Innovative Boiler Design to Reduce Capitel Cost andConstruction Time

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Power for Generations Siemens Power Generation

Authors: Joachim Franke Rudolf Kral

Power-Gen 2000

Presented originally at

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Introduction – 3

Power Plant Design – 3

Once-Through Operation – 6

Vertical BENSON Evaporator Design – 7

Horizontal Furnace – 10

Studies for the New ConceptConcentrate on Four Aspects – 10

Summary – 12

Content

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Innovative Boiler Design to Reduce Capital Cost andConstruction TimeJoachim Franke, Rudolf Kral Siemens Power Generation

Introduction

The requirements to be met by the next generation of power plants are determined on thebasis of various criteria. The most important factors on the assessment scale are efficiency andenvironmental protection, operational flexibility and power generation costs. The steam genera-tor is of major significance as no other power plant component is more cost-intensive or hassuch a major impact on availability.

The economic efficiency of steam power plants is enhanced by elevating steam parameters tothe supercritical level, making the use of once-through boilers necessary. Among the varioustypes of once-through boilers, the BENSON boiler for which Siemens is licensor exhibits clearadvantages based on its suitability for flexible operating modes with rapid load changes andbrief start-up times.

The market demand for a reduction in manufacturing and installation costs led to the develop-ment of a new BENSON boiler concept incorporating a furnace with vertical tubing.

Power Plant Design

In recent years, the requirements for coal-fired power plants in countries outside of Europehave also shifted from base load towards intermediate peaking and peak load duty. Frequentstartups and shutdowns as well as rapid load changes necessarily lead to a change in operatingmode from constant to variable pressure. The reason for this is that under such conditions,large temperature changes and hence high material loads on the HP turbine can be preventedonly with variable-pressure operation (Fig. 1). In addition, omission of the complex turbine con-trol stage is only possible with variable-pressure operation.

Figure 1 Comparison of different operation modes

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The transition to variable pressure has also had a decisive influence on boiler design as themain steam pressure, which must be increased to over 200 bar in order to enable achievementof higher efficiencies (Fig. 2). Different requirements thus also lead to different power plantconcepts (Fig. 3). While all available types of boilers and turbines can be implemented for base-load operation, for intermediate peaking and peak load duty only once-through boilers suitablefor variable-pressure operation remain. Various typical power plant types have established them-selves based on the differing requirements in force in the various regions (Fig. 4).

Previously, only base-load plants of intermediate efficiency were required in the USA as well asin Southeast Asia. Drum-boiler technology is correspondingly widespread in these areas.

Figure 2 Efficiency potential of advanced coal-fired power plants

Figure 3 Power plant design is defined by the requirements

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A start was made along the road to higher pressures and thus to higher efficiencies in the USAwith a boiler technology requiring supercritical pressure in the evaporator over the entire loadrange. For intermediate peaking duty this method has the disadvantage that part-load operationeither entails a significant reduction in plant efficiency – with throttling downstream of the evap-orator, turbine in variable-pressure operation – or that the number or rates of load changes mustbe significantly reduced – no throttling downstream of evaporator, turbine with control stage infixed-pressure operation.

In Europe and Japan, the combined requirement of higher efficiency and more operating flexibil-ity in intermediate peaking and peak load duty lead to the early implementation of once-throughboilers with supercritical steam parameters and suitability for variable-pressure operation.

The availability of such plants with supercritical pressures is on the same level as that of sub-critical plants (Fig. 5), and the large number of reference plants also demonstrates the maturityof this technology.

Figure 4 Typical power plant design

Figure 5 Energy unavailability not postponable (EU) of German power plants

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Once-Through Operation

With approx. 1,000 units constructed, the BENSON boiler is the once-through boiler with thegreatest representation worldwide. The various types of once-through boilers differ primarily intheir evaporator systems (Fig. 6).

Since the 60s, the evaporators in BENSON boilers have been equipped with tubes welded toform membrane walls in a spiral configuration around the furnace. This method, which hassince been adopted by most boiler manufacturers worldwide, enables parallel upward flow athigh mass flux through all evaporator tubes, and can thus be operated at both subcritical andsupercritical pressures.

In contrast, the Universal Pressure (UP) or Combined Circulation systems introduced in the USAin the 50s with vertical-tubed evaporators and higher mass velocity are suitable only for opera-tion with supercritical pressure in the evaporator. Variable-pressure operation is possible only bythrottling downstream of the evaporator and is thus associated with efficiency losses.

The development of new once-through evaporator concepts with vertical tubes began in the80s. A concept developed by MHI is characterized by a mass velocity in the evaporator tubeswhich is reduced but is still more than double that in natural-circulation systems, and by anadditional evaporator in the convection section of the boiler.

