Combustion Analysis of Nitric Dioxide

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Combustion Analysis of Nitric Oxide (NO), Acetylene (C2H2), and ​​ Hydrogen Cyanide (HCN) in the presence of high concentration moisture based on FTIR spectroscopy.

 

While the general purpose of using a long path gas cell is to increase the sensitivity of an FTIR Spectrometer, in the case of combustion analysis,​​ it tends to affect the measurements of some components. Up until at least a decade ago, it was common​​ thought, that to achieve detection limits in the low ppm range a long path gas cell was required.

ISO standard 19702 was mainly developed under those premises, where the overwhelming overlapping of moisture’s absorbance spectra with some of the absorption species becomes a hard problem to solve.

The upcoming new revision of ISO19702:2014 states on section 6.7 that:

​​ “6.7​​ A resolution of 0.5 cm-1​​ or finer is​​ recommended​​ for minimization of interferences in fire gas analysis, although a​​ coarser resolution (>0.5 cm-1) is allowed as long as spectral interference from overlapping compounds are identified and corrected for by the software.

Measurement of Nitric Acid(NO)

​​ As an example, Figure 1 shows the absorption intensity of 30% of moisture between 1970 to 1770 cm-1. The moisture absorption is so strong,​​ that​​ 75% of the infrared light has been absorbed​​ in this portion of the spectrum​​ by the moisture itself!. This leaves very few regions that effectively can be used for the measurement of NO.​​ Hence the recommendation of using a resolution of 0.5cm-1​​ or higher, as a way to mitigate the spectral​​ overlap and to aid the separation of​​ the​​ moisture’s​​ absorption spectra from the absorption spectra of NO​​ and other overlapping species.​​ 

It is worth​​ pointing​​ out that most of the absorption peaks of moisture are showing a​​ very​​ strong non-linear behavior,​​ which is​​ hard to compensate​​ for using​​ multivariate quantification analysis algorithms.​​ This problem is independent of the resolution used for the analysis.​​ Even using a finer resolution will not allow the software to separate the spectra if most of the light is absorbed and the absorption peaks behaving extremely non-linear.

Trying to go to longer pathlength gas cells to achieve more sensitivity only makes the problem worst. Is there a better way to solve this problems?

The short answer is: ​​ Yes!

The use of FTIR spectrometers​​ with higher sensitivity allow us to explore other alternatives.

Figure​​ 1​​ Collection Conditions: 0.5cm-1 resolution, 5.1meter pathlength, 1 atm, 120 Deg C, 1, 30% Water, and 1ppm of NO​​ 

Over the years, new FTIR spectrometers have been introduced into the market. One of them is the ABB’s MB3000. After many years of research and experience,​​ they​​ were able to develop an FTIR spectrometer with a great increase in sensitivity​​ due to the superior optical and electronic design.

The higher sensitivity of the spectrometer allows,​​ in the combustion analysis arena, the use of a shorter​​ pathlength to obtain the same level of sensitivity​​ as​​ seen in older FTIR spectrometers using long path gas cell.

The main advantage of the use of a short path gas cell is​​ that​​ the absorption​​ intensity​​ of moisture at high concentrations​​ is greatly reduced, and it follows a more linear behavior that is easy to compensate for. (See Figure 2.)

Figure 2 shows that the absorption spectrum of moisture now is more clear, and does not show​​ signs​​ of oversaturation in the same area. In fact, only 12% of the total infrared light in the area has been absorbed​​ by moisture alone. The moisture absorption in the area still strong, some of the stronger peaks do display​​ slightly​​ non-linear behavior, but it can be compensated​​ for​​ by multivariate quantification analysis algorithms. This alone makes the use of​​ lower resolutions​​ absorption spectra​​ perfectly feasible to separate the overlapping of NO​​ from the Moisture, as stipulated by ISO19702:2014 section 6.7 – “…​​ coarser resolution (>0.5 cm-1) is allowed as long as spectral interference from overlapping compounds are identified and corrected for by the software.

 

Figure​​ 2​​ Collection Conditions: 2cm-1​​ resolution, 0.15 meter pathlength, 1 atm, 120 Deg C, 1,​​ 30% Water, and 1ppm of NO

Another related problem of using a long path gas cell​​ is​​ that it restricts​​ the useful​​ upper​​ range to measure​​ other combustion gases.​​ The absorption intensity of those gases, at high concentrations grow so much, that they may show a strong non-linear behavior.

In the case of NO, the​​ absorption intensity​​ at high concentration (see Figure 3)​​ has grown​​ so much, that it really shows a non-linear behavior that is its difficult to compensate; for​​ even with the use of a non-linear correction curve (See Figure 4). At first​​ sight, the non-linear correction curve seem to go through all the calibration data points, but it can be observed that the curve is being over compensated. This overcompensation of the non-linear compensation curve​​ is​​ more evident when we look at the curve in a logarithmic scale (See Figure 5), and it can be seen that it will increase the reading errors of NO at​​ low concentrations. ​​ Again, in the case of NO, a high resolution spectrum is not helping to alleviate this problem.

