They`re he-e-re (almost): the 60% efficient combined cycle
By Timothy B. DeMoss, Associate Editor
Your next combined-cycle plant is not likely to be taken over by super-high-efficiency poltergeists (not that this would be a bad thing). But I wouldn`t turn my back.
At POWER-GEN Europe 1995, General Electric (GE) executives unveiled the company`s latest contribution to the combined-cycle efficiency race, the “H” technology gas turbine. Never mind that the technology had yet to be demonstrated; the promise of 60-percent net combined-cycle thermal efficiency in the near future was enough to turn plenty of heads. Since that time, the buzz has begun to subside. However, the lack of noise about 60-percent efficiency is not the result of promises undelivered. It is now clear that 60-percent efficiency is no longer an “if,” but a “how soon.” We now wait in quiet anticipation for someone to herald a 60-percent efficiency demonstration.
Though 60-percent efficient combined-cycle plants are hardly ubiquitous, since the GE announcement, nearly all the major gas-turbine players have at least hinted that they have 60-percent technology under development. Companies are building gas-turbine, combined-cycle plants with 56- to 58-percent efficiencies today, and considering that such plants were only promises themselves not long ago, the industry goal of 60-percent efficiency by 2000 doesn`t seem far-fetched. In fact, Power Engineering reported in June (“Gas turbines aim at world power market dominance”) that an ABB-equipped, 2,000-MW combined-cycle plant for Korea Electric Power Corp. is rated at 58 percent, but ABB expects the plant to achieve an overall fuel efficiency of “nearly” 60 percent.
Continuing to advance gas-turbine technology may ultimately fall to the hands of materials developers to allow even higher firing temperatures than the air- and steam-cooled turbines currently being developed. However, boosting combined-cycle efficiency does not necessarily have to happen in the gas-turbine industry alone. Combined-cycle designers may have only begun to wring efficiency points from the remaining cycle components and the cycle concept itself.
Figure 1 depicts GE`s “H” technology combined cycle. This basic design–gas turbine, steam turbine, heat recovery steam generator (HRSG)–is the foundation for almost every combined-cycle design. This efficient combination of the Rankine and Brayton cycles is easy and relatively inexpensive to construct. The challenge for today`s plant designers is how to maintain this cost effectiveness while finding innovative ways to increase cycle efficiency and keep emissions at a minimum.
Several options exist to continue pushing the efficiency envelope. Siemens categorizes the options this way:
z increased gas-turbine inlet temperature,
z advanced gas-turbine cooling techniques,
z auxiliary power consumption rate reduced,
z hydrogen-cooled generators,
z multiple-pressure steam cycles with reheat,
z better HRSG design and
z fuel preheating.
Of the three basic design components, gas turbines have certainly received the lion`s share of attention lately. Advances in gas-turbine cooling techniques are at the heart of the latest efficiency increases. However, progress is being made in steam-turbine and HRSG efficiencies (see sidebar) and no stone is being left unturned in the combined-cycle efficiency race. Innovative engineering is bringing to the table a number of technologies and ideas which look beyond the obvious and which will long keep combined-cycle design from becoming a “dead” science.
New tricks for old technologies
For example, combustion-turbine inlet-air cooling is no longer limited to conventional means such as evaporative cooling, which is less effective in humid conditions, and refrigeration, which is far from the goal of reducing auxiliary power consumption. The latest technique involves applying thermal energy storage technology similar to that used for some district cooling applications. By cooling the inlet air with ice made during off-peak hours, power plants can boost capacity and efficiency during on-peak hours. Once thought only applicable to peaking turbines, the technique has reaped large economic benefits for combined-cycle baseload applications, according to Strategic Energy Ltd.
The move to raise Rankine-cycle efficiencies is also picking up steam. Albert Kruetzer, Siemens fossil plant marketing head, stated, “In addition to further gas-turbine development, it appears that just the introduction of relatively straightforward system improvements, such as the use of supercritical steam cycles, will make it possible to achieve a combined-cycle efficiency of 60 percent.” Compared to a “base” combined-cycle design, which has a dual-pressure subcritical steam cycle (1,100 psi, 1,000 F), a triple-pressure subcritical cycle will have an overall efficiency gain of 0.6 percent and a triple-pressure subcritical cycle with reheat will have a 1.2-percent gain. According to Siemens, a supercritical cycle (3,600 psi, 1,000 F) compared to the base design will have a 1.9-percent gain with dual pressure, a 2.3-percent gain with single pressure and reheat, and a 2.4-percent gain with triple pressure at 1,050 F.
