Impact of Burner Yaw and Tilt Angles on the NO Emissions and Slagging in a 330 MW Tangentially Fired Boiler Utilizing Zhundong Coal: A Numerical Study

Abstract

Combustion of Zhundong coal in utility boilers is frequently challenged by ash-related issues (fouling/slagging) and stringent NO emission control requirements. This study conducted numerical simulations to investigate burner configuration effects, specifically yaw angle adjustments (modulating the imaginary tangential circle diameter, ITCD) and downward tilt angles, on the NO emissions and slagging propensity in a 330 MW subcritical tangentially fired boiler. The results reveal a compromise mechanism between NO emission control and furnace slagging mitigation. ITCD reduction via yaw angle optimization increases furnace exit NO concentrations by 1.5–3% but decreases total deposition rates by 1.3–2%. By altering the burner downward tilt angle to 15° and 25°, NO emissions increase by 8% and 19%, respectively, with about a 7% reduction in total particle deposition rate. An optimal burner yaw and tilt angle is identified to achieve a compromise between NO emission and furnace slagging control. The findings provide some guidance for the combustion optimization of Zhundong coal-fired boilers.

1. Introduction

Zhundong coal exhibits distinctive fuel properties as a premium steam coal resource, with notable advantages including reduced ash/sulfur concentrations and favorable combustion characteristics such as rapid ignition kinetics and complete burnout efficiency. Nevertheless, combustion optimization faces technical challenges arising from its ash composition. The elevated concentrations of alkali and alkaline earth metallic elements (AAEMs) in coal ash promote the generation of multiple low-temperature molten phases during thermal conversion processes. These molten compounds subsequently initiate ash deposition mechanisms through particle adhesion, ultimately leading to operational challenges in boiler systems through heat transfer surface contamination and gas passage blockage [1,2,3]. This ash-related problem has stimulated significant research focus on developing effective countermeasures against combustion-related deposition problems in Zhundong coal-fired power plants.

Controlling and optimizing boiler operating conditions is an important strategy to mitigate fouling and slagging in Zhundong coal-fired furnaces [4,5]. Temperature plays a critical role in slagging behavior. It modulates the volatilization kinetics and phase partitioning behaviors of sodium and other alkali metals during thermal conversion of Zhundong coal [6,7]. The thermophoretic force acting on fine particles near boiler heating surfaces changes with temperature, thereby altering particle deposition behavior [8]. High temperature will also lead to a decrease in the viscosity of ash particles and an increase in slagging tendency [9]. Due to the combined effects of gaseous alkali metal vapor condensation, adhesion of molten particles, and chemical transformation driven by temperature gradients, the slag formed on the boiler platen superheater exhibits a layered structure when burning high-alkali coal [10]. Based on temperature control principles, some combustion optimization methods have been proposed. Wu et al. [11] implemented the secondary air thermal regulation combined with the fuel supply strategies adjustment, successfully stabilizing the combustion gas thermal profile at the platen superheater ingress point within the critical threshold of 1250 °C, effectively alleviating heating surface slagging. Pei et al. [12] developed a wall wind-assisted combustion system that injects some secondary air perpendicular to the water-cooled wall into the main combustion zone. This reduces the actual tangential circle diameter (ATCD), suppresses thermal accumulation near the water-cooled wall, and lowers flame centers to address superheater overheating. Combustion atmosphere also significantly impacts slagging. Several research groups [13,14,15] reported enhanced slagging mitigation characteristics in oxygen-enriched combustion systems, attributed to increased fine particles generation. Liu et al. [16] found that elevated SO₂/SO₃ levels in oxy-fuel environments enhance alkali metal sulfation, forming stable high-melting-point sulfates that reduce slagging. In addition, co-firing is also a widely used method to mitigate slagging when utilizing Zhundong coal [17,18,19,20].

