Cogeneration plant owners all require different levels of flexibility from their plants, and they all need to see a return on their investment. An analysis shows how different cycle configurations can meet different needs.

A combined cycle cogeneration facility can be quite complex as there are many different configurations that can be selected and optimized to provide the desired flexibility, reliability, and rate of return for the owner. In competitive markets, these are important factors, and so selecting the right cycle is paramount.

The three most common cycle configurations using gas turbines in combined cycle applications to meet process steam demands have been evaluated by Bechtel for thermal performance and financial analysis. The results indicate different levels of flexibility, performance, and economics for each cycle.

Cycle configuration

Certain assumptions have been made in the evaluation: all cycles are based on F-technology, advanced gas turbines (GTs) in a plant configuration consisting of two GTs and two heat recovery steam generators (HRSGs). One steam turbine (ST) is included in two of the configurations. The steam cycles are non-reheat, dual-pressure cycles.


Figure 1. Steam and power map for cycle configuration 1
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Cycle configuration 1: The first cycle configuration is simple, consisting of two GTs, each exhausting to a dual-pressure level HRSG. The high pressure (HP) section in each HRSG is designed to provide the desired process steam to the host, while the low pressure (LP) section is designed to provide sufficient steam for deaeration of the cycle makeup water. Duct burners are included in the design. This facility also includes a dump condenser for steam rejection, providing greater flexibility to reduce or eliminate process steam to the host while maintaining equivalent power production.


Figure 2. Steam and power map for cycle configuration 2
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Cycle configuration 2: This is more complex, consisting of two GTs exhausting to two dual-pressure level HRSGs plus one backpressure ST. HP steam is supplied to the backpressure ST to generate additional power, while exhaust steam at desired conditions is sent to the process host. The LP section of the HRSG is used for deaeration of the cycle makeup water. The steam cycle is designed for maximum process flow as determined by maximum duct burner firing, and includes a dump condenser to allow for flexibility of process steam and power production.


Figure 3. Comparison of cycle efficiencies. Although less process steam is produced, configuration 3 provides the greatest overall cycle efficiency
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Cycle configuration 3: This consists of two GTs exhausting to two dual-pressure level HRSGs and one extraction-condensing ST. The HRSGs are capable of maximum duct firing to achieve maximum steam flow. Both HP and LP steam pressure levels are admitted to the ST. The process steam is extracted from the ST at the desired pressure level and is desuperheated to achieve the desired temperature level. The deaeration of cycle makeup water is accomplished in a separate vacuum deaerator using steam extracted from the ST.

Power and steam

For each cycle configuration, performance was evaluated at three process steam conditions to illustrate the range of power and process steam output that can be produced. The steam conditions were:

  • Low pressure: 965 kPa, 205°C
  • Medium pressure: 2860 kPa, 288°C
  • High pressure: 5620 kPa, 343

    All figures presented in this analysis are based on medium pressure steam conditions.

    Cycle configuration 1: Variations in output range from approximately 80 MW to 340 MW due to GTs operating at loads of 50 per cent to 100 per cent, with either one or both GTs in service. Process steam, ranging from approximately 76 to 273 kg/s can be produced based on GT exhaust energy plus HRSG duct firing. Steam in excess of steam demand can be diverted to a dump condenser, thus the total process steam range can vary from 0 to 273 kg/s.

    Cycle configuration 2: The output varies from approximately 80 MW to 420 MW while the steam production is in the range of 76 to 239 kg/s. The maximum power output is higher than for cycle configuration 1 because steam is first passed through the ST before being exhausted and then exported to the process steam host. Again, excess steam can be diverted to a dump condenser which allows no loss in power production when steam demand is low. Since the backpressure ST requires a higher throttle pressure, HP steam is produced in the HRSG at a higher pressure, resulting in a maximum process steam production of 239 kg/s.

