Gas turbines based on aviation technology offer an ideal fit for cogeneration applications through their high flexibility and efficiency as well as low emissions, argues Daniel A. Loero of GE.

Growing global electricity demand and the need to cut emissions have made dependable, efficient, compact and flexible heat and power a critical need for industrial and utility operators. And the US, Europe and other global regions offer large, untapped potential for developing highly efficient combined heat and power (CHP), with heat and power cogeneration currently contributing only 10% of global electricity, according to the International Energy Agency.

Aeroderivative technology is derived from an aviation heritage based on the GE CF6 aircraft engine, which powers most wide-body aircraft worldwide. This reliable, efficient, fast-starting technology is trusted for powering remote oil and gas facilities and also reliably generates electricity and steam.

Aeroderivative technology in CHP

Over their 40-year history, aeroderivative gas turbines have gained broad acceptance in CHP applications, where they can deliver net plant efficiency of 80–90%, which is far higher than that of typical simple- or combined-cycle power plants.

Figure 1. Electrical output and thermal load ranges for GE turbines

Aeroderivative gas turbines can also meet low emissions requirements, while providing great operational flexibility with fast starts, cyclic operation and high part-load efficiency (see Figure 1 for electrical output and thermal load ranges for GE turbines). With products ranging from 18 MW to 100 MW, aeroderivative gas turbines can also offer a wide range of thermal energy for CHP solutions.

Recent innovations and advances in gas turbine material and cooling technology have raised firing temperature without affecting maintenance intervals or component life. These enhancements have given the next generation LM6000 (PG & PH models) 18% more exhaust energy and 25% more power, leading to greater overall combined-cycle performance.

For example, GE’s latest LM6000 PH with Dry Low Emission (DLE) technology in a 2-on-1 combined-cycle configuration can generate 125 MWe net power with a thermal efficiency greater than 54%, and can approach 90% CHP net plant efficiency depending on configuration. Similar advances have allowed the LM2500+G4 aeroderivative to reach output levels of 34 MW at 41% efficiency in simple-cycle applications.

Aeroderivative CHP installations

A district heating plant in Denmark provides a current example of integrating aeroderivative gas turbines for CHP. The plant consists of a 2xLM6000-PD configuration, with one steam turbine and two heat recovery steam generators (HRSGs) with bypass stacks.

An output of more than 100 MW of electric power and 82 MW of thermal energy takes overall net plant efficiency above 89%. In the summer months, when district heating is in low demand, the HRSG bypass stack provides the flexibility for using the aeroderivative gas turbines in simple cycle for peak demand, which can still bring a high thermal efficiency of more than 40%.

In Halle, Germany, two CHP boiler steam turbine units have been replaced by two LM2500+ aeroderivative gas turbines that can deliver more than 40 MW of electrical power and a net plant efficiency of about 85%. The repowering’s other significant benefits include a drop in NOx gases to 25ppm, as well as lower carbon dioxide (CO2) emissions. Dry low emissions (DLE) combustion technology has also eliminated the need for water to abate emissions.

These are two of many success stories of aeroderivative gas turbines providing dependable, flexible CHP solutions at airports, hospitals, universities and refineries, delivering power together with steam for heating and cooling or process.

Technology for flexible applications

Aeroderivative gas turbine technology can provide dependable, high performance and flexible CHP to meet the growing demand for flexible power plants. Capabilities derived from their aviation heritage enable GE’s LM2500, LM6000 and LMS100 families of gas turbine engines to reach full power in less than 10 minutes. Planned enhancements on the LM6000 are set to enable full power to be reached in five minutes.

Known for fuel efficiency as high as 44% in simple cycle, these gas turbines can cycle – that is, start and shut down – several times per day without incurring additional maintenance needs.

With a high part-load efficiency and a unique ability to ramp up or down by 50 MW per minute, aeroderivative gas turbines let their operators chase required demand load within seconds. When baseload demand fails, this rapid response capability can allow stable electrical power to be supplied to the local grid in island mode.

A multi-shaft design gives the gas turbine an exceptional ability to hold power loads during frequency variations. Packaged in modules, these aeroderivative gas turbines also occupy a relatively small footprint, which enables them to be placed closer to demand load centres. Fuel flexibility is another key benefit, enabling the use of a wide range of liquid and gas fuels.

By opting to use these gas turbines’ high exhaust energy in the form of heat, for steam cogeneration or district heating applications, operators can increase overall plant net efficiency. These products also offer a wide range of solutions in terms of electric power vs thermal energy.

A specific thermal load can be met through using condensers, duct firing and turndown. With a natural capability derived from aviation jet engines to meet required power at a moment’s notice, the new fast-start fast-load will be at five minutes, meaning operators can hit full power in just five minutes, starting from zero speed.

