CEMS: why gas conditioning is the basis of reliable emission measurement

Flue gases from industrial processes are hot, wet and contaminated. Good CEMS gas conditioning is therefore the basis of accurate emission measurement, because moisture in the gas stream damages sensors, disrupts measurement results and causes blockages. In this article, we explain what the dangers are and which solutions are available.

⏱ approx. 5 minutes reading time

What is CEMS?

CEMS stands for Continuous Emission Monitoring System: a continuously operating measuring system that measures and records emissions of harmful substances from industrial sources. Think of power plants, waste incineration plants, cement factories and chemical plants. Typical measured parameters are SO₂, NOₓ, CO, CO₂, HCl, HF, O₂ and volatile organic compounds.

Emission data from a CEMS are used for statutory reporting to environmental authorities and for process optimisation. The quality of that data depends directly on the quality of the gas conditioning: a poorly conditioned sample gives an unreliable measurement result, no matter how good the analyser is.

Extractive CEMS: the most widely used system

In an extractive CEMS, a gas sample is actively extracted from the stack gas stream, filtered, conditioned and routed to the analysers. This type offers the most flexibility in the choice of analysis technique, but places the highest demands on gas conditioning. The gas must be brought from stack conditions to analyser conditions.

The route from stack gas to analyser

An extractive CEMS sample stream passes through a series of conditioning steps. The aim of each step is to further clean and condition the gas without losing the components to be measured:

The heated probe and line ensure that the gas stays above the dew point. Condensation at this stage would remove water-soluble components such as SO₂, HCl and HF from the sample before they can be measured. The gas is only dried after the gas conditioning system.

The dangers of moisture in a CEMS gas stream

Flue gas typically contains 5 to 20% water vapour by volume. That moisture is the greatest threat to the reliability of the system. Problems occur as soon as the gas cools below the dew point, and this can happen at several places in the system.

Problem 01

Blockage of lines and valves

Condensed water clogs narrow lines, valves and flow meters. Pump and valve failures are the most common cause of CEMS downtime.

Problem 02

Loss of soluble analytes

SO₂, HCl, HF, NOₓ and NH₃ dissolve in liquid water. Condensation in the sample line selectively removes these substances from the sample; the analyser reads too low.

Problem 03

Damage to optical sensors

Water vapour or droplets on the optical windows and lenses of NDIR or UV analysers weaken the signal and cause measurement drift or complete failure.

Problem 04

Ammonia salt formation

NH₃ + SO₂ + H₂O → NH₄HSO₃. In the presence of both ammonia and water, ammonium salts form that can irreparably clog lines and analysers.

Problem 05

Interference in IR analysis

Water vapour absorbs in IR bands that overlap with CO₂, CO and SO₂. Moisture that is not removed causes direct interference in the spectroscopic measurement.

Problem 06

Corrosion

Condensed water in combination with SO₂ or HCl forms sulphuric acid or hydrochloric acid respectively. This corrodes metal components in the sample line and the analyser rack.

Downtime costs more than gas conditioning

A CEMS that goes out of specification due to moisture problems does not just produce incorrect data. In most jurisdictions, downtime must be reported and covered with conservative substitution methods. The fines and reputational damage of unreliable emission data far exceed the investment in good gas conditioning equipment.

The solution: Nafion technology from Perma Pure (via Salaera)

Perma Pure (Salaera) offers an extensive range of gas conditioning products based on Nafion membrane technology. Nafion is a fluoropolymer that selectively passes water vapour based on partial pressure, while virtually all other analytes in the gas stream are retained. This makes it ideal for CEMS applications in which the sample must not be contaminated or affected.

Nafion membrane: what passes through and what does not

Remains in the sample

CO, CO₂, SO₂, NOₓ, O₃

HCl, HF, HNO₃, H₂SO₄

Hydrocarbons (alkanes)

Aromatics, esters, ethers

COS, H₂S, mercaptans

Ar, He, H₂, N₂, O₂

Is removed

H₂O (water vapour)

NH₃ (partially)

Alcohols, DMSO, THF

Variable: acids, aldehydes, amines, ketones

Additional information

Learn more about Salaera's various solutions for conditioning the gas stream in CEMS applications in the following brochure: