6 Pulverized-Coal—Fired Power Plant Performance and Operating Flexibility Available to Purchase

The design parameters of the 750-MW pulverized-coal-fired power plant as discussed in Chapter 2 provide a net plant efficiency of 45% at rated output conditions. However, it is of utmost importance that good performance is sustained over a wide operating range of the power plant. A high operating flexibility, which becomes more and more important, must be warranted for load cycling as well as startup operation. For achieving this goal, advanced power plants are designed to operate in a hybrid pressure mode and are equipped with main and reheat steam bypass systems.

Variable pressure operation, also referred to as sliding pressure operation, is performed over a wide power output range, normally from about 30% to 100%. This includes the evaporation system of once-through boilers without any boiler internal throttle valve. Below 30% output, constant pressure operation is used with the steam turbine control valves for power plant startup and stable low-load operation. Advanced supercritical once-through steam generators can be operated stably in the variable-pressure operational mode in the load range from 20% to 100%. Figure 6-1 shows schematically a Benson-type steam generator with rifled vertically arranged boiler tubes, which provide this operating flexibility, with only a steam/water separator downstream of the evaporating section for startup and low-load operation. This supercritical steam generator with vertical rifled tubing provides the flexibility to operate at supercritical as well as at subcritical pressure conditions over such an extended load range.

Variable pressure operation avoids throttle losses and therefore achieves the best heat rate performance during part-load operation. This fact is illustrated in Figure 6-2 with the heat rate curves for HP turbine operation in a nozzle-controlled mode at constant pressure and a throttle-controlled HP turbine operating at constant and variable pressure. The nozzle-controlled HP turbine has a control stage, which at 100% load does not achieve the same low heat rate as a throttle-controlled full-arc admission turbine. At part load, the performance of the nozzle-controlled turbine with 3 valve points at about 100%, 90%, and 70% performs better below about 80% when both units are operated in a constant pressure mode. However, the throttle-controlled full-arc admission turbine provides the lowest heat rate over the entire load range when operated in a variable pressure mode. The locus of valve best points curve indicates that better performance of the nozzle-controlled HP turbine would be possible below 60% if there would be a further valve point. However, large steam turbines do not have such low valve points because of the high steam forces related to such operation. Figure 6-3 compares a throttle and nozzle-controlled HP turbine design, with the throttle-controlled unit featuring a fullarc admission chamber in the inner casing and a tilted first-stage stationary blade row. Only 2 main steam admissions and stop and control valve combinations are needed. The nozzle-controlled partial-arc admission HP turbine features a control stage for operation with 3 valve points, namely at all 4 valves open, at 3 valves open, and at 2 valves open. This HP turbine design features 4 steam admissions to the 4 nozzle segments and 4 stop and control valve combinations.