FBC Repower     Simple Description     Detailed Description     APFBC Specs     GTs for APFBC

Click on picture to enlarge

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

Contents:

Please send me to the NON-TECHNICAL DISCUSSION about APFBC instead!

Click on the technical discussion areas below, or simply browse down
in this page:

APFBC Repowering Considerations

• Process Sketch
• APFBC Components
• High Energy Efficiency
• Excellent Environmental Performance
• Cost Comparable to a Pulverized Coal Plant

APFBC Repowering Considerations

Click on picture to enlarge

Process Sketch

While all elements of the APFBC plant have been tested, APFBC technology is still under development.  Test programs are in place that will soon show commercial operations for all parts.  The most important of these are the DOE Power Systems Development Facility in Wilsonville, Alabama, and two DOE Clean Coal Technology projects:  Piñon Pines, testing ceramic filters, and the APFBC CCT project.  DOE views the following as the development status of the various elements for application in a year 2002 repowering plant installation:

Component Summary
Follow these links to find out about:

• Foster Wheeler Fuel Forwarding System

• Foster Wheeler Carbonizer

• Foster Wheeler Char Transfer System

• Foster Wheeler PFBC Combustor

• Foster Wheeler Fluid Bed Heat Exchanger

• Foster Wheeler Ash Transport System

• Siemens Westinghouse Hot Gas Particulate Cleanup

• Hot Gas Valves

• High-Temperature Pipes Manifolds, Expansion Joints, and Ducting

• Siemens Westinghouse Topping Combustor and Multi-Annular Swirl Burners

• Modified Siemens Westinghouse W501F
  Gas Turbine

• Existing Steam Turbine System

Back to Contents      Back to TOP

Foster Wheeler Fuel Forwarding System

The fuel forwarding system has been successfully tested using dry coal and coal paste, including on-line switching from dry coal-sorbent feed to paste feed.

Back to Component Summary      Back to Contents

Foster Wheeler Carbonizer

Click on picture to enlarge

The carbonizer, shown as the squatter vessel in red to the left above, provides fuel gas for the topping combustor.  This is a synthesis gas made by a mild gasification process.  Tests run by Foster Wheeler in September 1995 demonstrated the technical viability of integrated carbonizer-CPFBC operation. Minor drain line plugging encountered during the tests was cleared on the fly. Over the 120 hours of the test, feed coal was separated into fuel gas and char in the carbonizer, and the char was continuously transferred by the char transfer system from the reducing carbonizer to the oxidizing CPFBC, where it was burned together with coal.  The tests included three coal feedstock blends:  3.5 percent sulfur petroleum coke with Plum Run dolomite; 3.4 percent sulfur Kentucky No. 9 coal with Three Rivers limestone; and 1.5 percent sulfur Pittsburgh No. 8 coal with Three Rivers limestone.

A dry-feed system is presently planned for the CCT (APFBC) carbonizer because paste feed to a carbonizer has not yet been demonstrated.  (The paste feed systems originally planned for Wilsonville were deleted as a cost-saving measure).

Back to Component Summary       Back to Contents

Foster Wheeler Char Transfer System

The mild gasification in the carbonizer leaves char and ash behind.  This needs to be burned in the pressurized fluidized-bed combustor.   Tests run by Foster Wheeler in September 1995 demonstrated the technical viability of transferring char from the reducing carbonizer to the oxidizing CPFBC for 120 hours without problems using a rotary valve to control the transfer rate.  Tests conducted by IGT for Foster Wheeler in 1993 demonstrated that an N-valve transfer system would operate well at commercial size feed rates over 2000 lb/hour.  A variation of this N-valve scheme has been incorporated into the Wilsonville APFBC design.

Back to Component Summary      Back to Contents

Foster Wheeler PFBC Combustor

The PFBC circulating fluidized-bed combustor completes the burning of the char, and provides hot vitiated air for the topping combustor.  Early Foster Wheeler carbonizer-CPFBC tests were hampered by blockages in the CPFBC cyclone J-valve/loop seal.  Resolution came by changing the fluidizing pattern in the loop seal, and a piping modification that reduced the seal height.  The circulating PFB combustor has been successfully operated for hundreds of hours with both dry feed and paste feed.  The PFBC combustor is the taller red vessel to the center of the illustration shown in the carbonizer discussion.

