In most cases, methane distributed through pipeline networks is decompressed in substation through the use of throttling valves.
An alternative process for decompressing natural gas consists of using turboexpanders: as the gas moves through the machine, the energy is removed from the fluid by the reduction in pressure and temperature; it’s then converted to mechanical energy by means of a rotor; the mechanical energy is transferred to the shaft and then converted to electrical energy.
This paper illustrates how a part of the energy lost through decompression can be recovered by means of peripheral turbines. These non-conventional turboexpanders can handle a range of applications, characterized by high expansion ratios and low flow rates, which traditional movers haven’t covered because their use wouldn’t be economically feasible.
Peripheral turbines offer specific advantages as prime movers whenever and wherever smaller mass flows and high expansion ratios are involved. Moreover, the presence of high pressure coefficients and low specific speeds support the choice of this mover in those cases which demand low level power at low revolutions from energy sources under high pressure. On the other hand, one negative aspect is that the level of isentropic efficiency is rather low. Nevertheless, owing to the small overall dimensions of the peripheral turbine, the mover can be usefully applied in a modular construction system, in which a number of machines can be assembled, either in series or in parallel, without an appreciable increase in price. The fact that the units are modular makes the system extraordinary versatile and adaptable.
In order to verify the technical feasibility of the project, detailed engineering was performed for a hypothetical pilot plant, based on data for a specific gas distribution network. Then a feasibility economic study was performed for the pilot plant. The evaluation of the pay back period and the NPV demonstrate the validity of the investment.