ME6701 – POWER PLANT ENGINEERING

INTERNAL ASSESMENT EXAMINATIONS – I

PART-A

1. The alternator is used in a power plant which converts...... (a) Electrical (b) Electrical (c) Mechanical (d) Mechanical Energy into energy into Energy into Energy into Mechanical Solar Energy Electrical Nuclear Energy Energy Energy 2. For forced draught system, the function of chimney is mainly...... (a) To produce (b) To discharge (c) To reduce the (d) None of the draught to gases high up temperature of above accelerate the in the the hot gases combustion of atmosphere to discharged fuel avoid hazard

3. Pulverized fuel is used for...... (a) Better (b) Saving fuel (c) Obtaining fuel (d) For economy burning

4. Ideal ‘Rankine Cycle’ is a ______process. (a) Reversible (b) Irreversible (c) Both of the (d) None of the mentioned mentioned

5. What is the most preferable dryness fraction of the exhaust steam? (a) 0.99 (b) 0.77 (c) 0.66 (d) 0.88

6. The vaccum obtainable in a condenser is dependent upon...... (a) Capacity of (b) Quantity of (c) Any of the two (d) Temperature ejector steam to be is possible of cooling handled water

7. Fluidized bed combustion helps to reduce (a) boiler size (b) pollution (c) both (A) and (d) None of the (B) above 8. The Otto cycle consists of (a) two constant (b) two constant (c) two constant (d) none of the pressure pressure and volume mentioned processes and two constant processes and two constant entropy two constant volume processes entropy processes processes

9 Which of the following is true for the Brayton cycle? (a) first sir is (b) heat is added (c) air expands in (d) all of the compressed reversibly at turbine mentioned reversibly and constant reversibly and adiabatically pressure adiabatically

10. The diesel plants are mainly used ______(a) As peak load (b) As base load (c) As standby (d) Both peak and plants plants power plants stand by plants

PART-B

Define steam and heat rate.  Steam is water in the gas phase, which is formed when water boils or evaporates. Steam is invisible; however, "steam" often refers to wet steam, the visible mist or aerosol of water droplets formed as this water vapour condenses. If heated further it becomes superheated steam. 11.  Heat rate is the common measure of system efficiency in a steam power plant. It is defined as "the energy input to a system, typically in Btu/kWh, divided by the electricity generated, in kW." Mathematically: Efficiency is "a ratio of the useful energy output by the system to the energy input to the system."

Why thermal power plants are not suitable for fluctuating loads?  All power plants have some measure of response time. Plants with boilers or nuclear 12. reactors might be more like hours, because the boiler has to take more fuel in, then heat more water into steam and that mostly takes time.

Give the example for once through boiler  Once-through boilers are generally associated with high pressure operation and the feed 13 water enters at high sub-critical >180 bar or supercritical pressure whilst superheated steam leaves at a pressure some 20–30 bar lower.

Differentiate stocker firing and pulverized fuel firing. 14  Pulverized Fuel Firing Due to pulverization, the surface much area of coal becomes larger, and in this method air required for combustion is much less. As the quantity of required air and fuel both are less, loss of heat in this method of boiler firing is much less.  A mechanical stocker is a mechanical system that feeds solid fuel like coal, coke or anthracite into the furnace of a steam boiler. They are common on steam locomotives after 1900 and are also used on ships and power stations. Characterize compounding of steam turbines.  steam's pressure drop and it will increase its velocity. This high-velocity strikes the turbines rotor and the speed of the rotor becomes high. Compounding of steam 15 turbine is used to reduce the rotor speed. It is the process by which rotor speed come to its desired value

Outline the quality of steam.  A steam quality of 0 indicates 100 % liquid, (condensate) while a steam quality of 100 16 indicates 100 % steam. One (1) lb of steam with 95 % steam and 5 % percent of liquid entrainment has a steam quality of 0.95.

Define boiler mountings and accessories.  Boiler mounting are the components generally mounted on the surface of he boiler to have safety during operation Safety valve: it is a mechanical device used to safeguard 17 the boiler, in case the pressure inside the boiler rises above its normal working atmosphere

List any four applications of diesel power plant.  Diesel power plant is used for electrical power generation in capacities ranging from 100 to 5000 H.P. 18  They are commonly used for mobile power generation and are widely used in transportation systems consisting of railroads, ships, automobiles, and airplanes.  They can be used as standby power plants.

Define cut-off ratio and expansion ratio.  The cutoff ratio is the ratio of the volume after combustion to the volume before 19 combustion. The compression ratio is the ratio of the maximum volume to the minimum volume. Efficiency goes up with compression ratio and down with cutoff ratio.

Mention the major differences between Otto and Diesel cycle.  Otto cycle is used for petrol or spark ignition engine while diesel cycle is used for diesel or compression ignition engine. The main difference between Otto 20 cycle and Diesel cycle is that in Otto cycle heat addition takes place at constant volume and in diesel cycle heat addition takes places at constant pressure. PART-C

21. Draw a Rankine cycle for a coal fired and steam thermal power plant. State the various means of a increasing the efficiency of the plant.

