Fuel Processing Technology 91 (2010) 88–96

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Fuel Processing Technology

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Numerical simulations of the slagging characteristics in a down-fired, pulverized-coalboiler furnace

Qingyan Fang a, Huajian Wang a, Yan Wei a, Lin Lei b, Xuelong Duan b, Huaichun Zhou a,⁎a State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR Chinab Hunan Electric Power Test and Research Institute, Changsha 410007, PR China

⁎ Corresponding author. Fax: +86 27 87540249.E-mail address: hczhou@mail.hust.edu.cn (H. Zhou)

0378-3820/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.fuproc.2009.08.022

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 April 2009Received in revised form 26 June 2009Accepted 27 August 2009

Keywords:Pulverized coalDown-fired boilerSlaggingNumerical simulation

Numerical studies of the slagging characteristics under different operational conditions in a 300 MWdown-firedboiler were carried out using slagging models coupled with gas–solid two phase flow and combustion models.Combinedwith the real operating conditions; comparative and detailed analysis on the slagging position, extent,and causes ispresented. The results show that the serious slagging ismainly on the sidewalls of the lower furnace.Because of the more rapid expansion of the flue gas under the higher temperature, the flue gas in the furnacecentermakes theflue gas on both sides deflect andflow to the sidewalls; and the pulverized-coalflame impingeson the side walls. This results in the slagging on the side walls. Under off-design operating conditions, such asstopping some burners, the local flow field is asymmetric and impinges on the local arch burner, front and rearwall regionswhere the stopped burners are located. It leads to slight slagging on the arch burner regions and thefront and rear wall regions of the lower furnace. Based on the investigation, it has been found that the seriousslagging on the side walls can be effectively alleviated by cutting off the burners close to the side walls, reducingboiler load and burning low slagging-tendency coals.

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The arch-fired (AF) boiler, also named down-fired or W-flameboiler, is one of the main boiler types that burn low-volatile andanthracite coals for electric power generation in China. Currently,more than 70 AF boilers, having a total capacity of approximately27,000 MW, are either in service or under construction. The AF boilerhas excellent characteristics of stable ignition and combustion of thepulverized coal in furnaces. However, its practical operation stillsuffers from the problems of high carbon content in the fly ash, highNOx emission and serious slagging [1,2]. In order to better understandthe combustion characteristics in AF boilers, several experimental andnumerical investigations have been carried out in recent years [1–6].The results of these works are of benefit to the optimum design andoperation of similar boilers.

Ashdeposits not only reduce the boiler heat transfer efficiency to theworking fluid, but also cause unscheduled boiler shutdowns in severecases. Therefore, ash deposits have been widely focused on [7–21]. Inaddition to the slagging tendency of coals, the ash depositions are alsoaffected by several other factors, such as furnace structural size andthermal parameters, burner type and secondary air distribution mode.

For the actual operating problems of utility boilers caused by slagging, itis necessaryfirst to gain insight into themain causes and then todevelopsome effectivemethods to alleviate or prevent it from happening. Someresearch has been conducted on the ash deposition in coal-fired utilityboilers [7–14]. Numerical methods have been successfully applied tostudy the position, extent and causes of the ash deposits in large-scalecoal-fired utility boilers [15–21]. However, to the authors' knowledge,there has been only one literature [20] to simulate the deposit growthunder slagging conditions for an AF boiler by Babcock & Wilcoxcompany's techniqueup to now.More research is necessary to elucidatethe causes of slagging in AF boilers.

In this paper, numerical simulations of the slagging characteristicsunder different operational conditions in a 300 MW AF boiler werecarried out by use of slagging models coupled with gas–solid twophase flow and combustionmodels. Combinedwith the real operatingconditions, comparative analysis on the ash deposit position, extentand causes is discussed in detail.

