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Open Access Journal

Journal of Power Technologies 92 (1) (2012) 2733

journal homepage:papers.itc.pw.edu.pl

Distribution of solids concentration and temperature in the combustion

chamber of the SC-OTU CFB boiler

Artur Baszczuk, Maciej Komorowski, Wojciech Nowak

Environmental Protection and Engineering, Czestochowa University of Technology69 Dbrowskiego Street, 42-201 Czstochowa, Poland

Abstract

One of the main parameters influencing heat transfer in a circulating fluidized bed boiler is the dis-tribution of the beds solids concentration within the contour of the boilers combustion chamber.This paper contains an analysis of the impact of the solids concentration in a fluidized bed on thevertical temperature profile in the combustion chamber of a SC-OTU CFB boiler. Investigations wereconducted at different loads of nominal generator power: 100%, 80%, 60% and 40%. Solids concen-tration was determined through pressure sensor measurements. Pressure measurements were taken at11 measuring ports situated on the front wall of the combustion chamber. The vertical temperatureprofile was produced on the basis of temperature measurements from various furnace elevations. The

combustion process was carried out at various primary to secondary air ratios with a constant excessair coefficient.

Keywords: temperature distribution, solids concentration, CFB boiler, bed pressure, unit loads

1. Introduction

Solids concentration in the contour of the com-bustion chamber has a significant impact on thework of the whole CFB unit. The solids concen-tration is affected by the particle size diameter of

materials fed to the boiler: limestone, bed ma-terial make-up sand, fuel supply, re-circulatedfly ash/bottom ash and internal material recircu-lation depending on the efficiency of the separa-

Paper presented at the 10th International Conferenceon Research & Development in Power Engineering 2011,Warsaw, Poland

Corresponding authorEmail addresses: [email protected]

(Artur Baszczuk), [email protected]

(Maciej Komorowski), [email protected](Wojciech Nowak)

tor. The particle size distribution of the mate-rials supplied to the combustion chamber affectsthe course of the combustion process, but is neg-ligible from both the hydrodynamic point of viewand the economics side [1].

The wide range of particle size distribution af-fects the segregation of solids, which is seen in thesituation when light particles, i.e. recirculated flyash, are elutriated by the fluidization gas evenin small fluidization velocities and coarse parti-cles sink to form a dense phase in CFB boilers.Also the relative amount of very light materialssupplied (having a very small d50) can lead toa situation where even in mid-range fluidizationvelocities, bigger particles are transported to the

higher levels of the combustion chamber by clus-ters of fine particles. Hence, the proportion of

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coarse material to fine particles has to be opti-mized to obtain good gas-solid, solid-solid mix-ing, and to avoid the problems associated withthe outflow of coarser materials, for example coal

particles, which in such case could be combustedin the cyclone instead of the combustion chamber.

Reflecting the vertical distribution of solids par-ticles, the combustion chamber is divided intobroad zones such as: bottom bed zone, splashzone and transport area. Research [2] deliveredthe information that the bottom bed zone is simi-lar to the bubbling bed, with constant concentra-tion and exploding bubbles, and above it is thesplash zone. Johnsson et al. [3] found that aver-

age bulk density of the bottom bed is about 800to 1200 kg/m3 and these values correspond to bedporosity of 0.54 to 0.70. The part of the bed whichis characterized by the constant pressure drop isthe bottom bed height. Bottom bed density isdetermined by the pressure drop, with negligi-ble influence from gravitational acceleration [4].The splash zone can be characterized by the ex-ponential decay of the solid particles concentra-tion in the vertical direction and with strong solidbackmixing throughout the cross-section. Datapresented by Zhang et al. [5] showed that thecore wall layer structure which is present in thetransport zone in large capacity units is similarto the core annulus structure which is presentin a circular cross-sectional combustion chamberwith small capacity. From [5] investigations [5],the biggest backmixing is in the wall region andflow is low in the core area downward. In thisarea, particles which attained the furnace exitform the externally recirculated flow while par-

ticles from the wall-layer form the internally re-circulated flow. According to [6] the particle con-centration in the wall-layer is about three timesas high as the cross-section average density.

