Regulations and Economics Drive Wet FGD Upgrades

Today’s coal-fired power plants face the twin challenges of improving their wet flue gas desulfurization (FGD) systems’ emission control capabilities in order to comply with environmental regulations while at the same time cutting their operational and maintenance costs. Smart strategies for retrofitting existing FGD systems can help plant personnel meet both of these objectives.

“Boosting SO2 removal rates and cutting other emissions, such as SO3, is one of today’s top drivers for upgrading wet flue gas desulfurization (FGD) systems at U.S. power plants,” said Michael Mengel, lead process engineer, Marsulex Environmental Technologies (MET). Mengel was one of the members of the “FGD Retrofits, Upgrades, and O&M” session panel at ELECTRIC POWER 2010 who provided useful information about successful approaches to implementing wet FGD system upgrades in order to maintain environmental regulatory compliance. The session was co-chaired by Melanie Green, PE, senior manager, CPS Energy, and Peter Macios, global account executive, GE Water & Process Technology.

Reasons to Retrofit

FGD is a technology used for removing sulfur dioxide (SO2) from the exhaust flue gases of coal-fired power plants. Typically, SO2 is removed from flue gases by wet scrubbing, using a slurry of alkaline sorbent, usually limestone or lime, to scrub the gases.

In addition to the need to cut SO2 emissions, today’s savvy power plant staffs are retrofitting their wet FGD systems for the following other reasons, Mengel explained:

  • To switch fuels in order to use higher-sulfur coal that is less expensive.
  • To eliminate the bypass process that is no longer allowed under new environmental regulations.
  • To upgrade units because of their advanced age.
  • To reduce maintenance and operation costs.
  • To enable a facility’s by-products to be sold to other industries.

“Power plant personnel who want to increase the performance of their facility’s wet scrubber system should assess the need for possible improvements throughout the entire system,” Mengel said.

How to Plan for a FGD System Upgrade

Mengel recommended that power plant staff assess and consider improvements to a number of areas in their facilities’ wet FGD systems, which might include upgrading the ductwork and improving the efficiency of the booster fans. He also focused on improving the functioning of the mist eliminator, and pointed out the following characteristics of this equipment’s performance:

  • Absorber gas velocities are generally limited by mist eliminator performance.
  • Supersaturated liquid film on mist eliminators will cause precipitation of solids and scaling.
  • Water quality, spray coverage, and optimized sequencing act together to minimize build-ups while keeping emissions low.

“Personnel should also perform computation fluid dynamic model testing to assess the current system’s condition and assess the impact of the proposed system upgrades,” he said.

In order to improve the efficiency of older wet FGD systems, plant managers should also consider either redesigning or replacing older absorber recycle sprays. This can involve the replacement of end-of-life headers or the use of self-support headers, Mengel said. Doing this can lead to the following positive results: better spray density, spray coverage improvement, and elimination of impingement problems.

When contemplating retrofits of their existing reagent systems, plant staff should consider a number of options, Mengel advised:

  • Assessing an upgraded reagent system’s capacity for higher SO2 burdens.
  • Optimizing the grind of the limestone to improve efficiency.
  • Evaluating the potential of new additives, such as ammonia salts, DBA, and adipic acid.

Mengel emphasized that checking the operational efficiency of oxidation systems is a high priority. He suggested that plant personnel check the performance levels of older air spargers because they might be plugged and cause oxidation problems that could lead to outage maintenance. He also urged facility managers to consider replacing older air spargers with new air lance/agitator systems that can be retrofitted to older wet FGD systems.

Enhancing FGD Systems’ Performance

“Power plant personnel need to focus on what potential upgrades their facility’s wet FGD systems need in order to optimize efficient plant operations and ensure compliance with current environmental regulations,” Mengel said.

Even in this time of tight budgets, strategic upgrades of wet FGD systems could easily pay for themselves through increased efficiency and less potential regulatory exposure for power generators.

—Angela Neville, JD, is POWER’s senior editor.

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