Working and Components of Thermal Power Plant

A thermal power plant is a Conventional way of generating electricity. In a thermal power plant, the heat and combustion energy do mechanical work. There are various components involved in a thermal power plant. We can divide the thermal power pant parts into four groups as follows. In this post, we’ll discuss each component of these groups in detail.

• Fuel Components: Fuel, equipment for fuel handling, and pulverization.
• Air System Components: Components for the draft system, air preheater, etc.
• Gas Components: In include boiler, super-heater and turbines.
• Feed Water and Cooling System: Condenser, spray pond, cooling tower, etc.

Site Selection of Thermal Power Plant

• The cost of land should be reasonable.
• An abundant amount of cooling water should be available
• Fuel transportation facilities should be available.
• The power station should not be very far from the distribution center.

Thermal Power Plant Working

The working diagram of the thermal power plant is shown below figure. The first step is to prepare the fuel for the thermal plant in the coal handling area of the plant. Then prepared coal goes to the boiler and the combustion process takes place. After that, the heat released from the combustion process is used to convert high-pressure water into steam. This converted steam passes to the turbine via a super-heater. The steams expand inside the turbine and rotates the rotor which drives the generator. This conversion of heat energy to mechanical energy results in electricity generation. After that, the ash remained in the boil passed to the ash handline and disposal area. Combustion gases from boiler go-to chimney from air-preheater. The expanded steam passed to the condenser from the turbine and the condensed water is reused.

In the working of the thermal power plant, the main components are the boiler, turbine, and generator. Other components like ash handling pants, air-heater, condenser, water-heater, cooling tower, chimney and etc. are auxiliary components that improve the efficiency of the plant. These additional auxiliaries are helpful in reduction of pollution and handling the waste of the plant.

Fuel Handling Components in Thermal Power Plant

Thermal Power Plant Fuel

Normally coal, oil, or gas is used as fuel in thermal power plants. Gaseous fuels have all the properties of oil and coal. But they are not economical due to their high cost. Oil is used only where the availability of oil is plentiful and it is available at a low cost. Coal is the most commonly used fuel in thermal power plants in India.

Analysis of Coal: Proximate analysis of coal gives a good indication of the heating and burning properties of coal. The ultimate analysis is a test that enables us with the chemical composition of coal.

Classification: Different types of coal have different heat values. Types of coal in increasing order of heat content is as follows: peat –> lignite –> bituminous –> semi-bituminous –> semi anthracite –> anthracite.

High ash content in coal reduces the thermal efficiency of a boiler because of loss of heat content through unburnt carbon, excessive clinker formation, more heat in ash, and difficulty in the disposal of hot ash.

Fuel Handling

In a thermal power station, half of the total operating cost is on account of coal. Therefore problems associated with coal handling require careful consideration in thermal plans. The reliable availability of coal, preparation of coal for the plant, storage, and maintenance are some of the main requirements of good coal handling in a thermal plant. The following diagram explains the complete coal handling process from delivery of coal to in-plant coal handling.

Pulverizing

In modern thermal power plants, coal is pulverized to dust-like size and carried to a furnace. Pulverization of coal expands the surface area of coal for the reaction of oxygen and consequently helping the combustion. Following are a few pros and cons of pulverization:

• Pulverization reduces the rate of combustion and less air is required for fule burning.
• Coal breaking losses are reduced and low-grade coal can be used in plants after pulverization.
• Requirement of extra pieces of equipment for pulverization which increases initial investment.
• There is a risk of explosion in pulverization, so it needs to be handled carefully.

Thermal Power Plant Components in Air System

Draft System

The combustion in the boiler required a supply of a sufficient quantity of air and the removal of exhaust gases. The differences in pressure cause the circulation of air, known as the draft. A differential in the draft is needed to cause the flow of gases through the boiler. Following are two types of draft systems generally used in thermal plants.

Natural Draft

A natural draft is provided by the chimney. Chimney produces a draft so that air can flow into the boiler. Then it releases the product of combustion gases into the atmosphere. Chimney delivers the product of combustion and fly ash to a high altitude which reduces air pollution. The efficiency and intensity of the natural draft depend on the height of the chimney and the difference between the temperature of inside and outside air.

Mechanical Draft

Large-size thermal plants use very huge boilers of capacity above 106Kg/hr. Such big boilers need a tremendous volume of air. The mechanical draft consists of forces or induced draft. In force draft system fan is installed near the base of the boiler. Throughout the system, the pressure of air is above atmospheric pressure, and forced air flows through the system. While in the induced draft, the fan is installed near the base of the chimney. The sucks out burnt gases out of the boiler, which reduces pressure inside the boiler. A balanced draft uses both forces (near the base of the boiler) and induced (near the base of the chimney) draft.

