Products Summary    Atmospheric Products    Hybrid Products    Pressurized Products

Contents:

• Introduction
• Cycle Studies
• Turbomachine Components
• PFBC Boiler and Related Components
• Land-Based Configurations
• Barge-Mounted Configurations
• Economics

Introduction

Several concepts of power plants that use pressurized fluidized-bed combustion (PFBC) technology have been evaluated by the Combustion Technologies group. These use a circulating-bed approach, under development by Foster Wheeler, which relies on a high-efficiency hot gas filter to remove particulates from the gas path, and uses commercially available gas turbine equipment.  This web page describes a series of conceptual designs based on the selection of industrial turbomachinery components that are combined to form a gas turbomachine.  This provides the designer the latitude to tailor the gas thermodynamic cycle for more optimal thermal and economic performance.   The turbomachinery is selected to fit the needs of the cycle, instead of fitting the cycle to the available gas turbines specifically designed for other purposes.   This web page describes the turbomachine components, the circulating-bed PFBC concept, and the integration of these components into a series of industrial land-based and barge-mounted designs spanning a range of outputs from 15 to 300 MWe.

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Cycle Studies

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This study began with the modeling of four variations of the standard Brayton gas turbine cycle, including a simple cycle, and intercooled, recuperated, and intercooled-recuperated versions of the Brayton cycle.   The turbine inlet temperature used for the cycle optimization process was set at 1575°F, the same level as the PFBC bed exit temperature.  Performance levels were set for each major component, based on the current state of the art.  A series of cases was evaluated over a range of cycle pressure ratios to determine the optimum cycle performance, based on maximum efficiency and maximum work per pound of mass flow.

A steam cycle was used as a bottoming cycle for the initial studies, with steam conditions of 2400 psig/1000°F/1000°F. The steam cycle was coupled to the gas (Brayton) cycle by heat recovery from the exhaust gas, forming a combined cycle.  Capital costs and operating costs were estimated for each configuration, and the overall cost of electricity (COE) was used as a figure of merit to determine the best design point (pressure ratio) for each configuration.

The results of these cycle studies indicate that the intercooled cycle (without regeneration), at a compressor pressure ratio of between 25 and 30 to 1, provides the lowest overall COE.  The other configurations tended to optimize at lower pressure ratios, but with higher COE.

The Foster-Wheeler PFBC design used in this evaluation is capable of operation at absolute pressures up to approximately 17 atmospheres.   This limiting pressure is based on current operational experience.  The evaluation process also identified a line of commercially available turbomachine components that could be mixed and matched on a common shaft to synthesize the desired gas path cycle.  The system pressure ratio may be set as high as 22:1, based on the design of the turbo-expanders selected in this study.  The ability to set the pressure ratio by changing the design of the turbomachine components enables the plant design to compensate for altitude effects up to an elevation of 7,000 feet.

Previous evaluations of circulating-bed PFBC configurations have focused on simple gas path cycles, provided by adapting existing commercial gas turbines, which have pressure ratios in the range of 12 to 15 to 1.  The higher pressure ratios evaluated in the study tend to increase efficiency and power output, while reducing the overall size of the PFB boiler and the pressure vessel that contains it.   This tends to reduce capital costs and operating costs, providing a more cost-effective design.

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Turbomachine Components

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The Dresser-Rand Corporation (D-R) is a joint venture between the Ingersoll Rand Corporation and Dresser Industries, and is a leading manufacturer of turbomachinery.  The product line of D-R includes hot gas expanders of several types, axial flow compressors, high-pressure centrifugal compressors, and steam turbines, with related components.  Other D-R product offerings include reciprocating engines and compressors.  The full line capabilities of D-R enable the design and fabrication of complete, integrated turbo- compressor machinery trains to suit a wide variety of thermodynamic and process requirements.

The gas power trains for the large size (over 120 MWe in this study) PFBC power units are driven by one or two CAES hot gas expanders.  This expander is currently operating in daily service at the Alabama Energy Cooperative CAES (Compressed Air Energy Storage) plant in McIntosh, Alabama.   The CAES expander, modified for the PFBC applications by removing the combustors.   The individual transition ducts are replaced by a single hot inlet scroll.   The expander casing is designed for through-circulation of compressor discharge air to cool and thermally isolate the outer case from the hot gas path.  The PFBC boiler process furnishes the process gas to the expander for power generation.

