Thermal pumping gas viscometer for Wobbe Index determination

Abstract

Natural gas is defined as a combustible mixture of hydrocarbons coming from ground deposits and it is mainly used as a source of energy for burners and motors. Its consumption is increasing worldwide, as this represents a cheap and abundant source of energy, while releasing less CO2 emissions than other fossil fuels, in addition to low emissions of particulates. The diversification of the supply sources of gas and the increase of international trading through pipelines and liquefied natural gas (LNG) shipments makes the gas quality vary more than before, which is a rising challenge for the natural gas industry, as currently the vast majority of gas appliances and motors are unable to adapt to changes in gas quality. This will even worsen in the upcoming years with the multiplication of biogas sources and power-to-gas systems that will be added to the pipeline networks. In order to accomodate the appliances to changes in gas quality, compact and low cost natural gas quality transducers have to be developped for the natural gas industry. The present Thesis aimed at investigating the existing methods to determine natural gas quality for combustion, focusing on the determination of the Wobbe Index (WI), which is a key parameter to set up the air-fuel ratio of the combustion in burners and motors, and to qualify a new type of dynamic viscosity transducer developped at Laboratoire de Production Microtechnique (LPM) of EPFL for the determination of the combustion properties. Inference transducers are a new kind of transducers that allow determining the combustion parameters of the gas by relating to fundamental thermo-physical parameters, and not through the determination of the concentration of the gas constituents, as in gas chromatographs and spectrometers. Here, the dynamic viscosity is chosen as the most promising thermo-physical property of a natural gas to relate to the WI and the Higher Heating Value (HHV). The viscometer takes measurements proportional to the resistance to flow of a gas through a capillary under the action of pulses of heat, and it can be therefore be described as a thermal pumping gas viscometer. A mathematical model and a finite element model (FEM) are used to predict the behavior of the system and for thermal optimization, which are combined by a thermal characterization of the system. The miniature heater used to generate the heat pulses inside the viscometer is a key element of the transducer, and after reviewing potential technologies, the design choices are reported and the fabrication process is exposed, followed by a mechanical and thermal characterization. In order to determine and validate the conversion of the WI from the dynamic viscosity, theoretical relations are analyzed and empirical characterizations are done with a test and calibration setup allowing to create given mixes of gases. Finally, the degradation of the transducer to contaminants of natural gas is investigated, and the analysis is focused to the characterization of the corrosion by the sulfur compounds of natural gas. Test vehicles are developed to study the effect of sulfur corrosion on metallic parts, electronic substrates, silicone adhesives and wirebonds materials, and experiments are held with the dedicated test setups developed at LPM of EPFL.