Solar Power Technology

The boiler is the central component of a fossil-fueled steam power plant. Most modern boilers are of the water wall type, in which the boiler walls are almost entirely constructed of vertical tubes that either carry feed water into the boiler or carry steam out of the boiler. The first water wall boiler was developed by George Babcock and StephenWilcox in 1867. The early water wall boilers were used in conjunction with reciprocating piston steam engines, such as used in old locomotives.

Only in the twentieth century, with the advent of the steam turbine, and its requirement for large steam pressures and flows, has the water wall boiler been fully developed. In a modern water wall boiler the furnace and the various compartments of the boiler are fully integrated.

Water from the high pressure feed water heater at a temperature of 230–260 ◦C is further heated in the economizer section of the boiler to 315 ◦C, then flows into the steam drum, which is mounted on top of the boiler. The steam drum measures typically 30 m in length and 5 m in diameter. In the steam drum liquid water is separated from the steam, usually by gravity. From the steam drum, liquid water flows down the downcomer tubes into the header. From there, the hot pressurized water flows upward (because of a negative density gradient) through the riser tubes, where the actual boiling of water into steam occurs. The separated steam passes another section of the boiler, called the superheater, where its temperature is raised to 565 ◦C at a pressure of 24 MPa.

At this point the temperature and pressure are higher than the critical temperature (Tc = 374 ◦C) and pressure (pc = 22 MPa) of water. The supercritical steam drives the high-pressure turbine. The exhaust steam from the high-pressure turbine flows through the reheater section of the boiler, where the temperature is raised again to about 500 ◦C at a pressure of 3.7 MPa.

This steam drives the low-pressure turbine. The superheater and reheater sections of the boiler are usually situated past
a bend in the boiler, called the neck. In order to optimize thermal efficiency, the combustion air is preheated to a temperature of 250–350 ◦C in the air preheater section of the boiler. Near the burners, heat is transferred from the combustion gases to the boiler tubes by radiation. Away from the burners, heat is transferred mainly by convection. Coal and oil flames are highly luminous in the visible portion of the spectrum because of the radiation from unburnt carbon and ash particles.

Natural gas flames are less visible because of the absence of particles in the flame. However, most of the radiative transfer of heat from all flames occurs in the nonvisible infrared portion of the spectrum. The theoretical (Carnot) thermodynamic efficiency of a heat engine that works between a temperature differential of 838 K (565 ◦C) and 298 K (25 ◦C)—that is, between the temperature of the superheater and the condensation temperature of water in the condenser—is
η = (TH − TL)/TH = 64%.

However, as mentioned in the introduction, typical efficiencies of steam power plants are in the 33–40% range. Because heat is added to the water and steam at all temperatures between these limits, the Rankine cycle efficiency is necessarily lower than the Carnot value. Furthermore, parasitic efficiency degradation occurs because of heat losses through the walls of the boiler, ducts, turbine blades and housing, and frictional heat losses.