A fossil fuel power plant is a system of devices for the conversion of fossil fuel energy to mechanical work or electric energy. The main systems are the steam cycle and the gas turbine cycle. The steam cycle relies on the Rankine cycle in which high pressure and high temperature steam raised in a boiler is expanded through a steam turbine that drives an electric generator. The steam gives up its heat of condensation in a condenser to a heat sink such as water from a river or a lake, and the condensate can then be pumped back into the boiler to repeat the cycle. The heat taken up by the cooling water in the condenser is dissipated mostly through cooling towers into the atmosphere.
General Layout of Thermal Power Plant
The general layout of thermal power consists of mainly four circuits. The four circuits are:
In this circuit, the coal from the storage is fed to the boiler through coal handling equipment for the generation of steam. Ash produced due to combustion of coal is removed to ash storage through ash-handling system.
Air and Gas Circuit
Air is supplied to the combustion chamber of the boiler either through forced draught or induced draught fan or by using both. The dust from the air is removed before supplying to the combustion chamber. The exhaust gases carrying sufficient quantity of heat and ash are passed through the air-heater where the exhaust heat of the gases is given to the air and then it is passed through the dust collectors where most of the dust is removed before exhausting the gases to the atmosphere.
Feed Water and Steam Circuit:
The steam generated in the boiler is fed to the steam prime mover to develop the power. The steam coming out of the prime mover is condensed in the condenser and then fed to the boiler with the help of pump. The condensate is heated in the feed-heaters using the steam tapped from different points of the turbine. The feed heaters may be of mixed type or indirect heating type. Some of the steam and water are lost passing through different components of the system, therefore, feed water is supplied from external source to compensate this loss. The feed water supplied from external source to compensate the loss. The feed water supplied from external source is passed through the purifying plant to reduce to reduce dissolve salts to an acceptable level. This purification is necessary to avoid the scaling of the boiler tubes.
Cooling Water Circuit:
The quantity of cooling water required to condense the steam is considerably high and it is taken from a lake, river or sea. At the Columbia thermal power plant it is taken from an artificial lake created near the plant. The water is pumped in by means of pumps and the hot water after condensing the steam is cooled before sending back into the pond by means of cooling towers. This is done when there is not adequate natural water available close to the power plant. This is a closed system where the water goes to the pond and is re circulated back into the power plant. Generally open systems like rivers are more economical than closed systems.
Working of the Thermal Power Plant:
Steam is generated in the boiler of the thermal power plant using heat of the fuel burnt in the combustion chamber. The steam generated is passed through steam turbine where part of its thermal energy is converted into mechanical energy which is further used for generating electric power. The steam coming out of the steam turbine is condensed in the condenser and the condensate is supplied back to the boiler with the help of the feed pump and the cycle is repeated. The function of the Boiler is to generate steam. The function of the condenser is to condense the steam coming out of the low pressure turbine. The function of the steam turbine is to convert heat energy into mechanical energy. The function of the condenser is to increase the pressure of the condensate from the condenser pressure to the boiler pressure. The other components like economizer, super heater, air heater and feed water heaters are used in the primary circuit to increase the overall efficiency of the plant.
Site Selection of a Thermal Power Plant:
The important aspect to be borne in mind during site selection for a thermal power plant are availability of coal, ash disposal facility, space requirement, nature of land, availability of water, transport facility, availability of labor, public problems, size of the plant.
Thermal Power Plant Engineering
In the thermal power plant the coal is shipped in by means of rail. The coal is obtained primarily from coal fields. This is done in most cases, but coal may also be shipped by trucks or by pipelines. Inside the power plant there is an unloading dock where the coal is unloaded. The coal is stored in huge heaps or piles of a half a mile in diameter. The reason for stocking this much of coal is because the loss due to loss of generation due to lack of coal is very high. The upper layer of the coal heap has to be compacted to make it into an airtight surface to prevent loss of coal due to oxidation. Other methods of preventing oxidation are by keeping it under water or by spraying chemicals on it.
In plant coal handling is a very important aspect of power plant safety. In variably the coal is not exposed as it can pollute the air and release poisonous gases like carbon monoxide. The coal from the heaps is moved into the plant by means of long conveyors that are electrically operated. There are many different types of conveyors and coal-handling devices like screwing conveyors, bucket elevators, grabbing bucket conveyors etc.