Each material supplied to the boiler undergoesprocesses in the combustion chamber such as:solid mixing, fragmentation, attrition and com-bustion. Polydisperse solids materials which aremixtures of heterogeneous (in diameter and den-sity) loose materials, raised up by fluidization gas,

move with different velocities. The variety of par-ticle velocities leads to collisions, causing erosion,

attrition and destruction of particles.According to [3] for solid mixing, the axial profilecan be described by the expression:

(h) =[d exp(a(HHd))] (1)

exp(asplash(h Hd))

+ exp(a(Hh))

The fragmentation process can be divided intotwo components primary and secondary frag-mentation [1]. Primary fragmentation starts af-ter fuel is injected into the combustion chamber,leading to losses of mass material (moisture and

volatiles) in the combustion chamber due to heat-ing, drying and devolatilization. The coal com-bustion process leads to secondary fragmentationdue to weakening and breaking up. This processoccurs in the whole combustion chamber and isvery important due to the simple fact that in thebottom region the particles are much coarser thanin the upper zone of the combustion chamber.

Particles of much smaller particle size diameterare created from the original, bigger particles dueto surface abrasion in the fluidized bed boilers andthis whole process is termed particle attrition.Various studies state that the presence of smallerparticles increases the attrition of bigger particles,but not vice versa [1].

The combustion process starts after fuel is in-jected into the combustion chamber, firstly frommoisture loss and devolatilization, then char com-bustion. The place in the combustion chamberwhere these processes take place is a very impor-tant feature with regard to the forming of the pri-

mary combustion products. According to [6] fuelfeeding in each point of the combustion chambereffects the axial gradients of the fuel concentrationdue to solids mixing. In a very large combustionchamber although the solid mixing is very quick,quite high axial gradients of the fuel concentrationcan occur [7].

Sekrets research [6] shows that there is signifi-cant particle segregation in the combustion cham-ber, not only in the contour of the furnace (coarser

particles falling to the dense zone and light parti-cles transported to the dilute zone), but also axial

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segregation in the dilute zone (between the coreand wall layer). Sekrets investigations show thatthe low part of the CFB combustion chamber isthe place where the predominant mass of fuel par-

ticles is burnt. The author in the work [6] showsthat in the wall layer of the dense zone the rate offuel particles is 55%, which means that this is anarea of intensive devolitilization. In the core re-gion of the dense area, the reduction rate of fuelparticles was higher: 77% This is a zone of in-tensive fragmentation processes, both mechanicaland thermal. This leads to the conclusion thatin the axial direction, the dense region is an areaof intensive solids mixing. In the dilute area, the

rate of fuel particles reduction is much lower: inagreement with results from paper [6] it is 10%and 3% respectively for the core and wall layer.The biggest mass losses in the core regard smallparticles (due to the combustion process), and inthe wall layer regard coarse particles (due to frag-mentation, which is intensified by the backflow ofgas near the wall and the constant particle masstransfer between the core and hydrodynamic walllayer). In a large scale unit like the Lagisza SCOTU CFB boiler, the higher velocities of falling

particles in the hydrodynamic wall layer and thekick-out of the walls in the bed part of the fur-nace improve internal particle circulation in thebed zone. In large scale boilers, which have alower HK/De ratio, the mixing process is more in-tensified, because of the particles falling in thehydrodynamic wall layer.

Fuel preparation and feeding vital to main-taining the combustion process [8] at the desiredoperating conditions are very important in both

emission and efficiency control. Fuel injectedshould have suitable parameters for the boiler,such as: low heating value, ash content, corro-sion potential of combustion by-products (chlo-rine content). The fuel should also be prepared.Fuel feeding is important for keeping the com-bustion process at steady state conditions: overlylarge particles can block fuel preparation and fuelfeeders, resulting in non-homogenous fuel feed-ing. Very coarse bed material can lead to prob-

lems with combustion and heat transfer, becauseit has a deleterious effect on the fluidization pro-

cess. Sintering and slagging cause defluidizationand are caused by the chemical composition offuel fed to the furnace (especially alkali content).

