Synopsis:Clean Coal Combustion for India:Circulating Fluid Bed and Advanced Supercritical Technologies

Authors: ( All of ALSTOM Utility Boiler Business)Dr. G. Scheffknecht, Technical Director J. Seeber, Head of Fluidized Bed Firing

Mark Palkes, Senior Consultant John M. Banas, Consulting EngineerGerhard Weissinger, Head of Thermal Engr. Werner Kessel, Director, Boiler Engineering

Electricity generation costs, like all forms of energy conversion, are influenced by fuel and operating costs and

by the added necessary costs of protecting the environment. Due to the relatively higher cost of natural gas and

oil, coal will continue to be a major resource for India's power generation needs. The challenge to today's power

plant owners and operators is to select coal generation technologies that provide reliable, efficient and clean

power at competitive costs.

This paper will discuss two Clean Coal technology options for achieving these goals. The first of these is the

use of supercritical steam cycles to achieve higher plant efficiencies that translate directly into lower fuel usage

and lower emissions per Kwhr produced. The design and performance of state-of-the-art supercritical steam

generators capable of sliding pressure operation is reviewed. Details are given on major components of the

boiler design including environmental performance.

The second Clean Coal technology to be discussed is Circulating Fluid Bed (CFB), which is an effective way to

utilize lower cost indigenous Indian coals with a high sulphur content. Meeting environmental requirements

while using these coals requires high desulphurisation efficiency. The paper describes the design of a 2 x 125

MW CFB plant currently under construction in India. Local lignite with a sulphur content of more than 8% (daf)

will be utilised. The targeted sulphur capture ratio of 97 % and low NOX values will create an environmentally

friendly plant with lower fuel costs and good reliability. .

Both of these technologies provide effective options for the Indian power industry to continue to meet the needs

of industrial, commercial, and residential customers for clean, reliable energy.

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1.0 State-of-the-Art Sliding Pressure Supercritical Steam Generators

The new generation of supercritical boiler designs offers full sliding pressure, true cycling capability andsimplified start-up systems without complex throttling valves of past designs. Circulation system design andoperating considerations for waterwall protection throughout the load range are described. The choice betweenconventional vertical waterwall and spiral wall arrangements is governed by plant size (MW), steam cycleparameters, fuel properties, and firing system design. Actual test and field experience are reviewed. Futuredevelopment of supercritical boiler technology for advanced steam conditions is also included.

2.0 Introduction to Supercritical

Due to fuel prices and environmental concerns, plant heat rate plays an important role in the selection of themost cost-effective thermodynamic cycle. A supercritical steam cycle enables higher plant efficiencies from theuse of increased operating pressure coupled with elevated steam temperatures. Figure 2.1 illustrates theefficiency advantage of supercritical power plant cycles over subcritical designs at a range of steamtemperatures and pressures. In addition, plant efficiency could be further improved by approximately 2% withthe installation of a second reheater that had been used on some supercritical designs in the past.

The corollary of higher efficiency is lower fuel consumption and lower emissions for the same unit of electricaloutput. It also means that for every pound of coal that does not have to be burned, there is a pound of coal thatdoesn’t need to be purchased, transported, stored and pulverized. If that coal is not burned, it is not necessary tocollect and dispose of the residues of combustion. As a direct function of efficiency, CO2, NOx, SOx,particulates, VOC, CO and trace metal emissions are reduced in proportion to improved efficiency.

Net Efficiency vs. Design Data

35

36

37

38

39

40

Net

Effi

cien

cy (H

HV)

[ %

]

Live Steam Press. 2407 3625 3915 [ psig ]Live Steam Temp. 998 1005 1050 998 1005 1050 998 1005 1050 1050 1050 1085 [ °F ] Reheat Temp. 1000 1040 1112 1000 1040 1112 1000 1040 1112 1112 1112 1148 [ °F ]Feedwater Temp. 500 525 555 [ °F ]Numb. of Heaters 7 8 7 8

Design Data

(500) - 600 - [700] MW ClassCond. Pressure : 1.23 psi

Figure 2.1Net Efficiency (HHV) vs. Steam Cycle Design

Supercritical designs represent approximately 20% of today's coal-fired steam plants market. New projectinquiries are increasingly specifying supercritical technology and it is anticipated that the share of supercriticalcycles will continue to increase.

ALSTOM has extensive experience in the design and development of supercritical technology and has thelargest market share worldwide, offering a range of design options for state-of-the-art supercritical boilers. Thenew generation of coal fired supercritical steam generators offers full sliding pressure operation and cyclingcapabilities.

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3.0 Historical Experience of Supercritical

3.1 U.S. Experience with Supercritical Steam Cycles

The modern pulverized coal boiler is the product of over 80 years of design development and improvement.The first generation of supercritical units was designed for steam temperatures of 1000oF (538oC). Continuedadvances in metallurgy allowed steam conditions to be increased. In 1959, this progress culminated with theEddystone No. 1 unit of the Philadelphia Electric Company, with a boiler supplied by ALSTOM. Designed withsteam conditions of 5300 psig (365 bar), 1210oF (654oC) and double reheats of 1050oF (566oC), Eddystone 1had the highest steam conditions and efficiency of any electric plant in the world. The generating capacity (325MW) was equal to the largest commercially available unit at the time. However, as a result of limitations ofmaterials of that era, main steam throttle conditions were reduced to 4300 psig (297 bar) and 1125o F (608oC) topermit more reliable operation. Nevertheless, the plant represents the first benchmark in what has been aprogressive advancement in steam conditions.

In the 1960’s, the industry retreated from these high steam conditions to more conservative 3500psig/l000oF/1000oF (241 bar/538oC/538oC) designs as a compromise between the heat rate benefits of advancedconditions and the attendant metallurgical limits of existing turbine and boiler materials. It was in that periodthat ALSTOM introduced Combined Circulation® boilers. An improvement on the earlier supercritical design,these boilers combined the design features of previous once-through boilers with those of subcritical ControlledCirculation® type units. Recirculation flow was superimposed on the once-through flow during start-up, low andintermediate loads, assuring adequate cooling protection to the waterwalls (Figure 3.1.1).

Figure 3.1.1Simplified flow diagram of Combined Circulation unit

Both the early and the later designs, however, had one limitation -- they were essentially base loaded withlimited cycling capability. While sliding pressure operation could be achieved in the superheater sections, fullload pressure had to be maintained in the furnace walls to avoid film boiling and tube overheating which couldoccur at subcritical pressures. Constant supercritical pressure was maintained in the furnace walls by boilerthrottling valves (BT and BTB valves). In the earlier designs, sliding pressure operation in the superheater wasrestricted to 30% rating and below as illustrated on Figure 3.1.2.

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Figure 3.1.2Constant pressure program for Combined Circulation steam generators

In later designs, the throttle valve complex of BT and BTB valves was redesigned to enable sliding pressure inthe superheater over a wider range while still maintaining the waterwalls at full pressure. Figure 3.1.3 shows asystem designed for sliding superheater pressure up to 80% rating. This design did provide some of theadvantages of sliding pressure, including reduced turbine thermal stresses and an extended reheat temperaturecontrol range. Feed pump power, however, was not reduced and the valves were a source of operationalcomplexity and maintenance.

Figure 3.1.3Sliding pressure program for Combined Circulation steam generators

While the 1960's saw considerable orders for supercritical power plants, especially in North America, a decadelater the demand for them had plummeted. No supercritical units have been ordered in the US from 1978 to2001. There were a number of reasons why the U.S. industry retreated from the supercritical plant. Among themwere shifts in the market environment caused by the increase in nuclear power for baseload application and theavailability of relatively inexpensive fossil fuel. The existing supercritical power plants lacked the operationalflexibility needed for load following duty required by the U.S. electric grid.

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In addition, the industry's belief at the time was that the supercritical plants were less reliable and available thanthe subcritical units. This belief has been dispelled by a number of comprehensive studies. These independentstudies concluded that there is no significant difference in availability due to subcritical/supercritical steamparameters for today’s plant designs (References 1, 2, 3).

Figure 3.1.4

Comparison of Subcritical and Supercritical Cycle Availability(Data Source: NERC)

3.2 European Experience with Supercritical Steam Cycles

Due to special conditions of the European markets, steam generators had to be designed for cycling duty. Theseboilers enabled quick start-up, shutdown and rapid load changes. To permit this mode of operation in aneconomically acceptable manner a once-through flow boiler with a spiral-wound furnace configuration wasdeveloped. The design provides sliding pressure operation and has been successfully used for both subcriticaland supercritical designs. The boiler throttling valves are completely eliminated from the design and the furnacewalls are allowed to enter the subcritical pressure range along with the superheater circuits over the entire loadrange as required.

Figure 3.2.1 shows four typical 700-750 MW bituminous coal fired once-through steam generators. They areinstalled at the Scholven F, Bergkamen A, Bexbach I and Heilbronn Unit 7 power plants in Germany.

