Advanced Electric Power Generation
Fluidized Bed Combustion

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Participant
The Ohio Power Company

Location
Brilliant, Jefferson County, OH (Ohio Power Company's Tidd Plant, Unit No. 1)

Plant Capacity/Production
70 MWe (net)

Coal
Ohio bituminous, 2-4% sulfur

Technology
The Babcock & Wilcox Company's pressurized fluidized-bed combustion (PFBC) system (under license from ABB Carbon).

Additional Team Members
American Electric Power Service Corporation
designer, constructor and manager

The Babcock & Wilcox Company
technology supplier

Ohio Coal Development Office
cofunder

Project Funding

Project Objective
To verify expectations of PFBC economic, environmental, and technical performance in a combined-cycle repowering application at utility scale; and to accomplish greater than 90% SO₂ removal and NOx emission level of 0.2 lb/10⁶ Btu at full load.

Technology/Project Description
Tidd was the first large-scale operational demonstration of PFBC in the United States. The project represented a 13:1 scaleup from the pilot facility.

The boiler, cyclones, bed reinjection vessels, and associated hardware were encapsulated in a pressure vessel 45 feet in diameter and 70 feet high. The facility was designed so that one-seventh of the hot gases produced could be routed to an advanced particulate filter (APF).

The Tidd facility used a bubbling fluidized-bed combustion process operating at 12 atm (175 psi). Pressurized combustion air is supplied by the turbine compressor to fluidize the bed material, which consists of a coal-water fuel paste, coal ash, and a dolomite or limestone sorbent. Dolomite or limestone in the bed reacts with sulfur to form calcium sulfate, a dry, granular bed-ash material, which is easily disposed of or is usable as a by-product. A low bed-temperature of about 1,600 °F limits NOx formation.

The hot combustion gases exit the bed vessel with entrained ash particles, 98% of which are removed when the gases pass through cyclones. The cleaned gases are then expanded through a 15-MWe gas turbine. Heat from the gases exiting the turbine, combined with heat from a tube bundle in the fluid bed, generates steam to drive an existing 55-MWe steam turbine.

Tidd PFBC Demonstration Project Process Flow Diagram
Larger jpeg or wmf version

Results Summary

Environmental

• Sorbent size had the greatest effect on SO₂ removal efficiency as well as stabilization and heat transfer characteristics of the fluidized-bed.

• SO₂ removal efficiency of 90% was achieved at full load with a calcium-to-sulfur (Ca/S) molar ratio of 1.14 and temperature of 1,580 °F.

• SO₂ removal efficiency of 95% was achieved at full load with a Ca/S molar ratio of 1.5 and temperature of 1,580 °F.

• NOx emissions were 0.15-0.33 lb/10⁶ Btu.

• CO emissions were less than 0.01 lb/10⁶ Btu.

• Particulate emissions were less than 0.02 lb/10⁶ Btu.

Operational

• Combustion efficiency ranged from an average 99.3% at low bed levels to an average 99.5% at moderate to full bed levels.

• Heat rate was 10,280 Btu/kWh (HHV, grass output) (33.2% efficiency) because the unit was small and no attempt was made to optimize heat recovery.

• An advanced particulate filter (APF), using a silicon carbide candle filter array, achieved 99.99% filtration efficiency on a mass basis.

• PFBC boiler demonstrated commercial readiness.

• ASEA Stal GT-35P gas turbine proved capable of operating commercially in a PFBC flue gas environment.

Economic

• The Tidd plant was a relatively small-scale facility, and as such, detailed economics were not prepared as part of this project.

• A recent cost estimate performed on Japan's 360-MWe PFBC Karita Plant projected a capital cost of $1,263/kW (1997$).

Project Summary
The Tidd PFBC technology is a bubbling fluidized- bed combustion process operating at 12 atmospheres (175 psi). Fluidized-bed combustion is inherently efficient because the pressurized environment enhances combustion efficiency, allows very low temperatures that mitigate thermal NOx generation, promotes flue gas/sorbent reactions that increase sorbent utilization, and produces flue gas energy that is used to drive a gas turbine. The latter contributed significantly to system efficiency because of the high efficiency of gas turbines and the availability of gas turbine exhaust heat that can be applied to the steam cycle. A bed design temperature of 1,580 °F was established because it was the maximum allowable temperature at the gas turbine inlet and was well below temperatures for coal ash fusion, thermal NOx formation, and alkali vaporization.

Coal crushed to one-quarter inch or less was injected into the combustor as a coal/water paste containing 25% water by weight. Crushed sorbent, either dolomite or limestone, was injected into the fluidized bed via two pneumatic feed lines, supplied from two lock hoppers. The sorbent feed system initially used two injector nozzles but was modified to add two more nozzles to enhance distribution.

In 1992, a 10-MWe equivalent APF was installed and commissioned as part of a research and development program and not part of the CCT Program demonstration. This system used ceramic candle filters to clean one-seventh of the exhaust gases from the PFBC system. The hot gas cleanup system unit replaced one of the seven secondary cyclones.

The Tidd PFBC demonstration plant accumulated 11,444 hours of coal-fired operations during its 54 months of operation. The unit completed 95 parametric tests, including continuous coal-fired runs of 28, 29, 30, 31, and 45 days. Ohio bituminous coals having sulfur contents of 2 - 4% were used in the demonstration.