Controlling and optimizing boiler temperature distributions and combustion atmospheres is critical for NO emission reduction. Air-staged combustion is a widely adopted approach for NO control in pulverized coal combustion systems. Deep air-staged combustion through regulation of primary/secondary air ratios [21] and multiple air-staged combustion [22] can achieve significant NO reductions in the down-fired furnace. Bian et al. [23] implemented air-staged combustion technology retrofit in a 600 MWe lignite-fired boiler through OFA rate optimization, achieving NO emission reduction. Jiang et al. [24] and Zhu et al. [25] further reported that air-staged combustion maintains its NO reduction effectiveness under reduced boiler loads. Jin et al. [26] investigated the SOFA angles’ impact on NO and CO concentrations in a 1000 MW dual-circle tangentially fired boiler. They found that adjusting SOFA angles to 5° yaw and 0° tilt could achieve simultaneously low CO and NO emissions.

Burner adjustment is another widely used method for boiler combustion optimization [27]. It changes the thermal distribution and combustion atmosphere inside the boiler furnace. Li et al. [28] proposed a novel burner layout featuring concentric multi-stage air injection used in a 660 MW tangentially fired boiler. Their design reduced flue gas temperature deviation and improved combustion efficiency but increased NO emissions. Tan et al. [29] observed that the NO emissions at the furnace outlet significantly increase with the burner upward tilting, a trend corroborated by subsequent studies [30,31,32]. However, the NOx emission characteristics associated with downward burner tilting remain contentious. Sankar et al. [30] reported increased NOx emissions with downward tilt configurations, whereas Jameel et al. [31] observed a 5% NOx reduction when the burner was tilted downwards 15°. Intriguingly, Jo et al. [32] detected insignificant variations in NOx emission when comparing downward-tilted burners to baseline (0° tilt alignment).

While prior studies have partially elucidated the NO emission characteristics associated with upward burner tilt angles [29,30,31,32], the impact of downward burner tilting on NOx formation remains inconclusive. The interactions between thermal NO and fuel NO pathways under different temperature distributions and oxygen concentrations critically govern final NOx emissions, requiring systematic quantification. Regarding furnace slagging, the burner tilting effects are primarily evaluated through gas-phase temperature distribution, lacking comprehensive simulations integrating the slagging process. Consequently, there remains a knowledge gap in systematic assessments of burner tilting configurations on both NO emissions and furnace slagging. This study examines a 330 MW corner tangentially fired boiler using Zhundong coal. By adjusting the burner yaw angle (modifying the ITCD) and downward tilt angle (regulating the flame center height in furnace), the impacts on the NO emission and furnace slagging are investigated. The findings aim to provide insights for optimizing the operating conditions of boilers firing Zhundong coal.

2. Boiler Descriptions

The investigation focuses on a sub-critical 330 MW corner tangentially fired boiler, as schematically depicted in Figure 1a. The boiler has a single furnace, with a width of 14.707 m, a depth of 13.743 m, and a height of approximately 55 m. A staged combustion system is employed, with 17 burner nozzles strategically positioned across the four corners of the combustion zone (Figure 1b). The burner array employs hierarchical grouping. Three primary air nozzles (A–C) integrated with four secondary air nozzles (AA, AB, BC, CC) form the lower group. Two primary air nozzles (D, E) coupled with secondary air nozzles (DD, DE, EE, OFA) form the upper group. Each primary air nozzle features concentric peripheral air shrouding. Four separated overfire air (SOFA) nozzles are deployed above the OFA level to enhance staged combustion. The platen superheaters are installed in the upper furnace. As illustrated in Figure 1c, primary air streams establish dual concentric tangential circles, with ITCD of 548 mm and 1032 mm. Figure 1d illustrates the actual tangential circle flow pattern formed in the furnace, visualized via fireworks tracing techniques. The secondary air jets and separated SOFA also form two tangential circles with the same ITCDs but with opposite rotation directions.