Figure 3.2.1Bituminous Coal Fired Once-Through Boilers of Capacity Class 700 – 750 MW

0

2

4

6

8

10

12

14

EFOR %

Plant (Super) 13.347 12.077 9.668 7.685 7.534 7.482Plant (Sub) 10.405 9.439 8.16 6.793 7.103 7.013Blr (Super) 8.441 7.285 5.823 4.872 4.434 4.023Blr (Sub) 5.928 5.464 4.344 3.811 3.926 4.018

1982-1984 1985-1987 1988-1990 1991-1993 1994-1996 1997

These plants went into operation in the years between 1979 to 1985. The boilers are of a tower type and aredesigned for sliding pressure operation down to 35% minimum once-through load. The boilers are equippedwith the tangential firing systems. Each boiler has 16 fuel nozzle pairs. The nozzles are installed in the cornersat four elevations with four twin nozzles per elevation. The firing system includes four bowl mills installedalong one of the sides of the boilers. One mill delivers coal to a level of nozzles. The Bexbach I plant isequipped with an induced draught fan, a forced-draught fan, a primary air fan and an air preheater all sized at100% capacity. The other three have a more traditional arrangement and are equipped with two of the abovecomponents each one sized to operate at 50% capacity. Figure 3.2.2 shows a supercritical unit at the GKM Mannheim Central Power Station commissioned in Germanyin May 1982. Design conditions are shown. A requirement for high efficiency was imposed on the design ofthis plant. Because of this requirement, a double reheat cycle was selected. In addition, the design incorporatesa recuperative heat exchanger installed outside the boiler proper. It was applied in place of a desuperheater forreheat steam temperature control. Desuperheating of reheat steam is generally not desirable because of thenegative effect on plant heat rate. In the heat exchanger design solution, excess temperature is removed bytransferring heat from the superheater to the high pressure reheater and to the low pressure reheater.Consequently, heat losses associated with spraying water into the reheat steam flow are eliminated.

475 MW Once-Through

Of more recent designs there are two steam gGermany in 1993 (Figure 3.2.3). These superccross section of 24X24 m and a height of 161 mgenerators in operation worldwide.

Live Steam 3990 psig /275 bar (design pressure)

986 °F /530°C3,015,000 lb/h/ 1345 tonnes/hr

Reheater Steam (1st Stage) 1260 psig//86 bar 1005 °F / 541°C 2,640,000 lb/h / 1178t onnes/hr

Reheater Steam (2nd Stage) 261 psig/ 18 bar 986 °F /539°C 2,068,000 lb/hr/ 923tonnes/hr

Feedwater 590 °F/ 310°C

Fuel: Ruhr and Saar Coals

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Figure 3.2.2Boiler (Double Reheat) with DeNOx Plant

enerators for the Schwarze Pumpe power station constructed inritical boilers were designed to fire brown coal. With a furnace, these steam generators are among the physically largest steam

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Live Steam4130Psig/285 bar (design pressure)

1017 °F/547°C

5,420,800 lb/h /2420tonnes/hrReheater Steam

827 Psig/57 bar1050°F/565°C4,745,000 lb/ 2118 tonnes/hr

Feedwater523 °F/273°C

Fuel Brown coal

Figure 3.2.3Power Station Schwarze Pumpe

Another recent design is a 1000 MW coal fired boiler, Niederaussem K, in Germany. The most importanttechnical data is summarized in Table 3.2.1.

Table 3.2.1Once-Through Boilers Reference List – Typical Units

sioning [MW] [KPPH/tonnes/hr [psig/bar)[°F]

Patnow 2004 Brown Coal 4603012/1345 4205/290

Wai Gao Qiao 2003Bituminous

Coal2 x 900 (980)

6247/2789 4045/279 1008/1055

Yonghung 2003Bituminous

Coal2 x 800

5410/2415 3930/271 1056/1056

Niederaußem K 2002 Brown Coal 1,0125860/2662 4205/290 1075/1112

Florina 2002 Lignite 3302278/1017 3800/262 1010/1008

Mai Liao 2000Bituminous

Coal2 x 600

4368/1950 3841/265 1005/1055

Schwarze Pumpe 1997 Brown Coal 2 x 8005420/2420 4135/285 1017/1050

Poryong 3 & 4 1993 ..Bituminous

Coal2x 500

3852/1720 3840/265 1005/1005

Vestkraft Unit 3 1992Bituminous

Coal400

2420/1080 4000/276 1040/1040

Shidongkou II 1992Bituminous

Coal2 x 600

4250/18973885/268

1005/1055

GKM MannheimBoiler 18

1982Bituminous

Coal475 3068/1370 3990/275

986/1004/986

Scholven, Unit F 1979 ..Bituminous

Coal4 x 750 4928/2200 3335/230

995/995

TemperatureDesign PressureSteam CapacityElectrical OutputFuel TypePlant Commis-

Year ofSH/Reheat

[°C]

544/568

542/568

569/569

580/600

543/542

540/569

541/569

541/541

547/565

1010/1055

560/560

530/540/530

535/535

7

3.3 Asian Experience with Supercritical Steam Cycles

During the past two decades supercritical technology has captured a growing share of the Asian market. Themajor driver for the increased commercial activities in supercritical plants is due to high fuel cost andheightened environmental awareness including growing concern about greenhouse gas emissions. In Japan,essentially all of the new steam power plants of the 1990s are supercritical. South Korea, China and Taiwanhave accepted supercritical technology on par with subcritical drum type designs. All supercritical unitsconstructed in Asia are capable of variable waterwall pressure. Some of the plants were designed to operate atsteam turbine inlet temperature as high as 1112o F (600o C). Most recent designs are represented by 2X800 MWboilers for the Korean Electric Power Company utility at Yonghung, 2X900 MW boilers for Shanghai Electricat WaiGaoQiao in Shanghai, China, as well as 2X600 MW boilers for Formosa Plastics at Mai-Liao, Taiwan(Figure 3.3.1).

Figure 3.3.1 Mai-Liao, Formosa – 2 x 600 MW – Coal fir4.0 Once-through Supercritical Technology for Flexible Oper

The design of a thermal plant is governed by the operating duty specdemands high operational flexibility from power plants which, in turfor a variable load program and two-shift operation (i.e. load followthese requirements, excellent dynamic behavior and high load gradie

Modern supercritical designs have greater operational flexibility as asuited for cycling duty. However, the requirement for daily cycundesirable thermal stresses especially in the steam turbine. Thessliding pressure operation that significantly minimizes temperature ddesigning more flexible units as well as extending component life operation with once-through boiler systems.

There are a number of variable pressures versus load programs programs differ mainly in the level of unit load at which sliding pressure operation (Program 3 in Figure 4.1) requires that the turbinoperation. Consequently, turbine throttle pressure is proportional toperation, however, has a disadvantage -- boiler response to changesnot satisfy the power-system control requirements. Compared wsignificantly larger thermal storage capacity. For this reason, the mopressure program (Program 2 in Figure 4.1) is more frequently usedoperation, a turbine is operated with a certain amount of throttle ressection at the turbine is altered briefly when the load is varied, sdischarged at once.

Live Steam3841 psig/265 bar (design pressure)1005 °F/541°C3,948,000 lb/h /1763 tonnes/hr

Reheater Steam1056 °F/569°C

Fuel: Bituminous coal

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ed Sliding Pressure Supercriticalation

ified by the plant owner. Optimal economyn, often requires that the plants are suitableing/cycling operation). In order to satisfy

nt accommodation are absolutely essential.

result of their lower thermal inertia and areling and/or two shift operation can createe concerns can be minimized by adoptingifferences in the turbine. The emphasis of

has led to the wide use of sliding pressure

available as shown in Figure 4.1. Thesepressure operation occurs. Natural slidinge inlet valves are fully open during normalo the steam flow. Natural sliding pressure in load demand is relatively slow and mayith the steam turbine, the boiler has ade of operation known as modified sliding. In the modified sliding pressure mode oferve for rapid change. The admission crosso that accumulated steam in the boiler is

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psig4350

3625

2900

2175

1450

725

0

Figure 4.1Pressure operation mode at boiler outlet

5.0 Furnace Walls Design for Supercritical Steam Cycles

5.1 Basic Design Considerations

Furnace walls are formed by finned or fusion welded tubes that form a continuous water-cooled envelope. Themajor concern for once-through operation is designing for sufficiently high mass velocity to ensure cooling ofthe furnace tubes.

Drum units maintain the proper mass flows by the use of either natural or forced circulation. These boilers aredesigned to generate steam in the furnace walls under nucleate boiling conditions. Nucleate boiling ischaracterized by formation and release of steam bubbles at the surface-liquid interface with the water continueswetting the inner surface of the tube

In a once-through boiler, waterwall mass flow changes in direct proportion to steam flow. In the supercriticalpressure region, the fluid inside the tubes is heated and the heat is directly converted into a higher temperature.In the subcritical once-through pressure region the process of heat transfer is more complicated and the processinvolves a change in phase from liquid to steam as well as superheating. In most practical situations, a fluid at atemperature below its boiling point at the system pressure enters a furnace tube in which it is heated so thatprogressive vaporization and slight superheating occurs. The process of heat transfer during vaporizationdepends on many variables. As the quality of the steam-liquid mixture increases, various two-phase flowpatterns are encountered as illustrated on Figure 5.1.1.

