Steam Temperature Control - Term Paper Example

Name: Instructor: Course: Date: Steam Temperature Control Abstract Tightening up discharge restrictions to the atmosphere is ceaselessly bringing in new disputes to a steam power plant design and evolution. Researchers and Engineers are gaining more interest in studying power plants as a result of increased need for power in the highly industrialized society we are living in. This paper is a comprehensive literature review on steam temperature control. Introduction A steam power station also known as thermal power station uses heat energy brought forth from burning coal to generate electrical energy. This type of power station is the most common type of power generating technique used around the world. Some power stations burn fossil fuels such as coal, oil and gas. The fuel is burned in a giant furnace releasing a lot of heat energy which in turn generates electricity (Alexander 20). The heat produced is thereafter used to heat water into steam at high temperatures and pressure. This pressured steam is used to turn the turbine connected to an electricity generator. This turbine is invented to convert the steam's energy into kinetic energy and for this turbine to work effectively; the steam must enter it at very high temperature and pressure and leave at very low temperatures (Moore 33). This is to ensure maximum extraction of energy present in the steam. Extracting this energy from these fuels takes place in stages and in the process, some amount of energy is lost at every stage. These stages are: Stage 1: Coal & Ash handling Stage 2: steam generating plant Stage 3: steam turbine and alternator/ generator Stage 5: feed water and cooling Basically, this power generating station uses the Rankine cycle to produce steam in the boiler. A boiler has a furnace which burns the fuel to generate heat. Due to supercritical pressure, the boiler is fitted with the following apparatus: Safety valve pressure measurement, the main steam Stop valve , Feed check valves, Water gauge, Low-water alarm, Low water fuel Cut-out, Inspector's Test Pressure Gauge Attachment and a feed water pump. These are very vital fittings because they prevent over pressurization of the boiler and indicate insufficient water in the boiler which may cause overheating and boiler failure. The steam is then taken to the steam turbine and from the turbine; the steam is cooled back to water in the condenser. The leaving water is channeled back into the boiler to perform the cycle again. Due to abundance of fuel (coal), this kind of power station can be used to generate large amounts of electrical energy. The station must be located close to coal mines to reduce transportation cost of fuel and near a river bank or canal for continuous water supply. Cheap coal used reduces the cost of generation. However, the plant must be well constructed to reduce air pollution from smoke fumes (Russel 20). Literature Review Modern steam turbines needs cyclic operations especially when setting up the system to control the temperatures of the steam chest in steam turbines which is set to reduce the stresses that are caused by thermal changes in the system. To control these thermal stresses, steam admission is allowed to balance the steam turbine temperature with the temperature of the steam that is being admitted. The steam turbine walls should be free from expansion and contraction when subjugated to high temperatures and should have enough thickness to withstand pressures that are exerted by the highly pressurized steam. It is generally a vital procedure and so one should undertake precautionary measures when controlling the temperatures by lowering the temperatures to point below the saturation temperature by use of de-superheaters (Lindsly & IEE 136). The temperature can also be lowered to slightly above or greater than the saturation point by the use of attaemperator. Though lowering temperature of the system is important, it affects the efficiency and the working principles of the steam turbine but not much. The cooling of the generator turbine ranges between 75% and 100% and so it does not make the system behave abnormally (Lindsley & IEE 137). The system has extraction valves that control the intake of the steam. It also has throttle valve which links the steam route flow between the source of the steam and the steam chest which is responsible for governing the steam course of steam over a predetermined ran, in a certain level of steam flow rates. Testing the temperatures is done with a sensor which is coupled at the steam chest for supplying indications of the temperature changes in the steam chest of the turbine system. Also, a gate which is leak proof is connected via the steam chest and includes: a valve for controlling the continuous flow of the steam from the steam chest to the leak proof gate attached to the system. There is a need to couple a restrainer to the system which necessitates good combination between the throttle valve and the flow control valve set for restraining the inflow and outflow of the steam from the steam chest. Lastly, the control system is connected to accommodate the signals transmitted by the temperature change sensors helping in the control of the turbine steam chest. This invention relates very well to a combined heat recovery steam generator and steam turbine (Bloch 18). An adaptive temperature controller establishes a target temperature for controlling the boiler temperature so that the steam admitted on the boiler follows a constant enthalpy when trying to match the temperature of steam and rotor. Discussion Generally, most power plants use water boilers to provide steam used in driving turbines and this makes steam turbine generators the major producers of world’s electricity. A steam boiler can be regarded as an enclosed vessel in which water and/or other fluids are heated to produce steam which in turn is used to drive turbines in power plants (Kehlhofer 13). An ideal steam turbine operation is usually regarded as an isentropic process or as a constant entropy process. Here, the entropy is usually the measure of energy of steam that is coming into the turbine. Basically, there cannot be a steam turbine which is purely isentropic that is, the system cannot be naturally stable without increase or decrease of entropy. There are isentropic efficiencies between the ranges from 20%-90% which are considered during the applications of steam turbine. The internal part of the steam turbine consists of two major categories of blades which include: the stationary blades which are connected to the turbine casing and the rotating blade which is connected to the turbine shaft. These two categories join each other with given least allowances which depend on the sizing and configuring of the two categories Steam boilers can be classified into the following: Pot boilers- these are used to store water and produce steam at very low pressure, which cannot be used to drive the turbines. Fire tube boilers- this is used in most steam automobiles to drive engines to enhance their locomotive nature. They are heated from fire source provided from a furnace which