Fuel Processing Technology 91 (2010) 789–798

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Fuel Processing Technology

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Gas flow analysis in a Kraft recovery boiler

D.J.O. Ferreira a,b, M. Cardoso a,⁎, S.W. Park b

a Federal University of Minas Gerais — Chemical Engineering Department, Espírito Santo st., 35, CEP 30160-030, Belo Horizonte, Brazilb São Paulo University — Chemical Engineering Department, Luciano Gualberto 380 av., trv.3, CEP 05508-900, São Paulo, Brazil

⁎ Corresponding author. Tel.: +55 31 34091783.E-mail address: marcelocardoso@globo.com (M. Car

0378-3820/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.fuproc.2010.02.015

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 June 2009Received in revised form 13 February 2010Accepted 20 February 2010

Keywords:Computational fluid dynamicsKraft recovery boilerAir supply arrangementsFlue gas flowQuaternary air arrangements

Demands for optimal boiler performance and increased concerns in lowering emission have always been thedriving force in the reevaluation and evolution of the Kraft boiler; specifically the air distribution strategies thatare directly related to achieving increased residence time of flue gas combustion inside the furnace which inturn lowers atmosphere emission levels and enhances boiler operation. This paper presents the results of astudy that analyzes the interaction of the different multilevel air injections have on flue gas flow patternsincluding various quaternary air supply arrangements. Additionally, this study assesses the performance of theCFD (Computational Fluid Dynamics) model against data available in literature. Simulations were performedconsidering isothermal and incompressible flows, and did not take into account thermal phenomena orchemical reactions. The numerical solutions generated proved to be coherently related to the data available inliterature, and provided proof of the efficiency of tertiary level air injection, as well as revealed that quaternaryair injection ports arranged in a symmetrical configuration is most suitable for optimal equipment operation.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Themain functionof thefiber processing line inKraft pulpingmill is toremove lignin fromwood and, in most plants, to achieve high brightnesspulp in the end of the bleaching sequence. This process also produces aby-product calledblack liquorwhich is extracted fromthedigester duringthe wood cooking process, and is sent to the recovery boiler.

The chemical recovery cycle is necessary to make the pulpingprocess economically feasible. Here the black liquor, which has beenconcentrated by a multi-effect evaporation system, is burned in thefurnace of the chemical recovery boiler. The combustion of the organiccomponents of the black liquor generates energy, in the form of highpressure superheated steam, which is used in the plant, while thecombustion of inorganic material goes through reduction reactions torecover Na2S and Na2CO3 for reuse in the wood pulping process.

The number of recovery boilers that have built from 1970 up to2006 with the firing capacity of over 1000 dry solids ton/day is veryimpressive [1,2]. It is estimated that a 1000 dry solids ton/day recoveryboiler produces around 95 MWof energy, sufficient for the demands ofa medium size city. Only part of this energy is converted to electricityby co-generation. More steam at higher pressure and electricity arerequired from this boiler, so operational safety [3,4] is an importantissue. The present work aims to improve our understanding of reco-very boiler by using CFD modeling.

The boiler is composed of the furnace, where the black liquorcombustion occurs, and the auxiliary equipment: economizers, boiler

banks, and superheaters. A detailed description of this equipment isfound in the literature [1,5] and showed in Fig. 1.

Multilevel combustion air injection strategies in the furnace of therecovery boilers strongly influence the reduction reactions and vaporproduction. Furthermore, these strategies directly affect the behaviorof the flue gas flow patterns within the furnace where each air levelhas a specific function in supplying the oxygen mix, maintainingoperational stability and reducing the atmosphere emissions. The airis usually fed into the boiler through ports located in the followingfour main levels:

Primary— At this level, the air, which is fed through air ports on fourwalls, supplies oxygen for burning the black liquor producing heat andproviding a reducing environment for the reduction reactions thatoccurring in the char bed.Secondary — This is the level where the air is fed through interlacedports in oppositewalls, usually the front and rearwalls, to increase theeffect of the bull nose on the flue gas flow. This secondary air injectionreduces the formation of theflow channeling [5], defines the height ofthe char bed, and aids in increasing the gas mixture in the furnace.Tertiary—Air is fed in a similarway to the secondary one (interlacedin opposite walls) and is located above the liquor guns. The mainfunction of tertiary air is supplying complementary oxygen, used inthe black liquor combustion, and mitigating any undesirableturbulent edges formed below. Therefore, this is the level mainlyresponsible for the stability of the liquor burning inside the furnace.Quaternary— The quaternary air is usually present in larger boilerswhere it is fed near the furnace exit close to the bull nose. Itsprimary function is to provide supplementary oxygen which

Fig. 1. Kraft recovery boiler; detailed view from a pulp and a paper mill.Source: Flávio Paolielio [6].