Figure 5.1.1DNB and DO Phenomenon

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Boiler designers must be concerned with two conditions that may occur when a boiler operates in the subcriticalpressure region. These conditions are Departure from Nucleate Boiling (DNB) and Dryout (DO). DNB and DOare characterized by formation of a flow of steam which covers the inner surface of a tube resulting in a sharpdecrease in heat transfer coefficient and consequent high metal temperature rise. DNB typically occurs at highersubcritical pressures and high heat fluxes and low steam contents and flow velocities. DO occurs at relativelyhigh steam content and flow velocity. Both DNB and DO may occur during transitional (subcritical) operationwithin some waterwall areas in a once-through boiler. However, through the use of sophisticated furnace designprograms, ALSTOM boiler designers mitigate the impact of DO/DNB conditions. In addition, materials areselected which accommodate these conditions and assure reliable operation. Once-through operation brings about a second design challenge; namely avoiding potentially damaging stressesresulting from temperature differences at the furnace wall outlet. A drum unit always operates with saturationtemperature in the waterwalls and all circuits are at the same fluid temperature. With a once-through design, thesteam outlet of the furnace walls is slightly superheated, i.e. outside of the steam dome or saturation region.Therefore, tube circuits can be at different temperatures caused by variation in heat absorption patterns aroundthe furnace perimeter. These temperature differences must be maintained within acceptable limits. Designstrategies to deal with the above concerns have been well proven and are described below.

5.2 Spiral Wall Design for Supercritical Steam Cycles

The spiral wall design has over thirty years of experience and can be applied to all unit sizes, pressures andfuels. The basic concept of the spiral wall is to increase the mass flow per tube by reducing the number of tubesrequired to envelop the furnace wall without increasing the spacing between the tubes. Figure 5.2.1 illustratesthis concept.

• Reduced number oftubes with pitch.

• Increased massflow.

• Mass flow rate canbe chosen bynumber of paralleltubes.

Features

Figure 5.2.1Spiral Wall Design

Figure 5.2.2 shows a comparison between the inclined tube arrangement and the vertical tube arrangement.

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Figure 5.2.2Evaporator Wall Design

The spiral furnace wall system employs smooth bore tubes that require high mass velocity to provide acceptabletube cooling. The high mass velocity produces high film conductance that ensures low metal temperatures andthus economical selection of tube materials. Fewer and longer tubes combined with higher mass flow rate,however, produce higher pressure drop in the furnace walls.

For a given furnace size and fuel selection, the expected heat flux rate determines the mass velocity rate requiredto ensure cooling of the furnace wall tubes. With a spiral wall design, the number and size of the tubes isselected to provide sufficient cooling over the entire load range. Additionally, by spiraling around the furnace,every tube is part of all four walls, which means that the difference in length between the furnace tubes isminimized and that the heat pickup by individual tubes is approximately the same. This makes the spiral wallsystem less sensitive to changes in the heat absorption profile in the furnace. The measured steam temperaturesat the outlet of the spiral wall tubing for an 800 MW supercritical boiler are shown in Figure 5.2.3.

Figure 5.2.3Evaporator Temperature at Spiral Outlet

0

100

200

300

400

500

Tem

pera

ture

at s

pira

l out

let 100 % Load

leftside wall front wall

rightside wall rear wall

40 % Load

930

oF750

570

390

210

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Practical design considerations require a vertical waterwall configuration in the upper furnace region. Thistransition to a vertical wall is accomplished in a zone where heat fluxes are relatively low and the requirementson tube cooling are not as high as in the lower furnace zone. The transition requires the use of an intermediateheader or bifurcated/trifurcated fittings. Typical waterwall construction includes 1.5”(38.1 mm) OD tubes in thespiral portion and 1.25”( 31.75 mm) OD tubes in the vertical portion of the furnace.

Because the furnace wall tubes are at an angle, there is a need to transfer some of the weight load from the spiralfurnace tubes to support straps. The support straps are welded to the spiral walls by means of scalloped clipsand transfer the collected load to the vertical wall tubes by means of finger straps (Figure 5.2.4).

Figure 5.2.4Wall Furnace Supporting Structure

HorizontalBuckstay

VerticalBuckstay

TensionStrap

TransitionZone

SpiralTubes

FingerStraps

CornerAssembly

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5.3 Vertical Wall Rifled Tubing

As an alternative to the spiral wall designs, ALSTOM has developed a design that uses conventional verticaltube walls for ease of construction and maintenance. The design is a derivative of the Combined Circulation®

design that ALSTOM has continually improved since its introduction in 1961. Rifled tubing is used in thefurnace walls.

A vertical wall configuration typically employs either 1-1/4” (31.8mm) or 1-1/8” (28.6mm) O.D. internallyrifled tubes (Figure 5.3.1).

Figure 5.3.1Rifled Tube

Rifled tubing offers significant advantages over smooth tubing in the evaporation range of subcritical pressureboiler operation. The advantages are best summed up by Figure 5.3.2.

Figure 5.3.2Comparison of wall temperature between rifled and smooth tubes (pressure 2990 psig)

Figure 5.3.2 compares the inside wall temperatures of smooth and rifled tubes for three different heat flux rates

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at a pressure of 2900 psig (200 bar). It is evident from this figure that for smooth tubes the wall temperaturerises steeply, even at relatively low steam qualities. The increase occurs earlier in the case of higher heat fluxrate and the maximum values are higher. With rifled tubes, the sudden rise of the wall temperatures is displacedtowards higher steam qualities. The use of rifled tubing therefore permits much lower mass flows with the samemargin of protection against DNB and overheating.

Extensive testing at ALSTOM’s Power Plant Laboratories has characterized the heat transfer and flow behaviorof rifled tubing for vertical tube furnace application. Actual size and rib configuration tubes were subjected tothe full matrix of heat flux, mass flow, and pressure conditions which will be experienced by these units duringsliding pressure mode of operation (Figures 5.3.3 and 5.3.4). Of particular interest was flow and heat transferbehavior in the transition zone between the supercritical and subcritical pressure. Engineering development ofvertical wall rifled tubing steam generators has involved detailed analytical evaluations of the furnace walls foroperating parameters shown on Figures 5.3.3 and 5.3.4.

Figure 5.3.3ALSTOM rifled tube test program – heat flux vs. pressure

Figure 5.3.4ALSTOM rifled tube test program – mass flow vs. pressure

Rifling promotes turbulence and aids in wetting the inside tube surface, consequently, increasing the nucleateboiling quality for a given heat flux, mass velocity, and pressure. Similar to the spiral-wound design, thewaterwall panels can be formed by either fin or fusion welded tubes. Typical average mass velocity per tube ismuch smaller than for the spiral arrangement. However, more than adequate margin of safety is provided by thecombined effect of a smaller diameter tube and inside tube rifling.

All supercritical units must be designed to minimize temperature differences in the furnace walls. Waterwalloutlet temperature deviations for vertical wall sliding pressure supercritical units are minimized in the samemanner as on the earlier Combined Circulation® designs. This is accomplished by installing individual tubeorifices, which distribute flow in conjunction with heat absorption by each circuit. Designers are able to select

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correct size orifices and prescribe required quantity of cooling flow because ALSTOM’s tangential firingsystem generates a well-defined heat absorption pattern across the furnace wall width. A typical heat absorptionprofile, which is constructed applying empirical data obtained from numerous field tests, is illustrated in Figure5.3.5 for a MCR load. Similar standards are available for lower loads.

Percent of wall dimension, corner to corner

Figure 5.3.5Predicted lateral water wall heat accumulation

Detailed waterwall analysis based on operating experience shows that satisfactory temperature differentialsthroughout the entire operating load range can be achieved with sliding pressure operation. The temperaturedifference between the maximum and minimum value is well within acceptable range. In addition, orifices areinstalled in a supply sphere (Figure 5.3.6). These orifices insure that the prescribed amount of flow isdistributed to each wall. Their main task is to ensure adequate flow distribution to each wall at lower loads.

The viability of a vertical wall rifled tube technology was demonstrated in Japan in the successful design andoperation of 700–1000 MW coal fired and 700 MW gas fired boilers.

Figure 5.3.6Furnace Waterside Arrangement

Hea

t Abs

orpt

ion

Q E

FF /

QA

VG

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6.0 Boiler Concepts for Reheat Steam Generators

6.1 Available Boiler Concepts

Two principal arrangements of heating surfaces are utilized by ALSTOM for reheat steam generators. These arethe pendant panel surface (two-pass) and the horizontal surface (tower) designs, both of which are used insubcritical and supercritical cycle applications. Regardless of the boiler concept, the location of the varioussuperheat, reheat, and economizer sections in the flue gas path is determined by:

• the temperature difference between the gas and internal fluid• the method of steam temperature control• the allowable metal temperature limits

The decision to use a pendant panel design versus a horizontal design is not dependent on the cycle choice (i.e.,subcritical vs. supercritical). Each of these configurations has its advantages and allows for customer preferenceas a factor in the final arrangement of the heating surface. The following briefly highlights each design.