The Rankine cycle closely describes the process by which steam-operated heat engines commonly found in thermal power generation plants generate power. Power depends on the temperature difference between a heat source and a cold source. The higher the difference, the more mechanical power can be efficiently extracted out of heat energy, as per Carnot's theorem. The heat sources used in these power plants are usually nuclear fission or the combustion of fossil fuels such as coal, natural gas, and oil, or concentrated solar power. The higher the temperature, the better. The efficiency of the Rankine cycle is limited by the high heat of vaporization of the working fluid. Also, unless the pressure and temperature reach super critical levels in the steam boiler, the temperature range the cycle can operate over is quite small: steam turbine entry temperatures are typically around 565 °C and steam condenser temperatures are around 30 °C. This gives a theoretical maximum Carnot efficiency for the steam turbine alone of about 63.8% compared with an actual overall thermal efficiency of up to 42% for a modern coal-fired power station. This low steam turbine entry temperature (compared to a gas turbine) is why the Rankine (steam) cycle is often used as a bottoming cycle to recover otherwise rejected heat in combined-cycle gas turbine power stations. The cold source (the colder the better) used in these power plants are usually cooling towers and a large water body (river or sea). The efficiency of the Rankine cycle is limited on the cold side by the lower practical temperature of the working fluid. The working fluid in a Rankine cycle follows a closed loop and is reused constantly. The water vapor with condensed droplets often seen billowing from power stations is created by the cooling systems (not directly from the closed-loop Rankine power cycle). This 'exhaust' heat is represented by the "Qout" flowing out of the lower side of the cycle shown in the T–s diagram below. Cooling towers operate as large heat exchangers by absorbing the latent heat of vaporization of the working fluid and simultaneously evaporating cooling water to the atmosphere. While many substances could be used as the working fluid in the Rankine cycle, water is usually the fluid of choice due to its favorable properties, such as its non-toxic and unreactive chemistry, abundance, and low cost, as well as its thermodynamic properties. By condensing the working steam vapor to a liquid the pressure at the turbine outlet is lowered and the energy required by the feed pump consumes only 1% to 3% of the turbine output power and these factors contribute to a higher efficiency for the cycle. The benefit of this is offset by the low temperatures of steam admitted to the turbine(s). Gas turbines, for instance, have turbine entry temperatures approaching 1500 °C. However, the thermal efficiency of actual large steam power stations and large modern gas turbine stations are similar.

The four processes in the Rankine cycle T–s diagram of a typical Rankine cycle operating between pressures of 0.06 bar and 50 bar. Left from the bell-shaped curve is liquid, right from it is gas, and under it is saturated liquid–vapour equilibrium. There are four processes in the Rankine cycle. The states are identified by numbers (in brown) in the T–s diagram.

 Process 1–2: The working fluid is pumped from low to high pressure. As the fluid is a liquid at this stage, the pump requires little input energy. In other words Process 1-2 is [Isentropic compression in pump]

 Process 2–3: The high-pressure liquid enters a boiler, where it is heated at constant pressure by an external heat source to become a dry saturated vapour. The input energy required can be easily calculated graphically, using an enthalpy–entropy chart (h–s chart, or Mollier diagram), or numerically, using steam tables. In other words Process 2-3 is [Constant pressure heat addition in boiler]

 Process 3–4: The dry saturated vapour expands through a turbine, generating power. This decreases the temperature and pressure of the vapour, and some condensation may occur. The output in this process can be easily calculated using the chart or tables noted above. In other words Process 3-4 is [Isentropic expansion in turbine]

 Process 4–1: The wet vapour then enters a condenser, where it is condensed at a constant pressure to become a saturated liquid. In other words Process 4-1 is [Constant pressure heat rejection in condenser]

In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine would generate no entropy and hence maximize the net work output. Processes 1–2 and 3–4 would be represented by vertical lines on the T–s diagram and more closely resemble that of the Carnot cycle. The Rankine cycle shown here prevents the state of the working fluid from ending up in the superheated vapor region after the expansion in the turbine, [1] which reduces the energy removed by the condensers. The actual vapor power cycle differs from the ideal Rankine cycle because of irreversibilities in the inherent components caused by fluid friction and heat loss to the surroundings; fluid friction causes pressure drops in the boiler, the condenser, and the piping between the components, and as a result the steam leaves the boiler at a lower pressure; heat loss reduces the net work output, thus heat addition to the steam in the boiler is required to maintain the same level of net work output.

Variables

Heat flow rate to or from the system (energy per unit time)

Mass flow rate (mass per unit time)

Mechanical power consumed by or provided to the system (energy per unit time)

Thermodynamic efficiency of the process (net power output per heat input, dimensionless) Isentropic efficiency of the compression (feed pump) and expansion (turbine) processes, dimensionless The "specific enthalpies" at indicated points on the T–s diagram

The final "specific enthalpy" of the fluid if the turbine were isentropic

The pressures before and after the compression process

Equations In general, the efficiency of a simple rankine cycle can be written as Each of the next four equations is derived from the energy and mass balance for a control volume. defines the thermodynamic efficiency of the cycle as the ratio of net power output to heat input. As the work required by the pump is often around 1% of the turbine work output, it can be simplified. When dealing with the efficiencies of the turbines and pumps, an adjustment to the work terms must be made:

Real Rankine cycle (non-ideal)

Rankine cycle with superheat In a real power-plant cycle (the name "Rankine" cycle is used only for the ideal cycle), the compression by the pump and the expansion in the turbine are not isentropic. In other words, these processes are non-reversible, and entropy is increased during the two processes. This somewhat increases the power required by the pump and decreases the power generated by the turbine. In particular, the efficiency of the steam turbine will be limited by water-droplet formation. As the water condenses, water droplets hit the turbine blades at high speed, causing pitting and erosion, gradually decreasing the life of turbine blades and efficiency of the turbine. The easiest way to overcome this problem is by superheating the steam. On the T–s diagram above, state 3 is at a border of the two-phase region of steam and water, so after expansion the steam will be very wet. By superheating, state 3 will move to the right (and up) in the diagram and hence produce a drier steam after expansion.

