THERMAL POWER PLANTBy

Ashvani ShuklaC&Ireliance

INTRODUCTION Thermal power generation plant or thermal power station is the most conventional source of electric power. Thermal power plant is also referred as coal thermal power plant and steam turbine power plant. Before going into detail of this topic, we will try to understand the line diagram of electric power generation plant.A thermal power station is a power plant in which heat energy is converted to electric power. In most of the world the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this is known as a Rankin cycle. 

THEORY OF THERMAL POWER STATION

The theory of thermal power station or working of thermal power station is very simple. A power generation plant mainly consists of alternator runs with help of steam turbine. The steam is obtained from high pressure boilers. Generally in India, bituminous coal, brown coal and peat are used as fuel of boiler. The bituminous coal is used as boiler fuel has volatile matter from 8 to 33 % and ash content 5 to 16 %. To increase the thermal efficiency, the coal is used in the boiler in powder form.

In coal thermal power plant, the steam is produced in high pressure in the steam boiler due to burning of fuel (pulverized coal) in boiler furnaces. This steam is further supper heated in a super heater. This supper heated steam then enters into the turbine and rotates the turbine blades. The turbine is mechanically so coupled with alternator that its rotor will rotate with the rotation of turbine blades. After entering in turbine the steam pressure suddenly falls and corresponding volume of the steam increases. After imparting energy to the turbine rotor the steam passes out of the turbine blades into the condenser. In the condenser the cold water is circulated with the help of pump which condenses the low pressure wet steam. This condensed water is further supplied to low pressure water heater where the low pressure steam increases the temperature of this feed water, it is again heated in high pressure. For better understanding we furnish every step of function of a thermal power station as follows, 1) First the pulverized coal is burnt into the furnace of steam boiler. 2) High pressure steam is produced in the boiler. 3) This steam is then passed through the super heater, where it further heated up. 4) This supper heated steam is then entered into a turbine at high speed. 5) In turbine this steam force rotates the turbine blades that means here in the turbine the stored potential energy of the high pressured steam is converted into mechanical energy.

6) After rotating the turbine blades, the steam has lost its high pressure, passes out of turbine blades and enters into a condenser.7) In the condenser the cold water is circulated with help of pump which condenses the low pressure wet steam. 8) This condensed water is then further supplied to low pressure water heater where the low pressure steam increases the temperature of this feed water, it is then again heated in a high pressure heater where the high pressure of steam is used for heating.9) The turbine in thermal power station acts as a prime mover of the alternator.

Rankin cycle

WORKING OF RANKIN CYCLEA typical Thermal Power Station Operates on a Cycle which is shown below.The working fluid is water and steam. This is called feed water and steam cycle. The ideal Thermodynamic Cycle to which the operation of a Thermal Power Station closely resembles is the RANKINE CYCLE. In steam boiler the water is heated up by burning the fuel in air in the furnace & the function of the boiler is to give dry super heated steam at required temperature. The steam so produced is used in driving the steam Turbines. This turbine is coupled to synchronous generator (usually three phase synchronous alternator), which generates electrical energy. The exhaust steam from the turbine is allowed to condense into water in steam condenser of turbine, which creates suction at very low pressure and allows the expansion of the steam in the turbine to a very low pressure. The principle advantages of condensing operation are the increased amount of energy extracted per kg of steam and thereby increasing efficiency and the condensate which is fed into the boiler again reduces the amount of fresh feed water.The condensate along with some fresh make up feed water is again fed into the boiler by pump (called the boiler feed pump). In condenser the steam is condensed by cooling water. Cooling water recycles through cooling tower. This constitutes cooling water circuit. The ambient air is allowed to enter in the boiler after dust filtration. Also the flue gas comes out of the boiler and exhausted into atmosphere through stacks. These constitute air and flue gas circuit. The flow of air and also the static pressure inside the steam boiler (called draught) is maintained by two fans called Forced Draught (FD) fan and Induced Draught(ID) fan.

RANKIN CYCLE

TYPE OF THERMAL POWER PLANT 1. CO-GENERATION POWER PLANT 2. CAPTIVE POWER PALNTS 3. SUBCRITICAL POWER PLANTS 4. SUPER CRITICAL POWER PLANTS 5. ULTRA SUPERCRITICAL POWER PLANTS

CO-GENERATION POWER PLANT Cogeneration is also called as combined heat and power or combine heat and power. As it name indicates cogeneration works on concept of producing two different form of energy by using one single source of fuel. Out of these two forms one must be heat or thermal energy and other one is either electrical or mechanical energy.

Cogeneration is the most optimum, reliable, clean and efficient way of utilizing fuel. The fuel used may be natural gas, oil, diesel , propane, wood, bagasse, coal etc. It works on very simple principle i.e. the fuel is used to generate electricity and this electricity produces heat and this heat is used to boil water to produce steam , for space heating and even in cooling buildings. In conventional power plant , the fuel is burnt in a boiler , which in turn produces high pressure steam. This high pressure steam is used to drive a tribune, which is in turn is connected to an alternator and hence drive an alternator to produce electric energy. The exhaust steam is then sent to the condenser, where it gets cool down and gets converted to water and hence return back to boiler for producing more electrical energy. The efficiency of this conventional power plant is 35% only. In cogeneration plant the low pressure steam coming from turbine is not condense to form water, instead of it its used for heating or cooling in building and factories, as this low pressure steam from turbine has high thermal energy. The cogeneration plant has high efficiency of around 80 - 90 %. In India, the potential of power generation from cogeneration plant is more than 20,000 MW.

