General Description of Thermal Power Plants - Power Plant Engineering Notes

 

 

A nuclear energy plant is a kind of power station in which heat energy is switched over completely to electrical energy. In a steam-creating cycle, heat is utilised to bubble water in a huge strain vessel to deliver high-pressure steam, which drives a steam turbine associated with an electrical generator. The low-pressure exhaust from the turbine enters a steam condenser, where it is cooled to deliver hot condensate, which is reused in the warming system to create more high-tension steam. This is known as a Rankine cycle. The plan of nuclear energy plants relies upon the expected energy source: petroleum derivatives, atomic and geothermal power, sunlight-based energy, biofuels, and squandered human cremation are undeniably utilised. Certain nuclear energy plants are likewise intended to deliver heat for modern purposes; such as region warming; or desalination of water, as well as create electrical power. Energises, for example, petroleum gas or oil can likewise be scorched straightaway in gas turbines (inward ignition). These plants can be of the open cycle or the more productive consolidated cycle type.

 

Nuclear Power Plant

Energy generation by a nuclear fuel takes place either by the process of nuclear fission of heavy fissile elements in a nuclear reactor, resulting in chain reactions, or by the process of nuclear fusion, in which simple atomic nuclei are fused together to form complex nuclei, as in the case of fusion of hydrogen isotopes to form helium. The process of nuclear fusion is also known as thermonuclear reaction, which is difficult to control even on date. As a result, the main source of nuclear energy is available at the present time mainly from nuclear fission. In the heart of a “nuclear power plant” there is a nuclear reactor, wherein a controlled chain reaction of nuclear fission of heavy elements takes place. The most common fissile radioactive heavy metals are the naturally occurring isotope of uranium, U²³⁵, artificial isotope of uranium, U²³³, and artificial element plutonium, P²³⁹. In a nuclear reactor, plutonium is produced from naturally occurring isotope of uranium, U²³⁸, and U²³³ is produced from naturally occurring element thorium, Th²³²

 

The nuclear energy thus liberated is converted into heat that is removed from the reactor by a coolant, eg, liquid sodium. Hot liquid sodium is then passed through another heat exchanger where water is circulated as a coolant agent, which absorbs heat, resulting in generation of steam. This steam generator emits virtually no carbon dioxide, sulfur, or mercury. Nevertheless, a major concern of a nuclear power plant is that the area surrounding the nuclear reactor is potentially radioactive. Further nuclear wastes, if not disposed of taking special care, may cause a devastating effect on living beings and inanimate objects, including the environment.

 

Hydel Power Plant

Hydroelectric or Hydel power plants comprise of hydraulic turbines, which can be of either vertical shaft or horizontal shaft. The preference for a horizontal shaft lies with the impulse-type while a vertical shaft, with the reaction types. The hydraulic turbine converts the potential energy of supplied water into mechanical energy of a rotating shaft, which in turn drives a generator to produce electricity. In the impulse turbine (Pelton Wheel), the static head is completely transformed into a velocity head in the guide vane. This type is of relatively low (specific) speed, suitable for higher heads. Impulse turbines receive their water supply directly from the pipe line.

In the reaction turbine, the static head is partly transformed into a velocity head in the guide vane. The radial flow type (Francis Turbine) is of relatively medium (specific) speed, suitable for medium heads. In Francis Turbine, high-pressure water enters the turbine with radial inflow and leaves the turbine axially. The high-pressure water, while passing through guide vanes, rotates the shaft to produce power.

 

The propeller type (Kaplan Turbine) is of relatively high (specific) speed, suitable for low heads; thus, it is essential to pass large flow rates of water through the Kaplan turbine to produce power. Incoming water enters the passage in the radial direction and is forced to exit in the axial direction that in turn rotates the shaft for producing power.

 

Renewable Energy Power Plant

Renewable energy resources draw natural energy flows of the earth, that is, solar, wind, geothermal, ocean thermal, ocean wave, ocean tidal, biomass, and storage energy. They recur, sometimes periodically, almost inexhaustible and are free for the taking. They are clean, barring biomass, almost free from causing environmental pollution, and sometimes even noise-free.