2. Mathematical models

2.1. Gas–solid two phase flow and combustion models

The mathematical model is based on a Eulerian description for thecontinuum phase and a stochastic Lagrangian description for the coalparticles. All the Eulerian partial differential equations that govern the

Nomenclature

ϕ variablek turbulent kinetic energy (m2/s2)ε turbulent dissipation rate (m2/s3)μ turbulent viscosity (kg/m.s)ρ density of gas (kg/m3)Γϕ diffusion coefficientSϕ mass source from gas phase (kg/s)Spϕ mass source from coal particles (kg/s)pi the sticking probability of particles of composition iTps impacting particle temperature (K)ps the sticking probability of deposit surfaceTs surface temperature (K)N the number of particle size groupsR90 pulverized-coal finenessAF arch-firedPA primary airVA vent airSA secondary air

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conservation of mass, momentum and energy can be written in thefollowing general form:

∂∂xi

ðρuiϕÞ =∂∂xi

Γϕ∂ϕ∂xi

� �+ Sϕ + Spϕ ð1Þ

where ϕ stands for the three momentum components, the turbulentkinetic energy k and its dissipation ε, the enthalpy, and the mixturefraction and its variance in the conservation equations of differentform. Γϕ is the diffusion coefficient of the transported variable ϕ. Forthe particle case of themass conservation equation, variable ϕ is set tounity and the right-hand side of the equation is zero. Sϕ is the sourceterm of the mass from the gas phase and Spϕ is the source term for themass of the coal particles.

The k–εmodel was used to model the turbulent flows in the furnace.The stochastic particle trajectorymodel was used to simulate themotionof the coal particles in the furnace. The energybalanceof the coal particleswas used to calculate the time dependent particle temperature and todescribe the coal evolution. Volatile releasewas computedusing the two-parallel-reactionmodel. Char combustionwasmodeledusing thekinetic-diffusion reaction model. The turbulent combustion of gas phase wasmodeledusingamixture fractionmodel. The radiativeheat transfer in thefurnace was calculated by the discrete transfer method. More detaileddescription of these numerical models can be found in Refs. [22,23]. Thedetailed computational evaluation of low NOx operating conditionsin a 350 MW Foster Wheeler (FW) AF boiler has been conducted inthe past research [6]. Therefore, in the present work, only the slaggingcharacteristics in FW AF furnaces is simulated and discussed.

An in-house code has been employed to conduct the simulations.The governing equations of gas flowwere discretized over a staggeredgrid using the first-order finite difference method with centraldifference and integrated over each control volume in the computa-tional domain. Each of the equations described in the above generalform were tri-diagonal and thus can be solved using TDMA solvers.The equations were solved by successive under-relaxation iterationsuntil the solution satisfies a pre-specified tolerance and the SIMPLERalgorithm of pressure correction was applied to consider the couplingof velocity and pressure fields. The gas phase was affected by thesource term that originated from the combustion of the particle phase,and the solutions of momentum and energy equations for the particlephase were based on the flow and temperature fields of the gas phase.

2.2. Slagging models

In the slagging models, the particle transport process was modeledusing a stochastic trajectorymodel,which accounted for the influence ofgas phase turbulent flows on particle trajectory through randomparticle–eddy interaction [18–20]. The model is also suitable for theinteraction of particle and gas phase turbulent flows within theturbulent boundary layer, where the momentum equation of theparticle is integrated to the wall with particle–eddy interaction time asthe integration time. This model can obtain relatively more accurateresults due to being able to consider the physical mechanism [17].

For the particle sticking model, the particle sticking probability isprimarily concerned with particle viscosity [17]. Critical viscosity wasadopted to estimate the influence of the particle viscosity on thesticking probability. In the present work, the critical viscosity of105Pas was employed, which has been extensively used in previousstudies [17,18,20]. If the ash particle viscosity is less than the criticalviscosity, the particle sticking probability on the wall is 1; otherwise itis the ratio of the critical viscosity to the practical particle viscosity.The detailed formula is as follows:

piðTpsÞ =μrefμ

μ N μref

piðTpsÞ = 1 μ ≤ μrefð2Þ

where pi(Tps) is the sticking probability of particles of composition i,Tps is the impacting particle temperature, μ is the particle viscosity,μref is the critical viscosity. The model is simple and can fit manyengineering applications. A temperature subarea method was appliedto obtain the particle viscosity [24]. The formulas of the high- and low-temperature viscosities are different, and then themaximum viscosityof these two viscosities is selected. This model is precise enough if theparticle critical viscosity in the interval is 104–109Pas, and it is just inthe range of the current viscosity; therefore, this model can providecreditable results and has a wide application.

The particle sticking probability is also affected by the depositsurface stickiness. If the temperature of the deposit surface is above1450 K, the surface is totally viscous, and the sticking probability ofthe deposit surface is 1, otherwise the surface is not viscous, and thusit is 0 [25].