The smaller PFBC plants (less than 100 MWe) use a second D-R product, the nitric acid hot gas expander, originally designed for power recovery from hot gas streams in nitric acid production.  The nitric acid expander is a five-stage unit, with a combination of impulse and reaction staging.  The expander is available in four frame sizes, based on the disc diameter of the rotating stages.

The turbomachinery train is completed by selection of axial flow low-pressure (LP) compressors and centrifugal flow high-pressure (HP) compressors from the D-R line of equipment, and the integration of these machine elements onto a common shaft, along with necessary auxiliaries such as controls, lube oil package, etc.

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PFBC Boiler and Related Components

The PFBC units described in this web page use a combined cycle for conversion of thermal energy from the fluid bed to electric power.  An open Brayton cycle using air and combustion products as working fluid is used in conjunction with a conventional sub-critical steam Rankine cycle.  The cycles are coupled by generation, superheat, and reheat of steam in the fluid-bed heat exchanger surfaces within the PFBC vessel, and feedwater heating in the gas turbine heat recovery unit (HRU).

Inlet air passes through an inlet filter, and then passes into an axial flow LP compressor.  The airflow exiting the compressor flows through an intercooler (a shell and tube heat exchanger, with air on the shell side and condensate from the steam cycle on the tube side), and then into a single centrifugal HP compressor.   A small portion of the air (3.8 percent) is boosted to a higher pressure (280 psig) for use in the lock hopper injection system for fuel and sorbent.   The main air stream exiting the HP compressor is sent to the PFBC vessel to provide O₂ for combustion reactions and fluid momentum for material transport.

An array of ceramic filters removes the dust from the PFBC gas discharge.  The cleaned hot gases from the PFBC are returned to the turbomachine at a temperature of 1560°F.  The hot gases are conveyed to the inlet of the expander section of the machine, where they enter and expand to produce power to drive the compressor and an electric generator.  The expander exhaust gases are conveyed through an HRU to recover the thermal energy that remains, and then to the plant stack.

The steam Rankine cycle selected for each PFBC configuration described below reflects conventional practice, with throttle pressure and temperature, and reheat temperature (where included) matching values that are widely prevalent in power generating facilities.

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Land-Based Configurations

Notes:

1. Performance shown at ambient dry bulb temperature of 59°F for 300, 84, and 44 MWe cases, -20°F for 16.7 and 8.9 MWe cases.  The 44/84 MWe cases are the same system with/without export steam; the 8.9/16.7 MWe cases are the same system with/without export steam.

2. Capital cost data for land-based units not available as of July 2000.

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Barge-Mounted Configurations

Notes:

1. Capital cost is bare erected basis, including barge, plus engineering and construction management.  Does not include certain land-based facilities.

2. COE based on 15-year book life, 80 percent capacity factor.

Click on picture to enlarge

The figure shows the 140 MWe PFBC power barge in a broadside view, with locations of the total unit (barge plus PFBC power plant) center of gravity shown in relation to the metacenter.

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Economics

The PFBC industrial component designs, both land-based and barge-mounted, show significant potential for competitive deployment in the evolving deregulated market for electric power.  The current high costs of oil and gas ($5.67 and $4.56 per 10⁶ Btu, respectively, as of July 2000) result in total COE values in excess of 40 mills per kWh by gas turbine combined cycle generating plants.  This figure is based on an estimate applicable to an F class machine operating in the combined cycle mode in the mid-50 percent efficiency range, with the above-noted oil and gas prices.  Total COE values for the largest PFBC units (300 MWe) are on the order of 40 mills per kWh, based on coal delivered to the site at $1.30 per 10⁶ Btu.  If the comparison is based on variable costs of production (fuel plus variable operation and maintenance costs), the advantage in favor of the coal-fired plants is greater.  However, the relative risk is greater, since the capital costs are significantly higher, and require higher capacity factors over a relatively long investment time span to recover those costs.