Flow Diagram of a Steam Thermal Power Plant

A thermal power generating plant on the Rankine Cycle turns heat into electricity. The coal-fired flow diagram uses three streams: coal, air and water.

This page is the flow diagram of a coal thermal power generating plant. Coal burns in the furnace and supplies the heat that raises steam. Other fuels follow a similar steam cycle with a different fuel path.

Air brings oxygen for combustion and then, as flue gas, carries heat across the boiler surfaces. Water in the boiler becomes steam. The steam drives the turbine.

The turbine shaft turns a generator. The rest of this page follows the three circuits: coal, air and water-steam.

Coal Circuit

Coal arrives from the mine or a stockpile into the plant coal yard. Conveyors then take it to the pulverisers.

Tramp metal and stones are taken out before milling. The mill grinds the coal to a fine dust so it burns in suspension. Bottom ash and fly ash then go to the ash-handling plant and on to the ash yard or a reuse store.

Air Circuit

Forced-draught fans push combustion air toward the furnace. On most large boilers that air first passes through an air preheater.

In the air preheater, leaving flue gas heats the incoming air before the air reaches the burners.

In the furnace the air supplies oxygen for combustion. The hot flue gas then flows over the boiler tube banks and gives up heat to the water and steam.

Most of the furnace heat enters the evaporator tubes. The gases then pass the superheater, where steam from the drum is heated above saturation temperature.

The gases then reach the economiser. Remaining heat raises the temperature of feedwater before that water enters the boiler drum or the water wall inlets.

After the economiser the gases pass the air preheater again, this time on the gas side, and give more heat to the incoming air.

Induced-draught fans then pull the cooled gases to the chimney. Dust collectors or a bag filter sit in that path on most modern plants.

Large thermal power plants usually run both a forced-draught fan on the air inlet and an induced-draught fan on the gas outlet. The two fans together are balanced draught. Some smaller boilers use only one fan.

Water Steam Circuit

The water-steam circuit is almost closed. Steam that leaves the turbine is condensed and pumped back as feedwater. Only makeup water replaces leaks, soot-blowing and blowdown. Circulating cooling water is a separate large flow through the condenser tubes.

Raw water often comes from a river, a reservoir or a bore. Coastal plants may use seawater for the condenser and a smaller treated stream for the boiler.

Boiler makeup is clarified, filtered and demineralised so salts do not scale the tubes. Treated water then joins the condensate and goes through the economiser into the boiler.

In the boiler, water becomes saturated steam. The steam then goes to the superheater and is heated above saturation. Superheated steam then enters the turbine. Impulse stages use nozzles. Reaction stages expand the steam in the moving blades as well.

In an impulse stage the steam expands in the nozzles and leaves at high speed. The kinetic energy turns the turbine discs. The drop is staged, not a single sudden dump from boiler pressure to condenser pressure.

The turbine is coupled to a generator. The generator sends alternating electricity to the grid through the unit transformer.

Exhaust steam leaves the last turbine stage and enters the condenser. Circulating water in the tubes condenses it. The cooling water is itself cooled in a cooling tower or taken from a once-through river or sea intake.

Condensate is pumped back through low-pressure heaters and the economiser to the boiler. Fresh makeup stays small because the same mass of water keeps circulating in the thermal power generating plant steam cycle.