Turboexpanders at Pressure Letdown Stations
Gas Pressure Letdown: Turboexpander vs. Pressure Regulator
Two similar devices that recover wasted energy in gas pressure letdown applications, with energy savings dependent upon gas flow rates, temperature, and pressure drop.
Item ID: 456
Process Loads & Appliances--Municipal Processes
Turboexpanders and air motors (which are similar, smaller devices, and are included below in references to “turboexpanders”) can be used to recover energy in gas pressure letdown applications.
Large turboexpanders are very common in the oil and gas, cryogenic gas separation, and chemical industries, but are less well known or applied in other sectors or applications. GE Power Systems claims to have produced over 3,500 units for a wide range of oil and gas industry applications. Turboexpanders (basically backwards-running axial fans or air compressors) are produced by many companies including Airco, Atlas Copco, Cryostar, Mafi-Trench, Hunter, Lotema, Linde, Sulzer, Texas Turbines, Dresser-Rand, GE Rotoflow and Turbodyne. Turboexpanders are available from 1 hp to 15,000 hp.
Turboexpanders can be used with hot gases, or pressurized room temperature gases including hydrocarbons, tail gases, hydrogen and carbon monoxide. High-speed rotating turboexpanders can be used in cryogenic, natural gas, petrochemical, and air separation industry applications. Turboexpanders for the natural gas industry must be leak-tight and safety-rated. Turboexpanders can be used to produce electricity with generators or serve as variable speed equipment drive motors.
Potential applications for turboexpanders include natural gas transmission pipeline pressure letdown stations (citygates) where natural gas at several hundred psig is reduced to distribution pressures. Power output is dependent upon gas flow and pressure ratio. Other applications include pharmaceutical plants, where air is compressed and admitted to a reactor where its oxygen content is consumed. The residual lower pressure nitrogen gas can produce power by being released through a turboexpander. A compander combines expansion-turbine and compression stages into one unit. Companders drive processes such as LNG-reliquefaction, create energy from excess gases in fertilizer production, and simultaneously chill and compress gases in liquefied natural gas (LNG) processes.
Power output is dependent upon gas flow and pressure ratio while energy production is dependent upon hourly, daily, and seasonal variations in flow. This is not a classical energy efficiency technology but is along the lines of an energy or waste heat recovery technology. Recovered energy will be in the form of electricity or machine drive horsepower (which offsets the need for a plant to purchase electrical energy).
Turboexpander installations are very site-specific and do not lend themselves easily to pre-packaged designs or standard configurations. No commercial turboexpander projects exist at city gates in the U.S. pipeline system although perhaps 20 are in operation in Europe (Hedman). Five turboexpander demonstration projects were installed in the 1980's with power outputs of 260 kW to 2,800 kW). Projects are generally not viable due to a combination of high capital costs, low electrical energy purchase prices, and wide variations in natural gas flow and pressure that negatively impact turboexpander performance. Projects may be cost-effective in areas where renewable portfolio standards provide incentives for green energy (and when the turboexpander project qualifies). Note that the isentropic expansion of the natural gas through the turboexpander results in a greater temperature reduction than would occur when the gas passes through a letdown valve necessitating the need to pre- or post-heat the natural gas to maintain minimum temperatures. This can result in a re-heat fuel consumption of 3,000 to 5,000 Btu/kWh (Hedman).
A turboexpander installed at a pressure letdown station in Germany accepted methane gas at an inlet pressure of 653 psia, a temperature of 167 deg F, and a flow rate of 28,000 standard cubic feet per minute (scfm). The recovered power is 2,530 kW. An expander at an ammonia fertilizer plant accepts a flow of 313,000 lbs/hour, at an inlet temperature of (-)200 deg F and pressure of 501 psia to produce 634 kW (Source: Atlas Copco brochure).
Manufacturer's Energy Savings Claims:
The U.S. natural gas pipeline network is comprised of more than 305,000 miles of interstate and intrastate transmission pipeline, more than 1,400 compressor stations that maintain a pressure (linepack) of 1,000 to 1,750-psig, and more than 11,000 delivery points (Renewable Energy World). As the gas nears populated areas, pressure must be let down to acceptable levels for use by gas distribution utilities and commercial and industrial customers. This pressure reduction is often done in stages (1000 psig to 600 psig; 600 psig to 250 psig; 250 psig to 100 psig). The pressure is reduced at letdown stations by devices called pressure control valves or regulators. An alternative is to reduce the pressure through routing gas flows through a power conversion device such as a turboexpander or screw-expander.
Note that energy can be produced 24/7 and the letdown stations are often located close to population centers. No fuel is required to produce this electrical energy. The turboexpanders are driven by the pressure in the transported gas.
Best Estimate of Energy Savings:
Operating efficiencies of over 60% are said to be routinely achieved.
Energy Use of Emerging Technology:
Currently no data available.
Currently no data available.
The installed cost of a Gas Letdown Generator is claimed to be less than $2,000 per kW. Economies of scale exist, as the larger the generating capacity, the lower the cost on a cost per kilowatt basis.
Simple payback, new construction (years): N/A
Simple payback, retrofit (years): N/A
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.