Gas Concentration Measurements Using Photoacoustic Sensors for Industrial Solid Oxide Fuel Cell Process Control

Tunable diode laser spectroscopy (TDLS) has proven to be a suitable optical sensing system for the measurement of methane and water vapour concentrations in Solid Oxide Fuel Cells (SOFCs).  The unique advantages of TDLS include the ability to measure gases, including water vapour, in-situ and in real time, providing an ideal measurement system for large-scale fuel cell research and process control on an industrial scale.  In this paper we present a technique similar to TDLS, known as photo-acoustic spectroscopy (PAS), which is capable of achieving much higher concentration measurement sensitivities with reduced sensor dimensions, enabling the possible integration of remotely controllable sensors within the SOFC enclosure.  In order to achieve even greater sensor sensitivities, mid-infrared optical sources are required due to the higher probability of optical absorption.  Here we present our recent work on miniaturised 3D printed PAS trace gas sensors (see figure), which includes the use of a mid-infrared quantum cascade laser as optical source. The 3D printed sensors have outside dimensions of approximately 25x15x10mm, with the optical interrogation coupled to the sensor through optical glass fibres or direct free-space coupling.  The potential for PAS as a process control strategy for measuring SO₂ concentration during the desulphurisation of a natural gas SOFC fuel source is presented.  Results are also shown for PAS measurements of CH₄ and CO₂ concentrations in the mid-infrared and the near-infrared respectively, showing the capability of using PAS as a replacement for a GC in fuel cell research and process control.