8 JUNE 2006
Coal provides more than half of the American electricity supply. Because coal's
energy comes from oxidizing carbon, 
coal makes more carbon-dioxide than any
other source of electricity -- and therefore has the biggest global-warming
punch.
But coal raises other environmental questions. What, for example, should we do with the ash produced when coal's non-carbon components, including limestone, iron, aluminum, silica sand and clay, are heated and oxidized?
Coal-fired electric generators produce most of American electricity -- and about 100 million tons of ash per year. This is the Boundary Dam plant in Saskatchewan, one of the first sites in the world to capture carbon dioxide from lignite coal. Courtesy HTC Purenergy
American manufacturers must dispose of about 100 million tons of coal ash each
year. Some is already recycled for roofing material or as a replacement for
gravel in roadbeds or concrete, 
but the rest is dumped into landfills. But
good landfills are expensive, landfills are notorious for polluting groundwater,
and they soak up land that could grow trees or crops.
This machine quickly consolidates coal ash into concrete products; 90 percent of the product is recycled ash. Courtesy Resonant Shock Compaction
So here's a bright idea: take some ash, add water and a dash of portland cement, then intensely shock the mix to force the particles closer together. The result is a strong, durable concrete, says Robert Amme, director of the Environmental Materials Laboratory at Denver University. "We have used the compactor for fly ash consolidation, to make panels that are competitive in most physical properties with concrete. As a result of the very high level of compaction, the particles become very intimately associated, are much closer together, so you need less water and portland cement" than you would for regular concrete.
A concrete idea
Cement companies already add some fly ash (the stuff that's captured in the smokestack) to improve the quality of portland cement, but the shock-compaction method could gobble fly ash -- and the heavier "bottom ash," by the truck-load. "It can, in four to five seconds, turn a granular material into a solid block that contains 90 percent recycled material," Amme says.
The shocking takes place on a gadget that can whack a mold of ash-crete with a force 100 times as strong as gravity, says Amme. Vibration usually causes particles to separate by size, but these shocks happen too quickly for that. In tests, the ash-crete had the same compressive and tensile strength as regular concrete, Amme says, and it also survived a freeze-thaw test, essential for concrete exposed to the environment in bridges, say, or roads. 
The ash-crete accepts steel reinforcement bars, and does not seem to weaken over time, adds Amme, who has been collaborating for nine years with Resonant Shock Compaction LLC, which has patented the technology.
One use for the compaction technology is to make blocks of contaminated soil, to reduce leaching of toxics into the environment. Courtesy Resonant Shock Compaction
The compaction process could be used to make concrete panels, blocks, or shapes, says Amme, but it's not likely to be scaled up for large hunks like bridge beams, which would require a monster shaker. The panels can be insulated, and built on site of a building project. Because the major raw material is something that any coal power plant would be happy to give away, a recent study showed that a building panel would cost half what wood would for the same job, Amme says.
Shock compaction can also consolidate hazardous wastes into a block to reduce or stop the leaching of toxic chemicals into the environment. That could be interesting to cleanup efforts at former nuclear sites, among other places.
Portland cement is an energy-intensive product: To make
a ton of cement, this kiln in Seattle may release 1 ton of carbon dioxide. Crushed
rocks and minerals are heated to 3,400 degrees F. in a massive rotating kiln,
making a gray cement powder that hardens with water. Photo: Portland
Cement Association
But the ash-crete project seems particularly relevant now that coal is on such a roll. Despite the ongoing fear over global warming, China and the United States both have big plans for new plants. Aside from the obvious benefit of cutting down on the need for landfills, turning ash into concrete could also reduce the number of pits dug for the sand and gravel that the ash could replace, so less land would be disturbed. And the compaction process also needs less portland cement. Making this fine powder accounts for 0.8 percent of U.S. greenhouse gases.
-- David Tenenbaum
Bibliography
• Vibratory Shock Compaction of Granular Nuclear Waste, Amme, Robert C., American Physical Society, April Meeting, 2004, abstract #B15.005.
• Coal vs. wind
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• Mercury: Coal's other pollution
• Global warming