Considering both the particle viscosity and deposit surfacestickiness, the capture efficiency or the portion of the impactingparticles adhering to the surface is approximated by [25];

fdep = ∑N

i=1piðTpsÞ + ½1−∑

N

i=1piðTpsÞ�psðTsÞ ð3Þ

where pi(Tps) is the sticking probability of particles of composition i, Tpsis the impacting particle temperature,ps(Ts) is the sticking probability ofthe deposit surface, Ts is the surface temperature, andN is the number ofparticle size groups.

3. The utility boiler and conditions for simulation

3.1. The utility boiler for simulation

The studied AF boiler, manufactured by Dongfang boiler company inChinawith FW company's technique, is subcritical pressure, onemiddlereheat, natural circulation, double arches and a single furnace in a300 MW unit at Zhuzhou Power Plant, Hunan Province, China. Aschematic diagram of the boiler is shown in Fig. 1. The zone of thefurnace below the arches is the lower furnace (pulverized-coal burningzone). That above the arches is the upper furnace (pulverized-coalburnout zone). The dimensions of the upper furnace are7.63×24.76×23.82 m, and those of the lower furnace are dimensionsof 13.73×24.76×15.48 m. The lower furnace is 4108m3 in volumewith

Fig. 1. Schematic diagram of the boiler. Half of the boiler is shown in the present figure because of the symmetry in the furnace structure. (PA — primary air, VA — vent air, SA —

secondary air).

Table 1Simulation conditions.

Case Coal Load (%) Mills Burners

1 1 100 4 242 1 100 4 203 1 75 3 184 1 50 2 125 2 100 4 246 3 100 4 24

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639 m2 of refractory coverage on the walls. A directly firing pulverized-coal preparation system is used with 4 double-entry double-exit ballmills. The furnace is, on thearches, equippedwith24 FWdouble-cyclonearch burners which can enrich the coal/air mixture. Themixture flow ofthe pulverized coal and primary air is divided into a fuel-rich streamanda fuel-lean stream. The fuel-rich stream is down injected into the lowerfurnace through the burner nozzle. The fuel-lean stream is downinjected into the lower furnace from the vent air (VA) pipe nozzle.Throughnozzles A, B, C, D, E and F secondary air (SA) is supplied into thefurnace according to the need of staging air for gradual and completecombustion. About 70% of the secondary air, divided into three streamsD, E and F, is supplied into the furnace under the arches. The remainingsecondary air, divided into three streams A, B and C, is introduced toports concentric with the fuel-rich nozzle, fuel-lean nozzle and oiligniter. Thedesign coal type is a blended coalwith a dry-ash-free volatilematter of 11.22% and an as-received lower heating value of 20,990 kJ/kg.The designed R90 value of pulverized-coalfineness is about 8%. There aresomeobservingports on the sidewalls. These are convenient tomeasurethe local temperature andobserve the slagging status inside the furnace.

3.2. Simulation conditions

The numerical procedure for slagging was performed as a post-process, which is based on the modeling of the flow, combustion andheat transfer. Gas emissivity is changed with flue gas temperature.The values of 0.3 and 0.15 were adopted for the absorption andscattering coefficients of particles respectively. The scattering ofparticles was assumed to be isotropic. Wall function method andtemperature wall were employed respectively for velocity and thermalboundary conditions. Because theheat exchange between theflame andthe water-cooled walls is mainly by heat radiation inside the furnace,the effect of thedeposit on the heat transferwas estimatedbypresettingappropriate thermal radiation boundary conditions, such as the

temperature and emissivity values of the water-cooled walls and therefractory belts. Due to the symmetry in the furnace structure and in theaerodynamic fields inside the furnace, the simulations were carried outon a half-boiler furnace along the width. Grid-dependent tests havebeen performed and a 79×166×65 orthogonal and non-uniformmeshwasfinally used. Themeshwas refined near the burners for the purposeof more accurate prediction. A typical 15,000 particle trajectories arenecessary to calculate the particle impacting information [17]. With therapid progress in the computing ability of computers, it is possible totrackmore particles. The present simulation tracked 1,536,000 particles.