With the components in a single-shaft design, the cycle offers more than just higher efficiency and capacity compared to its multi-shaft counterpart. According to a Siemens study, it also provides better reliability and maintenance figures. Siemens is building a single-shaft, combined-cycle plant in Tapada do Outerio, Portugal, that combines several efficiency-boosting options. The plant will feature the company`s V94.3A gas turbine with a triple-pressure, reheat steam cycle, fuel preheating and a hydrogen-cooled generator. Siemens guarantees 55.4-percent efficiency for the plant, scheduled to begin operation in 1998, with the potential for 56-percent efficiency. These numbers aren`t even flirting with 60 percent, but the groundwork is being laid for proving the viability and reliability of the technologies that can make 60 percent achievable.
Kalina combined cycle
GE, which kick-started the race for 60 percent by announcing plans for the “H” technology gas turbine, is looking beyond the obvious for ways to surpass even the 60-percent milestone. The company signed a worldwide exclusive license with Exergy Inc. in 1993 to develop and commercialize the Kalina combined cycle. ABB holds the license for direct-fired Kalina plants.
The Kalina cycle, invented by Alexander Kalina, president of Exergy, was only a fringe concept just a few years ago. Today, it offers a way to wrangle precious efficiency points out of the combined cycle by changing the working fluid of the bottoming cycle. This change improves efficiency by increasing the thermodynamic availability, or exergy, of energy transferred from the topping cycle to the bottoming cycle.
How it works
At POWER-GEN Americas 1995, Robert Bjorge, leader of GE`s Power Systems Market Development; James Corman, general manager of GE`s Power Generation Systems; and Raub Smith, manager of GE`s Combined-cycle Systems Technologies, presented a paper which studied the performance of the Kalina cycle applied to gas-turbine combined cycles.
In a conventional combined-cycle system, gas-turbine exhaust provides the energy to produce steam for expansion in the steam turbine and subsequent condensation. In a Kalina combined cycle, gas-turbine exhaust energy vaporizes an ammonia-water working fluid mixture, which then expands through a vapor turbine. According to GE, the key features of the Kalina cycle are its ability to exchange energy from topping to bottoming cycle and to reject heat from bottoming cycle to heat sink in a non-isothermal manner. Figure 2 illustrates the temperature differences between the exhaust gas and the bottoming cycle working fluid, resulting in exergy loss and reduced power generation potential. Even a single-pressure Kalina cycle reduces these losses significantly.
In addition to exergy loss caused by gas/steam temperature differences, the combined cycle also suffers exergy loss as a result of HRSG exhaust gas temperatures compared to stack temperatures. Power output for a Rankine cycle is proportional to recovered exergy in the HRSG and made available to the steam turbine. Multiple steam-pressure levels make it possible to reduce exergy losses by matching more closely the exhaust-gas temperature profile in the HRSG, but additional levels add incrementally smaller performance benefits while adding increased cost and complexity.
The Rankine cycle fails to capitalize on matching profiles because it uses a single working fluid. This requires all evaporation to take place at a constant temperature, limiting exergy efficiency to a theoretical limit of 68 percent in power generation applications.
Advanced Kalina combined cycle
The Kalina combined cycle reduces exergy loss and increases bottoming cycle output by mixing two working fluids (ammonia and water) and varying their composition, tailoring the fluid properties at each point in the cycle to achieve non-isothermal evaporation. The resulting efficiency is an improvement compared to a single-pressure Rankine cycle, but by adding reheat and a regenerative evaporator (Figure 3), the advanced Kalina cycle achieves significantly higher efficiency than the best triple-pressure Rankine bottoming cycle. Adding reheat and diverting a fraction of preheated working fluid to a regenerative boiler before superheating in the heat recovery vapor generator (HRVG), provides a better match between the heat transfer profiles of the working fluid and the exhaust gas. By carefully balancing the flows, pressures and fluid composition, it is possible to achieve nearly optimum, parallel heat transfer profiles throughout the heat recovery system.
The mixed working fluid does have its downside in terms of fluid condensation. Because the approximately 80-percent ammonia mixture would require a 22 F temperature for condensation, a distillation condensation subsystem is necessary to regenerate the working fluid. This system requires three stages to return the fluid to a liquid composition suitable for return to the HRVG for heating. However, while complicating the condensation process, using a mixed working fluid does allow selecting a composition which results in a low-pressure turbine exhaust pressure above atmospheric at all operating conditions. This reduces the cost of the low-pressure turbine and simplifies other system aspects.
Kalina combined- cycle performance
Using the Kalina cycle, GE has recorded efficiency gains up to 2.5 percent with its “F”-class turbine combined cycle and gains of up to 4 percent with its LM6000 aeroderivative gas turbine, according to Bjorge. He also said compared to a Rankine-cycle system, the GE STAG 207FA Kalina-cycle system produced an additional 25 MW of net power. Table 1 shows how the Kalina and Rankine cycles compare head-to-head using a STAG 207FA.
