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Applied Thermal Engineering

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Research Paper

Causes and mitigation of gas temperature deviation in tangentially firedtower-type boilers

Peng Tana, Qingyan Fanga,⁎, Sinan Zhaoa, Chungen Yinb, Cheng Zhanga, Haibo Zhaoa,Gang Chena

a State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, ChinabDepartment of Energy Technology, Aalborg University, 9220 Aalborg East, Denmark

H I G H L I G H T S

• A full-scale CFD simulation was performed for a 1000MWe tangentially fired tower-type boiler.• Gas temperature deviation characteristics in the tower-type boiler is revealed.• Two novel causes of the deviation in tower-type boilers are identified.• A new method, i.e., adding an arch nose, is proposed to mitigate the deviation.

A R T I C L E I N F O

Keywords:Tangentially coal-firedTower-type boilerGas temperature deviationNumerical simulation

A B S T R A C T

Gas temperature deviation is a common problem for tangentially fired boilers due to the residual swirling at thefurnace exit. It can induce overheating or even explosion of superheaters and/or reheaters tubes. Tower design oftangentially fired boilers is expected to abate the deviation. However, large gas temperature deviation still existsin tower-type boilers. In this study, a numerical method that is validated by experimental data and designedvalue is used to investigate the gas temperature deviation in a 1000MWe tangentially coal-fired tower-typeboiler, from which the two main causes of the deviation are identified and a new method to mitigate the de-viation is proposed. First, the traction of the induced draft fans skews the flue gas flow towards the rear wall, onwhich the furnace exit is located. Such gas flow skewness becomes aggravated along with the furnace height.Second, the dense panel heaters obstruct the gas flow, which further aggravates the gas flow skewness. Both incombination result in a highly non-uniform gas flow in the panel heaters zone and thus a large gas temperaturedeviation. In the 1000MWe tower-type boiler under investigation, the gas temperature deviation between theleft side and the right side on the entrance cross-section of the final superheater reaches 120 K. A new method,i.e., adding an arch nose under the furnace exit, is proposed to mitigate the deviation, in which the height of thearch nose is a decisive factor and needs to be carefully determined. A too low arch nose cannot eliminate theskewness of flue gas towards the rear wall while a too high arch nose skews the flue gas towards the front wall.The new mitigation method has been proven effective.

1. Introduction

Tangentially coal-fired boilers are widely used in thermal powergeneration industry. In such combustion systems, the coal particles andcombustion air form a concentric swirling fireball in the center of thefurnace, which ensures intense turbulence for effective mixing andsufficient residence time of the coal particles. Therefore, high com-bustion efficiency, good flame stability and fullness, and good adapt-ability to loads and coal types are secured [1]. However, tangentially

firing system often suffers from gas temperature deviation in thecrossover pass, which can induce tube overheating or even explosion ofsuperheaters and/or reheaters [2,3]. Gas temperature deviation is be-lieved to be an inherent characteristic of tangentially fired boilers [4,5]and to increase with the increase in boiler capacity: 100–150 K in200MWe boilers, 150–200 K in 300MWe boilers, and 200–250 K in600MWe boilers [6].

As an important topic that may endanger boiler operation, the gastemperature deviation in large-scale tangentially coal-fired boilers has

https://doi.org/10.1016/j.applthermaleng.2018.04.131Received 4 January 2018; Received in revised form 20 March 2018; Accepted 26 April 2018

⁎ Corresponding author.E-mail address: qyfang@hust.edu.cn (Q. Fang).

Applied Thermal Engineering 139 (2018) 135–143

Available online 27 April 20181359-4311/ © 2018 Elsevier Ltd. All rights reserved.

