Demisters for FGD and Power Plant Scrubbers

Flue gas desulfurization (FGD) and related power scrubbers are harsh on mist eliminators: high liquid load, solids and corrosive condensates.

The mist eliminator is part of the absorber chemistry

Droplets reaching an FGD mist eliminator can contain limestone or lime reagent, calcium sulfite or sulfate, fly ash and dissolved salts. Deposits may arrive as suspended solids, continue reacting on the surface or crystallize as water evaporates. A replacement therefore has to be reviewed together with reagent utilization, slurry solids, oxidation, wash-water quality and absorber hydraulics.

The device also protects downstream ducts, reheaters and stacks from liquid and solids carryover. The site should state the actual consequence being controlled: visible carryover, duct deposits, corrosion, fan load, water balance or an emission-related requirement. These are not interchangeable performance criteria.

Why mesh-only designs struggle

Gypsum and ash bridge knitted mesh, ΔP rises, and cleaning cycles shorten. Sites sometimes respond by buying denser mesh — which can plug faster.

Deposit appearance can guide investigation but does not establish one cause. Hard scale, soft mud-like solids, edge buildup and clean gas lanes point to different checks. Preserve a sample and operating history before high-pressure cleaning removes the evidence.

Style and material notes

• Vane packs with open, hooked or multi-channel blade profiles are common where liquid and solids dominate.
• PP mesh or PP vanes can be considered where metal corrosion is unacceptable and temperature allows. The applicable temperature depends on the raw material producer, the named commercial grade, load, duration and chemistry; “PP” as a polymer type does not establish a service limit.
• Combined arrangements are used when fine mist and high liquid load both exist.

Open vane passages generally provide more room for solids and washing than dense knitted media, but blade spacing, hooks, orientation and drainage still affect plugging. A combined system adds another stage to wash and support. Select the arrangement around deposit behavior and the required droplet range, not around a generic product hierarchy.

For polymers, check continuous operation, start-up, outage steam or hot-water cleaning, mechanical load and exposure time. Freeze the raw material producer and named commercial grade when the design depends on grade-specific temperature or chemical data.

Washing is a design input

Record which faces are washed, nozzle locations, spray coverage, pressure and flow available at the nozzles, water or liquor quality, sequence, frequency and drainage route. A stated wash-water flow alone does not prove full coverage. Blocked nozzles, shadowed areas and poor drainage can create local deposits even when total water use is high.

Do not copy wash settings from a different absorber. Historical EPA pilot and inspection documents show that mist-eliminator reliability depended on the deposit type, reagent system and tested wash arrangement. Those results support systematic testing, not one universal wash frequency or flux.

What to capture before a turnaround buy

• Photos of deposits and blade/mesh condition
• Wash system status
• ΔP trend
• Whether carryover is damaging downstream ducts

Add fuel and reagent context, absorber type, slurry solids and chemistry, gas-flow range, temperature, liquid loading, pressure-drop limit, deposit analysis, current blade or mesh geometry, support survey, access route and required downstream result. Include normal operation and credible upset or bypass cases.

Buyer controls for a replacement

A replacement RFQ for FGD should say “FGD / wet scrubber” explicitly — a generic “SS304 demister” quote is the wrong starting point for most of these towers. Freeze module geometry, blade spacing and hooks or mesh construction, flow direction, material grade, support and wall seals, wash interface, section map, dimensions and packing.

The quoted inspection scope should list agreed measurements, material or batch evidence, module marking, assembly and packing photos, and packing list. A manufacturing inspection proves the offered construction; it does not prove absorber chemistry, wash coverage or process performance. If carryover or pressure drop is contractual, state the inlet loading, operating cases, condition of the device and acceptance method.

Use the wet scrubber guide for wash-system checks and the fouling guide to build a deposit investigation.

Authoritative operational evidence

The U.S. EPA’s FGD Inspection and Performance Evaluation manual includes mist-eliminator inspection, gas-side pressure drop, slurry solids and gypsum-scale mechanisms. EPA/TVA pilot reports also document how tested top- and bottom-wash sequences controlled different deposits. These sources are historical operating evidence rather than current project design standards; use them to frame diagnosis and site testing, not to copy old plant settings.