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Suited for Repowering Existing Power Plants with Advanced Pressurized Fluidized-Bed Combined Cycles

APFBC combined cycles have high energy efficiency levels because they use modern, high-temperature, high-efficiency gas turbines as the core of a combined power cycle.  This web page discusses a current U.S. Department of Energy project that is evaluating combustion turbines suited for repowering existing steam plants.

The natural-gas-fueled version of the Siemens Westinghouse Power Corporation W501F. Modified versions of this gas turbine core are suited for operating in APFBC power plants.

Contents:

• Introduction

• APFBC Repowering Considerations

• Aeroderivative Prospects for APFBC

• APFBC Components for Gas Turbine Operations

  • Boost Compression System

  • Hot Gas Valves and Control Systems

  • Topping Combustor and Multi-Annular Swirl Burners

• APFBC-Modified Gas Turbine Cores

  • Siemens Westinghouse Power Corporation W501F

  • Siemens Westinghouse Power Corporation V64.3 and V84.3

  • Rolls-Royce Industrial Trent

  • Pratt & Whitney Turbo Power FT8 Twin Pac

• Specifications for APFBC Service

Introduction
This web page describes design issues for selecting gas turbines to repower existing steam power plants with an advanced coal-fired technology.   The discussion focuses on the integration of advanced circulating pressurized fluidized-bed combustion combined cycle technology (APFBC) as a repowering option for an existing steam turbine generator.

Gas turbine configurations, ranging from 70 MWe to 250 MWe simple cycle output, are evaluated to assess their suitability for application in APFBC systems.  The paper considers the application of both large-frame and aeroderivative gas turbines for APFBC repowering, and shows the design features needed for this service.   These considerations summarize conclusions developed from several detailed assessments, and show how these are being applied in evaluations that are now underway.

Today’s commercially offered pressurized fluidized-bed combined cycles (PFBC) use low-temperature (1500°F) ruggedized industrial gas turbine expanders for gas expansion, and are often referred to as "1^st-Generation PFBC" systems. APFBC instead uses the latest gas turbines with higher operating temperatures, providing a significant improvement in energy efficiency and reduction in emissions.  With the application of the advanced gas turbine and its associated coal-to-syngas conversion, the APFBC is also often referred to as "2^nd- Generation PFBC."  APFBC uses modified versions of the high-performance, high-temperature (2400°F) gas turbines common in natural gas-fueled combined cycle service today.  APFBC, however, employs a carbonizer for the production of the low-Btu syngas used to reach the high combustion temperatures needed by these gas turbines for high efficiency.  This mild gasification allows fluidized- bed temperatures to remain the same as today’s commercial PFBC equipment.

Coal-fired APFBC repowering has been found to be an efficient, economical, and environmentally superior technology option for power generators who are considering maintaining or adding baseload coal-fired capacity.  These considerations were developed in a series of DOE-sponsored APFBC repowering concept evaluations at the Progress Energy L.V. Sutton power station Units 1 and 2, the Duke Power Dan River power plant Unit 3, AES Greenidge power station Units 3 and 4, Nebraska Public Power District Units 1 and 2, and the Arizona Public Service Four Corners station Units 1, 2, and 3.  Capital investment is lower with APFBC repowering when compared to new pulverized coal plant construction, since a significant amount of existing equipment is retained.  In addition, operations with APFBC have significantly lower operating costs, and APFBC use significantly improves environmental performance of existing units.  Utility company production costing evaluations have shown that APFBC technology promotes a low-use unit from 10 percent capacity factors to first-dispatched baseload status.

However, repowering an existing site with this technology raises a number of plant integration issues directly impacting the selection of the gas turbine that are important considerations for any generator contemplating this type of plant upgrade.  This web page focuses on a number of these issues, including the following:

• Determining how combustion turbine type and size affects thermal and economic performance.

• Defining the operating characteristics of today’s commercial gas turbine offerings that are attractive for APFBC applications.

