ROOM TEMPERATURE WORK, BENEFITS OF AND TECHNIQUE FOR
Flue gases, engine exhausts, incinerator effluents and other products of combustion are hot, wet, and laced with toxic gases. There is a widespread impression that to do a spectroscopic analysis of such gas mixtures, one needs to maintain the high sample temperature. There is concern that if the gas sample is cooled, the water will condense and alter the sample by dissolving some of the toxic pollutants. It is the experience of Infrared Analysis,Inc. that such concern can be avoided by proper sample handling. Working at room temperature is feasible in almost all cases.
Keeping the combustion gases hot complicates the handling of the sample by requiring heated sampling lines and a heated absorption cell. Furthermore, the spectroscopic analysis is made difficult by the large amount of high temperature water vapor in the optical path. To allow a good infrared analysis, the heavy pattern of water vapor lines must be subtracted out of the spectrum. To do a good subtraction, you need to have a water reference spectrum at the same temperature as the sample spectrum and with nearly the same pathlength-concentration product; this is difficult to obtain. For matching the temperatures, you must have equipment to control the temperature and to monitor the temperature during recording of both the sample spectrum and the reference spectrum.
Another difficulty arising from the high temperature is that the calibration and reference spectra used in the quantitative analysis must be recorded from gases at the same high temperature as the sample. These spectra are difficult to obtain. Most available digitized reference spectra have been recorded only at room temperature.
How much easier things are if one is able to do the analyses at room temperature! The heated lines and heating jackets are gone! The temperature controller and temperature monitor are gone! The reference spectra for subtraction of water and CO2 interference are available! The calibration spectra for the quantitative analysis are available!
What are the obstacles to doing the analysis at room temperature? None! It is an easy thing to dilute the sample as it is being taken in. Dilution should be done with nitrogen. This may be taken from a tank or from the head-space in a container of liquid nitrogen. The only significant impurity in such nitrogen is a trace of carbon monoxide. This does not cause very much trouble. For details see the QASoft instructions. When a hot sample is being taken in and diluted, it is being cooled down. The dilution must be great enough that the water does not condense. This means that at room temperature the water content of the sample must be reduced to less than three per-cent, by volume. Let us say that a combustion effluent is 10 per-cent water vapor. Then the dilution ratio will have to be about one-in-four (25% sample and 75% nitrogen). This dilution will be easy to perform with the GAS MEGA SYRINGE.
One may ask: isn't the one-in-four dilution going to reduce your measurement sensitivity by a factor of four? The answer is no. The dilution will reduce all absorbances by a factor of 4, but this includes the interfering water lines. Measuring compounds like NO, NO2 and SO2, whose bands are badly mixed in with the water, will be made easier by the dilution. For these compounds, the dilution may reduce their measurement sensitivity by a factor of 2. Anyway, in analyzing combustion gases, measurement sensitivity is not the problem. With a good infrared spectrometer, an optical path of a few meters will give measurement sensitivities below one part per million for most toxic gases. If you want to go still lower, just lengthen the path.