790 D.J.O. Ferreira et al. / Fuel Processing Technology 91 (2010) 789–798

contributes to the increase in the residence time of the flue gas,thereby providing better heat exchange and gas recirculationinside the furnace. Thus, the quaternary air supply regulates theflue gas temperature profile (which, in turn, is strongly related tothe combustion kinetics), resulting in emission reduction ofatmospheric pollutants and inhibiting the formation of NOX bythermal and prompt reaction mechanisms [7]. The quaternary airinjects additional air at upper furnace decreasing the rate of underreacted fuel/air, increasing residence time and increasing flowstability. It is noteworthy to add that proper utilization ofquaternary air can reduce NOX emissions by half [8].

The main NOX production mechanisms are the Thermal NOX, FuelNOX, and Prompt NOX. In a Kraft recovery boiler, the dominantmechanisms are the Fuel NOX and the Prompt NOX. In the ThermalNOX, the N2 present in the air is converted to NO at high temperatures,above 1400 °C [7]. The furnace temperature on chemical recoveryboilers isn't greater than 1000 °C, therefore, in this equipment, there isno NOX formation by Thermal mechanism. In the Fuel NOX, thenitrogen present in the fuel is released and reacts in low temperaturesand locally fuel-rich environments [7], such as, for example, betweenchar bed and secondary air level region. Prompt NOX is a mechanismresulting from the reaction of hydrocarbons with molecular nitrogen[7]. It happens in fuel-rich conditions where temperatures arerelatively low and residence times are short, such as instable flowsat high velocities with under reacted gas. So, in a boiler with flowchanneling inside furnace, there are NOX formation due to acombination of Fuel and Prompt NOX production mechanisms.However, the analysis of NOX chemistry is beyond of the scope ofthis work.

Analyses of the air injection strategies and their effects on the fluegas patterns are done with Computational Fluid Dynamics (CFD)techniques, which have been used in chemical recovery boilers since1989 by studying the flue gas flow inside the furnace [9]. Since then,

the evolution of the boiler has always coincided with technologicaladvances in computer processing capacity and the development ofmathematical methods for solving differential equations. The firstworks had the objective of validating the CFD methodologies appliedon Kraft recovery boilers [10,11]. Studying the air supply levels beganwith Jones and Chapman [12] by testing different air supply strategies,then Yang et al. [13], evaluating computational mesh to simulateprimary and secondary air levels, Abdullah et al. [14], comparingisothermal simulation data with cold flow experimental results and,Lien and Horton [15], studying design modifications based onisothermal simulation results. Olausson [16] and Janka et al. [17]published patents for new air supply configurations in which the firstproposed a new quaternary air arrangement and the secondpresented an arrangement that promotes the formation of fourvortices spinning in opposite pairs in at least one air level. Recently,Fontes et al. [18] workedwith experimental and simulation results fordifferent configurations and supply strategies for secondary air level.They studied the influences of interlaced air ports in two oppositewalls and in four walls that lead to a rotational flow. Bymeasurementstaken above the bull nose in the convective heat transfer section andCFD simulations, it was observed that interlaced secondary airconfiguration in two walls provides a more balanced distributionwith lower temperatures close to superheaters that minimize particledeposition.

CFD simulations that specifically study quaternary air supplystrategies began with Walsh et al. [19], whose work described thedevelopment of projects and optimization of an industrial recoveryboiler. Among the changes implemented, there are the addition of thequaternary air level and new arrangements and sizes of the air ports ofthe tertiary air. Works such as those of Baxter et al., Bergroth et al. andMueller et al. [20–22] also used CFD to analyze the insertion of thequaternary air. At the same time in which quaternary air advances areused in boiler operations, the boilers capacity and the dry solidscontent have been increasing and their negative effects have beenbecomemore important. So, works studying NOX emissions [23], fume

Fig. 2. Kraft recovery boiler dimensions considered in the computational domain.

Fig. 3. Geometry details used to represent lower furnace for the computational domain.

