Natural gas demand is projected to continue growing in the long-run and the gas distribution networks are intended to expand with it. The gas compression, along the pipeline, is usually performed in centrifugal compressors driven by gas turbines. In a typical installation, a significant portion of primary energy introduced with natural gas is discharged into the atmosphere with gas turbine exhaust gases, as wasted heat. Since the important investment of the last years, it is of major interest to study solutions for compressor stations, in order to reduce the primary energy consumption and the operative costs. A promising way to enhance the process efficiency, achieving the aforementioned goals, involves recovering compressors drivers wasted heat and converting it into mechanical or electrical energy through an Organic Rankine Cycle (ORC).
In this study, the feasibility of adding additional compressor capacity inside the station, with the help of an ORC, as waste heat recovery technology, is studied. In particular, the Authors propose a procedure to identify the bottomer cycle optimal size and to re-define the optimal distribution of driver’s loads inside the station. The strategy consists in the resolution of a minimum constrained problem, such as the loads are re-allocated between gas turbines and ORC, in order to minimize the fuel consumption of the station. Constraints of the problem are the load balance of the system and the regulation limits of each units.
The objectives are: (i) to identify the optimal sizes for ORC and electric motor driven compressor to be installed; (ii) to redefine the optimal distribution of the loads based on an annual operating profile of compressors; (iii) to quantify the environmental savings in terms of CO2 avoided compared to the original set-up of the facility; (iv) to assess the economic feasibility in the presence of additional aspects, as, for example, a carbon tax.
A typical interstate gas compressor station, with about 24 MW of mechanical drivers installed is taken as case study. Results of the study show that, for the investigated case study, the optimal ORC size turns out to be close to 5.3 MW, which correspond to an additional compressor power consumption of 4.8 MW that can be provided to the ORC driven compressor. Thus, resulting ORC design allows to produce — via an electric motor generator, connecting the ORC and the user — the 18 % of the yearly station mechanical energy demand. A reduction of 22 % of CO2 emissions, compared to the original arrangement is achieved. The economic feasibility of the proposed solution turns out to be very dependent on the natural gas cost and on the carbon tax, if applied. As expected, higher prices lead to higher avoided costs, thus to higher saving and lower payback periods (4 years), whilst low gas prices and no carbon tax can increase the payback period up to 20 years.