CEM - Abstract

Abstract Title: Continuous in-line measurements of gaseous species at low concentrations using quantum cascade lasers (QCLs).
Presenter Name: Dr Giuseppe Smirne
Co-authors:Dr Paul Black
Dr Ruth Lindley
Dr Beth Livingstone
Company/Organisation: Emerson Automation Solution
Country: United Kingdom

Abstract Information :

Continuous in-line monitoring of the concentration of gas emissions is an effective way to ensure that a process output is in compliance with industrial regulations. At Emerson we develop techniques for the continuous measurement of multiple gases that absorb in the mid-infrared between 2.5 and 11 microns. Our measurement technique is based on direct absorption spectroscopy performed within microsecond long QCL pulses, each covering spectral windows of up to a couple of cm-1 and our instruments can fit up to 6 different laser wavelengths on one single unit, effectively being able to offer the monitoring of gas concentrations for between one and ten molecules of interest in CEMs applications. We take advantage of their unique spectral fingerprint in the mid-IR to measure, in a variety of ranges, molecules such as HCl, HF, NH3, SO2, CH4, N2O, CHOH, NO, NO2, CO, CO2, H2O and O2. Each instrument combines the flexibility of selecting the absorption path length and, in many cases, being able to access a variety of spectral lines for the same molecule, each with different absorption strength, to effectively achieve the required measurement ranges and sensitivities for multiple species in a single unit. The measurement cell can also be heated up to ~200C allowing for hot-wet measurements of samples whose composition may be compromised if cooled and dried prior to analysis. We present here the general principle by which we measure multiple gases in one stream and how we deal with cross interferences. Our measurement is based on a residuals minimization technique that fits, in real time, the acquired gas spectra with the simulated spectral profiles. The fitting spectra model the gas composition based on several, fixed or monitored, external parameters. Overall, these combine to offer a fast and accurate method for in-line monitoring of the concentration of the sample composition using instrument configurations that require low maintenance and offer long life cycles.