    Cycle configuration 3: Power production ranges from 90 MW to 500 MW due to the larger ST which is designed to condense all steam produced by the unfired HRSG trains. The maximum process steam flow in this configuration is relatively lower, with process steam varying from 0 to 202 kg/s. This lower process steam production is not only due to higher ST throttle pressure, but also due to ST minimum steam flow requirements for last stage blade cooling.

    While configuration 1 provides the greatest steam production capability, it also operates at a relatively low cycle efficiency since the GT exhaust energy is not fully recovered within the HRSG. Configuration 2 recovers a greater amount of the GT exhaust energy within the HRSG and uses the recovered energy to produce additional power in a backpressure ST. This results in an improved cycle efficiency as compared with configuration 1.

    Conversely, configuration 3 is designed for the HRSG to recover the maximum amount of GT exhaust energy which is then utilized in an extraction-condensing ST. Although less process steam is produced, configuration 3 provides the greatest overall cycle efficiency.

    Economic factors

    For any owner, the primary goal is to maximize return on equity (ROE) for the facility. Since the primary operating cost for any power plant is fuel, and the sources of revenue are based on the sales of power and steam, small variations in fuel, electricity and steam prices can result in very different levels of ROE. Therefore, the plant owner must establish the desired ROE to support project viability while taking into account the prevailing industry prices of fuel, electricity, and steam.

    Since it is assumed that any cogeneration facility will be required to meet a range of process steam demand, the evaluation was performed based on the varying operating dispositions within the capability range of each configuration. Financial calculations were performed based on two operating dispositions.


    Figure 4. Return on Equity versus electricity price for Disposition A
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    The first scenario (Disposition A) depicts a situation for which the primary purpose is an almost continuous supply of process steam to the host (80 per cent of the time).


    Figure 5. Return on equity versus electricity price for Disposition B
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    The second scenario (Disposition B) represents a contrasting situation in which process steam supply is critical, but is only required 50 per cent of the time.

    The results illustrate the important relationships between fuel price, electricity price, steam price, and ROE for operating at Dispositions A and B. As parameters are varied over typical market values, the data can be used to evaluate and select the best cycle configuration for specific project criteria.

    ROE vs. fuel price: This illustrates the ROE relationship with changes in fuel price for the three configurations, and provides guidelines for the owner as to how much ROE will be achieved based on a given fuel price. For low fuel prices, Disposition A showed that a similar ROE will exist for configurations 1 and 2; however, there will be a large gap between those and the ROE of configuration 3. This indicates that even though configuration 3 has the best efficiency, a low fuel price is not able to compensate for the large differences in steam production capability.

    If the fuel price increases, the ROE will decrease between configurations 1 and 2, and configuration 3. This is due to cycle efficiency becoming a more dominant factor in the equation.

    For Disposition B, which is based on a lower operating process steam demand, power production becomes a more dominant factor in the equation. Since power is valued, and configuration 3 has the best cycle efficiency for the greatest level of power production, it will maintain the highest ROE over the range of fuel prices.

    ROE vs. process steam price: For Disposition A in the low steam price range, ROE for configurations 2 and 3 will be almost equal, both having greater ROE than configuration 1. This is because configuration 1 has the lowest efficiency and lowest output relative to the other configurations. As the price of steam increases, the ROE of configuration 1 will improve, and will eventually exceed the ROE of the other configurations due to its ability to produce greater amounts of process steam. This demonstrates the value of greater process steam production over power and efficiency.

    For Disposition B, configuration 3 will have a higher ROE compared with configurations 1 and 2 in the lower steam price range. If steam prices become very high, configuration 1 will develop the better ROE due to highest steam production capability. This trend is similar to that described for Disposition A.

    ROE vs. electricity price: Figures 4 and 5 show the ROE relationship among the three configurations with change in electricity price. Figure 4 shows configuration 3 to have the lowest ROE over the electricity price range considered. For the parameters and operating disposition selected, the steam production ability of configurations 1 and 2 remains the dominant factor and does not outweigh the greater power production and better efficiency of configuration 3. Configuration 2, which is a combination of both relatively high power output and steam production consistently results in the greatest ROE.

