constant and sliding-pressure options for new supercritical plants

8/13/2019 Constant and Sliding-pressure Options for New Supercritical Plants

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Constant and sliding-pressure options for newsupercritical plants

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

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 andvapor, 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



Design for sliding pressure

Market conditions in Europe and Japan—including shutdowns and rapid and continual load ramping of

supercritical coal-fired plants—foster priorities and operating practices different from those in the U.S. In

part, these conditions have justified the development and expense of sliding-pressure designs overseas. For

instance, to handle rapid and continual load ramping (which is of particular value due to high local fuel

costs), turbine temperature transients are minimized by operating in sliding-pressure mode. This requires

certain drastic adaptations of the steam generator design, which—for current steam conditions—are

apparently worth the investment given European and Japanese market realities (except that the implied low

capacity factor means a longer payback period for the higher capital investment).

In sliding-pressure operation, because the steam generator operates under both supercritical and subcritical

conditions as load is varied, the furnace must be designed to accommodate both single- and two-phase fluid

flow. Because the two pressure regimes and the wide variation in fluid specific volume make continual

forced recirculation rather impractical, it is appropriate to use a once-through design, in which flow rate

through the furnace is directly proportional to load. Steam flow rate and velocity through the furnace tubes

are critical for cooling the tubes, and with flow proportional to load, low-load operation presents a challenge

to proper furnace tube cooling.

Further, in sliding-pressure mode at low load, the fluid is subcritical, posing specific challenges to heat

transfer and tube cooling. Both departure from nucleate boiling and steam dry-out carry the potential for

elevated tube metal temperatures. These conditions are mitigated or avoided, in part, by providing sufficient

steam mass flow density at subcritical, once-through, low loads. Designing for proper steam cooling effect atlow loads produces very high steam mass flow density and pressure drop at full load in a once-through

design. Therefore, specifying minimum once-through load should be done with careful consideration of its

consequences at full load. Below the minimum design once-through flow rate, recirculation pumps are

usually used to protect the furnace.

Sufficiently high steam mass flow density at once-through loads is provided by use of a small flow area.

Because the furnace perimeter has certain minimum limitations due to conventional firing configurations and

slag control, the challenge of providing a small flow area to envelop a relatively large furnace enclosure

requires special plumbing arrangements. But because sliding pressure operation involves two-phase fluid

over most of the load range, multiple furnace passes with up-down-up flow direction become difficult to

manage, making a single upward flow progression preferable.

The upward flow progression in a single pass is achieved with fewer tubes by laying the wall tubes down at

a low inclination angle rather than hanging the tubes vertically. A given transverse dimension of a furnace

wall normally covered by nine vertical tubes and membrane fins can be spanned by only three inclined tubes

of the same tube and membrane size (Figure 2). Although the furnace cross-section remains rectangular,

this inclined tube arrangement is often called a "spiral" design due to the overall progression of each tube

upward and around the furnace. The tube inclination angle is typically 10 to 20 degrees from horizontal, so

the tube length is three to five times greater than the vertical distance gained.



Source: Riley

Power Inc.

2. Spiral arrangement. The furnace circuit flow area and the tube count can be reduced by inclining the wall

tubing at a low angle.

Special internally rifled tubing could allow a lower steam mass flow density and the use of vertical tubes, but

the range of operating conditions under sliding-pressure operation would make such a system design quite

challenging.

Figure 3 is an example of a sliding-pressure unit designed for Powder River Basin (PRB) coal, with a spiral

arrangement in the high heat-flux zone of the lower furnace. Although much experience has been gained

and many lessons learned from such a furnace wall design, it remains a complicated structure to design,

fabricate, erect, and maintain. Once the tubes rise into a sufficiently low heat-flux zone, the expensive

arrangement is terminated and a transition is made to vertical tubes in the upper furnace. The transition is

commonly accomplished by a ring of forgings around the perimeter of the furnace and an external ring

mixing header. The walls composed of inclined tubes are not self-supporting, so an "exoskeleton" support

system is used, consisting of vertical support straps and load transfer by many welded lugs over the wall

surfaces.



Source: Riley

Power Inc.

3. Sliding-pressure, once-through furnace construction. The lower walls with inclined tubing are supported

by external support straps.

Constant pressure

Two-phase heat transfer crises are not encountered in furnaces maintained at supercritical pressure, so

constant-pressure operation allows greater flexibility and the use of a conventional design. By employing

furnace recirculation smoothly over the entire operating range, low load does not dictate furnace design. As

a result, a furnace can be designed with:

• Vertical, self-supporting, smooth-bore tubes.• A single upward pass with the same simple construction as a conventional drum

unit.•

No intermediate mixing or external piping.

Figure 4 shows a 400-MW Riley Power recirculating supercritical unit with these features. It has powered

South Carolina Electric & Gas Co.'s Wateree Station Units 1 and 2 since 1970.



Source: Riley

Power Inc.

