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Turbo-expanders, also referred to as expansion turbines, provide a way to capture the energy lost in natural gas facilities and refineries. Turbo-expanders have a range of applications, but this template focuses on the use of turbo-expanders for energy recovery and power generation. Virtually any high-temperature or high-pressure gas is a potential resource for energy recovery. Generator-loaded expanders can be custom engineered to recover the maximum amount of useful energy available in the process.
The expander principle relies on converting kinetic energy to useful energy / electricity by using turbines and electrical generators. As the gas flows from the high pressure stream into the turbo-expander, the gas spins the turbine, which is coupled to a generator that produces electricity. Thus, by replacing a conventional control valve or regulator with a turbo-expander, the energy contained in the motive stream, that would otherwise be lost, can be converted to electricity. It is noted that energy output is proportional to pressure ratio, temperature and flow rate of the stream. The higher the flow rate and pressure differential, the higher the potential energy output, as shown in the following graph.
Therefore, the location of this technology must be carefully studied in order to optimize power generation potential and efficiency. Turbo-expander systems can be found at the following types of oil and gas facilities:
- Liquified petroleum gas (LPG)
- Natural gas liquids (NGL)
- Dew-point control (DPC)
- Liquified natural gas (LNG)
- Nitrogen rejection
- Fluid catalytic cracking (FCC)
- Pressure let-down
By using a turbo-expander, the gas will be cooled via the Joule-Thomson effect. Several processes, such as dewpointing, refrigeration and natural gas liquification, can benefit from the additional cooling capability that a turbo-expander provides over the simple Joule-Thomson valve. In certain situations, modern turbo-expander installations utilize efficient methods for coping with this temperature loss. Turbo-expanders can be coupled with a second power generator such as a fuel cell or conventional fuel combusting generator. This secondary generator produces waste heat that can be used to offset the cooling effect of the turbo-expander. This symbiosis between the turbo-expander and secondary generator increases the net efficiency of the entire system.
There are a wide variety of turbo-expander-generator designs, with two basic configurations: one has the generator mounted directly on the turbine shaft; the other involves a connection via speed-reducing gears. An integral gearing option provides the additional benefit of multi-staging, allowing multiple expander stages to be mounted on a single gearbox. In most cases, the turbo-expander-generator unit can be completely skid-mounted to simplify transportation and reduce installation costs. The different types of turbo-expanders are as follows:
- Direct drive: The direct drive option, when feasible, eliminates the need for speed reduction, gear boxes and associated equipment.
- External gearbox: Expanders with an external gearbox feature GE patented bearings, with a common oil supply system for the complete package. The installed fleet ranges from 50 kW to 15 MW.
- Integral gearbox: This arrangement mounts the expander wheel directly on the high-speed pinion, eliminating the need for a high-speed coupling. Standard designs are available up to 15 MW.
- Multi-stage: High pressure ratios and/or high flow rates require the multi-stage arrangement. Standard expander-gear designs can accommodate up to four expanders on a common integral gearbox.
|Commercially available?: ||Yes |
|Offshore viability: ||Yes |
|Brownfield retrofit?: ||Yes |
Project examples in the industry
This technology has been implemented globally by several turbo-machinery manufacturers. One of the world’s largest single concentration FCC power recovery installation (150,000 HP / 112MW) was installed in 2006 (Reference 4).
Range / applicability of technology (for the Key Metrics section, below):
Table 1: Example turbo-expander-generator product range
|Pressure ||Up to 3,000 psia (200 BarA) |
|Temperature ||- 450⁰F to 925°F (-270°C to 500°C)|
|Power ||Up to 20,000 hp (15,000 kW) per stage|
|Expansion ratio ||Up to 1 |
|Power ||Up to 20,000 hp (15,000 kW) per stage|
|Process fluid ||All pure or mixed fluids including natural gas, petrochemical products, hydrogen, air, steam, etc.|
The following are technologies that provide similar benefits and may be considered as alternatives to turbo-expanders:
- Alternatives to power generation, from renewable energy to fuel fired generation.
Industry case studies
Case study 1: Turbo-expander evaluation of potential in Iran (Reference 5)
For this study, natural gas monthly flow data was taken from a single city gate station in Shahrekord, Iran and used in a computer simulation to determine the potential for installing a turbo-expander in parallel with existing gas pressure reduction systems to produce energy economically. Key considerations were natural gas flow rates, the amount of natural gas needed for preheating, and power produced by the system.
Baseline scenario: Use of throttle valves to reduce natural gas pipeline pressure at a city gate station.
Energy efficiency project activity: Installation of a turbo-expander and generator in place of a throttle valve to reduce natural gas pipeline pressure to capture the associated energy production from the natural gas expansion. The natural gas needed for preheating before expansion, and the variability of natural gas flow rates, were taken into account.
- Turbo-expander maximum power produced = 1.8 MW , 6000 MW-hr/year
- Turbo-expander efficiency = 85%
- Estimated energy cost savings = $463,000/year (2009 cost basis)
- Capital costs = $730,000
Case study 2: Enbridge Plant (Reference 6)
Energy efficiency project activity: A Direct Fuel Cell Energy Recovery Generation (DFC-ERG) power plant was constructed at Enbridge. A Direct FuelCell® provides non-combustion thermal energy to preheat the high-pressure gas by internally reforming 0.5% of the natural gas throughput to hydrogen. A back-up boiler is also used to preheat the gas. A process flow diagram and picture of the facility are provided below. The turbo-expander-generator and fuel cell provide ultra-clean, low impact electricity to the grid.
- Design flow rate = 1.8 MMscfh and 43.2 MMscfd
- Inlet pressure = 375 psig
- Outlet pressure = 175 psig
- Design power output = 1,000 kW
- Speed = 26,700/3,600 rpm
- Turbine building size = 25' x 40'
- Capital costs = $3.63 million (2008 cost basis); includes Phase 2 provisions for fuel cell
- Avoided CO2 emissions = 5,100 tonnes CO2e/yr (based on incremental carbon savings of 1.6 lb per kWh from fossil fuel generation)
- General Electric Company (2008). ‘Turboexpanders-Generators: For natural gas applications’.
- Rheuban, J. (2009). ‘Turboexpanders: Harnessing the Hidden Potential of Our Natural Gas Distribution System’.
- ‘Journal of Mechanical Engineering’, Vol. ME 41, No. 2, December 2010. Transaction of the Mech. Eng. Div., The Institution of Engineers, Bangladesh.
- Dresser-Rand (2007). ‘FCC Power Recovery Expanders’.
- Ardali, E.K. and Heybatian, E. (2009). ‘Energy Regeneration in Natural Gas Pressure Reduction Stations by Use of Gas Turbo Expander; Evaluation of Available Potential in Iran’.
- Eber, S. and Cavanagh, C. (2008) ‘Energy Recovery from Gas Distribution Operations’. AERTC Conference, 20 November 2008.