In contrast, the Siemens development of vertical evaporator tubes is characterized by low massvelocities comparable with those in natural-circulation boilers and the simplest possible struc-tural design.

Figure 6 Once-through steam generators

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Vertical BENSON Evaporator Design

Furnaces with spiral-wound tubes can look back on more than 30 years of development, andoperating experience with several hundred boilers. With their high availability, they representthe current state of the art. The only criticism in comparison with vertical tubing is the highercost of manufacturing and installation due to welding of the support straps and the numerousfield welds required. Global efforts therefore target replacement of the spiral-wound tube con-figuration with vertical tubes.

The vertically-tubed furnace with Low Mass Flux design was not feasible until Siemens per-formed further development of rifled tubes to give improved heat transfer. Fig. 7 shows whyheat transfer in a rifled tube is so good, especially during evaporation: Centrifugal force trans-ports the water fraction of the wet steam to the tube wall. The resulting wall wetting causesexcellent heat transfer from the wall to the fluid. This has the following advantages over smoothtubes:

Figure 7 Wall temperatures and boiling crisis of tubes

No deterioration of heat transfer even in the range of high steam quality

Very good heat transfer even at low mass flux

Only slight increase in wall temperature in case of film boiling near critical pressure (intervalfrom about 200 bar to critical pressure)

Potential for increased heat transfer by optimization of rifling geometry.

Changes in rifling geometry permit significant improvements in heat transfer to be achieved(Fig. 8). A Siemens high-pressure test rig – the largest in the world, with an electrical heatercapacity of more than 2,000 kW – was used to generate more than 150,000 data points in aninvestigation of standard commercial rifled tubes and of tubes with modified rifling geometry.For unchanged inner wall temperatures it proved possible to reduce the mass velocity byapproximately 25% compared to the use of standard commercial rifled tubes.

Rifledtube

Rifledtube

Fluid

Steam quality

1.0

0.8

0.6

0.4

Smoothtube

Smoothtube

Pressure: 150 bar

Mass flux: 500 kg/m2s

Heat flux: 300 kW/m2

100 200 300 400 500 600

Inside wall temperature (°C)

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The Low Mass Flux design mentioned in the foregoing not only enables downward extensionof the output limits for vertical tubes to 300 or 200 MW and use of large-diameter tubes, but inparticular it also changes the flow characteristic of a once-through system: With increased heat-ing of an individual tube, the throughput of that tube increases instead of decreasing.

Figure 8 Optimized rifled tubes reduce wall temperatures or allow mass flux reduction

This flow behavior – well known from drum boilers – is called a natural circulation or positiveflow characteristic.The standard flow characteristic of once through boilers, where excess heating impedes the flowin individual tubes, is transformed into a natural circulation characteristic when the full-load massflux is reduced to the value known from drum boilers.For an evaporator tube with 25% higherheat input than an average tube this transition is plotted in Fig. 9 as a function of mass velocity.The example is valid for a supercritical 600 MW BENSON boiler with vertically tubed furnace. Anatural circulation characteristic is established below a mass flux of roughly 1,050 kg/m2s. Belowthis value the increase in outlet temperature is more or less compensated by a correspondingincrease in mass flux in the individual tube. At values above 1050 kg/m2s the throughput de-creases with increasing heat input and the outlet temperature increases disproportionately.

The theoretical conclusions for this concept regarding flow distribution with non-uniformheating were tested in practice in the supercritical 320-MW FARGE plant.

A 47.00 m high furnace heat exchange surface of Low Mass Flux design was in trouble-freeoperation at the FARGE plant for more than 10,000 hours. This confirmed the calculation funda-mentals and at the end of trial operation the tubes were still practically as good as new, withthe rib profile not smoothed by deposits.

In addition, the thermohydraulic principles of Low Mass Flux design have already been provenin commercial operation in the horizontal heat recovery steam generator at the COTTAM GUD®combined-cycle power plant. The parallel tubes of the BENSON evaporator for the HP and IPstages arranged sequentially in the exhaust flow path are characterized by extremely differentheat uptakes to which the mass flow automatically adjust (Fig. 10).

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Figure 9 Mass flow characteristic of vertically tubed BENSON boilers

Figure 10 Heat and mass flow distribution in the HP evaporator of a once-through HRSG

The characteristics of the vertically tubed furnace can be summarized as follows:

Mass flux reduction from 2000 to 1000 kg/m2s flow characteristic as in drum boilers:increased heat input to an individual tube increases throughput in that tube

Cost-effective fabrication and assembly

Minimum BENSON output: 20%

Simple startup system for 20% evaporator throughput

Reduced slagging on furnace walls

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Horizontal Furnace

The scientifically founded design basis, the results of the practically oriented large-scale testand the operating experience with the BENSON heat recovery steam generator at the CottamCCPP have enabled the establishment of a solid knowledge base for the next step of innova-tion, the horizontal furnace boiler (HF-boiler).

