Quantum Cascade Laser development reportedly represents a step-change in continuous emissions measurement performance over traditional technologies

Exhaust gas emissions monitoring has become increasingly important as environmental legislation tightens on shipping. Measuring such gases as NOx, SOx, CO2, CO, NH3 and H20 has traditionally relied on a suite of different technologies, including Non Dispersive Infra Red (NDIR) and Fourier Transform Infra Red (FTIR).

Poor sensitivity/selectivity combined with concerns over cross-interference and measurement accuracy, however, means that such technologies are struggling to meet industry demands, according to Iain Howieson, chief technology officer at Scotland-based Cascade Technologies. The development of the Quantum Cascade Laser (QCL), he reports, has recently been harnessed to create a range of innovative mid-infra red gas sensors.

Operating at ambient temperatures with high output powers and excellent spectral quality, the QCL sensor has opened up many new applications for laser-based gas sensing due to its compact size, robustness, sensitivity and low power requirements. The applications include emissions monitoring where recent trials of commercial products have demonstrated a step change in performance over established technology.

QCL gas sensors rely on infra red optical absorption spectroscopy to determine both the identity and quantity of gases. Absorption spectroscopy is a well used and understood technique which is exploited in many gas sensing technologies, including NDIR and FTIR. Using a low noise, narrow band optical source such as the QCL, however, yields valuable advantages, such as significantly improved sensitivity, excellent selectivity, immunity to cross-interference and fast response time.
Patented methods for further exploiting QCLs for gas detection, recently developed by Cascade Technologies’ researchers, have enhanced those merits, delivering wide spectral tuning, simultaneous measurement of multiple gases and the ability to execute up to one million gas measurements per second. These in turn secure an extended environmental operating envelope and immunity to turbulence and vibration.

Recently launched by the company, the CT2000 is claimed to be the world’s first commercially available compact and robust QCL-based gas sensor developed specifically for industrial applications.

A solid-state construction with no moving parts is said to ensure that both lifetime and robustness expectations in the field will be easily met. The lasers are semi-conductor devices not unlike those found in CD/DVD players and recorders. An extrapolated lifetime greater than 20 years is foreseen by Cascade which worked with several laser manufacturers and a telecoms leader to secure quality systems for QCL production.

Weighing less than 5kg, the sensor is shoebox sized and control is accessed from a centralised PC via either a private or existing secure network. The control PC can communicate simultaneously with any number of sensors, allowing large-scale infrastructure to be mapped out.
Each sensing unit itself can hold up to four individual lasers to facilitate full user-configurability in terms of gas species measurement and sampling rate at individual measuring points.

Data from each sensing unit is stored centrally by the control PC, but modbus communication to other PCs and/or dedicated data loggers is also facilitated, enabling a single and secure point of data collection for legislative compliance.
Cascade Technologies’ first market – marine monitoring – was targeted in conjunction with BP. Legislation governing the emissions of NOx and SOx from ships has stimulated the introduction of continuous onboard monitoring.

While the CT2000 can be configured for both extractive and in-situ measurement, health and safety concerns relating to the extraction of exhaust gases in enclosed spaces – combined with longstanding concerns on representative measurement with this technique – has favoured in-situ measurement as the methodology of choice.
Exhaust stack conditions including gas temperatures ranging from 100°C to 400°C, with external ambients up to 70°C, are typical.
Cascade says its sensor recently passed all the tests required for type approval for the marine continuous emissions monitoring market, these being carried out at the UK’s National Physical Laboratory. An example of the raw spectrum output from one of the tests is illustrated. Also shown is a spectrum recorded with a low resolution NDIR technique typically associated with more traditional technology.
The QCL gas spectrum clearly shows individual absorptions associated with multiple gases, including methane and nitrous oxide. This is in stark contrast to the conventional measurement which cannot pick out the individual gas signatures due to its inherent low resolution, Cascade points out.

Cascade further exploits the high resolution of its QCL source to derive gas concentration from theoretical first principles as set out by the Beer-Lambert law. This is demonstrated by the theoretical spectrum generation for the same wavelength region, also illustrated.
An excellent fit between measured and theoretical spectra enables both identity and quantity of complex gas mixtures to be derived from first principles. Since these principles do not change with time, calibration remains both fixed and repeatable from one sensor to the next.
The ability to directly relate the spectral output to long-established theoretical principles also removes the need for correction factors for cross-interference, linearity, temperature, pressure and zero and spin drift. This was evidenced, Cascade reports, by the recent award from the UK’s Maritime & Coastguard Agency of type approval certification; testing was performed over a period of four months without using correction factors or gas calibration.