791D.J.O. Ferreira et al. / Fuel Processing Technology 91 (2010) 789–798

and particulate deposition [24–26] in function of temperature andflow behavior have been more frequent.

Nowadays, CFD simulations are considered the best auxiliary tool touse in assisting in the investigationof operational strategies andprojectsfor new boilers, as well as in retrofit works for older boilers. Therefore,we have developed a CFD modeling of the flue gas flow in a Kraftrecovery boiler furnace to produce the informationneeded for analyzingthe interaction between air injection at various levels and flue gas flowpatterns. The tertiary air insertion was studied in function to extinctionturbulent eddies created by interlaced secondary air supply. Therefore,the objective of this work is testing and comparing strategies ofquaternary air insert and analyzing the flue gas flow inside the furnaceboiler. The effectiveness of several quaternary air arrangements wasanalyzed and evaluated also in function of internal turbulent eddies.Some new quaternary air arrangements were tested for a boiler withinterlaced secondary and tertiary air supply levels.

2. Computational fluid dynamics models

In this study, CFD simulations were performed in order to analyzethe positioning of the secondary and tertiary air ports as well as severalquaternary air configurations. Therefore, the simulations were carriedout in both a transient and steady state which considered incompress-ible and isothermal flow. Whereas, it wasn't necessary to take intoaccount the thermal exchange, chemical reactions or drag of particulatematerial inside the boiler. The following are the procedures andconsiderations taken in the main steps in creating the CFD simulation.

The equipment considered is an industrial boiler, located insoutheastern Brazil, whose current operation has only three air levels.The boiler has a capacity of processing 75 ton/h of black liquor(1200 ton/day black liquor dry solids). The boiler dimensions andsome geometry simplifications chosen for the computational domainare showed in details at the Figs. 2 and 3.

As the addition of quaternary air was evaluated in this study, theblack liquor delivery system is not represented. The char bed isrepresented by a four sided pyramidal cone, for simplicity, no higherthan the lower part of the secondary air level (according to Adamset al. [5] and Saviharju et al. [27]). The air ports of the various levelswere represented as follows:

• The 144 primary air ports, due to the close proximity of each other,are simplified by using 4 slots, one in each wall of the furnace.

• The secondary air ports are distributed in two opposite walls (frontand rearwalls) around 2 to 3 m above the primary air portswhere it isdivided into two levels of 3 ports and positioned so that each port isinterlaced with two others on the opposite wall affording aninterlaced air mix.

• The 12 tertiary air ports are placed 7 to 11mabove the upper secondaryair level in an arrangement similarly to that of the interlaced secondaryair levelwithports onoppositewalls producing interlaced air streams intwo levels.

• The quaternary air was added in proportion to the boiler heightfollowing Olausson [16]. Thus, the quaternary air ports werepositioned 12.6 m above the upper tertiary air level. Four differentarrangements were evaluated as shown in Fig. 4:

Interlaced arrangement 3 ports (with dimensions of 0.22×0.44 m)on the front wall and 2 ports on the rearwall;

Concentric arrangement 1 port (from 0.44×0.88 m) in each wallthat directs the air flow in counter-clockwise with a 30° angle with the wall;

Tangential arrangement 1 port of 0.4×0.88 m in each wall close tothe furnace corners that directs the airflow counter-clockwise;

Crossed ports arrangement 5 ports (each one with dimensions of0.22×0.44 m) organized in such a waythat a picture of a cross is formed in thecentre of each wall.

Fig. 4. Kraft recovery boiler where the geometric representation used in the simulationis shown together with the four different quaternary arrangements: a) interlaced;b) concentric; c) tangential and d) crossed ports.

Table 1Number of cells for computational mesh used on the simulation cases including thetested quaternary air arrangements.

Simulation case Number of cells

Two air levels 998,910Three air levels 998,910Four air levels - interlaced 998,910Four air levels - concentric 870,615Four air levels - tangential 829,879Four air levels - crossed ports 1,070,802

Table 2Characteristic size in each computational mesh region and their respective localizations.

Region Cell size (m) Localization

Primary air 0.2 Slots representing four groups of primary air portsSecondary air 0.2 Secondary air ports

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The unstructured computational mesh generated for the compu-tational domain is composed of tetrahedral elements which arerefined near the char bed, the air ports and the bull nose as displayedin Fig. 5. Sincewe are only interested in the specific behavior of the gasflue flow, it was not necessary to use a prismatic mesh in the boilerwalls or the char bed surface. It is also imperative to note that anyeffects arising from the walls were ignored due to the large differencebetween the order of magnitude of these effects in comparison withthe turbulent eddies, which are studied in this work and arecomparable to the size of the equipment.

