How a Coal Plant Works

The following diagram shows the major components coal-fired power generation plant. Click on the diagram for a larger detailed view.

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Coal is transported to the site generally by truck or rail.  The coal goes through primary and secondary crushing before it is placed on a conveyor and stored inside the plant in a coal silo. The silos hold a several hour supply of coal and can continue to supply coal to the boiler in the event there is a problem downstream in the coal handling system.

Inside the plant, coal is dropped out of the silos into pulverisers.  Multiple pulverisers crush the coal into a powder which is blown into the boiler by air.  As the coal burns, it forms a fireball in the boiler.  Radiation from this fireball heats water flowing up tubes in the sides of the boiler.  In a sub critical boiler water is separated from the steam in these tubes and is recirculated through the boiler.  The steam created is further superheated in tubes by the hot gases passing through the top of the boiler.  The superheated steam is then sent to the steam turbines.  Heavier ash components from the burning coal drop to the bottom of the boiler and are collected. 

In a supercritical boiler, hot gases heat hot water at the top of the boiler into the supercritical phase of water.  This fluid has a temperature of about 600C and is at a pressure of about 25,000 kPa.  This supercritical fluid is sent to the steam turbines.  This high pressure fluid is diverted onto the blade of the turbine, forcing the turbine to spin.  There may be two or three steam turbines driving a generator.  The generator produces electricity at a modest voltage.  A transformer is used to increase the voltage to allow the power to be put into the transmission system.

The low pressure steam exiting the low pressure steam turbine must be condensed so the resulting liquid water can be recirculated back to the boiler.  Water from a cooling pond, lake, river or ocean flows through the inlet canal into the condenser.  This cool water condenses the low pressure steam into water.  The cooling water is diverted via the outlet canal back to where it came from. 

The warm flue gas exiting the boiler must be processed to remove harmful materials before the flue gas is discharged into the air.  The hot gases may be sent through a selective catalytic reduction unit to remove NOx.  SOx compounds may also be removed in a flue gas desulfurization (FDG) unit.  Mercury may be removed with activated carbon or by some other means.  Finally, most of the very fine ash particles are removed in a baghouse or an electrostatic precipitator.  This ash is collected and can be used for other industrial purposes.  The warm flue gas is composed mostly of nitrogen and steam and contains about 15% carbon dioxide.  The flue gas is blown up the stack where it is dispersed high in the air.

To see a video describing the operation of a supercritical coal plant please visit
http://www.midamericanenergy.com/virtualtour.aspx?tour=coal&id=100&player=wm&res=high