Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler

FUPROC-04624; No of Pages 13 Fuel Processing Technology xxx (2015) xxx–xxx

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Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler Dengfeng Tian a, Lijin Zhong b, Peng Tan a, Lun Ma a, Qingyan Fang a,⁎, Cheng Zhang a, Dianping Zhang b, Gang Chen a,⁎ a b

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China Zhuhai Power Plant, Zhuhai 519000, PR China

a r t i c l e

i n f o

Article history: Received 19 April 2015 Received in revised form 26 June 2015 Accepted 4 July 2015 Available online xxxx Keywords: Tangentially fired boiler Reheat steam temperature deviation Gas temperature deviation Tilt angle

a b s t r a c t In the present paper, computational fluid dynamic modelings were established to research a newly presented reheat steam temperature deviation solution on the basis of FLUENT 6.3.26 considering a 700 MWe tangentially fired pulverised-coal boiler, which confronted with severe flue gas and re-heat steam temperature deviation. The model was solidly validated by grid independence test and comparison with the experimental data obtained from a series of on-site measurements. Upon reliable validation, the model was further used to investigate the forming mechanism of re-heat steam temperature deviation as well as the influence of burner tilt angle on it. The conclusions mainly include (1) Residual swirling flow in the upper furnace caused the flue gas velocity and temperature deviations in crossover pass. For a typical anticlockwise tangential firing system, the flue gas velocity and temperature were lower in left part of crossover pass. The deviation of flue gas further generated the convective heat transfer imbalance of final re-heater, therefore, the temperature deviation of re-heat steam was severe. (2) Tilting the burner upward can effectively reduce the intensity of residual flow as well as the flue gas deviation degree. The +11° tilt angle of burner was relatively optimum considering the flue gas deviation and final re-heater overheating potential. Specifically, the intensity of residual swirl flow dropped 44% with burner tilting upward for +11°. Practical operation of boiler demonstrated that the reheat steam temperature deviation was reduced from 22 °C to 10 °C in this case. (3) When the tilt angle of additional air (AA) was bias set, the flow field of upper furnace was changed. Consequently, the residual swirl flow intensity and the flue gas deviation were reduced considerably. On-site measurements indicated that the combination of tilting burner upward for +11° and setting the bias of AA tilt angle for 10° can further reduce the re-heat steam temperature deviation to 4 °C. © 2015 Published by Elsevier B.V.

1. Introduction Tangentially fired boilers are most widely used in power generation industry. To a tangentially fired boiler in which the burners were installed at the four corners separately, the air jet flow and fuel inlets from each corner interact with each other and form a concentric swirling fire ball in the middle of the furnace. This combustion method ensures sufficient residence time of the coal particles, high combustion efficiency, good flame stability and fullness, and good adaptability to loads and various coal types. However, the main disadvantage of this method is the residual swirling of flue gas at entrance of the platen zone, which was widely believed to be the primary cause of flue gas temperature and velocity deviation in crossover pass [1–6]. Zhang [7]

⁎ Corresponding authors. E-mail addresses: [email protected] (Q. Fang), [email protected] (G. Chen).

investigated the temperature deviation from the perspective of nonlinear flow characteristics and concluded that the flue gas temperature deviation was inevitable due to nonlinear flow in the furnace. The flue gas deviations further cause steam temperature deviation and increase the failure potential of heat exchanger pipes. Therefore, the economic performance and safety of the boiler operation are seriously influenced [8–11]. As an inherent feature of a tangentially fired boiler, the flue gas temperature deviation was also found to increase considerably with increasing boiler capacity [12]. Ghen and Zheng [13] found a positive correlation between the voluminal heat load of upper furnace qv. up and the deviation of the flue gas temperature by analyzing a large amount of reliable operational data. The higher voluminal heat load of the upper furnace signifies that the more fuel was burned and more flue gas was produced in the upper furnace per unit time and unit volume. It partially explained the substantially increase of flue gas temperature and velocity deviations along with the raised voluminal heat load of the upper furnace.

http://dx.doi.org/10.1016/j.fuproc.2015.07.002 0378-3820/© 2015 Published by Elsevier B.V.

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002

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D. Tian et al. / Fuel Processing Technology xxx (2015) xxx–xxx

Nomenclature σ n P ρ ni nj xi yj Δx Δy Δh Vxi Vyj q l Δt h A S Q

standard deviation grid number of P7 swirling momentum moment of flue gas, N m flue gas density, kg/m3 grid number of x axis grid number of y axis distance between the ith grid of x axis and the center of furnace, m distance between the jth grid of y axis and the center of furnace, m grid space of x axis, m grid space of y axis, m unit of furnace height, m tangential velocity of ith grid of x axis, m/s tangential velocity of jth grid of y axis, m/s heat transfer quantity of flue gas per unit mass through flue which is l meters in length, J length of flue gas pass, m equivalent heat transfer temperature difference, K coefficient of convective heat transfer, W/(m2 K) equivalent area of heat transfer, m2 equivalent sectional area of flue gas pass, m2 equivalent total amount of heat transfer of flue gas pass, J

employed computational fluid dynamic code FLUENT to investigate the usage of brown coal and beneficiated semicoke in a 600 MW ultra-critical tangentially fired boiler. They numerically analyzed the blending coal combustion characteristics by changing variables including coal injection location, brown coal blending ratio, air staging ratio and blending ratio of beneficiated semicoke. To date, electricity power generation industry has been the major source of greenhouse gas emissions, and it is necessary to take effective measures to reduce the greenhouse gas emission through the application of carbon capturing technology. Available carbon capturing technology includes the pre-combustion capture, postcombustion capture and oxy-fuel combustion. Since the oxy-fuel combustion was widely considered most viable technology applicable in coal-fired power plant, scholars have done intense research on this subject by means of computational fluid dynamic code recently [22–26]. As far as the authors are aware, no numerical research has been conducted about the relationship between the flue gas temperature deviation of tangentially fired boiler and the burner tilt angle. The major objective of this paper is to propose a new method that tilt the main burners upward and set the tilt angle of additional air (AA) bias in each corner to alleviate the flue gas and reheat steam temperature deviation. Validated computational fluid dynamic modeling considering a 700 MW low-NOx tangentially fired pulverised-coal boiler was established and the model was further used to intensively analyze the mechanism of this newly presented method with particularly attention on the flow pattern within the furnace. 2. Computational methodology

