Sources: ChK Group, Inc., Plano, Texas; J.C. Roumain LLC, Louisville, Colo.; CP staff
With an eye toward converting landfill-bound agricultural waste into concrete-grade pozzolan, environmental technology company ChK Group Inc. has completed piloting high-temperature processing of rice hulls to yield a fine, high-silicate (>90 percent) ash with performance properties similar to silica fume. Following 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 now is seeking investors and strategic partners in the cement and concrete industries to commercialize the carbon-neutral product.
The National Science Foundation grant-backed 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. We see this product as the next step in green building and hope to have the first plant built in the next couple of years. 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 rice hull burned as an alterative fuel for generating electricity indicated that the resulting ash was too carbon heavy as a cement substitute. But, explains independent consultant Jean-Claude Roumain, recently retired from a corporate product manager position at Holcim (US) Inc., Those hulls were not burned to produce ash for concrete. However, at the right temperature Û 800_C (or 1,472_F) Û 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 megajoules/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 obtain an optimum amount of amorphous silica in the final product somewhere in the 96 to 98 percent range.
The physical, chemical and mineralogical characteristics of ash were analyzed to examine concrete speciman properties, including compressive strength, splitting tensile strength, water absorption, total charge-passed derived from rapid chloride permeability test and rate of chloride ion penetration in terms of diffusion coefficient. Results of an early round of testing (unrelated to the results given in late June) indicated that RHA consists of 87 percent silica, mainly in amorphous form and has an average specific surface area of 36.47 sq meter/g. The study's test data indicate that up to 30 percent of RHA could be advantageously blended with cement without adversely affecting the strength and permeability properties of concrete.
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, he asserts. 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 says ChK Group already has lined up a rice mill in Arkansas that annually generates 80,000 tons of hull waste, with the potential to process 15,000 tons of concrete-grade powder the first year of operation. Since rice mills are plentiful in Arkansas, he envisions furnaces erected at various locations around the state, next to mills. 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. This process also is being looked at closely in China and India, where rice and concrete consumption are especially concentrated.