Introduction
Energy is a key driver of any nation’s economy. The extent to which it contributes to the performance of all activities surrounding the human life is beyond measure. This is because energy contributes almost fully to the economic development of the economy. In its physical form, energy is classified either as renewable or non-renewable. Renewable energy forms the green energy that can be reused again and is less toxic to the environment. On the other hand, non-renewable energy is exploited once it’s used and mostly pollutes the environment. All this energy forms can be either used directly to power devices or be converted to other useful forms. One of the notable form of conversion that yields great power is the use of fossil fuels to produce steam that is finally converted to electricity. This energy production process is very popular since the world is entirely run by electricity whose uses range from domestic uses to the manufacturing uses as well. The whole process of electric energy production actually depends on the other existing fuels such as fossil fuels or nuclear energy that is used to heat water in the boiler to produce steam that is harnessed by turbines into electricity. The process of steam production in the boiler involves a series of steps with specific working condition aimed at producing superheated steam at highly elevated temperature and pressure so as to achieve a desirable steam efficiency even at the turbines. Stem power generation therefore encompasses a series of equipment with different functions which shall be deeply analysed in this excerpt. This research therefore works to give a critical description of steam power generation, the equipment involved and the results obtained.
Steam turbine power generation system and the individual components involved.
The steam turbine power generation system consists of a number of equipment ranging from the boiler, turbine, generator and exhaust system as illustrated in the flow process below. All this flow process are geared towards electricity generation. It is important to note that electrical energy generation by using steam turbines entails three energy conversion processes with the first step involving the process of extracting thermal energy from the fuel so as to use it to raise steam (Grant, 1981). This is followed by the second step of converting the thermal energy obtained from the steam into kinetic energy in the turbine and finally using a rotary generator in order to convert the mechanical energy from the rotating steam turbine into electrical energy as in the flow diagram below.
Boiler
The boiler forms the central part of steam generation in the steam turbine power generation system. This is the point whereby steam is generated by the combustion of fuel to heat water in the boiler up to superheated steam. There are a number of fuels used to heat steam in the boiler as discussed in this excerpt. However, the three main types of fuel sources currently used include fossil fuels such as coal, oil, and natural gas as well as nuclear, and solar energy fuels. Each of these fuels operates very differently although they all serve the very same purpose of combustion in the boiler. To begin with coal is the most commonly used type of fuel for most of the steam generation boilers although there are other fossil fuel boilers also used. This is due to the fact that coal produces very steady and highly reliable energy at a relatively lower price compared to other fuel types. Moreover, there are still huge unexploited deposits of coal in a number parts of the world, which can be transported so easily by use of trains and other barges around the world. It is important to note that when coal is normally delivered to a plant, first of all it’s stored in the main yard. The remaining excess coal is then stored for the purpose of use when emergency outages occur or in the times of high electricity demand. The coal is then sent through a specific crusher in which it is pulverized so as to achieve a fine powder. The fine coal powder is again mixed with very hot air which is then pumped into the given boiler so as to fuel the combustion process. The purpose of the combustion reaction is to produce the energy that is needed by the boiler to steadily heat the cold or warm water in the pipes or water tubes into superheated steam. Burning of the fossil fuel i.e. coal takes place in a large furnace so that the fuel can burn in a large mass flow rate. The burning process emits large quantities of ash which is collected at the disposal end. Large quantities of fans are used to harness the combustion air and feed in fresh air continually while eliminating used gases of carbon dioxide and carbon monoxide. The incoming air is first of all heated and cleaned so as to ensure a good combustion efficiency with minimal amounts of environmental pollution. After combustion has taken place, the hot combusted gases are then directed to the heat exchange zone whereby the hot air heats the warm water to steam and the used air finally discharged to the atmosphere.
The boiler has an exchange zone that enables the exchange to take place with the tubes carrying the water to be heated and the hot air running outside the tubes. Most of the times a counter flow of the water and hot combusted air is preferred at the exchange zone to increase the rate and efficiency of heat exchange. The subcooled water obtained as a result of the exchange is then led to the location where by the gases are relatively cool while the superheated steam leaves the exchange zone at the place where by the gases are hottest in order to maintain the high superheat temperature. This specific arrangement of counter flow has however some notable limitations. To begin with, the achieved intensity of the radiant heat flux that is obtained from the combustion zone is only sustained by water cooled walls alone. Moreover, if no initial cooling is done, the very high temperatures of gases which are produced in the combustion zone might be way too high the tube banks of the heat exchanger carrying steam. The partially cooled gases are then led to the super heater and the reheater tube banks located above the furnace at the very top location of the boiler structure. While relatively cool, the hot gases are then passed through the economizer located at the back of the furnace. The economizer preheats the incoming water which is sub cool. This shows that the both the combustion zone and the heat exchange zones of the boiler are integrated in nature. Also, it can be deducted from the arrangement that the whole resultant arrangement of the two zones i.e. the combustion zone and the heat exchange zone is ideal when analyzed from heat exchange point of view. Another limitation evident with the use of fossil boilers is that the materials used force heat to be substantially transferred over a large temperature difference thereby resulting in a large loss of energy or available work potential.
