Alexander Fateev of project partner DTU has provided an update illustrating some of the work progressing between February and November 2018 under WP1.

Part I: UV measurements

1. Introduction

Proper ammonia and formaldehyde optically-based measurements in stacks require either the measurements have to be performed in situ (i.e. directly in the gas) or by bypassing of the gas through a system which is kept at least at the same (or higher) temperature as the stack gas in order to avoid any cold spots where water condensation may occur. Moreover as short as possible heated lines between a gas sampling point and the measurement system is highly desirable in order to eliminate or at least minimize any surface-promoted loses molecules of interest. This is especially important for “sticky” molecules as e.g. NH3. In situ measurements with use optical absorption spectroscopy tools without stack gas flow disturbances normally require two access ports where an emitter (light source) and a receiver (spectrometer or detector) are placed on opposite sides of the gas flow. For some technical reasons it was not possible to establish an additional access port on opposite side of to the existing ones at INERIS bench. Therefore the measurements have been performed by bypassing of the stack gas through two independent systems. The first system is based on use UV absorption spectroscopy technique, Fig.1 and the second one on FTIR absorption spectroscopy, Fig. 2

 

 

For both systems the gas has been taken from the middle point of the flow (across the flow, from access ports available at the bench) with a hated to 150C Teflon line with about 2m (UV-system) and 1m (FTIR-system). No filters have been used, i.e. stack gas flew directly into the cells. UV and IR cells have been heated to 150C. Stack gas temperature during all measurements was measured close to gas sampling locations and it was varied between 129C and 140C. Because low overpressure in the stack a pump was used to force gas flow through both cells. The flow rate was 1.2 l/min for UV and 2 l/min for IR cells.

The UV cell was coated with a Teflon-like thick coating (in um-thickness) whereas the IR gas cell was coated with SilcoNert 2000 coatings (few hundreds of nm). The last one has been validated in the EUROMET funded project MetNH3 in regard to NH₃ surface adsorption. It was shown that both SilcoNert2000 and Teflon have least NH₃ adsorption rates compare to other materials. Because working temperature in UV/FTIR systems is 150C it is therefore expected that ammonia/formaldehyde surface adsorption will be further suppressed.

UV/IR NH₃ measurements have been performed with various NH₃ amounts added to the stack gas in 1) natural gas, 2) diesel and 3) biomass combustion cases. UV/IR H₂CO measurements have been performed with various H2CO amounts added to the stack gas in natural gas combustion case only. Some measurements have been done with NH3/H2CO in the hot gas (no combustion source).

Here the results of NH₃ and H₂CO UV absorption measurements are presented.

 

2. NH₃ UV absorption measurements

2.1 Validation of the INERIS bench

An example of NH₃ spectra in hot air at 140C (H₂O and CO₂ traces) measured with UV-system is shown in Fig. 3. A clear difference between spectrum #1 (blue) and spectrum #2 (olive) of 1.1 ppm can easily be observed because difference in NH3 concentrations in the hot gas. The spectrum #1 or #2 can perfectly be fitted with a reference one measured at DTU. In the Fig. 3 fit of the #1 spectrum with the reference spectrum is shown by the red line. This can be considered either as a validation of the INERIS bench and NH3 injection system or DTU’s UV system the reference data sets.

 

 

2.2 Natural gas combustion

An example of UV absorption spectra measured with 50 cm gas cell on stack gas (stack gas temperature 140C) in 180-190 nm and 190-230 nm is shown in Fig. 4 and 5 respectively. Because high water concentration in the stack gas (H₂O (13%)), the UV light will be totally blocked at 185nm. However in the hot air (H₂O (1.25%)) the measurements can be extended down to about 181nm.

 

 

The same spectra in 190-230nm are shown in the Fig.5. One can see that in 198-218 nm there is nearly no contribution from H₂O and CO₂ into the total spectra and therefore this spectral range can be used for only NH₃ measurements if there is no interest for other gases. NO absorption bands marked have very distinguished structures and can be easily accounted in the spectral data analysis.

 

 

2.3 Diesel combustion

Diesel combustion gives additional challenges in NH₃ measurements in tacks, namely presence of SO₂ which has broad absorption features in 170-350 nm and particles (as for example soot and nano particles). Particles give some light scattering and almost wavelength-independent light attenuation 220-350 nm and most fine ones (few nm in size) may give a continuum-like absorption feature below 220 nm. SO₂ has however distinguished structures well seen at moderate to high resolution of the UV spectrometer.

An example of the stack gas spectrum with light-diesel combustion in 190-230 nm is shown in Fig. 6 in blue together with it fit by NH₃+SO₂ (red). As in the Fig. 5, the 198-218 nm spectral range is free from CO₂ and H2O contributions and therefor can be used for NH₃, SO₂ and NO simultaneous measurements in stacks. This is particular in interest for NO_x reduction process monitoring where NH₃ or urine is used for NO_x reduction in the hot flue gases. No significant contribution from the particles has been found in the spectra.

 

 

 

The same spectrum as in the Fig. 6 is shown in Fig. 7 in 184-193 nm range.

 

 

The absorption in this range is dominated by H₂O and O₂ and therefore this spectral range can be used for their on-line measurements in addition to NH₃ and SO₂. Because very distinguished O₂ absorption features compare to that for H₂O or nano-particles, the 184-189 nm spectral range can be used for accurate O₂ measurements.

