By Tony Fitzpatrick
November 2, 2001
 Pratim Biswas, Ph.D., the Stifel and Quinette Jens Professor of Environmental Engineering Science, professor of chemical and civil engineering and head of the University's environmental engineering program, adjusts the electrical field in an instrument in the Aerosol and Air Quality Research Laboratory to measure on a real-time basis the size distribution of nanoparticle sorbent agglomerates. |
Pratim Biswas, Ph.D., has patented a technique that uses nanoparticle agglomerates, or clusters, to firmly bind and remove mercury from fossil fuel combustion exhausts.
Biswas, the Stifel and Quinette Jens Professor of Environmental Engineering Science, professor of chemical and civil engineering and head of the University's environmental engineering program, introduces a vapor phase precursor of titanium into the combustion chamber. This results in the formation of nanoparticle agglomerates of titanium dioxide.
In the presence of ultraviolet light (as found in industrial-pollutant-capturing devices called electrostatic precipitators) Biswas found that the titanium dioxide nanoparticles trap the mercury and firmly bind it on the surface. These agglomerates are readily removed in conventional particle control devices, thus preventing the emission of mercury.
A nanometer is roughly one-billionth of a meter, a thousandth of a micrometer. In contrast, a strand of human hair is typically 50 to 100 micrometers thick.
Mercury is one of the metals that the U.S. Environmental Protection Agency (EPA) has stipulated must be controlled in combustion systems. The EPA has recognized the health problems associated with mercury and has proposed stringent regulations to restrict mercury emissions because of its volatility, toxicity and its tendency to bioaccumulate, or invade the food chain.
According to a 1998 EPA report, mercury emissions produced by human activities rival or exceed natural mercury inputs. In the United States alone, the total mercury emission for 1994-95 was 158 tons; worldwide mercury emission is estimated to range from 1,000 tons to 6,000 tons per year.
According to Biswas, approximately 80 percent of all U.S. mercury emissions come from coal combustors and municipal-, medical- and hazardous-waste incinerators.
Mercury that is released gets deposited in water and finds its way into fish and livestock. It can remain airborne for more than a year and be transported over thousands of miles before being deposited into the ground and water.
Due to its high toxicity and long residence times in the environment, the EPA is planning to propose regulations for control of emissions of mercury from coal combustion systems.
"Currently, the most widely touted technique for mercury capture involves the use of activated carbon, but in a nutshell this is very expensive and not always efficient, especially under high temperatures encountered in combustion exhausts," Biswas said. "The search is on for less-expensive technologies, and we believe this one will work very well."
Biswas developed the process on the basis of a fundamental understanding of particle formation and growth in combustion environments. He leveraged this understanding to produce this high-surface-area sorbent material, titanium dioxide, that is so effective at trapping mercury emissions.
Other sorbents, such as silica- and calcium oxide-based compounds, are not as effective as titanium dioxide, primarily due to the non-reactivity of mercury. Biswas and his co-workers successfully exploit the photocatalytic properties of titanium dioxide to oxidize the mercury on the surface and bind it strongly. The binding is so strong that it is also not expected to leach out when the spent sorbent is disposed in a landfill.
Another key advantage of the nanoparticle-agglomerate-based process developed by Biswas is that a relatively low amount of sorbent is very effective at trapping and removing the toxic species; thus a large amount of waste is not generated, unlike conventional sorbent processes.
The proposed sorbent, titanium dioxide, is nontoxic and a versatile compound used in many commercial applications ranging from paint to toothpaste. Millions of tons are manufactured yearly. However, it should be noted that conditions have to be controlled carefully to obtain the right phase, size and morphology of the sorbent particles for it to be effective.
Biswas and his colleagues tested titanium dioxide, calcium oxide and silicon dioxide nanoparticles in a simulated flue-gas system. They found that the engineered titanium dioxide captured 98 percent of available mercury; the calcium-based sorbents removed 33 percent, and the silicon dioxide none.
Biswas was granted a patent this summer for his technique ("Process for the enhanced capture of heavy metal emissions," U.S. Patent 6,248,217). The technique is described in the April 2001 issue of AIChE Journal.
Due to its photochemical properties, titanium dioxide also oxidizes toxic organic compounds and may provide additional benefits for removal of other pollutants.
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