The temperature and velocity deviation of flue gas and its influence on the steam temperature has been a significant research topic for decades, massive amount of experience accumulated. The existing solutions to flue gas temperature deviation can be divided into two categories. One category of solution involves reduction of the heat transfer deviation from the side of the steam by optimizing the structure and re-arranging the platen heat exchanger [4]. The other involves weakening the residual swirling intensity, usually by opposing the tangential swirling direction of the air jet flow from the over fire air (OFA) or the separated over fire air (SOFA) [14–16]. In addition, the application of a dual-tangential-circle firing system within a single chamber is also helpful to decrease the residual swirling intensity via two counteracting tangential flue gas flow systems in opposite directions [17,18]. Although partially opposing the tangential circle direction of OFA or SOFA has been proven to be effective solution to decrease the gas temperature deviation, it is not applicable for boilers installed with burners that cannot change their yaw angle horizontally. The present paper describes a suitable method, i.e., tilting the main burners upward and setting the bias of the tilt angle of the additional air (AA) in each corner, to alleviate the flue gas and reheat steam temperature deviation. As an effective research approach, computational fluid dynamic (CFD) has been widely used in acquiring and analyze the combustion, flowing and heat transfer characteristics in tangentially fired boiler. In recent study, researchers mainly focus on application of brown coal, blended multifuel combustion and the numerical prediction of oxyfuel combustion. Audai et al. [19] developed a computational fluid dynamic modeling for the combustion characteristic of brown coal in a 550 WMe tangentially fired boiler under different operation scenarios on the basis of AVL Fire CFD code. Their work revealed the combustion characteristics of brown coal within 550 MWe boiler and the solution to optimize the operation. Dodds et al. [20] numerically analyzed the wear distribution due to coal particle and sand within the mill duct system of a brown coal fueled power plant by means of computational fluid dynamic code. The authors found the greater erosion on the upper leg swirl vanes compared with the lower leg due to the orientation of secondary flows attributed to the mill-duct geometry. Jian et al. [21]

2.1. The utility boiler This study has been conducted considering a 700 MW tangentially fired pulverised-coal utility boiler at the Zhuhai Power Station, Guangdong Province, China. The boiler was manufactured by Mitsubishi Heavy based on Mitsubishi Advanced Combustion Technology (MACT), which combined a Multiple Pollution Minimum (MPM) primary air burner and deep air staging combustion. The schematic diagram of the boiler is shown in Fig. 1 The boiler is equipped with the following nozzles: six primary air (PA) nozzles, six secondary air (SA) nozzles, three OFA nozzles and three AA nozzles located at each corner. Approximately 30% of the total combustion air was supplied into the furnace through AA nozzles acting as deep air staging combustion nozzles to achieve low NOx combustion. The tilt angle of the PA, SA, and OFA nozzles can be changed from −25 to +25 continuously in the vertical direction, and the tilt angles of the AA nozzles can be changed from −30 to +30 vertically. However, the yaw angle of the AA nozzles cannot be regulated horizontally. As shown in Fig. 1, the horizontal planes P1 (52 m) and P2 (48 m) are located at the middle and bottom of the platen zone, respectively. P3 (43 m) is located at the entrance cross-section of platen zone. P4 (38 m) is located exactly above the upper most AA burner. The vertical planes P5 and P6 were the longitudinal cross-sections of the left and right sides of the furnace, respectively, and the vertical plane P7 is the entrance cross-section of the final re-heater. All these planes are crucial for the following analysis. The practical operating parameters of this boiler indicate that the reheat steam temperature of the right side was 20 °C higher than that of the left side and that the reheat steam overheating on the right side occurred frequently for the final re-heater, resulting in desuperheating water spraying system being frequently acting and the local right-side tube over overheating during boiler operation. It has negative influence on the security and economic performance of the boiler operation. Generally, the reheat steam temperature will rise with increasing of burner tilt angle, while the reheat steam and metal temperatures may exceed their design values with an excessive tilt angle. Therefore, the

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002

D. Tian et al. / Fuel Processing Technology xxx (2015) xxx–xxx

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AA 3 AA 2 AA 1

OFA 3 OFA 2 OFA 1 F EF E DE D CD C BC B AB A AA

(a)

(b)

(c)

Fig. 1. Schematic diagram of boiler (a), arrangement of burners (b) and grid of the horizontal cross-section in the burner zone (c). (x, y and z axes stand for the directions of depth, width, and height, respectively).

burner tilt angle for practical operation should be properly set, considering the reheat steam temperature, metal temperature, and boiler efficiency.

The numerical simulation was performed on the basis of CFD code known as Fluent 6.3.26. Pulverised coal combustion processes within the boiler furnace contain several closely coupled sub-processes, including turbulent flow, solid particle transport and combustion, gas phase combustion and multiform of heat transfer. In the present study, the turbulent flow was calculated using the standard k-ε model [30,31]. Solid particle transport was simulated by Stochastic Particle Trajectory (SPT) model, which considered the influence of the turbulent fluctuation of the continuous phase on the particle trajectories. Particle radiation interaction was also considered. Radiation heat transfer within furnace was calculated by P-1 model, which considered the radiation scattering effect and was suitable for combustion equipment with large optical thickness and complex geometry. The emissivity of flue gas was calculated by weighted-sum-of-gray-gases model (WSGGM) [32] which has been widely used in computational fluid dynamics and reached good balance between calculating efficiency and accuracy [33, 34]. WSGGM assumed that the emissivity of flue gas was decided by local temperature and partial pressure of gas species. Specifically, the emissivity of flue gas ε(T,χi) can be expressed as a sun of the emissivity of several hypothetical gray gas weighted by temperature-depended factors: Iþ1 X

  α εi ðT Þ 1−e−ðai pþBTcÞs

K v1

ð2Þ

K v2

ð3Þ

CoalðsÞ → α 1 Volatileðg Þ þ ð1−α 1 ÞcharðsÞ

2.2. Mathematical models

εðT; χ i Þ ¼

model [35], i.e., one reaction controls at low temperature and the other reaction at high temperature:

ð1Þ

i¼1

where αεi(T) is the weighting factor of ith gray gas, s is the path length, ai is the absorption coefficient of gas mixture, and p is the sum of partial pressure of gas species e.g. H2O and CO2 [26]. Coal particle heterogeneous combustion consisted of devolatilization and char combustion processes. Devolatilization was modeled by two-competing-reaction

CoalðsÞ → α 2 Volatileðg Þ þ ð1−α 2 ÞcharðsÞ:

Char combustion was controlled by both oxygen diffusion and the surface chemical reaction rate [36,37]. The recommended kinetic data of devolatilization and char combustion were available in Table 1. Non-premixed combustion model was adopted to simulate the gas phase turbulent combustion, the transport equations of mixturefraction were solved instead of a series of reactions for individual species. The concentration of species was then derived from calculated mixture-fraction on the basis of chemical equilibrium assumption. Influence of turbulence fluctuation on chemistry was taken into consideration by β probability density function (PDF) [40,41]. In this study, the formation of thermal NOx and fuel NOx was considered, while prompt NOx was ignored [25–29]. The thermal NOx was formed by oxidating of N2 in combustion air and simulated by a set of strongly temperature-dependent chemical reactions [26,43] from extended Zeldovich mechanism. Fuel NOx was simulated by De Soete model [44], assuming that the fuel-bound nitrogen was distributed within char and volatile. The nitrogen from volatile was firstly released as intermediates HCN and NH3, then the intermediates were oxidated to NO or reduced to N2, while nitrogen from char converted to NO directly.

