Steam Power Station-Electricity generation

Steam Power Station Electricity generation

Electricity generation

The necessity of Electricity in our modern world is at the point of becoming part of our life. This has a lot to do with Energy, and how other forms of energy can be converted into electrical energy. Meanwhile, the usage of electricity is found in almost all segments in the world. Ranging from domestic, commercial, and industrial purposes. 

However, since it’s vivid that everyone needs electricity, therefore, it’s expected that electricity should be produced in bulk. Although, from the study of energy, part of the importance of electrical energy is the cheap nature. Meanwhile, despite its great necessity and availability, Electric power generating station or power plants still have to ensure economical bulk production of electricity. 

Generating Station or Power Plants

Well, a simple definition for generating station or power plants, is where electricity is produced in bulk. Meanwhile, during the design of a power plant, it’s expected that such plant lay considerations on two important aspects. Part of which is the selection and placing of suitable and necessary power generating equipment. While the other consideration is to ensure a perfect system of operation, as regards the economical generating of electricity, it’s availability, and continuity. 

However, a simple design arrangement of the generating station is the Prime mover-Alternator arrangement. Remember I made mention of other forms of energy being converted into electrical energy. The whole process starts from the energy source which includes, the sun, water, wind, fuel, and nuclear energy. Meanwhile, the prime mover help to the conversion of these energies into mechanical energy, for which the alternator act. The alternator then converter this mechanical energy into a three(3) phase electrical energy(electricity).

It may be emphasized that, apart from the prime mover-alternator arrangement as explained above, other auxiliary instruments and equipment are required. This additional equipment has a lot to ensure that, electricity is being produced at a cheap rate. Also, to ensure high reliability and continuity in operation. 

Classification of Generating Station 

Electric power generating stations can be of a different kind. Meanwhile, they are classified based on the form of energy conversion into the electrical energy required here. These classification includes:

  • Steam(Thermal) power generating station 
  • Hydroelectric power generation station
  • Diesel power generating station 
  • Nuclear power generating station 

Meanwhile, this article encompasses more of steam power generating station. So, let’s get more insight about it.

Steam Power Station

We have seen that part of the energy source utilized for the conversion process into electrical energy, is that of coal from fuel. Meanwhile, a power generation station that makes use of the heat energy from coal combustion is known as a steam power station. Basically, this heat energy is being converted into electrical energy, following a process known as the Rankine cycle.

However, this process isn’t a direct process, because there is conversion going in between. Firstly, steam is being produced in the boiler from coal combustion. The steam is directly transferred to the prime mover for expansion. Meanwhile, the prime mover is also considered as a steam turbine. 

Steam Power Station

The prime mover converts the heat energy(steam) into mechanical energy. Then with the help of an alternator, which converts this mechanical energy into three-phase electrical energy.

Meanwhile, from physics, energy can’t be created or destroyed. As such, part of the steam not utilized is then condensed by the condenser and sent back to the boiler to be utilized again. Thereby creating a cycle process known as the Rankine cycle.

However, the suitability of this type of power generation station is located at places where both coal and water is in abundance. Therefore, resulting in a large amount of electricity being produced.

Advantages of Steam Power Station 

  1. This type of power makes use of coal as its source of energy. However, among all forms of energy sources, coal is quite cheap.

2. This power station can be built anywhere, irrespective of the presence of coal or not because coal can always be transported anywhere.

3. When compared to the hydroelectric power stations, it occupies lesser space.

4. Meanwhile, the initial starting cost is cheaper when compared to other power generating stations.

5. Compared to diesel power generating stations, the cost of generating is lesser. 

Disadvantages of steam power station 

  1. Despite the initial cost of economical, the running cost isn’t.

2. Coal produces a large number of fumes and smoke, so this becomes a threat to the human atmosphere.

Stages of Steam Power Station Arrangement 

As earlier mentioned, this power station utilizes heat energy from coal combustion and convert it into three-phase electrical energy. Meanwhile, the operational process isn’t a direct process. Due to high efficiency and proper working, the operational process encompasses other arrangements. These arrangements have been divided into stages as seen below:

  • The coal and ash handling arrangement
  • Steam power plant 
  • Steam turbine 
  • Alternator 
  • Feed water 
  • Cooling arrangement 
Steam Power Station

Let’s have more insight on the following.

