Natural Gas Expansion, Single Stage Axial Turbine, Partial Admission, Geothermal, Power Generation
The efficient transportation of natural gas takes place with high pressure before it is distributed to the consumer with a lower pressure. Generally, pressure reduction in gas grids is realized in pressure control stations with a regulating valve, where the dissipative energy remains as heat in the fluid. Nevertheless, the temperature of natural gas decreases during the pressure reduction due to the Joule-Thomson-Effect, depending on pressure difference of the regulating valve. For higher pressure differences it is necessary to preheat the natural gas to prevent condensate formation and freezing of pipelines. Instead of using a regulating valve, an expansion machine can be used for pressure reduction and power generation apart from pressure control stations. For this purpose, a single stage axial flow turbine is developed for power generation in a natural gas pipeline. The “preheating” of natural gas is realized solely with geothermal energy out of the upper layer of earth and thus without additional heating equipment. The temperature of natural gas at the inlet of the expansion machine is given by geothermal energy as e.g. Tinlet = 278 -283 K, which varies dependent of the temperature in the ground. The outlet temperature downstream of the turbine should not fall below e.g. Toutlet = 273 K. The design criterion for the expansion machine is therefore the temperature difference instead of a pressure difference as it is common for expansion processes. Essential criteria for the whole concept are a simplified integration in a pipeline and an economical solution. Design of the axial flow turbine is carried out with mean diameter calculations followed by an empirical prediction method for the turbine efficiency. Using an empirical method the loss of total pressure in a cascade of blades is obtained. One-dimensional simulations are carried out to determine design parameters and inner power of the turbine. Subsequently the upper and lower partial load are examined. The axial flow turbine is designed in a compact module (generator inside pipeline) for a natural gas pipeline with relevant safety standards. A first prototype of a single stage axial turbine with partial admission is manufactured and tested initially in a test rig with air flow and in a pressure control station for natural gas. Results from field measurements and simulation are compared and analyzed with respect to the operational behavior of this new concept of an axial turbine for natural gas without preheating equipment.