Mar. 24, 1999 Anaheim, Calif. --High-value carbon products like activated carbons may become a commercially viable by-product of the new, environmentally friendly methods used to burn coal, according to a Penn State researcher.
"To meet environmental standards for low nitrogen oxide (NOx) emissions, we have redesigned the way we burn coal," says M. Mercedes Maroto-Valer, research associate in Penn State's Energy Institute. "We resolved the environmental problem, but we created other problems."
Coal-fired plants now use low NOx burners to reduce emissions. These burners do the trick, but increase the amounts of unburned carbon left after combustion. Power plants are left with a mixture of fly ash and unburned carbon.
"Before low NOx emission requirements came into being, power plants marketed the fly ash remaining after burning to the cement industry. However, with the higher levels of unburned carbon, this waste by-product can no longer simply be sold," Maroto-Valer told attendees today (Mar. 23) at the annual meeting of the American Chemical Society in Anaheim, Calif. "Now power plants must dispose of the fly ash and carbon mixture and disposal is expensive."
Maroto-Valer; Darrell N. Taulbee, industrial support coordinator for the Center for Applied Energy Research, University of Kentucky, and Harold H. Schobert, professor of fuel science and director of Penn State's Energy Institute, are investigating both fuel and non-fuel uses of coal for the 21st century. One use for both combustion waste and anthracite coal is as activated carbon.
Wood is the traditional source for activated carbon which is sometimes called activated charcoal. Activated carbon traps impurities found in both gases and liquids in pores within the carbon. These carbon products have a huge market with 350,000 tons sold each year for water treatment, gas purification and gold extraction among others. Products as diverse as air conditioning systems, household water purification pitchers and cigarette filters use activated carbon.
"We know we can separate the fly ash from the unburned carbon, and sell the fly ash to cement manufacturers," says Maroto-Valer. "However, until recently, uses for the remaining carbon were unknown because no one had characterized the unburned carbon."
The researchers used both the unburned carbon and anthracite coal to create activated carbon and compared the results from both. After separation from the fly ash, they activated the unburned carbon with steam at 850 degrees Celsius. The unburned carbon contained few volatile components because it had already been heated while in the combustor. The researchers crushed the anthracite before treating it with steam.
"It appears that the unburned carbon is suitable for manufacturing activated carbon products," says Maroto-Valer. "We get high surface area after short activation times and with product yields over 70 percent."
Activated carbon from wood products has about 10 percent yield. The anthracite coal activated for the same amount of time as the unburned carbon had about 59 percent yield, but higher surface area, and the anthracite activated for slightly longer had 33 percent yield and even better surface area.
"If we activated the unburned carbon for longer, we would probably get better surface area at the expense of some yield," says Maroto-Valer.
While both anthracite and unburned carbon can produce acceptable activated carbon, unburned carbon is probably less expensive and better for the environment. Unburned carbon, separated from fly ash, does not need cleaning or crushing, nor does it need heating to remove volatile components. Also, while anthracite sells for about $50 a ton, the waste from power plants can be separated for $10 to $15 per ton, and the fly ash could be sold to cement manufacturers.
"The combustion of 920 million tons of coal generates about 80 million tons of fly ash and unburned carbon as combustion by-products," says Maroto-Valer. "Separating this waste and using both components is certainly more economical and environmentally friendly than simply disposing of the waste."
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