Variations of the basic Rankine cycle The overall thermodynamic efficiency can be increased by raising the average heat input temperature of that cycle. Increasing the temperature of the steam into the superheat region is a simple way of doing this. There are also variations of the basic Rankine cycle designed to raise the thermal efficiency of the cycle in this way; two of these are described below.Rankine cycle with reheat Rankine cycle with reheat The purpose of a reheating cycle is to remove the moisture carried by the steam at the final stages of the expansion process. In this variation, two turbines work in series. The first accepts vapor from the boiler at high pressure. After the vapor has passed through the first turbine, it re-enters the boiler and is reheated before passing through a second, lower-pressure, turbine. The reheat temperatures are very close or equal to the inlet temperatures, whereas the optimal reheat pressure needed is only one fourth of the original boiler pressure. Among other advantages, this prevents the vapor from condensing during its expansion and thereby reducing the damage in the turbine blades, and improves the efficiency of the cycle, because more of the heat flow into the cycle occurs at higher temperature. The reheat cycle was first introduced in the 1920s, but was not operational for long due to technical difficulties. In the 1940s, it was reintroduced with the increasing manufacture of high-pressure boilers, and eventually double reheating was introduced in the 1950s. The idea behind double reheating is to increase the average temperature. It was observed that more than two stages of reheating are generally unnecessary, since the next stage increases the cycle efficiency only half as much as the preceding stage. Today, double reheating is commonly used in power plants that operate under supercritical pressure. Regenerative Rankine cycle

Regenerative Rankine cycle The regenerative Rankine cycle is so named because after emerging from the condenser (possibly as a sub cooled liquid) the working fluid is heated by steam tapped from the hot portion of the cycle. On the diagram shown, the fluid at 2 is mixed with the fluid at 4 (both at the same pressure) to end up with the saturated liquid at 7. This is called "direct- contact heating". The Regenerative Rankine cycle (with minor variants) is commonly used in real power stations. Another variation sends bleed steam from between turbine stages to feed water heaters to preheat the water on its way from the condenser to the boiler. These heaters do not mix the input steam and condensate, function as an ordinary tubular heat exchanger, and are named "closed feed water heaters". Regeneration increases the cycle heat input temperature by eliminating the addition of heat from the boiler/fuel source at the relatively low feed water temperatures that would exist without regenerative feed water heating. This improves the efficiency of the cycle, as more of the heat flow into the cycle occurs at higher temperature.

12 Draw the general layout of thermal power plant and explain the working of different circuits.

and then properly disposed. Periodic removal of ash from the boiler furnace is necessary for the proper combustion.

Boiler: The mixture of pulverized coal and air (usually preheated air) is taken into boiler and then burnt in the combustion zone. On ignition of fuel a large fireball is formed at the center of the boiler and large amount of heat energy is radiated from it. The heat energy is utilized to convert the water into steam at high temperature and pressure. Steel tubes run along the boiler walls in which water is converted in steam. The flue gases from the boiler make their way through superheater, economizer, air preheater and finally get exhausted to the atmosphere from the chimney. Superheater: The superheater tubes are hanged at the hottest part of the Almost two third of electricity requirement of the world is fulfilled by thermal power plants (or thermal power stations). In these power stations, steam is produced by burning some fossil fuel (e.g. coal) and then used to run a steam turbine. Thus, a thermal power station may sometimes called as a Steam Power Station. After the steam passes through the steam turbine, it is condensed in a condenser and again fed back into the boiler to become steam. This is known as ranking cycle. This article explains how electricity is generated in thermal power plants. As majority of thermal power plants use coal as their primary fuel, this article is focused on a coal fired thermal power plant. Typical Layout And Working Of A Thermal Power Plant A simplified layout of a thermal power station is shown below.

. Coal: In a coal based thermal power plant, coal is transported from coal mines to the generating station. Generally, bituminous coal or brown coal is used as fuel. The coal is stored in either 'dead storage' or in 'live storage'. Dead storage is generally 40 days backup coal storage which is used when coal supply is unavailable. Live storage is a raw coal bunker in boiler house. The coal is cleaned in a magnetic cleaner to filter out if any iron particles are present which may cause wear and tear in the equipment. The coal from live storage is first crushed in small particles and then taken into pulverizer to make it in powdered form. Fine powdered coal undergoes complete combustion, and thus pulverized coal improves efficiency of the boiler. The ash produced after the combustion of coal is taken out of the boiler furnace boiler. The saturated steam produced in the boiler tubes is superheated to about 540 °C in the superheater. The superheated high pressure steam is then fed to the steam turbine. . Economizer: An economizer is essentially a feed water heater which heats the water before supplying to the boiler. . Air pre-heater: The primary air fan takes air from the atmosphere and it is then warmed in the air pre-heater. Pre-heated air is injected with coal in the boiler. The advantage of pre- heating the air is that it improves the coal combustion. Steam turbine: High pressure super heated steam is fed to the steam turbine which causes turbine blades to rotate. Energy in the steam is converted into mechanical energy in the steam turbine which acts as the prime mover. The pressure and temperature of the steam falls to a lower value and it expands in volume as it passes through the turbine. The expanded low pressure steam is exhausted in the condenser.