The computational mesh used around 900,000 internal cells. Thenumber of cells used in each simulation case, including four

Fig. 5. Computational mesh with detail of refined mesh in the quaternary air level.

quaternary air arrangements, are showed in Table 1 and thecharacteristic size in each boiler region are illustrated on Table 2.

In all computational meshes, the number of cells can be consideredenough to obtain grid independence. Stanmore and Visona [28] statethat around 240,000 cells are sufficient to give grid independence forcoal furnace simulations considering thermal reactions involvingcarbon combustions and seven reactions only for NO chemistry withthree nitrogen species. Therefore, the high number of cells used in thiswork was chosen in order to give a good definition of flow profileswhich can be considered adequate to simulate isothermal flow insidefurnace.

The CFD methodology solves numerically the conservativeequations for mass, momentum, energy and other properties. Theequations considered in Table 3 are presented according to formalismfrom Wilcox [29], Versteeg and Malalasekera [30] and Hill and Smoot[31].

To solve the differential equations of computational fluid dynam-ics, the finite volume method [30] was used as implemented in thecommercial software CFX 10.0. The turbulence was simulated bymeans of the RANS model (Reynolds Average Navier–Stokes) two-

Tertiary air 0.2 Tertiary air portsQuaternary air 0.2 Quaternary air portsBull noise 0.4 Bull noise surfacesOut 0.8 Flue gas outlet surfaceBottom 0.6 Lower furnace walls where are the primary

air ports, secondary air ports and the surfacethat represents the char bed

Walls 0.8 Other furnace walls

Table 3Differential equations considered for the study of isothermal CFD simulations for Kraftrecovery boiler [29,30,31].

Generic conservation equation

∂ ρϕð Þ∂t + div ρϕuið Þ = div Γgradϕð Þ + Sϕ

Equation Expression

Continuity ∂ρ∂t + div ρuið Þ = 0

u momentum ρ ∂u∂t + ρ:uidiv uð Þ = −∂p

∂x + μ :lap uð Þ + ρgxv momentum ρ ∂v

∂t + ρ:uidiv vð Þ = −∂p∂y + μ :lap vð Þ + ρgy

w momentum ρ ∂w∂t + ρ:uidiv wð Þ = −∂p

∂z + μ :lap wð Þ + ρgz

Navier-Stokes ρ DuiDt = − ∂p

∂xi+ div μ:grad uið Þð Þ + SM;i

k ∂ ρkð Þ∂t + ∂

∂xjρkuj� �

= − 23 ρkδij

∂ui∂xj

−ρε + ∂∂xj

μ + μTσk

� �∂k∂xj

h i

ε ∂ ρεð Þ∂t + ∂

∂xj ρεuj� �

= − 23C1ε

εkρkδij

∂ui∂xj −C2ερ ε2

k + ∂∂xj μ + μT

σε

� �∂ε∂xj

h i

Turbulentviscosity

μt = ρCμk2ε

Turbulentvariables

Cμ=0.09; σk=1.00; σε=1.30; C1ε=1.44; C2ε=1.92

Table 5Boundary and initial conditions for the three air levels steady state and transientsimulations.

Steady state and transient simulations

Domain StationaryReference pressure (atm) 1Isothermal and incompressible flowTemperature (°C) 150

Inlets Primary air (kg/s) 37.5Secondary air (kg/s) 35.0Tertiary air (kg/s) 10.0

Walls Velocity (m/s) 0Roughness (m) 0No slip

Outlet Total mass flow (kg/s) 82.5

Transient simulation

Domain Time (s) 30Initial time (s) 0Time step (s) 0.03

Initial conditions (t=0) Velocity u (m/s) 0Velocity v (m/s) 0Velocity w (m/s) 0Pressure (atm) 1Kinetic energy (m2/s2) 0 (default)Kinetic energy rate dissipation (m2/s3) 0 (default)

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equation k -ε turbulence model. This model is suitable for this workbecause it is robust, economical, and provides reasonably accurateresults for a large variety of turbulent flows. Moreover, it is the mostpopular and widely used method in industrial problems [7,30,32].

