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THE FUTURE OF GAS TURBINES: FRAMES OR AERODERIVATIVES?
Nowadays, the trend is definitely toward gas turbine power generation units. Gas turbines, after all, offer high power density and have enabled significant unit train capacity increases, both for power generation and mechanical drive. Neither coal power plants with carbon capture or nuclear power plants can compete with gas in the current economic and political climate. The question is, do aeroderivatives or heavy-frame machines represent the future?
If high fuel costs are expected, the selection of high-efficiency aeroderivatives begins to play a more important role in the lifecycle cost evaluation process. And for simple-cycle applications, the efficiency advantage of aeroderivative gas turbines is clear.
When it comes to using aeroderivatives for combined-cycle base-load applications, though, their advantages (besides space and weight) are often questioned. However, aeroderivative-based combined-cycle plants utilizing commonly used systems could offer slightly higher efficiency compared to frames in the range of 20 MW to 100 MW.
Further aeroderivative advantages include greater flexibility, faster start-up and easier module replacement. Theoretically, complex aeroderivative combined-cycle arrangements can be designed to reach much higher efficiency compared to frames. In other words, for aeroderivative gas turbines, the firing temperatures are higher compared to heavy frames, which theoretically equates to efficiency.
The reality is that modern large-block combined-cycle gas turbines with thermal efficiencies ranging above 60% currently employ many advanced features of aeroderivatives. Thus there is room for aeroderivatives to play a more prominent role in the market. However, the conservative nature of the turbomachinery industry has dictated that there are fewer references for aeroderivative applications. Once sales pick up, the combined effect of more references and more hours of operation will translate into lower initial prices. After all, further refinements and lower costs are far more likely on a unit that has achieved market popularity. In the future, then, we will see far more applications of aeroderivative gas turbines.
A dual gas turbine set-up (or a twin-pack arrangement), which means a gas turbine at each end of an electrical generator, offers more power for a train, better redundancy and easier maintenance. It is a good arrangement for deploying smaller gas turbines in a larger power train. This set up could have a positive effect on the aeroderivative market, enabling them to compete with frames in larger block sizes.
Future developments can also be expected via advances in aircraft/aerospace engines. This works to the benefit of aeroderivatives in the long run as they form a relatively small portion of the immense aircraft/aerospace engine industry. Thus limited R&D dollars can be spent on how to convert a successful aircraft engine to the industrial gas turbine marketplace in terms of required power, speed range and characteristics. That said, this remains a challenging task.
Discussions are ongoing, too, on whether future developments are likely to come from wide-body or narrow-body aircraft engines. Whatever the outcome, the likelihood is that aeroderivatives operating beyond 80 MW will become more competitive compared to similarly sized frame machines.
A considerable amount of heat is wasted as gas turbine exhaust gas. One of the future goals in gas turbine development is to achieve the best use of the heat discharged along with the exhaust gas. The integration of the gas turbine into the power system or process unit, however, represents a continuous challenge as regards technical issues such as flexibility at different operating scenarios. Integration of gas turbines into a downstream system which could be a heat recovery system (usually a waste heat recovery steam generator), a cogeneration unit, an afterburning system, burners of a furnace or similar, should consider the performance, lifetime, operation, and safe running of the plant.
The oxygen concentration in the turbine exhaust gas is usually around 9% to 19% of the volume. The fact that the combustion process consumes only a small part of the oxygen from the intake air flow makes possible the application of supplementary firing to increase the heat rate of the exhaust gas and gain operational flexibility.
In a combined-cycle design or an integrated design, the performance of a downstream system depends on the gas turbine’s working regime which makes the downstream system difficult to control. On the other hand, downstream operation could affect the exhaust gas pressures/parameters and consequently may impact the gas turbine. The main problem is how to integrate a gas turbine and a downstream system in such a way that each can work with the flexibility required.
Flexibility is one of the limiting factors which calls for special attention. In many process or power units, aeroderivative gas turbines are preferred. The possibilities are limited for using relatively heavy liquid fuels, which could support the use of frame gas turbines. The aeroderivative can offer better flexibility which is crucial for integration and proper operation of a combined system.
In the short term, though, peaking power is likely to be a major usage of aeroderivatives. But longer term, I expect either aeroderivative gas turbines or gas turbines with more aeroderivative features like the LMS100 to replace a considerable number of frames in base-load applications. In addition, aeroderivatives should become prominent in mechanical drive, too.
Amin Almasi is a Chartered Professional Engineer in Australia, Queensland and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.