reasons for automating the operations of a combined cycle unit

Copyright 2005 ISA. All rights reserved. www.isa.org Presented at the 15th Annual Joint ISA POWID/EPRI Controls and Instrumentation Conference

48th Annual ISA POWID Symposium, 5-10 June 2005, Nashville, TN

Reasons for Automating the Operations of a Combined Cycle Unit

Dale Evely Principal Engineer Southern Company Generation 42 Inverness Center Parkway Birmingham, Alabama 35242

Bob Kelly Engineering Manager Southern Company Generation 42 Inverness Center Parkway Birmingham, Alabama 35242

Harrison Manning Senior Engineer Southern Company Generation 42 Inverness Center Parkway Birmingham, Alabama 35242

John Sorge Research Engineer Southern Company Generation 600 N 18th Street Birmingham, Alabama 35202

Cyrus Taft Chief Engineer EPRI I&C Center 714 Swan Pond Road Harriman, TN 37748

KEYWORDS

Combined Cycle, Automation, HRSG, Combustion Turbine ABSTRACT

As of the end of 2003, there were more than 500 operating combined cycle units in the United States representing approximately 175,000 MW of generation. Most combined cycle units were designed for base load operation with only infrequent startups and shutdowns. Natural gas prices have risen to the level where it is now uneconomic to operate the combined cycle units in this mode and these units are now being asked to operate in two-shift operation much of the year. Whereas these units were expected to start only several times a year, they are now being called on to start more than 100 times per year and this trend is expected to continue for the next several years. Problems resulting from this cycling include additional wear and tear on plant components and increased fuel consumption. Combined cycle units generally have very modern, comprehensive distributed control systems. However, as with the balance of the plant, the design focus was for base load operation and, therefore, startup automation was not considered a priority. EPRI and Southern Company are participating in a project to develop and demonstrate enhancements to the combined cycle unit's distributed control system (DCS) to highly

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automate startups and shutdowns. Potential benefits include: (1) faster startups; (2) reduced equipment wear and tear; (3) more uniformity between startups; and (4) formalization of operational best practices in the DCS. This paper will discuss drivers and plans for the project. NOMENCLATURE

HRSG - Heat Recovery Steam Generator DCS - Distributed control system CT - Combustion turbine EPRI - Electric Power Research Institute HP - High pressure LP - Low pressure IP - Intermediate pressure

INTRODUCTION

As of the end of 2003, there were more than 500 operating combined cycle units in the United States representing approximately 175,000 MW of generation or nearly 20% of total generation.1 Most combined cycle units were designed for base load operation with only infrequent startups and shutdowns. Natural gas is the most common combined cycle fuel and gas prices have risen to the level where it is now uneconomic to operate the combined cycle units in a base load mode and these units are now being asked to operate in two-shift mode for much of the year. Whereas these units were expected to start only several times a year, they are now being called on to start more than 100 times per year and this trend is expected to continue for the next several years. Problems resulting from this cycling include additional wear and tear on plant components and increased fuel consumption. Combined cycle units generally have very modern, comprehensive distributed control systems. However, as with the balance of the plant, the design focus was for base load operation and, therefore, startup automation was not considered a priority. EPRI and Southern Company are participating in a project to develop and demonstrate enhancements to the combined cycle unit's distributed control system (DCS) to highly automate startups and shutdowns. Potential benefits include: (1) faster startups; (2) reduced equipment wear and tear; (3) more uniformity between startups; and (4) formalization of operational best practices in the DCS. UNIT DESCRIPTION

The host site for the demonstration is Southern Power's Plant Harris located in Autaugaville, Alabama. This facility consists of two combined cycle units (Unit 1 and Unit 2) each capable of generating approximately 620 MW when operating at standard ambient condition. The units began commercial operation in 2003. The combined cycle units are assembled as blocks of generation with each combined cycle unit consisting of two combustion turbines (CT), two heat recovery steam generators (HRSG), and one steam turbine (ST) (Figure 1). Natural gas is the fuel for both the combustion turbines and supplementary firing in the HRSGs. Normal operation is either with both CTs operating (i.e., 2-on-1 operation) or with a single CT operating (1-on-1 operation). The unit is not designed to operate in simple cycle mode (i.e., without the HRSGs and steam turbine). A design basis for the unit was for base load operation with only a few starts per year, however, with escalating natural gas prices, the unit is now being asked to operate in two-shift operational mode for much of the year.

1 Includes combined cycle units of all sizes and all fuel types.



A list of the major physical components of the unit is shown in Table 1. The general configuration of the system consists of two CT/HRSG trains supplying steam to a single reheat turbine (Figure 2). In normal operation, each HRSG supplies approximately 50% of the steam requirements of the steam turbine. The combustion turbines are GE Model PG7241FA with simple cycle power outlet of 172 MW at rated ambient conditions (65°F, 60%RH) and with the inlet evaporative cooler operating. The CT controls are a GE supplied and configured Mark V SPEEDTRONIC system. The steam turbine (ST) is an Alstom supplied 3,600 RPM reheat, single shaft condensing machine with high pressure (HP), intermediate pressure (IP), and low pressure (LP) sections. The ST control system is an Alstom supplied and configured TGC 820 system. The hot exhaust gases from each CT pass through a natural circulation, finned tube, HRSG. In addition, a natural gas fired duct burner is installed in each HRSG. The HRSG is a triple drum design and is supplied by Deltak. The controls for the HRSG and the balance of plant are a Southern Company design and are implemented in an Emerson Ovation distributed control system (DCS). The Ovation, Mark V, and TGC 820 systems are interconnected by both analog loops (for critical and shared inputs) and a serial digital link (for less critical data). Control system coordination is through the Ovation system (Figure 3).

