New advances in folded pathlength technology for Process Tunable Diode Laser Absorption Spectrometers (TDLAS)

New advances in folded pathlength technology for Process Tunable Diode Laser Absorption Spectrometers (TDLAS) Jean-Nicolas Adami, PhD Head of Strategic Product Group Gas Analytics Mettler-Toledo GmbH, Process Analytics

1 Introduction 2 Pro's and Cons of conventional Gas Analysis Technologies 3 Tunable Diode Lasers Absorption Spectrometers (TDLAS) Principles 4 TDLAS: Bridging Accuracy with Compactness 5 Summary & Conclusion

Gas Analyzers are at the crossing point of critical plant operations topics: Less process downtimes Enhanced Safety In-Situ Ability Reliability Minimized Maintenance

Pro's Good price/performance ratio Single flange, In Situ probe design makes installation easier Can be fitted with combustion (COe) sensor for coarse trim control Designs allow long probes and high-temperature alloys Con's Cell catalyst electrodes can be poisoned by reducing conditions, sulfurs in low quality brown coal, increasing response times Not very accurate, therefore must use high safety margins; Loss of sensitivity over time (slope) High temperature sensor can provide an ignition source Diffusion types have to be removed from process for cell replacement Flammable gases (i.e. methane, alcohol, CO), can cause combustion resulting in large measurement errors Limited tolerance to pressure; - 5 to + 5 kpa gauge

Catalytic "Pellistor" Combustion Sensors 4 1 3 5 2 1.Resistor element 2.Ceramic coating 3.Catalyst 4.Sample flow 5.Exothermic reaction 6.Resistance changes due to heating causing change in electrical current 6

Pro's Low-cost Single, In -situ probe design makes installation easier Can be fitted with combustion sensor for rough trim control Designs allow long probes and high-temperature alloys Con's Pellistors not perfectly matched Radiative heat losses will be different Dirt in sample builds up on sample pellistor but not on reference = drift Simple catalyst easily poisoned or inhibited Results in crude measurement eg 0-2500ppm ± 125ppm Does require regular calibration (Weekly) and maintenance Measurement of COe (equivalents) only, i.e. unspecific for CO CO catalytic sensors do not work below 0.5% Oxygen concentration (API RP 556-2007)

Paramagnetic Analyzers There is a material constant which tell us how strong this effect is, the magnetic susceptibility : M = H < 0: diamagnetic gases; these were repelled from a magnetic field > 0: paramagnetic gases; these are attracted to a magnetic field Liquid Oxygen Across A Magnet

Pro's Accurate and reliable if sample system maintained Can be repaired without process interruption Sample system usually remote, at ground level, easy access Readings stable since not exposed to process dynamics Works well at lower O 2 levels if controlled pressure, temp and flow Con's Sample conditioning system creates maintenance headaches Extractive technology - so no fast with dynamic process data Cells can be damaged by ingress of water or particulates or pressure surges (" sudden death") Relatively slow response, even slower with long sample lines Subject to interferences Cells can not be cleaned or repaired

Dual Beam NDIR Analyzers Reference beam to compensate for light source decay Still not able to differentiate between dust and concentration changes Needs ultra clean sample Venting to air or header Sample Cell I.R. Detector (sample) I.R. Source Beam splitter From sampling system Sealed N2 cell I.R. Detector (reference)

Pro's Multi-gas capable Accurate & Reliable if sample systems are maintained Large selection of possible gases: CO, CO 2, CH 4, Hydrocarbons (HC's) (not O2) Multiple measurement possible with optical filters Con's Sample conditioning system creates maintenance headaches Extractive technology Slow response-typically Instrument 10sec (T 90 ) + sample system lag Higher maintenance - Usually has motor and hot wire IR source Large foot print Does require regular calibration and maintenance

Inherent Benefits of TDLAS Reliability: no Cross interferences, no drift Minimized maintenance: no calibration, no sample conditioning In-situ measurement ability: fast, representative, no shelter footprint required

Light Absorption Cause & Effect Symmetrical stretching Anti-symmetrical stretching Scissoring Rocking Wagging Twisting

Tunable Diode Lasers Diode LASER (Light Amplification by Stimulated Emission of Radiation) NIR to MIR Laser Safety Class 1M => eye safe TDLs can be "tuned" to emit light at a very narrow wavelength band This wavelength scan band can be programmed, within a certain range

Cross-Stack TDLAS in small pipes Possible self-dilution in bypass configuration Extension arms prone to vibrations

Pro's Originally designed for long path length applications, i.e. combustion, DeNOx & emissions monitoring Average measurement over whole path length Ability to measure in directly large combustion chambers without possible air leakage errors Longer path lengths improve the detection limit Con's Process Applications tend to be more challenging Needs alignment, and re-alignment after verification Challenge to keep alignment with varying temperatures or with vibrations Self dilution effect when installed in bypass (Beam parallel to flow) Generally limited to larger pipes >DN300 High purge gas consumption >50 l/min Flanges need to be aligned too Can be large and heavy needing re-enforced mounting to maintain alignment Double costs for scaffolding, platforms..

Cross stack TDLAS Two flanges, mounted opposite to each other Purging and wiring necessary on both sides Accurate (+/-1 ) alignment of welded flanges required Probe-type, folded-path TDLAS Only one flange, easy/fast installation Purging and wiring on one side No alignment of system by user Regular re-alignment of system after verification No re-alignment after verification Short paths difficult, 0.3-0.5m minimum High purge consumption, 20-120l/min Possible with very short (folded) paths Very low purge consumption, 1-5 l/min

Bridging Compactness with Precision Can be fitted between analyzer and process interface, but not in contact with process gas Prism shape determines Multi-reflexion factor: 2, 4, 8 or 16 Beer`s law: I I o exp [ S T ) G(, P, T ) L ( n ] S : Line strength G : Line profile L : Optical pathlength n : density concentration

TDLAS: mature, versatile analyzers TDLAS have the disruptive power to replace conventional technologies for low TCO Their maturity resides in their inherent performance but also they have significantly enlarged their range of possible applications