How Wet Flue Gas Desulfurization Scrubbers Change Mist Eliminator Design

Wet flue gas desulfurization systems create one of the more demanding environments for mist eliminators.

The separator is not handling only clean water droplets.

Depending on the process, entrained liquid may contain:

• gypsum or other reaction solids;
• dissolved salts;
• chlorides;
• suspended particles;
• slurry droplets.

At the same time, the flue gas volume can be very large and the pressure-drop allowance may be limited.

A mist eliminator in wet FGD service must therefore balance four competing requirements:

droplet removal, low pressure drop, solids tolerance, and washability.

This is very different from selecting a fine wire mesh pad for a clean laboratory gas stream.

Why FGD Mist Is Difficult

In a wet FGD absorber, flue gas contacts circulating slurry or scrubbing liquid.

The gas leaves the contact zone carrying droplets.

Those droplets may contain both:

• dissolved material;
• suspended solids.

When they reach the mist eliminator, the separator captures not just liquid but also the nonvolatile material contained inside that liquid.

As water drains or evaporates, solids can remain on the separator surface.

The mist eliminator therefore operates in a combined droplet-separation and fouling environment.

Slurry Droplets Can Deposit Solids

A captured clean-water droplet may simply drain away.

A slurry droplet behaves differently.

When it impacts a vane or chevron surface, suspended particles may remain behind even after most of the liquid drains.

Repeated exposure gradually produces deposits.

These deposits can:

• narrow flow passages;
• obstruct drainage channels;
• increase pressure drop.

This makes long-term cleanability one of the central design requirements.

Why Open Geometry Is Valuable

Fine knitted wire mesh contains a large collecting surface and can provide excellent fine-droplet capture.

In heavy FGD slurry service, however, narrow internal passages can become difficult to keep clean.

Vane or chevron-type mist eliminators are often attractive because they provide:

• larger gas passages;
• defined drainage paths;
• better access for washing.

This does not mean that every FGD system must use the same vane geometry.

Blade spacing and profile still need to balance:

• droplet capture;
• fouling tolerance;
• pressure drop.

A very tight vane profile may achieve stronger initial separation but foul more rapidly.

Two-Stage Separation Can Be Useful

Large FGD absorbers may use more than one mist-removal stage.

The first stage can remove:

• heavier slurry droplets;
• higher liquid loading.

A second stage can provide additional polishing.

The advantage is that the final stage receives a cleaner and lower-liquid-load gas stream.

But a second stage also adds:

• pressure drop;
• washing requirements;
• mechanical complexity.

Multistage separation should therefore be used because the process duty requires it—not simply because “more stages must be better.”

Wash Systems Are Part of the Separator

In slurry service, the wash system should be treated as part of the mist eliminator design.

Important variables include:

• nozzle coverage;
• wash-water flow;
• washing frequency;
• water quality;
• drainage capacity.

If one part of the vane pack receives insufficient washing, deposits begin there.

Resistance increases.

Gas shifts toward cleaner sections.

Local velocity then rises in those areas.

The original fouling problem develops into a gas-distribution problem.

Washing Can Temporarily Increase Liquid Load

During washing, the separator receives both:

• normal process droplets;
• additional wash liquid.

If the wash rate is excessive, the demister may temporarily become hydraulically overloaded.

This can produce:

• pressure-drop spikes;
• increased downstream liquid carryover.

Wash-system design therefore needs sufficient cleaning power without overwhelming the drainage system.

Chlorides Add a Material Challenge

FGD liquid can contain significant chloride concentration.

Material selection must therefore consider actual process chemistry.

The active separator is not the only component that matters.

The same review should apply to:

• frames;
• support beams;
• fasteners;
• wash-system components.

One unsuitable component can become the corrosion weak point of the assembly.

Polymers or corrosion-resistant alloys may be considered depending on temperature and chemistry.

Large Gas Volumes Make Distribution Critical

FGD absorbers can have very large cross-sectional areas.

An acceptable average gas velocity does not guarantee uniform loading.

Possible causes of maldistribution include:

• inlet momentum;
• spray distribution;
• absorber geometry;
• upstream internal arrangement.

One section of the mist eliminator may receive much greater gas and liquid load than another.

This can cause local:

• erosion;
• fouling;
• re-entrainment.

Large separator area makes gas-distribution engineering particularly important.

Pressure Drop Affects Fan Power

The separator is part of the overall flue-gas pressure-loss system.

A design with unnecessarily high pressure drop can increase:

• fan power;
• operating cost.

Fouling makes the situation worse.

A clean mist eliminator may have acceptable resistance, but deposits accumulated between wash cycles can increase system DP significantly.

The design target should therefore consider the sustainable operating pressure drop, not only the clean value.

Deposit Patterns Can Reveal System Problems

During shutdown, inspect the mist eliminator before washing.

If one region contains much heavier gypsum or slurry deposits, investigate:

• local gas velocity;
• spray distribution;
• wash coverage.

Uniform fouling suggests a different mechanism from strongly one-sided fouling.

The separator surface effectively records the absorber flow history.

Support Structures Need Corrosion and Load Review

Large FGD mist eliminators can carry significant wet and fouled weight.

Supports must handle:

• separator mass;
• retained slurry;
• deposits;
• differential-pressure force.

Corrosion or long-term deformation of supports can produce sagging and module gaps.

Mechanical integrity is therefore part of hydraulic reliability.

What Should an FGD Mist Eliminator RFQ Include?

Useful inputs include:

• actual flue-gas flow;
• operating temperature;
• absorber dimensions;
• expected droplet or slurry loading;
• solids concentration;
• chloride concentration;
• wash-water system;
• allowable pressure drop;
• material requirements;
• access and segmentation limits.

Existing fouling and wash history are especially valuable for retrofit projects.

Final Engineering Perspective

Wet FGD mist eliminators operate in a high-volume, solids-bearing, chemically aggressive environment.

The best separator is not simply the one with the highest clean-condition collection efficiency.

It must remain:

• open;
• washable;
• drainable;
• mechanically stable

through the operating cycle.

Reliable FGD design therefore balances droplet removal, slurry tolerance, washing, material compatibility, gas distribution, and long-term pressure drop.