FGD and DeNOx
NEWSLETTER       

January 2010
No. 381

Hot Topic Hour on November 12 was Wet vs. Dry FGD

Mike Meadows of Black & Veatch spent two days talking about this subject in South Africa recently. So it was a challenge for all the speakers to condense the comparison into a two hour webinar. With the potential of new MACT rules, limits on FGD wastewater discharges and CO₂ capture cleanliness requirements, there is the big question as to whether the right choice for this year is the best choice for the longer term.

Terence Ake of Babcock Power said that the choice of scrubbing technology depends on the age of the boiler, space constraints, the availability of water, and other site specific facts.  It’s important to consider the use of the byproduct. The co-benefits of a technology to remove sulfur trioxide, mercury, and flyash may become important in answering the question of whether a boiler can be built or must be retired. Roosevelt Huggins and Mike Meadows (Black & Veatch) provided a good comparison between the two technologies. They cautioned that a retrofit situation can be different in terms of capital cost. If you already have a precipitator and are installing a wet scrubber, the cost may be lower than installing a fabric filter and dry scrubber.

Bob Nicolo of Hitachi Power compared the conventional technologies and then discussed the advanced venturi pre-scrubber which avoids ESP rebuilds, enhances gypsum purity and can provide salable hydrochloric acid. This technology is covered in Patent 5429808. In the discussion period it was pointed out that the mercury can be stripped from the acid bleed stream. This eliminates concern about metal contaminants in the gypsum or wastewater.

Pete Honeycutt of Kiewit Power discussed the Flash Dry Absorber (FDA or NID) design among others. This is available in modules of 100 MW, so multiple units would be needed for large boilers. CFB scrubbers are now able to achieve up to 98 percent removal with inlet SO₂ as high as 6 lbs/MMBtu. They can use poor quality  water sources for humidification.

Greg Carleton of Pollution Control Services pointed out basic differences for the two technologies in relationship to the operating parameters for existing coal-fired generators.   Understanding how numerous boiler modifications and fuel supply changes can either positively or negatively affect plant operational costs with both technologies is crucial. The two basic technologies need to be compared over the equipment life cycle because capital costs and O&M costs are considerably different. Since the rules are continuing to change, the selection parameters will change as well.

Gordon Maller of URS Corp. took up the issue of SO₃ removal and pointed out that with SBS and other additives the wet scrubber can achieve high SO₃ removal. Previous speakers had displayed the higher efficiency of the CFB scrubber for SO₃. Gordon stated that the wet scrubber would achieve higher removal efficiencies on the class of air toxics that pass through the particulate collector in the vapor phase, i.e., selenium, HF and HCl.

Emmett McDermott of Dedicated Transport explained that his company is delivering byproduct lime from municipal water plants to utilities for their FGD systems. This reduces pollutants at municipal water plants and provides a low cost source of reagents. Over 249,000 tons were delivered to FGD and waste-to-energy (WTE) systems in Florida last year.

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