1. The coal and ash handling arrangement

Since coal can be transported, therefore it can also be stored. Every thermal power station has a coal storage plant where coal is being stored. Meanwhile, the whole essence of coal is to prepare for days of shortages. However, due to the initial size of coal, it will reduce in size for combustion purposes. So, for this reason, coal is being pulverized, this implies the crushing of coal into smaller pieces. 

Meanwhile, the pulverized coal help to increase surface exposure, which is necessary to increase the combustion. Although, this pulverized coal still has to be subjected to combustion. As such, it’s being conveyed through a belt conveyor to the boiler. However, after complete combustion, the ash produced is being moved to the ash handling plant. Well, for proper burning of coal, the coal from ash handling plant, it’s then delivered to the ash storage plant disposal. 

2. Steam power plant 

Here, both the boiler and other auxiliary equipment operate in the form of a cycle for the utilization of the flue gases. This equipment includes:

  • The boiler 
  • Superheater 
  • Economizer
  • Air preheater 

The boiler 

The pulverized coal conveyed to the boiler is then subjected to combustion. Meanwhile, the heat from the coal combustion is used to convert the water feed to the boiler into steam at high temperature and pressure. Then the steam is fed to the steam turbine but before then, let’s talk a look other auxiliary equipment for which the flue gases find its way to the chimney.

Moreover, the flue gases journey its way to the chimney, starting from the superheater, to the economizer, then to the air preheater before reaching the chimney.

Superheater

The superheater is placed between the boiler and the steam turbine. The superheater helps to convert the wet steam from the boiler into dry steam, with a very high steam temperature. Meanwhile, the superheater utilizes the flue gases to convert the wet steam into dry steam. Also, this is done to increase the overall efficiency and avoid blade corrosion which can result from wet steam. 

Economizer

The water which is then converted into steam is conveyed from the economizer to the boiler. Invariably, the economizer is a feedwater heater, what it simply does is that it utilizes the same fuel gases as that of the superheater. But here, part of fuel gases is used to increase the water temperature before feeding it to the boiler.

Air preheater

Again, for combustion to take place, there should exist the presence of oxygen(air). Air is being conveyed to the boiler through the air preheater. Meanwhile, the air preheater extracts air from the atmosphere through a forced draught fan. Also, the air preheater utilizes part of the fuel gases to increase the temperature of the air before feeding it to the boiler. Although, other necessities of the air preheater are to increase the steam efficiency and surface capacity per square meter of the boiler.

3. Steam turbine 

Through the main valve, the dry steam from the superheater is conveyed to the steam turbine. Meanwhile, this dry steam exerts a rotational force on the steam turbine blades to create mechanical energy. Although, after utilizing the steam, the steam is then subjected to a condenser that condenses the steam through cold water circulation.

4. Alternator 

The whole process is to achieve electrical energy, and this is done with the help of an alternator connected to the steam turbine. The alternator helps to convert the mechanical energy from the blade of the turbine to three-phase electrical power. Meanwhile, the electricity passes through the bus bars, transformer, circuit breakers, and isolators to the indoor substations then to the consumers.

5. Feedwater

We said earlier that, water is needed in the boiler. Technically, it passes through from the condensate of the condenser to the boiler. Although, this feed water still has to pass through a water heater(economizer) before reaching the boiler. Meanwhile, the essence of passing this medium to the boiler is to increase the plant overall efficiency.

6. Cooling arrangement 

The necessity of this arrangement is done to increase plant efficiency. After the whole process, the exhausted steam from the turbine is being condensed through a condenser. This condenser envelope water is drawn from natural sources. This exhausted steam comes into the condenser, the water present there becomes hot, thereby cooling down the steam. 

Well, the hot water in the condenser is then conveyed back to a suitable point of the natural source(river). Although, during cases of low availability of water, instead of sending back the hot water to the river. This hot water is sent instead to the cooling tower, which cool down the hot water and reused for the same process.

Choose of site selection for Steam Power Generation Stations

It’s necessary that when setting up a steam power station, some economical consideration has to be made. This consideration for the site includes:

Availability of coal: Although, I made mention that, coal can be transported irrespective of the location of the station. Well, it will be more economical if the power station is situated close to the coal mines. This will help to reduce the cost of transporting coal.