Proactive power boost, also known as pro-boost, will provide a spinning reserve, enabling a short-term control limit ‘bump’ of an additional 2 MW power above normal control limits for a short duration. This is a controls approach, to what has been a mechanical solution with higher-inertia equipment, that enables aeroderivatives to serve grid power demands and balance grid frequency during system disruptions.

LM2500 product line

GE’s LM2500 has been one of the top selling aeroderivative gas turbine in its class for more than 40 years and continues to evolve to provide increased customer value. Offering 18–35 MW of power generation with up to 41% efficiency, the LM2500 can serve the typically demanding and diverse power needs of applications in industry and in oil and gas.

Proven reliability and availability, dual fuel capability, outstanding emissions as low as 15 ppm NOx and fast load response enable the LM2500 to meet the demands of a wide range of industries.

The LM2500 and LM2500+ feature a lightweight and compact design that allows for fast installation and ease of maintenance. The product family boasts more than 1500 installed units and more than 65 million hours of operating experience across a variety of operating applications ranging from cogeneration to land-based power generation and mechanical drive to platform power generation and mechanical drive.

Derived from the CF6-6 and TF39 aircraft engines used on wide body jetliners, the LM2500 family is a hot-end drive, two-shaft gas generator with free power turbine.

The LM1800e is designed for 18 MW applications. The lower rating allows a lower firing temperature, further extending the hot section and combustor life. Like all members or the LM2500 family, the six-stage power turbine allows direct drive operation at 50 or 60 Hz (3000 or 3600 rpm), as well as mechanical drive.

Figure 2. Enhancements to the LM6000-PH

The LM2500+ adds a 17th compressor stage to the LM2500, increasing the engine pressure ratio and airflow to raise the total power output of the gas turbine to more than 31 MW. LM2500+G4 is the fourth generation of the LM2500 product. It operates at higher rotational speeds and pressure ratios than the LM2500+ to deliver up to 34 MW.

LM6000 product line

The LM6000 aeroderivative gas turbine was introduced in the early 1990s and is derived from the CF6-80C2 aircraft engine, the most widely accepted engine flying in the popular Boeing 767 and 747 wide body aircraft.

Since its introduction, the LM6000 gas turbine has established itself with more than 1000 units ordered and more than 22 million operational hours at reliability greater than 99.8%. The LM6000 targets power generation needs between 35 MW and 60 MW.

Current LM6000 production models include the PC, PF and PH. The PC is a 42 MW model with a standard combustion system that can use liquid and gas fuels and is capable of 25 ppm NOx emissions.

The LM6000 PF has an efficiency of 41% and includes DLE technology that can guarantee NOx emissions at 15 ppm and enables lower fuel consumption per unit of power output than competing technologies.

High fuel efficiency and DLE technology can offer significant environmental benefits. For example, in the 60 Hz segment, when compared with a simple-cycle gas turbine in the 35–60 MW range with 35% efficiency operating at 25 ppm NOx, GE’s LM6000 PF:avoids 15,000 tonnes of CO2 emissions over a 3000-hour peaking season;reduces natural gas consumption by more than 264,000 MMBtu (280,000 GJ);reduces NOx emissions by 370,000 kg by operating at 15 ppm NOx; saves the 37.5 million litres of water per year that a typical 60 Hz, simple-cycle turbine consumes as a diluent.

The next generation LM6000, the PG and PH model type, offer a 25% simple-cycle power increase and an 18% boost in exhaust energy for cogeneration applications (see Figure 2).

The LM6000 PG and LM6000 PH provide combined-cycle power in the range of 65 MW with efficiencies ranging from 52% to more than 54%, depending on selected emissions control methods. The power increase comes from the same 4.5 x 21.5-metre package footprint as existing 50 Hz LM6000 technology, yielding a power density improvement of nearly 20%.

Figure 3. Established technologies are integrated in the LMS100

The first engine to test (FETT) exceeded the expected maximum output of 53.2 MW (corrected ISO conditions) with 41% efficiency and NOx levels at 25 ppm. The comprehensive validation test then progressed to assessing engine performance operating on gas fuel only, with water injection for NOx control added later. Upon completion of testing with gas and water, the programme switched to operation on diesel fuel without water, and later with water for NOx control.

Figure 4. An intercooled system enhcances the LMS100’s hot-day performance

The LM6000 PH, the DLE equivalent of the LM6000 PG, completed validation test in 2011. The LM6000 PH is to deliver 51 MW (w/SPRINT) of output with +42% efficiency, and achieve NOx levels at 15 ppm. The first production LM6000-PH engine is scheduled to ship in late 2013 and will be installed at a launch oil and gas customer facility due to operate from mid-2014.