The simulation conditions are listed in Table 1. The slaggingbehaviorunder 100% load with all 24 burners in service was first simulatedburning Coal 1 in Case 1. Combined with the actual operational status,detailed analysis on the slagging position, extent and causes waspresented. Then, to identify the effects of various operating conditionson the slagging characteristics, the other five cases have also beenconducted. Case 2was carried outwith the four burners close to the sidewalls cut off at full load with the corresponding D, E and F secondary airstill being supplied. Cases 3 and 4 were performed under 75% and 50%loads; and Cases 5 and 6 were as for Case 1, but for different coal types(Coals 2 and 3, respectively).

Table 2Coal properties.

Item Coal 1 Coal 2 Coal 3

Proximate analysis, wt.% (as received)Volatile matter 4.83 5.47 4.50Moisture 7.0 8.0 8.69Ash 33.50 34.47 23.42Fixed carbon 54.67 52.06 63.39Lower heating value (kJ/kg) 20,040 19,210 22,310

Ultimate analysis, wt.% (as received)Carbon 55.98 53.57 64.37Hydrogen 0.73 1.13 1.53Oxygen 0.61 1.65 0.55Sulfur 1.58 0.53 0.80Nitrogen 0.60 0.65 0.64

Table 3Ash component, fusion properties and slagging tendency of the coals.

Item Unit Coal 1 Coal 2 Coal 3

Ash componentSiO2 % 56.97 60.25 54.54Al2O3 % 26.92 26.61 29.30TiO2 % 0.95 1.58 1.15Fe2O3 % 5.98 3.92 5.04CaO % 1.41 1.31 1.66MgO % 0.64 0.78 1.50K2O % 3.76 2.59 4.75Na2O % 1.09 0.95 0.50SO3 % 0.67 0.35 0.12MnO2 % 0.19 0.44 0.04

Ash fusion pointDT °C 1220 1320 1450ST °C 1300 1460 N1500FT °C 1400 N1500 N1500

DT — deformation temperature.ST — softening temperature.FT — flow temperature.

Table 4Operational parameters.

Item Nozzle size Temperature(K)

Velocity(m/s)

Flow(kg/s)

Air rate(%)

Coal concentration(kg/kg)

PA Φ0.253 m 366 19.8 43.2 12.8 0.946VA Φ0.253 m 366 4.8 10.7 3.2 0.199A SA Φ0.330 m 599 35.2 27.4 8.1 –

B SA Φ0.330 m 599 35.2 27.4 8.1 –

C SA Φ0.600 m 599 5 19.4 5.8 –

D SA 0.30 m2 599 8.1 33.6 9.7 –

E SA 0.30 m2 599 8.1 33.6 9.7 –

F SA 0.92 m2 599 11.3 145.6 42.6 –

Vent air valve opening is 30%.The adjustable vane is located at the lowest position of the primary air nozzle.

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The properties of the coals used are given in Table 2. The ashcomponent and fusion temperatures of the coals are illustrated inTable 3. The operating parameters of the boiler are listed in Table 4. Inorder to compare the effects of various operating conditions on theslagging, the same coal and the same particle size distribution wereused. The pulverized coal particles were divided into 10 groupsaccording to their initial sizes as shown in Table 5.

Table 5Particle sizes and mass percentage of the pulverized coal.

Diameter (μm) 5 15 35 60 9Mass percentage (%) 21.50 26.40 25.80 14.80

4. Results and discussion

4.1. Validation for simulation results

Fig. 2 shows the velocity field, temperature and oxygen concen-tration distributions over the central cross section of the burner. It isapparently seen that the velocity field in Fig. 2(a) presents a W shape,which indicates that the practical aerodynamic characteristics insidethe AF boiler furnace are reflected properly. Below the regions of thefurnace arches, there exist two recirculation zones, which can entrainhigh-temperature flue gas, enhance the heat transfer between thepulverized coal particles and high-temperature flue gas, and thusbenefit the stable ignition and combustion of the pulverized coal. It canbe observed in Fig. 2(b) and (c) that, due to the heating by theconvection of the high-temperature fuel gas and by the flame radiation,the pulverized coal particles are ignited in time, and then combustintensively to release a great amount of heat. Due to many refractorybelts, a high-temperature and low-oxygen-concentration zone isformed in the central region of the lower furnace, where thetemperature is between 1700 K and 1900 K and the oxygen concentra-tion is below6%. They are favorable conditions for the stable ignition andcombustion, and burnout of the low-volatile and anthracite coal. Theradiative and mixed superheaters in the upper part of the furnace andabove the nose were not included in the model established. This maylead to the slight increase in the flue gas temperature in the upperfurnace. But it will not affect the flue gas temperature of the lowerfurnace because the length between the lower furnace and thesesuperheaters is more than 10 m. In order to validate the simulationresults of the flow, combustion and heat transfer in general, a suctionthermocouple was used to measure the local temperature through theobserving ports along the furnace height when Coal 1 was burned. Themeasurement points were 1.0 m away from the right side wall. Fig. 3shows that the calculated temperatures agree well with the measuredvalues. Therefore, it is deemed that the present numerical results arereasonable and reliable.

