FTIR: testing resolution

Reliable single-component continuous measurement techniques are available for SO2, NOx, CO2, CO and VOCs when using proper sample conditioning and calibration procedures. However – especially in the case of field studies – it can be tedious and time consuming to build up a

measurement system for several components using single-component analysers.

On-line analysis of stack gases with FTIR (Fourier Transform Infra-Red) spectrometry has received a great deal of interest because of the multi-component analysis capability together with speed and sensitivity of the method. The FTIR technique can give results comparable to the results of the established single-component methods together with a substantial cost saving. In field studies, it is an advantage to use rugged, easy-to-use instrumentation that can be rapidly set up.

Real alternative

Traditional laboratory-designed ‘high’ resolution FTIR spectrometer-based gas analysers do not necessarily lend themselves to stack monitoring because of the high level of expertise which is needed to calibrate the analyser and to develop the quantitative analytical methods.

‘Low’ resolution FTIR spectrometry, however, offers some valuable advantages compared to the traditional high-resolution FTIR gas-phase spectrometry, especially for industrial applications. First, a low-resolution FTIR spectrometer can be made using a very simple and rugged design. This aspect is most important when reliability of the instrument is considered in the harsh real world of stack gas monitoring. Second, high signal-to-noise ratio spectra can be acquired without the use of the more traditional liquid-nitrogen-cooled detectors. Third, spectral analysis speed is increased and data storage requirements are substantially reduced. Fourth, the dynamic range for quantitative analysis is larger for low-resolution spectrometry than for high resolution, due to the lower absorbance values and lower noise levels.

The use of a transportable, fully integrated low-resolution FTIR gas analyser for continuous monitoring of all these stack gases in field conditions, is now a real alternative to traditional methods and, in addition, difficult components such as HCl, HF, HCN, and NH3 can also be measured in hot, wet stacks. TšV approval for a low resolution FTIR-based CEM (Continuous Emission Monitor) has also been achieved.

The recurring question has been: “How can I measure HCl with FTIR when water vapour partly overlaps the HCl bands?”

The answer is that low concentrations of HCl in high humidity can be detected and quantified due to the high signal-to-noise ratio of the low resolution FTIR spectra and by the use of multicomponent analysis routines specifically developed for low resolution FTIR spectra.

The computer-based spectral analysis methods used differ from common spectral fitting programmes, in that there is no need to define specific analytical spectral regions for each component. Basically, all data points in the spectra can be used in the fit (and spectra are generated very fast – typically ten per second) maximising the spectral information content to perform the analysis.

Studies have been carried out where HCl analysis results using FTIR and liquid absorption methods are compared. In one study, average HCl concentration during a one-week monitoring period measured by the liquid absorption method was 16.3ppm, compared to 19.3ppm measured by the FTIR analyser, thus being well above the respective method detection limits. The water vapour concentration was approximately 25% during the monitoring period. The results indicate an excellent agreement between the two methods and show that a low-resolution FTIR gas analyser can give results consistent with approved analysis methods for

low levels of HCl in high-humidity exhaust gases over extended periods of time.

Low-resolution FTIR spectrometry is a powerful tool in emission monitoring at the ppm level. Because the FTIR analyser’s gas cell and the whole sampling system is heated, water vapour removal is not needed. Water vapour concentrations are continuously measured by the FTIR gas analyser, which provides more accurate dry-gas based emission rates. Low concentrations of HCl, NO, NO₂, N₂O, NH₃, SO₂, CH₄, CO and CO₂ can be continuously and accurately measured by low-resolution FTIR.