Numerical solution of non-isothermal compressible natural gases mixed with hydrogen in pipeline using method of lines

Abstract

The effect of hydrogen injection into natural gas pipelines is investigated in particular the pressure and temperature conditions, real gas effects, Joule–Thomson effect, linepack and energy consumption of the compressor station. Real gas effects are a form of compressibility factor calculated using a particular predictive group contribution method. It is being incorporated with the conventional mixing rule, which in turn being used as a parameter in the non-isothermal transient flow model. A non-isothermal gas flow is a two-way interaction between gas flow and heat transfer. Although many other engineering cases isothermal model is usually adopted in situations over a long pipeline system, the non-isothermal case is considered here. When a gas is subjected to a temperature change, properties such as density and viscosity, change accordingly. In some situations, these changes are large enough to have a substantial influence on the flow characteristics. Because the gas transports heat, the temperature is in turn affected by changes in the flow characteristics. While it is convenient to treat that flow in pipeline is steady because it is easier to solve and under many conditions produces adequate results, gas pipeline system does not usually operate under steady condition. As such, an assumption that the gas flow in pipeline is in an unsteady condition is made. The gas flow is described by a set of partial differential equations (PDE) resulting from the conservation of mass, momentum and energy. The PDE are solved using the Method of Lines (MOL) and which in turn resulted in ordinary differential equations (ODE). The ODE is solved using the fourth order Runge Kutta Method. The Yamal-Europe gas pipeline on Polish territory is selected as case study, where results for the thermodynamic properties of mixed hydrogen-natural gas significantly differed from those of natural gas. The presence of hydrogen in natural gas mixture impacted the pressure and temperature gradient in the pipeline as hydrogen injection into the pipeline reduced the molecular weight of the gas mixture. Besides that, Joule-Thomson effect dominates the temperature profile and even causes a temperature drop below the soil temperature.