Pratim Biswas, Ph.D., Stifel and Quinette Jens Professor of Environmental Engineering Science and professor of chemical engineering at Washington University, introduces a vapor phase precursor of titanium into the combustion chamber and 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) he 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. 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, total mercury emissions for 1994-95 was 158 tons; worldwide mercury emissions are estimated to range from 1,000 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 to the ground and water. Due to its high toxicity and long residence times in the environment, 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 nustshell this is very expensive and not always efficient, especially under high temperatures encountered in combustion exhausts," said Biswas. "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 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 non-toxic 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, and 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, who holds joint appointments in the Washington University civil and chemical engineering departments,was granted a patent in the summer of 2001 for his technique ("Process for the enhanced capture of heavy metal emissions", US 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.
Journal
AIChE Journal