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been carefully studied. For example, Yin et al. [6] studied the effect ofresidual airflow swirling at furnace exit, super-heaters panels, coalparticle trajectories and their combustion histories on gas temperaturedeviation, from which two new methods (i.e., a nose on front-wall andre-arranged super-heater panels) are suggested to mitigate the devia-tion. Xu et al. [7] concluded via a detailed analysis that reducing thediameter of tangential circle formed by secondary air jets or opposingthe tangential direction of some secondary air jets (if properly de-signed) can mitigate the temperature deviation. Zhou et al. [8] studiedthe effect of opposing tangential primary air jets with different biasedangles on the aerodynamic field and flue gas velocity deviation in thecrossover pass, from which an optimal opposing tangential angle ofprimary air jets and ratio of opposing tangential momentum flux totangential momentum flux are recommended. He et al. [9] mitigatedthe gas temperature deviation by introducing downward-tiltingcounter-flow jets and doubling the air box pressure difference. Parket al. [10] proposed a deviation reduction method by adjusting the yawand tilt angles of over-fire air. Tian et al. [1] found that properly tiltingthe burner upward can reduce the gas temperature deviation. Liu et al.[11] concluded that a larger degree of air staging can lead to moreintense residual swirling at the furnace exit, which eventually results ina higher deviation in the final superheater. However, all these studieshandle gas temperature deviation exclusively in two-pass type (or Π-shaped) boilers.

Due to the pressing need on energy saving and emission reduction,the boiler technology has advanced from subcritical to supercritical andultra-supercritical units [12–14]. Among the large-scale units, tower-type boilers have been widely used instead of two-pass type boilers[15]. In tower-type boilers, the superheaters, reheaters, and economizerare arranged directly above the combustion zone, and the furnacebelow the exit is symmetrical. The upward particle-laden gas flow doesnot need to bend horizontally or downward when traveling through thesuperheaters, reheaters, and economizer [16]. As a result, it is expectedthat the in-furnace velocity and gas flow is more uniformly distributedand the gas temperature deviation is effectively mitigated in tower-typeboilers. However, large gas and steam temperature deviations are stilloften reported in operation of tower-type boilers [17–19]. Due to thedifferent structural layout of tower-type boilers including the arrange-ment of panel heaters, the causes of temperature deviation in tower-type boilers may be different from those in two-pass type boilers. To thebest of the authors’ knowledge, there is a lack of detailed investigationof gas temperature deviation in tower-type boilers in literature.

This study provides a deep insight into the characteristics andcauses of gas temperature deviation in a 1000MWe tower-type boiler. Afull-scale numerical model of the boiler under investigation was de-veloped and validated, based on which the distributions of gas tem-perature and gas flow in the panel heaters zone were carefully ex-amined. The causes of gas temperature deviation in the tower-typeboiler are identified and mitigation strategies are proposed to reduce oreliminate the deviation.

2. Methodology

2.1. Utility boiler

The boiler under investigation is a 1000 MWe ultra-supercriticaltangentially coal-fired tower-type utility boiler, as shown in Fig. 1. Thefurnace width between the two side-walls, depth between the front walland the rear wall, and height are 23.16m, 23.16m and 113.4 m, re-spectively. A low NOx concentric firing system comprising 48 en-hanced-ignition pulverized-coal burners at the corners of the furnace isused. The arrangement of burners, secondary air (SA), close-coupledover-fire (CCOFA) and separated over-fire air (SOFA) ports along thefurnace height at each of the four corners are shown in Fig. 1c. Sixmedium-speed mills are employed to pulverize the raw coal and eachmill provides coal powders to eight burners (at two elevations). The

coal and air streams from the four corners form imaginary circles in aclockwise direction at horizontal cross-sections in the furnace as shownin Fig. 1b. Three superheaters, two reheaters, and one economizer arearranged in the vertical flue pass in the upper furnace. Some horizontalcross-section planes (P1-P4) and lines (L1-L4) for results presentationare also shown in Fig. 1a. Planes P1-P3 are at the entrance of the pri-mary superheater, the final superheater, and the final reheater, re-spectively, while plane P4 is at the middle of the final reheater. L1-L4are the centerlines of P1-P4 along the width direction between the twoside-walls. In actual operation of the boiler, the main steam tempera-ture deviation between the left side and the right side usually rangesbetween 10 K and 15 K under 100% load, implying large gas tempera-ture deviation along the width direction in the final superheater zone.

