Pressure Let Down Generator (Pressure to Power)

At some of the existing DRS (District Regulating Stations) within the gas distribution network, it is possible to harness energy lost in existing pressure let down arrangement by replacing pressure reducing valves with a turbo expander

Existing systems at DRS operates through a Pressure Reducing Valve (PRV), which is an isenthalpic process resulting in energy lost through pressure let down. However, if the gas is allowed to pass through a turbo expander at a higher pressure, as the gas expands through the expander, its static energy is converted into kinetic energy through rotation of vanes inside the expander. This kinetic energy will be converted into electrical energy by attaching a generator, which will generate power

Thus, in the process, pressure (static energy) is converted into electrical energy as pressure is reduced to the desired lower level. As gas expands through the turbo expander, the temperature drops across the system and the output temperature is lowered substantially. The gas needs to be pre-heated before it enters the turbo expander to avoid hydrate and ice formation downstream from the turbo expanders

As many of the DRS are located near the CNG station, where compressors are powered through gas engines, exhaust from gas engine (waste heat) is used to preheat the gas to the required level. No additional gas needs to be used for preheating. It is possible to install a turbo expander in parallel to an existing pressure let down system (PRV) for ease of maintenance and reliability issues. Following is the schematic of the system:

Based on our understanding of turbo expanders available from various suppliers, 250 KW size is the smallest one available as standardized module. This is roughly equivalent to 10,000 SCMH of peak flow. However, if required, it is possible to design the system for various capacities upwards of 250 KW