Back to Component Summary      Back to Contents

Foster Wheeler Fluid-Bed Heat Exchanger

The fluid-bed heat exchanger is part of the circulating fluid-bed PFBC system. It generates steam for the steam cycle.  The Foster Wheeler fluid-bed heat exchanger has been designed and tested without problems.  The PFBC fluid-bed heat exchanger is the large orange horizontal vessel to the center of the illustration shown in the carbonizer discussion.

Back to Component Summary      Back to Contents

Foster Wheeler Ash Transport System

The Foster Wheeler ash transport system has been designed and tested without problems.

Back to Component Summary      Back to Contents

Siemens Westinghouse Power Corporation Hot Gas Particulate Cleanup

Click on picture to enlarge

A developmental focus of the PFBC plant has been the hot gas cleanup system, which in this application uses high-temperature ceramic candle filters. These are used to clean the fuel gas from the carbonizer and the exhaust gas from the PFB combustor, and to clean the vitiated air from the PFBC exhaust before it is used as combustion air for the topping combustor.  Technical challenges for hot gas cleanup operating at temperatures of 1400ºF include material creep, brittle fracture, alkali destruction of binder, bridging, and ash drain stoppage.

Although hot gas filters are not yet used on first-generation PFBCs, they are necessary to reach the efficiency goals of second-generation / advanced PFBC plants.  Of the five PFBC demonstration plants that have operated or are in operation, only two of them -- Tidd and Wakamatsu -- tested particulate removal devices other than cyclones.

Demonstration testing must wait for the completion of coal gasification testing at the Piñon Pine CCT demonstration in Reno, Nevada; APFBC testing at the Power System Development Facility in Wilsonville, Alabama; and the APFBC CCT demonstration.  

Back to Component Summary      Back to Contents

Hot Gas Valves

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.  In the CCT (APFBC) design, the carbonizer gas is cooled to 1400ºF to allow use of commercially available valving.  This APFBC repowering design uses similar temperatures; consequently this is not expected to be a development issue.

Back to Component Summary      Back to Contents

High-Temperature Pipes, Manifolds, Expansion Joints, and Ducting

The APFBC evaluated in this study has several classes of high-pressure (250 to 300 psig) air and gas piping needs:

• Conveying about 700ºF gas turbine compressor discharge/boost
  compressor "cold" air.

• Conveying about 1400ºF CPFBC flue gas/vitiated air to the topping combustor.

• Conveying about 1400ºF carbonizer fuel gas to the topping combustor.

These are designed for the following needs:

• Providing pressure containment under all conditions.

• Avoiding thermal growth damage. Since the high-temperature piping experiences substantial growth as it moves from cold conditions to operation at high temperature, attention to providing low-friction, non-binding support design is important.  The pipe, refractory, and inner liner, if present, have different growth rates, and pipe connections to component fixed points need adequate flexibility.

• Preventing gas turbine damage from spalling from the refractory-lined pipes by using an inner-liner design.

• Resisting oxidation, reducing gas attack, corrosion, and erosion.

• Minimizing pressure drop, heat loss, and cost by using short low-pressure drop run.

• Providing transient over-pressure/over-temperature protection should compressor surge or other upset events occur.

The "cold" air pipe design is common practice.

The design of the CPFBC high-temperature vitiated air and gas piping is important, but not considered a risk item.  The chosen refractory-lined design for this APFBC repowering draws heavily from work performed for NETL for PFBC applications.  Other single refractory- or alloy- lined insulated piping system designs exposed to severe operating conditions are providing satisfactory service with no abnormal maintenance requirements at numerous petrochemical and power generation facilities.  The chosen single-pipe design uses materials and some features similar to those that have been proven in service in the co-annular pipe system at the American Electric Power Corporation's Tidd PFBC facility.

