Switch to Membranes Yields Big Energy Savings

FABRIC
FILTER NEWSLETTER

August 2006
No. 370

Switch to Membranes Yields Big Energy Savings

Jim Fisher of Advanced Filtration Technologies in a presentation at ICESP X in Australia showed that replacement of conventional fiberglass bags with membrane bags in a reverse air unit saved significant energy.

On average, the results show a 41 percent decrease in fabric filter drag that allowed actual stack gas flow to increase by five percent and baghouse pressure differential to decrease by 39 percent. Results suggest the lower drag is attributable to improved ash removal from the bag during cleaning and not due to lower resistance through the filter media itself. Maximum electric power generation increased by 2.8 percent during the peak summer demand period due to the higher gas flow achieved with the ePTFE membrane bags.

Originally, Colorado Springs Utilities (CSU) installed an ESP at the Drake Station to control emissions from their Unit 6 boiler. Due to concerns about increased opacity the utility replaced the ESP with a baghouse filter in 1977. The No. 6 baghouse uses the reverse gas cleaning method. Sonic horns are available to assist the cleaning, if needed. The first set of filter bags were woven fiberglass (10 oz/sq yd) with a Teflon B finish cleaned with reverse gas. Glass provides excellent resistance to acid attack, can easily withstand the maximum inlet gas temperature, and is dimensionally stable. Operating experience showed that the baghouse pressure drop increased as a result of the addition of Powder River Basin coal to the fuel mix. In order to reduce the higher than expected baghouse pressure differential and related gas flow restrictions, CSU decided to replace the existing conventional woven fiberglass bags with ePTFE membrane filter bags during the October 2004 maintenance shut down.

Membrane filter bags start with a lower permeability (6 to 8 CFM/sq ft) than conventional glass, but may drop to 3 to 4 CFM/sq ft in a few months. While the permeability is important to measure when assessing the filter bag performance, unless the bag is blinded (permeability less than 1 CFM/sq ft) the loss through the fabric itself does not account for a substantial amount of differential pressure across a filter bag. The biggest advantage membrane bags can offer over conventional media is better cleaning.

Increased power generation was CSU’s expected goal when they decided to use the membrane bags. Actual results show that CSU was able to increase the maximum power generation to 85.0 MW with the membrane bags versus 82.7 MW with conventional fiberglass bags. When using the conventional fiberglass bags, CSU had to decrease generation in the summer to maintain NO_x and CO at the appropriate levels. Because of the decreased generation in the summer months CSU’s only option was to purchase power on the open market at a premium price. The true benefit of the membrane bags is the 2.3 MW (2.8 percent) power increase that was not achievable with the conventional fiberglass bags.

Using a conservative figure of $50/MWhr for purchase power cost, the cost savings in the first year of operation during the summer period (April 15 to September 15) approached $345,000.

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