Capabilities and challenges of optical measurements Jane Hodgkinson Centre for Engineering Photonics School of Engineering Cranfield University

Why use optical absorption? gas light source detector window

Non-dispersive infra-red (NDIR) sensing Broadband light source is optically filtered to target particular gases Commercially available components; filters can be specified Small footprint (20mm D x 16mm H) Typically configured for CO2 * 20 mm 16.5 mm 3 3.2 3.4 3.6 3.8 4 4.2 20 40 60 wavelength / μm gas absorption / cm-1 filter transmission / % CH4 absorption band measurement filter reference filter * Hodgkinson J, Smith R, Ho WO, Saffell JR, Tatam RP. Sensor. Actuat. B 186, 580– 588, 2013.

Potential for discriminating alkanes 60 methane ethane 40 propane butane absorption / cm-1 pentane 20 hexane 3.1 3.2 3.3 3.4 3.5 3.6 wavelength / μm 6 Lower hydroacrbons can be discriminated (C1-3) Higher hydrocarbons cannot so easily Total absorption corresponds to C-H bonds - good correlation with CV for C2+ 2 point measurement would give CV Full spectrum would also give full information 4 Total absorption / arb 2 1 2 3 4 5 CV / 103 kJ mol-1

Tunable diode laser spectroscopy (TDLS) Laser emission wavelength may be tuned over a narrow range, enabling the gas spectrum to be measured in a very small region Near IR region uses telecomms components (lowest cost) 1.62 1.64 1.66 1.68 1.7 50 100 wavelength / μm transmission / % This gives: Gas specificity Immunity to baseline drifts Calibration stability Configurable for accuracy or sensitivity

Potential for discriminating C1-C3 Methane + ethane measurement previously implemented in this region[1], accuracy for %LEL measurement of NG was as predicted from gas compositions (± 3%) [2] Methane, ethane, propane demonstrated in principle by Siemens [3] Width of single laser scan methane ethane 0.2 propane absorption / cm-1 butane pentane 0.1 hexane 1.680 1.685 1.690 wavelength / μm [1] Hennig O, Strzoda R, Magori E, Chemisky E, Trump C, Fleischer M, Meixner H, Eisele I. Sensor. Actuat. B 95, 151-156, 2003 [2] Hodgkinson J, Pride R D. Meas. Sci. Technol. 21, 105103 (10pp), 2010. [3] Fleischer M, Strzoda R, Magori E, Meixner H, Hennig O. EP1174705, 2000 (ceased 2007) Bachmaier G, Hennig O, Magori E, Meixner H, Strzoda R, Tump C. EP1447657, 2004 (withdrawn 2010)

Commercial implementation Geotech – ppm methane detection in landfill gas (high CO2 matrix) Use of long pathlength cell and 1.65μm laser, lod ~ 1ppm Kannath A, Hodgkinson J, Gillard RG, Riley RJ and Tatam RP Proc SPIE 7952, 7942-14, 2011. GMI / Strathclyde NDIR with 3.3μm LED Longer path (12 cm) gives lower limit of detection than needed for this application (100ppm) Massie C, Stewart G, McGregor G and Gilchrist JR. Sensor. Actuat. B 113 830–836, 2006.

Summary – capabilities and challenges Detection capabilities Good signal to noise ratios, long-term reliability, field proven technology Possible to predict and model response to gas mixtures Specific measurement of C1-C3 using laser spectroscopy With NDIR, inferential measurement of CV NDIR can also measure CO2 levels Challenges for gas quality measurement How to measure total inert gases (N2) How to predict / validate performance if compositional variation is not known or specified (this is particularly a UK issue)

Way forward? Test measurement strategies on paper Requires good data on potential gas compositions Industry-wide consensus on required performance Needs engagement from all stakeholders (safety, regulation, operators) Translation of errors in CV, Wobbe index etc into errors in measurands, via knowledge of compositional variation Key step to enable instrumentation companies to invest Approach has been used previously in UK for %LEL accuracy Methane – only measurement found to be inadequate Goal-setting company standard produced, test compositions identified Performance of optical technologies predicted and validated experimentally; methane + ethane gave required performance