Constant and sliding-pressure options for new supercritical plants

Brian P. Vitalis, Riley Power Inc., a subsidiary of Babcock Power Inc.

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The drivers may be different, but the destination—higher efficiency—is the same worldwide. As a primary component of current efforts to reduce the environmental impact of burning low-cost coal, new and more-efficient steam plant designs are once again being considered by the U.S. generation industry.

Even though current market conditions in the U.S. tend to favor diversification of technologies and operating capabilities, the lowest-cost generating units will still be first in line for dispatching. The present and expected makeup of regional generating fleets in the U.S. generally indicate that any modern supercritical, coal-fired unit will have a significant fuel cost advantage and could be dispatched at costs approaching those of current nuclear plants.

Although seasonal and daily load reductions could be plausible in the long term, much of any new supercritical coal-fired capacity will not be frequently shut down or continually load-cycled. This is one major difference between the market conditions and practices of the U.S. and Europe, and a main reason why it should not be assumed that the pressure-control mode and technology prevalent in Europe should be embodied in the bulk of new unit construction in the U.S.

To advance plant efficiencies to 40% (HHV) and beyond, supercritical steam conditions (higher than 3,208 psia) are employed. Operation at these pressures, where there is no phase distinction between liquid and vapor, requires unique steam generator design features, most notably in furnace circuitry and components. Within this category of steam generators, the design is also very much influenced by the intended operating mode: constant pressure or sliding pressure (see box).

Beyond the apparent differences in component and construction design features, the choice of mode has broader implications, for example, on overall furnace sizing differences and materials options. These less-discussed differences can have a noticeable impact on cost and can become even more significant as steam conditions are gradually advanced toward ultra-supercritical conditions in pursuit of greater efficiency and reduced emissions. Plant designers should factor these steam generator design implications into their strategic planning and their development of specifications for new plants to arrive at the most cost-effective generation portfolio for particular U.S. and regional market environments.

Steam pressure vs. load

Constant pressure implies stable pressure of the steam generator and main steam line over the unit's load range. Meanwhile, the basic nature of a simple, rotating turbine is to require less pressure as load and flow rate are reduced, and if the main steam pressure is limited to only that required for each load, this mode is referred to as pure sliding pressure.

However, when we speak generally of "sliding pressure," we often mean modified sliding pressure, as shown in Figure 1. This mode has a limited amount of pressure throttling to provide a modest amount of fast-response load reserve. A unit under constant pressure will have significant load reserve at any reduced load, due to its significant pressure throttling or the availability of admission valve(s). By opening the throttle valve or an admission valve, the pressure in the turbine and steam generator move toward equalization. The sudden reduction of pressure in the steam generator prompts an instantaneous expulsion of steam mass due to the increase in a specific volume of steam within the confines of the system, and it provides a temporary load increase even before the fuel-handling and -firing system can be loaded to support any sustained higher load.

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1. Steam generator operating modes

Pure sliding-pressure operation does not offer this kind of load or frequency response and is therefore generally not practiced. Note that for a typical 3,800-psia steam pressure rating, a (modified) sliding-pressure steam generator operates at subcritical pressures at all loads below about 73% maximum continuous rating (MCR).

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