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The notion that thermal power generation assets are optimized to operate at maximum efficiency within relatively narrow and static parameters close to full load has been central to the energy sector since time immemorial.
Theoretically, at least, such a philosophy enables designers, engineers and equipment manufacturers to deliver systems with the best return on capital and operating expenditure, with each megawatt of capacity delivering the maximum MWh per year for the minimum in fuel and maintenance costs.
Efficiency is the watchword for any plant owner and operator, and today a modern gas-fired combined cycle unit can achieve impressive efficiencies of 60 per cent and above.
Obviously this is all highly commendable, but for the last decade and more the operational demands on power plants have been radically changing – so much so that today, a more accurate interpretation of the ‘efficiency’ watchword may be ‘flexibility’.
Naturally, peak efficiency is still a central consideration, but it is vying for top spot with a host of other factors as operators weigh their options for a successful economic strategy under the prevalent commercial environment.
It seems that, to some extent, full load peak thermal efficiency is being decoupled from peak economic efficiency – or, at the very least, the two are no longer necessarily equivalent. Instead, operators nowadays are more typically looking to extend the operating range, increase their ramp rates, achieve high efficiency at low power ratings, deliver on fuel flexibility and so on.
As a result of this demand from their customers, original equipment manufacturers are now offering a range of retrofit options that can enable asset owners to further optimize operational performance, rapidly respond in a far more dynamic marketplace, cut commercial risks and keep operating costs to a minimum.
A key driver behind this trend is the ever-increasing volume of variable-output renewable energies, such as wind and solar, feeding into the grid. The attendant requirement for spinning reserve capacity to manage unexpected lulls in output, as well as the ability to turn down thermal output rapidly as potentially more energy than anticipated is delivered from renewable installations – which typically have market despatch priority – makes for challenging grid compliance architecture. Coupled with long-term policy support and rapidly falling costs, further growth in this sector is widely expected to continue and even accelerate.
This is a theme picked up by Alstom’s Head of F-Fleet Product Management, Leone Tessarini, who says: “Many plants are online trying to catch the market spikes during the day, getting to the peaks of electricity prices late in the morning and afternoon and hanging over for the rest of the day.”
Tessarini also points to other factors, such as the low carbon price in Europe and withdrawal of carbon pricing policies on Australia, as well as subsidies for several other power segments such as coal, which have created “what we’ve called ‘a perfect storm’ for Europe”. This has led to examples of brand new gas-fired units that have been mothballed after just one year of operation because they were generating losses. Tessarini says: “It’s clear that flexibility today is one key aspect for the existing gas plant.”
Demand for flexibility
Ed Bancalari, Director of Siemens’ Large Gas Turbine Service Product Line, also notes a host of drivers placing similar demands for flexibility on the North American market. Influences include pricing spikes that are a regular occurrence in the central region of the US, variations in fuel quality and fuel constraints on the east coast, and renewables growth along the west coast and the Ercot transmission region covering Texas.
As Nathan Race, F-Class Marketing Program Manager for GE Power Generation Services, says: “We’re really seeing the impact of renewables in a lot of different markets, predominantly in North America and Europe; it’s driving a need for flexibility”.
And, in a view no doubt echoed by every other competing OEM, Tessarini states: “It’s of particular importance that we keep our plants as competitive as possible in the difficult market we are in today.”
Kenneth Engblom, Marketing Director of Power Plant for Finland’s Wärtsilä, notes the inherent flexibility available from internal combustion engines and their dynamic capabilities.
“These engines can start to full load within five minutes, and stop within a minute with no additional maintenance cost. Efficiency is very high in part load and cycle load operation. These engines are perfect for following any type of load and coping with the additional challenges that the wind and solar put on the stability of the power systems.”
Siemens’ Bancalari explains that in any upgrade and refurbishment programme, efficiency is key, and previously this has led to a focus on the gas turbine elements of the power plant.
Now, though, increasing attention is being directed towards the balance of plant systems that can limit operational flexibility. Bancalari cites an example in which Siemens uses hot start on the fly, a highly integrated start of gas and steam turbine to allow short combined-cycle startup times, with a couple of installations in Europe already operating.
In order to have solutions for various economic and plant scenarios, Siemens has developed upgrade solutions that range from interval extensions made during the part refurbishment process to the FD6 upgrade, which increases plant output by more than 77 MW and reduces heat rate by 200 BTU on a 2×1 plant.
