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Turbo-expanders

Sectors: Downstream, Upstream

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.

Figure 1: Turbo-expander performance
(Figure adopted from Reference 6; the original figure was Courtesy of Twisted Oak Corporation)

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.

 

Figure 2: illustration of a turbo-expander 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.

 

Technology maturity

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).

 

Additional notes

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.

 

Key metrics

Range of application:
See ‘Additional Notes’ section, above.
 
Efficiency: Up to 90%
 
Guideline capital costs: Turbo-expanders with large output capabilities cost substantially less on a per-kilowatt basis than smaller turbo-expanders.
 
Guideline operational costs: Long life and easy maintenance
 
Typical scope of work description: The scope of work begins with collection of application data, which includes the flow and physical properties of the high pressure stream (temperature, pressure) and the allowable discharge conditions. In addition, for power generation applications, it will be necessary to exercise due diligence with respect to the electrical infrastructure and loading. Although the turbo-expander may have the capacity to generate a significant portion of the electrical power needs, the power generation line-up may require some spinning reserve for process reliability and to manage large loads that may come online or offline.
Sizing the turbo-expander is typically done by vendors using proprietary software packages that optimize the performance within the frame sizes available given the process data above. After preliminary sizing and budgeting of the capital equipment, a conceptual design can be prepared that includes preliminary process flow diagrams, mass and energy balances, and a list of major equipment. A very rough order-of-magnitude cost estimate can also be provided. Some companies refer to this type of work as FEL-0 and FEL-1, where FEL refers to Front End Loading.

Decision drivers

Technical: Inlet/outlet pressure
Inlet/outlet temperature
Volume of flow
Operational: Improved energy efficiency
Optimized utility consumption
Reduced greenhouse gas (GHG) emissions
Commercial: Carbon market
Price of electricity
Environmental: Reduced environmental footprint by saving energy. Expanders are considered as green energy systems.

Alternative technologies

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.

 

Operational issues/risks

  • The power recoverable from expansion is small compared to the power that would be gained from a gas-fired power plant.
  • The flow rates may vary widely, which makes maintaining a steady power output difficult.
  • The well head stream contains a mixture of gas, condensate and water. Therefore, turbo-expanders may not work efficiently at the well head with un-processed gas.
  • Pre-heating before expansion is almost always necessary to avoid hydrate formation.

 

Opportunities/ business case

  • While turbo-expanders may only capture a few megawatts at a time, the widespread deployment of turbo-expanders could serve an important function in the greater agenda of a more efficient and greener energy system.
  • By using turbo-expanders on natural gas distribution systems, even if the power obtainable at individual locations is not large compared to the conventional thermal power plants, the sum of all locations can be substantial.

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.

Performance specifications:

  • Turbo-expander maximum power produced = 1.8 MW , 6000 MW-hr/year
  • Turbo-expander efficiency = 85%

Estimated costs:

  • 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.

Figure 3: DFC-ERG Enbridge Plant process flow diagram
(Figure adopted from Reference 6; the original figure was Courtesy of Enbridge)

 

Figure 4: Completed DFC-ERG Enbridge Plant
(Photo adopted from Reference 6; the original photo was Courtesy of Enbridge)

Performance specifications:

  • 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'
     

Estimated costs:

  • 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)

References:

  1. General Electric Company (2008). ‘Turboexpanders-Generators: For natural gas applications’.
  2. Rheuban, J. (2009). ‘Turboexpanders: Harnessing the Hidden Potential of Our Natural Gas Distribution System’.
  3. ‘Journal of Mechanical Engineering’, Vol. ME 41, No. 2, December 2010. Transaction of the Mech. Eng. Div., The Institution of Engineers, Bangladesh.
  4. Dresser-Rand (2007). ‘FCC Power Recovery Expanders’.
  5. 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’.
  6. Eber, S. and Cavanagh, C. (2008) ‘Energy Recovery from Gas Distribution Operations’. AERTC Conference, 20 November 2008.