A multi gas analyzer for in line composition evaluation has been developed and optimized for the detection of fuel gas main components. The system has been designed for unmanned and remote operation on natural gas pipelines and biogas production plants as a low cost and low maintenance alternative to current industry standard gas chromatographic methods which are used to evaluate the mixture heating value as well as to detect undesired contaminants. The system does not require any consumable supply and could also be considered as a competitor of most available non-analytical techniques for the determination of heating value, which in our case can be obtained from actual gas composition through application of the relevant calculation as described by ISO 6976 norm.
A high-power, lighting grade multimode laser diode pump has been used to reduce both manufacturing and operation costs as well as power supply requirements. Sample gas is flowing in a cell rated for high pressure operation and the spectral analysis is performed by a custom design, high throughput lens based spectrometer developed for this application featuring an industrial CMOS camera as a low cost, high sensitivity and low noise focal plane array.
The use of a multimode pump operating over a wide temperature range and a non-cooled detector array are the source of several issues in the recorded spectra which have been addressed with the developement of custom image processing and fitting software routines to cope with broadened, drifting Raman spectra. This approach has been lab calibrated against the single components (such as methane, ethane, propane, n-butane, iso-butane, nitrogen, oxygen, water vapour, carbon dioxide, carbon monoxide, hydrogen) and tested with certified gas mixtures made to simulate the concentration levels commonly found during field operation at total absolute pressures ranging from 1 to 6 bar.