Tessarini explains that, based on customer demand, Alstom has gone to considerable lengths to make their GT24 and GT26 plant more flexible and extend the operating range, especially in the low power part of the operating regime.
The GT26 architecture consists of a two-stage compressor, and then a first combustion chamber and a one-and-a-half-stage high pressure turbine. This is followed by a secondary combustion chamber and a four-stage low pressure turbine. The secondary combustion chamber includes 24 discrete burners, which can be individually switched off to reduce the power of the machine.
“The advantage is that you have an inlet temperature in the secondary combustion chamber, which is already high because it’s coming from the outlet of the high-pressure turbine. Instead of de-rating or reducing the temperature on all 24 burners, of the secondary combustion chamber, you switch off some of them while maintaining the other burners at the same optimal combustion conditions without generating detrimental emissions, so the CO and the NOx limits are well maintained within the regulations,” says Tessarini.
He adds that, at the moment, the machine can reduce the number of operating burners to eight – a two-thirds reduction – but “we’re further working to reduce that number to further improve the machine turndown capability.”
“With the sequential combustion we have this additional element of flexibility that allows us not only to go so low with overall combined-cycle power, but as well remaining at a very high level of efficiency.”
He further states that, at significantly lower loads than 30 per cent of the full load, the system can deliver 45 per cent efficiency in a 400 MW single shaft combined cycle power plant, adding that the unit can be operated “without any restriction or preparation time from the 30 per cent load or lower up to 100 per cent or any additional penalization on the lifetime and no penalization on the efficiency”.
Echoing Bancalari, he notes: “For an operator it’s important what can be achieved on a combined cycle not only on the GT, only because they are operating combined cycle plants and not GT [single cycle] plants.”
GE also offers a number of retrofit options for its machines, including both hardware and software upgrades for its F-class units.
Race explains that the Power FlexEfficiency portfolio is a series of upgrade solutions designed “to really expand all the different elements of performance of the gas turbine that could include more common things like output and efficiency, and other things like expanding load range, increasing turndown capabilities and changing ramp rates.”
Available upgrades for the 60 Hz 7F machine, for instance, include an entirely new hot gas path section for the turbine, with more than 100 installations completed to date. A second piece of the upgrade portfolio is the Dry Low NOx 2.6+ combustion system which Race says offers “a significant increase in fuel flexibility as well as an extension of the maintenance inspection intervals”.
However, Race continues by saying that perhaps the most important upgrade from a flexibility perspective is the OpFlex software. A series of control software upgrades, it includes elements such as automated tuning of the combustion system, and is built on a platform called Model Based Control.
“The controller is running a real time simulation of the engine. It’s feeding in all the sensor data that it gathers while the unit is operating, and by combining that sensor data with the model it allows you to remove a lot of the operating margin that you used to have to maintain, based on not having a detailed understanding on how the unit was behaving. That’s the enabler for improved flexibility.”
He adds that the system allows additional opportunities for flexible operation in the event of a failure: “It allows for a more robust system in terms of trip avoidance, in terms of remaining on-line and offering a high level of performance.”
Race explains that, in part, the strategy was to get away from a plant control mindset where the tendency is to have equipment trip offline in the event of, say, a sensor fault, and instead deliver a more robust ‘limp-home’ operating mode.
He references GE’s aviation heritage: “If you think of a jet engine, a jet engine can’t trip offline. It has to get home, regardless of what sensor fault you suffer. The goal would be to bring the unit back to a safe condition where it can operate. It would still remain online [but] you might not be able to produce 100 per cent power.”
A third GE Frame F hardware upgrade component is its Advanced Compressor, launched in early 2014. It offers about a 12 per cent increase in output as well as about a 2 per cent improvement in the gas turbine heat rate, Race says, though it has fewer compressor stages, from 18 down to 14. Furthermore, all the stages are now field replaceable: “We’re leveraging some of our experience from aviation to include field replaceable blades in our industrial compressors,” he explains, again referencing GE’s aviation platforms.
Siemens is also offering more in-service and commercial products in its bid to respond to customer demand. This includes elements such as flexible service agreements where operators do not have the benefit of a long-term power purchase agreement.
In addition, Bancalari explains that the company has changed the tools and the approaches it uses for servicing and maintaining such machines. As an example, he refers to Siemens’ semi-robotic turbine disassembly tool that relies on automated techniques to disassemble a gas turbine more quickly, at lower cost and with increased safety margins for service engineers and equipment. The system is currently being deployed in Germany following a validation trial at a project in South Korea on an H-frame machine.
