth state of matter - plasma - which effectively is this very high temperature, ionized volt. So it breaks down to the atomic level, and then with a little cooling gets to the molecular nitrogen, hydrogen, carbon monoxide, etc. That's very effective, and it's far different from anything oxidized that would be called ignition, combustion, fire or anything. It doesn't create ash - it's something different - but it's easy to confuse because it's a matter of degrees on the Celsius scale."
It dissociates atoms and breaks them down into their basics, Prinzing added. "Of course we know that matter is neither created nor extinguished. Matter's mass is conserved. So we just recombine it. In a sense, it's a form of recycling of the unrecyclable portion of the waste because we're getting beneficial things - whether it's energy from the breaking of those bonds that we can turn into electricity; or from producing transportation fuels, like ethanol and hydrogen for fuel cells. Then, with the solid product, you can make ceramic tiles and rock wool, which is a form of insulation. It makes excellent roadbed. It's not exactly a high-value output for that, but there are other things you can do to take advantage of this. So really, it's like the ultimate form of recycling."
For a plasma gasification plant to succeed, it must do one critical thing: produce more value in its output than it takes to operate it. The negotiations between the USST and Sacramento currently call for a plant that can handle 750 metric tons of municipal solid waste each day. A steam turbine and a gas turbine would combine to produce 66 megawatts (MW) of electricity. The plant itself will require 13 MW to operate, leaving the city with a 53-MW surplus it could sell back into the power grid.
In California, the demand on cities to recycle keeps increasing. The state's cities must, by law, recycle half their trash. Rinehart said he doesn't want to change Sacramento's recycling plans. Instead, he thinks plasma gasification can significantly increase the amount of waste the city can recycle.
"California right now has legislation that all cities are to recycle at least 50 percent of their municipal waste," he said. "Everything that is being considered for this technology is post-diversion. So this is all for material that would have gone to the landfill, not the recyclable bin."
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