The PFBC demonstration at the repowered 70-MWe unit
at Ohio Power's Tidd Plant led to significant refinements
and understanding of the technology.

Environmental Performance
Testing showed that 90% SO₂ capture was achievable with a Ca/S molar ratio of 1.14 and that 95% SO₂ capture was possible with a Ca/S molar ratio of 1.5, provided the size gradation of the sorbent being utilized was optimized. This sulfur retention was achieved at a bed temperature of 1,580 °F and full bed height. Limestone induced deterioration of the fluidized-bed, and as a result, testing focused on dolomite. The testing showed that sulfur capture as well as sintering was sensitive to the fineness of the dolomite sorbent (Plum Run Greenfield dolomite was the design sobent). Sintering of fluidized-bed materials, a fusing of the materials rather than effective reaction, had become a serious problem that required operation at bed temperatures below the optimum for effective boiler operation. Tests were conducted with sorbent size reduced from minus 6 mesh to a minus 12 mesh. The result with the finer material was a major positive impact on process performance without the expected excessive elutriation of sorbent. The finer material increased the fluidization activity as evidenced by a 10% improvement in heat transfer rate and an approximately 30% increase in sorbent utilization. In addition, the process was much more stable as indicated by reductions in temperature variations in both the bed and the evaporator tubes. Furthermore, sintering was effectively eliminated.

NOx emissions ranged from 0.15-0.33 lb/10⁶ Btu, but were typically 0.2 lb/10⁶ Btu during the demonstration. These emissions were inherent in the process, which was operating at approximately 1,580 ºF. No NOx control enhancements, such as ammonia injection, were required. Emissions of carbon monoxide and particulates were less than 0.01 lb/10⁶ Btu and 0.02 lb/10⁶ Btu, respectively.

Operational Performance
Except for localized erosion of the in-bed tube bundle and the more general erosion of the water walls, the Tidd boiler performed extremely well and was considered a commercially viable design. The in-bed tube bundle experienced no widespread erosion that would require significant maintenance. While the tube bundle experienced little wear, a significant amount of erosion on each of the four water walls was observed. This erosion posed no problem, however, because the area affected is not critical to heat transfer and could be protected by refractory.

The prototype gas turbine experienced structural problems and was the leading cause of unit unavailability during the first three years of operation. However, design changes instituted over the course of the demonstration proved effective in addressing the problem. The Tidd demonstration showed that a gas turbine could operate in a PFBC flue gas environment.

Efficiency of the PFBC combustion process was calculated during testing from the amount of unburned carbon in cyclone and bed ash, together with measurements of the amount of carbon monoxide in the flue gas. Combustion efficiencies averaged 99.5% at moderate to full bed heights, surpassing the design efficiency of 99.0%.

Using data for typical full-load operation, a heat rate of 10,280 Btu/kWh (HHV) was calculated. This corresponds to a cycle thermodynamic efficiency of 33.2% at a point where the cycle produced 70 MWe of gross electrical power while burning Pittsburgh No. 8 coal. Because the Tidd plant was a repowering application at a comparatively small scale, the measured efficiency does not represent what would be expected for a larger utility-scale plant using Tidd technology. Studies conducted under the PFBC Utility Demonstration Project showed that efficiencies of over 40% are likely for a larger, utility-scale PFBC plant.

In summary, the Tidd project showed that the PFBC system could be applied to electric power generation. Further, the demonstration project led to significant refinements and understanding of the technology in the areas of turbine design, sorbent utilization, sintering, post-bed combustion, ash removal, and boiler materials.

Testing of the APF for over 5,800 hours of coal-fired operation showed that the APF vessel was structurally adequate; the clay-bonded silicon carbide candle filters were structurally adequate unless subjected to side loads from ash bridging or buildup in the vessel; bridging was precluded with larger particulates included in the particulate matter; and filtration efficiency (mass basis) was 99.99%.

Economic Performance
The Tidd plant was a relatively small-scale demonstration facility, so detailed economics were not prepared as part of this project. However, a recent cost estimate performed on Japan's 360-MWe PFBC Karita Plant projected a capital cost of $1,263/kW (1997$).

Coal and sorbent conveyors can be seen just
after entering the Tidd Plant.

Commercial Applications
Combined-cycle PFBC permits use of a wide range of coals, including high-sulfur coals. The compactness of bubbling-bed PFBC equipment allows utilities to significantly increase capacity at existing sites. Compactness due to pressurized operation reduces space requirements per unit of energy generated. PFBC technology appears to be best suited for applications of 50 MWe or larger. Capable of being constructed modularly, PFBC generating plants permit utilities to add increments of capacity economically to match load growth. Plant life can be extended by repowering with PFBC using the existing plant area, coal- and waste-handling equipment, and steam turbine equipment.

The Tidd project received Power magazine's 1991 Powerplant Award. In 1992, the project received the National Energy Resource Organization award for demonstrating energy-efficient technology.

Contacts

Michael J. Mudd
  American Electric Power
  1 Riverside Plaza
  Columbus, OH 43215
  (614) 716-1585
  (614) 716-1292 (fax)
  mjmudd@aep.com

Victor Der, DOE/HQ, (301) 903-2700
  victor.der@hq.doe.gov

Donald W. Geiling, NETL, (304) 285-4784
  donald.geiling@netl.doe.gov