NOx Monitoring in Humid Exhaust Gas Using Non-Dispersive Infrared Spectroscopy

This PhD thesis is concerned with the measurement of NOX in moist exhaust gas onboard ships using non-dispersive infrared (NDIR) spectroscopy. In such a measurement one of the major challenges is spectral interference from water vapour which is present in high concentrations in the exhaust. The PhD study investigates a possible solution to this problem, which is to balance out the signal contribution from water vapour by means of carefully designed and manufactured optical bandpass filters.

The thesis, presents a thorough theoretical description of the NDIR sensor concept together with simulations suggesting that it is possible but challenging to measure NOX in moist exhaust gas using NDIR. The characteristics of optical filters tend to change with temperature, and since this compromises the water signal balancing, much of the work presented in the thesis is devoted to the design of optical bandpass filters in general, and temperature invariant filters in particular.

This work has led to the derivation of the thermo-optical expansion coefficients of embedded PbTe and ZnSe thin films, frequently used in optical bandpass filters for the mid-infrared range. Knowledge of these coefficients allows accurate prediction of temperature invariant filters, previously realised based on experience and trial and error.

Finally, the absorption spectrum of CO2 at elevated temperatures and pressures has been experimentally investigated, and the absorption cross section of CO2 at 1000 K and 100 bar is revealed for the first time. Information about CO2 absorption and emission in combustion conditions is highly demanded for accurate modelling of heat transfer processes during combustion in large ship engines which in turn is crucial for understanding the formation of emission gasses, in particular NOx.