Fig. 4 shows the sticking particle masses on the right side and frontwalls under 100% load in Case 1. The sticking particle masses on theright side wall of the lower furnace are much larger than those on theright side and front walls of the upper furnace, and those on the frontand rear walls of the lower furnace is near zero. This indicates that theslagging mainly occurs on the side walls at heights between 5 and17.5 m, where the refractory belts are laid in the lower furnace. In theregion of heights between 9 and 13 m, where the D, E and F secondaryair nozzles are located, the slagging is very serious. There is noslagging on the front and rear walls of the lower furnace. Foulinghappens on the surfaces of water-cooled walls in the upper furnace.To validate the rationality of the simulation results of the slagging, theactual slagging pictures taken through the observing ports (in Fig. 5)at the elevations of 9.74 m and 12.24 m indicate that the slagging wasserious on the side walls of the lower furnace at the height range ofthe D, E and F secondary air nozzles. The slag is dense and hard to clearoff. But, it can be inferred from the pictures at the heights of 17.84 mand 21.60 m that it is mainly fouling on the water-cooled walls of theupper furnace. These are consistent with the predicted results.

Detailed analysis of the effects of the aerodynamic field, temper-ature and oxygen concentration distributions on the slagging position,extent and causes are discussed next. Fig. 6 illustrates the temperatureand oxygen concentration distributions in the regions close to the sidewalls. It can be seen that there exists a region below the furnacearches, where the temperature is up to 1800 K while the oxygen

0 130 170 205 235 2506.30 2.90 1.40 0.60 0.20 0.10

Fig. 2. Computed results over the central cross section of the burner, (a) velocity field (m/s), (b) temperature distribution (K), and (c) oxygen concentration temperature distribution(vol.%).

92 Q. Fang et al. / Fuel Processing Technology 91 (2010) 88–96

concentration is only about 1%, implying a strongly reducingatmosphere. It indicates an obvious slagging tendency in the regionsof the side walls. Both the simulation results and practical operationshow that the serious slagging mainly occurs on the partial side wallsof the lower furnace in the studied boiler.

4.2. Analysis of slagging causes

Fig. 7 shows the typical velocityfields on the cross sections at variousdepths. It can be observed that the flue gas flow deflects obviouslytoward and impinges on the right side wall. The velocity fields on thecross sections atdifferentheights in Fig. 8 alsodemonstrate that theflowimpinges on the right side wall. The impinging is more serious in theregions of the right side wall around the heights of the D, E and Fsecondary air nozzles, while slighter in the regions of the right side wallabove the arches. Due to the intensive combustion of the pulverizedcoal, a high-temperature zone is formed in the center of the lowerfurnace. Because the flue gas temperature in the central zone is higherthan that on both sides, the expansion of high-temperature flue gas inthe central zone is more rapid. Furthermore, there exists a low-temperature zone 3.0 m long between the side walls and the burners

Fig. 3. Calculated and measured temperatures.

close to the side walls, where no air flow enters at the beginning of thecombustion. Therefore, the more rapid expansion of flue gas in thefurnace center makes the flue gas on both sides deflect, driving the flow

Fig. 4. Slagging masses on (a) the side and (b) the front walls under 100% load.

Fig. 5. Actual slagging and fouling pictures taken through the observation ports, (a) Z=9.74 m, (b) Z=12.24 m, (c) Z=17.84 m and (d) Z=21.60 m.

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towards the side walls, and it impinges on the side walls. It is the mostimportant reason for the flue gas flow impinging on the side walls.

