NDIR & FTIR Infrared Spectroscopy | Specialty Gas | Coregas NZ

NDIR & FTIR infrared spectroscopy

NDIR and FTIR are common IR spectroscopy techniques used in environmental monitoring applications for gaseous emissions. They both require high precision calibration gas mixtures.

Many chemical bonds absorb light in the IR spectrum. This is the principle behind the infrared spectroscopic techniques such as non-dispersive infra-red (NDIR) and Fourier transform infrared (FTIR).

IR gas detection

Methane and other flammable gases can be detected with IR sensors in modern gas detection devices. These are extremely small and highly portable. With most sensor types, the sensor itself is directly exposed to the gas, often causing the sensor to drift or die prematurely. The main advantage of IR gas sensors is that the detector does not directly interact with the gas sample. When the gases to be detected are corrosive or reactive this is a tremendous advantage. The functional components of the detector are protected with optical parts so that only the sample cell is directly exposed to the gas sample stream. These wetted parts can be treated to make them corrosion resistant and the device can be designed such that these parts are easily removable for maintenance. As with all gas sensors, these IR based gas detectors will require a regular functional test and periodic calibration with specialty gases calibration mixtures.

NDIR instrumentation

NDIR is a highly robust and simple technique that is suited to a range of commonly-analysed molecules such as CO2, CO, CH4 and SO2. The calibration of such an instrument simply requires a zero gas, such as Nitrogen 5.0 or Zero Air, and a span gas which will often be specified at 90% of full-scale deflection. So a device that is configured to read 0 to 100 ppm of CO2 will typically be calibrated with a calibration gas mixture, or span gas, containing 90 ppm of CO2.

Metals heat treatment relies on accurate process control of the various gases inside the heat treatment furnace. Carburising and nitro-carburising applications require CO and CO2 levels in the furnace to be controlled through the measurement of CO or CO2 with an NDIR and subsequent adjustment of the carbon source (often carbon dioxide) injection rate. Specialty gases calibration gas mixtures for this process control application are required.

In the food and beverage sector, NDIR sensors are often used in food packaging applications to determine the concentration of CO2 in the modified atmosphere that is required to keep the food fresh. They are also useful in process safety. For example: in the fermentation of wine and beer, large quantities of CO2 are generated which could create a toxic atmosphere for employees if there is insufficient ventilation to dilute the CO2 with fresh air. CO2 measurement using NDIR gas detection instrumentation is common to raise an alarm and prevent hazardous situations. As with all gas detectors employed in safety applications, these NDIR CO2 sensors will require frequent functional tests and occasionally they will require a more precise calibration with suitable specialty gases calibration mixtures.

NDIR-based CO2 sensors are common in breweries and wineries for safety

FTIR analysers

FTIR instrumentation is slightly more exotic and sensitive but has the advantage that one instrument can be configured to measure more than ten components simultaneously. And the species that can be detected are some of the most noxious pollutants such as hydrogen fluoride or hydrogen chloride. The ability to measure these exotic pollutants has given FTIR a unique status as the instrument of choice for CEMS in waste incineration or co-generation combustion applications. In these situations, unlike the burning of relatively clean natural gas, the emissions footprint of the combustion process is highly variable and may include a broad range of species that should be monitored to ensure public health and environmental protection.

Use of mirrors to create a long optical path length and use of a pressurised sample chamber both increase the sensitivity of the FTIR instrument so that its detection limit can be extremely low. This feature enables it to be applied to sophisticated R&D applications and ambient air quality monitoring.

Since the FTIR can measure multiple components it will require a suite of calibration gas mixtures. If it is to be used in CEMS for environmental management it will generally be calibrated to a high standard using traceable accredited calibration gas mixtures. At Coregas we have developed our NATA-accredited ISO 17034 product scope to serve the demanding needs of this application.

We are one of the few specialty gases companies globally to offer accredited calibration gas mixtures containing the main pollutant gases NO, SO2, CO and CO2 in the same cylinder. These accredited multi-component calibration gas mixtures are ideal for CEMS calibration applications and avoid the need for users to purchase and handle multiple cylinders for these species, thereby saving time and money in logistics and procurement. Furthermore, a single calibration event using a multi-component gas mixture is more productive than repetition of the calibration with three or four separate calibration gas mixture cylinders. Coupled with high purity Nitrogen 5.0 as a purge gas, these multi-component accredited calibration gas mixtures combine to make a great solution for the calibration of FTIR instrumentation used in CEMS applications.

Victor Chim, Business Development Manager for Coregas explains how his specialty gases team rises to the challenge of providing these high-tech products: "our multi-component calibration gas mixtures are manufactured by choosing the most suitable materials for the cylinder and valve and then employing the utmost discipline in our cylinder preparation, filling and analytical techniques. We customise our process according to the components used in each mixture to ensure optimal results. After all, the calibration accuracy can never be better than the accuracy of the calibration gas mixture, so our customers rely on us to get this right every time."

The presence of oxygen interferes with the FTIR measurement of the pollutant species. In many CEMS applications oxygen will be present at perhaps 6 to 10% in the flue gas because the combustion process is generally run with an oxygen rich stoichiometry. In order to compensate the measurement accurately, the oxygen concentration in the FTIR sample will be measured and that result fed to the FTIR signal processing software to make a precise adjustment to the analytical results. Oxygen measurement in this hot, wet CEMS application would be done with a zirconia analyser, which is able to withstand the high flue gas temperatures and is not troubled by the high water vapour in the flue gas. In addition to the calibration gases for the pollutant species, one additional calibration gas mixture is required for the zirconia analyser and this will typically be in the range of 6% to 12% oxygen in nitrogen. This calibration gas mixture is one of the most typical specialty gases used for combustion process control applications.