One very important issue is the link between

the monitoring of temperature inside the combus-tion chamber and the control measures for gasemissions. As regards large combustion plantsthe permissible level of gas emissions in the Euro-pean Union is regulated by the Large CombustionPlant Directive (2001/80/EC LCP). The mainflue gas components are as follows: SO2, NOx,CO and solid particles. The temperature withinthe combustion chamber depends on the follow-ing parameters: particle size distribution of boiler

materials (i.e. fuel, sorbent, make-up sand), solidhold-up within the combustion chamber, primary-to-secondary air ratio, the height of the bed inthe lower part of the combustion chamber, ex-cess of supplied air, circulation of bed materialbetween the combustion chamber and return leg,fuel moisture content and low heating value. Typ-ical work temperatures of the boiler lie in therange 750900C. This temperature range is themost suitable range for sulphur oxide bonds andlow level NOx emissions. The degree of the desul-phurization process depends on the following fac-tors: quantity of limestone (i.e. reactivity index),combustion temperature, bed height, fluidizationvelocity, primary-to-secondary air ratio, fly ashrecirculation and the qualities of the coal and ash.Combustion temperatures of below 750C lead tolower NOx emissions but higher emissions of COand hydrocarbons. The higher combustion tem-perature is due to the fact that the efficiency ofthe sulphur bond is lower in higher temperatures

in many cases the efficiency of desulphurizationis much lower above 850C and the emission levelof NOx is much higher as the combustion temper-ature increases.

An important factor for the correct and sta-ble fluidization process especially in the grid zoneof the boiler is the need to avoid agglomerationphenomena. Agglomeration has multiple causes:the chemical composition of fuel and ashes, overlyhigh furnace temperature. Internal and exter-

nal material circulation within the furnace volumeand between the combustion chamber and recir-

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From Economizer

INTREX -RH IIINTREX -SH IV

SH IIISH IIIBenson

Bottle

Water/Steam

Separator

To SH I

SH

II

Figure 1: Schematic diagram of the 460 MWe OUT-SCCFB boiler at Lagisza Power Plant [9].

culation system has a significant influence on heattransfer and is responsible for creating the tem-perature profile in the contour of the combustionchamber. This statement holds true for such basicassumptions as: layout of the heat exchange area,fuel and sorbent feed points, physical-chemicalproperties of the fuel and sorbent being stable

during boiler work. Changes in the heat trans-fer coefficient within the combustion chamberthrough monitoring the mass flux and tempera-ture of the recirculated material from the externalheat exchangers is also a control measure and canaffect the furnace temperature profile.

2. Experimental setup

Investigation were carried out at the worlds

largest fluidized bed combustion boiler, which isthe second generation of once-through supercriti-cal circulating fluidized bed (CFB OUT-SC) withcapacity of 460 MWe, which is installed in theLagisza Power Plant of Poludniowy Koncern En-ergetyczny S.A. in Poland. A schematic diagramof the 460 MWe OUT-SC CFB boiler is shown inFig. 1, while basic constructional parameters areshown in Table 1.Lagiszas boiler is designed to burn bituminous

coal as its main fuel. The fuel properties for theboiler are shown in Table 2. There is an option

Table 1: Design parameters of 460MWe OTU-SC CFBboiler at Lagisza Power Plant [10].

Parameter Unit DataCapacity MWe 460

Net electrical efficiency - 44Boiler type - OTU-

SCMain steam flow kg/s 360Main steam pressure MPa 27.5Main steam temperature C 560Reheat steam flow kg/s 307Reheat steam pressure MPa 5.5Reheat steam temperature C 580Feed water temperature C 289.7

to burn additional fuel, in particular coal slurry.Coal slurry can be combusted in a maximum 30%share by fuel heat input [10]. Light oil is used asa start-up fuel.

Unlike typical CFB boilers, this boiler is equippedwith Benson vertical-tubes which are a new su-

percritical steam technology. At the height of8.95 m from the grid, the combustion chambercross section area has a diameter of 27.6 m10.6 m(widthdepth). The total height of the combus-tion chamber is 48 m. On the bottom of the com-bustion chamber there is a fluidization grid withprimary air nozzles. Secondary air nozzles are lo-cated at three elevations above the grid (SA rightwall and SA left wall).

