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Infrared gas analyzer

Infrared (IR) gas analysis is one of the most widely used techniques for measuring gas concentrations in industrial processes — from kiln inlet oxygen and CO monitoring in a cement plant to combustion efficiency checks on any fired process. This post covers how IR gas analyzers work, the two main analyzer types, and the physical layout of a typical sampling system. For the detailed working principle of the non-dispersive (NDIR) technique specifically, see the dedicated NDIR Gas Analyzer post; for oxygen measurement via zirconia cells, see Working Principle of Zirconia Gas Analyzer.

How Infrared Gas Analysis Works

An infrared gas analyzer measures the concentration of a specific gas in a sample by determining how much of an emitted infrared light source that gas absorbs. Different gas molecules absorb different, specific wavelengths of infrared light — so a sample with a higher concentration of a target gas absorbs more of that gas's characteristic wavelength, and the analyzer reports that absorption as a proportional concentration reading.




Two Main Types of IR Analyzer

  • Non-Dispersive IR (NDIR) — the wavelength isn't pre-filtered before entering the sample chamber; instead, an optical filter sits in front of the detector to isolate the wavelength of interest. This is by far the more common design in process instrumentation.
  • Dispersive IR — the light is split into its component wavelengths before reaching the sample, typically using a prism or diffraction grating, allowing more precise wavelength selection at the cost of more complex optics.

Working Principle

The core components of an IR gas analyzer are:

  1. An infrared source
  2. A sample chamber (light tube)
  3. A wavelength-selective detector chamber

IR light from the source is directed through the sample chamber toward the detector. In parallel, a reference chamber containing an inert reference gas — typically nitrogen — provides a baseline. An optical filter mounted in front of the detector blocks all IR wavelengths except the one the target gas molecule absorbs; other gases in the sample are assumed not to absorb significantly at that wavelength. The IR source signal is chopped or modulated so that steady thermal background radiation can be electronically separated from the actual measurement signal.

Gas Sampling System Components

A complete extractive gas analysis system — as used for kiln inlet or stack gas monitoring — involves considerably more than just the analyzer cell itself. A typical sampling train includes:

  • Sampling probe — stainless steel, sized to the application (e.g., ~60 mm probe diameter within a 100 mm assembly, 20 mm wall thickness)
  • Gas extraction heater and SS filter — rated up to 150°C to prevent condensation at the extraction point
  • Heated sampling tube — PVC or copper, heated up to 100°C to keep moisture from condensing in the line en route to the analyzer
  • Purge MOV (motor-operated valve) — 110V, with limit switches for open/close position feedback
  • Drain separator — with a water-filled drain pot to knock out condensate
  • Gas conditioner
  • Sampling pump / gas aspirator — typically drawing around −250 mbar suction
  • Gas cooler — chills the sample to around 2°C to drop out remaining moisture before it reaches the optics
  • Calibration gas SOVs (solenoid-operated valves) — switch in span/zero gases on demand
  • Rotameter — sets and monitors sample flow, typically around 0.6 L/min
  • Membrane filters — fine final filtration ahead of each analyzer cell
  • IR analyzer cell(s) — for CO, NOx, etc.
  • NO2-to-NO converter — needed ahead of NOx measurement, since standard NDIR cells respond to NO but not NO2 (see the field-notes companion post for the converter's temperature requirement and fault symptoms)
  • Zirconia oxygen cell — for O2 measurement, operating at high temperature (see the dedicated zirconia post for the working principle)

Benefits of Gas Analysis in Combustion Processes

  • Fuel savings through optimized combustion air ratio
  • More complete combustion, reducing unburnt fuel losses
  • Pollution control (CO, NOx monitoring and compliance)
  • Product quality improvement through more consistent process conditions

Common Analyzer Brands and Models

Commonly encountered in cement and process plants: Fuji ZRJ / ZFK series, and ABB Magnos 14/25 (oxygen) and Uras 17/27 (NDIR) series.

The Bottom Line

IR gas analysis gives you a fast, continuous read on combustion gas composition without needing to physically sample and lab-test the gas stream. The analyzer cell itself is only part of the picture, though — sample conditioning (heating, filtering, cooling, drying) is just as critical to a reliable reading, since a fouled probe or a wet sample line will throw off the measurement long before the analyzer cell itself is at fault. For calibration procedures, PID controller settings, and field troubleshooting notes drawn from real kiln inlet and coal mill applications, see the companion Gas Analyzer Fault Tracing post.


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