Table 1 Reaction kinetic parameters [38,39]. Reaction

Char combustion Devolatilization 1(α1 = 0.3) Devolatilization 2 (α2 = 1)

E Þ K ¼ A ; expð− RT

A

E

0.0043 kg/m2 s Pa 3.75 × 105 s−1 1.46 × 1013 s−1

8.37 × 107 J/kmol 7.366 × 104 J/mol 2.511 × 105 s−1 J/mol

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002

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The partial equilibrium approach was adopted to calculate the concentration of [O] and [OH] [26–28]. Since the concentration of NO was far lower than main species in coal combustion processes, the NOx formation was usually decoupled from coal combustion processes and the transport equations for NO, HCN, and NH3 were calculated on the basis of given convergent flow-field solution [21,26–28]. Audai [26] simulated the NOx formation together with the coal combustion processes by solving the transport equations simultaneously with the coal combustion reaction. They found that whether the NOx formation was decoupled from coal combustion processes or not has little impact on NOx simulation results. The influence of turbulent flow temperature and species concentration fluctuation on NO formation was taken into consideration by the adopting of β probability density function (β PDF) [42].

Table 3 Coal property. Proximate analysis, (wt.%), as received (ar) Volatile matter Moisture Ash Fixed carbon Low heating value, (kJ/kg)

27.0 16.30 8.77 47.93 23,035.0

Ultimate analysis, (wt.%), as received (ar) Carbon Hydrogen Oxygen Nitrogen Sulfur

61.30 3.65 8.90 0.78 0.30

2.3. Cases studied set up In actual operation of the boiler, the burner was usually tilted upward for specific angle in order to increase the steam temperature, while the reheat steam temperature and metal temperatures may exceed their design values with an excessive tilt angle. Therefore, the burner tilt angle should be properly established. The numerical simulations were conducted for 8 cases presented in Table 2, which were established on the basis of practical operating parameters. Cases 1–4 were conducted to study the influence of the burner tiling up angle on the flue gas deviation. Cases 3, 5, and 6 were conducted to study the influence of the bias setting of the AA tilt angle. Cases 3, 7, and 8 with 100% load, 75% load and 50% load, respectively, were performed to validate the confidence of the numerical simulation. The coal properties are presented in Table 3. 2.4. Boundary condition The boundary condition of the burner inlets, the mass flow rate and the temperature of the air inlets were properly established according to practical operating parameters. Operating parameters are presented in Table 4. The wall function method was used to consider the near-wall effect, and the temperature boundary condition was used for the heat transfer boundary condition of the wall. In this study, 24 surface type of injections were set up in each of the primary air inlets. The pulverised coal particle diameter distribution obeys Rosin-Rammler algorithm with an average diameter of 65 μm and a spread parameter of 1.5 achieved form the pulverised-coal sampling. Total number of tracked particles was 25,920. The SIMPLE algorithm of pressure correction was used to consider the coupling of the velocity and pressure fields [45]. The governing equations were calculated with appropriate underrelaxation and TDMA line-by-line iterations. A first-order finite difference scheme was used and the calculation didn't end until the solution satisfies the pre-specified tolerances, which were set to 1 × 10−6 for interactions of energy, radiation heat transfer, NO, and 1 × 10−4 for other equations.

Main burner tilt angle (°)

1 2 3 4 5 6 7 8

Special attention was paid to the grid system to improve the accuracy of the numerical calculation. The structured grids were used because this approach can ensure the high quality of grid system. Grid independence test was conducted for case 1 in order to reach the balance between computational accuracy and computing cost. Three grid systems with 1,270,946, 2,501,812 and 3,466,378 cells were considered respectively. The latter two grid systems were achieved by refining the meshes in burner region and upper furnace that represent regions of high variables and flow gradients. All the grid systems were assessed by comparing the gas velocity component Vy, gas temperature along the line 1 (x = 0–18.60 m, y = 10.73 m, z = 18.16 m in the B layer PA cross-section) and line 2 (x = 0–18.60 m, y = 10.73 m, z = 21.70 m in the D layer PA cross-section). It is evident from Fig. 2 that the computational results of grid system with 2,501,812 cells and 3,466,378 cells are almost identical. Therefore, the grid system with 2,501,812 cells was adopted in present study considering computational accuracy and computing cost. Besides, the grids of the horizontal cross-section were properly designed, i.e., the grid lines were set approximately along the flow direction to decrease the pseudo-diffusion error [46] as shown in Fig. 1.

2.6. Full-scale experimental tests The experimental measurements of combustion were conducted under different loads. Some performance parameters, such as the carbon content in the fly ash, the oxygen concentration in the flue gas, and NOx emissions, were measured. A MSI EURO-type flue gas analyzer was used to measure the components of the flue gas. Flue gas and fly ash samplings were conducted at the entrance of the air pre-heater. Coal sampling was performed at the coal hopper exits. Pulverised coal sampling was conducted using the sampling equipment installed on the

Table 4 Operating parameters.

Table 2 Simulation cases.

Case Case Case Case Case Case Case Case

2.5. Grid independence test

AA tilt angle (°)

#1

#2

#3

#4

#1

#2

#3

#4

0 +4 +11 +22 +11 +11 +11 +11

0 +4 +11 +22 +11 +11 +11 +11

0 +4 +11 +22 +11 +11 +11 +11

0 +4 +11 +22 +11 +11 +11 +11

0 +4 +11 +22 +12 +6 +11 +11

0 +4 +11 +22 +16 +16 +11 +11

0 +4 +11 +22 +16 +16 +11 +11

0 +4 +11 +22 +12 +6 +11 +11

(“+” stands for tilting up the burners).