2.2. Numerical simulation

In this study, comprehensive numerical simulation was performedto investigate the characteristics and causes of gas temperature devia-tion in the tower-type boiler and to propose feasible deviation mitiga-tion strategies. To ensure the reliability of the simulation results, all thekey issues, e.g., the geometric model and mesh, boundary conditions,various sub-models, numerics and convergence, are properly taken intoaccount. A high-quality mesh comprising 4,130,000 hexahedral cellswas employed, in which the mesh in regions with large variations in keyparameters (e.g., the near burner zone) is refined in order to properlyresolve the large variations. Our previous work [20] shows that thismesh is fine enough to obtain practically grid-independent CFD resultsfor this boiler.

The seven computational cases, for which CFD results are presentedand discussed in this paper, are summarized in Table 1, among whichthe comparison between Case 1 (baseline case) and Case 2 is mainly toidentify the causes of gas temperature deviation in the tower-typeboiler. To better visualize the differences in the computational cases,the local view of the upper furnace part used Case 1 - Case 5 is shown inFig. 2. The bottom part including the concentric firing system zone isprecisely the same in all the cases.

For 100%, 70% and 50% load, the operation conditions of the boilerare given in Table 2, among which the 100% conditions are employedin Case 1–5 as the inlet boundary conditions, while 70% and 50%conditions are used in Case 6 and Case 7, respectively. The properties ofthe coal fired in the boiler are given in Table 3. The pre-exponentialfactor and activation energy for devolatilization are 312,000 s−1 and7.4×107 J/kmol, respectively, while the pre-exponential factor andactivation energy for char combustion are 0.004 kg/(m2 s Pa) and8.37×107 J/kmol, respectively, all of which are taken from our pre-vious study for the same type of coal [20]. The coal particle size dis-tribution is obtained from a sieving analysis. It is found to follow theRosin-Rammler distribution with the minimum, maximum, mean dia-meter and the spread parameter of 10 μm, 250 μm, 60 μm and 1.15,respectively, in which 10 discrete particle sizes are used in the simu-lations as a good compromise between the simulation accuracy andcomputational efficiency. The domain-based weighted sum of greygases model (WSGGM) is employed for the gas radiative properties. Theparticle emissivity and particle scattering factor are 0.9 and 0.6, re-spectively. For the furnace walls, the temperature and internal emis-sivity are set to 700 K and 0.8, respectively. The panel heaters weremodeled as constant-temperature double-side walls. The furnace outletis set as pressure outlet with gauge pressure of −80 Pa, which is de-termined from the experimental condition.

The key sub-models used in the computational cases are summar-ized in Table 4. The SIMPLE algorithm is used for the velocity-pressurecoupling [32]. The first order upwind scheme is used to speed upconvergence, before the second order upwind scheme is finally em-ployed for improved accuracy. The simulations are performed usingANSYS Fluent 16.0.

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3. Results and discussion

3.1. Simulation verification

To verify the reliability of the numerical model, the CFD-predictedresults are compared against the available the experimental data [20].The measurement error in the experiment is about± 0.1% for O2

and± 20 ppm for NOX. For the three cases corresponding to real boileroperation, the CFD-predicted O2 concentration at the furnace exit, un-burned carbon content in the fly ash and NOX emission agree well withmeasured values, as shown in Table 5. Moreover, the CFD-calculatedflue gas temperature at the entrance of the primary superheater in Case1 is compared against the designed value: 1439 K by calculation vs.1485 K in design. The good agreement between the CFD predictions and

Fig. 1. Overview of the studied boiler.

Table 1Summary of the computational cases.

Case No. Case description Purpose

Case 1 The real boiler under 100% load – the baseline case To validate the numerical model by experimental dataCase 2 Assuming no panel heater in upper furnace, 100% load To study the effect of panel heaters on gas temperature deviationCase 3a Adding a 6.6m-height arch nose below furnace exit, 100% load To propose mitigation strategy for gas temperature deviationCase 4a Adding a 8.6m-height arch nose below furnace exit, 100% load To propose mitigation strategy for gas temperature deviationCase 5a Adding a 10.6 m-height arch nose below furnace exit, 100% load To propose mitigation strategy for gas temperature deviationCase 6 The real boiler operated under 70% load To validate the numerical model by experimental dataCase 7 The real boiler operated under 50% load To validate the numerical model by experimental data

a Due to the arrangement of the arch nose between the economizer and furnace exit, the furnace height is increased by 6.6 m.