In a substantial upgrade on a 5000 F machine requiring rotor removal and replacement, this patented tooling resulted in a 30-40 per cent decrease in the time required to implement the upgrade to the low-pressure end of the turbine, which includes changing the exhaust chamber casing and blade seals, Bancalari says. In addition, the tooling reduces the need for the high-capacity cranes previously required to split a turbine case.
A further avenue of flexible operations that OEMs are exploring is fuel flexibility. Bancalari explains: “One thing we’re seeing is the need for fuel flexibility, especially in the Middle East where liquid fuels or gas that was previously flared is now being sourced for power generation. Fuel flexibility will become more important in the future, for example with the advent of biofuels in Latin America and Brazil in particular.”
Wärtsilä’s Engblom expounds that internal combustion engines can run on everything from “some of the thickest, most aggressive fuel oils to the cleanest gas, and they can switch from one fuel to another while running”.
Engblom also highlights the development of the gas infrastructure, but warns of uncertainties as such infrastructure is being developed. “With a dual-fuel engine you are pretty much secure, you can always go back to liquid or continue to run until gas is there,” he says, “and then if you see a sudden drop in oil price you could even switch back to oil for economic reasons if environmental legislation allows. With a dual-fuel engine you have all the options.”
Wärtsilä completed a 600 MW dual-fuel power plant in Jordan last year. This plant is now starting up on HFO with the intention to change over to natural gas as soon as the gas infrastructure is in place.
Furthermore, Engblom concludes, existing Wärtsilä heavy fuel oil engines can be retrofitted to accommodate gas or dual fuels with modifications or replacement of the fuel supply and injection system and the cylinder heads.
Indeed, Wärtsilä is now also very much involved in LNG supply logistics. “That’s where we can help our customers a lot to ensure that we make it easy for them to convert to gas by ensuring availability of gas for them. There will be more gas everywhere, with many of our clients shifting from heavy fuel oil to gas. We’ll see that the gas will be more easily available thanks to LNG. That’s why we are also moving into this LNG logistic chain now, to help speed up availability of LNG all over the world,” Engblom says.
While OEMs have gone to considerable lengths to develop suitable products that will allow their customers further commercial opportunities, Race explains that a retrofit upgrade must be considered as part of a long-term asset management strategy.
Referring to GE’s advanced compressor, he says: “It’s not the sort of upgrade that anybody would buy, it’s more to do with what are your long-range plans for your plant or operator, and that would include things like rotor life extension, capacity options, or long-term capacity planning if you’re a regulated utility. It’s about long-term asset management philosophy and how do you then structure the upgrade and improvement of your equipment and the balance of plant equipment.”
He continues: “The goal in this Power FlexEfficiency portfolio is really to accommodate and serve customers’ different situations, and to help them reach specific needs without a ‘one size fits all’ solution.”
For example, Race notes the growth in the number of flange-to-flange turbine replacements over the last five years. He says: “Where you have older gas turbine centre line equipment that’s reached its end of life, whether that’s technical or economic, customers have approached us looking for upgrade solutions.”
In some cases, he says, “If you look at the number of things that have to be changed, you’re better off just buying a new gas turbine with all of the capability of the latest and greatest equipment. It creates a situation where the owner has the opportunity to take advantage of all the investment and capital that surrounds the gas turbine.
“By coming in with just the new prime mover, you can then capitalize on the remaining life of all the residual equipment around the plant, and you can do it with a much higher level of efficiency based on the performance of that new and clean gas turbine, for a fraction of the cost of doing a full tear-down and rebuilding it.”
Such an upgrade could typically be expected to deliver a 5 per cent or more improvement in performance, he concludes.
It is clear that power plant owners and operators are increasingly placing a premium on opportunities for flexible operations which can yield significant economic benefits. As a result, OEMs are making available a range of options for upgrades and retrofits which can offer valuable opportunities for operators to regain their commercial competitiveness. However, there is the potential for wider benefits to emerge from a fleet-wide retrofit programme given that such products can technically be applied in every single machine installed.
Taking Alstom’s GT26 as an example, there are some 150 units installed globally with a total installed capacity of about 35 GW, with about half of this in Europe alone. As Tessarini says, “such an increase in operating range over such a relevant installed capacity, it’s significant”.
David Appleyard is a freelance journalist specializing in the energy, technology and process sectors.
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