Mass flow rate (kg/s) Total air flow rate Primary air Secondary air OFA AA Pulverised coal feed rate Inlet temperature (K) Primary air and coal mixture Secondary air, OFA and AA

Case 1 (100% load)

Case 7 (75% load)

Case 8 (50% load)

681 172 175 108 226 76

538 142 141 89 162 58

380 95 108 51 126 40

338 578

338 578

338 578

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002

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Fig. 2. Grid independence test: (a) gas velocity component Vy and (c) gas temperature along line 1 (x = 0–18.60 m, y = 10.73 m, z = 18.16 m in the B layer PA cross-section) and (b) gas velocity component Vy and (d) gas temperature along line 2 (x = 0–18.60 m, y = 10.73 m, z = 21.70 m in the D layer PA cross-section).

pipes of the primary air and the pulverised coal mixture. The bottom ash was also sampled. The flue gas temperature measurement was conducted using a suction thermocouple through four observation ports along the furnace height. The measurement points were 1.0 m away from the front wall and 2.6 meter away from the right side wall. In addition, the flue gas temperatures were also measured through seven observation ports along the furnace width at the platen zone. These measuring points were 1.0 m away from the front wall and 1.0 m above the furnace nose.

3. Results and discussion 3.1. Validation of the numerical simulations Validation of the numerical simulations after achieving convergent solutions is mandatory. The calculated flue gas temperatures were compared with the measured values along the furnace height and at the platen zone. Fig. 3 indicates that the calculated temperatures were consistent to the measured values, with maximum error less than 9%. The comparison of the calculated and measured values of the carbon

Fig. 3. Comparison of the simulated and measured temperature profiles of case 3: (a) profile along the furnace height, (b) profile along the furnace width.

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002

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Table 5 Comparison between the calculated and experimental results. Item O2 (vol.%) NOx (mg/m3, 6% O2) Carbon content in fly ash (%) CO (ppm)

Calculated Measured Calculated Measured Calculated Measured Calculated Measured

Case 3

Case 7

Case 8

2.74 2.60 132.8 147 1.46 1.17 495 653

3.62 3.80 146.7 153 1.27 1.41 439 395

3.91 3.84 138.7 148 1.12 0.88 84 75

content in the fly ash and O2, the CO concentration, and the NO emissions in the flue gas is presented in Table 5. The calculated values of the carbon content, O2 concentration and NOx emissions are in good

agreement with the measured values. The calculated results can properly reveal the variation characteristics of the carbon content and the NOx emissions. These results indicated that the models adopted in the present study are suitable for correctly investigating the flow, combustion, and heat transfer characteristics of the boiler. 3.2. Flue gas deviation characteristics of case 1 Fig. 4 shows the flue gas velocity and temperature distributions in the boiler for case 1. The four horizontal planes are cross-sections of PA, OFA, and AA and the entrance of the platen zone. As shown in Fig. 4, the distribution of velocity and temperature are in good agreement, demonstrating the existence of a tangential fire ball inside the furnace. The penetration depth of the AA air flow was larger than those of the PA and the OFA for its larger mass flow rate and

Fig. 4. Gas velocity and temperature distribution in the boiler: (a) velocity (m/s), (b) temperature (K).

Fig. 5. Velocity and temperature distributions of the inlet cross-section P7 of the final re-heater: (a) velocity (m/s), (b) temperature (K).

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002

D. Tian et al. / Fuel Processing Technology xxx (2015) xxx–xxx

(a) P5(left)

7

(b) P6(right)

Fig. 6. Streamline distributions of the vertical cross-sections in the upper furnace (m/s): (a) P5 (left), (b) P6 (right).

momentum. The tangential flow in the furnace was intensified by AA air inlets, and tangential flow at the entrance of the platen zone, i.e., the residual swirling momentum of the flue gas, was also observed. Since P7 is inlet cross-section of final re-heater, the flue gas flow and temperature distribution characteristics of P7 have crucial impact on heat transfer characters of final re-heater and reheat steam temperature deviations. Fig. 5 shows the simulated velocity and temperature distributions of P7. This figure indicates the obvious deviations of flue gas velocity and temperature. To perform quantitative analysis of the flue gas deviations, P7 was divided into four parts by center lines along the width and height directions. Flue gas velocity and temperature distributions were relatively more uniform in the upper part of P7 and non-uniform in the lower part. The velocity and temperature in the right zone of the bottom part were larger than that in the left zone. To conduct quantitatively evaluation of the flue gas deviation degree; the non-uniformity coefficient M and the deviation factor E are introduced. MT represents the non-uniformity degree of temperature distribution of P7. Mv represents the non-uniformity degree of velocity distribution of P7. MT and Mv are defined as follows: MT ¼

T ave þ 3σ T T ave

ð4Þ

(a) P2

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u 1 X σT ¼ t ðT i −T ave Þ2 n‐1 i¼1

Mv ¼

V ave þ 3σ v Vave

ð5Þ

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u 1 X ðV −V ave Þ2 σv ¼ t n‐1 i¼1 i where Ti is the temperature in the ith cell of P7, Vi is the velocity in the ith cell of P7, and Tave and Vave are the average temperature and velocity values of P7. ET and Ev represent the deviation degree between the right part and left part of P7, respectively; these quantities are defined below. ET ¼

T ave:right T ave:left

ð6Þ

Ev ¼

V ave:right : V ave:left

ð7Þ

(b) P1

Fig. 7. Streamline distributions of the cross-sections P1 and P2 in the platen zone (m/s): (a) P2, (b) P1.

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002

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introduced to conduct quantitative analysis of swirling intensity of flue gas for different cases and different positions. P is defined as follows: 0 1 nj ni X 1 @X 2 2 P¼ ρ V x ΔxΔh þ ρ j V y j y j ΔyΔhA 2 i¼1 i xi i j¼1

Fig. 8. Tube metal temperature of the outlet header of the final re-heater.

Based on the analysis above, the flue gas deviation was mainly found in the lower part of P7, while the distribution was relatively more uniform in the upper part. For this case, the deviation factor of the upper and bottom parts, Eup and Ebottom, respectively, is introduced. The definitions of Eup and Ebottom are of similar form, with the only difference that Eup is defined as the ratio of the average value of corresponding variable in upper-right to that in the upper-left and Ebottom was defined as the ratio of that in the bottom-right to the bottom-left. The values of Eup and Ebottom in case 1 were ET.up = 1.059, ET.bottom = 1.309, Ev.up = 1.434, Ev.bottom = 7.881, MT = 1.599, and Mv = 2.611. It is clear that the values of E, Eup, Ebottom, and M exhibit high degree of consistency with gas deviation distribution characteristics. Therefore, in this paper, these coefficients and factors will be used synthetically to conduct quantitative evaluation and comparison of flue gas velocity and temperature deviations for different cases.

