Nov-2022

Process gas analyser for measuring hydrogen concentration

A process gas analyser, based on near-infrared tunable diode laser absorption spectroscopy, has been developed to measure hydrogen in production environments.

Airat Amerov and Michael Gaura
AMETEK Process Instruments

Article Summary

The human population of planet Earth is expected to reach eight billion by the end of 2022 or early in 2023, depending on the reference source. This means that, since the birth years of the authors, nearly twice as many people are occupying the same amount of space. However, most of us do not realise that, in the same time frame, global energy consumption has tripled. Thankfully, energy suppliers have continued to adapt and provide us with reliable and safe power, as well as heating, cooling, and transportation fuels, as our population and energy consumption habits have increased. As our understanding of the environmental impacts of the increasing energy requirements has evolved, a focus has been placed on reducing carbon utilisation and release from energy suppliers as well as end users.

One ongoing transition that has secured billions of dollars in investment and spending is replacing some portion of fossil fuel or coal-based energy sources with hydrogen. Recognising that there are a variety of processes to produce hydrogen (the ‘rainbow’ — green, blue, brown, grey), this article focuses on a technology (tunable diode laser absorption spectroscopy (TDLAS)) that can be used to analyse the concentration of hydrogen in almost any method of hydrogen production, transportation, and storage.

A new process gas analyser, based on near-infrared TDLAS, was developed and tested for the measurement of hydrogen in production environments. TDLAS is a non-contact optical technique with long-term stability, high specificity, and considerable sensitivity. TDLAS has been proven for several decades in many energy production and emissions monitoring applications. The low operating expense (OpEx) — no consumables, long-life optical components, and minimal maintenance requirements — has driven its acceptance as a preferred measurement technology.

Measurements of hydrogen were made with this gas analyser in sample matrices corresponding to nitrogen. Reliable performance was demonstrated over a wide range of analyte concentrations and under a variety of pressure levels in the sample cell of the analyser. The hydrogen measurements yielded an accuracy of 2% full-scale range over a concentration range of 0-100%.

The principal objective of the work reported here is to characterise a new TDLAS-based extractive analyser with an all-digital protocol for the modulation of the laser drive signal and the demodulation of the detector response. The analyser was configured for hydrogen measurements in nitrogen, with an environmental temperature range of -20°C to +50°C, and a sample cell pressure range of 10-25 psia.

Instrument design
The TDLAS instrument evaluated in this work was an Ametek 5100HD analyser (see Figure 1) which was modified to operate with a multi-pass Herriott cell, having an optical path length of 20 metres and a volume of 1 litre.

A schematic representation of the instrument is shown in Figure 2. The measurement of hydrogen was performed with a distributed feedback (DFB) laser. This laser produced an optical power of approximately 3 MW, and optical attenuators were used to reduce the output power to usable levels. The output of the laser was coupled into single-mode optical fibres, which in turn were connected to a fibre-optic beam splitter. The splitter was used to divide the optical power in a 50/50 ratio for use in the sample and reference measurements, respectively. Gradient refractive index (GRIN) lenses, with a beam divergence of 1.8 mrad (milliradians), were used to collimate the output of the single-mode fibres and direct the resulting beams through the sample and reference cells. The sample and reference cells each contained 0.5 mm2 InGaAs-photodiode detectors, which were connected to separate input channels of the electronics unit. With this configuration, it was possible to make simultaneous measurements of unknown samples and known references, which were used to lock the output wavelengths for both lasers. 

The sample cell temperature was controlled with an accuracy of +/- 0.1°C and could be set in the range of 60°C by setting the temperature of the oven in the sample cell compartment. The reference cell is not located in the sample compartment, but the temperature was maintained above 40°C. The laser and photodiodes were also located in the main electronics compartment, isolated from the heated sample oven.

The wavelength modulation spectroscopy (WMS) experiment was implemented by using a digitally sampled sine function, summed with a staircase, and the resulting signal was used to drive the tunable DFB laser diode. Signals produced by the detectors were digitised prior to applying signal processing (such as phase-sensitive detection and smoothing). In contrast with the common practice of using second harmonic detection (2F), the detection/demodulation in this analyser was performed at the laser-modulation frequency (i.e., ‘1F’ detection). Using the 1F-detection scheme enabled the normalisation of the spectra without the need for a separate measurement of the laser power. Specifically, the magnitude of the power envelope of the laser output is contained in the spectra produced by 1F demodulation. After the 1F spectra were normalised, they were differentiated; the resulting derivative spectra approximate the second derivative of the absorption spectrum of the analyte and were referenced as 2F signals in this work.

The scan parameters for the laser (such as injection current range, modulation depth) were set to match the desired wavelength range required to cover the width of the ro-vibrational transition. Line width was determined from hydrogen data published in spectral libraries. The wavelength-locking algorithm employed by the instrument was based on two nested levels of temperature control employed to maintain the operation of the laser diode at the proper wavelength. The first level was a simple PID control loop, which maintained the laser at a target temperature. In the second level, the outer control loop, the spectra of the analyte samples in the reference cells were monitored. Minor shifts in the observed peak positions were used as a feedback signal for the temperature set point of the inner control loop. Thus, the outer control loop provided a fine adjustment for the inner control loop.

Wavelength selection for hydrogen measurements was dictated by the spectral position and intensity of the hydrogen absorption lines in the near-infrared range, the requirement of minimal spectral interference with other components of the gas stream at a refinery or ammonia production plant (as examples) and, of course, by the availability of laser diodes. From this point of view, the hydrogen ro-vibrational spectral line was the most attractive for measurements. As was demonstrated earlier by others, this line is not only the strongest line in the fundamental vibration band of hydrogen electric-quadrupole transitions, but is also the line with minimal spectral interference with methane, ammonia, and carbon dioxide. In addition, this region of the spectrum corresponded to high sensitivity for extended InGaAs detectors.