Need for Cogeneration a) Cogeneration helps to improve the efficiency of the plant. b) Cogeneration reduce air emissions of particulate matter, nitrous oxides, sulphur dioxide, mercury and carbon dioxide which would otherwise leads to greenhouse effect.

c) It reduces cost of production and improve productivity. d) Cogeneration system helps to save water consumption and water costs. e) Cogeneration system is more economical as compared to conventional power plant Types of Cogeneration Power Plants In a typical Combined heat and power plant system there is a steam or gas turbine which take steam and drives an alternator. A waste heat exchanger is also installed in cogeneration plant, which recovers the excess heat or exhaust gas from the electric generator to in turn generate steam or hot water. There are basically two types of cogeneration power plants, such as- • Topping cycle power plant • Bottoming cycle power plant Topping cycle power plant- In this type of Combine Heat and Power plant electricity is generated first and then waste or exhaust steam is used to heating water or building . There are basically four types of topping cycles. a) Combined-cycle topping CHP plant - In this type of plant the fuel is firstly burnt in a steam boiler . The steam so produced in a boiler is used to drive turbine and hence synchronous generator which in turn produces electrical energy . The exhaust from this turbine can be either used to provide usable heat, or can be send to a heat recovery system to generate steam, which maybe further used to drive a secondary steam turbine.

b) Steam-turbine topping CHP Plant- In this the fuel is burned to produce steam, which generates power. The exhaust steam is then used as low-pressure process steam to heat water for various purposes.

c) Water- turbine topping CHP Plant- In this type of CHP plant a jacket of cooling water is run through a heat recovery system to generate steam or hot water for space heating. d) Gas turbine topping CHP plant- In This topping plant a natural gas fired turbine is used to drives a synchronous generator to produce electricity. The exhaust gas is sent to a heat recovery boiler where it is used to convert water into steam, or to make usable heat for heating purposes.

Bottoming cycle power plant - As its name indicate bottoming cycle is exactly opposite of topping cycle. In this type of CHP plant the excess heat from a manufacturing process is used to generate steam, and this steam is used for generating electrical energy. In this type of cycle no extra fuel is required to produce electricity, as fuel is already burnt in production process.

Configuration of Cogeneration Plant • Gas turbine Combine heat power plants which uses the waste heat in the flue gas emerging out of gas turbines. • Steam turbine Combine heat power plants that use the heating system as the jet steam condenser for the steam turbine.

• Molten-carbonate fuel cells have a hot exhaust, very suitable for heating. • Combined cycle power plants adapted for Combine Heat and Power.

2. CAPTIVE POWER PLANTA captive power plant is a facility that is dedicated to providing a localised source of power to an energy user. These are typically industrial facilities or large offices. The plants may operate in grid parallel mode with the ability to export surplus power to the local electricity distribution network. Alternatively they may have the ability to operate in island mode; i.e. independently of the local electricity distribution system. Captive power plants are a form of distributed generation, generating power close to the source of use. Distributed generation facilitates the high fuel efficiency along with minimising losses associated with the transmission of electricity from centralised power plants.Captive power plants are used to generate the power for ourselfs or out plant load or house load.it will be synchronized to grid for import and export the power depend upon our requirement.

SUB CRITICAL POWER PLANT

In a coal based power plant coal is transported from coal mines to the power plant by railway in wagons or in a merry-go-round system. Coal is unloaded from the wagons to a moving underground conveyor belt. This coal from the mines is of no uniform size. So it is taken to the Crusher house and crushed to a size of 20mm. From the crusher house the coal is either stored in dead storage( generally 40 days coal supply) which serves as coal supply in case of coal supply bottleneck or to the live storage(8 hours coal supply) in the raw coal bunker in the boiler house. Raw coal from the raw coal bunker is supplied to the Coal Mills by a Raw Coal Feeder. The Coal Mills or pulverizer pulverizes the coal to 200 mesh size. The powdered coal from the coal mills is carried to the boiler in coal pipes by high pressure hot air. The pulverized coal air mixture is burnt in the boiler in the combustion zone.Generally in modern boilers tangential firing system is used i.e. the coal nozzles/ guns form tangent to a circle. The temperature in fire ball is of the order of 1300 deg.C. The boiler is a water tube boiler hanging from the top. Water is converted to steam in the boiler and steam is separated from water in the boiler Drum. The saturated steam from the boiler drum is taken to the Low Temperature Superheater, Platen Superheater and Final Superheater respectively for superheating. The superheated steam from the final superheater is taken to the High Pressure Steam Turbine (HPT). In the HPT the steam pressure is utilized to rotate the turbine and the resultant is rotational energy. From the HPT the out coming steam is taken to the Reheater in the boiler to increase its temperature as the steam becomes wet at the HPT outlet. After reheating this steam is taken to the Intermediate Pressure Turbine (IPT) and then to the Low Pressure Turbine (LPT). The outlet of the LPT is sent to the condenser for condensing back to water by a cooling water system. This condensed water is collected in the Hotwell and is again sent to the boiler in a closed cycle. The rotational energy imparted to the turbine by high pressure steam is converted to electrical energy in the Generator.

