Paul A. Berman
Westinghouse Electric Corporation
Combustion Turbine Systems Division
Manager, Systems Engineering
Joseph C. Dille
Westinghouse Electric Corporation
Combustion Turbine Systems Division
The development of pressurized fluidized bed combustors (PFB) as an efficient, economical, and environmentally acceptable means of utilizing the nation's coal reserves has been underway for a short time. Although at least a dozen PFB facilities are in existence or under construction, and other PFB plants (1) are being designed, the technology is not fully developed.
This paper presents the results of an investigation into achieving high efficiency with a PFB system. The base case was a novel cycle, with the PFB combustor is located at an intermediate pressure between the maximum cycle pressure and atmospheric pressure. This configuration eliminates many of the technical problems associated with conventional PFB systems, and has a higher heating value net efficiency of 38 percent. Gas reheat, topping combustion, carbonizer systems, and compressor intercooling are added to the base system to yield a cycle efficiency of 45 percent.
Thirteen PFB experimental plants with six-inch diameter or larger combustors have been or are being constructed worldwide. One of three designs -steam-cooled, air-cooled, or adiabatic (cooled with excess combustion air) is used to remove heat from the fluidized bed. The largest steam-cooled plant, 85 MWt, is currently undergoing hot commissioning, and operation is scheduled for 1982. The largest air-cooled plant, 40 MWt, is scheduled for completion in 1983. The largest adiabatic plant, 8.7 MWt, constructed in the early 1970s to burn municipal waste, was operated on coal in 1974 and 1975.
Three PFB combined cycle (PFB/CC) experimental plants are in the engineering design phase, and the nearest to construction is a steam-cooled 170 MWe plant. Conceptual designs for air- and steam-cooled 1000 MWe PFB/CC plants have been or are being prepared to compare their cost with the cost of integrated gasification combined cycles or pulverized coal plants with flue-gas desulfurization.
The PFB power generation system can burn high sulfur coal in an environmentally acceptable way, more efficiently than any other coal burning systems currently under development. However, the operation and efficiencies of PFB systems are limited by one or more of the following characteristics.
Because of these limitations there is an incentive to design systems that are more efficient, lower in cost, and more reliable through advanced, innovative PFB concepts. This paper explains such a con- Table 1 cept, and its advantages over other coal-burning concepts.
The base cycle, shown in Figure 1, differs from conventional PFBC cycles in that the combustor is located at an intermediate pressure, lower than the maximum cycle pressure. This arrangement has several advantages over systems with full pressure combustors.
|Net Power Output||320406 kW|
|Net HHV Efficiency||37.72%|
|HHV Heat Rate||9048 Btu/kWh (2651 kj/mws)|
|Compressor Air Flow||755 lb/s (351 kg/s)|
|Bed Operating Temp.||16000F (8700C)|
|Air Temp Leaving Bed||15000F (8150C)|
|HP Inlet Temperature||15000F (8150C)|
|LP Inlet Temperature||11000F (5930C)|
|Bed Pressure||4.0 atm|
|Steam Power Fraction||81.8%|
|Excess Air in PFBC||30%|
| Figure 1, The base PFB cycle
In the base cycle, air is compressed to 14 atmospheres and. 7000F (3700C), is then heated to 15000F (18150C) in a heat exchanger located in the PFBC. The hot clean air is expanded in the high pressure expander to the combustor pressure, four atmospheres, where it is used to support combustion in the 16000F (8700C) fluidized bed. The hot products, after being cleaned in the hot gas cleanup (HGCU) system, are expanded to atmospheric pressure in the low-pressure expander. Most of the heat released in the combustor is used to generate, superheat and reheat steam for the 2400/1000/1000 psia/0F/0F steam power cycle (166.6/538/538 bar/0C /0C). The performance for the base cycle is shown in Table 1.
Starting with this base case, various system options that have the potential to achieve higher efficiency, lower power costs, and improved reliability were investigated. The options included a circulating PFB, topping combustion, carbonizer - PFB combustion, gas system reheat, and intercooling.