Availability of water: This type of power station utilizes a large amount of water in the condenser. This is needed to reduce the pressure of the exhausted steam. As such, it becomes paramount that the location of this type of power station should be where water is close by.

Land cost and type: Again, for economical purposes, it becomes advisable to set up a power generation station in places where the cost of land is cheap. Also, with the tendency of land extendibility. Meanwhile, the soil strength of such an area should be strong enough because of the massive equipment to be installed.

Transportation: Both types of machinery and materials has to be transported. So, for this reason, the road to the power station has to be good enough. Else, it will result in unnecessary funds to repair faulty vehicles affected by the bad road.

Isolated from the populated area: Part of the disadvantages of the steam power station is that the coal produces fume and smoke. This poses a threat to humans. So, it’s advisable to be located far away from a populated area, to avoid future health issues.

However, for cases where direct current(d.c) supply, distance from the transmission station has to be put in place. That’s the power station has to be situated close to a transmission station. Meanwhile, this consideration can be neglected for a.c power supply.

Steam Power Station Efficiency 

For the steam power station, both the thermal efficiency and the overall efficiency is very low. Meanwhile, the overall efficiency is 29% and that of the thermal efficiency is 30%.

The relationship which exists among these efficiencies:

Overall efficiency = Electrical efficiency * Thermal efficiency 

However, this low efficiency of this station is as a result of the heat energy lost in the condenser. Also, the losses of energy in the system plants. Moreover, the losses experienced in the condenser can’t be avoided. The reason being that, for heat energy to be converted to mechanical energy, there should exist a temperature difference. Meanwhile, from the thermodynamics law, which implies that the greater the temperature difference, the greater the conversion. Well, the whole essence of the condenser, is to ensure a low temperature of the steam. Again, the more the temperature difference, the higher the heat energy lost. So, either way isn’t favorable, the reason why the overall efficiency of a steam power generating station is quite low.

Thermal efficiency: This is the ratio between the equivalent heat of the mechanical energy to the shaft and the heat of the combustion of coal.

As earlier mentioned, the thermal efficiency of the steam power station is 30%. This implies that, if the heat from 100calories of coal undergoes complete combustion, only 30calories of heat will be utilized on the mechanical shaft of the turbine. The others will be lost, meanwhile, 50% of the heat lost is experienced in the condenser.

Overall efficiency: This is the ratio of the heat equivalent of the electrical output to the heat of the coal combustion. Meanwhile, the overall efficiency is about 29%, which is a bit lower than the thermal efficiency. This is because 1% of the heat is lost in the alternator. 

Steam Power Station Equipment 

These equipment includes:

  1. The Steam generating equipment 

2. Condenser 

3. Primer mover(steam turbine)

4. Water treatment plant

5. Electrical equipment 

Although, there are lots of equipment and auxiliaries present in the steam power station. But the above listed are the main equipment and we shall see more about it.

1. The Steam generating equipment

The whole conversion process starts here, making this set of equipment very important. In other words, it’s where water is converted into steam with the heat produced from the combustion of coal. This equipment encompasses the boiler, boiler furnace, superheater, economizer, and the air preheater.

Boiler 

The boiler is sort of an enclosed vessel, where water is being converted into steam with the help of the heat from steam combustion. Meanwhile, there are two types of boiler and this includes that of the water tube boilers and the fire tube boilers

These two boilers have advantages over each other. Although, that of water tube boilers has more advantages compared to the fire tube boilers. Therefore, the usage of the water tube is more preferable. These superseding advantages include:

  • Less space occupied 
  • Size of drum and tube is smaller when compared 
  • High working pressure 
  • Less liable to explosion 

Boiler furnace

This is a supportive and protective enclosure for the boiler. It also acts as a chamber where the fuel undergoes complete combustion to produce the heat required for the process. Nevertheless, the proper constructive measure has to be put in place as regards the furnace wall. This constructive measure is due to the heat produced inside the furnace, as such, the boiler furnace wall is made up of refractory materials. That is materials that have to ability to resist the effect of heat. Example of these materials includes silica, fire clay e.t.c. 

