A discussion on boilers would be incomplete without mentioning the Rankine cycle. The steam-based Rankine cycle has been synonymous with power generation for more than a century. In the United States, utility boilers typically use subcritical parameters (2400 psi, 1050/1050°F), whereas in Europe and Japan, supercritical plants are in vogue (4300 psi, 1120/1120°F). The net efficiency of power plants has increased steadily from 36% in the 1960s for subcritical coal-fired plants to 45% for supercritical units commissioned in the 1990s. Several technological improvements in areas such as metallurgy of boiler tubing, reduction in auxiliary power consumption, improvements in steam turbine blade design and metallurgy, pump design, burner design, variable pressure condenser design, and multistage feedwater heating coupled with low boiler exit gas temperatures have all contributed to improvements in efficiency. An immedi­ate advantage of higher efficiency is lower emissions of CO2 and other pollutants. Current state-of-the-art coal-fired supercritical steam power systems operate at up to 300 bar and 600°C with net efficiencies of 45%. These plants have good
efficiencies even at partial load compared to subcritical units, and plant costs are comparable to those of subcritical units. At 75% load, for example, the efficiency reduction in a supercritical unit is about 2% compared to 4% for subcritical units. At 50% load, the reduction is 5.5-8% for supercritical versus 10-11% for subcritical. These units are of once-through design. Cycle efficiencies of 36% in the 1960s (160 bar, 540/540°C) rose to about 40% in 1985 and to 43-45% in 1990. These gains have been made through [1-3]

Increases in the main and reheat steam temperatures and main steam pressure, including transitions to supercritical conditions Changes in cycle configuration, including increases in the number of reheat stages and the number of feedwater heaters Changes in condenser pressure and lowering of the exit gas temperature from the boiler (105-115°C)

Reductions in auxiliary power consumption through design and develop­ment

Improvements in the performance of various types of equipment such as turbines and pumps, as mentioned above

One of the concerns with the steam-based Rankine cycle is that a higher steam temperature is required with higher steam pressure to minimize the moisture in the steam after expansion. Moisture impacts the turbine performance negatively through wear, deposit formation, and possible blockage of the steam path. As can be seen in Fig. 1.6, a higher steam pressure for the same temperature
results in higher moisture after expansion. Hence steam temperatures have been increasing along with pressures, adding to metallurgical concerns. This implies a need for higher boiler tube wall thickness and materials with higher stress values at high temperatures. Multistage reheating minimizes the moisture concern after expansion; however, this adds to the complexity of the boiler and HRSG design. Also with HRSGs, the steam-based Rankine cycle limits the effectiveness of heat recovery, because steam boils at constant temperature and significant energy is lost, which brings us to the Kalina cycle.