Table 1 - Major Component List Equipment Vendor Description Combustion Turbine GE 7FA with GE hydrogen cooled generator HRSG Deltak Triple pressure with deaerator and supplementary firing Steam turbine Alstom HP, IP, and LP sections, condensing Boiler feed pumps Sulzer Horizontal, multistage, variable speed, with intermediate stage extraction Control System Emerson Ovation distributed control system (configured by Southern Company) CT controls GE Mark 5 (configured by GE) ST controls Alstom TGC 820 (configured by Alstom)

Figure 1 – Combined Cycle Process Diagram



HP Turbine IP Turbine LP Turbine

Generator

Condenser

Condensate Pumps

HRSG Feedwater Pumps

LPDrum

IPDrum

HPDrum

To StackFrom CTCombustion

Exhaust

B HRSGB HRSG

B HRSG

IP BypassHP Bypass

B HRSG

Vent Vent Vent

Duct BurnerNatural Gas

LP Bypass

Figure 2 – Combined Cycle Process Diagram / "A" HRSG

Ovation DCS

GT A ControlMark 5

GT B ControlMark 5

ST ControlTGC 820

Figure 3 – Control System Coordination



PROJECT DRIVERS

Changing Operations The rise in natural gas prices in relation to coal prices has led to combined cycle units that were designed for base load operation being used in two-shift operation mode. Although combined cycle units have significantly higher thermal efficiency than coal fired units (50% vs. 35%), due to high gas prices, economics still tend to favor dispatching coal units over combined cycle units. Although other aspects (e.g. reduced emissions from combined cycle unit) factor in the dispatch, based on fuel cost alone, coal units are more economic than natural gas units below a gas/coal price ratio of 1.2, and therefore expectations are that combined cycle units will be dispatched after coal fired units for several years (Figure 4). A general indication of the impact of changing fuel prices on combined cycle operations is shown in Figure 5. 2 As shown, the number of combined cycle units approximately tripled from 2000 to 2003 with the corresponding number of starts increasing by near an order of magnitude. As another example, one unit has gone from a mostly base loaded unit in 2001 to a unit that experienced over 150 starts in 2003 (Figures 6 and 7). For this unit and many other similar units, the situation is particularly grave in that damage due to cycling is generally accelerated on modern combined cycle units running at elevated temperatures and pressures [Left1][Left2][EPRI1]. These units have higher efficiencies and maximum load capability and are generally of higher overall value to the generation fleet than older combined cycle designs or those designed specifically for cycling. If

2 As of March 2005, the data for 2004 was not available.

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Figure 4 – Coal & Gas Price Projections



projected starts are true, a unit will spend a substantial amount of its operating time in startup mode (possibly 10% or more). Similarly, there is the drive to reduce minimum load on combined cycle units. Operating permits for some combined cycle units limit the time in startup mode because emission rates are higher during this phase of operation. This could be the result of either: (1) high NOx emissions rates from the CT during startup for which the SCR was not designed to accommodate or (2) the SCR not being operable at the lower temperatures sometimes observed during very low load operation. Also, at low loads, control of the unit is generally more difficult due to non-linearities in the process. An example of this is the increased tendency of spraying steam flows into saturation during startup and low loads.

Combined Cycle Unit Historical Generation and Startups Source: EPA National Allowance Database (NADB) (for Acid Rain Program units)

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1. EPA database: http://cfpub.epa.gov/gdm/index.cfm?fuseaction=iss.emissions 2. For the NADB database, many CC units are reported as multiple units since they have a reporting CEM per train. 3. Includes only those units that have a maximum generation greater than 150 MW.

Figure 5 – Historical Generation and Startups



Figure 6 – Unit "A" Load Profiles from 2002 through 2004

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Figure 7 – Unit "A" Hours of Continuous Operation from 2002 through 2004



The detrimental impacts of cycling generally fall into the two broad categories of reduced thermal efficiency during startup and increased wear and tear on the unit. For the latter, cycling can exacerbate damage caused by the mechanisms listed in Table 2.

