Q2Power Generation Sources
Question
Q.2. Describe basic schemes and working principles of thermal power plants with suitable diagrams.
Answer
A thermal power plant operates on the Rankine cycle, with major subsystems being the coal-and-ash handling circuit, air-and-flue-gas circuit, feed-water-and-steam circuit, and cooling-water circuit; feed water is boiled into superheated steam, expanded through the turbine to generate electricity, then condensed and recirculated.
A coal-fired thermal power plant converts the chemical energy of coal into electrical energy through four principal interconnected circuits, all working together on the basic Rankine thermodynamic cycle.
1. Coal and Ash Handling Circuit
Raw coal is delivered by rail/road/conveyor to the coal yard, crushed to the required size in crushers, and fed via conveyors to bunkers and then pulverizing mills, where it is ground to a fine powder to increase the surface area for efficient combustion. Pulverized coal is blown into the boiler furnace along with preheated primary air. The resulting ash (bottom ash from the furnace and fly ash carried with flue gases, captured by electrostatic precipitators) is collected and disposed of via ash-handling systems, often to ash ponds or used in cement/brick manufacture.
2. Air and Flue Gas Circuit
A forced-draft (FD) fan supplies combustion air, which is preheated in an air preheater (using waste heat from the flue gases) before entering the furnace. Combustion of pulverized coal releases heat, which is transferred to the boiler tubes. The resulting flue gases pass through the superheater, economizer and air preheater (recovering heat at each stage), then through electrostatic precipitators or bag filters to remove particulate matter, and finally through the induced-draft (ID) fan and chimney (stack) to the atmosphere.
3. Feed Water and Steam Circuit
Feed water is pumped by the boiler feed pump through the economizer (where it is preheated using flue-gas waste heat) into the boiler drum. In the boiler, water circulates through water-wall tubes surrounding the furnace, absorbing heat and converting to saturated steam, which collects in the steam drum. This steam passes through the superheater, raising its temperature well above saturation to produce dry, superheated steam (typically 540°C and 150-250 bar in modern plants), which is fed to the high-pressure (HP) turbine. After expansion in the HP turbine, the steam may be reheated in the boiler's reheater and sent to the intermediate-pressure (IP) and low-pressure (LP) turbine stages for further expansion, extracting maximum work. The exhaust steam from the LP turbine is condensed to water in the condenser (under vacuum, maintained by extracting non-condensable gases), and this condensate is returned via condensate extraction pumps, low-pressure heaters, deaerator and boiler feed pump back to the boiler, completing the closed steam-water cycle.
4. Cooling Water Circuit
Cooling water (drawn from a river, sea or cooling tower) is circulated through the condenser tubes to absorb the latent heat of the exhaust steam, condensing it to water; the warmed cooling water is then cooled in a cooling tower (natural or induced draft) by evaporative heat rejection to the atmosphere before being recirculated, or is discharged (once-through systems) back to the source water body if permitted.
Overall, the plant achieves the energy conversion chain: chemical energy (coal) → thermal energy (steam) → mechanical energy (turbine rotation) → electrical energy (generator output), with typical overall thermal efficiencies of 33-40% for modern supercritical/subcritical units.
Auxiliary systems further support this basic scheme: instrumentation and control systems continuously monitor drum level, steam pressure/temperature, and turbine speed, adjusting fuel and air feed rates through a distributed control system (DCS); a generator excitation and protection system regulates output voltage and trips the unit under fault conditions; and environmental control equipment (electrostatic precipitators/bag filters for particulates, flue-gas desulphurization for SO₂, and increasingly selective catalytic reduction for NOₓ) is fitted in the flue-gas path to meet emission norms before the gases are released through the chimney. Efficient integration of these coal-and-ash, air-and-flue-gas, feed-water-and-steam, and cooling-water circuits, together with heat recovery at the economizer and air preheater, is what allows a modern thermal station to approach its rated thermal efficiency while meeting reliability and environmental compliance requirements.