Fig. 9 shows the typical trajectories of the pulverized coal particles.The pulverized coal particles fire gradually during the downwardmovement. When reaching the central zone of the furnace, they turnand move upward due to the effects of the D, E and F secondary airflows. Then they enter the upper furnace, the fuel-burnout zone, andgradually burn out, and leave the furnace finally (Fig. 9(a)). FromFig. 9(b), it can be observed that the pulverized coal particles,

Fig. 6. (a) Temperature (K) and (b) oxygen concentration (vol.%) distributions on thecross section close to the side wall.

especially those from the burners close to the side walls, movetowards and impinge on the side walls following with the flue gaswhile burning at the same time. They burn intensively in the regionsclose to the side walls, where a high-temperature and low-oxygen-concentration zone (see Fig. 6) is formed because of the refractorycoverage of the side walls. Therefore, the high-temperature and evenmolten coal and ash particles directly impact on the side walls, whichis the essential reason for the slagging or the existence of a slaggingtendency on the side walls.

Fig. 7. Velocity fields on the cross sections along the furnace depth, (a) X=3.790 m, (b)X=6.865 m.

Fig. 8. Velocity fields on the cross sections along the furnace height, (a) Z=10.0 m, thecenter of F SA nozzle, (b) Z=12.20 m, between D and E SA nozzles.

94 Q. Fang et al. / Fuel Processing Technology 91 (2010) 88–96

There is no slagging tendency on the front and rear walls of thelower furnace, in the region where the temperature is below 1200 Kand the oxygen concentration is above 15%, presenting an obviouslyoxidative atmosphere. There are many secondary air slot nozzles (D, Eand F secondary air nozzles) located uniformly on the front and rearwalls of the lower furnace. The secondary air from these slot nozzlescan form air curtains which effectively prevent the impact of coal andash particles and consequently the slagging on the front and rear wallsof the lower furnace. It can be also seen from Fig. 2(b) and (c) that theash particles are less likely to deposit on the arches, in a region wherethe temperature is not high and the oxygen concentration is over 15%.In the upper furnace, the heat exchange between the flame and thelow-temperature water-cooled walls can cool the high-temperatureflue gas, char and ash particles. Therefore, only fouling occurs on thewater-cooled walls of the upper furnace. As discussed above, there

also exists a small slagging tendency on the wing walls. The largequantity and momentum of the D, E and F secondary air can preventthe flow of the mixed pulverized coal and air from the arches fromdirectly impinging on the water-cooled walls of the ash hopper.Therefore, slagging does not occur on the water-cooled walls of theash hopper.

4.3. Slagging characteristics under different operational conditions

As analyzed above, slagging mainly occurs on the side walls of thelower furnace. Therefore, the slagging intensity on the right side wallof the lower furnace was used to study the slagging characteristicsunder different operational conditions. Here, the slagging intensity isequal to the ratio of the total masses of the sticking particles to theslagging area in a unit time on the right side wall of the lower furnace.In Case 2, the four burners close to the side walls were cut off. Theprimary and secondary air from the arches is not supplied, but the D, Eand F secondary air corresponding to the burners cut off is stillsupplied from the lower furnace. Compared to the slagging in Case 1,when all 24 burners were in service, the slagging intensity issignificantly reduced from 4.99×10−5kg/m2h to 2.53×10−5kg/m2h.As discussed above, one of the essential reasons for the serious slaggingon the side walls is that the pulverized coal particles, especially thosefrom the burners close to the side walls, impinge on the side wallsfollowing the high-temperature flue gas and burn intensively in theregions close to the sidewalls. By cutting off the burners close to the sidewalls, the coal particles impinging on the side walls will be largelyreduced, and the combustion temperature in the regions close to theside walls will also be decreased effectively. As a result, the slaggingextent can be alleviated. Furthermore, the D, E and F secondaryair corresponding to the burners cut off can increase the oxygenconcentration in the regions close to the sidewalls,which can also lowerthe slagging tendency on the sidewalls. Therefore, it is concluded that itis effective to prevent the slagging on the side walls by cutting off theburners close to the side walls, which has been validated by the actualoperating experiences burning Coal 1 and a similar AF boiler burning alow-volatile and high slagging-tendency coal at Dafang power plant,Guizhou Province, China.