3.3. Mechanism analysis of the flue gas temperature deviation Fig. 4 shows the anti-clockwise residual swirling of flue gas before it enters into the platen zone. The swirling momentum moment P [1] is

ð8Þ

where ρ is the gas density, Vxi and Vyj are the tangential velocity of flue gas in x and y directions, respectively, xi and yj are the distances between the a cell and the furnace center in x and y directions, respectively, Δx and Δy are grid spacings in the x and y directions. Δh is the unit of furnace height. The flue gas swirling momentum moment P in P4 and P3 is 4.558 N m and 2.504 N m, respectively, indicating rapid decay of flue gas swirling movement intensity in the furnace nose area. The vertical velocity component Vz increased with the furnace height, and the horizontal components (Vxi and Vyj) reduced at the same time because of the gradually reducing passage area of the furnace arch. Consequently, the calculated value of P decreases considerably. Fig. 6 shows the simulated flow field of the upper furnace. Planes P5 and P6 are vertical cross-sections of the left and right sides of the furnace respectively. As shown in Fig. 6, the flue gas in the right side (P6) flows directly into the horizontal flue gas pass, while the flue gas in the left side (P5) tends to flow towards the front wall. This difference of flow pattern in the upper furnace is mainly caused by the residual swirling of flue gas, which causes the horizontal velocity component Vx of flue gas in the left side point towards the front wall, while in the right side, Vx points towards the horizontal flue gas pass. This flow field difference of upper furnace influences the distribution of the horizontal component Vx as well as the flue gas flow rate distribution along the width of the horizontal flue gas pass. Fig. 7 shows the flow pattern of the platen zone by using streamlines of the flue gas. The horizontal cross-sections P1 and P2 are located in the upper and lower part of platen zone respectively. Fig. 7(a) indicates that the flow pattern of the right and left sides of the horizontal flue gas pass is remarkably different. In the left side, a part of flue gas flows towards the front wall and further flows across the platen heat exchanger because of the residual swirling, forming some vortices between the plates. Moreover, a fraction of flue gas in the left part flows through the gaps between the front wall and division platen super-heater to the right part of flue gas pass. However, the flue gas in the right part flows along the arrangement of plates. It implies that the velocity and net flow rate deviation of flue gas in the horizontal flue gas pass is

Fig. 9. Vertical velocity component Vz and Vx distributions along the width of the cross-section P4: (a) Vz, (b) Vx.

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002

D. Tian et al. / Fuel Processing Technology xxx (2015) xxx–xxx

9

the flow deviation of flue gas is located mainly in lower half of the horizontal flue gas pass. 3.4. Mechanism analysis of the re-heat steam temperature deviation The residual swirling of the flue gas causes the deviation of the velocity and the net flow rate of the flue gas in the horizontal flue gas pass along its width, as detailed above. These deviations result in the heat transfer deviation for the re-heater and further cause the deviation of the reheat steam temperature. Because the final re-heater is a convective heat exchanger, only the convective heat transfer is considered here. The equivalent heat transfer quantity of flue gas per unit mass through flue pass, which is l meters in length, is q, and it is expressed as follows: q ¼ hAΔt

Fig. 10. Swirling momentum moment distributions along the furnace height.

mainly caused by the residual swirling of flue gas. As shown in Fig. 7(b), the flow pattern of the upper part of the platen zone (P1) is relatively more symmetrical than that of P2, which indicates that the swirling intensity of flue gas weakens rapidly due to the influence of the plates and

l Vx

ð9Þ

where h is equivalent surface coefficient of the convective heat transfer between flue gas and final re-heater, A is the equivalent area of heat transfer, and Δt is the equivalent heat transfer temperature difference. The equivalent surface coefficient of the convective heat transfer can be expressed as follows [47] (Re b 1.0 × 105): 0:65

h ¼ αV x

ð10Þ

Fig. 11. Evaluation indices of flue gas deviation of P7: (a) indices of temperature under different burner tilt angles, (b) indices of velocity under different burner tilt angles, (c) indices of temperature with AA bias setting, (c) indices of velocity with AA bias setting.

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002

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D. Tian et al. / Fuel Processing Technology xxx (2015) xxx–xxx

where coefficient α is independent of V x. The formulas (7) and (8) show the negative correlation between q and Vx. Therefore, with greater flue gas velocity, the heat release per unit mass of flue gas becomes smaller, i.e., the flue gas temperature becomes higher. In this case, the flue gas in the right part of the horizontal flue pass (in which flue gas velocity and the net flow rate are greater than those in the left part) has the higher temperature. The total amount of equivalent heat transfer Q within flue gas pass can be expressed as follows: 0:65

Q ¼ αV x

AΔt

l 0:65 SV x ¼ αV x AlSΔt Vx

ð11Þ

where S is the equivalent sectional area of flue gas pass and SV x is the equivalent flow rate of flue gas. This qualitative formula indicates that the total amount of convective heat transfer in the right part of flue pass is greater than that in the left because of the larger Vx. As a result, the reheat steam temperature of the right part is higher than that in the left part. Based on the analysis above, the gas temperature and reheat steam deviations are basically determined by the flow deviation of the flue gas and the flow deviation is caused by the residual swirling of the flue gas. The tube metal temperatures of the final re-heater were determined from the distributed control system (DCS) under different loads. The measuring points are located at the inlet tube of the outlet header of the final re-heater and are evenly distributed along the width of the furnace roof. Fig. 8 shows the tube metal temperature of the final re-heater outlet header. The temperature profiles of tube metal basically coincided with the flue gas temperature distribution characteristics shown in Fig. 5. As shown in Fig. 8, the temperature of the 13th point at the right is found to be relatively higher than the temperature of the other points and closest to the warning value of 610 °C. This high temperature is mainly caused by the heat transfer imbalance described previously. Therefore, the reduction of the flue gas deviation can also be helpful to reduce the excessive temperature of the tube metal of the final reheater.