In their POWER-GEN paper, the GE authors discussed the economic evaluation of the Kalina cycle versus the conventional Rankine cycle, assessing life-cycle cost in terms of capital, fuel, and operations and maintenance costs. Based on improved efficiency and assuming a 10-percent discount rate, a 20-year life, an 18-percent fixed charge rate and a 4-percent fuel cost escalation, the Kalina cycle provides an equivalent total combined-cycle capital cost savings of $30/kW to $100/kW. However, considering additional power output alongside efficiency increases, this value is enhanced further, said the authors. Figure 4 shows the total value in incremental $/kW of the Kalina cycle above the Rankine cycle for a range of operating hours and fuel price scenarios.
The benefit shown is the sum of incremental power output value ($500/kW in the figure) together with the efficiency value, because the incremental power output is produced with no incremental operating cost. Assuming an incremental cost of $27 million more than the Rankine cycle, the horizontal line at $1,060/kW in Figure 4 presents this cost on a $/kW basis for the Kalina cycle. For example, assuming $4/MBtu fuel cost, Kalina would be the economic choice for a plant operating more than 3,300 hours per year.
What`s to come
In a recent interview with Power Engineering, Bjorge said GE is trying to resolve some materials issues and drive down system costs for Kalina combined cycle, but the company is ready to build a commercial-scale plant to demonstrate the technology. He said the system would work especially well for high fuel-cost generating plants.
By 2000, it is conceivable that many of the technologies discussed here will be commercially available. As engineers continue to combine these technologies, reaping efficiency gains from each one in the process, 60-percent combined-cycle efficiency may become run of the mill–a thought which is far from scary. z
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Improving HRSG performance
By V. Ganapathy, ABCO Industries
Gas-turbine heat recovery steam generators (HRSG) are peculiar in that their gas-to-steam ratio is very large compared to conventional steam generators. This difference occurs because the inlet gas temperature (800 to 1,000 F) from the turbine is much lower than the inlet temperature from a boiler (3,000 F). In addition, the exit gas temperature is high in single-pressure units because the HRSG economizer is a small heat sink. This elevated exit temperature results in lower steam production at increased steam pressure and temperature. This is the reason for using multiple-pressure-level steam generation in large, high-pressure HRSGs. Because this is a thermodynamic problem irrespective of surface area, the gas/steam temperature profile cannot be significantly changed.
Furthermore, in gas turbines exhaust gas flow and gas temperature vary with ambient conditions and load. At low turbine loads, gas flow does not vary much, but inlet gas temperature decreases, resulting in lower steam production. This means the economizer is a smaller heat sink at low load, so the exit gas temperature increases. Thus we see the interesting trend that as the inlet gas temperature to the HRSG decreases, the exit gas temperature increases, and vice versa. These aspects require new ways to improve energy recovery.
Consider a base case single-pressure HRSG generating steam at 600 pounds per square inch gauge and 650 F using feedwater at 240 F from a deaerator. Table A describes the base case and subsequent improvement options. The exit gas temperature is 374 F. Steam is sent to a turbine where deaeration steam is extracted. This steam is condensed and returns to the deaerator after makeup water is added. Comparing the following modifications to this simplified model, the base case generates the lowest electric power.
In the first option, adding a condensate heater to preheat the makeup water requires less turbine steam for deaeration, generating more power. This option works well with a natural-gas-fired turbine. Using distillate or heavy oil could result in acid condensation in the condensate heater. The log-mean temperature difference across the heater is high and as a result, surface area required for this option is small and hence inexpensive.
In the next option, all steam required for deaeration is generated in the low-pressure evaporator. As a result, no extraction steam is required. This option generates the maximum power output, but the controls, steam drum and instrumentation make this option expensive. Nonetheless, by operating at a temperature close to or above the acid dew point, this option alleviates acid condensation concerns.
The last option in Table A uses a heat exchanger to cool feedwater for the economizer while heating the makeup water. This option is not as expensive as the low-pressure evaporator option because another economizer is not necessary, only a larger one. Steam is still required for deaeration, but the quantity is reduced due to the higher makeup water temperature entering the deaerator. This option may not be feasible if the mixed water temperature is close to the feedwater temperature. Cross conditions in the exchanger may make it large or uneconomical.
Auxiliary firing in the HRSG improves the steam generation efficiency for two reasons. First, because the gas flow does not vary significantly with steam generation, raising the inlet temperature generates additional steam. This increases the heat sink, thereby reducing the exit gas temperature and increasing the efficiency. Second, as excess air increases, heat losses increase, decreasing efficiency. By firing the HRSG, excess air decreases and efficiency goes up. Table B shows some results of auxiliary firing as calculated by the software “HRSGs.”
Note that the fuel utilization efficiency in an HRSG is nearly 100 percent, while that in a conventional gas-fired steam generator is 90 to 93 percent (lower heating value). Thus, one of the simplest ways to improve HRSG efficiency is to design it as an auxiliary-fired unit. Raising the inlet temperature for these units to approximately 1,700 F from 900 to 1,000 F requires minimal additional costs compared to an unfired unit.
HRSG performance …