On the design of an ORC axial turbine based expander working as a mechanical driver in gas compressor stations

In this work, the feasibility of increasing the capacity of a natural gas compressor station by means of an Organic Rankine Cycle (ORC) is studied. In the proposed configuration, the ORC recovers natural gas compressor drivers' wasted heat and converts it into mechanical energy. Thus, as innovative approach, the ORC generated mechanical power will be used to drag an additional gas compressor. 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 the 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: about 2700 hp (2 MW) of capacity could be added to drive a compressor. For a comprehensive investigation, ORC off-design operating range is explored too assuming one 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 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 at design and off design point conditions.