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onsite measurements in the oxygen content, carbon in the fly ash, andNOX concentration as well as the flue gas temperature at the entrance ofsuper-heater indicates that the CFD simulations are reliable enough toextract more detailed information and to achieve an improved under-standing of the underlying sub-processes.

3.2. The gas temperature deviation characteristics

Fig. 3 shows the temperature distribution on planes P1-P4 for Case 1and Case 2. To facilitate the analysis, each of the planes is divided intofour regions, i.e., F-L (region bounded by front wall and left side-wall),F-R (by front wall and right side-wall), R-L (by rear wall and left side-wall) and R-R (by rear wall and right side-wall). In Case 1 (the 4 con-tours on the top), the temperature distribution of the four planes arequite irregular, from which the consistent trend can be observed: lowesttemperatures in the region F-L, highest temperatures in the region R-R,and temperature increase from the left side-wall to the right side-wall.With the increase in height (i.e., from P1 to P4), the average tem-perature decreases, which is more pronounced in regions F-L, R-L and F-R while less pronounced in region R-R. The difference in the averagegas temperature between the left and the right side is over 100 K in thesuperheater zone, and reaches 120 K at the entrance cross-section of thefinal superheater (i.e., plane P2).

For quantitatively evaluation of the gas temperature deviation inCase 1, the temperature deviations on lines L1-L4, which is defined as

the temperature difference between any point at the right side of theline and its symmetric point at the left side [7], are shown in Fig. 4. Indifferent furnace height, the temperature deviations near the side-wallare higher than that near the furnace center. Six meters away from thefurnace side-walls is a critical point, where the temperature deviationdecreases rapidly. With the increase in furnace height, the deviationsfirst increase and then decrease. The deviation on L2, the centerline ofthe final superheater entrance along the width direction, is the largestand is over 400 K, which is quite remarkable.

When all the panel heaters are removed, the calculated temperaturedistributions on the planes, P1-P4, are much more symmetric betweenthe two side-walls and the temperature deviation is almost negligible,as shown in the bottom four contours in Fig. 3. When moving upwardsfrom plane P1 to P4, the high temperature region only slightly shifts tothe R-R region.

Fig. 2. The top half of the computational domain used in Case 1–5.

Table 2Operation conditions of the boiler at 100%, 70% and 50% load.

Load 100% 70% 50%

Mills in service ABCDE ABCDE ABCDTotal coal flow rate (kg/s) 103.3 73.0 52.7Primary air flow rate (kg/s) 179.1 160.0 124.4Annular fuel air flow rate (kg/s) 69.8 66.3 50.2Auxiliary air flow rate (kg/s) 401.2 297.0 258.1CCOFA flow rate (kg/s) 32.1 26.6 23.9SOFA flow rate (kg/s) 198.0 135.7 103.3Leaking air flow rate (kg/s) 36.3 26.5 21.6Temperature of primary air (K) 349.7 349.7 349.7Temperature of secondary aira (K) 616.0 616.0 616.0Temperature of leaking air (K) 300.0 300.0 300.0

a Secondary air includes annular fuel air, auxiliary air, CCOFA, and SOFA.

Table 3Coal properties.

Ultimate analysis Proximate analysis Qnet.ar (kJ kg−1)

Car (%) Har (%) Oar (%) Nar (%) Sar (%) Var (%) Mar (%) Aar (%) FCar (%)

63.25 3.40 11.18 0.64 0.50 26.20 14.00 7.04 52.76 23,390

Table 4The sub-models used in the numerical study.

Item Model

Turbulence Standard k-ε model [21,22]Standard wall functions [23]

Gas phase chemical reaction Mixture fraction/PDF method [24,25]Particle-tracking model Stochastic particle trajectory model [26,27]Radiation P-1 algorithm [28]Devolatilization Single kinetic rate model [29]Char combustion Kinetic/diffusion model [30]NOX formation Extended Zeldovich mechanism [31]

De Soete mechanism [31]

Table 5Comparison between the measurement results and CFD calculations.