Table 2 – Damage Mechanisms

EPRI has estimated that more than 80% of HRSG pressure part failures are the result of poor cycle chemistry and damaging thermal transients [EPRI2]. Unit efficiency is also much lower during startup, when only the combustion turbine is operating, than during normal operation with the disparity being the greatest for the advanced technologies [EIA1]. For example, for an "F-class" combustion turbine, simple cycle heat rate is near 10,500 Btu/kWh as compared to around 7000 Btu/kWh when operating as part of a combined cycle unit. Depending on the duration, type, and number of startups, the incurred delta fuel cost over the life of the unit may be very substantial. Cycling damage and reduced efficiency is mitigated on units designed for the cycling mode of operation (though possibly at the expense of lower efficiencies or higher capital costs) [Blan1][EPRI1][McMa1]. The later designs may include a once-through HRSG, gas bypass systems, auxiliary boilers, reduction in thickness of thick wall components, and automated vent and drain valves. Startup Consistency Startups are generally classified as to whether they are cold, warm, or hot. Definitions vary, but as may be inferred by their name, the basis of this classification is the temperature of one or more components of the combined cycle unit just prior to initiation of the startup.3 In practice, these classifications are generally simplified as follows:

• Cold start - more than 48 hours since the gas turbine was last synchronized • Warm start - between 8 and 48 hours since the gas turbine was last synchronized • Hot Start - less than 8 hours since the gas turbine was last synchronized

An example of startup duration variability for one unit is shown in Figure 7. During this period (~1 year), startup duration varied from one hour to more than six hours. Although there are formal

3 Such as a steam turbine 1st stage or reheat bowl inner stage metal temperatures. The rate at which these metal temperatures may be changed without incurring significant material damage can be the limiting factor in startup times.

Thermal Mechanisms Chemistry Mechanisms - Corrosion fatigue - Thermal quench-induced fracture - Thermal fatigue - Creep fatigue

- Corrosion fatigue - Flow accelerated corrosion (FAC) - Under-deposit corrosion - Pitting corrosion



procedures addressing unit startup and each operator is well trained in these procedures, there are many variations in the startups, some of which can be attributed to operator preferences and actions. Automation Concept and Design The existing Ovation control system will be modified to provide a high level of startup automation for the HRSG’s and balance-of-plant (BOP) equipment at Plant Harris. There are expected to be little or no modifications made to the CT or ST control system because these are already highly automated. The concept of the new automation system is to approach a single-button startup for the HRSG and BOP equipment provided that extensive modifications to field equipment are not required. The entire startup procedure will be subdivided into small sequences of activities to make the logic development more manageable. Each sequence will be initiated by the operator or by the successful completion of a prerequisite procedure. Feedback from field devices will be used extensively to ensure that commanded operations actually occur as desired. Operator screens will be developed for all sequences that show all inputs and outputs of the sequence along with the status of all permissives necessary for the procedure. Any permissive which is inhibiting the sequence will be highlighted for the operator. An example of the operational sequences required to start up the plant are shown in Figure 9.

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Figure 8 – Unit "A" Startup Duration



Valve Alignment

CT On Turning Gear

ST On Turning Gear

Vacuum System

Lead CT and HRSG

Instrument Air System

Condensate System

Feedwater System

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Figure 9 – Cold Startup Sequence / Lead CT and HRSG

SUMMARY

Combined cycle units in the United States are rapidly changing from base load operation to daily cycling operation due to the large increase in natural gas prices. Daily cycling will decrease the plant’s efficiency and will likely increase the wear and tear on the plant equipment. In order to capture best practices of operators and improve operational consistency, an automation system is being developed and demonstrated at Southern Power’s Harris Plant. The project will modify the plant’s Ovation control system to add nearly single button startup capabilities to the HRSG and balance of plant equipment. The project is scheduled for completion in early 2006.

REFERENCES

[Blan1] Blankenship, S., "Fast-Starting Combined Cycles Must Beat the Drum", Power Engineering, May 2004.

[EIA1] Impacts of the Kyota Protocol on U.S. Energy Markets and Economic Activity, Energy Information Agancy, U.S. Department of Energy, October 1998.



[EPRI1] Cyclic Operation of Combined Cycle Plants – Design, Maintenance, Reliability, and Cost Impacts, EPRI, Palo Alto, California, February 2004. 1004796.

[EPRI2] Diagnostic/Troubleshooting Monitoring to Identify Damaging Cycle Chemistry or Thermal Transients in Heat Recovery Steam Generator Parts, EPRI, Palo Alto, California, March 2005. 1008088.

[Left1] Lefton, S.A., Besuner, P.M., Grimsrud, G.P., and Peltier, R.V., "The real cost of cycling powerplants: what you don't know will hurt you", Power, Nov/Dec 2002, Vol. 146, Issue 8.

[Left2] Lefton, S.A., Besuner, P.M., Grimsrud, G.P., and Kuntz, T.A., Experience in Cycling Cost Analysis of Power Plants in North America and Europe, Aptech Engineering Services, Inc., Sunnyvale, CA., Report TP133.

[McMa1] MCManus, M.T. and Baumgartner, R., Integration of Advanced Gas Turbine and Combined Cycle Technologies for High Efficiency with Operational Flexibility, presented at PowerGen 2003, Las Vegas, Nevada, December 2003.

[Pijp1] "HRSGs Must be Designed for Cycling", Power Engineering, May 2002.

reasons for automating the operations of a combined cycle unit

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