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.
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