O2 content(vol.%)

Unburned carboncontent (%)

NOX emission (mg/m3, 6% O2)

Case 1 Measured 2.9 0.85 187Calculated 2.78 0.76 180

Case 6 Measured 4.13 0.45 330Calculated 4.09 0.42 335

Case 7 Measured 6.32 0.35 390Calculated 6.23 0.32 397

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3.3. The causes of gas temperature deviation

Fig. 5 shows the flue gas streamlines in the panel heaters zone forCase 1 and Case 2. In both cases, the flue gas skews towards the rearwall, and a backflow zone is formed in the top and front region of thefurnace, both of which are more pronounced in Case 1 than in Case 2.The skewed flue gas and the formation of the backflow zone are pri-marily induced by the traction of induced draft (ID) fan. The area of thefurnace exit is only about 30% of that of the furnace horizontal cross-section, the flue gas in the panel heaters zone skews towards the rearwall and flows into the rear flue pass under the traction of ID fan. At thesame time, a low-pressure zone forms in the top and front region of thefurnace, resulting in the flue gas backflow. In Case 1, the dense panelheaters restrain the rotation of the flue gas and the traction effect be-comes more obvious.

Fig. 6 shows the streamlines on planes P1-P4 for Case 1 and Case 2.In Case 2, the flue gas streamlines on the planes are generally uniformand symmetrical, though the rotation center slightly moves from thefurnace center to the R-R region with the increase in the furnace height.In Case 1, both the flue gas streamlines and velocity are highly asym-metrical. The rotation center of the flue gas on plane P1 significantlyskews to the R-R region. Moreover, the rotation intensity decreases

rapidly under the impeding effect of the dense panel heaters. No cir-cular streamlines can be observed on planes P2-P4, and the velocity offlue gas decreases with the increase in furnace height.

Table 6 gives the flue gas flow in each region on planes P1-P4 forCase 1 and Case 2. In Case 2, the flow gas flow is even at the entrance ofthe panel heaters zone (plane P1), but with the increase in furnaceheight, the flow in the rear side (region R-L and R-R) increases con-tinually while the flow in the front side decreases. This is consistentwith the previous analysis that the flue gas skews towards the rear wallunder the traction of ID fan. In addition, the flow in Region R-R is largerthan that in Region R-L, and the deviation increases with the increase infurnace height. This is because the traction of ID fan inhibits the fluegas flowing from Region R-R to Region F-R. In Case 1, the rotationmomentum of flue gas is attenuated by the dense panel heaters. Thetraction effect is more obvious. Thus, the deviation of the flue gas flowin the four regions becomes greater. As seen from the streamlines onplanes P2-P4 in Fig. 6, the flue gas in region F-L primarily flows intoregion R-L. Part of flue gas in region R-L flows into region R-R under the

Fig. 3. Temperature distribution on planes P1-P4 for Case 1 (the top four contours) and Case 2 (the bottom four contours).

Fig. 4. Gas temperature deviation along the four horizontal centerlines (L1–L4)in Case 1.

Fig. 5. Gas streamlines in the panel heaters zone for Case 1 and Case 2.

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effect of residual swirling, and the rest flow towards the rear wall. Inregion R-R, due to the traction of ID fan, part of the flue gas flows toregion F-R, and then flows back to region R-R. The rest makes a “U”turn and remains in region R-R. The majority of the flue gas in region F-R does not flow to region F-L but to region R-R. Thus, the flue gas flowin the rear side is larger than that in the front side and peaks in regionR-R. Meanwhile, due to the continuous lack of flue gas flowing intoregion F-L, a low-pressure zone is formed in this region, which leads tothe backflow of the flue gas. Thus, the main stream flows downwards inthis region.

In summary, the furnace structure of tower boiler is symmetricalexcept for the furnace exit. The flue gas flow skews towards the rearwall under the traction effect of ID fan, and the dense panel heatersimpede the gas flow and intensify the skewness, which in combinationyield significant non-uniformity of gas flow in the panel heaters zone.The non-uniformity distribution of gas flow further leads to the tem-perature deviation.

