A sensor for carbon monoxide measurement has been developed using a mid-infrared quantum-cascade (QC) laser operating in the fundamental band (?v= 1) of CO near 4.5 ?m. The fundamental band was chosen due to its stronger absorption line-strengths compared to the overtone bands near 2.3 ?m and 1.55 ?m. The mid-IR sensor was applied to a high-pressure combustor to determine the time-history of carbon monoxide and compare it to other measured combustor parameters to fully characterize the combustor's performance. A mixture of natural gas, oxygen, and carbon dioxide was used for combustion. An emissions probe was connected to the exhaust of the combustor that fed into an absorption cell with a known path length and two optical ports to allow mid-IR laser access for absorption. The temperature and pressure of the cell were measured, and the laser wavelength was monitored by a separate calibration process using a known CO/N_(2) mixture. The operating wavelength was measured at the R(12) transition of 2190.01667 cm^(-1) (4.56618 ?m) within ? 0.0008 cm^(-1) (? 0.01625?m) from CO absorption measurements using the known CO/N_(2) mixture. This operating wavelength uncertainty was used to measure the uncertainty of the calculated CO absorption coefficient. The mid-IR sensor measured a CO concentration of 67 ppm ? 12 ppm at a steady rig temperature and rig pressure of approximately 2300?F and 1500 psig, respectively. This measured CO concentration was comparable to equilibrium calculations with respect to the rig stable conditions.
The QC laser was also applied to validate the absorption time-history of a CO/Ar mixture at elevated temperatures and pressures in a shock tube at Texas A&M University. In addition to direct absorption measurements for CO in an absorption cell, the mid-IR sensor was also adapted to measure the hydrocarbon fuel concentration using a 3.39 ?m HeNe laser. This method was used to validate the laser for potential mid-IR laser diagnostics for unburned hydrocarbon (UHC) concentration measurements. A fiber-coupled mid-IR sensor method was proposed for the direct CO absorption measurements through the combustor exhaust liner. The spectroscopic parameters of CO were determined using HITRAN. The broadening behavior of CH_(4)-CO_(2) was experimentally determined on-site for mid-IR sensor calibration for hydrocarbon concentration measurements. The mid-IR sensor proved to provide successful CO time-histories during combustion at both transient and steady combustor conditions. The uncertainty in CO concentration can be reduced by eliminating condensation within the gas lines. The sensor also proved to capture the fuel flow variations while showing room for improvement in the broadening behavior. These tests using direct absorption spectroscopy (DAS) demonstrated the mid-IR sensor capability for species concentration measurements on a high-pressure combustor.