Condenser: The exhausted steam is condensed in the condenser by means of cold water circulation. Here, the steam loses it's pressure as well as temperature and it is converted back into water. Condensing is essential because, compressing a fluid which is in gaseous state requires a huge amount of energy with respect to the energy required in compressing liquid. Thus, condensing increases efficiency of the cycle.

Alternator: The steam turbine is coupled to an alternator. When the turbine rotates the alternator, electrical energy is generated. This generated electrical voltage is then stepped up with the help of a transformer and then transmitted where it is to be utilized.

Feed water pump: The condensed water is again fed to the boiler by a feed water pump. Some water may be lost during the cycle, which is suitably supplied from an external water source.

This was the basic working principle of a thermal power station and its typical components. A practical thermal plant possess more complicated design and multiple stages of turbine such as High Pressure Turbine (HPT), Intermediate Pressure Turbine (IPT) and Low Pressure Turbine (LPT). Advantages And Disadvantages Of A Thermal Power Plant Advantages: . Less initial cost as compared to other generating stations. . It requires less land as compared to hydro power plant. . The fuel (i.e. coal) is cheaper. . The cost of generation is lesser than that of diesel power plants. Disadvantages: . It pollutes the atmosphere due to the production of large amount of smoke. This is one of the causes of global warming. . The overall efficiency of a thermal power station is low (less than 30%). 13 Explain in detail about different types of Fluidized Bed Combustion.

Fluidized bed combustion (FBC) is a combustion technology used to burn solid fuels. In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone etc.) through which jets of air are blown to provide the oxygen required for combustion or gasification. The resultant fast and intimate mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed. FBC plants are capable of burning a variety of low-grade solid fuels, including most types of coal and woody biomass, at high efficiency and without the necessity for expensive fuel preparation (e.g., pulverising). In addition, for any given thermal duty, FBCs are smaller than the equivalent conventional furnace, so may offer significant advantages over the latter in terms of cost and flexibility.

FBC reduces the amount of sulfur emitted in the form of SOx emissions. Limestone is used to precipitate out sulfate during combustion, which also allows more efficient heat transfer from the boiler to the apparatus used to capture the heat energy (usually water tubes). The heated precipitate coming in direct contact with the tubes (heating by conduction) increases the efficiency. Since this allows coal plants to burn at cooler temperatures, less NOx is also emitted. However, burning at low temperatures also causes increased polycyclic aromatic hydrocarbon emissions. FBC boilers can burn fuels other than coal, and the lower temperatures of combustion (800 °C / 1500 °F) have other added benefits as well. There are two reasons for the rapid increase of FBC in combustors. First, the liberty of choice in respect of fuels in general, not only the possibility of using fuels which are difficult to burn using other technologies, is an important advantage of fluidized bed combustion. The second reason, which has become increasingly important, is the possibility of achieving, during combustion, a low emission of nitric oxides and the possibility of removing sulfur in a simple manner by using limestone as bed material. Fluidized-bed combustion evolved from efforts to find a combustion process able to control pollutant emissions without external emission controls (such as scrubbers-flue gas desulfurization). The technology burns fuel at temperatures of 1,400 to 1,700 °F (750-900 °C), well below the threshold where nitrogen oxides form (at approximately 2,500 °F / 1400 °C, the nitrogen and oxygen atoms in the combustion air combine to form nitrogen oxide pollutants); it also avoids the ash melting problems related to high combustion temperature. The mixing action of the fluidized bed brings the flue gases into contact with a sulfur-absorbing chemical, such as limestone or dolomite. More than 95% of the sulfur pollutants in coal can be captured inside the boiler by the sorbent. The reductions may be less substantial than they seem, however, as they coincide with dramatic increases in polycyclic aromatic hydrocarbons, and possibly other carbon compound emissions.[citation needed] Commercial FBC units operate at competitive efficiencies, cost less than today's conventional boiler units, and have NO2 and SO2 emissions below levels mandated by Federal standards. However, they have some disadvantages such as erosion on the tubes inside the boiler, uneven temperature distribution caused by clogs on the air inlet of the bed, long starting times reaching up to 48 hours in some cases. 1. FBC has a lower combustion temperature of 750 °C whereas an ordinary boiler operates at 850 °C. 2. FBC has low sintering process (melting of Ash). 3. Lower production of NOx due to lower temperature. 4. Lower production of SOx due to capture by limestone. 5. Higher combustion efficiency due to 10 times more heat transfer than other combustion processes because of burning particle. 6. Less area is required for FBC due to high coefficient of convective heat transfer. 7. Iso-thermal bed combustion as temperature in free belt and active belt remain constant.

Types FBC systems fit into essentially two major groups, atmospheric systems (FBC) and pressurized systems (PFBC), and two minor subgroups, bubbling (BFB) and circulating fluidized bed (CFB). Fluidized Bed Combustion Atmospheric fluidized beds use limestone or dolomite to capture sulfur released by the combustion of coal. Jets of air suspend the mixture of sorbent and burning coal during combustion, converting the mixture into a suspension of red-hot particles that flow like a fluid. These boilers operate at atmospheric pressure. Pressurized Fluidized Bed Combustion The first-generation PFBC system also uses a sorbent and jets of air to suspend the mixture of sorbent and burning coal during combustion. However, these systems operate at elevated pressures and produce a high-pressure gas stream at temperatures that can drive a gas turbine. Steam generated from the heat in the fluidized bed is sent to a steam turbine, creating a highly efficient combined cycle system. Advanced PFBC