• Evaluating the value of either matching existing steam conditions or selecting
  a new steam turbine generator.

• Defining the most cost-effective thermal power cycle without compromising production and emission enhancements.

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Considerations for APFBC in Repowering Application

Click on picture to enlarge

Repowering Applications, Design Compromises and Considerations

An APFBC plant in repowering application, however, is not an all-new steam cycle.  Rather, the steam cycle exists and has fixed design and part-load characteristics.  Here, it is important to select a combustion turbine, also of fixed size, to maximize combined cycle performance.  Some of these effects are illustrated in the sketch below, which presents a series of designs possible with a fixed-size gas turbine. One trajectory is An/Bn/Cn, which illustrates different plant designs with -- as you move toward the right -- larger and larger all-new, custom-designed steam turbines and heat recovery systems. The second trajectory, A/B/C, illustrates the series of designs that could be considered when a fixed-size turbine is used to repower an existing steam station.

It is unlikely that the steam turbine will match exactly the thermal integration as required for operation at point "A," where the combustion turbine power output is large relative to the steam turbine power.   The illustration above shows the likely circumstances for a repowering, compared to the same combustion turbine that has been perfectly matched to an all-new steam turbine system "An." [Weinstein and Travers, 1997]

Point "A" supplies all of its coal to the PFB carbonizer.  Only the char left over from the carbonizer’s mild gasification process is passed on to the PFB combustor and fluid-bed heat exchanger to raise superheat/reheat steam.  The further one moves to the right in the illustration, an increasing amount of coal supplements the char to PFB combustor to generate additional steam in the fluid-bed heat exchanger.  That increment in coal does not add energy to the gas turbine, so combined cycle efficiency is compromised, although output increases substantially at a low addition in plant cost.

In the example illustrated, point "A," most of the available energy goes to the combustion turbine, and does not generate enough steam to fully power the existing steam turbine.  The steam turbine is at part-load conditions, and loses energy efficiency by being at off-valve point of operation, and at other than design flow conditions.  In a repowering, where the steam turbine size is fixed, the real "peak efficiency" point may occur with some added coal to the PFBC to generate steam that picks up the steam cycle efficiency.   This is point "B" on the exhibit.

Notice that none of the points on the repowered plant "trajectory" is as high as an all-new "greenfield" steam cycle designed to perfectly match the capability of an APFBC system.  This is due to several reasons:

• The existing steam turbine system has suffered wear and tear, and is unlikely to be performing in its "as-new" condition.

• Even if the existing turbine were refurbished to its "as-new" condition, it would be less efficient.  For example, the existing turbine was built with 1950s technology; new equipment would use modern steam turbine generator designs that are inherently more efficient.

• The existing steam turbine cycle is usually optimized for feedwater heaters in service, whereas efficient APFBC integration uses economizer duty to replace feedwater duty.  An all-new system would be optimized for the specific conditions of the APFBC application, whereas an existing system will not be, and almost always involves some compromise in repowering.

Another consideration is the relationship of the size of the existing steam turbine compared to the selected combustion turbine added for APFBC repowering.  Adding coal to the PFBC increases steam production, but an existing steam turbine has a fixed maximum flow capability (particularly with feedwater heaters out of service).  Once this is reached, no further output capability exists at the site.   An all-new design would simply add a larger steam turbine, allowing a decision to move to output point "Cn."  At some sites, such as the L.V. Sutton station repowering and the Greenidge station repowering, there is more than one steam turbine that can be repowered at the site.  If more output is needed at such sites, it is feasible to add more coal to the PFBC, and develop sufficient steam to repower two steam turbines, allowing operation at point "C."

