Sources: ChK Group, Inc.; J.C. Roumain LLC; Staff reports

With an eye toward converting landfill-bound agricultural waste into concrete-grade pozzolan, environmental technology company ChK Group completed piloting high-temperature rice hull processing to yield a fine, high-silicate (>90 percent) ash with performance properties similar to silica fume. With a preliminary round of testing already completed (the results of which were presented in late June at the Green Chemistry and Engineering conference in College Park, Md.), the company is now seeking investors and strategic partners in the cement and concrete industries to commercialize the carbon-neutral product.

With a National Science Foundation research grant, the pilot scale study was performed at the headquarters of furnace manufacturer Harper International, Lancaster, N.Y., with additional cement standards testing taking place at Clemson (S.C.) University and the University of Miami. “The National Science Foundation, which funded the initial research, may match 50 cents for every dollar invested in the pilot program up to $1 million,” says ChK Group President R.K. Vempati. He envisions the product being sold in 1-ton bags initially, but welcomes a partner who is prepared to market bulk sales as well.

Early tests done on risk hull burned as an alterative fuel for generating electricity indicated that the resulting ash was too carbon heavy as a cement substitute. But Jean-Claude Roumain, recently retired as corporate product manager at Holcim (US) Inc. and now working as an independent consultant, explains, “Those hulls were not burned to produce ash for concrete. However, at the right temperature--800°C (or 1,472°F)--for the right period of time in an aerobic furnace environment, we can eliminate carbon, leaving an almost pure silica product behind. In fact, you can use this product at the same cement replacement rate as you would silica fume, anywhere from 6 to 10 percent.” Further, rice hulls have 15 MJ/kg of energy, therefore, the process does not consume a lot of energy to burn.

Roumain says the resulting fine, white ash requires no additional grinding out of the furnace and can be used in a ternary blend with fly ash or slag cement. “This product results in high ultimate strength and low permeability,” he says, adding that one of the pilot program’s goals is to have the optimum amount of amorphous silica in the final product be somewhere in the 96 to 98 percent range.

Prasad Rangaraju, professor of civil engineering at Clemson University and a collaborator on this project, adds that the pozzolanic reactivity of rice hull ash can be further improved by grinding the ash to less than 10 microns. With the ground rice hull ash, portland cement replacement rates as high as 10 to 15 percent could be used in concrete, resulting in improved early and long-term strength and durability.

The CO2 emissions from this process are considered carbon neutral, according to Roumain, so there is no need to capture or treat it. Nor is there a need for SOx or NOx scrubbers. “This is cradle-to-cradle technology in its purest form,” explains Roumain. “You use food waste to create a byproduct after burning, and the small amount of carbon dioxide that results can go right back into new plants. It’s a constant cycle.” Vempati adds that if the pilot program is successful and production scaled up to use all the rice hulls produced in the United States, the company could ship 2.1 million tons of ash per year.