<p>The world is facing a long-term prospect of a rising demand for energy. Growing population in developing countries will require a massive expansion in energy sources. Natural gas remains one of the most exploited energetic sources for power production, despite of the important development of sustainable energies over the last years. The International Energy Agency’s new gas scenario forecasts that, between 2008 and 2035, primary natural gas demand will increase by 60 % globally. Consequently, natural gas transportation lines are continuously expanding to follow the growing demand. For instance, four major pipeline development initiatives have started globally. In the aforementioned scenario, it is of major interest to study solutions to increase both capacity and operating efficiency of natural gas compressor stations, still containing climate-change gas emissions. A promising way to enhance the process efficiency involves recovering compressors drivers (i.e. gas turbines) waste heat and converting it into mechanical or electrical energy through a Rankine cycle. In particular, in the temperature range of 300-600°C, Organic Rankine Cycle (ORC) technology represents a competitive and feasible solution for waste heat recovery applications, as extensively documented in literature. In this work therefore, the feasibility of increasing the capacity of natural gas compressor stations by means of an ORC is studied. In the proposed configuration, the ORC recovers the natural gas compressor drivers’ waste heat and converts it into mechanical energy that is used to drag an additional gas compressor. Accordingly, a case study representative of a medium-size on-shore facility is taken as reference. The mechanical drivers’ arrangement is composed of four recuperated GTs of PGT5 R type (three units continuously operating and one used as back-up) and two smaller engines of Solar Saturn 20 type. Assuming the actual operation of the station, the addition of an ORC, as bottomer cycle, is designed to recover the exhaust heat from the three PGT5 R running units. According to Authors’ preliminary investigations and state of the art MW-size parameters, a regenerative sub-critical ORC cycle is selected. Therminol 66 and Hexamethyldisiloxane (MM) are chosen as intermediate and working fluids, respectively. The design ORC key cycle parameters are identified, along with the additional compressor capacity to be installed equal to about 2 MW. For a comprehensive investigation, ORC off-design operating range is explored assuming one and two out of three topper cycle units out of service. Since a direct coupling of the ORC driver and the gas compressor is expected, thus excluding thus the use of gearboxes to avoid losses, an ORC axial turbine based expander is designed that accommodates variable speed operation. The referred design includes mean-line calculations and three-dimensional computational fluid dynamics (CFD) based numerical simulations.</p>

On the Design of an Orc Axial Turbine Based Expander Working as a Mechanical Driver in Gas Compressor Stations

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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