 A 1½ generation PFBC system increases the gas turbine firing temperature by using natural gas in addition to the vitiated air from the PFB combustor. This mixture is burned in a topping combustor to provide higher inlet temperatures for greater combined cycle efficiency. However, this uses natural gas, usually a higher priced fuel than coal.  APFBC. In more advanced second-generation PFBC systems, a pressurized carbonizer is incorporated to process the feed coal into fuel gas and char. The PFBC burns the char to produce steam and to heat combustion air for the gas turbine. The fuel gas from the carbonizer burns in a topping combustor linked to a gas turbine, heating the gases to the combustion turbine rated firing temperature. Heat is recovered from the gas turbine exhaust in order to produce steam, which is used to drive a conventional steam turbine, resulting in a higher overall efficiency for the combined cycle power output. These systems are also called APFBC, or advanced circulating pressurized fluidized-bed combustion combined cycle systems. An APFBC system is entirely coal-fueled.  GFBCC. Gasification fluidized-bed combustion combined cycle systems, GFBCC, have a pressurized circulating fluidized-bed (PCFB) partial gasifier feeding fuel syngas to the gas turbine topping combustor. The gas turbine exhaust supplies combustion air for the atmospheric circulating fluidized-bed combustor that burns the char from the PCFB partial gasifier.  CHIPPS. A CHIPPS system is similar, but uses a furnace instead of an atmospheric fluidized-bed combustor. It also has gas turbine air preheater tubes to increase gas turbine cycle efficiency. CHIPPS stands for combustion-based high performance power system.

14 Briefly discuss about Draught and the types of Draught.

The difference between atmospheric pressure and the pressure existing in the furnace or flue gas passage of a boiler is termed as draft. Draft can also be referred to the difference in pressure in the combustion chamber area which results in the motion of the flue gases and the air flow. Types of draft Drafts are produced by the rising combustion gases in the stack, flue, or by mechanical means. For example, a blower can be put into four categories: natural, induced, balanced, and forced.

 Natural draft: When air or flue gases flow due to the difference in density of the hot flue gases and cooler ambient gases. The difference in density creates a pressure differential that moves the hotter flue gases into the cooler surroundings.  Forced draft: When air or flue gases are maintained above atmospheric pressure. Normally it is done with the help of a forced draft fan.  Induced draft: When air or flue gases flow under the effect of a gradually decreasing pressure below atmospheric pressure. In this case, the system is said to operate under induced draft. The stacks (or chimneys) provide sufficient natural draft to meet the low draft loss needs. In order to meet higher pressure differentials, the stacks must simultaneously operate with draft fans.  Balanced draft: When the static pressure is equal to the atmospheric pressure, the system is referred to as balanced draft. Draft is said to be zero in this system.

Importance/significance For the proper and the optimized heat transfer from the flue gases to the boiler tubes draft holds a relatively high amount of significance. The combustion rate of the flue gases and the amount of heat transfer to the boiler are both dependent on the movement and motion of the flue gases. A boiler equipped with a combustion chamber which has a strong current of air (draft) through the fuel bed will increase the rate of combustion (which is the efficient utilization of fuel with minimum waste of unused fuel). The stronger movement will also increase the heat transfer rate from the flue gases to the boiler (which improves efficiency and circulation).[3]

Drafting in steam locomotives Since the stack of a locomotive is too short to provide natural draft, during normal running forced draft is achieved by directing the exhaust steam from the cylinders through a cone (“blast pipe”) upwards and into a skirt at the bottom of the stack. When the locomotive is stationary or in a restricted space “live” steam from the boiler is directed through an annular ring surrounding the blast pipe to produce the same effect. 15 Explain with neat sketch about the Super critical boiler.

A supercritical steam generator is a type of boiler that operates at supercritical pressure, frequently used in the production of electric power. In contrast to a subcritical boiler in which bubbles can form, a supercritical steam generator operates at pressures above the critical pressure – 3,200 psi or 22 MPa. Therefore, liquid water immediately becomes steam. Water passes below the critical point as it does work in a high pressure turbine and enters the generator's condenser, resulting in slightly less fuel use. Technically, the term "boiler" should not be used for a supercritical pressure steam generator as no "boiling" actually occurs in the device.

History of supercritical steam generation Contemporary supercritical steam generators are sometimes referred to as Benson boilers. In 1922, Mark Benson was granted a patent for a boiler designed to convert water into steam at high pressure. Safety was the main concern behind Benson’s concept. Earlier steam generators were designed for relatively low pressures of up to about 100 bar (10 MPa; 1,450 psi), corresponding to the state of the art in steam turbine development at the time. One of their distinguishing technical characteristics was the riveted water/steam separator drum. These drums were where the water filled tubes were terminated after having passed through the boiler furnace. These header drums were intended to be partially filled with water and above the water there was a baffle filled space where the boiler's steam and water vapour collected. The entrained water droplets were collected by the baffles and returned to the water pan. The mostly-dry steam was piped out of the drum as the separated steam output of the boiler. These drums were often the source of boiler explosions, usually with catastrophic consequences. However, this drum could be completely eliminated if the evaporation separation process was avoided altogether. This would happen if water entered the boiler at a pressure above the critical pressure (3,206 pounds per square inch, 22.10 MPa); was heated to a temperature above the critical temperature (706 °F, 374 °C) and then expanded (through a simple nozzle) to dry steam at some lower subcritical pressure. This could be obtained at a throttle valve located downstream of the evaporator section of the boiler. As development of Benson technology continued, boiler design soon moved away from the original concept introduced by Mark Benson. In 1929, a test boiler that had been built in 1927 began operating in the thermal power plant at Gartenfeld in Berlin for the first time in subcritical mode with a fully open throttle valve. The second Benson boiler began operation in 1930 without a pressurizing valve at pressures between 40 and 180 bar (4 and 18 MPa; 580 and 2,611 psi) at the Berlin cable factory. This application represented the birth of the modern variable-pressure Benson boiler. After that development, the original patent was no longer used. The "Benson boiler" name, however, was retained. 1957: Unit 6 at the Philo Power Plant in Philo, Ohio was the first commercial supercritical steam-electric generating unit in the world, and it could operate short-term at ultra-supercritical levels. It took until 2012 for the first US coal plant designed to operate at ultra-supercritical temperatures to be opened, John W. Turk Jr. Coal Plant in Arkansas.[4] Two current innovations have a good chance of winning acceptance in the competitive market for once-through steam generators[citation needed]:

 A new type of heat-recovery steam generator based on the Benson boiler, which has operated successfully at the Cottam combined-cycle power plant in the central part of England,  The vertical tubing in the combustion chamber walls of coal-fired steam generators which combines the operating advantages of the Benson system with the design advantages of the drum-type boiler. Construction of a first reference plant, the Yaomeng power plant in China, commenced in 2001. On 3 June 2014, the Australian government's research organization CSIRO announced that they had generated 'supercritical steam' at a pressure of 23.5 MPa (3,410 psi) and 570 °C (1,060 °F) in what it claims is a world record for solar thermal energy.

16 Why boiler water is to be treated? Explain briefly feed water treatment.

Boiler feed water is an essential part of boiler operations. The feed water is put into the steam drum from a feed pump. In the steam drum the feed water is then turned into steam from the heat. After the steam is used it is then dumped to the main condenser. From the condenser it is then pumped to the deaerated feed tank. From this tank it then goes back to the steam drum to complete its cycle. The feed water is never open to the atmosphere History of feed water treatment During the early development of boilers, water treatment was not so much of an issue, as temperatures and pressures were so low that high amounts of scale and rust would not form to such a significant extent, especially if the boiler was “blown down”. It was general practice to install zinc plates and/or alkaline chemicals to reduce corrosion within the boiler. Many tests had been performed to determine the cause (and possible protection) from corrosion in boilers using distilled water, various chemicals, and sacrificial metals.[1] Silver nitrate can be added to feed water samples to detect contamination by seawater. Use of lime for alkalinity control was mentioned as early as 1900, and was used by the French and British Navies until about 1935.[2] In modern boilers, treatment of feed water is critical, as problems result from using untreated water in extreme pressure and temperature environments. This includes lower efficiency in terms of heat transfer, overheating, damage, and costly cleaning.

Characteristics of boiler feed water Water has higher heat capacity than most other substances. This quality makes it an ideal raw material for boiler operations. Boilers are part of a closed system as compared to open systems in a gas turbine. The closed system that is used is the Rankine cycle. This means that the water is re circulated throughout the system and is never in contact with the atmosphere. The water is reused and needs to be treated to continue efficient operations. Boiler water must be treated in order to be proficient in producing steam. Boiler water is treated to prevent scaling, corrosion, foaming, and priming. Chemicals are put into boiler water through the chemical feed tank to keep the water within chemical range. These chemicals are mostly oxygen scavengers and phosphates. The boiler water also has frequent blow downs in order to keep the chloride content down. The boiler operations also include bottom blows in order to get rid of solids. Scale is precipitated impurities out of the water and then forms on heat transfer surfaces. This is a problem because scale does not transfer heat very well and causes the tubes to fail by getting too hot. Corrosion is caused by oxygen in the water. The oxygen causes the metal to oxidize which lowers the melting point of the metal. Foaming and priming is caused when the boiler water does not have the correct amount of chemicals and there are suspended solids in the water which carry over in the dry pipe. The dry pipe is where the steam and water mixture are separated.

Boiler feedwater treatment Boiler water treatment is used to control alkalinity, prevent scaling, correct pH, and to control conductivity. The boiler water needs to be alkaline and not acidic, so that it does not ruin the tubes. There can be too much conductivity in the feed water when there are too many dissolved solids. These correct treatments can be controlled by efficient operator and use of treatment chemicals. The main objectives to treat and condition boiler water is to exchange heat without scaling, protect against scaling, and produce high quality steam. The treatment of boiler water can be put into two parts. These are internal treatment and external treatment. The internal treatment is for boiler feed water and external treatment is for make-up feed water and the condensate part of the system. Internal treatment protects against feed water hardness by preventing precipitating of scale on the boiler tubes. This treatment also protects against concentrations of dissolved and suspended solids in the feed water without priming or foaming. These treatment chemicals also help with the alkalinity of the feed water making it more of a base to help protect against boiler corrosion. The correct alkalinity is protected by adding phosphates. These phosphates precipitate the solids to the bottom of the boiler drum. At the bottom of the boiler drum there is a bottom blow to remove these solids. These chemicals also include anti-scaling agents, oxygen scavengers, and anti-foaming agents. Sludge can also be treated by two approaches. These are by coagulation and dispersion. When there is a high amount of sludge content it is better to coagulate the sludge to form large particles in order to just use the bottom blow to remove them from the feed water. When there is a low amount of sludge content it is better to use dispersants because it disperses the sludge throughout the feed water so sludge does not form.