Back to Component Summary      Back to Contents

Siemens Westinghouse Power Corporation 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 is 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 the APFBC described in these DOE APFBC repowering studies, the topping combustor system employs several multi-annular swirl burners (MASBs) manufactured by 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).  This MASB is intended for use in the W251 (50 MW) and W501 services (100 MW and up) combustion turbines.  The MASB was tested in propane and synthetic fuel gas operations. 

The APFBC plant at the Wilsonville Power System Development Facility will provide the first operation of a gas turbine and topping combustor with hot pressurized fuel gas from the carbonizer and hot pressurized flue gas from the CPFBC.  Periodic examination of the gas turbine will allow the merits of hot gas cleanup for APFBC systems to be evaluated.

Back to Component Summary      Back to Contents

Modified Siemens Westinghouse Power Corporation W501F Gas Turbine

The type of turbine used in an APFBC repowering might be a unit similar to an APFBC-modified Siemens Westinghouse Power Corporation W501F.  Other gas turbines have been investigated, such as Rolls-Royce Industrial Trents, Pratt & Whitney FT8 Twin-Pacs, and Siemens Westinghouse Power Corporation V64.3 and V84.3 combustion turbines.   These machines also have characteristics that might be suitable for adaptation for APFBC operations.

The Siemens Westinghouse Power Corporation W501F series machines are commercial equipment items with no problems related to PFBC operation.  The aerodynamic core of the W501F is retained in this repowering, and should pose no problems as long as the particulate filtration operates as expected.

Click on picture to enlarge

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

Back to Component Summary      Back to Contents

Existing Steam Turbine System

The steam turbine systems in the repowering application are those already existing at the station.  Typically, these are 1950s vintage units operating at something like 1450 psig /1000ºF/1000ºF throttle and reheat conditions.   There is nothing unusual about the operations, except that almost all of the feedwater heaters are taken out of service, since there is a lot of energy available for economizing in the APBC system and the combustion turbine heat recovery steam generator.

Back to Component Summary      Back to Contents

High Energy Efficiency

Typical performance before and after APFBC repowering is shown below.

• If both Units 1 and 2 are repowered, efficiency drops about 2 points, but output increases to 360 MW -- an all-coal-fired 108 MW increment for only modest additional capital cost.

Back to Component Summary      Back to Contents

Excellent Environmental Performance

APFBC environmental emissions are excellent, and will usually improve existing unit.

Back to Component Summary      Back to Contents

Cost Comparable to a Pulverized Coal Plant

A power plant repowered with APFBC is expected to have costs ($/kW) similar to those for building an all-new pulverized coal steam plant of comparable capacity.  However, the fuel savings with APFBC operations would result in significantly lower operating costs, do to the exceptionally high energy efficiency of a unit repowered with APFBC.  This means that APFBC would likely be the more economical choice. The chart below shows the costs for two different units repowered with APFBC, each of these in two configurations.  A "utility" plant has a high degree of redundancy and parallel equipment trains and features for lower operating costs, at the expense of higher initial capital investment; a "merchant" plant sacrifices some of these features for lowest possible initial investment.

Click on picture to enlarge

APFBC, however, is not yet commercially offered in large sizes, so there are a number of uncertainties in its likely initial costs.  The cumulative risk assessment chart below illustrates the range of uncertainty in these estimates for one of the evaluations.

In Economic Dispatch Evaluations by the Generation Planning group at Progress Energy, the following conclusions were drawn:

• APFBC would become the flagship coal-fired unit, the first dispatched as baseload coal capacity.

• L.V. Sutton Unit 2, which without modification now dispatches at about 20 percent capacity factor, would move to 83 percent capacity factor with APFBC.

• In the next 10 years, there is sufficient baseload reserve at Progress Energy, so investment is only needed in natural-gas-fired combustion turbine peakers.

• APFBC repowering appears to have superior economics over a new conventional pulverized coal unit with FGD.

• When new coal-fired generation is needed, APFBC will be given serious consideration.