for approximately 30% of the total air, was added into the furnace from AA nozzles at approximately 32 m in height; as a result, the rapidly decreasing tendency of P is obviously reduced, and even a slight increase in the swirling momentum moment P is observed. In the furnace nose and the platen zone, the swirling momentum moment P decreases relatively faster. Overall, the swirling momentum moment P curves decrease with increasing in tilt angle. The swirling momentum moment P in entrance cross section of platen zone (P3) is 2.52 N m in case 1, 1.94 N m in case 2, 1.71 N m in case 3, and 1.41 N m in case 4. Fig. 11 shows the evaluation indices of the flue gas deviation under different cases. It clearly demonstrates that the evaluation indices of the flue gas deviation reduce with the increase of burner tilt angle. The variation of ET.bottom and Ev.bottom with the increase of burner tilt angle is particularly remarkable compared with the other corresponding items. Fig. 12 shows the temperature distributions of final reheater inlet cross-section P7. The temperature distribution becomes more evenly with the increasing of burner tilt angle. Based on analysis of each evaluation index in Fig. 11 and temperature distribution of final re-heater inlet cross-section presented in Fig. 12, we can draw

(a) Case 1

(b) Case 2

(c) Case 3

(d) Case 4

(e)

(f)

3.5. Effect of the burner tilt angle on flue gas temperature deviation Fig. 9 shows the profiles of Vz and Vx along the width direction of cross-section P4 under different burner tilt angles. As shown, the profiles of Vz for different cases under different burner tilt angles are similar. All of these profiles have the M-shaped overall curve, and the profiles are basically symmetrical. The vertical velocity component Vz of the flue gas considerably increases with the rising of burner tilt angle. This increase occurs mainly because the air jet-flow momentum in z direction provided from the air inlets increase with the tiling up angles of the burners, while those of the x- and y-directions decrease. Fig. 9 shows that the profiles of Vx under different burner tilt angles are also similar, with the same symmetrical double-peak curve. The horizontal velocity component Vx decreases with an increasing tilt angle, whereas the tangential circle of the swirling flue gas, which is defined as the distance between the two peaks of the Vx profile, obviously decreases. Fig. 10 shows the profile of flue gas swirling momentum moment P along the furnace height. As shown in Fig. 10, the profiles of P for the different cases have the similar form of a single peak curve. The swirling momentum moment initially increased with the furnace height. On the one hand, the momentum of the air jet-flow provided into the furnace from the burners is converted into the swirling momentum of the flue gas within the tangential firing system. On the other hand, the turbulent dissipation weakens the swirling intensity of tangential flue gas, i.e., the swirling momentum moment of flue gas is weakened to some degree. The maximum of P occurs in the center area of main burner zone at approximately 22 m in height due to the combined action of the two abovementioned factors. Meanwhile, because no air jet flow is added into the furnace between the upper-most OFA and the first layer of the AA, the moment P decreases rapidly. The AA, which accounts

Case 5

Case 6

Fig. 12. Temperature distributions of the inlet cross-section P7 of the final re-heater (K):(a) case 1, (b) case 2, (c) case 3, (d) case 4, (e) case 5, (f) case 6.

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002

D. Tian et al. / Fuel Processing Technology xxx (2015) xxx–xxx

(a) Case 3

(b) Case 5

11

(c) Case 6

Fig. 13. Streamline distributions of the cross-section P3 (m/s): (a) case 3, (b) case 5, (c) case 6.

the conclusion that the velocity and temperature deviation of the flue gas in the horizontal flue gas pass are reduced considerably by tilting up the burners. During the practical operation of this boiler, the tilt

(a) Case 3

angle of all the burners was normally set to +11°. In this circumstance, the reheat steam temperature of right side can reach the designed value, and the deviation of reheat steam temperature decreases from

(b) Case 5

(c) Case 6 Fig. 14. Streamline distributions of the cross-section P2 in the platen zone: (a) case 3, (b) case 5, (c) case 6.

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002

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D. Tian et al. / Fuel Processing Technology xxx (2015) xxx–xxx

approximately 20 °C to 10 °C. The frequency of the right-side reheat steam over-heating was also reduced. If burner tilt angle further increased, the right-side metal over-heating of final re-heater may occur. The tilting upward of burners is an effective measure to decrease the flue gas deviation. However, when the tilt angle of the burners reached approximately 11°, the reheat steam temperature deviation remained 10 °C, and a further increasing in burner tilt angle to reduce the flue gas deviation is not applicable considering the over-heating potential of the final re-heater right-side pipes. 3.6. Effect of AA tilt angle bias setting on flue gas temperature deviation As described above, titling the burners upward to further reduce the flue gas deviations and reheat steam temperature deviation is not applicable for the over-heating potential of final re-heater right-side pipes when the tilt angle exceeds +11°. In this work, a new measure to decrease the flue gas deviation is presented, which was achieved by setting the tilt angle of AA at each corner bias, as described in Table 2. The AA tilt angles of the first and fourth corners were less than those of the second and third corners. Fig. 13 shows the streamline distributions in the horizontal cross-section P3 of cases 3, 5, and 6. Fig. 13 clearly indicates that the center of the swirling flue gas is moved noticeably towards the bottom-right corner of P3 when the AA tilt angle was bias set. This result mainly occurred because the horizontal component of air jet momentum from the first and fourth corner is larger than that from second corner and third corner, so the center of the swirling flue gas is pushed towards the bottom-right corner. The moving of the center of the swirling flue gas before the platen zone further influenced the flow field of the platen zone. As mentioned previously, a part of the flue gas flowed across the platen heat ex-changer and formed some vortices between the plates because of flue gas residual swirling, which illustrated the flow field deviation of flue gas pass. As shown in Fig. 14, four typical vortices are observed in P2 of case 3, which are mainly distributed at the left part of the horizontal flue gas pass. In Fig. 14(b) and (c), four typical vortices are still observed, but they have moved rightward at different levels. This observation indicates that the region of platen zone in which the flow field is influenced by the flue gas residual swirling spatially moved rightward, i.e., the flow deviation was reduced to a certain extent. Fig. 15 shows the profiles of swirling momentum moment P of the flue gas along the furnace height for cases 3, 5, and 6. The figure clearly shows that the bias setting of the AA tilt angle has no significant influence on the profile of P below the position of AA, while the influence is observable after AA inputting position. The flue gas swirling momentum moments of case 5 and case 6 were less than that of case 3 after AA

