Common gas constituents to be quantified include carbon monoxide, nitrogen oxides and hydrocarbons. Other non-chemical properties that gas analysers often measure are temperature and velocity.
“When we analyse a boiler, we’re looking at the CO and the CO₂, which will give us a ratio, which is basically the CO divided by the CO₂ and then divided by 10,000,” states Derek Robins, gas engineer and trainer, TomKat Gas Training. He says that the safe reading is less than 0.004 for a condensing boiler, or 0.008 for a non-condensing boiler. He continues: “Now, if a boiler or a flued appliance fails its flue gas analyser readings, then in the unsafe situations procedure, that is deemed ‘at risk’. But, if a flueless appliance fails its flue gas analyser readings then it would be deemed immediately dangerous.” Testing should always be carried out in accordance with the gas analyser manufacturer’s instructions and BS7967:2015.
WHAT IS INFRARED SPECTROSCOPY
A spectrophotometer is an instrument which determines the chemical composition of a sample by measuring how light at different frequencies passes through it. These instruments use a light source to pass light through a sample, and a photodetector to measure the intensity of light reaching the other side. Infrared spectroscopy gas analysers are spectrophotometers that use light in the infrared spectrum.
Spectroscopy makes use of the way that substances absorb light. The amount of light absorbed by a substance depends on the interaction of photons with molecules. The molecules of each chemical substance have a unique set of energy levels resulting from their chemical bonds and nuclei. This causes them to more strongly absorb light at specific narrow frequency bands. By identifying the wavelengths that a sample most strongly absorbs, its chemical composition can therefore be determined.
There are basically two different ways of finding a narrow frequency band that a sample absorbs. Either the light source must produce light in a narrow frequency range or the detector must measure light in a narrow frequency range. In order to create a narrow bandwidth light source, a broad spectrum source can be combined with a monochromator and diffraction grating. If an analyser is configured to detect a single gas, then the diffraction grating can be set to the required frequency. For multi-gas analysers, the grating can be mechanically scanned though a range of frequencies, or an array of photodiodes can be used to measure different frequencies at the same time.
Spectroscopy can use a single beam or a double beam arrangement. Often a sample needs to be compared with a reference quantity of gas. If a single beam is used, then the sample and the reference need to be placed in the analyser in turn. Double beam arrangements allow samples to be compared with a reference simultaneously. The double beam is achieved by splitting a beam after it has been filtered into a narrow frequency.
An infrared spectroscopy gas analyser evaluates the proportional difference between a sample and a reference to detect the presence of a specific gas. They use non-dispersive infrared (NDIR) sensors that emit infrared radiation at tightly controlled wavelengths. The infrared light passes through the sample and reference gases, and the wavelengths which are absorbed by the gases are measured.
Gas analysers may be made to detect a single chemical constituent of a gas, or can be configured as multi-gas analysers. Gases that are frequently monitored include oxygen, carbon monoxide, carbon dioxide, methane, hydrocarbons, hydrogen, helium and nitrogen oxide.
Oxygen (O₂) analysers are often used to monitor process gas and flue gas. Combustion air oxygen enrichment can be used to achieve higher flame temperatures and reduce energy lost in heating nitrogen. Monitoring oxygen to control such combustion processes is therefore one important application. Oxygen analysers are also used to monitor the safety of working environments. Oxygen deficiency analysis is used to detect reduced levels of oxygen, which may occur when liquefied gas is released, leading to displacement of air. Helium and nitrogen are common gases which present a significant risk of oxygen displacement, due to their dramatic increase in volume between a cryogenic liquid and a gaseous state. Reduced oxygen levels can lead to impaired cognitive abilities, unconsciousness and death. Oxygen deficiency is defined as less than 18% oxygen in the air, compared with 21% under normal atmospheric conditions. Spectroscopy is not necessary to detect oxygen. Oxygen is typically detected using an electrochemical sensor in which a chemical reaction produces an electrical output proportional to the oxygen level.
Carbon monoxide (CO) analysers are often used for hazard detection and may also be combined with temperature and oxygen analysers to monitor combustion efficiency and performance. Both electrochemical sensors and infrared spectroscopy may be used.
Carbon dioxide (CO₂) analysers are used to monitor environmental releases, control processes and detect hazardous levels in working environments. Industries which produce large quantities of CO₂, which cannot be calculated from inputs such as fuel, may directly measure the CO₂ they release. Examples include agriculture and landfill.
USEFUL FOR COMBUSTION
For combustion processes in engines, furnaces and boilers, gas analysis may be used to ensure correct operation, often combined with other gases such as carbon monoxide, hydrocarbons, oxygen and nitrogen oxides. Processes used in industries such as pharmaceutical and beverage manufacturing may require an accurately controlled stream of CO₂. Carbon dioxide analysers usually use infrared spectroscopy.
Methane (CH₄) analysers are often used to detect environmental emissions from landfill, or monitor the composition of biogas or syngas production. Methane is a hydrocarbon and less sensitive analysers may simply measure the total hydrocarbon content while others can distinguish between different hydrocarbons, including methane specifically. These detectors use infrared spectroscopy. Hydrocarbon gas analysers are used to monitor the composition of biogas or syngas production, as well as exhaust gases.
Hydrogen (H₂) analysers often make use of the high thermal conductivity of hydrogen compared to other gases. This means that a simple thermal-conductivity detector can be used instead of spectroscopy. Electrochemical sensors can also be used. They are used to monitor the production of hydrogen and syngas, as well as in other industrial processes where hydrogen is used.
Helium (He) analysers also often use thermal conductivity rather than spectroscopy, as helium measurement is often only needed in binary gas mixtures.
Nitrogen oxide (NO, NO₂, and other NOx) analysers are often used to monitor the emission of these dangerous pollutants from combustion equipment (engines and boilers). Nitrogen oxides can be detected using spectroscopy or using the chemiluminescence effect when reacted with ozone.
Other gases that are commonly analysed include sulphur dioxide (SO₂), chlorine and hydrogen chloride. Hazard analysers may be set to provide an alert before the concentration of a gas becomes combustible, known as the lower explosive limit (LEL).
Gas analysers are used in a wide range of applications, including flue gases, vehicle exhausts, process flows, landfill emissions. Infrared spectroscopy is the most versatile method.