6.2 Pendant Panel Design

A pendant supercritical unit surface arrangement is illustrated in Figure 6.2.1. It shows superheat panels andplatens above the furnace. The superheat panels are the primary superheater surfaces, while the platens are thesecondary. The temperature entering the primary superheater on a supercritical unit is relatively high since thefluid leaving the furnace walls is already slightly superheated. The temperature is approximately 800ºF (427ºC).This high fluid temperature makes it impractical to place the primary superheater in a low gas temperature zone.The difference between the fluid temperature in the tubes and the gas temperature over the tubes must bemaintained at a sufficient level to ensure efficient convective heat transfer.

The final superheater is located in the tunnel between the final reheat section and the backpass. Because ofhigher operating pressures, supercritical designs require tubes with thicker walls and/or a smaller diameter whencompared to subcritical designs. The thicker walls result in higher metal temperatures and consequently requireapplication of more advanced alloys. The installation of the finishing superheater in the tunnel, in a lower fluegas temperature zone and away from high radiation of the furnace, enables the designers to maintain lowermetal temperatures, thus minimizing the use of expensive alloys.

The final reheater is installed before the final superheater. In the design shown in Figure 6.2.1, reheattemperature is controlled primarily by fuel nozzle tilts. This system provides for altering the gas temperatureleaving the furnace by changing the location of the fireball inside the furnace. Fuel nozzle tilts rapidly adjust tochanging furnace conditions in order to maintain the desired steam temperature leaving the reheater. In order toeffectively control reheat steam temperature through the use of fuel nozzle tilts, the heating surface must belocated such that it is exposed to both radiant and convective heat. In this way the fuel nozzle tilt has amaximum effect on the reheater surface.

The first stage of reheat, because of its lower inlet temperature, is placed just ahead of the economizer in thebackpass resulting in lower overall boiler surface requirements. In addition, with this heating surfacearrangement, control of reheat steam temperature at lower loads is improved. By placing a reheaterhorizontally in the rear convective pass, the reheat temperature can be further controlled by varying excesscombustion air in the furnace. Thus, reheat steam temperature is controlled by both fuel nozzle tilt and byexcess combustion air.

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Figure 6.2.1 Arrangement of Supercritical Pendant-Panel Unit

6.3 Tower Type Design

In a tower design, all convective heat transfer surfaces including superheater and reheater sections and theeconomizer are arranged in the upper part of the boiler. Only the regenerative air preheater is located at thelower part of the second pass. A typical design of a tower-type boiler for a 900 MW supercritical unit is shownin Figure 6.3.1.

Figure 6.3.1WaiGaoQiao, 2 x 900 MW

Live Steam

4050 Psig/279 bar (design pressure)1008 °F/542°C

6,247,360 lb/h/r/2789tonnes/hr

Reheater Steam 1000 psig/ 69 bar 1055 °F/568°C 5,445,000 lb/h2431 tonnes/hr

Feedwater 523 °F/272 °C

Fuel: Bituminous Coal

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The economizer is the furthest downstream section and is installed in the parallel flow type arrangement. Theflow from the economizer is fed to the hopper inlet header that supplies the furnace tubes. The flow from thefurnace walls is collected and is piped to the water separators. Steam from the separators is fed to the hangertubes that support the convective sections. Typically superheating and reheating is carried out in three mainsteam superheater and two reheater stages. More detailed information is shown in the water/steam diagram for a900 MW supercritical unit in Figure 6.3.2.

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� � �

� � �

� � � �

� � � �

� � � �

� � � �

Figure 6.3.2Water/Steam Diagram

With the exception of the SH and RH final stages, which are arranged in parallel flow, all other superheat andreheat sections are arranged in counterflow. The parallel flow of the final stages was selected for two reasons.The first one is to prevent potential flow of condensed steam (that will occur during prolonged unit shutdown)into the main steam and reheat piping during boiler restart. The second reason is that parallel flow results in alower heat flux and, therefore, a lower metal temperature of the last sections of the finishing superheater andreheater tubes.

6.4 Modified Tower DesignsIn addition to the “pure” tower arrangement, a modified arrangement exists where the economizer is located inthe second (rear) pass above the air preheater. This affords reduced boiler structure, counterflow economizerflow and cased economizer surface. Figure 6.4.1 illustrates this arrangement.

Figure 6.4.1 – Modified Tower Arrangement

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6.5 Steam Temperature Control

In both pendent panel and tower arrangements, primary steam temperature is controlled by establishing theproper ratio of firing rate to feedwater flow. In addition, tilting fuel nozzles may be used to control reheateroutlet temperature. Depending upon the severity of operational requirements, including transient conditions, oneor two spray water stations on the primary and reheater steam sides can also be used to assist steam temperaturecontrol.

7.0 Start-up Systems for Supercritical Steam Cycles

7.1 General Design Considerations

Today’s supercritical power plants are designed to follow a rigorous load program that often includes two shiftor cycling operations. To accommodate this operating requirement most effectively, a supercritical steamgenerator must be capable of sliding pressure operation in the entire system. This means that during low loadand start-up the steam generators are operated in a subcritical pressure range. Therefore, to facilitatesatisfactory service, a low load start-up system is provided. Selection of a minimum once-through flow dependson such factors as mode of operation, circuit stability, and tube materials.

For boilers which are primarily base loaded, the once-through minimum load should be selected as high aspossible. This results in the lowest pressure drop in the waterwalls at a full-load condition. Steam generatorswhich are required to cycle must be designed for a lower once-through minimum load so that once-through flowoperation is extended to the lowest load practical. Commercial experience with a minimum once-through flowdown to 30% to 40% load has proven to be successful. Lower once-through loads are also feasible.

The start up system includes a water separator system located between the waterwalls and the primarysuperheater, a water storage tank and a drain water discharge system with heat recovery capabilities. The waterseparator consists of one or more vertical vessels with tangential inlets (Figure 7.1.1).

Figure 7.1.1Steam-water separator system and water storage tank

Steam outlets are located in the upper part and the drain is discharged through the lower part. The waterseparator is in “wet condition” when it operates in flow recirculation mode and it is “dry” when the flow isonce-through.

The drain discharge systems with heat recovery capabilities can be of two types: • indirect heat recovery • direct heat recovery

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There are also two types of direct heat recovery systems that are available. The first is a system with a low loadrecirculation pump; the second is a system that includes a drain return line via a heat exchanger into thedeaerator/feedwater storage tank.

The suitability of each system depends on the economic evaluation associated with operational requirements ofa steam generator. For example, base loaded units will typically not benefit from a direct heat recovery system.The need to minimize heat and water losses during start-up is important for two-shift and daily start/stop units.However, increased equipment cost due to added equipment must be weighed against the operational benefitssuch as reduced fuel consumption and retained water saved by a direct heat recovery system.

7.2 Drain Discharge with Indirect Heat Recovery System

In this type of start-up system the minimum required cooling flow is assured by the feedwater pump. Thefeedwater flows into the economizer, the waterwalls and then to the water/steam separator and the storage tank(Figure 7.2.1).

Figure 7.2.1Drain Discharge with Indirect Heat Recovery System

From there water returns through a control valve to the feedwater (deaerator) tank and partly through one of thetwo control valves located on the boiler house flash tank.

During a cold start, as water begins to swell, the HWL and WL valves open and return water from the waterseparator storage tank into the boiler house flash tank as well as into the deaerator. As the pressure in the waterseparator increases, the return water flow will increase into the feedwater tank. As the return water temperatureincreases, the related heat will be transferred to the feedwater. Thus the heat produced in the boiler during start-up will be largely conserved in the feedwater system, thereby reducing the start-up time and associated losses.

Under ideal operating conditions (e.g. acceptable feedwater tank level and pressure) the flash tank valves areclosed allowing the entire drain water flow into the feedwater tank. In the event that the pressure limitations areexceeded in the feedwater tank, the flow will be rerouted into the flash tank thus maintaining acceptableseparator level.

Drain water flow from the flash tank is routed through a receiving tank, through a drain transfer pump and backinto the condenser.

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7.3 Drain Discharge with Direct Heat Recovery Utilizing Low Load Recirculating Pump System

Systems that include a small low load recirculation pump, utilize this pump to maintain the required minimumflow in the economizer and waterwalls during start-up and “wet condition” operation (Figure 7.3.1).

Figure 7.3.1Drain water return system with low load circulation pump

The recirculation pump takes its suction from a mixing tee located downstream of the water storage tank. Thetee combines the flow of the feedwater system with the drain water flow from the storage tank. In this start-upsystem the feedwater pump and circulation pump are installed in series. A parallel arrangement is availablealso. In the parallel system the feedwater flow is combined with the recirculation flow at the circulation pumpdischarge.