Before the coal is sent to the plant it has to be ensured that the coal is of uniform size, and so it is passed through coal crushers. Also power plants using pulverized coal specify a maximum coal size that can be fed into the pulverizer and so the coal has to be crushed to the specified size using the coal crusher. Rotary crushers are very commonly used for this purpose as they can provide a continuous flow of coal to the pulverizer.
Most commonly used pulverizer is the Boul Mill. The arrangement consists of 2 stationary rollers and a power driven baul in which pulverization takes place as the coal passes through the sides of the rollers and the baul. A primary air induced draught fan draws a stream of heated air through the mill carrying the pulverized coal into a stationary classifier at the top of the pulverizer. The classifier separates the pulverized coal from the unpulverized coal.
The ever increasing capacities of boiler units together with their ability to use low grade high ash content coal have been responsible for the development of modern day ash handling systems.The hydraulic system carried the ash with the flow of water with high velocity through a channel and finally dumps into a sump. The hydraulic system is divided into a low velocity and high velocity system. In the low velocity system the ash from the boilers fall into a stream of water flowing into the sump. The ash is carried along with the water and they are separated at the sump. In the high velocity system a jet of water is sprayed to quench the hot ash. Two other jets force the ash into a trough in which they are washed away by the water into the sump, where they are separated. The molten slag formed in the pulverized fuel system can also be quenched and washed by using the high velocity system. The advantages of this system are that its clean, large ash handling capacity, considerable distance can be traversed, absence of working parts in contact with ash.
A well defined operations system backed up by a comprehensive maintenance programme can mitigate many of the loss exposure scenarios in respect of turbines and generators. However, as sophisticated as the online performance monitoring and testing may be, the only way to examine many parts of the turbine is by dismantling and overhaul.. Peaking turbines will require more frequent overhaul due to the additional stress imposed by frequent start-ups and shut-downs as well as load changes. Records of all disassemblies, run-downs and run-ups, vibration signatures, photographs and other supporting information can be used to assist in tracking conditions between overhauls. An oil analysis is one reliable preventive maintenance tool to identify problems arising with the installation.
A formal program for water/cool vapor induction protection in accordance with current standards of the American Society of Mechanical Engineers (ASME) in the United States, or the equivalent in other countries, is needed to control the hazard of cool water ingress. GE Global Asset Protection Services (GE GAPS) Guidelines recommend providing a three-hour rated fire separation, full automatic sprinkler protection with a density of 12.2 l/min/m² over 279 m². Foam water sprinklers might be more effective than water only and are more desirable where adequate drainage is not granted.
The duration of foam application should be designed according to the time required to run the turbine on turning gear. The bearings of turbine generator sets should be protected with directional spray nozzles. The heads should provide a density of 0.25 gallons per minute (gpm) over the protected area. Lube oil lines within the turbine lagging (skirt) should also be sprinkler-protected with a density of 0.30 gpm over the protected area. Exciter housings, which are directly connected, should be protected with automatic CO2 systems. With full protection under turbine sprinkler NFPA 850 does not consider separate protection for spot oil hazards necessary. GE GAPS Guidelines recommend both.
Electro Hydraulic Control Systems (EHC systems) operate the main steam stop valves and control valves, reheat and intercept valves. Pressures on these hydraulic systems can be as high as 200 bar. Low hazard oil is typically used now, but old plants and some foreign machines may use oil directly out of the main lube oil reservoir. Sprinkler or water spray systems should be used.
Buildings and areas where hydrogen is stored or transferred should be equipped with adequate venting facilities and gas sensors to exhaust and indicate gas leakages. Hydrogen concentration inside generators must be kept above the upper explosion limit. Under normal conditions the explosion range of hydrogen is 4% to 77% in volume. Due to the conditions inside the generator - pressure, temperature and water vapor content - the explosive range can be higher than under normal conditions. Often hydrogen concentration is maintained above 98% in volume inside the generator. GE GAPS Guidelines recommend forcing hydrogen out of the generator through dump valves and introducing an inert gas such as nitrogen, CO2 or another in case hydrogen concentration drops under 95% in volume.
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