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A power station operates using a closed steam power cycle, where water undergoes various thermodynamic processes in a cyclic process.One half of the cycle consists of the boiler (or heat source) and its auxiliaries; the other, the thermodynamic cycle, consists of turbine, generator, condenser. feed pump and feed-water heaters.Feed-water is supplied to the boiler, where water is boiled and converted into dry saturated steam. This dry steam is further super-heated and then fed to the 111 cylinder of theturbine. The steam expands in the turbine giving up heat energy, a high proportion of which is transferred into work energy on the turbine shaft. The shaft turns an electrical generator which produces electric power. Steam leaving the HP cylinder returns to the boiler, where it is reheated. The reheated steam is further expanded in the IP and LP cylinders, before passing into the condenser.In the condenser, which is a large surface-type heat exchanger. the steam is condensed by transferring its latent heat of vaporization to the cooling water (CW). The main steam, having been condensed in the condenser. is now in a liquid state at a very low pressure and approximately saturation temperature. This water drains from the condenser, where it enters the hot-well. The water in the hot-well is pumped by the condensate extraction pump through the low pressure feed-heating system to another pump, the boiler feed pump.In a modern regenerative cycle, some of the steam passing through the turbine cylinders is bled from a series of extraction belts located after selected moving blade stages and fed to the condensate and feed-water heaters. This steam is used to heat the condensate in the LP heaters and the feed water in the HP heater.m which are of a surface type.The boiler feed pump increases the water pressure to a level in excess of the drum pres sure， to provide for the pressure loss in the boiler circuit and HP feed-heating train. The cycle is now complete.Practical cycle using superheatThe first development of the Rankine cycle into a more practical steam cycle involves raising the pressure and temperature of the steam entering the turbine.In the superheat cycle, the saturated dry steam leaving the boiler drum is further heated before entering the turbine. Hence. there is an improvement in the cycle efficiency. This superheat cycle is chosen to have the same turbine exhaust conditions as the previous Rankine cycle. However, a major advantage of superheating steam is that for increasing cycle temperature and pressure, the exhaust wetness in the turbine can be maintained within the physicallimits.4. 1. 2 Reheat cycleThe desire for further increases in cycle conditions and consequent increases in cycle efficiency. led to the addition of steam reheat during turbine expansion. In the reheat cycle, steam at a given initial temperature is partially expanded through the turbine doing some work, and then is fed back to the reheater, where it is reheated to about original temperature. The heated steam is then fed through the remainder of the turbine before being condensedThe reheat cycle incorporates an improvement in thermal efficiency over the superheat cycle. The reheat cycle benefits from reduced wetness in the turbine exhaust, but presents an increased capital outlay in terms of reheater pipe-work to, from and within the boiler. The turbine is usually split into HP and LI cylinders to avoid the high thermal gradients which would be introduced between stages of reheat on a single-cylinder machine.4. 1.3 Regenerative feedheatingTo complete the cycle development of the steam cycle, the inclusion of regenerative feedheating must be discussed. Physically, a proportion of the steam is bled from various points of the turbine. which is then condensed to heat feed-water on its return to the boiler. The improvement in thermal efficiency for a simple Rankine cycle is by virtue of the bled- steam releasing all of its heat to the feed water, and little or none to the condenser. There will be a small loss of work available front the bled steam not expanding in the turbines how ever, this loss is out weighted by the gain in cycle efficiency.The greater the number of feed heaters installed, the greater the improvement in thermal efficiency. However. the incremental gain for each additional tied heater reduces with the number of heaters increases.4. 1. 4 Supercritical plantOne effective way of achieving increased thermal efficiency is to increase steam pressure. The limits of a natural circulation boiler are around 2608. 2psi (18MPa). and although assisted circulation may be used at higher pressures, an overall improvement in station efficiency is not achieved unless the pressure is advanced to about 3477. 6psi (24MPa), i, e. above the critical pressure of water/steam 3205. 2psi (22.12 Mla). Although the use of supercritical pressure requires special,consideration in the design of the boiler, the implications for the turbine only concern the higher pressure.A further improvement tray he obtained by increasing steam temperature. Most of the supercritical plant in service worldwide operates at 1000. 