Air Pre-heater

After the fuel gases leave the economizer they are used to heat incoming air for combustion. Cooling of fuel gas by 20^oC raises plant efficiency by 1%. A regenerative air pre-heater used a cylindrical rotor made of a corrugated steel plate. The rotor is fixed on the shaft which rotates at a slow speed of 2-4rpm. Rotor passes through fuel gases and air zone alternatively. The rotor element is heated by fuel gas and transfer this heat to the air in the air zone.

Gas Components in Thermal Power Plant

Boiler

The boiler is a close vessel, which converts water into steam. It is one of the major components of a thermal plant. The boiler is always designed to absorb maximum heat release in combustion. Conduction, convection, and radiation transfer heat to the boiler. The factors which influence the choice of the boiler are initial cost, maintenance cost, labor cost, etc.

Fire Tube Boiler

The product of combustion passes through tubes that are surrounded by water. They are classified as vertical and horizontal depends on the tube. This boiler is simple, compact, and rugged in construction. A vertical fire tube boiler occupies little floor space. The initial cost of a fire tube boiler is low.

Water Tube Boiler

Water flows inside the tube and hot gases outside the tube. These are classified as vertical, horizontal, and inclined depends on the tube. Circulation in the boiler may be natural or forced. Force circulation has advantages like lighter tubes, cheaper foundation, uniform heating of all parts, increase efficiency of the boiler, and better control of temperature.

Super-heater

Superheated steam contains more heat than saturated steam at the same pressure. This additional heat provides more energy to the turbine and thus electric power output is more. The function of a super-heater is to remove the last traces of moisture from a saturated stream leaving the boiler tube and also raise the temperature of steam. The heat of combustion gases from the furnace is used for superheating. The convection super-heater is placed in the gas stream and receives most of the convection. A radiant super-heater is placed in or near a furnace where receives most of the heat from radiation.

Steam Turbine

The turbine converts the heat energy of steam into mechanical energy which drives the generator. Steam attains high velocity during expansion and the velocity depends on the initial and final heat content of the steam. The difference between initial and final heat content represents the heat energy converted into kinetic energy. There are two types of turbine impulse and reaction. Both have a number of stages in which pressure drops takes place. The number of stages in the reaction is more. Steam turbines are manufactured in a variety of sizes up to 106hp and have a horizontal configuration. The standard speeds of the turbine are 3000 rpm and 1500 rpm for coupling a 50Hz generator.

Reaction Turbine

In this only partial expansion takes place in the nozzle and further expansion takes place as steam flows over the rotor blade. The relative velocity increases as it expands while passing over blades. There is a difference in pressure on the two sides of moving blades.

Impulse Turbine

Steam is expanded in a turbine nozzle and attains high velocity. Steam jet impinges on blades fixed on rotor periphery. In this turbine, complete expansion of steam takes place in the nozzle, and steam flows over the turbine blades.

Thermal Plant Components in Feed Water and Cooling System

Condenser

The condenser does the job of condensing exhaust steams from the boiler and helps in maintaining low pressure. Exhaust steam is condensed and used as part of feed water for the boiler. Maintenance of a high vacuum in the condenser is essential for efficient operation. A surface condenser consists of an air-tight cylinder shell having a camper at each end. Water-tube extends between campers and colling water flows through the tube. For efficient operation temperature rise in cooling water passing through the condenser should be around 10^oC.

Spray Pond

It consists of a tank of water in which condensed hot water passes through pipes. The hot water in pipes is sprayed through nozzles at suitable pressure. A small amount of cooling takes place in a spray pond due to conduction and radiation.

Cooling Towers

In the cooling tower, the amount of water splits in smaller quantities, practically in the size of drops. These drops fall to the bottom of the cooling tower. The splitting of water in drops, the draught provided by the tower, and the large evaporating surface help to cool water very quickly. Practically the cooling takes place during the time when water droplets descend to the bottom of the tower. Water from the base of the cooling tower goes back to the condenser and the same cycle repeats.

Feed Water Heater

Natural water contains impurities, so it can’t be used for the generation of steam in a boiler. Water needs to clean up to remove impurities for use in the boiler. The water condensate in the condenser returns to the boiler as feedwater but some quantity of natural water is still required to make up for the loss in water. The amount of extra water required is approx. 1% of total water needed. The total make-up water required for the 100MW plant will be of order 25-30tons. There are problems such as formation, corrosion, caustic embrittlement, etc. due to impurities in water. The heater heats the feed water before it goes to the boiler.

Advantages of Thermal Power Plant

• The fuse used in the thermal plants is cheaper.
• Less initial cost in setting up the plant
• Requires less area as compare to the hydroelectric plant.
• The cost of electricity generation is less than the diesel engine power plant.

Disadvantages of Thermal Power Plant

• The running cost of thermal plants is more than hydro-electric plants.
• Ash disposal handling required which causes pollution.
• Unlike closed-cycle gas turbine power plants and hydroelectric power plant, air pollution is more in thermal plants due to exhaust fuel gases and ash.