The choice of boundary conditions is an important step in theexecution of the CFD simulations. The chosen isothermal temperaturewas 150 °C because it is the pre-heated temperature that primary andsecondary air supplies are inserted in furnace. Eleven simulationswere done and their boundary conditions are described according toTable 4.

It is possible to observe in Table 4 that, the total mass flow for theboiler operating under only two air levels is 72.5 kg/s and for theothers cases it is 82.5 kg/s (three and four levels). The availableoperating data is for a boiler operating with three air levels. Theobjective of the two air level simulation is to investigate the con-sequences of the absence of tertiary air on the flow behavior andcompare it with earlier results from literature which states theprevention of the formation of flow channeling due to the primary andsecondary air interaction [12]. So, due to the lack of two air levelsoperating data, itwas decided to simulate this casewithout the tertiaryair supplymass flow. In order to supply air to the four levels, twomassflow rate simulations were performed for each quaternary airconfiguration considering both 10 and 15% of the total air mass flow.This was done so that their effect on the behavior and flow stabilitycould be observed. The overall amount of air was not altered since thisquantity is related to the mass flow of oxygen required to burn allorganic matter (defined by the boiler capacity of supplying dry solids)and to reduce all inorganic compounds. It was established that theprimary air mass flow rate should not be changed since it is calculatedin order to ensure a reducing environment near the char bedthroughout the furnace operation. Therefore, the air mass flowintroduced as a quaternary air was compensated for the decrease ofthe secondary and tertiary air. The boundary and initial conditions forthe steady state and transient simulations are described on Table 5.

3. Numerical results

3.1. Velocity field

The comparison of our numerical results with the theoreticalqualitative behavior as described in the literature data [1,5,12] isaccomplished through the transient simulation results. The transientsimulation was performed considering the three air supply levels inorder to visualize the evolution of the velocity field at the secondaryair level during the initial moments. Fig. 6 shows the interlacingresults of the secondary air as a function of time in the first 3.5 s.

Table 4Boundary conditions for the simulations of Kraft recovery boiler for different air supplystrategies.

Airsupply

Simulation Number ofsimulations

Mass flow(150 °C)

Arrangements

Two airlevels

One steady state andone transient state

2 Air 1°: 37.5 kg/s NoneAir 2°: 35.0 kg/s

Three airlevels

Steady state 1 Air 1° : 37.5 kg/s NoneAir 2º: 35.0 kg/sAir 3°: 10.0 kg/s

Four airlevels

One steady statesimulation for eachstudy case

4 Air 1°: 35.0 kg/s NoneAir 2°: 26.2 kg/sAir 3°: 7.5 kg/sAir 4°: 8.5 kg/s

One transientsimulation foreach study case

4 Air 1°: 35.0 kg/s NoneAir 2°: 26.3 kg/sAir 3°: 6.3 kg/sAir 4°: 12.4 kg/s

Obs: The definitions Air1°, Air 2°, Air 3° and Air 4° mean primary, secondary, tertiary,and quaternary air, respectively.

It canbeobserved that the secondary air interlacing,whosebeginningoccurs around the first 1.5 s of operation, has a velocity distributionsimilar to that describedbyAdamset al. [5]. In thefirst 3.5 s of simulation,it is already possible to observe the influence of the primary air on theflow channeling occurring in the center. Indeed, after this time, thesecondary air interlacing is disturbed and there is a turbulent eddytendency due to the flow channeling of the primary air in the centralregionof theplanes, shown in Fig. 6. The symmetric interlacingof thefluegas velocities, as described by Adams et al. [5] and Blue et al. [33], cannotbe seen in a steady state simulation, as canbeobserved at the last velocityfield of Fig. 6, because such anair configuration vanishes in less than4 s ofoperation. In the first 4s of simulation, the mass flow from primary andsecondary air inlets interact with each other by perturbations visualizedon the local flow. At this moment, the primary air just interacts withsecondary air interlacedprofile but it hasn't timeenough tobe influencedby tertiary air. As mentioned before, the main function of tertiary air iskeeping themixing below its level, so it is expected that tertiary air massflow can increases the flow velocities on secondary air level. Due to thisprobable positive interaction, the velocities on secondary air level atsteady state conditions are greater than their corresponding transientresults at 4.0 s but velocity profile structure is similar in both cases. So,there is an evidence of qualitative coherence for the obtained results.

