Thermal Power Plant Operation

Thermal Power Plant Basics

• Thermal power plants/steam power stations convert heat from coal combustion into electrical energy.
• They operate based on the Rankine cycle.

Processes in Thermal Power Plants

• Coal combustion produces steam in a boiler.
• Steam expands in a steam turbine (prime mover), generating mechanical energy.
• An alternator converts mechanical energy into electrical energy.
• A condenser cools and condenses the steam, returning it to the boiler.
• The plants are suitable for locations with abundant coal and water where large-scale electricity generation is needed.

Advantages

• Coal fuel is cheap.
• Plants can be installed anywhere, irrespective of coal source, as coal can be transported by rail or road.
• They require less space than hydroelectric power stations.
• Generation costs are lower than diesel power stations.

Disadvantages

• Atmospheric pollution occurs due to smoke and fumes.
• Running costs can be more expensive compared to some power plants.

Schematic Arrangement

• Steam power stations convert heat from coal combustion into electrical energy and rely on proper working and efficiency.

Coal & Ash Handling Plant

• Coal transported by road or rail and is stored in a coal storage plant for protection against strikes, transport issues, and shortages.
• Pulverized coal is fed into the boiler for combustion, then ash is removed and stored.
• A 100 MW station burns about 20,000 tons of coal per month, producing 2,000 to 3,000 tons of ash and fuel/handling accounts for 50%-60% of operating costs.

Steam Generating Plant

• Consists of a boiler for steam production with auxiliary equipment for using flue gasses.
• Major components include the boiler, super heater, economiser and air preheater.

Steam Turbine Operation

• Superheated steam passes through the main valve to the steam turbine.
• The steam's heat energy is converted into mechanical energy.
• Steam exhausts to the condenser and is cooled/condensed via cold water circulation.

Alternator Function

• The alternator is coupled to the steam turbine.
• The alternator converts mechanical energy into electrical energy.
• Output is delivered to bus bars through transformers, circuit breakers, and isolators.

Feedwater System

• Condensate from the condenser is reused as feedwater for the boiler.
• Lost water is replenished and preheated by water heaters and the economizer to improve plant efficiency.

Cooling Arrangement

• Steam from the turbine condenses in a condenser using water from natural sources to absorb heat or cooling towers when little water is available.

Choice Site Selection for Steam Stations - Fuel and Water

• Locate near coal mines to minimize fuel transportation costs or ensure transport facilities are available.
• They should be located near a river or canal to ensure a continuous water supply for the condenser.

Choice Site Selection for Steam Stations - Other Considerations

• The steam power station should be located on cheap land with potential for future expansion, which should have sufficient bearing capacity.
• Plants should be located near the center of the load, especially for DC supply systems because it reduces transmission costs.
• For AC systems, location relative to load centers is less important because AC power can be transmitted at high voltages.
• Plants should be located away from populated areas because a large amount of coal is burned in a steam power station, producing smoke and fumes.

Efficiency of Steam Power Stations

• Overall efficiency is low at around 29% due to heat losses in the condenser and elsewhere.
• A temperature difference is required to convert heat energy into mechanical energy and maintaining a greater temperature difference increases heat loss, leading to lower efficiency.

Thermal Efficiency Defined

• Thermal efficiency is the heat equivalent of mechanical energy transmitted to the turbine shaft divided by the heat of coal combustion.

Overall Efficiency Defined

• Overall efficiency is the ratio of heat equivalent of electrical output to the heat of combustion of coal.
• The overall efficiency, ~29%, is less than thermal efficiency due to alternator losses (~1%).
• Overall efficiency = Thermal efficiency x Electrical efficiency.

Steam Generating Equipment

• Consists of components that generate superheated steam.
• Includes like the boiler, boiler furnace, superheater, economizer, air pre-heater, and other heat-reclaiming devices

Boiler Specs

• A boiler is a closed vessel that converts water into steam by using coal combustion.
• Classifications of steam boilers: water tube boilers and fire tube boilers.

Boiler Furnace

• Fuel is burnt within to liberate heat energy.
• Furnaces provide support and enclosure for combustion equipment e.g. burners.

Superheater

• Superheats steam, raising its temperature above the boiling point of water.

Economiser Facts

• Heats the feed water on its way to the boiler, deriving heat from the flue gases.
• Doing this raises boiler efficiency, saves fuel, and reduces boiler stresses from the higher feed water temperature.

Air Pre-Heater

• They extract heat from flue gasses and transfer it to the air supplied to the furnace, raising furnace temperature and increasing plant thermal efficiency.
• Economizers cannot fully extract heat from flue gases, so pre-heaters are used to recover heat from escaping gases

Condensers and Functions

• Condenses steam at the turbine's exhaust.
• It creates a very low pressure, allowing the steam to expand.

Prime Movers Defined

• Converts steam energy into mechanical energy.
• There are steam engines and steam turbines which are more efficient, have better power-to-weight ratios, and smoother operation.

Water Treatment

• Boilers require clean/soft water for efficiency and longevity.
• Water is purified/softened through chemical treatment before use.

Alternators

• Each is coupled to a steam turbine and converts the turbine's mechanical energy into electrical energy.
• The alternator is hydrogen or air cooled and excitation is provided by main and pilot exciters.

Transformers

• Include main step-up, station, and auxiliary transformers.
• Main step-up transformers step-up the generation voltage for transmission of power.
• Station transformers are used for services such as lighting.
• Auxiliary transformers supply individual unit-auxiliaries.

Switchgear

• Locates faults and isolates the faulty section from the healthy section.
• Includes circuit breakers, relays, switches, and other control devices.