A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine, which either drives an electrical generator or does some mechanical work, like ship propulsion. After it passes through the turbine, the steam is condensed and recycled to where it was heated before; this is known as the Rankine Cycle. The greatest variation in the design of thermal power stations is the different fuel sources. Some prefer to use the term energy centre because such facilities convert forms of heat energy into electrical energy.
In the Rankine Cycle-based steam power plants are the major source of power generation. In a thermal power plant, the chemical energy stored in fuels such as coal, oil and natural gas is converted successively into thermal energy, mechanical energy and finally electrical energy for continuous use and distribution across a wide geographic area. In the Rankine Cycle, high pressure and high temperature steam, generated in a boiler is expanded through a steam turbine that drives an electric generator. Thermal Power plants have very high availability and can operate for more than a year between shutdowns for maintenance and inspections. Their unplanned or forced outage rates are very low.
Thermal Power Plant assets need to be flexible to meet rapidly fluctuating demand levels. In addition, they need to remain reliable and demonstrate that every effort has been made to minimise environmental impacts and maximise efficiency. Ensuring flexible, reliable operation with minimum forced outages, implementing innovative strategies that reduce emissions and dealing with volatile power markets while achieving the lowest operating costs possible are the new industry realities.
Basics of Coal fired Thermal Plants
More than half of the electricity generated in the world is by using coal as the primary fuel.
The function of the coal fired thermal power plant is to convert the energy available in the coal to electricity. Coal power plants work by using several steps to convert stored energy in coal to usable electricity that powers our lights, computers and sometimes, back into heat for our homes.
How Coal Power Plants Produce Electricity?
The conversion from coal to electricity takes place in three stages.
Stage 1
The first conversion of energy takes place in the boiler. Coal is burnt in the boiler furnace to produce heat. Carbon in the coal and Oxygen in the air combine to produce Carbon Dioxide and heat.
Stage 2
The second stage is the thermodynamic process.
- The heat from combustion of the coal boils the water in the boiler to produce steam. In modern power plants, boilers produce steam at a high pressure and temperature.
- The steam is then piped to a turbine.
- This high-pressure steam impinges and expands across a number of sets of blades in the turbine.
- The impulse and the thrust created rotate the turbine.
- The steam is then condensed and pumped back into the boiler to repeat the cycle.
Stage 3
In the third stage, rotation of the turbine rotates the generator rotor to produce electricity based on Faradays Principle of electromagnetic induction.
Layout of thermal power plants is as below.
There are four main circuits in any thermal power plant:
- Coal & Ash Circuit – This circuit deals mainly with feeding the boiler with coal for combustion purposes and taking care of the ash that is generated during the combustion process. It includes equipment and paraphernalia that is used to handle the transfer and storage of coal and ash.
- Air & Gas Circuit – Air is one of the main components of the fire triangle and hence essential for combustion. Since a lot of coal is burnt inside the boiler, it needs a sufficient quantity of air, which is supplied using either, forced draught or induced draught fans. The exhaust gases from the combustion are in turn used to heat the ingoing air through a heat exchanger before being let off in the atmosphere. The equipment, which handles all these processes, falls under this circuit.
- Feed Water & Steam Circuit – This section deals with supplying the steam generated in the boiler to the turbines and to handle the steam going out from the turbine by cooling it to form water in the condenser so that it can be reused in the boiler, plus making good any losses due to evaporation etc.
- Cooling Water Circuit – This part of the thermal power plant deals with the handling of the cooling water required by the system. Since the amount of water required to cool the outgoing steam from the boiler is substantial, it is either taken from a nearby water source such as a river, or is done through evaporation if the quantity of cooling water available is limited.
Concrete in thermal plants:
The object of the invention is a thermal power plant, especially with a view to the production of electric power, including at least one boiler house equipped with a boiler and at least one chimney for the evacuation of the fumes produced by the boiler, characterised by the fact that the boiler house and the chimney present a common lateral wall portion and are made up of a unitary concrete structure. One way of setting up the thermal power plant, including the boiler house with a tower and a chimney. A common supporting structure consists of foundations, which support a floor slab plate. The boiler house and the chimney reveal a vertical portion with a common lateral wall.
These structures are made of concrete and constitute a continuous assembly. The tower of the boiler house, in particular, consists of a layer of reinforced concrete about 20cm thick, surmounted by a belt of reinforced concrete. In this example, the tower has a cylindrical revolution shape. The concrete layer of this tower serves as a cover for the boiler house, supporting and sheltering it. The tower of the chimney as such consists of a revolution cylinder of reinforced concrete with a thickness of about 30cm.
A smoke flue is arranged concentrically inside the chimney while leaving a free space between the chimney and the flue. This flue is designed to protect the concrete of the chimney against heat stress, i.e., against the chemical attack of hot gases; the free space guarantees heat insulation and at the same time permits any possible necessary maintenance or repair work.
A version involving two cylindrical towers made of concrete and forming a continuous block, resting on the same foundations, ensures the stability of the chimney, which, if built separately, would require a conical structure with foundations and a floor slab plate of its own. This arrangement makes it possible to achieve considerable savings, especially in terms of the volume and the weight of the chimney tower and its construction cost. For example, using cylindrical sliding coffering, which is half the cost of conical sliding coffering. Second, it facilitates savings in terms of the weight and the volume of the common foundations as well as the floor slab plate surface, which is reduced. Finally, it is possible to use the concrete cover of the boiler house in place of a shingle covering. On the other hand, it should be noted that the time to build these structures is much shorter due to the usage of cylindrical sliding coffering; this is not without significance in terms of construction time.
By comparison,
Volume of concrete for shaft of boiler tower 1,200m3; Surface of coffering for shaft of boiler tower 8,300m2; Volume of concrete for tower belt 760m3; Surface of coffering for tower belt 950m3; Volume of reinforced concrete for chimney shaft 1,040m3; Surface of coffering of chimney shaft 8,300m2; Volume of concrete for foundations 660m3. The figures provided here would— in case of construction according to the invention—lead to a reduction in the cost of these comparative elements by 60%, compared to the cost of the conventional solution; this would hold true within the framework of the construction of a single boiler unit.