3.2. Flow channeling

The flow channeling is characterized by high ascendant velocitygradients in confined flows [14]. As a consequence of the flowchanneling, a fraction of the heatedflue gases situated in the bottomofthe furnace ascends quickly towards the upper boiler region. As thesegases rise fast from the burning zone to the upper furnace, there is notenough time to perform the thermal exchange with the char bed orwith the furnace water walls. This causes an undesirable gradient oftemperature inside the equipment and brings about an erosiveenvironment resulting in the corrosion of water walls and thesuperheater pipes, and, consequently, in damages and frequent breaksduring the operational procedures for maintenance. According withYuan et al. [34], high temperatures and flow instabilities, conditionspresent in furnaces with turbulent eddies as flow channeling, arefavorable to create cyclic oxidizing/reducing environments that cancause corrosion in the boiler walls. Furthermore, the low velocities inthe char bed surface allow the undesirable accumulation of particulate

Fig. 6. Velocity field evolution at the secondary air level in the furnace for the first 4.0 sof transient simulation and comparison with the velocity field for the boiler operatingunder same conditions at steady state.

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matter, increasing locally its height. This phenomenon can result in anincrease of the inactive layer of the char bed at the expenseof the boilerreduction efficiency and the increase of the particle dragging towardsthe superheat region.

It is possible to observe two main flow channelings in thesimulation results: a central flow channeling that decreases the flowused in combustion for warming the char bed, and a lateral flowchanneling, near the furnace walls, which increases the particulatedragging towards the upper region of the boiler [35]. Similar toconclusions presented by Sullivan et al. [36], the left side of furnacehas high tendency for instable flow due to the more intense flowchanneling presented in all numerical results for steady statesimulations. These flow channelings are created by the passage ofthe primary air through the pyramidal surface of the char bed and areusually mitigated by the secondary air interlacing and often vanishdue to the action (also interlaced) of the tertiary air [37].

Fig. 7 shows a comparison of the iso-surfaces for the velocity of2.5 m/s of flue gases inside the simulated recovery boiler, with two(Fig. 7a), three (Fig. 7b), and four (in the four configurations, Fig. 7c,d,e and f, for each air supply strategy with 10% and Fig. 7 g,h,i and j for15% total air supply on quaternary air respectively) air levels. Thevalue of 2.5 m/s for the iso-surface was chosen because it is a gasvelocity larger than the expected average that furnishes a satisfactoryrepresentation of the flow channeling inside the equipment. The fluegas flow behavior inside the furnace is shown in Fig. 7 where iso-surfaces with this particular value of velocity were generated.

In Fig. 7a, it can be seen that in the absence of tertiary air there, aflowchanneling occurs in the center of the furnace and along both the frontand left walls, above the lower furnace as described in [14]. Theinterlaced secondary air supply is not capable of preventing theformation of the flow channeling caused by the primary air. As theinjection of black liquor occurs between the secondary and the tertiaryair, the flow channeling or other eddies arewelcome in this region since

they don't exceed the tertiary air top level. However, this case, Fig. 7a,the turbulent eddies exceeds the desirable limit.

In Fig. 7b, it is observed that the presence of the tertiary airinfluences the secondary air flows, giving stability to the flue gas flowat this level. It is also interesting to note that the liquor combustionand the mixing of the gases should occur below the tertiary air level isa desirable situation. The tertiary air, under these operating condi-tions, has the function of concentrating the mixing only at the bottomfurnace. The insertion of tertiary air in two levels appropriately spacedof each other by approximately 4 m (in the case of this boiler) waseffective concerning the task of extinguishing the flow channeling.

As illustrated in Fig. 7c,d,e, and f (in which 10% of the totalquaternary air supply were considered), the quaternary air supplyresulted in the reappearance of the flow channeling that was removedby the boiler operation characterized by three levels of air (Fig. 7b). Allsimulated arrangements with the quaternary air presented aweakening of the tertiary air and, consequently, induced new flowchannelings. These flow channelings were envisioned at the left sideof the front wall, near the tertiary air top level. It is possible to observethat the crossed ports arrangement was the worst configurationbecause it exhibited the strongest flow channeling. The concentric andlateral arrangements presented the minor formation of flow chan-neling above tertiary air level, becoming possible good options for thequaternary air supply of the boiler using 10% of total air supply asquaternary air.