Aeroderivative Prospects for High-Efficiency APFBC Combined Cycles

Aeroderivative units are gas turbines originally designed as turbofan engines for aircraft propulsion.  These engines usually have lightweight frames, high pressure ratios (over 25 : 1), and high firing temperatures, needed for efficient aircraft propulsion and low thrust-to-weight ratios.  When converted to stationary service, the aerodynamic cores and much of the design are retained. Usually firing temperatures are lowered to preserve life in the different operating environment of power generation. Because of their high pressure ratios, these engines usually excel in simple cycle efficiency.  However, they generally suffer as combined cycles because their high pressure ratios result in low exhaust gas temperatures (see the illustration below) and less efficient heat recovery steam generation.

Overall Pressure Ratio vs. Exhaust Gas Temperature

APFBC has one advantage over natural gas combined cycles that may well leave aeroderivative units with excellent potential in an APFBC application.   Unlike natural-gas-fueled combined cycles, APFBC maintains combined cycle efficiency even with the low turbine exhaust temperature that comes from high pressure ratio combined cycles.  An APFBC unit does not need high gas turbine exhaust temperature to raise high-temperature steam.  In an APFBC unit, final superheat and reheat steam is generated in the high-temperature (1650°F) fluid-bed heat exchanger, not in the heat recovery steam generator (where conventional combined cycles must raise the steam).  This means that highest simple cycle efficiency from the aeroderivative gas turbine might have outstanding potential if demonstrated in an APFBC plant.  Further, the high pressure ratio might result in equipment cost savings due to the reduced volume in equipment, vessels, and piping in the APFBC island.

COMPONENTS SUPPORTING GAS TURBINE OPERATIONS IN APFBC

An APFBC system involves a number of components.  This section discusses those components outside the combustion turbine core that are most likely to be included in the scope of supply of the gas turbine manufacturer.  In the section following this one, the gas turbine core is discussed.

Boost Compression System

The gas-side pressure drop through the fluid beds, cyclones, candle filters, and piping for an APFBC plant is greater than the corresponding pressure drop through the combustor of a simple gas turbine fueled by natural gas.  Increased pressure drop in APFBC plants will adversely affect the aerodynamic matching and output of the gas turbine.  A boost compressor can be added between the gas turbine compressor and the PFBC to return the gas turbine rotor inlet pressure to its design value while only slightly reducing the overall efficiency of the APFBC plant.

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Hot Gas Valves and Control Systems

Two types of valves are associated with advanced PFBC systems:  emergency bypass valves and fuel gas control valves.  Emergency bypass valves are needed to prevent gas turbine overspeed.  These normally closed valves are located in "dead" sections of piping away from flowing hot gases, and can be kept relatively cool until they are needed.

Fuel gas control valves modulate the flow of high-temperature gas from the carbonizer.  The carbonizer gas is cooled to 1400 ºF to allow use of commercially available valving.  This APFBC repowering design uses similar temperatures, and is not expected to be a development issue.

The valves and control system with APFBC are different from operations with natural gas:

• High-temperature valves are needed with actuators sized to close with sufficient margin to provide the required overspeed and safety protection.
  • Gas turbine syngas control valve.
  • Gas turbine emergency vitiated air bypass valve.
  • Gas turbine emergency syngas bypass valve.
  • Gas turbine emergency fuel trip valve.
• Control valve must be capable of load control modulation with minimum pressure drop, and be capable of transition from natural gas firing at startup through syngas operations at full load.
• The APFBC has large gas volumes and substantial thermal "inertia."
• Syngas and vitiated air flow rates are linked; adjusting fuel-air ratio takes combined action of the gas turbine fuel control with the APFBC system controls.

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Topping Combustor and Multi-Annular Swirl Burners

This section describes the topping combustor.  Since there are two combustor systems in an APFBC plant and, in addition, burners that are subsystems, it is first important to understand the specific terms used.

Topping combustor.   The gas turbine that is part of the APFBC system is a combustion turbine.   Since the gas turbine is the "topping" cycle portion of the combined cycle (the steam cycle forming the "bottoming" cycle), the combustor for the gas turbine in an APFBC system is referred to as the "topping combustor."