4. Constant-pressure, recirculating unit. This design features vertical, self-supporting, smooth-bore furnace

tubing in a single upward pass.

Beyond plumbing

In addition to incorporating these constructional differences, a sliding-pressure furnace (evaporator system)

must be sized to yield a greater outlet enthalpy (energy content of steam), so it requires a greater heat duty

and furnace size.

To illustrate this, Figure 5 compares the steam generator operating conditions and trends on an enthalpy-

pressure steam diagram. This steam property diagram is used to trace the rising heat content (enthalpy) of

the steam as it flows and loses pressure through the boiler (the series of circled data markers and dashed

lines at right).



Source: Riley

Power Inc.

5. Enthalpy-pressure steam diagram. In sliding-pressure operation, the furnace must absorb proportionately

as much energy as a typical, 1,500-psia industrial boiler.

Sliding-pressure operation during load reductions moves the furnace operation into the subcritical, two-

phase region at loads below 70% to 75% MCR. The nearly horizontal dashed lines in Figure 5 indicate the

trend of furnace inlet and outlet conditions over the load range. To accommodate the two-phase boiling

condition of steam, there are specific steamside conditions that must be fulfilled at the minimum once-

through load, and so it is sometimes low load—rather than full load—that determines the heat duty and size

of the furnace or evaporator system. Those conditions are:

• The economizer size is limited to prevent steaming within it.• The furnace size must be sufficient to produce dry steam in once-through mode to

prevent introduction of liquid water into superheaters.

These requirements are indicated in Figure 5 at the 35% of MCR load condition. A furnace sized for a

certain minimum once-through load produces the indicated conditions at full load, including the total heating

duty (the arrow on the far right) and the furnace outlet enthalpy and temperature. Accordingly, the selection

of minimum once-through load has consequences not only on the steam flow area and the full-load pressure

drop; it also drives the overall furnace size and operating steam and metal temperatures. It is interesting to

note that the sliding-pressure furnace is essentially sized as one would size the evaporator system for a

1,500-psia industrial unit. Often, these medium-pressure industrial units employ a boiler bank or convectiveevaporator section to supplement the boiling heat duty while limiting the furnace size.



In contrast, constant-pressure units stay in the supercritical, single-phase region and therefore have no such

waterside sizing criterion. Figure 6 shows in blue the operating conditions of the constant-pressure, Riley

Power recirculating unit over the same load range. The usual gas-side furnace sizing criteria that apply to

any operating pressure unit—such as firing arrangement requirements, residence time and burnout,

emissions considerations, and exit gas temperature limits for slagging and fouling control—will dictate.

Depending on the particular fuel and fireside conditions, the constant-pressure furnace could be sized as

indicated (the large blue arrow). Note that, although the sliding-pressure furnace must be sized like an

industrial boiler, the constant-pressure furnace can be sized as one would a high-pressure subcritical,

natural-circulation unit (Figure 7).

Source: Riley

Power Inc.

6. Constant- and sliding-pressure operating trends. The constant-pressure furnace size is not driven by the

significant heat of vaporization at lower pressures.



graph Source:

Riley Power Inc.

7. Relative furnace heating duty. Although the sliding-pressure furnace must be sized like an industrial

boiler, the constant-pressure furnace can be sized as one would expect for a high-pressure subcritical,

natural-circulation unit.

But unlike natural-circulation units, the supercritical unit remains flexible in its performance, because it does

not have a fixed evaporator (furnace) end point. Evaporative and superheat duty can be shifted between

furnace and convective surfaces in response to changes in fuel, slagging, or other conditions. This feature is

not limited to Benson, Sulzer, or other once-through designs, and the constant-pressure design retains this

flexibility at all loads. By comparison, a sliding-pressure unit has less flexibility as pressure is reduced and

the margin above saturation (two-phase boiling) decreases.

Nearly as important as this size difference, the furnace outlet temperature of the constant-pressure unit can

be significantly less than that from the sliding-pressure unit (due to this enthalpy difference). Furthermore,

the thermodynamics of steam are such that, at the greater outlet enthalpy level required for the sliding-

pressure unit, temperature is much more sensitive to differences in enthalpy between furnace tubes. This

increased sensitivity is partly mitigated by the heat absorption equalizing effect of the spiral tube

arrangement around the sliding-pressure furnace.

These are especially important points for extension to ultra-supercritical conditions, where it is found that

sliding-pressure designs will have very high furnace outlet temperatures (approaching 1,000F to 1,100F)

and may require advanced alloys for the furnace walls. The various materials research efforts beingconducted worldwide for ultra-supercritical plants are struggling with this issue, partly due to the exclusive



assumption of sliding pressure. Though the furnace outlet temperature with constant pressure also

continues to rise, the potential reduction compared to sliding pressure becomes greater—and furnace

materials ooptions are comparatively broader—as the final steam conditions are advanced.

