03-25-10

90 Percent Mercury Removal is a Big Challenge say Hot Topic Speakers

The speakers at the Hot Topic webinar yesterday dealt with the question of obtaining 90 percent mercury removal. They pointed out that it may be difficult to determine what efficiency is being achieved. They also explored the complex variables which affect efficiency. There were several points brought out in the discussion which are quite important:

1. Adding activated carbon ahead of the air heater improves efficiency at little increase in maintenance.

2. Combination of technologies is needed.

3. Selenium can or cannot be removed at 90 plus percent efficiency.

4. A holistic approach is needed to determine impacts on soil and water as well as air for each option.

§ Adding activated carbon ahead of the air heater improves efficiency at little increase maintenance.

Graphs were presented to show considerable efficiency improvement when the activated carbon is injected ahead of the air heater instead of after it. There was debate whether this was from the additional mixing in the rotating type heater or from deposits on the heater surfaces. Utilities were concerned that adding another material in the air heater would result in maintenance problems. Others cited experiences to alleviate this fear. However, all agreed that there is little experience yet with injection ahead of the air heater.

§ Combination of technologies is needed.

Obvious combinations include the SCR and the scrubber, but there are others. For example adding halogens and then using unadulterated activated carbon could be an alternative to brominated carbons.

§ Selenium can or cannot be removed at 90 plus percent efficiency.

Many power plants are investing millions of dollars to remove selenium from FGD wastewater. If the activated carbon were to capture the selenium along with the mercury it could save lots of money. EERC testified to 97 percent removal while one of the carbon vendors says testing found only 50 percent removal. So this obviously should be a big area of investigation.

§ A holistic approach is needed to determine impacts on soil and water as well as air for each mercury control option.

The example with selenium above shows the necessity of viewing all the toxic removal options in terms of the impacts on all parts of the environment. If mercury ends up in the wastewater, the cost of removing it from the wastewater should be considered before choosing scrubbers. The co-removal of other metal toxics is very important. Fabric filters will remove particulate metals but selenium vapor will pass on through. So the addition of activated carbon makes the fabric filters efficient on both the vapor phase and particulate fractions.

Karin Madsen of Topsoe A/S displayed a successful model predicting the performance of an SCR catalyst as a control option for mercury removal. Her model takes into account possible reactions to oxidize the mercury such as the adsorption of Hg and then reaction with gaseous HCl or the reverse. She observed that the absorbing specie completes with NH₃ for binding on the V₂0₅. The model is proving useful in predicting the ability of the SCR catalyst to oxidize elemental mercury. There are many variables which can cause the rate to be as low as 50 percent or as high as 90 percent. But keep in mind that this is just the elemental fraction. If 80 percent of the mercury reaching the catalyst is already oxidized, then only 50 percent conversion is needed to reach 90 percent.

Stephen (Steve) Niksa Ph.D., President of Niksa Energy Associates LLC/ NEA, reviewed the statistical uncertainties for various Hg control strategies and examined whether or not 90 percent reduction in Hg emissions can even be measured. He pointed out that measuring the mercury in the coal is highly inaccurate. Measuring the mercury in the duct prior to the control devices is tricky because if you measure it before the SCR, you get one result, if you measure it prior to the air heater, you get another; and if you measure it before the precipitator, you get still another measurement. He questions whether accurate measurement of the inlet mercury can be made and maintains that the error range is 10-15 percent. So how can you know you removed 90 percent?

Jeffrey (Jeff) Thompson, Research Scientist at the University of North Dakota Energy and Environmental Research Center, discussed recent developments in mercury control technology including approaches which maximize removal with process changes as well a control technology. Dry FGD with a fabric filter can achieve 65-95 percent removal on bituminous coal but 0-20 percent removal on lignite. The wet FGD following a cold side ESP can remove 60-80 percent of mercury when burning bituminous coal but would remove less with other coals.

Rob Nebergall, Business Manager Emissions at Norit Americas, Inc., brought us up-to-date on the supply-demand situation with activated carbon and indicated supply is ample considering the new facilities coming online. He forecasts that the future for carbon companies is in supplying services to the utilities to help them increase removal rates and decrease carbon usage.

Sid Nelson, President of Albemarle Sorbent Technologies, explained that the company has a new partnership with Mead-Westvaco. This partner is the second largest activated carbon supplier. He stressed the difference between sorbents and used the case of one sorbent high in ash and water which did not have nearly the absorption as one with lower levels of non-carbonaceous material. He demonstrated the improvement in capture efficiency by injection of the carbon ahead of the air preheater.

The Bios, Abstracts and Photos can be viewed as follows:

BIOS, ABSTRACTS AND PHOTOS - MARCH 25, 2010.htm

The individual slides can be viewed through the following links.

Karin Mardsen – Haldor Topsoe

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