A topping combustor is a gas turbine combustion system capable of supporting stable, low-emission combustion of the APFBC combustion air and fuel streams.  This means that the topping combustor system must be capable of using the high-temperature (1400ºF) depleted-oxygen (about 16% oxygen) vitiated air from the PFBC exhaust as its combustion air supply.  The topping combustor system must also be able to use the low-Btu-content (about 135 Btu/scf) fuel gas from the mild gasification process in the carbonizer, and accommodate that fuel at the high delivery temperature (1400ºF).  Usually a topping combustor system will employ a number of individual burners to provide the total heat release needed by the gas turbine.  The topping combustor employs one to several several burners as subsystems. One type of burner is a "multi-annular swirl burner," or MASB.

PFBC combustor.   The PFBC combustor is an entirely separate circulating pressurized fluidized bed combustion (PFBC) system.  The PFBC burns char along with a sorbent (usually limestone) to capture sulfur.  The PFBC is part of the APFBC power block.   The PFBC combustor consumes the char, and provides hot vitiated air to the topping combustor.  The PFBC combustor is part of the APFBC power island.

MASB Burners.   In APFBC, the topping combustor system employs several multi-annular swirl burners (MASBs) manufactured by the Siemens Westinghouse Power Corporation.

Siemens Westinghouse Power Corporation successfully designed and tested a full-scale (18-inch) low-NOx MASB with natural gas and synthetic fuel gas at the University of Tennessee Space Institute (UTSI). 

The topping combustor is significantly different from a natural gas combustor:

• The topping combustor must accept hot (1400°F to 1700°F)low-Btu content (130 Btu/scf) syngas.

• The burners must be capable of the stable combustion of syngas and vitiated air over the combustion turbine's load range.

• Burner must be capable of sustaining stable combustion and low-NOx operations throughout the load range on syngas.

• Burners must be capable of starting and operating on natural gas, and capable of smooth transition to full syngas firing.

• The nozzles and connectors to the APFBC system fuel gas and vitiated air piping must be capable of handling the thermal growth loads imposed on them at the high delivery temperatures of these gases.

• Internal hoses interconnecting individual burners from the manifolds must be capable of conveying the high-temperature gases.

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Gas Turbines

APFBC operation differs considerably from natural gas operation:

• Modified casings and plenums are needed to allow the export of compressor discharge air to the APFBC system.

• Must have fixed-point nozzles with strength at high temperature (1400°F to 1700°F) to accept piping loads from thermal growth.

• Hoses/piping to individual burners must be capable of operating at syngas and vitiated air temperatures.

• Modified casings and plenums are needed to allow the import of hot vitiated air from the APFBC PFBC combustor exhaust.

• The topping combustor must accept hot (1400°F to 1650°F) pressurized vitiated air, depleted in oxygen (5 to 16 mole percent oxygen).

• Modified casings and plenums are needed to allow the import of hot syngas from the APFBC carbonizer.

In spite of the fact that there are many companies offering to supply gas turbines, there are only a few manufacturers with units in the size range suited for APFBC repowering applications.  However, not all of these have had the development testing that would qualify them for such service.  The table below lists the gas turbines considered to be candidates for repowering existing coal-fired plants of 100 to 180 MWe output, if their manufacturers were to become interested in supplying equipment for APFBC power plants.  The list was made on the following basis:

• Units with single-unit output over 70,000 kW.

• Gas turbines with the lowest heat rates, regardless of size.

• Units where the manufacturer is known to have considered export air applications (IGCC, HIPPS, recuperative cycles, etc.), regardless of size.

• Only 3600 rpm single-shaft machines considered for 60 cycle applications (domestic USA).  Units with separate-shaft power turbine expanders, regardless of rpm, were assumed to be easily modified to operate at 3600 rpm.

• The latest versions of the different frames.

