A Miniaturized Calorimeter as Sensor for Determining the Calorific Value of Natural Gas

摘要

As a consequence of the liberalization of the gas market in a lot of countries the need for miniaturized devices for the determination of heats of combustion of natural gases is strongly increasing. Classical combustion calorimeters for gases like the CUTLER-HAMMER calorimeter are extremely expensive. Known miniaturized devices that are useful in a sensor like manner are based on indirect methods (e.g. infrared absorption) or measure thermal effects of the catalytic oxidation of natural gas more or less quantitatively [1, 2]. In the presented work a first miniaturized calorimeter for the direct and accurate determination of heats of combustion of natural gas will be described. The natural gas is burned continuously in a micro-flame of approximately 3 W corresponding to a methane flow rate of 5 ml min-1. A mixture of the gas and oxygen is lead through a heat exchanger tube and enters the micro-combustion chamber through a capillary that is optimized in the diameter to ensure the stabilization of the flame. Special requirements for a reliable ignition are necessary. The steady-state heat flow is detected using high-temperature thermopiles. In order to prevent condensation of water the working temperature of the calorimeter is set near 100 degree centigrade. A precision of better than 1% was found. The applied method avoids the poisoning of catalysts and allows the use gases of a broad range of composition. In connection with adequate fluid control elements a portable device for in-field applications seems possible.