is injected. The conversion of gas jet momentum to swirling momentum requires good interaction of the air jet flow from each corner, i.e., the formation of a tangential firing system requires a symmetrical arrangement of the air jet flow of the four corners. When the bias of the tilt angle of AA was set, the air jet flow from each corner was no longer symmetrically organized, which partially inhibited the conversion of the gas jet momentum to the swirling momentum. The swirling momentum moment of the flue gas at the entrance cross section of the platen zone (P3) was 1.71 N m in case 3, 1.42 N m in case 5, and 1.08 N m in case 6, and the effectiveness of reducing the residual swirling momentum moment P by setting the bias of the AA tilt angle is of the same order of magnitude as the effectiveness of tilting the main burners upward detailed in Fig. 10. Fig. 11(c) and (d) shows the evaluation indices of flue gas deviations of cases 3, 5, and 6. Fig. 12 shows the temperature distribution of final re-heater inlet cross-section P7. Fig. 11 and Fig. 12 indicate that the bias setting of the tilt angle of AA can effectively reduce the flue gas deviations in crossover pass. During practical operation, the bias setting scheme of case 6 can reduce the reheat steam temperature deviation from approximately 10 °C in case 3 to less than 4 °C. At the same time, the super-heat steam temperature can reach the designed value without pipes over-heating, the boiler efficiency can reach approximately 94.72%, and the NOx emission is less than 147 mg/m3(φ(O2) = 6%). Therefore, the application of tilting main burners at + 11° upward and setting the AA tilt angle bias at 10° can effectively reduce the reheat steam temperature deviation to less than 4 °C ensuring high boiler efficiency and low NOx emission.

4. Conclusions In this study, CFD modelings were developed to investigate a newly presented solution of re-heat steam temperature deviation on the basis of FLUENT 6.3.26 considering a 700 MWe tangentially fired pulverised-coal boiler. The residual swirling of the flue gas in the upper furnace was found to cause flue gas deviation in the bottom part of the final re-heater inlet cross section. Flue gas velocity and temperature in the right part were higher than those in the left part for a typical anticlockwise tangential firing system. These flue gas deviations accounted for steam temperature deviation and frequent right-side steam overheating. Tilting the burner upward can effectively reduce the intensity of residual flow as well as the flue gas deviation degree. The burner tilt angle of +11° was relatively optimum considering the flue gas deviation and potential of final re-heater overheating. The intensity of residual swirl flow dropped about 44% and boiler practical operation demonstrated that the reheat steam temperature deviation was reduced from 22 °C to 10 °C in this condition. The newly presented methods involve tilting the burner upward and setting the bias of the AA tilt angles. When the tilt angle of AA was bias set, the flow field of upper furnace was changed. Consequently, the residual swirl flow intensity and the flue gas deviation were reduced considerably. During practical operation, when the burner tilt angle was set at +11° and the bias of the AA tilt angle was set at 10°, the reheat temperature deviation was reduced to less than 4 °C from the initial 20 °C of case 1. Besides, the frequency of the right-side re-heat steam overheating was also reduced while ensuring a high boiler efficiency and low NOx emission in this circumstance.

Acknowledgments

Fig. 15. Swirling momentum moment distributions along with the furnace height.

The authors gratefully acknowledge the Project on the Integration of Industry, Education and Research of Guangdong Province (No. 2012B091100173), Youth Foundation of Huazhong University of Science and Technology (No. 2014QN185) and the National Natural Science Foundation of China for funding this research (No. 51390494).

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002

D. Tian et al. / Fuel Processing Technology xxx (2015) xxx–xxx

References [1] Y.G. Zhou, M.C. Zhang, T.M. Xu, S.N. Hui, Effect of opposing tangential primary air jets on the flue gas velocity deviation for large-scale tangentially fired boilers, Energy Fuel 23 (2009) 5375–5382. [2] Z. Zhang, X.C. Xu, Closed streamline numerical simulation on two-phase flow field of upper furnace and superheater platen zones in tangential-firing boiler, J. Comput. Sci. Technol. 8 (2) (2002) 97–102 (in Chinese). [3] Y.F. Diao, B.X. He, J.Y. Xu, Three-dimensional motion of vortices in the platen zone of tangentially fired furnace, Proc. CSEE 23 (5) (2003) 170–175 (in Chinese). [4] C.G. Yin, L. Rosendahl, T.J. Condra, Further study of the gas temperature deviation in large-scale tangentially coal-fired boilers, Fuel 81 (2002) 1127–1137. [5] J.H. Zhou, G.L. Song, Y.B. Chen, Test and research of flue gas temperature deviation at the furnace exit of a 2008 t/h boiler with tangential firing, Thermal Power Gener. 6 (2003) 31–35 (in Chinese). [6] C.G. Yin, L. Rosendahl, T.J. Condra, Thermal load deviation model for superheater and re-heater of a utility boiler, Appl. Therm. Eng. 20 (2000) 545–558. [7] M. Yang, Y.Y. Shen, H.T. Xu, M. Zhao, S.W. Shen, K. Huang, Numerical investigation of the nonlinear flow characteristics in an ultra-supercritical utility boiler furnace, Appl. Therm. Eng. (2014) 1–11. [8] L.J. Xu, A.J. Kahana, Z.H. Chen, Thermal load deviation model for superheater and reheater of a utility boiler, Appl. Therm. Eng. 20 (2000) 545–558. [9] H.B. Vuthaluru, R. Vuthaluru, Control of ash related problems in a large scale tangentially fired boiler using CFD modelling, Appl. Energy 87 (2010) 1418–1426. [10] L. Yan, B.S. He, F. Yao, R. Yang, X.H. Pei, C.J. Wang, J.G. Song, Numerical simulation of a 600 MW utility boiler with different tangential arrangements of burners, Energy Fuel 26 (2012) 5491–5502. [11] H. Zhou, K. Zhou, Q. Tang, S.B. Chen, K.F. Cen, Using a core-vector machine to correct the steam-separator temperature deviations of a 1000 MW boiler, Fuel 130 (2014) 142–148. [12] Y.G. Zhou, T. Xu, S. Hui, Experimental and numerical study on the flow fields in upper furnace for large scale tangentially fired boilers, Appl. Therm. Eng. 29 (2009) 732–739. [13] G. Cheng, C.G. Zheng, Relation between the volume heat load of upper furnace and the fuel gas temperature deviation for tangential firing boiler, Proc. CSEE 22 (11) (2002) 146–148 (in Chinese). [14] M.H. Xu, J. Yuan, S. Ding, H. Cao, Simulation of gas temperature deviation in largescale tangential coal fired utility boilers, Comput. Methods Appl. Mech. Eng. 155 (1999) 369–380. [15] Y.P. Ho, H.B. Se, Y.J. Kim, Numerical and experimental investigation on the gas temperature deviation in a large scale, advanced low NOx, tangentially fired pulverised coal boiler, Fuel 104 (2013) 614–646. [16] B.X. He, L.Y. Zhu, J.W. Wang, S.M. Liu, Computational fluid dynamics based retrofits to re-heater panel overheating of No. 3 boiler of Dagang Power Plant, Comput. Fluids 36 (2007) 435–444. [17] C.M. Shen, Numerical Simulation of Pulverised Coal Combustion Process in a 1000 MW Dual Circle Tangential Firing Single Chamber Boiler, Harbin Institution of Technology, 2006. (in Chinese). [18] L. Sha, H. Hui, L.F. Xu, Q.X. Cao, Q. Li, S.H. Wu, Research on the elliptic aerodynamic field in a 1000 MW dual circle tangential firing single furnace ultra supercritical boiler, Energy 46 (2012) 364–373. [19] A.H. Al-Abbas, J. Naser, E.K. Hussien, Numerical simulation of brown coal combustion in a 550 MW tangentially-fired furnace under different operating conditions, Fuel 107 (2013) 688–698. [20] D. Dodds, J. Naser, Numerical study of the erosion within the pulverised-fuel millduct system of the Loy Yang B lignite fueled power station, Powder Technol. 217 (2012) 207–215. [21] J. Zhang, Q.Y. Wang, Y.J. Wei, L. Zhang, Numerical modeling and experimental investigation on the use of brown coal and its beneficiated semicoke for coal blending combustion in a 600 MW utility furnace, Energy Fuel 29 (2015) 1196–1209.