During initial start-up, when no steam is being produced, the feedwater pump is de-energized and the entire flowthrough the economizer and waterwalls is supplied by the drain system. As steam production begins, the level inthe storage tank begins to decrease. As this occurs, the flow through the evaporator begins to vary from entirelydrain water recirculation to a mix of feedwater and drain water recirculation. This trend continues until theminimum once-through load is reached at which point the circulation pump is taken out of service. Above theminimum once-through load, the steam generator has entered into the once-through mode where all of the wateris being evaporated into steam and the entire flow through the evaporator is supplied by the feedwater pump.

7.4 Drain Discharge with Direct Heat Recovery Utilizing a Start-up Heat Exchanger System

The drain discharge line via a start-up heat exchanger is equipped much like the system with indirect heatrecovery. The notable difference is the incorporation of a straight tube type heat exchanger integrated into thedrain line that leads from the separator/storage tank to the feedwater tank (deaerator). The heat exchanger allowsfor the direct recuperation of excess heat in the water from the waterwalls into the feedwater line to theeconomizer (Figure 7.4.1).

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oF

54

36

18

0

Figure 7.4.1Drain Water Return System with a Start-up Heat Exchanger System

Similarly to the system with the recirculation pump, this system has the advantage of minimizing the heat lossesduring start-up and low load operation and as with the pump system the operational savings must be comparedto the increased initial capital cost of the added equipment.

8.0 Operating Experience with Supercritical Steam Cycles

After a design has been completed and a boiler erected, operating testing provides an opportunity to determinewhether it meets performance guarantees and whether any design adjustments are required. Individual unitsshould follow load programs, participate in cycle load operations and should have capability for emergencyoperation following power plant disturbances. All these requirements put an increased demand on plant andboiler controls. The control quality of the main steam parameters is illustrated on the next few figures.

Figure 8-1 shows the results of field tests conducted on a 750 MW once-through tower type unit.

Figure 8.1Horizontal and Vertical Temperature Distribution for Superheaters 1,2,3 and 4 at 100%, 60% and 25%

Load

for Superheaters 1, 2, 3 and 4at 100 %, 60 % and 25 % Load

Horizontal and VerticalTemperature Distribution

SH 3

leftright left right

temp.

boiler load

temp.

temp. °F

boiler load

section A-Bsection A-B

section A-B

tem

pera

ture

°F

tem

pera

ture

°F

5th tube row from top

inlet

inlet

inlet

top

top

bottom

bottom

inlet

inlet

inletbottom

SH 1

rightleft

tem

pera

ture

°F

5th tube row from top806

788

770

752

770

752

734

716

698

680

662

644

626

608

932

914

896

878

860

914

896

878

860

842

914

896

878

860

842

644 680 716 752 788

842 860 878 896 914 932

828 826 829 822

829 806 842 824

822 808 846 824

SH 4boiler load

top11th tube row from top

SH 2

rightleft

temp.

boiler load

section A-B

tem

pera

ture

°F

bottom

top

14th tube row from top o = 9th tube from top + = bottom tube

896

878

860

842896

878

860

842

824

860

842

824

806

788

770806 824 842 860 878 896

869 873 880 867

862 867 880 858

905 860 860 858

1022

1004

986

968

1022

1004

986

968

1022

1004

986

968

950

932932 950 968 986 1004

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at 100 % and 25 % Load (4 Lines in parallel)

Heat Absorptionof the Superheater S tages

071 083p

Temperatures in °C

SH 4

SH 3

SH 2

SH 1 SH 1

SH 2

SH 3

SH 4

Separator 100 % load

Separator25 % load

1000990 988

869880

+121 +115

-20 -38 -27 -33

1000 1000

873867

+120 +133

900 900900

828 826

907

829 822

+72 +74 +78 +78

862 865860 858

-34 -31 -39 -36

770 770

770

761 777

777

761 770

+92 +95 +99 +81

748

748

736

738

+22+22+24

990 986997 997

905 860860 856

889 883 909 892

822 808846 824

835 846846 826

666 649655 664

664 666

655 649

534

534

550

550

+85 +126 +137 +141

+67 +63+75 +68

+169 +197 +191 +62

+126 +108

-29 -4 -32 -27

-13 0 -38 -2

The results show the horizontal and vertical temperature distribution at the outlet of the superheater tubes at100%, 60% and 25% loads. The temperature measurements were taken over the entire furnace width as well asover the superheater height for the center row tube. As can be seen, the temperature profiles and heatabsorption by individual tubes (Figure 8.2) are quite uniform indicating excellent gas side and steam side flowdistribution. Because of these excellent results, available design margins for the superheater tubes weremaintained.

Temperatures in oF

Figure 8.2Heat Absorption of the Superheater Stages at 100% and 25% Load (4 Lines in parallel)

Load transfer from the recirculation mode to once-through mode as well as reverse load transfer is fullyautomated. At the transfer point to once-through flow, control strategy shifts from controlling the water level inthe tank and maintaining minimum cooling flow to the waterwalls to the control of enthalpy in the separator.

Boiler transient load behavior was tested at an 800 MW unit. Figure 8.3 illustrates maximum fluctuation in thesteam temperature and pressure leaving the superheater and reheater as boiler load increases at 6%/minute.

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Figure 8.3 Load increase with over 6% Steam Flow Variation

It should be mentioned that these load changes were achieved at constant turbine inlet valve position. If thecontrol system was modified to account for stored heat capacity of the steam generator, successful operation ofthe steam generator could be achieved even with higher load change rates.

Boiler availability and minimum temperature variation during emergency conditions were tested also. Failure ofone of two forced or induced draught fans was simulated. Figure 8.4 shows the results of the load run-backfrom 100% to 50% in a time frame of a little more than one minute.

Figure 8.4Load Runback to 50% (Schwarze Pumpe)

The control quality attained was excellent. The reheat and superheat outlet temperature variations were withinallowable temperature differences. Over the past few decades, considerable body of experience becameavailable with the design and operation of supercritical sliding pressure steam generators. Figure 8.5 shows apartial list of ALSTOM steam generators that have been operating successfully in the base-load and cyclingmode of operation.

0

363

725

1088

1450

1813

2175

2538

2900

3263

3625psig

32

122

212

302

392

482

572

662

752

842

932

1022

1112°F

0

10

20

30

40

50

60

70

80

90

100

110

120

10:00 10:05 10:10 10:15 10:20 10:25 10:30

%

Total firing rate [%] SH Steam temperature [°F] RH Steam temperature [°F] SH Steam flow [%] SH Steam pressure [ psig ] Auxiliary line

64.8 %/10:10:19

6.7 %/min.

95.28 %/10:14:52

8:38:30:2080:248:30:288:30:328:30:368:30:368:30:408:30:448:30:488:30:528:30:568:31:008:31:048:31:088:31:088:31:128:31:168:31:218:31:248:31:288:31:328:31:368:31:368:31:408:31:448:31:488:31:528:31:56

0

10

20

30

40

50

60

70

80

90

100

8:30 8:45 9:00

Total firing rate [%] SH Steam pressure [psig] SH Steam flow [%] RH Steam temperature [°F] SH Steam temperature [°F]

%

0

363

725

1088

1450

1813

2175

2538

2900

3263

3625psig

932

968

1004

1040

1076

1112

1148

1184

1220

1256

1292°F

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Figure 8.5Year of Commissioning, Hours of Operation and No. of Start-Ups

9.0 Materials and State-of-the-Art Steam Parameters

Except for the waterwalls, where tubing made of low Cr alloy is used instead of carbon steel, materials appliedin supercritical boiler designs are similar to materials selected for drum-type steam generators. Creep rupturestrength and its oxidation limit often establish the temperature limits of a material. For a given temperature, thecreep rupture strength of a material decreases over time. As the steam parameters increase, available designmargin of many conventional alloys decreases and at some temperature level their application becomesimpractical. The use of alloys for critical pressure part components such as waterwalls, finishing superheat andreheat sections, and thick wall components including high pressure steam outlet headers and main steam pipingare reviewed in the next few sections.

9.1 Materials for Waterwalls

Furnace tubes are subject to the highest heat fluxes in the furnace. Typical tubing alloys applied in thecommercial designs are: 1.25 Cr-0.5Mo (T-12) material, a 2.25 Cr – 1.0 Mo (T22) material and a Europeandeveloped material 15Mo3. These alloys have excellent mechanical properties suitable for easy fabrication ofthe waterwall panels. Design limit of T12 was studied for a cycle with the superheater outlet steam conditionsup to 4060 psig (280 bar) and 1112°F (600° C). For non-corrosive coals and based on the furnace outlettemperature of 2280°F (1250°C), T12 can be applied in waterwalls up to the waterwall outlet temperature ofabout 880°F (470°C). T12 is used for lower temperatures in the panel designs.

For higher steam conditions and higher water wall outlet temperatures respectively, different materials arerequired. For example, T22 has slightly higher creep rupture strength properties and higher oxidation limits andis frequently used in place of T12. Its application, however, doesn’t enable any significant increase in cycleparameters.