 

Fuel Cell Power Plant

A fuel cell generates electrical power by continuously converting the chemical energy of a fuel into electrical energy by way of an electrochemical reaction. Fuel cells typically utilize hydrogen as the fuel and oxygen (usually from air) as the oxidant in the electrochemical reaction. The reaction results in electricity, along with by-product water and heat. The fuel cell itself has no moving parts, making it a quiet and reliable source of power. With their high-power generating efficiency (40–60%), ease in installing near consumers, and applicability for cogeneration (heat and electricity), fuel cells are expected to achieve substantial energy conservation. In addition, fuel cells can utilize natural gas, methanol, or coal gas as fuel, serving as a driving force to encourage the use of oil-alternative energy sources. These plants, however, suffer from generating capacity limitation.

 

Thermal Power Plant

In contrast to all types of power plants described in the foregoing paragraphs, a thermal power plant utilizes the “heat of combustion” of fossil fuels. Fossil fuels are hydrocarbons, primarily coal and liquid petroleum or natural gas, formed from the fossilized remains of dead plants and animals by exposure to heat and pressure in the earth’s crust over hundreds of millions of years. 

These fuels consist of a large number of complex compounds of five principal elements: carbon (C), hydrogen (H), oxygen (O), sulfur (S) and nitrogen (N). All fuels contain also mineral matter (A) and moisture (M) to a certain extent. However, there are just three combustible elements of significance in a fuel; these are carbon, hydrogen, and sulfur of which carbon is the principal combustible element. Fossil fuels generate substantial quantities of heat per unit of mass or volume by reacting with an oxidant in a combustion process. 

Two major constituents of a thermal power plant are a prime mover, which develops mechanical work, and a generator or an alternator, in which mechanical work is converted into electrical energy. Whatever may be the type of prime movers steam turbine, gas turbine, or diesel engine a generator is common to all of them for developing electrical energy. Generators of various plants may have different capacities, but their voltage and frequency essentially have to be identical and constant and shall be in conformance with the voltage and frequency of the high-voltage (132/220/400/700 kV) grid/bus, to which output power from a generator is transmitted through generator transformers, isolators, and generator circuit breakers for further distribution to different consumers. A typical electrical distribution system.

 

Steam Power Plant

A steam power plant constitutes a steam generator (also termed as boiler), a steam turbine, a generator (also known as alternator), condenser, heaters, pumps, fans, and other auxiliaries. In the steam generator, chemical energy available in fossil fuel (coal, fuel oil, and natural gas) is converted to heat energy by combustion. The heat thus liberated is absorbed by low temperature water to become high-energy steam in the steam generator. The thermal energy of the steam is converted to mechanical energy and then to electrical energy in the steam turbine and generator. The typical flow path of working medium, which is steam-water, in a steam power plant.

 

Air is fed into the furnace for combustion of fuel-forming products of combustion or flue gas. Flue gas passing over heating surfaces at various zones gets cooled and is discharged to atmosphere through a stack. The heat released by burning fuel is absorbed in different heat transfer surfaces superheater, reheater, and economizer, to heat the working fluid and in the air heater to heat the ambient air prior to entering into the furnace. The thermodynamic cycle that acts as the backbone of steam power plants is the Rankine cycle, which is described below.

 

Gas Turbine Power Plant

A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine and a combustion chamber in-between. In the compressor, atmospheric air is sucked in at point 1 in the figure; consequently air is pressurized. This pressurized air then enters the combustion chamber, point 2, wherein fuel is sprayed for combustion to take place. 

Energy is released when air is mixed with the fuel (either liquid or gaseous) and ignited in the combustor. As a consequence of the combustion at constant pressure, the temperature of air vis-à-vis the mixture is increased. High-temperature products of combustion of fuel and air are directed over the turbine blades, point 3, get expanded, thereby spinning the turbine and at the same time powering the compressor. Finally, the gases are passed through additional turbine blades, generating additional thrust by accelerating the hot exhaust gases by expansion back to atmospheric pressure, point 4, and performing useful work. Energy is extracted in the form of shaft power that is used to power electrical generators, aircraft, trains, ships, tanks, and so on.

Gas turbines are ideal for electricity generation in periods of peak electricity demand, since they can be started and stopped quickly, enabling them to meet energy demand. They are smooth running, and their completion time to full operation is the fastest compared to other power generating plants. When running in simple cycle mode, the heat rate of a gas turbine is very high compared to the heat rate of other prime movers. The heat rate can be substantially improved by operating the gas turbine in combined cycle mode. In a combined cycle plant, high heat content in gas turbine exhaust gas is recovered in a heat recovery steam generator (HRSG) to generate high-pressure, high-temperature steam, which is then passed through a steam turbine to produce additional power.