The most frequently used fuels for large-scale power generation are oil, natural gas, and coal. Figure3 illustrates the principal elements of a fossil fuel power plant. Fuel handling includes transport by rail, on ships, or through pipelines. A power plant usually maintains several days of fuel reserve at any one time. Oil and gas are stored in large metal tanks, and coal is kept in open yards. The temperature of the coal layer must be monitored carefully to avoid self-ignition.
Oil is pumped and gas is fed to the burners of the boiler. Coal is pulverized in large mills, and the powder is mixed with air and transported by air pressure, through pipes, to the burners. The coal transport from the yard to the mills requires automated transporter belts, hoppers, and sometimes manually operated bulldozers.
Two types of boilers are used in modern power plants: the sub-critical water-tube drum-type and the super-critical once-through type. The former operates around 2500 psi, which is below the water critical pressure of 3208.2 psi. The latter operates above that pressure, at approximately 3500 psi. The super-heated steam temperature is about 1000°F (540°C) because of turbine temperature limitations.
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| Figure: 3 Primary components of a fossil fuel power plant: (a) system block diagram; (b) common configuration of turbine/generator system. |
A typical subcritical water-tube drum-type boiler has an inverted-U shape, as illustrated in Figure 4. On the bottom of the rising part is the furnace where the fuel is burned. The walls of the furnace are covered by water pipes. The drum and the super-heater are at the top of the boiler. The falling part of the U houses the re-heaters, economizer (water heater), and air pre-heater, which is supplied by the forced-draft fan. The induced-draft fan forces the flue gases out of the system and sends them up the stack, which is located behind the boiler.
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| Figure: 4 Flow diagram of a typical drum-type steam boiler. |
This steam generator has three major systems:
FUEL SYSTEM
Fuel is mixed with air and injected into the furnace through burners. The burners are equipped with nozzles, which are supplied by preheated air and carefully designed to assure the optimum air-fuel mix. The fuel mix is ignited by oil or gas torches. The furnace temperature is around 3000°F.
AIR-FLUE GAS SYSTEM
Ambient air is driven by the forced-draft fan through the air pre-heater, which is heated by the high-temperature (600°F) flue gases. The air is mixed with fuel in the burners and enters the furnace, where it supports the fuel burning. The hot combustion flue gas generates steam and flows through the boiler to heat the super-heater, re-heaters, economizer, and other related systems. Induced-draft fans, located between the boiler and the stack, increase the flow and send the 300°F flue gases to the atmosphere through the stack.
WATER-STEAM SYSTEM
Large pumps drive the feed water through the high-pressure heaters and the economizer, which further increases the water temperature (400 to 500°F). The former is heated by steam removed from the turbine; the latter is heated by the flue gases. The preheated water is fed to the steam drum. Insulated tubes called Down-comers are located outside the furnace and lead the water to a header. The header distributes the hot water among the risers. These water tubes line the furnace walls. The water tubes are heated by the combustion gases through both connection and radiation. The steam generated in these tubes flows to the drum, where it is separated from the water. Circulation is maintained by the density difference between the water in the down-comer and the water tubes. Saturated steam, collected in the drum, flows through the super-heater. The super-heater increases the steam temperature to about 1000°F. Dry super-heated steam drives the high-pressure turbine. The exhaust from the high-pressure turbine goes to the re-heater, which again increases the steam temperature. The reheated steam drives the low-pressure turbine.
The typical super-critical once-through-type boiler concept is shown in Figure 5. The feed water enters through the economizer to the boiler, which consists of riser tubes that line the furnace wall. All the water is converted to steam and fed directly to the super-heater. The latter increases the steam temperature above the critical temperature of the water and drives the turbine. The construction of these steam generators is more expensive than the drum-type units but has a higher overall operating efficiency.
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| Figure: 5 Block diagram of a once through-type steam generator. |
The turbine converts the heat energy of the steam into mechanical energy. Modern power plants typically use one high-pressure and one or two lower-pressure turbines. High-pressure steam enters the high-pressure turbine to flow through and drive the turbine. The exhaust is reheated in the boiler and returned to the lower-pressure units. Both the rotor and the stationary part of the turbine have blades. The length of the blades increases from the steam entrance to the exhaust. Figure 6 shows the blade arrangement of an impulse-type turbine. Steam enters through nozzles and flows through the first set of moving rotor blades. The following stationary blades change the direction of the flow and direct the steam into the next set of moving blades. The nozzles increase the steam speed and reduce pressure, as shown in the figure. The impact of the high-speed steam, generated by the change of direction and speed in the moving blades, drives the turbine.
In a fossil fuel plant, the generator converts mechanical energy from the turbines into electrical energy. The stator typically has a laminated and slotted silicon steel iron core. The stacked core is clamped and held together by insulated axial through bolts. The stator winding is placed in the slots and consists of a copper-strand configuration with woven glass insulation between the strands and mica flakes, mica mat, or mica paper ground-wall insulation. To avoid insulation damage caused by vibration, the ground-wall insulation is reinforced by asphalt, epoxy-impregnated fiberglass, or Dacron. Most frequently, the stator is hydrogen-cooled; however, small units may be air-cooled, and very large units may be water-cooled. The solid steel rotor has slots milled along the axis. The multi-turn copper rotor winding is placed in the slots and cooled by hydrogen. Cooling is enhanced by sub-slots and axial cooling passages. The rotor winding is restrained by wedges inserted in the slots.
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| Figure: 6 Velocity and pressure variation in an impulse turbine. |
The rotor winding is supplied by dc current, either directly by a brush-less excitation system or through collector rings. The non-drive-end bearing is insulated to avoid shaft current generated by stray magnetic fields. The hydrogen is cooled by a hydrogen-to-water heat exchanger mounted on the generator or installed in a closed-loop cooling system.
The condenser condenses turbine exhaust steam to water, which is pumped back to the steam generator through various water heaters. This condensation produces a vacuum, which is necessary to exhaust the steam from the turbine. The condenser usually is a shell-and-tube heat exchanger, where steam condenses on water-cooled tubes. Cold water is obtained from the cooling towers or other cooling systems. The condensed water is fed through a de-aerator, which removes absorbed gases from the water. Next, the gas-free water is mixed with the feed-water and returned to the boiler. The gases absorbed in the water may cause corrosion and increase condenser pressure, adversely affecting efficiency. Older plants use a separate de-aerator heater, whereas de-aerators in modern plants are usually integrated in the condenser, where injected steam jets produce a pressure drop and remove absorbed gases.
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