Coal fired power stations are used to generate electricity. They are the most common type of power station and the largest global megawatt (MW) contributor of all power station types. This type of power plant has found widespread application due to its tried and tested design, as well as the cheap availability of coal. Unfortunately, coal fired power plants -particularly older plants- are not environmentally friendly compared to other types of power generation plants. Coal fired power plants are non-renewable (not ‘green’) types of plant.
How Coal Fired Thermal Power Stations Work
The below video is an extract from our Mechanical and Electrical Engineering Explained Online Video Course.
Coal is delivered to the plant via road, rail or ship, and deposited in a coal yard. Stacker reclaimers are used to gather coal and deposit it into hoppers, the hoppers then feed flatbed conveyors. Conveyors transport the coal from the coal yard to day silos within the main power station building. Each day silo contains enough coal for a set period of time when the power station is fully loaded e.g. one day silo may contain enough coal for one boiler at 12 hours of full load operation. The day silos ensure that any disruption to the supply chain from the coal yard to the day silos will not cause a disruption to the boiler and consequently power generation.
Larger power stations may have several large watertube boilers, steam turbines and generators. It is standard practice for each power generation building to be referred to as a ‘Block’ e.g. Block A, Block B etc. Firetube boilers are used to provide the initial heating of the power plant steam systems.
Day silos feed the boiler directly (old design and uncommon), or, by passing the coal through a coal pulverizer (standard design and common). Coal pulverizers increase the coal’s contact surface area with the air by grinding the coal into small pieces. The pulverizer also dries the coal in order that combustion can more easily occur (reduced moisture content). The heat to dry the coal is recovered from the boiler exhaust gas stream.
Pulverized coal from the pulverizer is blown into the boiler with the primary air stream. The coal at this stage is finely ground and very dry, both of these characteristics aid combustion. Combustion occurs and heat is generated (this plant is a ‘thermal power plant').
The heat generated by the water tube boiler is used to change the state of water to steam. The steam is then discharged to a condenser steam turbine.
The steam turbine is connected via a gearbox to an a.c. electrical generator. Alternating current then passes through switchgear prior to being distributed to an electrical transformer; the switchgear used will often be of the SF6 or vacuum design. The electrical transformer increases the output voltage and is referred to as a ‘generator step-up (GSU)’ transformer.
The GSU increases the output voltage to match that of the electrical grid, this may be several hundred thousand volts e.g. 110kV, 220kV etc. Increasing the voltage reduces transmission losses and reduces the thickness of the transmission cables required (higher voltage means lower amps, lower amps means thinner conductors/cables can be used).