In the HF-boiler, the convection section with the horizontal and vertical passes is located in thegas path downstream of the horizontal furnace, and is largely identical with proven two-passboilers (Fig. 11). The vortex burners are shown here in a front configuration. HF-boiler evaporatortubes are vertical. This concept is characterized by a very low building height. The potential re-duction in structural steelwork, connecting lines to the turbine as well as installation costs andtime in comparison with traditional single and two-pass designs is clearly evident.

Figure 11 Steam generator with horizontal furnace for 350 MW

Studies for the New Concept Concentrate on Four Aspects:

Firing: NOX formation and unburned carbon must be evaluated. In addition, the horizontalflame orientation may effect heat flux distribution and thus have an impact on the flowdesign of the furnace walls.

Furnace walls: The evaporator tubes can only be vertical. The influence of the large heatinput differences of 3:1 and more over the length of furnace on the outlet temperature ofthe evaporator tubes connected in parallel must still be investigated.

Transition from furnace to horizontal section: Large temperature differences occur herebetween adjacent tubes and tubes which are welded together.

Reinforcement of surrounding walls needs attention because of relatively large spans.

The problem of the water/steam side design of the furnace walls is characterized by the factthat an evaporator tube in the main combustion zone has a heat input roughly 3 times as greatas at the furnace outlet. Despite this unfavorable situation, the outlet temperatures from the in-dividual parallel tubes must under no circumstances exceed either allowable maximum values

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Figure 12 Mass flow distribution adjusts automatically to heat flux distribution

or set tolerance limits from other tubes. Once again, as already seen in case of HRSG Cottamthe Low Mass Flux design provides the solution to this problem (Fig. 12). The natural circulationcharacteristic described earlier results in a higher mass flux and thus greater throughput in atube with higher heat input than in a tube with lower heat input. This behavior is further sup-ported by different dimensioning of the tubes and fins.

Fig. 12 also illustrates the effect of this design on the tube outlet temperature: The temperaturedifference between any two tubes is less than 50 K, with that between adjacent tubes evenless, below 30 K. Tube throughput also automatically adjusts to new heating conditions, such asa change in heat input due to fouling or soot blowing.

A boiler with a horizontal furnace is only about 30 m high, which allows simple and fast installa-tion. The heights of comparable conventional steam generators are between 60 m and 90 m(Fig. 13).

The advantages of this horizontal, low-profile design are obvious:

Reduced cost of structural steelwork

Straightforward installation

Short installation time due to parallel installation of the furnace, lateral pass andvertical pass

Shorter steam lines between boiler and turbine.

The modular design of the boiler with its furnace, lateral pass and vertical pass also makesadaptation to other power ratings and other fuels a relatively simple matter. A 700-MW plantwith dual furnace, for example, has only twice the width but otherwise the same dimensionsas a 350-MW boiler.

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Summary

The requirements for modern power plants with regard to efficiency and operating behaviornecessarily lead to the implementation of once-through boilers. On the one hand, this boilertype enables supercritical steam conditions, while on the other it is especially suitable forvariable-pressure operation.

The further development by Siemens of the BENSON once-through boiler with vertical-tubefurnace walls constitutes a step in the direction of simpler and thus more cost-effective designwith improved operating behavior.

The development is based on extensive fundamental research in heat transfer and pressuredrop in rifled tubes. Optimization of the rifled tubes enables a significant reduction in mass fluxand hence a furnace wall flow design yielding increased throughput in tubes with higher heatinput. This behavior is known from natural circulation evaporators. In addition to the cost reduc-tion, the reduction of the minimum BENSON output to 20% is especially attractive, as thismakes night and weekend shutdowns unnecessary in many cases.

Figure 13 Size comparison of coal-fired steam generators with horizontal furnace for 550 MW output

The development of the HF boiler focused on reducing investment costs by incorporating theoperational advantages of vertical tubes. Investigations of possible problem areas were initiatedand solutions found. The HF boiler will constitute a milestone in boiler construction for manufac-turers and operators alike. The cost advantages continue to increase with increasing steam con-ditions.

Tower Two Pass Horizontal

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This paper is based on a lectureheld at Power Gen 2000

Published by and copyright 2000Siemens Power GenerationFreyeslebenstraße 191058 Erlangen, GermanyPhone: +49 9131 18-37 87E-mail: contact@erl11.siemens.dehttp://www.siemens.de/kwu

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