Waste from low-Nox burners makes saleable coal product

New ways of burning coal have been developed to meet environmental standards for low nitrogen oxide (NOx) emissions, but while resolving one environmental problem they have created others.

Coal-fired plants now use lowNOx burners to reduce emissions. These burners are effective, 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 force, 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 the annual meeting of the American Chemical Society in Anaheim, California. “Now power plants must dispose of the fly ash and carbon mixture and disposal is expensive.”

Maroto-Valer and associates, 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.

Activated 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 during combustion. 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.

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.”

Contact: Dr. Maroto-Valer