A Method to Determine Biogas Composition for Combustion Control

This paper presents a methodology for a rapid determination of biogas composition using easily detectable physical properties. As biogas is mainly composed of three constituents, it is possible to determine its composition by measuring two physical properties and using specific ternary diagrams. The first part of the work deals with the selection of two physical properties, which are easy and inexpensive to measure, from a group comprising thermal conductivity, viscosity and speed of sound. Then, in the second part, a model to express these properties in terms of ternary composition is presented. It is demonstrated that the composition of a ternary gas mixture can be determined with good precision using the above. The model is applied to specific situations such as the online determination of the lower heating value of biogas without any complicated apparatus like calorimeters or batch techniques (gas chromatographs). The error on the lower heating value and Wobbe index of biogas is less than 1% even when taking into account other constituents not specified in the ternary diagram like oxygen. The effect of small errors in the measurement of physical properties has also been highlighted.