The prime mover/ steam turbine
After leaving the boiler, the highly superheated and high pressure steam is then fed into the fast rotating turbine in which it passes along the turbine axis through multiple rows of the alternately fixed and fast moving blades. From the main steam inlet port of the steam turbine to the main exhaust point of the very same steam turbine, the turbine blades and the turbine cavity are designed in such a way that they are progressively larger in order to allow for the efficient expansion of the steam.
Also, the stationary blades of the turbine act as nozzles in where by the steam expands and then emerges at an extremely high speed but again at a relatively lower pressure. The whole process is a verification of the Bernoulli’s principle of conservation of energy which states that the Kinetic energy of a fluid increases while its pressure energy falls. As the incoming superheated steam impacts or falls on the fast moving blades at a higher pressure, the constituent force of the heated steam then imparts some of its kinetic energy to the fast moving blades. It is important to note the two basic steam turbine types i.e. the impulse turbines and the reaction turbines that have their blades designed to control the speed, pressure and the direction of the steam as it finally passes through the turbine as shown below.
The impulse turbine consists of steam jets that are directed at the point of the turbine's bucket shaped rotor blades such that the pressure exerted by the steam jets compels the rotor to exactly rotate and also to ensure that the velocity of the steam is reduced as it imparts its inherent kinetic energy to the turbine blades. The impulse turbine blades at the same time changes the direction of flow of the imparting steam while its pressure remains exactly constant as it passes along the rotor blades due to the constant cross section of the chamber between the blades. Impulse turbines therefore in this case are also referred to as constant pressure turbines. It is important to note that the next series of fixed impulse turbine blades reverses the direction of flow of the steam just before it again passes to the second row of the moving blades. On the other hand, the reaction turbine has its rotor blades shaped more like aero foils which are arranged in such a way that the cross section of the chambers formed between the reaction turbine fixed blades diminishes from the main turbine inlet side towards the end of the exhaust side of the turbine blades. Also, the chambers located between the reaction turbine rotor blades essentially form a set of nozzles such that as the superheated steam progresses through the deeper parts of the chambers, its velocity continually increases while at the its inherent pressure decreases as observed in the nozzles that are formed by the fixed blades. Therefore, the steam pressure decreases in both the turbine fixed blades and the moving blades. Also, as the hot steam emerges in form of a jet from between the turbine rotor blades, it is observed that the steam creates a strong reactive force on the turbine blades which also create the turning moment observed on the turbine rotor, as seen in Hero's steam engine (Pilavachi, 2000). This is a fulfilment of Newton's Third Law of motion which states that, for every action there must be an equal and opposite reaction force.
The Condenser
Generator
The power from the turbine axle is directly tapped by the generator which consists of two main sections i.e. the revolving section which is called the rotor and is directly coupled to the drive shaft steam turbine and the other section being the stator which is a series of wire coils that form a cylinder around the generator’s rotor. The rotor, that is actually an electro-magnet, rotates at an extremely high speed in order to generate electricity which in this case is the alternating current in the stator. Another completely separate static excite also energizes the wire coils of the rotor. The electricity generated by the generator is then transmitted to various destinations (Kehlhofer ET AL., 2009).
Transmission
Normally, electricity is produced in the recent generators at 23,000 volts. The produced electricity is then passed through a certain transformer which increases the production voltage to a higher voltage, as high as 500,000 volts. The stepped up voltage then passes into the adjacent electric power station switchyard where the transmission lines readily carry it to where it’s needed using a large network of interconnected and very high voltage transmission grid.
The impact on the environment
The steam turbine power generation system is also known to have some notable impacts on the environment. To begin with, the extensive filter system that is present between the boiler and the neighboring emission stack normally extracts nearly all of the ash particles obtained from the exhaust gases that are generated by the boiler. This filter system actually consists of a maximum of about 48,000 fabric bags whose total surface area is about 120 hectares. The system is almost one hundred percent efficient and again reduces emissions to the neighboring atmosphere to a barely visible level. Also, the smoke that is seen coming out of the power stations is actually steam and not pollution. This can be proved because the steam evaporates from the extremity instead of forming an extended plume across the sky.
Conclusion
Electricity plays a central role in the operation of all machines. The application of this form of energy is widely spread and cuts across all the spheres of life. It is therefore important the production process of this form of energy be extensively and continually analysed so as to ensure a constant supply of this form of energy. Steam turbine power generation systems have been deployed to produce electricity as explained from the above analysis. The efficiency of the electric energy produced by this system is at times average and needs to be closely monitored so as to ensure high voltage or high energy production schemes. Amongst the areas to take keen note on is the energy losses experienced as the steam leaves the boiler to the turbines. If close attention could be given to this critical points of production, then the efficiency of the final energy produced could be extremely high.
References
Grant, N. A. (1981). U.S. Patent No. 4,277,416. Washington, DC: U.S. Patent and Trademark Office.
Heppenstall, T. (1998). Advanced gas turbine cycles for power generation: a critical review. Applied Thermal Engineering, 18(9), 837-846.
Ishida, M., Zheng, D., & Akehata, T. (1987). Evaluation of a chemical-looping-combustion power-generation system by graphic exergy analysis. Energy, 12(2), 147-154.
Pilavachi, P. A. (2000). Power generation with gas turbine systems and combined heat and power. Applied Thermal Engineering, 20(15), 1421-1429.
Kehlhofer, R., Hannemann, F., Rukes, B., & Stirnimann, F. (2009). Combined-cycle gas & steam turbine power plants. Pennwell Books.