Deaeration of feed water Oxygen and Carbon Dioxide are removed from the feed water by deaeration. Deaeration can be accomplished by using deaerator heaters, vacuum deaerators, mechanical pumps, and steam-jet ejectors. In deaerating heaters, steam sprays incoming feed water and carries away the dissolved gases. The deaerators also store hot feed water which is ready to be used in the boiler. This means of mechanical deaeration is used with chemical oxygen scavenging agents to increase efficiency. Deaerating heaters can be classified in two groups: spray types and tray types. With tray type heaters the incoming water is sprayed into steam atmosphere to reach saturation temperature. When the saturation temperature is reached most of the oxygen and non- condensable gases are released. There are seals that prevent the recontamination of the water in the spray section. The water then falls to the storage tank below. The non-condensable and oxygen are then vented to the atmosphere. The components of the tray type deaerating heater are a shell, spray nozzles, direct contact vent condenser, tray stacks, and protective interchamber walls. The spray type deaerator is similar to the tray type deaerator. The water is sprayed into a steam atmosphere and most of the oxygen and non-condensables are released to the steam. The water then falls to the steam scrubber where the slight pressure loss causes the water to flash a little bit which also aids the removal of oxygen and non-condensables. The water then overflows to the storage tank. The gases are then vented to the atmosphere. With vacuum deaeration a vacuum is applied to the system and water is then brought to its saturation temperature. The water is sprayed into the tank like the spray and tray deaerator. The oxygen and non- condensables are vented to the atmosphere Boiler corrosion

Corrosive compounds, especially O2 and CO2 must be removed, usually by use of a deaerator. Residual amounts can be removed chemically, by use of oxygen scavengers. Additionally, feed water is typically alkalized to a pH of 9.0 or higher, to reduce oxidation and to support the formation of a stable layer of magnetite on the water-side surface of the boiler, protecting the material underneath from further corrosion. This is usually done by dosing alkaline agents into the feed water, such as sodium hydroxide (caustic soda) or ammonia. Corrosion in boilers is due to the presence of dissolved oxygen, dissolved carbon dioxide, or dissolved salts. Fouling Deposits reduce the heat transfer in the boiler, reduce the flow rate and eventually block boiler tubes. Any non-volatile salts and minerals that will remain when the feed water is evaporated must be removed, because they will become concentrated in the liquid phase and require excessive "blow-down" (draining) to prevent the formation of solid precipitates. Even worse are minerals that form scale. Therefore, the make-up water added to replace any losses of feed water must be demineralized/deionized water, unless a purge valve is used to remove dissolved minerals.

17 Explain about the essential components of diesel engine power plant.

• General layout • Components of Diesel power plant • Performance of diesel power plant • fuel system • lubrication system • air intake and admission system • supercharging system • exhaust system • diesel plant operation and efficiency • heat balance • Site selection of diesel power plant • Comparative study of diesel power plant with steam power plant

Diesel power plant • A generating station in which diesel engine is used as the prime mover for the generation of electrical energy is known as diesel power station. • In a diesel power station, diesel engine is used as the prime mover. The diesel burns inside the engine and the products of this combustion act as the working fluid to produce mechanical energy. The diesel engine drives alternator which converts mechanical energy into electrical energy. Used when: • Demand of power is less • Sufficient quantity of coal and water is not available • Transportation facilities are inadequate • This plants supply power to hospitals, radio stations, cinema houses and telephone exchanges. Advantages • The design and layout of the plant are quite simple. • It occupies less space as the number and size of the auxiliaries is small. • It can be located at any place. • It can be started quickly and it can pickup load in a short time. • There are no standby losses. • It requires less quantity of water for cooling. • The overall cost is much less than that of steam power station of same capacity. • The thermal efficiency of the plant is higher than that of a steam power station. • It requires less operating staff. Disadvantages • The plant has high running charges as the fuel (diesel) used is costly. • The plant doesn’t work satisfactorily under overload conditions for a longer period. • The plant can only generate small power. • The cost of lubrication is generally high. • The maintenances charges are generally high APPLICATION OF DIESEL POWER PLANT • They are quite suitable for mobile power generation and are widely used in transportation systems consisting of railroads, ships, automobiles and aero planes. • They can be used for electrical power generation in capacities from 2 to 50 MW. • They can be used as peak load plants for some other types of power plants. • Industrial concerns where power requirement are small say of the order of 500 kW, diesel power plants become more economical due to their higher overall efficiency. Mechanism: Components of Diesel power plant The essential components of diesel electric plants are: Engine: This is the main component of the plant which develops required power. The engine is generally directly coupled to the generator. Air-filter and supercharger: The function of the air filter is to remove the dust from the air which is taken by the engine. The function of the supercharger is to increase the pressure of the air supplied to the engine to increase the power of the engine. The superchargers are generally driven by the engines Exhaust system: This includes the silencers and connecting ducts. The temperature of the exhaust, gases is sufficiently high, therefore, the heat of the exhaust gases many times is used for heating the oil or air supplied to the engine. Fuel system: It includes the storage tank, fuel pump, fuel transfer pump, strainers and heater. The fuel is supplied to the engines according to the load on the plant. Cooling system: This system includes water circulating pumps, cooling towers or spray ponds and water filtration plant. The purpose of cooling system is to carry the heat from the engine cylinder to keep the temperature of the cylinder in safe range and extend its life. The essential components of diesel electric plants Contd.. Lubrication system. It includes the oil pumps, oil tanks, filters, coolers and connecting pipes. The function of the lubrication system is to reduce the friction of moving parts and reduce the wear and tear of the engine parts. Starting system: This includes compressed air tanks. The function of this system is to start the engine from cold by supplying the compressed air. Governing system: The function of the governing system is to maintain the speed of the engine constant irrespective of load on the plant. This is done generally by varying fuel supply to the engine according to load. Performance of diesel power plant The basic performance parameters are (see the class notes for details) : • Power & Mechanical Efficiency Indicated power (I.P) Brake Power(B.P) Mechanical Efficiency • Specific output • Specific fuel consumption • Thermal Efficiency: Site selection of diesel power plant The following Factors should be considered while selecting the site for a diesel power plant: • Foundation sub-soil condition: the condition of sub-soil should be such that a foundation at a reasonable depth should be capable of providing a strong support to the engine. • Access to the site: the site should be so selected that it is accessible through rail and road. • Distance from that load centre: the location of the plant should be near the load centre. This reduces the cost of transmission lines and maintenance cost. The power loss is minimized. • Availability of water: Sufficient quantity of water should be available at the site selected. • Fuel transportation: The site selected should be near to the source of fuel supply so that transportation charges are low. 18 In an air standard diesel cycle, the compression ratio is 16, and at the beginning of isentropic compression, the temperature is 15oC and the pressure is 0.1MPa.Heat is added until the temperature at the end of the constant pressure process is 1480oC. Calculate (i) The cut-off ratio (ii) The heat supplied per kg of air (iii) Cycle efficiency (iv) The M. e. p 19 Drive the air standard efficiency for Otto cycle.