is designed in such a manner that a kind of a fire tunnel is surrounded by the water which is to be heated and this in turn ensures that the temperatures of the heating surface are maintained below the boiling point of water. These types of boilers are mostly used for heating solid fuels. Water tube turbines produce high steam pressure which can be used to drive turbines but they have a disadvantage of low volume storage of water. Superheated steam boilers- in these boilers, water is first vaporized and then superheated in the super-heater present in the system which provides steam at greater temperatures. The super heat produced has a possibility of lowering the general thermal efficiency of the system since at high temperatures, the steam temperature also needs an elevated flue gas exhaust Super-critical steam boilers- these are boilers which operate at very high pressure ranged over 3,200Psi. At such great pressures, normal boiling stops taking place. These great pressures authorize steam driving pressures to the pressure turbine and then enter the generator’s condenser making it to be more efficient which in turn responds to minimized use of fuel for the steam production. Turbo-turbines, also referred to as turbo generators are a type of turbines that are steam driven and hence power produced by these turbines is proportional to the rate at which steam is generated (Moore 14). The turbines are directly connected to an electric generator which generates the electric power. This efficiency enhancement is also developed during the exhaust of gases which in turn are used to generate steam in combined cycle. The operation of turbo-turbine requires constant cooling since it is driven at a very high speed, thus hydrogen cooling is the best based on its classification as a coolant. Use of hydrogen gas as the main coolant increases the efficiency of the system by almost 99% due to the important physical properties of hydrogen which include: high specific heat, high thermal conductivity and low density of the hydrogen gas. Also, this gas is commonly found and can be manufactured easily using electric current. The boilers used in this case that is in the steam generation for power generation purposes mostly depend on mechanical drawing instead of natural drawing. Mechanical drawing is more preferred because it is economical. There is a need to ensure smooth running of the turbine and therefore it is important to ensure that the control of the turbine is enhanced by the use of the governor to control the movement of the turbine. It is necessary to ensure that the turbines are run slowly in order to prevent damages in some applications like current and voltage generation. Due to the speed of the turbine, the rotor can end up at a higher speed slip and this may lead to clogging of the valves of the nozzle that control the steam. Failure to this, the turbine might continue to speed up till it dismantles instantly without a warning to the operator. Most of the times, turbines are costly to make because of the manufacturing process and the attributes of the materials that are used. In synchronization with the electricity process in the normal operation with electric network, the power plants are governed with a five percent (5%) sag speed controlling motors. When the machine is operating at this sag speed control, the full load speed is 100% and the no-load speed of the generator is 105%. At normal operations, the speed changes are minor. The changes in the power output are built up slowly and thus increasing sag curve by changing the spring pressure on a centrifugal governor in the turbine (US Patents). Generally, this becomes a must for all power plants because the older and newer plants have to be compatible in response and to enhance proper changes in their frequency without changing the external connections. The steam power plant needs to be designed in such a manner to work in levels of considerations. There are generally five (5) stages involved in the steam power generation namely: Stage1: fuel /coal and ash treatment This stage acts as the entrance and exit of the system processes. In this stage, the fuel is administered in and ash is released from the system. Handling/treatment of the fuel is important before anything is done to ensure that there is constant flow in the system. Stage 2: steam generation Most of the energy that is generated by the steam is lost at this stage. The steam energy is used to release heat energy for the plant. Stage 3 and 4: steam turbine and the generator/alternator Steam energy that is developed is converted to rotational energy used for rotating the turbines thus this becomes the energy conversion stage. Stage 5: feeder water and process cooling At this stage, the used water is condensed and taken back to the boiler system for the process to repeat and thus the cooled water is re-used in the system and so this stage is recycling stage of the steam turbine system. Conclusion/Recommendation Low emission levels can be achieved by the use of well designed steam power plants and involvement of well trained staff. This process includes characteristic features such as in-furnace sulphur capture by simple injection of crushed limestone. This is necessary because sulphur oxides are very poisonous and when inhaled can cause death. There are also low emissions of NOx because of presence of low furnace temperature, staged combustion and fuel firing flexibility (Gene 6). To obtain the highest cycle ratio, the power stations should be maintained running at a range very close to the maximum efficiency round the clock unless the plant is being maintained and this will ensure effective operation. These plants should be designed to last between 30 to 35 years. To increase turbine efficiency, steam should be expanded to generate energy in a number of stages. Each of these stages is characterized by energy extraction mechanisms which can be referred as either impulse turbine or reaction turbine. Basically, most steam turbines use a variety of the reaction and impulse designs. Each stage behaves as either one or the other but the overall turbine uses both. Works Cited Lindsley, David & Institution of Electrical Engineers. Power-plant control and instrumentation: the control of boilers and HRSG systems. London: Institution of Electrical Engineering, 2000. Print. Bloch, Heinz. A practical guide to steam turbine technology. New York: McGraw-Hill, 1996. Print. US Patents. “Temperature control of a steam turbine steam to minimize thermal stresses”. US patents, 1991. Web. 18 March. 2011. Kehlhofer, Rolf. et al. Combined-cycle gas & steam turbine power plants. Oklahoma: Penwell Corporation, 2009. Print. Alexander, Leyzerovich. Wet-steam turbines for nuclear power plants. Oklahoma: Penwell Corporation, 2005. Print. Russel, James. Steam & Diesel Power Plant Operators Examinations. Livingston: James Russell Publishing, 2000. Print. Moore. Micro-turbine generators. New York: John Wiley, 2002. Print. Gene, Knight. Selective Catalytic Reduction Technology for the Control of Nitrogen Oxide Emissions from Coal-Fired Boilers. Pennsylvania: Diane Publishing, 2008. Print. Read More