How Are PTFE Exchangers Used in Preheating Combustion Air with Acidic Flue Gas?

Exhaust gases from a boiler or furnace are a mixture of hot, water vapor-saturated gases with acidic chemicals produced by sulfur, chlorine, and combustion products. If this gas is merely released to the stack, its sensible heat and the latent energy in the water vapour are lost. This corrosive low grade heat can be safely recovered and transferred into the incoming combustion air stream by means of a PTFE heat exchanger, fitted in the cold end of the flue system. The heat exchanger improves thermal efficiency and resists the harmful condensate that spoils conventional metal equipment.

The current PTFE exchanger preheat combustion air flue gas system operates in one of the most aggressive conditions in industrial thermal processing. The exchanger is placed at the most aggressive area of the chimney where the acid rain falls.

Why Acidic Flue Gas Kills Conventional Exchangers
As the combustion gases cool, water vapor condenses out of the exhaust.

Generation of acid dew point
Flue gas typically contains:

Sulfur dioxide (SO2)

Sulfur trioxide SO3

Hydrogen chloride HCl

Water vapor

As the temperature falls below the acid dew point these chemicals mix with condensed moisture to generate highly corrosive liquids such as:

Sulfuric acid;

* Hydrochloric acid

The precise acid dew point is determined by:

Fuel sulfur

Chlorine pollution

Concentration of moisture

Excess air for combustion

The composition of stack gases

In many systems this condensate zone rapidly degrades carbon steel and even stainless steel heat exchangers.

The Problem of Cold-End Corrosion
Acidic condensate assaults the surface of tubes in conventional metal economizers causing "cold-end corrosion."

Typical failure modes are:

Pitting corrosion

General thinning of the walls

Stress Corrosion Cracking (SCC)

Sulfate attack and deposits

Once condensation starts the corrosion rate increases rapidly.

Handling Acidic Condensation with a PTFE Exchanger
PTFE offers a fundamentally different answer in that it is chemically inert to acidic condensate.

PTFE Tubing Structure
For a typical design:

Flow of hot flue gas across shell side

Cold combustion air flushes the PTFE tubes.

As the flue gas cools below the acid dew point, acidic moisture will condense right onto the outside of the PTFE tubes.

This is not the case for metal surfaces, where PTFE is totally unaffected.

Resistance to Hydrochloric and Sulfuric Acids
PTFE has very strong resistance to:

Sulphuric acid condensate

Droplets of hydrochloric acid

Chloride-laden moisture

Acid sulphate deposits

The chemical immunity of the exchanger ensures its ongoing operation in the condensation region, where metal exchangers degrade quickly.

Flue Gas Latent Heat Recovery
Not only sensible heat recovery, but latent heat condensation gives the biggest efficiency improvement.

Condensation as a source of energy
On condensation of water vapour:

The release of large amounts of latent heat

This heat is transferred via the wall of the PTFE tube into the incoming combustion air flow.

The thermal conductivity of PTFE is rather low in comparison with metals but the exchanger compensates for:

Large area for heat transfer

thin wall tubing

Condensation heat transfer mechanisms

Preheating Combustion Air
The combustion air is then preheated and enters the burner system at a higher temperature.

This reduces the fuel needed to achieve:

Target flame temperature

Furnace temperature

Steam production in a boiler

The savings on fuel can be such as to enhance the overall thermal efficiency by several percentage points.

Advantages of Hydrophobic Surfaces
Another significant operational benefit of PTFE is that .

Drainage of Condensate
The smooth, non-stick PTFE surface promotes acidic moisture to:

Droplets

Drain well.

Avoid build up of film

This minimizes the production of large layers of insulating fluid which would otherwise cut down the heat transfer efficiency.

And the result:

Improved thermal performance

Less fouling .

Less frequent maintenance

Temperature Limits and Designing the System
However, the thermal limits of PTFE have still to be properly maintained.

Maximum temperature continuous
Limitations of PTFE are typically around:

Continuous service temperature 110 °C

Thus the position of the exchanger has to be at a point where the flue gas temperature is already below that limit or has been decreased to below this point.

Strategies for Pre-Cooling
Some common ways are:

Dilution air mixing

Gas cooling upstream

Heat recovery phases in-process

Bypass control of exhaust

Good thermal control protects the PTFE tubes from overheating and mechanical distortion.

Condensate Management
Acidic condensate treatment is an important aspect of the system design.

Design of Drainage System
The exchanger shall consist of:

Collection zones on a slope

Corrosion resistant drains

Continuous removal of condensate

Pooling of the condensate can impede heat transfer and promote fouling.

Requirements for Neutralization
Collected acidic liquid is usually:

Neutralised before disposal

Wastewater systems were treated

Compliance monitoring of pH

The condensate may contain concentrated sulfuric and hydrochloric acid components and in general cannot be released without treatment.

Note System
Cleaning Requirements on the Shell-Side
With time, particulate build-up of soot, ash or combustion residue on the shell side of the exchanger may occur.

To keep up performance, systems usually contain the following:

Sootblowers

Water wash spray systems

Chemical wash periodically

Accessible inspection openings

Because PTFE is chemically inert, rigorous cleaning processes can be used without harming the tube surfaces.

Benefits Compared to Metal Economizers
PTFE exchangers offer various operational advantages in acid recovery service.

Corrosion resistant
PTFE is not dependent on: unlike metallic systems:

Passivation oxide films

Corrosion inhibitors

sacrificial protective coatings

PTFE is not an issue for the cold-end corrosion that eats away at metal economizers.

Long term Reliability
PTFE exchangers can offer:

Extended working life

Steady state thermal performance

Lower maintenance expenses

Reduced chance of acid leaking

especially in sulfur- or chloride-containing exhaust streams.

Industrial Use
PTFE flue gas heat recovery systems are increasingly being employed in:

Incinerator for waste

Boilers with Fuel Oil Sulfur

Heaters for chemical processes

Biomass combustion systems

Furnaces for metal processing

These applications generally produce aggressive acidic condensate exhaust streams that are incompatible with conventional metallic heat recovery equipment.

Conclusion:
The PTFE heat exchanger offers a rugged corrosion resistant option for reclaiming waste heat from acidic flue gas streams. The exchanger allows for direct condensation of moisture on chemically inert PTFE tubes with collection of both sensible and latent heat, and safe resistance to assault by sulfuric and hydrochloric acid.

The recovered energy is supplied to the entering combustion air thereby lowering fuel consumption and boosting overall thermal efficiency. Of course design requirements such as good condensate drainage, periodic cleaning on the shell side and cautious temperature management remain important, particularly as PTFE is limited to continuous usage below about 110°C.

In corrosive stack conditions where conventional economizers are prone to cold-end acid assault, the PTFE technology converts a damaging condensation process into a beneficial source of recoverable energy. The route to lower fuel consumption and decreased carbon emissions is increasingly supported by materials that can withstand the most chemically aggressive corners of the thermal process sector.