In recent years new ferritic alloys became commercially available that could be applied to waterwallconstruction. They offer a substantial improvement in the creep strength and can be used instead ofconventional alloys or enable higher steam parameters cycle. These materials are a 2.25Cr-1.6W-V (T23) alloy(ASME code approved) developed by Sumitomo Metal Industries and a 7CrMoVTiB1010 (T24) alloydeveloped by Vallourec & Mannesmann Tubes. Allowable stresses of ferritic alloys are shown on Figure 9.1.1.For comparison, a 9Cr-1Mo-V (T91) alloy, used mainly in the construction of superheater and reheater surfaces,is shown also.

Commissioning Status Hours of operation No. of start-ups

Scholven F 1979 12/2000 137,200 1,830

Bergkamen A 1981 12/2000 147,096 311

GKM K 18 1982 12/2000 140,950 350

Bexbach I 1983 02/2001 106,156 2,007

Heilbronn Unit 7 1985 12/2000 83,230 1,108

Vestkraft 1992 06/2000 58,300 200

Shidongkou 1 & 21) 1992 12/2000 63,842/60,474 162/112

Poryong 3 & 41) 1993 12/2000 60,685/61,025 73/71

Poryong 5 & 61) 1993 12/2000 57,378/56,818 113/74

Schwarze Pumpe A/B 1997 06/2002 40,457/35,050 120/124

Hadong 1 & 21) 1997 12/2000 27,631/25,418 19/14Note: 1) Above data includes only startups after commercial operation, i.e., no commissioning phase startups

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A L L O W A B L E S T R E S S T 9 1 , T 2 2 v s . T 2 3

0

2 0

4 0

6 0

8 0

1 0 0

1 2 0

1 4 0

3 7 0 .0 4 2 0 .0 4 7 0 .0 5 2 0 .0 5 7 0 .0 6 2 0 .0

T e m p e ra tu re , d e g . C

Allo

wab

le S

tress

, MPA

T 9 1

T 2 3T 2 2

Figure 9.1.1Allowable stresses for ferritic alloys

9.2Materials for Superheater and Reheater Tubes

When selecting superheater and reheater tube materials, the creep strength of the selected alloys must be highenough to provide adequate margins of safety in the operating pressure and temperature range. In addition to therequirement for high strength, corrosion resistance, both on the flue gas side and on the steam side, must beconsidered. Oxides will always form on the inside and outside surface of a tube. It is a known fact thatexfoliating metal-oxide scale from the internal surfaces of superheater and reheater tubes, headers, and pipingcan lead to solid particle erosion in steam turbine blading and valves. Solid particle erosion predominantlyoccurred in the intermediate pressure stages and seldom in the high pressure stages of steam turbines and valves.The exfoliated oxides are mostly ferritic types. Low chrome ferritic alloys are predominant materials throughoutthe reheater system and are more prone to exfoliation than high chrome ferritic alloys like the 12% Cr materialX20CrMoV121 and austenitic alloys. ALSTOM’s design approach mitigates this exfoliation phenomena bylimiting application of low chrome ferritic alloys to relatively low temperatures and through conservativeselection criteria regarding allowable metal temperatures.

In the past ten years new higher Cr ferritic containing 9-12% Cr alloys became commercially available. Ferriticmaterials allow for more economical design and have the advantage of avoiding dissimilar metal welds and thelarge coefficient of expansion of austenitic steels that are typically used for higher temperature tubes. These highchrome ferritic alloys are continuously being improved for high strength and higher resistance to oxidation. Theaddition of Tungsten (W) and other carbide and nitride forming elements have led to considerable improvementsin the high temperature strength of the 9-12Cr steels. However, even for these higher strength alloys ALSTOMdesign practice is to limit their application to steam temperatures less than approximately 1060ºF (570ºC). Forhigher temperatures, austenitic alloys can be used.

A new generation of ferritic materials is being developed which will give enhanced temperature capabilitiesrelative to those now available. Very high operating steam temperatures, considered for higher efficiency cycles,will require the application of materials with greater creep strength and greater corrosion resistance than the bestboiler materials in use today. The most attractive alternative appears to be nickel-base alloys for the very highesttemperature components. The creep strength is sufficient to allow operation with steam temperatures close to1300°F (700°C).

In the USA, the Electric Power Research Institute (EPRI) has acted as the focus for development of advancedmaterials. Under the EPRI 1403-50 project, co-operative research and development with USA utilities andequipment manufacturers in the USA, Europe and Japan materials properties and performance have been studiedto enable alloys to be used for pressure parts operating at high temperature. Projects of similar nature have beenconducted in Europe and Japan. These projects have been successful in developing a number of material thatwere qualified and introduced into a number of power plants in Europe with maximum steam temperatures of1110ºF (600ºC). Figure 9.2.1 shows the strength of some of these alloys.

Allo

wab

le S

tress

, ksi

20.3

17.4

14.5

11.6

8.7

5.8

2.9

0 698 788 878 968 1058 1148

Temp oF

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Figure 9.2.1100,000 h Creep Rupture Strength for SH and RH Materials

Long term superheat and reheat tubing material tests have been conducted in the Danish Power Plant VestkraftUnit 3 since 1995. Many test superheaters made up of a number of material samples have been installed in theboiler. The goal of the project has been to investigate and validate high-temperature corrosion and oxidationresistance of new materials. Also, the effect of long exposure to high temperatures on microstructure ofmaterials and welds is being studied. The test program is being carried out in three phases with maximum steamtemperatures at 1150°F (620°C), 1180°F (635°C) and 1300°F (700°C). More than 35,000 operating hours havebeen accumulated. External corrosion rates and internal scales are being determined.

9.3 Materials for High-Pressure Outlet Headers and Piping

Traditional material used for manufacturing of the main steam outlet headers and piping is Grade 22 (2.25 Cr –1.0 Mo) and, in Europe, the 12% Cr alloy X20CrMoV121. In the past decade, P91 (9 Cr – 1.0 Mo-V) ferriticsteel with much higher creep rupture strength properties became available and has been used in power plantapplications. Oxidation limits of P22 and P91 alloys are 1075°F (580°C) and 1150°F (621°C) respectively. InEurope, X20CrMoV121 has been widely used for headers and piping with steam conditions up to 3750psig/1020°F (260 bar/ 550°C.

The allowable stresses of Grade 22 decrease significantly at higher temperatures, thus requiring thicker walledheaders and piping. This required increase in wall thickness limits application of this alloy in cyclic dutydesigns. For higher temperature and pressure, P91 is a better choice because it enables manufacturers to producecomponents of thinner wall and lower cost.

Higher creep strength ferritic alloys also have become available in recent years. These alloys are P92 (9 Cr –2.0 W) and P122 (12 Cr – 2.0W) and are ASME code approved for power plant use. For high steam conditions,application of these alloys results in thinner walls and lower cost. These materials can be used for steamparameters of approximately 4350psig/1110°F (300bar/600°C SH outlet).

A new generation of steels is being developed which will give enhanced capability relative to those nowavailable. Of particular interest are Save12 (10Cr-W-3Co), New NF12 (11Cr-2.6W-2.5Co), and VS2161A(11Cr-1.6W-4Co) which show a gain in strength of around 30% compared with current state-of-the-artmaterials. Figure 9.3.1 illustrates comparative strength of conventional materials and some of the new alloys.

560 590 620 650 680 710 740 770

Temperature

0

50

100

150

200

250

10

0,0

00 h

- C

reep

rup

ture

val

ues

(ave

rage

)

°C

N/mm2

TP 347 HFG

HCM 12

1.4910

NF 709

T 91

HR 3C

SUPER 304 H

X 20

SAVE 25

Tempaloy A-1Tempaloy A-3

Alloy 617: A 130

Alloy 617: VdTÜV

X 20: DIN 17175 and DIN 17176T 91: VdTÜV Material Sheet 511/2HCM 12: VdTÜV Material Sheet 5101.4910: DIN 17459Tempaloy A-1: NKKTempaloy A-3: NKK

SUPER 304 H: Sumitomo and ASME C.C. 2328TP 347 HFG: Sumitomo and ASME C.C. 2159NF 709: Nippon SteelSAVE 25: SumitomoHR 3C: Sumitomo and ASME C.C. 2115Alloy 617: VdTÜV Material Sheet 485Alloy 617: Research Project A 130

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986 1040 1094 1148 1202

Temperature

0

7.25

14.5

21.75

29

36.25

100

000

h -

Cree

p ru

ptur

e va

lues

(ave

rage

)

°F

X 20: DIN 17175 and DINP 91: VdTÜV Material SheetE 911:VdTÜV Material Sheet

HCM 12 A (P 122):Sumitomo and ASME C.C.P 92: ASME C.C. 2179NF 12: Nippon

ksi

E 911 HCM 12 A (P 122)

P 91

NF 12

X 20

P 92

Figure 9.3.1100,000 h Creep Rupture Strength for Pipe and Header Materials

10. Future Supercritical Cycle Developments

The fundamental need for improved cycle efficiency capable of variable pressure operation will requireincreases in steam temperatures and pressures. Available materials make cycles with steam conditions of 4350psig/1110ºF/1150ºF (300 bar/600ºC/620ºC) feasible in today’s market. A number of material developmentprograms in the US and Europe will enable superheat steam temperatures higher than 1110ºF (600ºC) in thefuture. For example, Ni bond alloys are being developed and tested for higher steam conditions.

