Thermal conductivity microsensor for determining the Methane Number of natural gas

As a result of the liberalization of the gas market, more important and frequent fluctuations of the natural gas composition are expected. Owing to this fact, the on-site control of the gas properties will become necessary for end-users. Methane Number (MN) is the parameter used to quantify knocking tendency of a gas, parameter especially relevant when natural gas is used as engines fuel. The present paper describes a measurement method to determine the Methane Number of natural gas as well as the microdevice developed to carry out these measurements. The method is based on the measurement of the gas thermal conductivity and the existing correlation between this parameter and the Methane Number of natural gas. The developed microsensor is ascribed to Microsystems Technology. It integrates two platinum thermoresistors, a sensing one and a reference one, each of them patterned on a microbridge. The latter is defined in turn in the silicon substrate by surface micromachining using porous silicon as sacrificial layer. In the range of natural gas compositions considered (60-100 MN), applying a 50 mA constant current to the sensing thermoresistor, obtained sensitivity has been 0.95 mV/1 MN. Furthermore, the microdevice has been mounted in a gas line in order to test it in field. Additional tests are still required but it can be concluded that the developed microsensor is a valid alternative to measure in situ the Methane Number of natural gas.