3.4. Mitigation strategy for the gas temperature deviation

To mitigate the gas temperature deviation in tangentially coal-firedtower-type boiler, a strategy is proposed by adding an arch nose belowthe furnace exit. Three computational cases, Cases 3–5 (also shown inTable 1 and Fig. 2) are conducted to evaluate the impact of arch nosesof different heights on the gas flow distribution and gas temperaturedeviation. For quantitative evaluation of the deviation, two indexes,referred as flow deviation coefficient EQ and temperature deviationcoefficient ET [1], are defined,

=EQQQ

ave right

ave left

,

, (1)

=ETTTave right

ave left

,

, (2)

where Qave right, and Qave left, donate the average gas flow rate in the rightand left side of the furnace’s cross-sections,Tave right, andTave left, donate theaverage temperature in the right and left side of the furnace’s cross-sections.

Fig. 7 shows the flue gas streamlines in panel heaters zone in Case 1and Cases 3–5. Compared to Case 1, the arch nose diverts part of fluegas before exiting the furnace, which reduces the size of the backflowzone and improves the fullness degree in the panel heaters zone. In Case3, the flue gas flows first towards the rear wall, and then into thehorizontal pass under the guidance of the arch nose. A small backflowzone is formed in the front and bottom side of the panel heaters zone. InCase 5, a backflow zone is formed near the rear wall below the archnose. The flue gas streamlines skew towards the front wall, which iscontrary to that in Case 3. In Case 4, flue gas streamlines well fill thepanel heaters zone with negligible backflow zone, which outperformsCase 3 and Case 5.

Fig. 8 shows the flue gas streamlines on plane P1 for Case 1 andCases 3–5. Case 3 slightly improves the streamline distribution overcase 1, in terms of less biased rotation center and less pronounced high-velocity zone in the R-R region. Comparatively, the streamline dis-tribution is obviously improved in Case 4 and Case 5. First, thestreamline distribution shows a more regular circular shape and therotation center is almost coincident with the furnace center. Second, thevelocity distribution across the plane is more even than that in Case 1 orCase 3.

Fig. 9 shows the streamlines distribution on plane P3 for Case 1 andCases 3–5. The velocity distribution in Cases 3–5 is more even than thatof Case 1. In Case 3, the rotation center of the flow gas moves from therear wall to the boundary of region F-R and region R-R when comparingwith Case 1. As analyzed previously, the flow of flue gas is primarilydetermined by the effect of residual swirling and the traction effect ofID fan. On the right side, the former effect drives the flue gas flow in thenegative x-direction and the latter in the positive x-direction. Themovement of rotation center indicates that the traction effect is wea-kened by the arch nose. In Case 4, the traction effect is further

Fig. 6. Gas streamlines on planes P1-P4 for Case 1 (the top four plots) and Case 2 (the bottom four plots).

Table 6The upstream flow rate of flue gas in different regions.

Plane Case 1 (kg/s) Case 2 (kg/s)

F-R F-L R-L R-R F-R F-L R-L R-R

P1 242 163 226 387 258 246 250 262P2 244 106 279 388 255 243 251 267P3 208 −16a 399 427 240 231 261 286P4 164 −62 447 470 221 222 272 301

a Negative value donates the main stream in this region flows downwards.

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attenuated by the arch nose and the flue gas basically rotates aroundthe furnace center. The flue gas streamlines distribution in Case 5 issomehow opposite to that in Case 3, that is, the rotation center locatesat the boundary of region F-L and region R-L. This is because the fluegas of the Case 5 skews to the front wall under the extrusion of the deeparch nose. The gas flow from region F-L to region R-L is inhibited.