In Case 3, three mills and 18 burners are in service for 75% load,and in Case 4, two mills and 12 burners for 50% load. Compared withthe slagging status at 100% load in Case 1, the slagging intensity isreduced from 4.99×10−5kg/m2h to 2.24×10−5kg/m2h, and then to1.91×10−5kg/m2h. It means that the slagging is obviously alleviatedin these two cases. Therefore, it is also effective to prevent or alleviatethe slagging on the side walls by reducing the load. This is mainlybecause fewer coal particles, flowing with the flue gas, impinge on theside walls and the furnace temperature is lower under lower loads,resulting in a lower slagging intensity. It is also found that there existsslight slagging on the partial front and rear walls of the lower furnaceand the burner regions of the furnace arches. Most of the primary andsome of the secondary air corresponding to the burners cut off will notbe supplied, and only a little secondary air is introduced to cool thenozzles. Under these operating modes, an asymmetric aerodynamicfield will be formed, resulting in deviant combustion in the furnace.The high-temperature flue gas and part of particles will impinge on thelocal regions on the arches and the front and realwalls corresponding tothe burners cut off, and consequently the slight slagging occurs in theselocal regions. So, the present simulations suggest that the operationalburners should be periodically switched on to prevent serious slaggingon the local regions of the front and rear walls in the lower furnace,whichmay block D, E and F secondary air nozzles and be harmful to thesafety and economic performance of the boiler.

Coals 2 and3withdifferent slagging tendencieswere employedunderfull load in Cases 5 and 6. Comparedwith the slagging status burning Coal1 in Case 1, the slagging intensity is reduced from 4.99×10−5kg/m2h to3.53×10−5kg/m2h, and then to 2.95×10−5kg/m2h. It is well known

Fig. 9. Typical particle trajectories in the furnace in the (a) depth direction, and (b) height direction.

95Q. Fang et al. / Fuel Processing Technology 91 (2010) 88–96

that burning low slagging-tendency coals can effectively improve theslagging characteristic. As the ash fusion points shown in Table 3, thesoftening temperaturesof Coals 2 and3arehigher than that of Coal 1. Thisindicates that the slagging tendencies of these two coals are lower thanthat of Coal 1. Therefore, burning these two coals will result in lowerslagging intensities on the side walls. It also suggests that a coal of highslagging tendency should be burned bymixing itwith one having a lowerslagging tendency.

5. Conclusions

Numerical simulations of the slagging characteristics under differentoperational conditions in a 300 MWFWAF boiler have been performedusing slagging models coupled with gas–solid two phase flow andcombustion models. The simulated slagging characteristics agreed wellwith the actual slagging status in the real furnace.

It has been found that the serious slagging is mainly on the sidewalls of the lower furnace. Because of the more rapid expansion ofthe flue gas under the higher temperature, the flue gas in the furnacecentermakes the flue gas on both sides deflect towards the sidewalls,and the pulverized-coal flame impinges on the side walls. It is theessential reason for the slagging on the side walls. Under off-designoperating conditions, such as cutting off some burners, the local flowfield is asymmetric and impinges on the local arch burner, front andrear wall regions where the stopped burners are located. It results inthe slight slagging on the arch burner regions and the front and rearwall regions of the lower furnace. Some fouling occurs on the water-cooled walls of the upper furnace. A small slagging tendency alsoexists on the wing walls. The present study suggests that cutting offthe burners close to the side walls, reducing load and burning thecoals with low slagging tendency are the effective measures toalleviate the serious slagging on the side walls.

Acknowledgements

The financial support of this research from the National NaturalScience Foundation of China (Nos. 50806024, and 50721005), the Hi-Tech Research and Development Program of China (“863” program,

contract No. 2007AA05306) and the Program of Introducing Talents ofDiscipline to Universities (“111” project, No. B06019), China areacknowledged.

References

[1] F. Ren, Z.-Q. Li, Y.-B. Zhang, et al., Influence of the secondary air-box damperopening on airflow and combustion characteristics of a down-fired 300-MWeutility boiler, Energy & Fuels 21 (2) (2007) 668–676.

[2] Z.-Q. Li, F. Ren, J. Zhang, et al., Influence of vent air valve opening on combustioncharacteristics of a down-fired pulverized-coal 300-MWe utility boiler, Fuel 86(15) (2007) 2457–2462.

[3] F. Ren, Z.-Q. Li, J.-P. Jing, et al., Influence of the adjustable vane position on the flowand combustion characteristics of a down-fired pulverized-coal 300 MWe utilityboiler, Fuel Processing Technology 89 (12) (2008) 1297–1305.