Listing of Candidate Gas Turbines for DOE APFBC Repowering Assessment

(Unless noted otherwise, information is drawn from the 1997 Gas Turbine World Handbook)

‡ This gas turbine engine is not yet offered commercially; the performance, OPR, and EGT are preliminary information from earlier DOE studies. [United Technologies, 1994]
† This engine is recuperated and intercooled; information is from Rolls-Royce brochure. [Rolls-Royce, 1996]

The discussions below describe some of the units evaluated for APFBC operations.

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APFBC-Modified Siemens Westinghouse Power Corporation W501F Gas Turbine

The Siemens Westinghouse Power Corporation has been involved in a number of tests of multi-annular swirl burners for the topping combustors and equipment suited for APFBC operation in their large frame "F" series of gas turbines (the W401F and W501F).  These combustors have been tested as simulated APFBC conditions in DOE-sponsored tests at the University of Tennessee Space Institute in Tullahoma, Tennessee, and are currently undergoing testing at the DOE and Industry Power System Development Test Facility (PSDF) in Wilsonville, Alabama.

The APFBC-Modified Siemens Westinghouse Power Corporation W501F has been evaluated in two APFBC repowering concept evaluations:   the repowering of  L.V. Sutton Unit 1 and Unit 2, and Dan River Unit 3.  In each application, a single W501F is employed.

Aerodynamic Core Unchanged.   At the core of the Siemens Westinghouse Power Corporation W501F series of machines are commercial equipment items with no problems related to PFBC operation.  The aerodynamic core of the W501F is retained APFBC repowering, and should pose no problems as long as the particulate filtration and alkali capture operate as expected.

Modifications Needed for APFBC Operations.  There are, however, modifications to the casing to accommodate the plenums for collection of high- pressure compressor discharge air, and to accommodate the topping combustor with its multi-annular swirl burners.   Integrated operation with a boost compressor and an APFBC system requires demonstration.  The problems are similar to those of integrated gasification combined cycle systems.

APFBC-Modified Siemens Westinghouse V64.3 and V84.3 Gas Turbines

Two different APFBC-Modified Siemens Westinghouse large-frame gas turbine configurations were evaluated in APFBC repowering concept evaluations at the Progress Energy's L.V. Sutton station.  In one configuration, two APFBC-modified V64.3 gas turbines would run in parallel to repower the station, while in the second configuration a single APFBC-modified V84.3 would be used.  Both gas turbines use off-board combustion chambers, which are well-suited for compressor air export and vitiated air / syngas import, though materials changes need to be considered due to APFBC operating temperatures.  The Siemens Westinghouse 3-series engines have off-board combustors that make an easier transition for APFBC air export, vitiated air and syngas import, and APFBC topping combustor operations.  The Siemens Westinghouse 3A-series engines use hybrid ring burners (TM).

Aerodynamic Core Unchanged.   At the core of the Siemens V64.3 or V84.3 series of machines are commercial equipment items with no problems related to PFBC operation. The aerodynamic core of these is retained for APFBC repowering, and should pose no problems as long as the particulate filtration and alkali capture operate as expected.

This view above shows, toward the center, the transition duct connection for the off-board
combustor (here removed) on one side.  A second off-board combustor is on the other side. 
The 3-series engines have the off-board combustors preferred for APFBC modification.

The view above shows the newer 3A-series, which employs hybrid ring burners (TM).

Modifications Needed for APFBC Operations. There are, however, modifications to the materials used in fabricating the casing and topping combustor that might be needed to accommodate the temperatures involved in APFBC operations.  Burners suited for APFBC syngas operation would need testing.   Integrated operation with a boost compressor and an APFBC system requires demonstration.  The problems are similar to those of integrated gasification combined cycle systems.

In the twin V64.2 integration concept, control and valving methods for distributing the split in vitiated air would need verification.