The boiler is equipped with 8 solid separators:

4 on both sidewalls. The separators are made ofmembrane walls, which are cased with a thin layerof erosion-resistant refractory. A similar construc-tion is used for the solid particles return leg to theINTREXTM chamber, which is integrated with theboilers sidewalls. In the INTREX chambers theheat is transferred with circulating material to thefinal superheater and reheater stages. In the com-bustion chamber there are: points of solid particle

returns from the INTREXTM chamber, points of

fuel and sorbent (limestone) feed, make-up sandand points of recirculation ash feeding.

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Table 2: Fuel properties for 460 MWe Lagisza OUT-SCCFB boiler [9].

Component Unit Overallrange

BituminouscoalLower Heating value(LHV)

MJ/kg 1823

Total moisture % 623Surface moisture % max. 10Ash content % 1025Carbon content % 5260Hydrogen content % 3.34.2Oxygen content % 6.18.9

Nitrogen content % 1.31.7Sulphur content % 0.71.7

3. Results and discussion

All tests were carried out for four different gener-ator loads (i.e. 100%MCR, 80%MCR, 60%MCRand 40%MCR). During each test, bed pressurewas at high values of about 6 to 7 kPa.Temperature measurements in the combustionchamber were carried out on the boilers frontwall at various unit loads and combustion cham-ber height (42.4 m; 31 m; 24 m; 8.3 m; 5 m; 2 m;1 m and 0.25 m). During investigations, the tem-perature distribution within the boilers combus-tion chamber was in the range 741C to 888C.Recorded temperatures were in the range of typ-ical temperatures for CFB boilers of 750900C,which is optimal in the thermal-flow conditionsfor the reduction of gaseous emissions. Verticalprofiles of temperature in the combustion cham-

ber are shown in Fig. 2. In the graph (Fig. 2)furnace temperature differences for all the unitloads can be seen on the X-axis. The relativetemperature differences in the range of all unitloads (i.e. 100%MCR, 80%MCR, 60%MCR and40%MCR) varied between 13C and 148C. Onthe basis of the data shown in Fig. 2, the sec-ond generation Lagisza boiler features lower rela-tive temperature within the combustion chamber.Temperature distribution in combustion chamber

is nonlinear in character. The result of secondaryair influence on the temperature distribution can

40

250200150100500

100% MCR unit load

80% MCR unit load

60% MCR unit load

40% MCR unit load

Fit polynomial

Distancefromt

hegrid[m]

Relative temperature [oC]

impact of secondary air flow

30

20

10

0

50

Figure 2: Temperature distribution in the combustionchamber for Lagisza OUT-SC CFB boiler.

200 400 600 800 1000

0

10

20

30

40

lower lean phase zone

upper lean phase zone

transition zone

dense phase zone

bottom/grid zone

Suspension density [kg*m-3]

Distancefromt

hegrid[m]

Distance from the grid, 100% load

Figure 3: Distribution of solids concentration in the com-bustion chamber at 100% unit load.

be seen in L of the combustion chamber heightabove the dense region. There are significant gra-

dients of temperature at the height of 10 m in thecombustion chamber. In particular, this situationcould be seen in the area of the dilute circulatingbed, where the weak mixing process of the solidphase with gaseous phase took place. This is alsoa confirmation of the low volumetric concentra-tion of the circulating bed.

Figures 3 to 6 present the distribution of solidsconcentration along the height of the combustionchamber. Solids concentration was determined

through pressure measurements in the combus-tion chamber using Aplisens sensors. Pressure

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200 400 600 800 1000

0

10

20

30

40

dense phase zone

bottom/grid zonetransition zone



Distancefromt

hegrid[m]




200 400 600 800 10000

10

20

30

40

bottom/grid zone

dense phase zone

transition zone



Distancefromt

hegrid[m]




measurements were carried out in 11 measuringports situated on the front wall of the combus-tion chamber. Due to the anticipated high ver-tical pressure gradient of the bed material in the

bottom part of the combustion chamber, the mea-surement ports were thickly spaced in this area.Five main zones along the combustion chambersheight can be readily identified, i.e. from the gridrespectively: bottom/grid zone, developed densephase zone, transition zone, lower lean phase zone(which contains fewer fine and more coarse parti-cles) and upper lean phase zone. From the data,a significant difference in solids concentration canbe observed between the grid zone and the up-

per part of the furnace. This difference is at itslowest for the 100%MCR load and its greatest for