Fig. 10 shows the flow deviation coefficient EQ along the height ofthe furnace. The EQ in Case 1 is between 1.6 and 1.7, which indicates

that the flue gas flow in the right side is far greater than that in the leftside. When the arch nose is added, i.e. Cases 3–5, the EQ reduces ob-viously. In Case 3, the deviation is still over 1.2. This is because the archnose is not high enough (or deep enough into the furnace) to neutralizethe traction effect of the ID fan. The EQ of Case 4 is close to one, in-dicating that the flue gas flows on the left and right sides are nearlyequal. The EQ of Case 5 deteriorates compared to Case 4, which rangesfrom 0.92 to 0.95, indicating larger flue gas flow on the left side than on

Fig. 7. Gas streamlines in the panel heaters zone.

Fig. 8. Gas streamlines on plane P1.

Fig. 9. Gas streamlines on plane P3.

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the right side. This is because the arch nose is too high, skewing the fluegas towards the front wall.

Fig. 11 shows the temperature profile on plane P3 for Case 1 andCases 3–5. In Cases 3–5, the temperature distribution is more even andsymmetrical than that in Case 1, and the high temperature zone shiftsfrom the R-R region to the center of the furnace. Among Cases 3–5, Case4 is better than others in terms of uniformity of the temperature dis-tribution, which is also consistent with the flow deviation coefficient. InCase 5, the temperature distribution is similar to that of Case 4, but alow temperature zone is formed near the rear wall.

The temperature deviation coefficient ET along the furnace height isplotted in Fig. 12. ET improves after adding the arch nose. Meanwhile,the decrement in ET diminishes with the increase in the arch noseheight. When the height of the arch nose increases from 8.6 m to 10.6m(i.e., from Case 4 to Case 5), ET improves little. The ET in Case 5 is lessthan 1, indicating the average flue gas temperature on the left side islarger than that on the right side. The arch nose is too high. This isconsistent with the previous analysis on the gas flow deviation.

The combustion characteristics after adding the arch nose are fur-ther examined. The average temperature along the furnace height isplotted in Fig. 13. The similar temperature profiles indicate that addingthe arch nose does not remarkably affect the combustion and overallheat transfer characteristics in the furnace. Moreover, O2 concentration,fly ash carbon content and NOX emissions at the furnace exit for Case 1and Cases 3–5 are summarized in Table 7. No distinct difference inthese parameters among the four cases also indicates that adding archnose does not affect the combustion characteristics.

Fig. 10. Gas flow deviation coefficient along furnace height.

Fig. 11. Temperature distribution on plane P3.

Fig. 12. Temperature deviation coefficient along furnace height.

Fig. 13. Average temperature along furnace height.

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4. Conclusion

A comprehensive numerical study of a 1000MWe tangentially coal-fired tower-type boiler is successfully performed, which has been vali-dated by experimental data and designed value. The numerical methodis used to investigate the gas temperature deviation in the furnace.Under full load operation, the gas temperature deviation between theleft and right side-wall on the horizontal entrance cross-section of thefinal superheater reaches 120 K, for which two causes are identifiedfrom the numerical analysis. One is the skewed gas flow towards therear wall where the furnace exit is located, and the other is the ob-struction of the gas flow due to the dense panel heaters. A new miti-gation strategy for such a gas temperature deviation is proposed via amodeling-based parametric study, i.e., adding an arch nose of appro-priate height below the furnace exit. For this furnace whose horizontalcross-section is 23.16m×23.16m, an arch nose of 6.6m in height isfound to best mitigate the gas temperature deviation while not com-promise the combustion, heat transfer and pollutant emissions from theboiler.

Acknowledgements

This work was sponsored by the National Natural ScienceFoundation of China (NO. 51676076 and NO. 51390494).

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Table 7Key parameters at furnace exit.

Case O2 content (%) Unburned carbon content(%)

NOx emission (mg/Nm3)

1 2.78 0.76 1803 2.80 0.79 1884 2.78 0.77 1845 2.76 0.75 186

P. Tan et al. Applied Thermal Engineering 139 (2018) 135–143

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Causes and mitigation of gas temperature deviation in tangentially fired tower-type boilersIntroductionMethodologyUtility boilerNumerical simulation

Results and discussionSimulation verificationThe gas temperature deviation characteristicsThe causes of gas temperature deviationMitigation strategy for the gas temperature deviation

ConclusionAcknowledgementsReferences