 

Figure​​ 3​​ Collection Conditions:​​ 0.5cm-1​​ resolution,​​ 5.1​​ meter pathlength, 1 atm, 120 Deg C, 1,​​ 30% Water, and​​ 0.5%​​ of NO

Figure​​ 4​​ NO's non-liner correction curve for the range of 0 to 0.5% in ​​ a 5.1m pathlegth gas cell and​​ 0.5cm-1​​ resolution

 

 

Figure​​ 5​​ NO's non-liner correction curve​​ (Logarithmic scale)​​ for the range of 0 to 0.5% in ​​ a 5.1m pathlegth gas cell and​​ 0.5cm-1​​ resolution

Once again,​​ the solution​​ to this problem arrives​​ with​​ the use of a spectrometer with a higher sensitivity, and the use of a short path gas cell.​​ The absorption intensity of NO at 0.5% is greatly reduced (See Figure 6​​ and 7).

The non-linear correction curve is practically linear, and it will not introduce error in the proper​​ quantification​​ and correction of the​​ concentration​​ of NO.

Figure​​ 6​​ Collection Conditions:​​ 2cm-1​​ resolution,​​ 0.15​​ meter pathlength, 1 atm, 120 Deg C, 1, 30% Water, and 0.5% of NO

Figure​​ 7​​ NO's non-liner correction curve for the range of 0 to 0.5% in ​​ a​​ 0.15m​​ pathlegth gas cell and​​ 2cm-1​​ resolution

Figure​​ 8​​ NO's non-liner correction curve (Logarithmic scale) for the range of 0 to 0.5% in a​​ 0.15m pathlegth gas cell and​​ 2cm-1​​ resolution

Measurement of Acetylene (C2H2) and Hydrogen Cyanide (HCN)

 

The ISO standard 19702:2014 also states on Annex G.3 that:

“G.3 It is important to acknowledge that FTIR determines the infrared absorption of a gas or mixture of gases over a wide range of wavenumbers (typically 400 cm-1​​ to 4 500 cm-1​​ (for KBr windows) and 650 cm-1​​ to 4500 cm-1​​ (for ZnSe Windows). ​​ The infrared absorption bands for the main fire gases are given in Annex I. ​​ It should be noted that many other gases found in fire atmospheres, e.g. hydrocarbons, also absorb infrared radiation over this region. ​​ It is therefore rare for a fire gas to absorb infrared within a given wavenumber region without interference from other fire gases in the same region. ​​ However using a spectrophotometer resolution of 0.5 cm-1​​ or higher, provides an acceptable solution for avoiding overlap of absorbances for fire gases such as HCN and acetylene (C2H2), and nitric oxide (NO) and water, that​​ would not be possible at lower resolutions.”

But they also added the following note to the passage:

NOTE 1: ​​​​ Measurements using spectrophotometer resolutions as low as 4 cm-1​​ have proved acceptable for measurements of fire gases, as long as spectrally interfering (overlapping) compounds can be addressed in the mathematical evaluation of the spectra [6].”

As mentioned earlier, the bulk of the ISO19702 document assumes the use of a long path gas cell for the analysis, and​​ focuses on​​ overcoming​​ the strong absorption intensity of moisture. As we have shown,​​ a​​ shorter path length and a more sensitive spectrometer provided​​ a better​​ solution since the absorption spectrum of moisture can be easily compensated by the software since it exhibits a more linear behavior.

This also applies to the overlapping of C2H2​​ with HCN. Both of the species overlap heavily with each other, and the use of a long path gas cell will make them behave very non-linear at high concentrations. The recommendation of the standard is to only use high resolution spectrometers​​ (See figure 9).​​ With this heavy overlapping, even ​​ at​​ high resolution, the whole spectrum cannot be used to measure HCN. The best option is to use the region between 3400 to 3360cm-1, and provide a non-linear compensation curve. To measure C2H2​​ it​​ is better to use the region between 3240 to 3200cm-1​​ while using a non-linear correction curve. Both non-linear correction curves​​ should​​ be as closer to linear as possible. I must point out, that the use of a lower resolution can also be use for the same regions while using a long path gas cell.​​ 

Figure​​ 9​​ Overlapping of the absorption spectrum of HCN and C2H2​​ at high concentration

The magnitude of this problem gets greatly reduced by use of a short path gas cell and a highly​​ sensitive spectrometer.

The absorption intensity of both gases​​ will​​ exhibit​​ almost linear behavior​​ at high concentrations,​​ and a multivariate quantification analysis algorithm should be able to separate them easily. No need for a high resolution spectrum here (See figures​​ 10 and 11)