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APFBC-Modified Rolls-Royce Industrial Trent Gas Turbines

Two APFBC-Modified Rolls-Royce industrial Trent gas turbines were evaluated for APFBC repowering of the AES Greenidge power plant.  The Trent is an aeroderivative gas turbine engine.  Rolls-Royce has developed a regenerative gas turbine, the WR-21, that shares aerodynamic similarity with the high-pressure section of the Trent.  This means that the plenums and chambers needed to export compressor air and import vitiated air are already designed, as well as the casing and shaft modifications for the high-pressure core.  These casings however, would need material upgrades to accommodate the APFBC operating temperatures.

Aerodynamic Core Unchanged.   At the core of the industrial Trent machines are commercial equipment items with no problems related to PFBC operation. The aerodynamic core of these is retained for APFBC repowering, and should pose no problems as long as the particulate filtration and alkali capture operate as expected.

Rolls-Royce Industrial Trent -- Photos courtesy of Rolls-Royce

Modifications Needed for APFBC Operations.  There are, however, development needs for a topping combustor and burners to accommodate APFBC operations on syngas.  Integrated operation with a boost compressor and an APFBC system requires demonstration.  Since twin Trents would be used, control and valving methods for distributing the split in vitiated air would need verification.

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APFBC-Modified Pratt & Whitney Turbo Power FT8 Twin-Pac
Gas Turbines

Four APFBC-Modified Pratt & Whitney Turbo Power FT8 gas turbines arranged as two FT8 Twin Pacs were evaluated in APFBC repowering concept evaluation variant at the Progress Energy's L.V. Sutton station.  The FT8 is an aeroderivative gas turbine.  In the original L.V. Sutton evaluations, a single large-frame Siemens Westinghouse Power Generation W501F was used, with quenched 1400°F filters.  In this evaluation two APFBC-modified FT8 Twin-Pacs were evaluated in a more aggressive, higher performance configuration with unquenched 1700ºF filters.

Aerodynamic Core Unchanged.   At the core of the FT8s are commercial equipment items with no problems related to PFBC operation.  The aerodynamic core of these is retained for APFBC repowering, and should pose no problems as long as the particulate filtration and alkali capture operate as expected.

Pratt & Whitney Turbo Power FT8  -- Photo courtesy of Pratt & Whitney

Modifications Needed for APFBC Operations.  There are, however, significant modifications to the casing to accommodate the plenums for collection of high-pressure compressor discharge air, topping combustor design needed, and burner development to accommodate the APFBC operating conditions and syngas.  Integrated operation with a boost compressor and an APFBC system requires demonstration.  Since four FT8s would be used in two Twin Pacs, control and valving methods for distributing the split in vitiated air between the four individual FT8s would need verification.

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Specifications

The tables that follow provide some of the gas turbine operational parameters that are expected if a gas turbine is operated in an APFBC environment.

Operations Parameters Specific to Case GT-A: Gas Turbine Suited for Greenidge Station Repowering

Note: The data below are an initial APPROXIMATION assessment for beginning the evaluation. The actual conditions will vary, depending upon the specific needs of the gas turbine chosen and its interaction with the APFBC power island.  Establishing these conditions requires cooperation between the gas turbine manufacturer and system integrator to develop an accurate system model to balance flows and compositions for the expected conditions.

Operations Parameters Specific to Case GT-B:
150+ MWe Gas Turbine for 1700°F Syngas Repowering Application

Note:  The data below are an initial APPROXIMATION assessment for beginning the evaluation. 
The actual conditions will vary, depending upon the specific needs of the gas turbine chosen and its interaction with the APFBC power island.  Establishing these conditions requires cooperation between the gas turbine manufacturer and system integrator to develop an accurate system model to balance flows and compositions for the expected conditions.

Operations Parameters Specific to Case GT-C:
190 MW APFBC Total Output Greenfield Application with Alternate Fuel

Note: The data below are an initial APPROXIMATION assessment for beginning the evaluation. 
The actual conditions will vary, depending upon the specific needs of the gas turbine chosen and its interaction with the APFBC power island.  Establishing these conditions requires cooperation between the gas turbine manufacturer and systems integrator to develop an accurate system model to balance flows and compositions for the expected conditions.