13

[22] A.A. Bhuiyan, J. Naser, Numerical modelling of oxy fuel combustion, the effect of radiative and convective heat transfer and burnout, Fuel 56 (2015) 268–284. [23] A.H. Al-Abbas, J. Naser, D. Dodds, A. Blicblau, Numerical modelling of oxy-fuel combustion in a full-scale tangentially-fired pulverised coal boiler, Procedia Eng. 56 (2013) 375–380. [24] A.A. Bhuiyan, J. Naser, Computational modelling of co-firing of biomass with coal under oxy-fuel condition in a small scale furnace, Fuel 143 (2015) 455–466. [25] A.H. Al-Abbas, J. Naser, D. Dodds, CFD modelling of air-fired and oxy-fuel combustion in a large-scale furnace at Loy Yang A brown coal power station, Fuel 102 (2012) 646–665. [26] A.H. Al-Abbas, J. Naser, Effect of chemical reaction mechanism and NOx modeling on air-fired and oxy-fuel combustion of lignite in a 100-kW furnace, Energy Fuel 26 (6) (2012) 3329–3348. [27] Q.Y. Fang, H.J. Wang, H.C. Zhou, L. Lei, X.L. Duan, Improving the performance of a 300 MW down-fired pulverised-coal utility boiler by inclining downward the Flayer secondary air, Energy Fuel 24 (2010) 4857–4865. [28] Q.Y. Fang, A.B. Musa, Y. Wei, Z.X. Luo, H.C. Zhou, Numerical simulation of multifuel combustion in a 299 MW tangentially fired utility boiler, Energy Fuel 26 (2012) 313–323. [29] L.I. Díez, C. Cortés, J. Pallarés, Numerical investigation of NOx emissions from a tangentially-fired utility boiler under conventional and overfire air operation, Fuel 87 (7) (2008) 1259–1269. [30] B.E. Launder, D.B. Spalding, Mathematical Models of Turbulence, Academic Press, New York, 1972. [31] B.E. Launder, D.B. Spalding, The numerical computation of turbulent flows, Comput. Methods Appl. Mech. Eng. 3 (2) (1974) 269–289. [32] H.C. Hottel, A.F. Sarofim, Radiative Transfer, McGraw-Hill, New York, 1967. [33] T.F. Smith, Z.F. Shen, J.N. Friedman, Evaluation of coefficient for the weighted sum of gray gases model, J. Heat Transf. 104 (4) (1982) 602–608. [34] C.G. Yin, Refined weighted sum of gray gases model for air-fuel combustion and its impact, Energy Fuel 27 (2013) 6287–6294. [35] H. Kobayashi, J.B. Howward, A.F. Sarofilm, Coal devolatilization at high temperatures, Symposium (International) on Combustion, 16 (1) 1977, pp. 411–425. [36] M. Baum, P.J. Street, Predicting the combustion behavior of coal particles, Combust. Sci. Technol. 3 (5) (1971) 231–243. [37] M.A. Fried, Rate of combustion of size-grated fractions of char from a low rank coal between 1200 K and 2000 K, Combust. Flame 13 (1969) 237–252. [38] Y.C. Guo, C.K. Lau, Numerical studies of pulverised coal combustion in a tubular coal combustor with slanted oxygen jet, Fuel 82 (2003) 893–907. [39] C.D. Sheng, B. Moghtaderi, R. Gupta, T.F. Wall, A computational fluid dynamics based study of the combustion characteristics of coal blends in pulverised coal-fired furnace, Fuel 83 (2004) 1543–1552. [40] W. Jones, J. Whitelaw, Calculation methods for reacting turbulent flows: a review, Combust. Flame 48 (1982) 1–26. [41] Y. Sivathanu, G. Faeth, Generalized state relationships for scalar properties in nonpremixed hydrocarbon/air flames, Combust. Flame 82 (1984) 211–230. [42] L.D. Smoot, P.J. Smith, Coal Combustion and Gasification, Plenum Press, New York, 1989. [43] S.C. Hill, L.D. Smoot, Modeling of nitrogen oxides formation and destruction in combustion systems, Prog. Energy Combust. Sci. 26 (2000) 417–458. [44] G.G. De Soete, Overall reaction rates of NO and N2 formation from fuel nitrogen, 15th Symposium (International) on Combustion, Pittsburgh, PA 1975, pp. 1093–1102. [45] S.V. Patankar, Numerical Heat Transfer and Fluid Flow, McGraw-Hill, New York, 1980. [46] M. Norbert, Computational modeling of a utility boiler tangentially-fired furnace retrofitted with swirl burners, Fuel Process. Technol. 91 (2010) 1601–1608. [47] Y.G. Zhou, T.M. Xu, S.E. Hui, Research on the forming mechanism of the flue-gas temperature deviation in the horizontal pass for tangentially fired boiler, Power Eng. 21 (5) (2001) 1422–1425 (in Chinese).

Please cite this article as: D. Tian, et al., Influence of vertical burner tilt angle on the gas temperature deviation in a 700 MW low NOx tangentially fired pulverised-coal boiler, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.002