LMS100 product line

The LMS100 merges two proven technologies: frame industrial gas turbines and aeroderivative gas turbines. An intercooler package takes the flow from the low-pressure compressor and reduces its temperature to increase flow density, resulting in the most efficient simple-cycle gas turbine at 44% thermal efficiency, capable of 102 MW output (see Figure 3).

Since the introduction of the intercooled LMS100 in 2005, the energy industry has seen several changes in terms of competing products, system needs and the application of this new technology.

Figure 5. LMS100-PB provides DLE combustion with minimal hardware changes

The LMS100 has surpassed major operational milestones with 120,000 hours and over 18,800 starts demonstrating the benefits that the hybrid, intercooled gas turbine brings. In addition the LMS100 is on track to meet its mature reliability and availability targets of 99.2% and 97.1% respectively.

The most noticeable feature is the off-engine intercooler, illustrated in Figure 3, which is comprised of the variable bleed valve exhaust system and the horizontal shell and tube heat exchanger. The system is sized to receive compressed air from the six-stage low-pressure compressor, cool it to about 149°C and return it to the high-pressure compressor resulting in significantly higher power output versus conventional cycles.

Another key benefit of the intercooled system is better hot-day performance, as illustrated in Figure 4. This results in higher simple-cycle efficiency, 10-minute starts, better part power performance and faster ramp rates.

High demand for flexible generation, along with industry acceptance, has accelerated the fleet maturity plan. Early installations consist of peaking, mid-merit and baseload applications in North America, South America and Europe.

Since the first LMS100 entered service in July 2006 in South Dakota, US, another 37 units have been ordered for a wide range of environmental and market conditions, which should further demonstrate the product’s capability across temperatures ranging from -30ºC to more than 40ºC.

In several installations, the gas turbine will be used for primary power due to its higher efficiency than existing older steam turbine units and its power fit with older combined-cycle plants. Similarly, at sites in regions with volatile power supplies, many units will leverage their fast 10-minute start capability to operate with several start/stop cycles and to accumulate hours without impacting planned maintenance costs.

In accordance with GE aeroderivative design practice, the LMS100 was introduced with a single annular combustor equipped with water injection for NOx control. In a parallel effort the combustion team continued work on a DLE system to enable 25 ppm NOx without water injection.

This effort utilizes proven aeroderivative multi-annular technology and experience from GE’s other aeroderivative models: the LM1600, LM2500, and LM6000. More than 450 DLE gas turbines now operate around the world, accumulating more than 10 million operating hours.

Lessons learned from these installations include the application to the LMS100 design of the enhanced heat shield design and optimized fuel distribution for improved fuel-air premixing that is used on the LM6000 PF and LM2500+G4.

The LMS100 PB is the DLE version with a higher flow and a higher firing temperature to meet its performance requirements. The first two production LMS100-PB gas turbines are scheduled to ship in mid-2012 and will be installed at a launch customer facility in Sochi, Russia, to help meet the power needs of the 2014 Winter Olympics venues. This launch site is due to start operating from early 2013.

The required component changes from a LMS100 PA (SAC) to a LMS100 PB (DLE) are rather minimal. The LMS100 PB, as with the other LM gas turbines with DLE combustors, uses a larger combustor than the SAC model. This larger combustor provides the volume to properly mix and control the combustion flame in order to provide the high efficiency and low emissions.

Utilizing the extensive experience of the GE DLE fleet, the LMS100 will use a dual annular premixed combustor, which will simplify the staging process and reduce the hardware required to meet the performance requirements. The balance of the package equipment and engine hardware will be identical to the LMS100 PA and will continue to benefit from experience gained on the LMS100 PA fleet.

The LMS100 PB will provide 25 ppm NOx without using water as a diluent. Additionally, the LMS100 PB offers customers a solution in this ever-demanding world of meeting more efficient power requirements while retaining a power industry-best heat rate and the proven flexibility of the LMS100 PA.

Conclusion

As the demand for electricity grows globally, aeroderivative gas turbine CHP technology can offer a unique solution of dependable, ultra-high efficiency, flexible power that can also help reduce gas emissions. LM2500, LM6000 and LMS100 technology offers a wide range of options in a cost effective manner.

CHP projects often run at partial loads, when steam load demand declines at a set period of the day, such as overnight when cooling demand falls. In such conditions, an aeroderivative gas turbine CHP plant typically outperforms industrial gas turbines through their higher degree of flexibility:

  • 10 minutes to full load (with a five-minute fast load option) – a faster reaction to grid demands;
  • pro-boost – spinning reserve power when needed;
  • high part-load efficiency – 36% efficient at 50% load;
  • rapid response load – 50 MW/min to meet changes in demand;
  • superior start-up reliability – over 99.8%;
  • high availability – 12 days of planned outage in 50,000 hours of operation.