200 400 600 800 1000

0

10

20

30

40

bottom/grid zone

dense phase zone

transition zone



Distancefromt

hegrid[m]




40%MCR, being ca. 140 times and ca. 730 timesrespectively. These data lead to the conclusionthat if the load is nearer to the nominal load, thedifference in solids concentration in the contourof the combustion chamber height is significantlysmaller than in the case of the lowest unit loads.

4. Conclusions

The circulating fluidized bed boiler at the PKELagisza Power Plant operates smoothly under theinvestigated conditions. The range of changes inrelative temperature was not significant and var-ied between 13C and 148C. During the inves-tigation, temperatures in the combustion cham-ber were in the range of typical temperatures forCFB boilers, i.e. 750900C. Increased temper-atures are seen at and especially above the level

of the secondary air feed to the combustion cham-ber. The solids concentration in the contour of thefurnace was determined from the registered pres-sure values. A significant difference in solids con-centration between the grid and the upper zoneof the combustion chamber was observed. Thisdifference increased as the unit load fell, beingca. 140 times and ca. 730 times respectively for100%MCR and 40%MCR load.

The results lead to the conclusion that units such

as the Lagisza Power Plant work flexibly acrossthe whole range of unit loads.

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Acknowledgements

The research leading to these results re-ceived funding from the European Com-munitys Seventh Framework Programme

(FP7/2007-2013) under grant agreement n

TREN/FP7EN/239188/FLEXI BURN CFB.

References

[1] Y. Hua, G. Flamant, J. Lu, D. Gauthier, Modellingof axial and radial solid segregation in a cfb boiler,Chemical Engineering and Processing 43 (2004) 971978.

[2] A. Svensson, F. Johnsson, B. Leckner, Fluid-dynamics of the bottom bed of circulating fluidizedbed boilers, in: Proceedings, 12 th Int. Conf. on Flu-

idized Bed Combustion, L. Rubow and G. Common-wealth, ed., San Diego, pp 887 897, 1993.

[3] F. Johnsson, B. Leckner, Vertical distribution ofsolids in a cfb-furnace, in: Proceedings of 13 th Inter-national Conference on Fluidized Bed Combustion,ASME, Orlando, May 7-10,pp. 671 - 679, 1995.

[4] B. Leckner, Fluidized bed combustion: mixing andpollutant limitation, Prog. Energy Combust. Sci. 24(1998) 3161.

[5] W. Zhang, F. Johnsson, Fluid dynamics boundarylayers in circulating fluidized bed boilers, report a91-193, issn 0281-0034, Tech. rep., Dept. of Energy Con-

version, Chalmers University of Technology, Gote-borg (1991).[6] R. Sekret, Warunki cieplno-przepywowe i emisje

zanieczyszcze w kotach z cyrkulacyjn warstw flu-idaln duej mocy, Zeszyty Naukowe, Politechnikalska, Gliwice 1694 (2005) 5118.

[7] R. Brown, E. Brue, Resolving dynamical featuresof fluidized beds from pressure fluctuations, PowderTechnology 119 (2001) 6880.

[8] J. Koornneef, M. Junginger, A. Faaij, Development offluidized bed combustion an overview of trends, Anoverview of trends, performance and cost, Progress inEnergy and Combustion Science 33 (2007) 1955.

[9] Materials of Foster Wheeler Energy Oy, Finland 2009.[10] A. Hotta, Foster wheelers solutions for large scale

cfb boiler technology. features and operational per-formance of lagisza 460mwe cfb boiler, in: Proc. 20th International conference on Fluidized Bed Com-bustion, pp. 59-70, May 18-21, Xian China 2009.

Nomenclature

(h) voidage along the bed height

d, particle volume fraction in the bottomzone and in the upper zone

asplash, a decay exponents respectively for thesplash zone and transport zone, both inthe function of the particle diameter

H the total height of the combustion cham-ber

h the height above the air distributor

Hd the bottom zone height

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