Tunable diode laser absorption spectroscopy for CO₂ and H₂O concentration measurements in biomass combustion systems

Measurements of biomass combustion products are performed using tunable diode laser absorption spectroscopy. A distributed feedback quantum cascade laser probes transitions in the wavenumber range between 3682 cm^-1 and 3687.5 cm^-1. Two infra-red transparent calcium fluoride windows provide optical access to the flue gas, where temperatures range from approximately 100 °F to 700 °F. A 0.75 GHz free spectral range solid germanium etalon is used to obtain a continuous, accurate wavenumber calibration for the entire laser scan range. A calcium fluoride beam splitter directs part of the beam into a test cell for baseline monitoring, where path length, pressure, temperature, and concentration can be fixed. A line-by-line code simulates the absorption spectrum by utilizing the HITRAN database for spectroscopic parameters. Pressure and absorbing path length are held constant, while temperature and concentrations are varied in the simulation input. The best fit simulation gives results for concentrations of probed species. These concentrations of CO₂ and H₂O agree well with data from hand-held gas analyzers as well as theory.