The European research project, Thermie, is aiming at the development of a cycle with steam conditions at 5440psig/1290ºF/1330ºF (375 bar/700ºC/720ºC). The boiler waterwalls will need to be constructed of tubes made ofhigher-strength, corrosion resistant martensitic steels. The high-pressure outlet headers, piping, and the finalstage of the superheater tubes will need to be fabricated of Ni-based steels. The Thermie project goal is to makethis cycle available in the next 10-15 years. Predicted plant efficiency will be more than 50% net based on netcalorif value.

.

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11.0 Circulating Fluid Bed Combustion for High Sulfur Fuels

Introduction

Despite the tendency of the world energy market to become more and more dependent on international fuelsupplies like imported coal, oil or gas, there is still the need for local fuel supply in different countries andregions.

Local fuel supply is especially important in cases where the infrastructure does not allow transportation of gas orcoal. Such application generally covers medium size power plants of about 50 MW to 300 MW. Most of thelocal fuels exhibit higher moisture content and / or lower heating values than international steam coals.Therefore, they are commonly not beneficiated in washery plants as is done for steam coals.

Besides peat, oil shale and biomass, the most typical local fuel is lignite, and many of the different types oflignite can contain significant amounts of sulphur. The main advantage of lignite is that it usually can beexcavated in open cast mines which makes it an economically very attractive fuel. Beneficiating of highsulphur lignite to reduce the ash and sulphur content is generally not applied for economical reasons.

High sulphur lignite is found in many countries around the world. Table 11.1 gives some data about utilizedhigh sulphur lignite.

Table 11.1 Typical Analysis of High Sulphur Lignites

ALSTOM, as the market leader for lignite fired power plants, has been involved in combustion of high sulphurlignite for more than 40 years.

The two boilers of the lignite fired power plant in Kutch in Gujarat, India, as an example, are operating with aPC firing system designed and supplied by ALSTOM. The lignite from the nearby open-cast mine has a netcalorific value of 12,7 MJ/kg and sulphur contents of up to 6 %. Based on the conservative design of the boilersand a firing system taking the high sulphur content (mostly pyrite) into account, the boilers have beencontinuously in operation for about 14 years.

OriginSite Country

NCV(MJ/kg)

Volatile matter(% maf)

Moisture(% ar)

Ash(% ar)

Sulphur(% maf)

Amyntheon Greece 5.2 59 55 18 2.4

Meliti Achlada Greece 8.0 62 36.8 27.36 2.6

Çayirhan Turkey 8.4 64 26.2 39.4 5.4

Kangal Turkey 5.4 69 51 20 8.0

Elbistan Turkey 4.8 75 52 23 8.0

Drmno Yugoslavia 7.3 60 43.9 22.3 3.0

Sostanj Slovenia 9.2 68 41 17.1 3.2

Akrimota India 12.0 59.1 35 21 8.8

Wählitz Germany 11.5 59.6 47.9 5.8 3.6

Tisova Czech Republic 10.7 52 37 20.3 3.2

Kutch India 12.7 57.1 39.7 12.4 5.6

Provence France 15.3 43.7 11.7 25.9 5.5

Çan Turkey 10.9 58 22 32 8.7

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12.0 Advantages of CFB firing for high sulphur lignite

The ability of CFB plants to achieve very high sulphur capture with moderate limestone consumption is not theonly advantage when firing high sulphur coals.

Other advantages are:

• Significantly lower SO3 emissions allow lower flue gas exit temperatures and thus increase the boilerefficiency,

• avoiding of slagging in the furnace and reducing of fouling in the backpass and the ability to utilize fuelswhich could not be utilized with conventional combustion technology.

13.0 Experience with existing PC fired plants and CFB fired plants

While PC fired boilers with high sulphur coals usually operate at flue gas exit temperatures of 320°F/160 °C orabove, the flue gas outlet temperature of CFB boilers can be reduced to 284°F/140 °C or less. As predicted fromchemistry, the SO3 capture via CaO is significantly higher than the capture of SO2. Measurements with Germanhigh sulphur lignite of approximately 4 % maf sulphur showed marginal SO3 levels of less than 0.1 ppm in theflue gas.

Nevertheless, special technical precautions must be taken at the cold end of the air preheater to avoid corrosion.With non-adequate tubular air preheater design, it has been found that even with marginal SO3 levels corrosioncan occur if the cold end of the air preheater is not protected properly or if certain operational conditions such asvery low ambient temperature or low load operation are not taken into consideration. Adequately sized steamcoil air preheaters can safely avoid these conditions.

In PC fired plants, when the sulphur in the lignite occurs as pyrite severe slagging of the furnace can occur. Inthe last fifteen years more than 10,000 MW of boiler capacity was modified to low NOX conditions byALSTOM. Generally, air staging was applied to limit NOX emissions. It turned out that furnace slaggingdecreased the more air staging was used.

CFB plants have shown that slagging in the furnace does not occur even when utilizing high sulphur coals. Theexperience with the plants Waehlitz, Germany, Tisova, Czech Republic and Gardanne, France (all these boilersare in operation for more than 4 years) showed that no slagging occurred in the furnace, cyclones or the loopseals.

14.0 CFB Plant Akrimota in India

In 1999, ALSTOM was awarded the contract for two CFB steam generators for the 2 x 125 MWe Steam PowerPlant in Akrimota, Gujarat/India. The owner of the plant is the Gujarat Mining and Development Company(GMDC).

The power plant is located in Chher Nani, near the city of Bhuj, close to the Arabian sea and not far from thelignite fired power plant Kutch. It is currently under construction and the first unit will be commissioned in2003.

The fuel supply is coming from the nearby open-cast mines Panandhro and Akrimota. The Pandandhro mine isalready in operation, while the Akrimota mine is to be opened with the construction of the power plant. Boreprobes across the coal field of the Akrimota mine showed significant variations in sulphur content in the coal,ranging from 3.9% up to 4.6% depending on location and seam. Table 14.1 depicts several of these analyses.

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Table 14.1 Typical Analysis Data of High Sulphur Lignites

The major forms of the sulphur are pyritic sulphur and organic sulphur. Both of these substances will reactcompletely to form SO2 when fired at temperatures of approximately 850 °C, which are prevailing in the CFBfurnace. The sulphate sulphur on the other hand will not further react in the CFB, SO2 from sulphate will bereleased only at elevated temperatures in excess of 1100 °C or higher, which is not the case in a CFB boiler.

A cross sectional drawing of the Akrimota boiler is depicted in Figure 14.1.

Figure 14.1 Power Station Akrimota 2x125 MW Circulating Fluid Bed

+ 50.0 m

Borehole No. 53 60 61 63

Total Sulphur % 4.43 2.00 2.33 7.84

Pyritic Sulphur % 54.8 37.3 55.2 4.8

Sulphate Sulphur % 27.0 10.6 6.7 25.5

Organic Sulphur % 18.2 52.0 38.0 69.7

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The CFB boilers are of well proven design, featuring several technical items which make them especiallysuitable for combustion of high sulphur lignite. These items include:

• Cyclones with high efficiency• Re-injection of screened bottom ash• Equal fuel injection and distribution• Equal limestone injection and distribution

These measures are required to achieve the guaranteed desulphurization rate of 97 % for the whole coal rangeand load range. Special importance was given to the cyclone design. As it is known, the desulphurizationreaction is an adsorption process of the gaseous SO2 and O2 penetrating into the CaO to form CaSO4. Therefore,the desulphurization reaction is highly time dependent, especially because the penetration of the gaseouscomponents into the limestone particle core is reduced due to the presence of a dense sulphate layer formed atthe outside of the CaO particle.

The routing of the cyclone inlet ducts and the shape and arrangement of the eccentric vortex finder was furtherdeveloped such to optimize cyclone efficiency. The aim of these measures is to increase the particle residencetime to a maximum, especially for small particles between 40 µm and 300 µm (typical limestone PSD).

The effects of the cyclone inlet duct shape on the cyclone efficiency have been tested in cold model tests in theALSTOM laboratory and by CFD analysis (see Figures 14.2 and 14.3).

Figure 14.2 Cyclone Test Rig at ALSTOM laboratories

Figure 14.3 CFD Analysis of Cyclone Performance

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First tests with eccentric vortex finder arrangement were performed in 1995 in the 250 t/h lignite fired CFBplant Berrenrath, Germany (see Figure 14.4). This testing improved the particle residence time and thus reducedthe loss of limestone and bed material through the cyclone significantly.

Figure 14.4 Eccentric Vortex Finder Arrangement

Figure 14.5 summarizes the various measures to improve the cyclone performance.

