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New catalysts scavenge nitrogen oxides emissions....

New catalysts scavenge, NO.sub.x emissions . . .

Recent data suggest that more aggressive controls on the nitrogen oxides (NO.sub.x) involved in acid rain may also offer a big payoff in smog-ozone prevention (SN: 9/17/88, p.180). By redesigning the catalyst used in one of the most effective commercially available anti-NO.sub.x technologies, chemical engineers at W.R. Grace & Co. in Columbia, Md., have increased its NO.sub.x-breakdown activity by 50 percent, according to L. Louis Hegedus, vice president of research at Grace. Preliminary data suggest this innovation will not only extend the catalyst's life but also broaden the range of industrial conditions under which it can efficiently operate.

While many, though not all, industrial boilers and electric power plants in the United States are subject to federal NO.sub.x limits, none of these limits is so stringent that it requires an add-on pollutant-scavenging device -- comparable to a stackgas scrubber for sulfur dioxide. The same is not true in Japan and West Germany, where selective catalytic reduction (SCR) devices -- though fairly expensive -- are widely used. The only commercially available exhaust-gas-treatment technology for NO.sub.x, SCR removes about 80 percent of the NO.sub.x emitted by industrial boilers. Because the U.S. government is contemplating more stringent NO.sub.x controls for boilers, Grace -- a leading catalyst manufacturer -- decided to investigate ways to make SCR technology more economical.

In SCR systems, exhaust gases, together with small quantities of ammonia, pass through a large, porous catalyst. As the gas and ammonia contact vanadium pentoxide on the catalyst's surface, NO.sub.x breaks down to nitrogen gas and water.

Grace computer analyses showed a reengineering of the size of the catalyst pores through which exhaust gases' flow could dramatically increase NO.sub.x breakdown. Though details are proprietary, Hegedus says, catalyst pores had to become more bimodal -- meaning a larger proportion of different-sized holes was needed. Though existing SCR catalysts are slightly bimodal, Hegedus says they should be drastically more so, with the trend toward inclusion of more large pores. However, the titanium dioxide, which supports the catalytically active vanadium pentoxide, was too crumbly to be reconfigured with the necessary pore changes, he says. So Grace engineers built new catalyst blocks of silica, then covered them with the titanium and vanadium layers. A pilot plant to test the new catalyst under typical industrial conditions should begin operating within a year.
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Title Annotation:Chemistry; report from American Chemical Society's spring national meeting
Author:Amato, Ivan
Publication:Science News
Date:Apr 29, 1989
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