Sustainability involves more than monitoring and reporting emissions.
Understanding and controlling the entire process chain is crucial for prolonged maintenance and support. Through optimisation we directly increase product quality and profit margin. However an often overlooked, secondary effect is the reduction in pollutant generation. Considering combustion, one of the most important industrial processes providing heat and power for operation, the primary goals of plant operators are fuel efficiency and safe operation. Optimising this process not only increases these factors, but the generation of pollutants such as COx and NOx is greatly reduced.
Gas analysers based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) have been used for many industrial process control and emission monitoring applications, however the use of TDLAS for combustion analysis has always been a challenge. Balancing operator goals requires carbon monoxide (CO) measurements to be made across a high dynamic range. PPM readings need to be measured with high sensitiy for clean combustion, while safe operation relies on accurately monitored percentage CO levels.
To better understand this challenge, we elaborate on the TDLAS technology. Gas concentration is determined by scanning a laser with a known wavelength across a spectral region where the gas to be measured absorbs light. The laser is directed across the column of gas towards a photodetector, where the light emitted from the laser is processed and monitored. The gas concentration is calculated by function of absorbed light and known process parameters such as optical path length, temperature and pressure. With the gas light absorption proportional to the concentration, less light received by the photodetector translates to a higher concentration of the target gas present in the process column.
Considering the aforementioned spectral region, all gases have absorption lines at different wavelengths. When implementing TDLAS, it is crucial to select an absorption line that is not shared by gases that may be present in the process stream. When scanning for CO there are several available absorption regions to select from, however, each has its own limiting characteristics.
Geiser et al (2019)1 write: ”While the band around 1,5 μm is too weak to achieve the desired sensitivity, the band around 4,6 μm is too strong, and thus limits the upper boundary of the measurement range. This leaves the 2,3 μm band, where not only methane (CH4) has strong absorption bands, but also some water vapour (H2O) absorption lines are very strong at high temperatures.”
In order to combat this challenge, NEO Monitors has developed a single combustion analyser, combining a new signal processing technique with two lasers in a single compact unit. A single laser is responsible for measuring O2 and temperature, with the additional laser measuring the remaining process stream constituents CH4, CO and CO2. Additionally, the ability to monitor CO levels has allowed the inherent benefits of general TDLAS to be fully utilised across burner applications.
The in-situ cross stack measurement, when compared to extractive sampling, is superior for accurately representing the entire combustion zone, while fast response times are essential for detecting rapid concentration changes in the highly dynamic combustion process. In comparison to alternative technologies, TDLAS technology can continue to measure target gases without the presence of other gases, providing critical feedback on safe/unsafe operation, while ensuring optimum air-fuel ratio control.
NEO Monitors’ LaserGas iQ provides cost-effective and reliable fine control of the entire combustion process, contributing to a reduction in unwanted emissions, and most importantly, a safer environment capable of operating well into the future.
For more information contact Stephen Scholtz, RTS Engineering Africa,
[1] Geiser, P., Avetisov, V., Wang, J., and Sieker, l., ‘Smart combustion analysis’, Hydrocarbon Engineering, (December 2019), pp. 54 - 58.
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