4 F (538C) , although some 1049 F (565t) plant exists. and there are some pioneering units with temperatures as high as 1166 F (630V). At the higher temperatures, the efficiency is often boosted still further by using double reheat. Apart from the efficiency benefit. this has the merit of reducing the turbine exhaust wetness from the high level that such advanced initial conditions would otherwise entail.The so-called ultra-supercritical plants of 350-1000 MW with steam conditions such as 4491. 9psi (31MPa) 1094 (590C), and later up to 5071. 5psi (35MIa) 1166 F (630C). all with double-reheat cycles are or will be in service.The use of the double-reheat cycle introduces additional complexity. First, additional boiler controls are required for steam temperature, and secondly the turbine must either have an extra cylinder or it must use a combined cylinder for the first two expansions. The extra cylinder increases machine length and cost. while the combined cylinder may give the possibility of problems due to sealing between the two expansions or the close proximity of sections at hot and cold reheat temperatures.None of these developments present technical problems, given sufficient time and resources. Their application in practice depends on potential customers being satisfied that the potential return in improved efficiency is not accompanied by additional risk either to plant life, operational flexibility, or availability. To this end. the development programs embody the full range of research, design. rig testing, and prototype component testing, which. coupled with the first full-size prototype unit, will give the necessary assurance.The rate at which such plant will be introduced is however uncertain, depending as it does on factors such as electricity demand, fuel costs, the economic environment, the extent of alternative energy sources, and the refurbishment of existing plant for extended life.4. 2 Modern Steam Power PlantMost applications of boilers involve the production of electricity or the supply of process steam. In some cases,a combination of the two applications, called cogeneration, is used. In each application, the boiler is a major part of a larger system that has many subsystems and components. Key subsystems include fuel receiving and preparation, steam generator and combustion, environmental protection, turbine generator, and heat rejection including cooling tower.Fig. 4 - 1 shows an advanced. pulverized coal (PC) unit that meets todays low, permitted emissions levels. The three main components of a PC unit are: (1) the boiler block where coal is burned to generate steam in the boiler tubes; (2) the generator block, which contains the steam turbine/electric generator set and manages the steam, condenser, and cooling water; and (3) the flue gas clean-up train, which removes particulates and criteria pollutants from the flue gas. The flue gas clean-up section contains Selective Catalytic Re- duction(SCR) for NO, removal, followed by electrostatic precipitation(ESP) to remove particulate matter, and wet flue gas desulfurization(FGD) to remove SO:. The choice of coal, and the design and operation of the flue gas units is to assure that emissions are below the permitted levels.The fuel handling system stores the fuel supply (coal in this example), prepares the fuel for combustion and transports it to the boiler. The associated air system supplies air to theburners through a forced draft fan. The steam generator subsystem, which includes the air heater, burns the fuel-air mixture, recovers the heat, and generates the controlled high pressure and high temperature steam. The flue gas leaves the air heater and passes through particulate collection and sulfur dioxide (S()L) scrubbing systems where pollutants are collected and the ash and solid scrubber residue are removed. The remaining flue gas is then sent to the stack through an induced draft fan.The boiler evaporates water and supplies high temperature, high pressure steam under carefully controlled condition. The steam is passed to a turbine-generator set that produces the electricity. The steam may also be reheated in the boiler, after passing through part of a multi-stage turbine system, by running the exhaust steam back to the boiler convection pass (rehcater not shown). Ultimately, the steam is passed from the turbine to the condenser where the remaining waste heat is rejected. Before the water from the condenser is returned to the boiler it passes through several pumps and heat exchangers (feedwater heaters) to increase its pressure and temperature. The heat absorbed by the condenser is eventually rejected to the atmosphere by one or more cooling towers. These cooling towers are perhaps the most visible component in the power system. The natural draft cooling tower shown is basically a hollow cylindrical structure which circulates air and moisture to absorb the heat rejec ted by the condenser. Such cooling towers exist at most modern power plant sites.
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