    Compare this with Figure 5, representing Disposition B, which again illustrates the importance of evaluating each facility for the specific operating mode anticipated. Here, configuration 3 becomes the most attractive option over the electricity price range considered due to longer operating periods demanding power output with minimal steam. Electricity production now becomes the driving force.

    Flexibility, performance, economics

    Each of the three configurations can illustrate the ROE relationships between different levels of flexibility, performance, and economics.

    Flexibility: Each configuration uses HRSGs designed with duct firing to increase the plant flexibility by meeting the desired level of steam and power. Among the three facilities, configuration 3 has the highest flexibility due to the condensing ST. In configuration 1, if the steam demand falls below the unfired HRSG mode of operation, then the GTs are required to run at part load in order to meet the lower steam demand, resulting in reduced power output.

    In configuration 2, the non-condensing ST will provide steam, as much as is required, based on HRSG duct firing level. If the steam demand falls below the maximum flow, HRSG duct firing is reduced, supplying less steam to the ST, and therefore resulting in reduced power output.

    Configuration 3, however, has the advantage over configurations 1 and 2, in that whenever steam demand falls below the maximum flow, more steam can be passed through the ST. This larger ST capacity results in greater output. So, in this configuration, desired steam demand can be traded off with power output, assuming that the plant owner can sell additional power. It should be noted that flexibility in configurations 1 and 2 can be increased due the inclusion of a dump condenser.

    Performance: Configuration 1, having the lowest power output and cycle efficiency, has the largest steam production because steam is produced at the desired process steam pressure level in the HRSG.

    In configuration 2, power output is relatively higher and steam production is relatively lower. Cycle efficiency is also improved due to steam being produced at a higher pressure level so that it can pass through the ST before it is exhausted to process.

    Configuration 3 produces the highest power output and greatest cycle efficiency, but with lowest steam production. This is due to the extraction-condensing ST which needs minimum steam to flow through the last stage blades for the purpose of cooling, in addition to a higher ST throttle pressure to allow for extraction. This results in minimum steam available for export when compared to configurations 1 and 2, but allows for generating maximum power output.

    Economics: The best configuration must be selected based on careful consideration of both anticipated operating modes and economic factors.

    For Disposition A, configurations 1 and 2 will result in higher ROE than configuration 3. Based on price level of electricity, steam and fuel assumed, the higher power output produced by configuration 3 does not compensate for the higher process steam export supplied by configurations 1 and 2. However, in a situation such as Disposition B where consistently higher levels of power production are desired in conjunction with process steam, configuration 3, which has higher output and better efficiency, becomes a more favourable option. In addition, for a desired ROE, Disposition B requires a lower steam price over Disposition A throughout the range of fuel and electricity prices because of the higher power output and lower fuel consumption rate.

    Primary products

    Based on economic parameters assumed, for Disposition A where the primary product is steam, configurations 1 and 2 should be the preferred designs based on higher ROE. Conversely, for Disposition B, where the primary product is power, or both steam and power, configuration 3 should be the preferred design due to better efficiency and greater power production. In addition, if plant flexibility is an important requirement, configuration 3 may again be considered a more attractive option.

    The selection of a cogeneration facility type and the economic parameters are very much site specific and are based on numerous variables such as site ambient conditions, the level of desired power output and steam demand, capacity factor, flexibility, power purchase agreement and steam purchase agreement requirements, and owner’s economic parameters for ROE. The configurations and economic parameters assumed are for illustration purposes only; it should be recognized that a specific project design needs to be optimized for actual project-specific criteria.

    However, this methodology of analysis for the selection of cycle configuration based on economic parameters and operating dispositions can be used to maximize facility ROE.

    Reference

    This article is based on a paper (2000-GT-301) presented at the 2000 ASME TurboExpo Conference, May 8-11, 2000, Munich, Germany, and is reproduced with permission of the American Society of Mechanical Engineers.