The design of the circulating bed, shown in Figure 2, separates the combustion and heat exchange
|Figure 2, Pressurized Circulating Fluidized Bed (PCFB) Comustor with boiler|
zones in the PFB. This arrangement greatly simplifies or eliminates many design problems that are associated with a heat exchanger in the combustion zone.
The reference PCFB system is composed of a single vertical refractory lined pressure cylinder and two sets of primary and secondary cyclones. Coal or char, and dolomitic limestone are fed into the bottom of the furnace above an air distributor. Combustion and S02 are removed in the furnace.
Because of the intense mixing of solids in the furnace, the circulating fluidized bed requires a minimum of fuel feed points. A 12-ft (3.7-m) diameter furnace requires only one fuel feed nozzle and one limestone feed nozzle. A mixed fuel/limestone feed through a single nozzle is also an option. Further, staged fuel pumps could replace the lock hopper type feed system.
A coal pump feed system is an attractive alternative system, with the single stage screw representing an advanced version of the base system. There have been investigations into adapting a single stage extrusion type screw to feed up to 14 atmospheres pressure. The system also offers the possibility of using additives to assist in coal pumping by improving the flow or packing density.
Solids entrained by the combustion gases enter the primary cyclones through horizontal ducting. Primary solids separation occurs in the first stage cyclone at about 95 percent efficiency. Gases leaving the first stage cyclone are directed to the second stage for further solids separation.
Solids collected in the primary cyclone flow by gravity through a heat transfer section with horizontal tube bundles below the barrel of the cyclone (Figure 2). The solids are then collected in a conventional hopper and returned to the PCFB furnace through a siphon duct arrangement.
A second stage of cyclone provides further cleanup. Air leaving this cyclone is finally cleaned as solids are let down through a lock hopper system. This ash contains little or no carbon and, after cooling, the ash is sent to waste disposal.
PCFB cyclone performance determines the solids level entering the gas turbine. Cyclone theory shows that performance is affected less by pressure, and more by particle size, inlet velocity, and barrel diameter. The reference cyclone design has a constant diameter convection section located directly below the main barrel section. Solids separated in the cyclone barrel will rain down over horizontal heat transfer tube bundles.
In a PCFB, combustion air from the high pressure gas turbine is split into primary and secondary streams. Primary air is injected into the bottom of the furnace under the air distributor. Secondary air is injected into the furnace above the primary combustion zone. Excess air (30 percent) is used to establish total air flow and gas turbine output in the system.
It is expected that pressurized operation will improve reaction kinetics. The furnace height should be more than adequate to achieve high combustion efficiency (99 percent). The S02 removal rate will also be improved. The circulating fluidized bed also allows the use of staged combustion to reduce the NOx formation and improve combustion efficiency.
The pressure drop across the PCFB system has been estimated to be 9.7 psi (0.67 bar), of which 5.5 psi (0.38 bar) is attributed to the furnace and grid plate and 3 psi (0.21 bar) from the two stages of cyclone. Duct losses cause the remaining pressure drop.
Inventory for the process is controlled by a solids diverter valve as used in the ACFB system. Solids are discharged through the valve to the solids cooler to keep the furnace pressure drop constant. Additional solids are removed from the system in the secondary cyclone and final gas cleanup system.
The results of preliminary feasibility studies indicate that the circulating fluid bed system can be adapted to pressurized operation. Because the volume of the PFB divided between the combustion section and the heat exchanger, the system may be fabricated in modules in a factory and assembled in the field.
The article goes on to describe several improvements to the base cycle, all add complexity and improve efficiency. In the end the effiency was up to 44.88% from the initial 37.72%.
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(1) Descriptions and status of the various facilities were taken from DOE report ANL/FE-81-65 prepared by Argonne National Laboratories under contract W-31109-Eng-38 printed in April, 1982.
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