Observing the Fig. 7 g,h,i, and g (in which 15% of the totalquaternary air supply were considered), there is a weakening ofsecondary and tertiary air level influences due to the decreasing ontheir respective mass flows. All simulated arrangements presentedflow channeling which were envisioned at the left side of the frontwall, near the tertiary air top level, with greater intensity for theconcentric and tangential quaternary air configurations. The mostundesirable behaviors were observed on the concentric and lateralarrangements which presented tendencies for flow channeling at thecenter and on the front-left corner walls inside furnace above tertiaryair level. The interlaced and crossed ports arrangements presented theminor formation of flow channeling above tertiary air level, becomingpossible good options for the quaternary air supply of the boiler using15% of total air supply as quaternary air. Note that the configurations7d and 7h have rotational air flow with air entrance by 30°, and theconfigurations 7e and 7i have “tangential” air flow with air entranceparallel to side wall, as it is sketched previously on the Fig. 4. Thischoice of configurations was tomagnify the impact of rotational air onthe flow at the quaternary level. The configuration “tangential”doesn't dump very well the ascending air from the tertiary level as theFig. 7i shows in the magnified mode.

Fig. 8 shows a comparison among the velocity profiles of the gasesin the vertical planes of the recovery boiler, with two (Fig. 8a), three(Fig. 8b), and four air levels (Fig. 8c,d,e,f in their various arrangements)with 10% of total air supply as quaternary air. These profiles representanother approach to view the preferred paths of the flow channelinginside the boiler.

In Fig. 8b, it is observed that the flue gas flow behavior is stableinside the furnace when compared with the velocity profile of Fig. 8a.For the boiler with three air levels, the flow is uniform andhomogeneous in the region between the air feed and the bull nose,with turbulence flow observed only below the tertiary air level. Thesecondary air plays the role of increasing the mixing degree at thebottom furnace, and the two levels of tertiary air avoid, in a jointaction, the passage of any turbulent eddies above their level.

Among the studied quaternary air arrangements (with 10% ofquaternary air, relative to the overall amount of air) the configurationsthat showed more pronounced flow channelings were the concentricand the tangential ones. In both cases, it is possible to observe lowvelocities in one side of the char bed and the presence of undesirablevelocity gradients in the superheater regionwhich in turnwill produce

Fig. 7. Steady state flue gas isosurfaces inside the furnace and corresponding to 2.5 m/s. Results: a) without tertiary and quaternary air supply, b) without quaternary air supply. With10% of the overall quaternary air in the following configurations: c) Interlaced, d) concentric, e) tangential and f) crossed ports. With 15% of the overall quaternary air in the followingconfigurations: g) Interlaced, h) concentric, i) tangential and j) crossed ports.

Fig. 8. Steady state vertical velocity profiles for flue gas flows with a) two, b) three and four air levels with 10% of total air supply as quaternary air considering c) interlaced,d) concentric, e) tangential and f) crossed ports arrangements.

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796 D.J.O. Ferreira et al. / Fuel Processing Technology 91 (2010) 789–798

the negative consequences mentions early. This undesirable flowchanneling is also observed in the interlaced quaternary air arrange-ment where the concentric configuration presented the mostpronounced velocity gradients.

However, the injection of the quaternary air in a crossed portsarrangement showed the best results among those tested. The flow inthis configuration is stable, with high velocities limited by the tertiaryair level. The crossed ports quaternary air arrangement showed lowvelocities, uniform flow and nearly absence of velocity gradients afterthe bull nose. As the air insertion occurs in several air ports (twenty),its air supply is slight and smooth, without any significant flowperturbation.

In spite of not presenting in this paper the quaternary air resultsfor 15% of the total air, such air supply was more efficient in reducingthe flow channeling above the furnace than the boiler operation with10% of the total air. As 15% of quaternary air (with respect to the totalamount of air) hasmoremass flow than 10%, its influence is enough tooffset the negative effects due to the decrease of the secondary andtertiary air mass flows. It was also observed that among the studied15% quaternary air arrangements, the concentric and the tangentialconfigurations showed the more pronounced flow channeling.According to Lien and Horton [15] air supply strategies with moresecondary air have tendency for present less particulate transport toconvective heat transfer section. So, on the 15% of total air supply asquaternary air cases, there is the necessity for symmetrical arrange-ments that works as sieve to particulate, fouling, ashes and othermaterials.