However, there are different type of refractory wall based on their contribution and they include: 

  • The plain refractory walls
  • Refractory walls made hallow with air cooling arrangement 
  • Water wall 

That of the plain refractory walls are applicable for small plants. In other words, a plant with a low furnace temperature. Therefore, for large plants where the furnace temperature is considered high, then the plan refractory won’t be applicable. The reason is that a high amount of heat can damage the material. So, in such a case, the refractory wall made hallow with an air cooling arrangement is used. Meanwhile, the air circulates through the hallow to keep the temperature in the low. 

With advanced development, the water wall was created. Here, tubes are connected at the top and bottom of the furnace with water in it. Meanwhile, the functionality of this water is to limit the heat to a certain level, so it won’t cause damage to the furnace wall.

Due to the shape and cost of the furnace, it’s size is always limited.

Superheater

The steam produced from the boiler is considered wet, this with time made poses a threat of corrosion to the turbine blade. For this very, the steam needs to be in its try form. This is achieved with the superheater, also helps to increase the overall efficiency and the thermal efficiency of the system. Meanwhile, the superheater utilizes the fuel gases traveling to the chimney to operate. Nevertheless, this superheater can be of two forms as listed below:

  • Radiant superheater
  • Convection superheater 

The radiant superheater is mounted in the furnace between the furnace wall and the heat which is generating from the complete combustion of coal by radiation. Although, there are some limitations to this type of superheater. One of which, for large plants with high furnace temperature, the superheater might get damage. So, proper care has to lay during its construction. Again, when the steam output increase, the temperature of this superheater reduces. This inverse proportionality has limited the utilization of the radiant superheater. Well, this isn’t so for that of convection superheater because as the steam output increase, the temperature of the heater increases also. 

Meanwhile, the convection superheater is situated inside the boiler tube tank and utilizes the heat from the coal combustion through convection. In addition, every superheater consists of a special group of tubes made of an alloy of steel.

The Economizer 

Here, the production of steam requires water being feed to the boiler. However, this feed water will have to go through the economizer. The economizer consists of a large number of parallel close steel tubes. The necessity of this is to increase the temperature of the feedwater before reaching the boiler. Invariably, this will help reduce the work of the boiler since the temperature of the feedwater is high already. 

The economizer utilizes the heat produced from the fuel gases traveling to the chimney. Meanwhile, the feed water flows inside the steel tube with the fuel gases traveling around it. Also, this will help increase both the overall efficiency and thermal efficiency of the system. 

Air Pre-heater

The presence of oxygen is essential for the combustion process. Although, the economizer and the superheater can’t extract the heat from the fuel gases traveling to the chimney literally. Still trying to find a way to increase the thermal efficiency of the system. Having the air preheater does the job perfectly, extracting the heat from the fuel gases while sending air to the boiler. It’s expected to know that the air is being expected from the atmosphere through a forced draught fan. 

Again, there are also different types of air preheater in accordance with the method through which heat is being applied to the furnace. These includes:

  • The Recuperative type 
  • Regenerative type 

Recuperative type of air preheater consists of a group of steel tubes enveloping the fuel gases. Having the air extracted from the atmosphere passing through it. Moreover, when this air comes in contact with the surrounding where these steel tubes are present, the temperature of the air increases.  

The Regenerative type of air preheater consists of a slowly moving drum made of corrugated metal. This drum has two sides, one for which the fuel gases travel through to the chimney. While the other side of the drum is where the extracted airflow through. Definitely, there will be heat transfer from the fuel gases to the air within. Resulting in to increase in temperature of the air before flowing to the boiler. 

2. The condenser 

The condenser helps to condense the exhausted heat at the turbine. Meanwhile, the condenser is filled with water from its natural source through the cooling tower. The condenser helps to liquidate the steam, thereby subjecting it to low pressure. Again, the serve as a passage of the feed water to the boiler. The condenser can be of two types and these include: 

  • The jet type condenser 
  • Surface type condenser 

In that of the jet type condenser, both the feed water and the condensate are mixed together. This then reduces the temperature of the condenser to that of the feed water. Although the condensate can’t be reused by the boiler, this poses a limitation on this type of condenser. Nevertheless, the initial cost is low, requires less cooling water and again, the floor area is minimal. 