n my previous article, I gave a short introduction to Otto cycle and discussed the processes in it. In this article, I will derive the air-standard efficiency of Otto cycle. p-V and T-s Diagrams of Otto Cycle: T-s Diagram p-V Diagram Basic terms used in derivation of air-standard efficiency of Otto cycle: Total Cylinder Volume: It is the total volume (maximum volume) of the cylinder in which Otto cycle takes place. In Otto cycle,

Total cylinder volume = V1 = V4 = Vc + Vs (Refer p-V diagram above) where,

Vc → Clearance Volume Vs → Stroke Volume Clearance Volume (Vc): At the end of the compression stroke, the piston approaches the Top Dead Center (TDC) position. The minimum volume of the space inside the cylinder, at the end of the compression stroke, is called clearance volume (Vc). In Otto cycle, Clearance Volume, Vc = V2 (See p-V diagram above) Stroke Volume (Vs): In Otto cycle, stroke volume is the difference between total cylinder volume and clearance volume.

Stroke Volume, Vs = Total Cylinder Volume – Clearance Volume = V1 – V2 = V4 – V3 Compression Ratio: Compression ratio (r) is the ratio of total cylinder volume to the clearance volume.Now that we know the basic terms, let us derive expressions for T2 and T3. These expressions will be useful for us to derive the expression for air-standard efficiency of otto cycle. For finding T2, we take process 1-2 and for finding T3, we take process 3-4.

Process 1-2: This process is an isentropic (reversible adiabatic) process. For this process, the relation between T and V is as follows:

Process 3-4: This is also an isentropic Process 1-2: Process 3-4: relation between T and V in process 1-2:

Air-standard efficiency of Otto cycle process. The relation between T and V in this process is similar to the relation between T and V in process 1-2:

Air-standard efficiency of Otto cycle: It is defined as the ratio between work done during Otto cycle to the heat supplied during Otto cycle.

Air-Standard Efficiency (thermal efficiency) of Otto cycle,

If you have any ideas or suggestions about air-standard efficiency of Otto cycle, you can comment on this article.

20 Compare and contrast the open and closed cycle Gas turbine power plant with suitable illustration.

S. Criterion Closed Cycle Gas Turbine Open Cycle Gas Turbine no

It works on closed cycle. It works on open cycle. The fresh charge is Cycle of The working fluid is 1 supplied to each cycle and after combustion operation recirculated again and and expansion. It is discharged to atmosphere. again. It is a clean cycle.

The gases other than the air like Helium or Helium- Working Carbon dioxide mixture Air-fuel mixture is used which leads to lower 2 fluid can be used, which has thermal efficiency. more favourable properties.

Since heat is transferred externally, so any type of Since combustion is an integral part of the Type of fuel; solid, liquid or system thus it requires high quantity liquid or 3 fuel used gaseous or combination of gaseous fuel for burning in a combustion these can be used for chamber. generation of heat.

Manner The heat is transferred Direct heat supply. It is generated in the 4 of heat indirectly through a heat combustion chamber itself input exchanger.

The heat can be supplied Quality from any source like waste It requires high grade heat energy for 5 of heat heat from some process, generation of power in a gas turbine. input nuclear heat and solar heat using a concentrator. High thermal efficiency for Efficienc Low thermal efficiency for same temperature 6 given lower and upper y limits. temperature liquids.

Part load Part load efficiency is Part load efficiency is less compared to Closed 7 efficienc better. cycle gas turbine. y

Size of Reduced size per MWh of Comparatively large size for same power 8 plant power output. output.

Since combustion products do not come in direct Direct contact with combustion products, the Blade contact of turbine blade, 9 blades are subjected to higher thermal stresses life thus there is no blade and fouling and hence shorter blade life. fouling and longer blade life.

Control on power Better control on power 10 Poor control on power production. producti production. on

Closed cycle gas turbine Open cycle gas turbine plant is simple and less 11 Cost plant is complex and costly. costly.