Oil and gas infrastructure is a significant source of emissions of methane and ethane, both of which are greenhouse gases. These emissions occur throughout oil and gas production processes, from extraction to transportation, due to unintentional leaks and intentional venting. Such releases not only intensify climate change – methane has a global warming potential more than 80 times that of carbon dioxide (CO₂) over 20 years (GWP-20) – but also represent a significant loss of valuable resources. These considerations make efficient methane and ethane leak detection systems indispensable. Such systems must be capable of rapidly identifying and locating leaks and providing operators with high-accuracy real-time data. By meeting these requirements, the risks associated with gas transport and storage can be mitigated, the environment and public safety protected and regulatory compliance ensured.

Why simultaneous detection of methane and ethane is important

The simultaneous detection and quantification of methane and ethane is essential in determining the origin of these gases. Biogenic methane is derived from natural sources such as wetlands, while thermogenic methane is produced from fossil fuels. In contrast, ethane is almost exclusively generated from fossil fuels. Therefore, a higher-than-expected ratio of ethane to methane can indicate that the methane emissions primarily originate from fossil-fuel sources rather than natural sources →01. A simultaneous measurement of both gases can determine this ratio.

Off-axis but on-target

ABB provides a wide range of gas analyzers based on unique and proprietary off-axis integrated cavity output spectroscopy (OA-ICOS) technology. OA-ICOS represents the fourth generation of cavity-enhanced tunable diode laser absorption spectroscopy (TDLAS). This technology has revolutionized the detection and quantification of gases. More specifically for natural gas leak detection, the laser-based analyzers offered by ABB can be deployed quickly to identify and quantify gas leaks in the field. ABB is the first and only company offering four different solutions for detecting, finding, quantifying and mapping leaks of natural gas while driving, walking, flying or stationary →02.

These ABB gas analyzers are based on so-called non-mode-matched optical cavities, in which only a very small amount of the laser light enters the cavity to interact with the gas sample. The research project described here aims to increase the amount of light entering the cavity – and thus available for measurement purposes – by using an optical reinjection technique. The reinjection arrangement will:

• Replace costly high-power laser sources with cost-effective and low-power alternative
• Enhance the signal-to-noise ratio, improving instrument precision to expand the sales volume and available marke
• Enable use of higher reflectivity cavity mirrors, thus increasing the optical path length and thus measurement sensitivit
• Tolerate a wider range of mirror reflectivity values, thus increasing allowable mirror manufacturing margins.

Simulate, design, build and test

The development and realization of optical reinjection in an OA-ICOS analyzer was carried out in several steps. Firstly, ABB developed an optical simulation tool to establish optimal and feasible configurations. Secondly, the team designed and set up an interband cascade laser (ICL)-based OA-ICOS analyzer that demonstrated optical reinjection. The ICL produces the wavelengths of interest for the spectroscopic examination of the gases of interest. The demonstrator is based on a commercially available ABB GLA231 series analyzer →03, in which key elements such as the laser source, the photodetector, the cavity mirrors, the collection lens and the reinjection optics, were replaced by custom versions appropriate to the aims of the project. 

Various optical reinjection scenarios suggested by the simulations were experimentally investigated and verified. Measurements of precision, accuracy, linearity and cross-interferences for methane, ethane and water were performed with and without optical reinjection. Although the optical reinjection technique is implemented in the methane/ethane OA-ICOS gas analyzer in this project, it can be applied to the whole ABB OA-ICOS platform for various applications.

Three mirrors and a cavity

The OA-ICOS analyzer with an optical reinjection system consists of an ICL laser source coupled to an optical cavity with two high-reflectivity mirrors, a photodetector to measure and monitor the signal, the laser control electronics, the data acquisition and processing system and a reinjection mirror →04.

Initially, the laser beam passes through a small hole in the reinjection mirror and enters the first cavity mirror. The interior of the “exit” side of this mirror is highly reflective (99.99 percent) so most of the light is reflected internally, back towards the reinjection mirror, where it is reflected back to the cavity mirror. Since this three-mirror configuration is optically stable, this process repeats itself continuously. Only about 0.01 percent of the available light enters the cavity at each pass. The light that does enter the cavity ‘bounces’ between the cavity mirrors in an off-axis manner. A small fraction of the light leaks out through the rear cavity mirror, giving rise to the ICOS signal that indicates the concentrations of the various gases present in the cell. The output light is then focused onto a photodetector with a suitable collection lens. The gas sample is pumped continuously through the cavity using a vacuum pump and the pressure inside the cavity is controlled and measured. The temperature inside the cavity is measured with a temperature sensor.

The optical system must be carefully designed, particularly with regard to the reinjection mirror. Further, given the space limitations of the final product, the dimensions of the reinjection setup are critical. To design an appropriate optical reinjection setup, ABB used an optical simulation software called Zemax that integrates all the features required to conceptualize, design, optimize, analyze and document any optical system. Zemax is widely used in the optics industry as a standard design tool.

The simulation and experimental setup are configured identically, having the same mirror curvatures, clear apertures, reinjection mirror hole size and offset, beam diameter and divergence, collection lens surfaces and detector size →05. The efficiency of optical reinjection is related to the position of the hole, the distance between the reinjection mirror and the cavity, the curvature of the reinjection mirror and the incident angles of the laser beam. In the end, ABB experimentally tested and verified six reinjection mirrors with various cavity-mirror distances.

A spectrum of results

→06a compares the signal with and without optical reinjection (by simply removing the reinjection mirror and keeping the laser alignment unchanged). Here, the alignment of the standard ICOS system may not be optimal in terms of signal amplitude and the signal-to-noise ratio. The comparison results after re-optimizing the standard ICOS (without the reinjection mirror) by adjusting the laser beam direction are shown in →06b. The enhancement of the signal amplitude is around a factor of four with optical reinjection. However, as shown in the optical simulation, more reinjection power is expected and can be obtained by further tweaking the incident angle of the laser beam →06c, though this significantly increases optical noise.

The concentration of the gas samples and the measurement precision for a given period can be obtained by fitting the spectra. First, the measured time-dependent transmission spectra must be turned into frequency-dependent absorbance spectra. A physics model is then fitted to the recorded spectra, using species-specific spectral data from the HITRAN database (a molecular spectroscopic library) as input. The proportionality between the area of the fitted line shape and the directly measured parameters (ie, gas temperature and pressure) allows direct inference of the gas concentration. →07 shows the absorbance spectra measured in ambient air, where the absorption from methane, ethane and water vapor can be observed. In the case of optimized optical reinjection (upper curve in →06b), the demonstrator is capable of reporting methane and ethane concentrations in ambient air continuously with a precision of 10 parts-per-billion (ppb) for methane and 15 parts-per-trillion (ppt) for ethane over one second of measurement time.

Reinjection perfection?

Various optical reinjection configurations have been simulated, set up, optimized and characterized to construct a mid-IR OA-ICOS analyzer for the simultaneous measurement of trace-level methane and ethane. Compared to incumbent methods, the precision achieved for the ethane measurement is enhanced by three orders of magnitude as a result of employing optical reinjection, a new laser source and optimization of the mirror and detector. With this high precision, the discrimination between thermogenic and biogenic sources in the field can be improved significantly. Based on gas dispersion simulations, the leak attribution accuracy can be improved to nearly 98 percent across all the simulated leak rates and conditions. This improvement reduces the time surveyors spend investigating biogenic emissions, allowing them to focus on actual natural gas emissions, thus enhancing the safety of natural gas grids. In addition, optical reinjection can be leveraged for the entire ABB ICOS product platform to improve overall performance on applications requiring high gas measurement sensitivity.