Figure 14.5 Cyclone Improvement Measures

The positive effect of these modifications on the cyclone performance is illustrated by the particle sizedistribution (PSD) of the furnace inventory. Figure 14.6 shows the PSD of three plants using a traditionalcyclone designed at the end of the eighties and the PSD of the improved cyclone design used in the MladáBoleslav plant located in the Czech Republic.

DownwardInclinedInlet Duct

H igh Perform anceRefractory for In let Area

Eccentric VortexFinder Arrangem ent

Advanced VortexFinder Shape

Second Pass

To Seal Pot

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Figure 14.6 Particle Size Distribution of Solid Inventory (Old vs. New Cyclone Designs)

The fine PSD of the inventory leads also to higher solid recirculation in the furnace itself and also higher soliddensities will be achieved. Therefore, the temperature homogeneity will be increased to a maximum and thefurnace temperature control could be improved. Both effects support the sulphation of CaO. Another side effectof the cyclone enhancement is an improved fuel utilization. Again, a higher particle residence time will explainthis effect. Even if for highly reactive lignite this impact on the boiler efficiency is very limited, there is asignificant influence for medium to low reactive fuels.

15.0 Recycling of screened bottom ash

The ash content of the Akrimota fuel can vary in a wide range from about 5 % to approx. 45 % in the boreprobes resulting in long-term variations of the coal fed to the boilers between 18 % and 35 %. The reason forthis wide range is due to the way the mine will be operated. The different seams of the mine have differentthicknesses and for certain smaller seams it will be more economical to excavate smaller seams which are closetogether in one batch including the intermediate ash rich layer which partly contains high sulphur levels, thusincreasing the overall sulphur content in the fuel even more.

The large variation of the ash quantity was decisive for the design of the ash handling system with the bottomash quantities varying by a factor of more than 10. Water-cooled fluidized bed ash coolers are installed whichare able to cool down large ash quantities without the need of high temperature water-cooled screw coolers(Figure 15.1).

10 µm100 1000Gra in size d

0.1

1.0(%)

10.020.030.040.050.060.070.0

80.0

90.0

99.0

99.9

Resi

due

R

old cyclone designnew cyclone design

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Figure 15.1 Water Cooled Ash Cooler

To avoid removing too much bed material containing CaO with poor sulphation a screening plant for the bottomash is planned by the customer. This screening plant allows screen off of the fines which contain the majority ofpoorly sulphated lime from the bottom ash and recycle it to the furnace or to the bed make-up silo.

Recycling of the screened bottom ash also allows influencing the PSD of the furnace inventory therebyproviding a tool for furnace temperature control, especially during times when low ash lignite is fed.

16.0 Equal fuel injection and distribution

Equal fuel distribution for high sulphur fuels is of utmost importance because:

• even fuel distribution avoids temperature unbalances in the fluid bed thus enhancing sulphur capture;• even fuel distribution leads to a balanced fuel / air mixing, thus ensuring that sufficient oxygen for the

desulphurization process is available.

In order to achieve an equal fuel mixing, the Akrimota furnace will be equipped with four fuel feeding chuteslocated on the return legs coming from the split loop seal. The split loop seal was specifically developed toimprove fuel / air mixing by doubling the number of fuel feeding points per cyclone (see Figure 16.1). The firstinstallation of double loop seals has been in the lignite fired 130 MW power plant Goldenberg, Germany in1992.

Return to furna ce Ash inle t duct fromfurna ce bottom

Conveyor a shto a sh silo Fluidizing a ir

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Figure 16.1 Split Loop Seal

17.0 Equal limestone injection and distribution

The limestone feeding system is designed redundantly. The crushed and dried limestone, stored in a day bunker,is dosed by rotary feeders and pneumatically transported to the furnace. The feeding lines are kept as short aspossible to reduce the power consumption. Each line is split into four further lines, which are equally distributedto all return legs from the loop seals. This gives a good limestone distribution regardless of whether only one ortwo limestone feeders are in operation.

18.0 Limestone characteristics

In case of high sulphur lignite, the limestone characteristics influence to a large extent the economics of theplant. While desulphurization rates of 97 %, as required for the Akrimota plant, demand high purity and highreactivity limestones, the economics ask for the lowest cost available limestone source which should be as closeto the plant as possible to minimize transportation costs. Luckily, for the Akrimota plant both requirementsmatch extremely well.

Geologic investigation has shown that for a large extent the surface layer of the coal mine consists of alimestone layer of several meter thickness totalling about 30 MM tons (coal about 200 MM tons).

Several samples from the limestone layer were taken and analyzed in the ALSTOM Power fuel lab. Testsshowed that despite the purity of the limestone of only approximately 91 % the limestone can be ranked as ahighly reactive limestone according to the ALSTOM Power based reactivity test. The ALSTOM Powerreactivity index is determined in a thermogravimetric analyzer (TGA). The amount of sulphur removed from agas containing SO2 and passing the limestone sample is measured for a certain time interval. This amount givesa reliable indication of the limestone reactivity, as field data from more than 30 plants have shown.

19.0 OutlookOne of the future trends in CFB combustion is going towards inexpensive local fuel. High sulphur lignite is oneof them. The Akrimota power plant will be India's first CFB plant operating on high sulphur lignite and

Coa lfrom Cyclone

to Furna ce

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expecting sulphur removal rates of 97 % using also local limestone. The technical concept used by ALSTOM isbased on previous experience with high sulphur lignites.

The next step of plants firing high sulphur lignite will be the 2 x 160 MWe Can power plant in Turkey, firinglignite with up to 14 % maf sulphur content.

This plant is currently being built by ALSTOM as turnkey supplier and will be commissioned in 2003.Figure 19.1 shows the CFB boilers. It will be the first large CFB plant in Turkey. Combustion tests wereperformed in the 1.2 MW test facility in the German Niederaussem power station which proved that the requireddesulphurization rate of 97 % can be achieved with the predicted amount of limestone. Table 19.1 summarizesthe main data of ALSTOM´s CFB plants for high sulphur lignite.

Figure 19.1 Can Power Plant 2x160 MW CFB

Table 19.1 ALSTOM's High Sulphur Lignite CFB Plants

Pla nt Wä hlitz Tisova Prove nce Akrimo ta Ça nStea m genera torca pa city MWel 1 x 40 1 x 90 1 x 250 2 x 125 2 x 160

Ca lorific va lue NCV MJ/ kg 10.5 - 12 .0 9.8 - 12.8 15.3 7.2 - 12.1 9.8 - 12.1Ash content % 4.5 - 8 .5 13 - 25 26 18 - 35 32Sulphur content ma f % 3.6 - 4 .8 0.7 - 9 .2 5.5 4 - 13 4 - 14Moisture content % 47 - 54 36 - 40 11.7 30 - 35 22

SH Stea m flow t/ h 150 350 700 405 457SH Tempera ture °C 535 505 567 538 543SH Pressure ba r 115 94 169 138 175

RH Stea m flow t/ h - - 654 375 407RH Tempera ture °C - - 565 537 542RH Pressure ba r - - 36 36 38

Commissioning Yea r 1994 1995 1995 2002 2003

Rema rks ConsortiumLurgi - ALSTO M

Engineeringby ALSTO M

ConsortiumLurgi - ALSTO M

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Conclusion

This paper has summarized the advantages of two Clean Coal technology options:

State-of-the-art Sliding Pressure Supercritical designs respond to the need for higher efficiency steamcycles, with their corresponding environmental benefits and fuel savings. Supercritical units have proven theirability to achieve high availability in both base load and load following operating modes throughout the world.

Circulating fluid bed designs provide the flexibility to burn high sulphur and difficult to burn fuels bothcleanly and reliably. The Akrimota power plant will be India's first CFB plant operating on such local Indianfuel and will achieve sulphur removal rates of 97 % using also local limestone.

The selection of a steam cycle, whether supercritical or subcritical, and a combustion technology - pulverizedcoal or fluidized bed, will, of course, be extremely fuel and site specific. Modern-day optimized designs strike abalance among plant economics, proven design experience, and prudent engineering selections. Ultimately,each plant owner will need to evaluate the most cost-competitive solution for their unique combination of fuelcosts, operating profile, power revenues, and financing structure.

ALSTOM looks forward to working with you to ensure that your steam cycle and combustion technologyselections meet your performance and environmental goals.

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Sadlon, Guenter Scheffknecht.5. Analysis and Summary of Rifled Tube Heat Transfer Date, Mark Palkes, J. H. Chiu6. State-of-the-Art Large Capacity Sliding Pressure Supercritical Steam Generators, Mark

Palkes, Edward S. Sadlon, A Salem7. First 900 MW supercritical boilers in China. A new generation of coal fired power stations. , Kessel, W.,

Scheffknecht, G.: Power-Gen Europe 2000, 20.-22. June 2000, Helsinki, Finland.8. Material issues for supercritical boilers , Scheffknecht, G., Chen, Q.:. PARSONS 2000, 5th International

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