A visible difference

A constant-pressure furnace designed according to the universal gas-side criteria results in a furnace outlet

steam enthalpy of about 1,050 Btu/lb (at 760F). The equivalent sliding-pressure furnace is about 20% larger

in order to yield the required outlet enthalpy of 1,150 Btu/lb (at 790 to 800F). Because the larger furnace is

effectively accomplishing some of the superheat duty at higher loads, the radiant superheater can be

reduced accordingly, but the net cost increase is positive. Additionally, a particular advantage of the Riley

Power recirculating supercritical design is that it does not require intermediate furnace mixing. That not only

reduces associated piping costs but also permits the use of a close-coupled backpass and eliminates the

tunnel section that would otherwise be required.

The primary differences in furnace construction and size are estimated to result in 4% to 5% greater overall

boiler cost for sliding-pressure designs. For a 650-MW unit, this differential amounts to about $6 million to $7

million, including materials and erection. This cost differential is due to only the tube circuitry, intimate

support, erection, and overall furnace size differences. It does not include further potential differences in

tube materials; tunnel pass elimination; cycling design requirements; and steel, building, or foundation

differences—all of which lead to even greater costs for a typical sliding-pressure design.

Is it worth it?

Can the additional capital investment in a sliding-pressure plant be recovered by operating cost advantages

in the U.S. market? With uncertainty about long-range load dispatching, the efficiency of new plants at low

loads becomes important for considering a plant's payback of capital and, indeed, for dispatch competition.

Many people have been under the impression that sliding-pressure units offer better efficiency (lower heat

rate) than constant-pressure units at reduced loads. The extent to which this is true depends greatly on the

turbine control mode, and so a closer review of heat rate differentials is in order.

Though old, throttle-control turbines at constant pressure indeed suffer in efficiency at part loads,

comparative data from turbine manufacturers indicate that modern, nozzle-control turbines at constant

pressure have nearly the same efficiency as at sliding pressure across the load range. This is mainly due to

the sequential use of the turbine admission valves, and at several loads (the "valve best points") the

remaining valves are fully open and there is negligible throttling loss before the first turbine stage.

Using differential heat rate data from turbine manufacturers, heat rates were evaluated for both constant-

and sliding-pressure systems, with both throttle and nozzle control. Plant operating costs were evaluated at

all loads for each turbine control mode using a detailed economic model including fuel, reagent, and

emissions costs according to typical U.S. conditions.

Even assuming a nightly load reduction to 35% to 80% every night over an entire 20-year evaluation period,

the present value of the difference in operating costs is calculated to be only $0.5 million for PRB coal firing



and less than $1 million for high-sulfur bituminous coal firing of a modern 650-MW unit with nozzle control.

As Figure 8 makes clear, the present value of 20 years of operating cost savings is not nearly enough to

justify the additional $6 million to $7 million capital investment required for the sliding-pressure steam

generator. Meanwhile, the sliding-pressure turbine cost savings are reportedly estimated to be on the order

of $0.5 million and would be partly offset by any additional feedwater heater and steam generator costs to

handle sliding pressure and any associated load and pressure cycling.

Source: Riley

Power Inc.

8. Investment payback. The chart shows simple 20-year present value of operating cost savings with sliding

pressure on a 650-MW unit. Additional cost for a sliding-pressure steam generator is estimated as $6 million

to $7 million.

For cycling service?

For completeness, it should be recognized that continual load cycling and fast start-up abilities may be of

particular value for a limited number of units in each region of the U.S., though the value is relatively difficult

to quantify. Sliding pressure may be justified and viable where such features are especially valued, but

development of these abilities with constant-pressure systems should not be overlooked. Nevertheless, it is

widely believed that any continual load cycling of new coal units, beyond controlled nightly reductions, will be

for a relatively small proportion, to be strategically determined for each grid region. The significant operating

cost advantage of new supercritical units will give these units preference for load dispatch.



In addition, America's installed natural gas–fired capacity—now almost 200 GW—represents a sizeable

sunk investment in generation that is well suited for peaking duty. Though it is expensive to operate, this

capacity is available to meet peak loads and is relatively easy to start up and shut down. This creates a

different environment from that of the 1970s, when such peaking capacity was not available and utilities

were caught not being able to easily cycle their baseloaded units when a recession hit. Independent power

producers considering new coal-fired units should recognize that—armed with economically efficient

generation fired by coal rather than by natural gas—their role in contributing to the regional grid load and

their priority on the dispatch curve will be entirely different, moving from the peaking role into the baseload

and average-load roles.

Regarding start-up, it should be noted that not all of the start-up systems and features employed on modern

generating units around the world are inherently or exclusively applicable to sliding-pressure operation, and

the expense of once-through sliding-pressure steam generators need not be assumed to gain such features.

The Riley Power recirculating units in operation since 1970 already prove the successful application of

recirculation to facilitate start-up of a constant-pressure supercritical unit. For the future generation of coal-

fired plants in the U.S., other modern start-up features can be developed and integrated with appropriate

plant designs for the range of expected domestic needs, for both constant- and sliding-pressure

applications.

constant and sliding-pressure options for new supercritical plants

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