[4] J.-R. Fan, X.-D. Zha, K.-F. Cen, Study on coal combustion characteristics in aW-shapedboiler furnace, Fuel 80 (2001) 373–381.

[5] J.-R. Fan, X.-H. Liang, L.-H. Chen, et al., Modeling of NOx emissions from aW-shapedboiler furnace under different operating conditions, Energy 23 (12) (1998)1051–1055.

[6] N. Fueyo, V. Gambon, C. Dolazo, et al., Computational evaluation of low NOxoperating conditions in arch-fired boilers, Journal of Engineering for Gas Turbinesand Power 121 (1999) 735–740.

[7] H. Naganuma, Control of ash deposition in pulverized coal-fired boiler, Proceed-ings of the Combustion Institute 32 (2) (2009) 2709–2716.

[8] A. Zbogar, F.J. Frandsen, Sheddingof ash deposits, Progress in Energy andCombustionScience 35 (1) (2008) 31–56.

[9] N.S. Harding, Ash deposition impacts in the power industry, Fuel ProcessingTechnology 88 (2007) 1082–1093.

[10] F. Wigley, The effect of mineral additions on coal ash deposition, Fuel ProcessingTechnology 88 (2007) 1010–1016.

[11] H.L. Wee, H.W. Wu, The effect of combustion conditions on mineral mattertransformation and ash deposition in a utility boiler fired with a sub-bituminouscoal, Proceedings of the Combustion Institute 30 (2005) 2981–2989.

[12] A. Zbogar, F.J. Frandsen, Heat transfer in ash deposits: a modelling tool-box,Progress in Energy and Combustion Science 31 (2005) 371–421.

[13] T.F. Wall, S.P. Bhattacharya, D.-K. Zhang, et al., The properties and thermal effectsof ash deposits in coal-fired furnaces, Progress in Energy and Combustion Science19 (5) (1993) 487–505.

[14] L. Yan, R.P. Gupta, T.F. Wall, The implication of mineral coalescence behaviour onash formation and ash deposition during pulverized coal combustion, Fuel 80 (9)(2001) 1333–1340.

[15] H. Zhou, K.-F. Cen, P. Sun, Prediction of ash deposition in ash hopper when tiltingburners are used, Fuel Processing Technology 79 (2) (2002) 181–195.

[16] T.A. Erickson, S.E. Allan, D.P. McCollor, et al., Modelling of fouling and slagging incoal-fired utility boilers, Fuel Processing Technology 44 (123) (1995) 155–171.

96 Q. Fang et al. / Fuel Processing Technology 91 (2010) 88–96

[17] H.F. Wang, J.N. Harb, Modeling of ash deposition in large scale combustionfacilities burning pulverized coal, Progress in Energy and Combustion Science 23(3) (1997) 267–282.

[18] F.C.C. Lee, F.C. Lockwood, Modeling ash deposition in pulverized coal-firedapplications, Progress in Energy and Combustion Science 25 (2) (1999) 117–132.

[19] G.H. Richards, P.N. Slater, J.N. Harb, Simulation of ash deposit growth in apulverized coal-fired pilot scale reactor, Energy and Fuels 7 (6) (1993) 774–781.

[20] J.-R. Fan, X.-D. Zha, P. Sun, et al., Simulation of ash deposit in a pulverized coal-fired boiler, Fuel 80 (5) (2001) 645–654.

[21] C.-F. You, Y. Zhou, Effect of operation parameters on the slagging near swirl coalburner throat, Energy and Fuels 20 (2006) 1855–1861.

[22] M.-H. Xu, J.L.T. Azevedo, M.G. Carvalho, Modeling of the combustion process andNOX emission in a utility boiler, Fuel 79 (2000) 1611–1619.

[23] R.K. Boyd, J.H. Kent, Three-dimension furnace computer modeling, 21st Symposium(International) onCombustion, vol. 21, The Combustion Institute, 1986, pp. 265–274.

[24] C.L. Senior, S. Srinivasachar, Viscosity of ash particles in combustion systems forprediction of particle sticking, Energy and Fuels 9 (2) (1995) 277–283.

[25] P.M. Walsh, A.N. Sayre, D.O. Loehden, et al., Deposition of bituminous coal ash onan isolated heat exchanger tube: effects of coal properties on deposit growth,Progress in Energy and Combustion Science 16 (4) (1990) 327–346.