Electricity production in natural gas pressure reduction by using turboexpanders

Natural gas is transported for longer distances through transit pipeline at high pressures (50 - 70 bar). In a place of consumption or at passing into a lower pressure pipeline the pressure of the gas must be reduced. In transmitting stations the pressure must be reduced from 50-70 bar to 15-40 bar (usually to 25 bar) into high pressure intrastate pipeline, then to approximately 3 bar into medium pressure intrastate pipeline. Standardly, gas pressure reduction is accomplished in throttle-valves, where the isenthalpic expansion takes place without producing any energy. Most gases cool during the expansion (Joule-Thompson effect). The temperature drop in natural gas is approx. 0.45-0.6°C per 1 bar, depending on gas composition and state. The gas must be preheated before the expansion to ensure that no liquid or solid phase condenses at the output temperature. When an expansion turbine driving a generator is used in place of the throttle valve, the energy in the gas can be used to produce electricity. The work the gas performs is gained from its internal energy (enthalpy) and the gas cools rapidly in the turbine. The temperature drop in the expansion turbine is approx. 1.5-2°C per 1 bar of pressure drop in transmitting stations from transit pipeline depending on gas composition and state, and on turbine's isentropic efficiency. When using an expansion turbine, gas outlet temperature must remain above hydrate zone and dew point. This means the gas must be pre-heated before it enters the turbine to temperatures higher than when using throttle valves, usually to 55-85°C. Reliability of the pressure regulating and reduction stations must be assured, and therefore the expansion turbines are installed parallel to existing conventional pressure reducing valves. In the pressure reduction stations by using expansion turbine and generator system, electricity can be produced with the expansion energy of natural gas. This application has already many samples in our country and also in all aroud the world. With using this system the lost energy by gas expansion can be returned effectively to electricity.