THERMAL POWER PLANT

Introduction of Thermal power plant

 

A thermal power plant station is a power plant in which heat energy is converted to electric power. In most of places in the world of turbine is steam- driven. Water is heated, turns into 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 Rankine cycle. 

Thermal power station

                                                               Almost all coal, petroleum, nuclear, geothermal, solar thermal electric, and waste incineration plants, as well as many natural gas power stations are thermal. Natural gas is frequently burned in gas turbines as well as boilers. The waste heat from a gas turbine, in the form of hot exhaust gas, can be used to raise steam by passing this gas through a heat recovery steam generator (HRSG). The steam is then used to drive a steam turbine in a combined cycle plant that improves overall efficiency. Power stations burning coal, fuel oil, or natural gas are often called fossil fuel power stations. Some biomass-fueled thermal power stations have appeared also. Non-nuclear thermal power stations, particularly fossil-fueled plants, which do not use cogeneration are sometimes referred to as conventional power stations.

            Thermal Power Plant Layout

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.

Layout of thermal station 

                                           The layout of the steam power plant is shown in figure below it consist of different circuits. These are Boiler, Generator, condenser, Hot well, Drain cooler, LP heater, Turbine, boiler feed pump, excitor, Chimney, Ash Handling Plant etc.

Process and cycle of thermal power station 

 In a Coal based power plant, Coal is   transported from coal mines to the power plant by railways in wagon. Coal is unloaded from the wagon to a moving underground conveyor belt. This coal from the mines is of no uniform size. So it is taken to the crusher house & 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 the live storage (for 8 hours coal supply) in the raw coal bunker in the boiler house. Raw coal from raw coal bunker is supplied to the coal mills by raw coal feeder. 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 boiler in the combustion zone.  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 super heater platen super heater and final super heater respectively for superheating. The superheated steam from the final super heater is taken to the steam turbine. In the turbine the steam pressure is utilised to rotate the turbine and the resultant is rotating energy. From the high pressure turbine the out coming steam is taken to the Reheater in the boiler to increase its temperature as the steam becomes wet at the high pressure turbine outlet. The outlet of the turbine is sent to the condenser for condensing back to water by a cooling water system. This condensed water is collected in the hot well and it again sent to the boiler in a closed cycle. The rotational energy imparted to the turbine buy high pressure steam is converted to electrical energy in the generator.

          Major equipment of thermal station

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

process & cycle of Thermal station 

A thermal power plant burns enormous amount of coal. A 200MW plant may require around 2000 tons of coal daily. In every plant there is enough storage of coal to last for 15 days or so.

PULVERISING  PLANT :- In modern thermal power plant , coal is pulverized i.e. ground to dust like size and carried to the furnace in a stream of hot air. Pulverisation is a means of exposing a large surface area to the action of oxygen and consequently helping the combustion. 

coal pulveriser 

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 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.

Reheater

Power station furnaces may have a reheater section containing tubes heated by hot flue gases outside the tubes. Exhaust steam from the high-pressure turbine is passed through these heated tubes to collect more energy before driving the intermediate and then low-pressure turbines.

STEAM TURBINE

A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. High pressure superheated 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

                            .

steam condenser is a device in which the exhaust steam from steam turbine is condensed by means of cooling water. The main purpose of a steam condenser in turbine is to maintain a low back pressure on the exhaust side of the steam turbine.

Condenser Hot Well

Steam enters the condenser where it cools and condenses into water before being sent to the low-pressure feed-water heater. The condenser hotwell serves as a water reservoir for the turbine cycle. When hotwell level reaches the low point, a valve opens to supply make-up water to the cycle. When hotwell level reaches the high end of the level range, a dump valve opens to move the condensate from the hotwell to a condensate storage tank.

Cooling towers

A cooling tower is a heat rejection device that rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of closed circuit dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature.

cooling towers are an effective way to remove unwanted heat from one place to another using water as transport media in applications where there is a need to have a reliable source of cool water year round in the 70°F to 100°F temperature range.  Cooling towers are commonly used to cool products and machinery in a multitude of applications.

                                             

# ASH HANDLING SYSTEM

In thermal power plants, coal is used as a fuel for generating electricity. After burning of coal, 40% of the total coal consumption is converted into ash which need to be properly disposed off from the thermal power plant.

TYPES OF ASH GENERATED IN THERMAL POWER PLANT

 

1.    Bottom Ash -  Ash generated below furnace of the thermal power plant is called the bottom Ash. The value of bottom Ash generated is around 20% of total Ash. Bottom Ash is mostly coarse in nature hence it needs to be further crushed before being transported to ash handling system.

2.    Fly Ash - around 80% of ash generated in thermal power plant is fly ash. It is in form of very fine particles which is collected by economiser hopper, air preheater Hopper and electrostatic precipitator.

 Ash handling plant or ash handling system in thermal power plant are used to cooled down the ash to manageable temperature, transferred to a disposal area or storage which is further utilised in other industries.

 

Working of Ash handling system in thermal power plant:

 Ash handling system generally divided into three types in the fly ash handling system, bottom handling system and ash slurry disposal system.

 

1. Fly ash handling system: fly ash is captured and removed from the flue gases by economiser, air preheater and electrostatic precipitator located at the outlet of the furnace and before the induced draft.The fly ash is penumatically transported from collection Hopper of economizer, air preheater and electrostatic precipitator to storage silo for subsequent transport by trucks.

 2. Bottom Ash handling system: it is collected bottom Hopper located under the furnace boiler. Due to course in nature it is treated through clinker and grinders to normalise the bottom Ash size. It is then transported to bottom ash silo for temporary storage or ash pond in slurry form

3.Ash slurry disposal system: Ash received from bottom ash handling system is mixed with water in bottom ash silo. Slurry formed due to mixing of water and ash is transported to ash disposal area by using two system:

1.    High concentration ash slurry disposal system:  Slurry pump is used to transport ash slurry to ash disposal area vai ash slurry transportation pipes.

2.    Lean ash slurry disposal system: Slurry transported to a dewatering bin by a Jet pump, and carried to an ash dump by truck after dewatering.

 

Wastewater from the dewatering bin goes into an ash sedimentation tank or an ash sedimentation pond for the sedimentation of ash particles and water without as particles is supplied to a jet pump by an ash handling pump. The ash particles which settle in the sedimentation tank and the water tank are returned to the dewatering bin by a sludge return pump.