Acoustic sensor for determining combustion properties of natural gas

Since natural gas is not refined its composition depends on when and where it is recovered. This variation in composition results in problems in the optimal performance of combustion systems using natural gas as a fuel. The speed of sound in natural gas can be used to measure the fuel quality, since the speed of sound in a gas changes with the molecular weight of the gas. The primary constituent of natural gas, methane, has a relatively low molecular weight resulting in a high speed of sound, whereas contaminants such as ethane, propane, nitrogen and carbon dioxide have a lower speed of sound. A bench-top prototype speed of sound sensor for natural gas was built, and the speed of sound was measured at temperatures from -30 degrees C to 65 degrees C for twelve representative gas compositions. The sound speeds using the prototype sensor match theoretical predictions. That speed of sound in the natural gas was correlated to several composition and combustion properties including the percentage methane composition, the molecular weight, the density, the volume stoichiometric air-fuel ratio, the volumetric higher heating value, the Wobbe index and the lower flammability limit.