Future regulations will, undoubtedly, focus on ways to improve accuracy and repeatability. One of the most obvious methods is the trend towards infrared and ultraviolet measurement technologies. Infrared, due to lower costs and more practical manufacturing methods will be more common. Not all gas components are optically active. One of the major exceptions is oxygen, and alternative methods will always be necessary in this case. There are enough means of measuring oxygen concentrations relatively accurately thanks to medical research anyway. The most common form of infrared analysis is the Non-Dispersive InfraRed. These sensors operate on the principle that a gas will absorb infrared radiation of a specific wavelength. Basically, it measures how much of a particular wavelength of light is absorbed over a set distance and relates this to the concentration of a particular gas. The practice is, naturally, slightly less simple due to various factors such as pressure and temperature effects as well as the non-linear response of such measurements in general.
The latest result is the portable infrared analyser. Infrared analysers were always bulky and expensive instruments requiring temperature-regulated enclosures and humidity control. Now it is possible to produce such an instrument in portable form and make this technology truly transportable. Instead of temperature and humidity control, there are measurement and electronic compensation of these factors, and such instruments can be made modular to allow the addition of extra components as it becomes necessary. Thus we have an analyser using the most modern technology at a fraction of the cost of earlier infrared fixed systems, yet retaining the accuracy associated with this technology. Particularly valuable for carbon monoxide and sulphur dioxide measurement (CO and SO2), it is also widely used for carbon dioxide and hydrocarbons such as methane, which are otherwise very difficult to measure.
It is still not possible to pull yourself up by your own bootstraps! To use this type of instrument for stack testing requires the use of a high-quality sample conditioning system including an appropriate heated hose. A portable infrared analyser is just as susceptible to fogging of the optics due to condensation as its fixed brethren. Without a sample conditioner there is no hope of producing reliable and repeatable results, regardless of the quality of the instrument. Such a system will keep the sample gas at a temperature above the dew-point until it reaches a special cooler, usually a Peltier element. Here the sample is cooled quickly and the resulting water removed rapidly by a peristaltic or other pump. This prevents the soluble components from being absorbed by the moisture and also ensures that no water can penetrate the analyser system. No water is perhaps an exaggeration. It is never possible to remove 100 % of the water, but, provided the Peltier element is the coldest part of the system, there can be no condensation formed due to the low vapour pressure at points downstream of here.
This type of system used as a stack monitor will also have to reach an equilibrium state before use. In practical terms, this means that the system will have to run for about 30 minutes until it reaches a stable internal temperature before being used for measurement. This temperature will then be taken as the "zero" for all following measurements and provides a known baseline. This effect will be exaggerated in areas where air conditioning or building heating are in general use. The instrument will have to reach the temperature of the surroundings before use. In extreme cases it will be necessary to heat the casing of the portable infrared analyser to ensure that no condensation is possible inside the unit. The link will lead you to a page containing some tips for use of an infrared analyser.
Nevertheless, this represents a vast breakthrough in the accuracy of portable measurement technology, being specially suited to EPA compliance testing and the measurement of carbon dioxide and methane or other hydrocarbons. Infrared measurements of NOx and CO are also presently possible, although it is not really possible to measure NO2 directly with infrared technology.
Such instruments are now available for use when needed and a description of a portable infrared flue gas analyzer can be found by following the link.
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