3.3. Flue gas velocity vector fields

The real furnace includes the superheater zone near the bull noseand it results on a dumping backpressure in the furnace, as it wastested as a porous domain. In the simulated furnace, this superheaterzone was removed to magnify the effects of quaternary air config-

Fig. 9. Steady state flue gas vector fields (in the horizontal and vertical planes) in the supquaternary air arrangements: interlaced (a), concentric (b), tangential (c) and crossed port

urations. Fig. 9 presents a comparison of the flue gas velocity vectorfields for the horizontal and vertical planes in the convective heattransfer section with 10% of total air supply for four quaternary airarrangements: interlaced (Fig. 9a), concentric (9b), tangential (9c)and crossed ports (9d).

It is possible to confirm that the crossed ports and the interlacedarrangements present more uniform flue gas flows. In both arrange-ments, it was verified a uniform horizontal velocity profile mainly inthe vicinities of the boundary of the computational domain, where thesuperheater pipes are located. The crossed ports arrangementpresents a significant zone of low velocities near the bull nose surface.Therefore, the flue gas velocity vector field exhibits a stable anduniform behavior with the boundary layer displacement immediatelyafter the passage of the gases through the bull nose. This behavior isideal for an efficient heat exchange as well as for low pressure lossesand to decrease the particulate depositions in the superheater pipesunder a controlled operation of the boiler and a few breaks formaintenance. The concentric and tangential arrangements presentrecirculation zones just above the bull nose that can provide local coldflow regions in the superheater section. In the recirculation zone, boththe convective and radiation heat transfer coefficients are reducedand the overall heat transfer is lower in this region [38].

Therefore, a more suitable insertion of the quaternary air into thefurnace is likely to occur in a symmetrical arrangement of the airports, which does not promote rotating effects in the flow. Among thestudied cases, the interlaced and the crossed ports arrangements weremore suitable for the boiler operation.

3.4. Residence time

By means of the last analysis, the residence time was evaluated for60 particle trajectories starting from the secondary air ports until theexit of the computational domain, determining the flue gas streamlines. As the black liquor injection nozzles are located just above the

erheater region considering the boiler operating with 10% of total air supply as fours (d).

797D.J.O. Ferreira et al. / Fuel Processing Technology 91 (2010) 789–798

secondary air flow, plotting the time through the streamlines thatleave the secondary air ports provides a rough estimation of theliquor's residence time. At the steady state, the streamlines coincidewith the trajectory of the particles that are dragged to the top of theboiler.

This analysis was interesting in order to evaluate possible effects ofthe quaternary air on the liquor combustion kinetics. The introductionof the quaternary air causes the decrease of the atmosphere emissionsdue to the increase of the residence time of the liquor particles insidethe furnace [26]. Table 6 displays the results for the residence timesfor each simulated boiler.

According to Table 6, it is possible to verify that due to theintroduction of the quaternary air there is a significant increase in theresidence time of the flue gases inside the furnace, in comparison withthe simulation done with only three air levels. Therefore, there iscompelling evidence that the quaternary air increases the recirculationof flue gas inside the furnace. According to Karvinen et al. [39], thereare little variation on the flue gas flow, O2 profile and particulatetracking inside the furnace for the boiler operating under different drysolids content, except due to the need for more residence time tohigher solids content. So, the increase on residence time due to thequaternary air supply is adequate to crescent demand for moreconcentrated black liquor burning.

The number of the particle trajectories tracked here is very low toconclude under the population balance assumptions, although themethods and observations agree with the operator's analysis. If it isnecessary, it can be calculated with more computationally powerfulresource.

4. Conclusion

Firstly, when comparing our simulation results with the expectedphenomena as described in literature, it was possible to findqualitatively similar features with computational fluid dynamics,therefore supporting its use as an appropriate and very useful tool forobserving the flow channeling in the Kraft recovery boiler. Secondly,the simulation results revealed the effectiveness of interlaced tertiaryair at two levels and further confirmed that quaternary air has apositive effect on the resident time of flue gasses and that among thetested quaternary air configuration, the interlaced and the crossedports arrangements were more effective in producing a more stableboiler operation and lower emissions.

Acknowledgments

The authors are grateful to the Brazilian agencies Fundação deAmparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) andCoordenação de Aperfeiçoamento de Pessoal de Nível Superior(CAPES) for sponsoring this work.

Table 6Residence time ranges for steady state simulation.

Three airlevels

Residence time (s)

23–55

Four air levels With quaternary air as 10% oftotal air

With quaternary air as 15% oftotal air

Arrangement Residence time (s)

Interlaced 32–64 32–70Concentric 39–65 32–69Tangential 25–65 30–70Crossed ports 30–60 34–67

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