SUPER CRITICAL POWER PLANT WHAT IS “CRITICAL” ABOUT SUPER CRITICAL POWER GENERATION “Supercritical " is a thermodynamic expression describing the state of a substance where there is no clear distinction between the liquid and the gaseous phase (i.e. they are a homogenous fluid). Water reaches this state at a pressure above around 220 Kg Bar ( 225.56 Kg / cm2) and Temperature = 374.15 C. In addition, there is no surface tension in a supercritical fluid, as there is no liquid/gas phase boundary.

WHAT IS “CRITICAL” ABOUT SUPER CRITICAL POWER GENERATION

By changing the pressure and temperature of the fluid, the properties can be “tuned” to be more liquid- or more gas like. Carbon dioxide and water are the most commonly used supercritical fluids, being used for decaffeination and power generation, respectively.

CHALLENGES FOR ADOTION OF SUPER CRITICAL TECHNOLOGY Up to an operating pressure of around 190Kg Bar in the evaporator part of the boiler, the cycle is Sub-Critical. In this case a drum-type boiler is used because the steam needs to be separated from water in the drum of the boiler before it is superheated and led into the turbine. Above an operating pressure of 220Kg Bar in the evaporator part of the Boiler, the cycle is Supercritical. The cycle medium is a single phase fluid with homogeneous properties and there is no need to separate steam from water in a drum. Thus, the drum of the drum-type boiler which is very heavy and located on the top of the boiler can be eliminated Once-through boilers are therefore used in supercritical cycles. Advanced Steel types must be used for components such as the boiler and the live steam and hot reheat steam piping that are in direct contact with steam under elevated conditions STEAM GENRATION IN NA

STEAM GENRATION IN NATURAL CIRCULATION & ONCE THROUGH BOILER

STEAM GENRATION IN NATURAL CIRCULATION & ONCE THROUGH BOILER

BOILER FOR SUPERCRITICAL ONCE THROUGH POWER PLANT

O Once through Boiler technology, which originated in Europe, has evolved into the most effective application for Supper Critical Steam condition.

There are no operational limitations due to once- through boilers compared to drum type boilers.

In fact once-through boilers are better suited to frequent load variations than drum type boilers, since the drum is a component with a high wall thickness, requiring controlled heating. This limits the load change rate to 3% per minute, while once-through boilers can step-up the load by 5% per minute.

This makes once-through boilers more suitable for fast startup as well as for transient conditions.

CHANGE FROM NATURAL CIRCULATION TO ONCE THROUGH IS MORE IMPPORTANT THAN THE SWITCH FROM SUB-TO SUPER CRITICAL

BOILER FOR SUPERCRITICAL ONCE THROUGH POWER PLANT

Once-through boilers have been favored in many countries, for more than 30 years.

They can be used up to a pressure of more than 300 Kg Bar without any change in the process engineering. Wall thicknesses of the tubes and headers however need to be designed to match the planned pressure level.

Once-through boilers have been designed in both two-pass and tower type design, depending on the fuel requirements and the manufacturers‘ general practice.

For the past 30 years, large once-through boilers have been built with a spiral shaped arrangement of the tubes in the evaporator zone.

The latest designs of once-through boilers use a vertical tube arrangement

BOILER CONCEPTS –SUPRCRITICAL BENSON TYPE

SUPERCRITICAL ONCE THROUGH POWER PLANT – TURBINE GENERATOR

The Turbine designs for a Super Critical plant are similar to the sub critical with the only special materials required for the casings and walls for withstanding high Temperatures and pressures.

High Pressure (HP) Turbine : In order to cater for the higher steam parameters in supercritical cycles, materials with an elevated chromium content which yield higher material strength are selected.

Intermediate Pressure (IP) Turbine Section: In supercritical cycles there is a trend to increase the temperature of the reheat steam that enters the IP turbine section in order to raise the cycle efficiency. As long as the reheat temperature is kept at 560 DEGC there is not much difference in the IP section of Sub critical and Super Critical plants.

Low Pressure (LP) Turbine Section: The LP turbine sections in supercritical plants are not different from those in subcritical plants.

CHALLENGES FOR ADOPTION OF SUERCRITICAL TECHNOLOGY O DNB (DEPARTURE FROM NUCLEATE BOILING) & DO (DRY OUT) O DAMAGING THERMAL STRESSES ARISING OUT OF TEMPERATURE DIFFERENCE AT EVAPORAOR OUTLET

SPIRAL WATER WALL,TUBING & HEAT FLUX

SPIRAL WATER WALL, TUBING & HEAT FLUX

TUBE TEMPERATURE EVAPORATOR OUTLET

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