Application of the Yokozeki thermodynamic model for the design of an innovative cryogenic system for biogas purification

The work set its basis on the European project Waste2Watts, that aims to design and build a large-scale biogas fuelled SOFC power plant. The biogas is supposed to be produced in a large farm and it consists mainly of methane and carbon dioxide. However, in order to guarantee the long-term correct operation of the SOFC, the biogas needs to be desulphurized and cleaned of other contaminants, like mercaptans e siloxanes. Here is where the efforts are put: the idea is to adopt a cryogenic cleaning system without use of chemicals or biological agents, in order to make the contaminants to condense and be suitably removed. Preliminary simulations have not clearly revealed the possibility to cool down the biogas while avoiding the sublimation of carbon dioxide, that plays a crucial role in the steam reforming reaction and therefore is highly expensive to be removed, in terms of cooling energy to provide. A scientific literature research has drawn attention to a thermodynamic model, Yokozeki equation of state, that has been widely used to describe the behavior of binary system, methane and carbon dioxide, and even ternary system, sulphuric acid included, with experimental validation. The work, then, has focused on the application of the Yokozeki model on the single components and to the binary system, trying to get useful information like the pressure which the system should work at, and the specific heat of the gaseous mixture. The application of the model to the ternary system, needed to verify the efficiency of the cryogenic removal, is left to further development.