Unit
- First law of thermodynamics: Energy cannot be created or destroyed, only converted from one form to another. In a thermal power plant, chemical energy stored in fuel is released through combustion, converted into thermal energy in the boiler, then transformed into mechanical energy by the turbine, and finally into electrical energy by the generator.
- Second law of thermodynamics: Heat cannot be completely converted into work in a cyclic process; a portion must be rejected to a low-temperature sink. This fundamental limitation defines the maximum theoretical efficiency of any heat engine, typically around 40-45% for modern thermal plants.
- Heat engine concept: A thermal power plant operates as a heat engine that absorbs heat from a high-temperature source (combustion gases), performs work through expansion in the turbine, and rejects remaining heat to a low-temperature reservoir (condenser cooling water). The net work output is the difference between heat added and heat rejected.
- Energy conversion chain: Fuel chemical energy is released as heat (combustion efficiency 85-95%), transferred to water/steam (boiler efficiency 80-90%), expanded through turbine (isentropic efficiency 85-92%), and finally converted to electricity by generator (efficiency 97-99%). Each step introduces losses that reduce overall plant efficiency.
- Thermal efficiency factors: The overall plant efficiency depends on steam temperature and pressure at turbine inlet, condenser vacuum level, boiler feedwater temperature, and auxiliary power consumption. Higher steam parameters increase efficiency but require advanced materials to withstand elevated temperatures and pressures.
- Power plant classification by fuel: Thermal plants are categorized by fuel type: coal-fired (subcritical, supercritical, ultra-supercritical), natural gas (combined cycle, open cycle), oil-fired, nuclear (fission heat source), and biomass/waste-to-energy. Each fuel type has distinct combustion characteristics, emission profiles, and operational constraints.
- Major plant subsystems: A thermal power plant comprises fuel handling and storage, boiler and steam generator, turbine-generator set, condenser and cooling system, feedwater heating system, emission control equipment, and electrical switchyard. Understanding subsystem interdependencies is essential for operational optimization and troubleshooting.
- Heat rate concept: Heat rate is the amount of thermal energy input (in kJ) required to produce one kilowatt-hour of electrical output. Typical heat rates for subcritical coal plants range from 9000-10500 kJ/kWh, while supercritical plants achieve 7500-8500 kJ/kWh. Lower heat rate indicates higher efficiency and lower fuel consumption per unit of electricity generated.
Process Flow
- Fuel combustion: Fuel enters the furnace and reacts with preheated air, releasing thermal energy at temperatures exceeding 1400°C. Combustion gases transfer heat to water-wall tubes through radiation and convection.
- Steam generation: Feedwater is heated to saturation temperature in the economizer, evaporated in the water-wall tubes forming two-phase mixture, and superheated to final steam temperature (typically 540-600°C) in superheater sections.
- Steam expansion: High-pressure superheated steam (160-250 bar) enters the high-pressure turbine, expands partially, returns to the boiler for reheating, then expands through intermediate and low-pressure turbines. The expansion converts thermal energy to rotational mechanical energy.
- Electrical generation: The turbine shaft drives the generator rotor at synchronous speed (3000 RPM for 50 Hz or 3600 RPM for 60 Hz) through a coupling. The rotating magnetic field induces alternating current in the stator windings.
- Condensation and heat rejection: Exhaust steam from the low-pressure turbine enters the condenser where cooling water absorbs latent heat, condensing the steam back to liquid water at vacuum conditions (0.05-0.1 bar absolute).
Quick Revision
- Energy conversion in thermal plants follows the chain: chemical to thermal to mechanical to electrical.
- The second law limits thermal efficiency; practical plants achieve 35-45% efficiency.
- Major subsystems include boiler, turbine, condenser, feedwater heaters, and emission controls.
- Heat rate is a key performance metric representing energy input per unit electrical output.
- Higher steam parameters (temperature and pressure) improve thermodynamic efficiency.