2.4 Biomass combustion (wood pellets)

As well-known biomass combustion is a common source of particle emission and a broad range of various PAH/VOC’s especially when combustion is not optimized. Fig 8 shows UV absorption spectra measured on stack gas from biomass combustion at about 140C with various added NH3 amounts. The measurements started with NH₃(49.2ppm) and continued with stepwise NH₃ concentration reduction down to NH₃(2.4ppm) (spectra from blue to red in the Fig. 8). Each NH₃ concentration in the bench has been hold stable for 30 min.

 

 

Because particles in the gas (soot and nano-particles) and no filters between gas sampling point and the UV gas cell, Fig. 1, it was observed a change of the base line with time because particles deposition on the windows of the gas cell. As one can see from the Fig. 8 while NH₃ concentration is going down, soot absorption on the windows is going up. This is in particular clearly seen in the 202-230nm. However it does not influence on NH₃ measurements because even at for example NH₃ (2.4ppm) the fine structure of the NH₃ band at 208 nm (marked vertical double arrow in the Fig. 8) can be used for NH₃ identification and quantification in the gas. As it was pointed above soot absorption gives continuum-like slow-varying absorption below 230 nm. This can be clearly seen from the last spectrum at NH₃ (2.4pppm), Fig. 9, red.

Apart of particle emissions, PAH/VOC’s emissions are also of environmental concern. Fig. 9 shows a part of UV absorption spectrum in 184-193 nm measured on the stack gas with NH₃ (2.4ppm) (blue).

 

 

One can see that the spectrum can be fitted by O₂ (13%)+H₂O (6.95%) (red) with some constant offset in the base line because soot absorption. Fitted H₂O and O₂ concentrations are in the very good agreement with those measured by INERIS with GASMET analysers. Because most PAH/VOC’s give strong absorption below 200 nm, the very good fit of the measured spectrum in the Fig. 9 show that the stack gas is clean from PAH/VOC’s.

 

2. H₂CO UV absorption measurements

H₂CO optical absorption measurements are most difficult at low H₂CO concentrations. Fig. 10 shows a compilation of low-temperature UV absorption cross sections for NH₃ and H₂CO in 140-360 nm and Fig. 11 shows H₂CO and NH₃ IR absorption cross sections measured with high-resolution FTIR spectrometer at PNNL.

 

 

 

One can see that NH₃ has about 17 times higher absorption cross section in UV (197nm) than in IR (around 1000cm^-1). This is why the UV technique is more suitable and can be recommended for low-NH₃ level measurements than the IR-based one.

In opposite, H₂CO has about 2.3 times higher absorption in IR (around 2900 cm^-1) compare to that in UV (300-360nm). However both cross-sections are very low and require a long absorption pathlength (more than 5m) either in UV or IR for low-concentration H₂CO measurements. The last mentioned spectral range is popular in atmospheric long-path H₂CO measurements with use so-called DOAS-technique where few ppb detection limit can be achieved. Because molecular structure of H₂CO the 240-360nm spectral range is the only one, where electronic excitation from the ground to an excited (valence) H₂CO states can be observed. This spectral range is also free from H₂O and CO₂ absorption bands those are always in interference either with NH₂ or H₂CO bands in IR.

As one can see from the Fig. 10, H₂CO has much higher absorption cross-sections in far UV (below 190nm). Thus for example the first Rydberg state absorption band at 175nm has about 164 times higher absorption cross section than that in IR at 2800cm^-1. Because high water content in the stack gas (8-11%) measurements below 180nm require much shorter gas cell (length of 1 cm or less). Therefore H₂CO measurements in far UV have been performed with 0.39cm gas cell. N₂ gas from a gas bottle has been used for purge of all optical paths.

An example of far UV absorption spectrum measured in stack gas in natural gas combustion is shown in Fig. 12.

 

 

Although fitted O₂ and H₂O concentrations match the GASMET data there is no sign of H₂CO absorption feature at 175nm either in the stack gas or in the hot air with H₂CO(52ppm). Measured spectrum of stack gas around H₂CO 175nm is smooth and dominated by H₂O absorption.

Comparison of predicted H₂CO(52ppm) absorbance calculated with use H₂CO reference data at 20C with that in the long-wavelength part of the spectra in the Fig. 12 is shown in Fig. 13.

 

 

One can see that that the absorbance values for H₂CO and O₂+H₂O in the stack gas and hot air are at the same level as one can expect to see for H₂CO(52ppm). Moreover the FTIR measurements with 5.7 gas cell, Fig. 2 confirm H₂CO presence in the stack.

 

As it was mentioned above H₂CO absorption at 175nm is related to transition from ground state to the first 3s excited 1B2 Rydberg state with complex and diffuse structure. The structure of this band is found to be caused by a vibrational excitation in the HCH angle which has very shallow deep by less than 1eV. Therefore absence any signature of H₂CO absorption in the 170-175nm may probably attributed to 1) band shape temperature-related changes at 150C and/or 2) it fast dissociation due to increased perturbation with other 1A’ non-planar H₂CO state, resulting from 1B1/1A1 mixing. The mixing becomes probably stronger with temperature rise. This requires further investigations in the lab.

 

3. Conclusions from NH₃ and H₂CO UV absorption measurements at INERIS

There are few conclusions from NH₃/H₂CO measurements with use UV absorption spectroscopy:

• UV spectroscopy technique for NH₃ measurements in 184-230 nm can be used for low- and high- NH3 measurements in stacks. NH₃ measurements can be done on-line or/and in situ;
• apart from NH₃ other gases as for example O₂, SO₂, NO, H₂O can simultaneously be measured;
• soot concentration can also be measured with a modified set up;
• Possibilities for H₂CO measurements in far UV should be further investigated.