The development of the quantum cascade laser (QCL) has led to the creation of a new generation of mid-infrared gas sensors. These open up new possibilities for laser based gas sensing thanks to their compactness, robust construction, superior reliability and high sensitivity, including the capability to measure in high temperatures and dusty atmospheres. Trials in real world applications have demonstrated a step change in performance over incumbent technologies.

The measurement of flue gases has become increasingly important with the introduction of ever more stringent requirements governing emissions (arising, for example, from Kyoto, US Clean Air Act and the European IPPC (Integrated Pollution Prevention and Control) and LCPD (Large Combustion Plant Directive).

The measurement of gases such as NOx, SOx, CO2, CO, NH3 and steam has traditionally relied on employing a suite of different technologies, including non dispersive infrared (NDIR), Fourier transform infrared (FTIR) and chemiluminesence.

Chemiluminesence techniques, for example, are commonly used to measure NO and can also indirectly measure NO2 by conversion. However, the technology cannot easily be applied to the detection of other gases and the requirement for extractive sampling and sample dilution, which requires the costly and time consuming installation of heated sample lines as well as raising concerns over representative measurement.

NDIR and FTIR lend themselves more readily to the detection of multiple gases (ie several gases at once), however, low resolution and poor sensitivity/selectivity combined with associated concerns over cross interference have prevented the widespread adoption of these technologies. Gas sensors based on quantum cascade lasers can overcome many if not all of these concerns.

QCL gas sensors rely on infrared optical absorption spectroscopy to determine both the identity and quantity of gases.

Absorption spectroscopy is a commonly used and well understood technique, which is currently applied to many gas sensing technologies, including NDIR and FTIR.

However, the use of a low noise, narrow band, optical source such as the quantum cascade laser, brings major advantages, including significantly improved sensitivity, excellent selectivity, immunity to cross interference and fast response time.

In the quantum cascade laser, first demonstrated at Bell Laboratories in 1994, electrons cascade down a series of quantum wells, created using very thin layers of semiconductor. Whereas a single electron–hole recombination, as used in conventional semicondustor lasers, can only ever produce a single photon, the quantum cascade laser electron can cascade down between 20 and 100 quantum wells producing a photon at each step. This “electronic waterfall” provides a step change in performance in terms of lasing efficiency.

New ways of further exploiting quantum cascade lasers for gas detection have recently been developed by Cascade Technologies, which afford the technology additional significant advantages including wide spectral tuning, simultaneous measurement of multiple gases, the ability to make up to 1 million measurements per second, extended environmental operating envelope and immunity to turbulence and vibration.

Typical outputs from the QCL gas sensor are shown in Figure 1. Industrially relevant gases such as SO2, NO2, N2O, H2S, CH4, CH2O and H2O are all recorded with excellent sensitivity and discrimination and the measurement only takes a microsecond.

Figure 2 shows a comparison between quantum cascade laser and NDIR spectroscopy. The multiple absorption features for each gas in the case of QCL spectroscopy provide excellent fingerprinting whilst significantly enhancing the instrument’s dynamic range. The sensor can be configured to operate in both closed path (sample line) and open path measurement for applications where there are known concerns over representative sampling such as stack monitoring.

Field trials and commercialisation

A system for marine application will be offered commercially in January 2007, following extensive sea trials.

For the power generation sector, Cascade Technologies has recently signed a distribution agreement with Clyde Bergemann Power Group. This will also include further development work, with Clyde Bergemann injecting funding during a preliminary six-month marketing and test site evaluation phase.

This will see the system being tested at a number of power stations in the UK, as from next year, with Clyde Bergemann also providing marketing intelligence and performance feedback. Following successful completion of the trial period, both parties will enter a commercial supply agreement that will give Clyde Bergemann an exclusive licence to market and distribute the system throughout the world – excluding USA, Canada and Mexico.

During 2005 and 2006, a quantum cascade laser based gas sensor configured to measure SOx, NOx and CO2 underwent development trials on an industrial stack. Traditional extractive sampling techniques have raised several concerns, including point of extraction, adsorption through heated sample lines and sample conditioning, which has left a question mark over representative measurement. In order to address these concerns the quantum cascade laser gas sensor was configured to measure directly across the stack.

Figure 3 shows the sensor hardware configuration used for across-stack measurements. The stack was modified to allow the QC laser head and detector to be set up diametrically opposite to one another. Factory pre-alignment and simple alignment protocol instructions allowed full installation to be carried out by a plant engineer in less than one hour. No drift or signal degradation was observed during the trial.

The NOx stack sensor was also configured to record water vapour simultaneously. Time evolutions for all three gases are shown in Figure 4. A clear anti correlation between water and NO concentrations is recorded. It is also noted that fast transients (< 5 second duration) are observed, such as those shown in the NO2 data. Such transients would not be seen by incumbent technologies where response times of the order of 5 minutes are typical for ppm detection sensitivities.

The sensor’s ability to record gas concentrations at up to 1 million measurements per second provides significant measurement head room that enables a single sensor to service multiple stacks simultaneously. A fibre coupled sensor (Figure 5) has also been developed, which can measure up to eight individual points (stacks) in real time. This should bring significant advantages in cost/reliability combined with significantly reduced installation overhead.

The technology’s capabilities have recently been demonstrated during UK Department of Trade and Industry (DTI) funded testing at the National Physical Laboratory (NPL) MCERTS facility. The tests included assessing the instrument’s linearity of response, susceptibility to cross interference, reading drift, instrument noise and inter-comparison between recorded and theoretically predicted data.

These initial sensor tests, measuring N2O and CH4, have proved to be very encouraging. For example instrument response with known determinand concentrations showed an excellent linear response. Sensor drift was also assessed by passing a fixed concentration of determinand gas (N2O) through the sensor for a period of 4 hours. The reading drift during that period was less that 2 ppb. Finally, for the ranges tested, the maximum effect observed for cross sensitivity due to interfering substances was less than 0.2%, comparing extremely favourably with the 4% MCERTS requirement.

Cascade’s instrumentation is currently being processed through the QAL1 stage of MCERTS with an expected completion of the certification process by the second quarter of 2007.

Step change in performance

The development of QCL based gas sensors has reached maturity. The benefits of high sensitivity and fast measurements coupled with multiple measurement points for numerous species in a single, robust and easily calibrated system requiring little maintenance is now a reality. These instruments have demonstrated a step change in performance over incumbent technologies.

Figure 1. Recorded spectra of various gases (SO2, NO2, H2S, CH4, CH2O and steam)
These spectra highlight the excellent signal to noise ratio and selectivity that can be achieved with the QCL based system Figure 2. Comparison of quantum cascade laser and NDIR spectroscopy Figure 3. Spectrometer hardware configuration used for direct across-stack measurement. The QC laser head and detector were set up diametrically opposite one another enabling concerns over representative sampling to be removed Figure 4. Time evolution for NO, NO2 and water shows a clear anti-correlation between water and NO within the stack.
Fast transients are also observed, which would not be picked up by conventional technologies Figure 5. A fibre coupled analyser will allow up to eight stacks to be measured simultaneously in the harshest of environments. The sensor uses a fully ruggedised probe to enhance reliability and minimise maintenance requirements, even in high temperature