Combustion control gas mixtures
Instrumentation is used to measure the oxygen concentration of the exhaust gases from power station burners and other combustion applications.
Typical combustion process control applications include power generation, cement kilns and waste incineration. Combustion process control instrumentation requires frequent calibration with specialty gases mixtures.
For stoichiometry control, where the air flow rate is adjusted to match in the incoming fuel load, a target concentration of 8% of oxygen in the burner flue gas is a typical control point. So a specialty gases calibration gas mixture composition of 8% oxygen in nitrogen is typical. The instrumentation used for this hot, wet application will typically be a zirconia oxygen analyser. It is robust and has a rapid response to changes in the oxygen concentration.
This is a process control application, not related to regulated emissions monitoring, so a general certified specialty gases calibration mixture is usually satisfactory and there is not usually a requirement for ISO 17025 or ISO Guide 34 accreditation for these mixtures.
A zero gas is also used. This should not be zero air because air contains oxygen at around 20.9%. The zero gas is therefore generally a high purity specialty gases grade of Nitrogen 5.0.
For both the calibration gas mixture and the zero gas a single-stage chrome-plated brass regulator is appropriate. Since these are one-shot calibration or zero-setting events there is no need to upgrade to a two-stage regulator. Furthermore, the gases involved here are not corrosive so there is no need to upgrade to a stainless steel regulator.