The surface type condenser is different from the jet type, in that, both the condensate and feedwater do not mix with each other. While this is achieved with the presence of a bank of horizontal tubes. The feedwater flow inside the tube, with the exhausted steam flowing around it. Part of its advantages is that the condensate can be used as feedwater. Although, despite its advantages, there are still some limitations to it. This includes high initial and maintenance costs, large surface area. 

3. Prime movers

The heat energy from the steam-generating arrangement is then converted into mechanical energy in the prime mover. Although, the prime mover for steam power generating station can be of two types:

  • Steam turbine 
  • Steam engine 

The application utilization of the steam turbine is far more than that of the steam engine. Due to the following:

  • Simple in construction 
  • initial and maintenance cost is low
  • Require reasonably floor area 
  • High efficiency 

However, the steam turbine can be divided into two, and both are based on the action of the steam on the moving blade of the turbine. Therefore, we have the following: 

  • The impulse steam turbine 
  • Reactions steam turbine 

For the impulse steam turbine, the steam expanse completely on the stationary nozzles. Meanwhile, the pressure on the moving blade becomes constant, this results in high velocity on the steam. Acting on the moving blade to create an impulsive force on the moving blade, causing the rotor to rotate.

Well, for that of Reactions steam turbine, the steam does not expand completely on the stationary nozzles. Hence, creating momentum on the steam to cause a reaction force on the moving blade. This whole process subjects the rotor to rotate.

4. Water treatment plant 

The water feed to the boiler is expected to be soft and clean. These attributes of the water will aid the longer-lasting duration of the boiler and also high efficiency. Certainly, the natural source of water for the steam power station can either be from a river or lakes. Knowing that water from this source contains some impurities, including dissolved impurities and dissolved gases. A way to purify this water is by subjecting it to different chemical treatments.

Firstly, the water is stored in a storage tank where chemical treatments are done. The dissolved impurities are removed through coagulation, sedimentation, and filtration. While that of the dissolved gases is removed through degasification and aeration.

The chemical treatments just mentioned, is done to clean the water from impurities. In other to soften the water, the water is also subjected to another chemical process to eradicate both temporary and permanent hardness. After the whole process of making the water clean and soft, the water is then fed to the boiler through the economizer. This process will increase the overall efficiency of the system.

5. Electrical equipment 

In every power generating station, there exist the presence of different electrical equipment. However, we will look at the main electrical equipment present in every power generating station. 

Alternator: 

The necessity of the steam power generating station is to convert energy from coal into electrical energy. Certainly, we have seen how the heat energy from the coal combustion is being converted into mechanical. Now, we need to convert this mechanical energy generated into electrical energy. Well, we can achieve this with the help of an alternator. An alternator converts mechanical energy into electrical energy. 

Here, the alternator which is connected to a prime mover(steam turbine or steam engine) converts the mechanical energy produce by the prime mover into a three(3) phase electrical energy(electricity). Moreover, the alternator needs to be excited and this can be achieved by means of main and pilot exciter. This exciter is directly connected to the alternator shaft.

Transformers: 

Basically, the primary function of a transformer is for either stepping up or down of voltages. Well, in the power generating station, there exist the presence of a different type of transformers with different purposes. Here, you can find the main transformer. The main transformer helps to step up the voltage generated from the whole process(output from the alternator) for transmission purposes.

You can also find the station(normal) transformer with the purpose of lighting the power station. Also, the presence of an auxiliary transformer, with the purpose of supplying to other sub auxiliaries(unit).

Switchgear:

The operational control of the various equipment in every power generating station can be remotely controlled. This is done with the help of switchgear. Meanwhile, the switchgear consists of some electrical and electronics equipment. These include: the circuit breaker, switches, relays e.t.c. Again, fault can be traced and controlled from the switchgear. 

The conclusion to Steam Power Station

The necessity of the steam power station is to convert heat energy gotten from the complete combustion of coal into electrical energy. Although, both the overall efficiency and thermal efficiency are low, as such, ways must be considered to at least maintain its maximum efficiency. 

Also, we got some other interesting articles on Electrical Engineering that will be of interest to you. So, feel free to check them out: Electrical Archive

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