fuel such as pulverized coal is air-blown into the furnace through burners located at the four corners, or along one wall, or two opposite walls, and it is ignited to rapidly burn, forming a large fireball at the center. the thermal radiation of the fireball heats the water that circulates through the boiler tubes near the boiler perimeter. the water circulation rate in the boiler is three to four times the throughput. as the water in the boiler circulates it absorbs heat and changes into steam. it is separated from the water inside a drum at the top of the furnace. the saturated steam is introduced into superheat pendant tubes that hang in the hottest part of the combustion gases as they exit the furnace. here the steam is superheated to to prepare it for the turbine.