3. Numerical Method

3.1. Mesh

A three-dimensional model of the target boiler was constructed and appropriately simplified to enable detailed analysis. The mesh for the furnace was generated using ANSYS ICEM CFD 19.2 software. As depicted in Figure 2a, a zonal partitioning method was employed for the overall mesh division. This method enables efficient and accurate meshing of complex geometries. The horizontal section of the furnace was meshed using body-fitted grids to minimize the influence of pseudo diffusion, as shown in Figure 2b). Local mesh refinement was applied to capture the intricate flow patterns and combustion characteristics accurately. To determine the number of grids for this simulation, a mesh independence study was conducted. The vertical temperature profile within the furnace was selected as the criterion. As illustrated in Figure 3, the results of the mesh system featuring 2.2 million grids were generally quite similar to those with 3.3 million grids, indicating further grid refinement does not lead to a substantial enhancement in simulation accuracy. Hence, 3.3 million grids are sufficient to obtain satisfactory simulation accuracy.

3.2. Models for Pulverized Coal Combustion

The steady-state combustion process in the corner tangentially pulverized coal-fired boiler was simulated using ANSYS FLUENT 19.2 software. The Eulerian–Lagrangian framework was adopted to model the gas-particle two-phase flow, incorporating the Realizable k-ε model for 3D turbulent gas flow simulation inside the furnace and the discrete particle model for tracking coal particle trajectories.

Table 1. Proximate and ultimate analysis of coal.

Coal *	Proximate Analysis/wt.%			Qar,Net,p	Ultimate Analysis/wt.%
	Mar	Aar	Vdaf	(MJ/kg)	Car	Har	Oar	Nar	Sar
Coal 1	8.70	18.94	37.84	22.1	58.43	3.86	8.48	0.91	0.68
Coal 2	20.90	15.74	33.54	17.99	50.11	2.50	9.70	0.52	0.53

* Coal 1 was used in the simulations of this study. Coal 2 was used for the field test, with experimental data employed for model validation.

presents the properties of the coal. The particle size distribution follows the Rosin–Rammler distribution function, with an average diameter of 65 μm and a dispersion coefficient of 1.27. The coal pyrolysis rate was represented by a single-rate equation, and the subsequent homogeneous combustion was described using a mixture fraction model. A kinetic-diffusion model was employed to simulate the char combustion process [33]. To calculate the radiative heat transfer within the furnace, the P1 radiation model was utilized.

3.3. NO Formation Model

NO concentrations generated in coal combustion are typically low. As a result, NO chemistry has negligible influence on the predicted flow, temperature fields and major combustion product distributions. Therefore, NO formation modeling is strategically decoupled from the core combustion-flow solver to maintain computational efficiency while preserving solution fidelity, termed a post-processing approach [26,27,28,29,30,31,32]. The thermal and fuel NO mechanisms are considered in the present study. The prompt NO mechanism was neglected due to its minor contribution to total NO formation in coal combustion.

Thermal NO in this analysis adheres to the extended Zeldovich mechanism [34], with the production rate computed according to

$${\dot{\omega }}_{N_2\to NO}=3\times {10}^{14}{c}_{N_2}{c}_{O_2}^{1/2}exp\left(-\mathrm{54,200}/RT\right)$$

(1)

where ${\dot{\omega }}_{N_2\to NO}$ is the thermal NO formation rate; $c_{N_2}$ and $c_{O_2}$ represent the molar concentrations of O₂ and N₂, respectively; R is the universal gas constant, with a value of 8.341 J/(mol·K); T is the temperature.

Fuel NO mechanism mainly considers the conversion of nitrogen in the volatile matter and char. Nitrogen present in the volatile matter is released in the form of intermediate HCN and NH₃, where the proportion of HCN to NH₃ is 9:1. Subsequently, these intermediate substances undergo oxidation to generate NO or reduction to yield N₂. Regarding the nitrogen in the char, it is directly transformed into NO, and the conversion coefficient of 1. The reaction rates are described by the de-Soete mechanism [35]. The reaction rates are as follows:

$${\dot{\omega }}_{HCN\to NO}=1\times {10}^{10}{c}_{HCN}{c}_{O_2}^{b}exp\left(-\mathrm{280,452}/RT\right)$$

(2)

$${\dot{\omega }}_{HCN\to N_2}=3\times {10}^{12}{c}_{HCN}{c}_{NO}exp\left(-\mathrm{251,151}/RT\right)$$

(3)

$${\dot{\omega }}_{{NH}_3\to NO}=4\times {10}^6{c}_{{NH}_3}{c}_{O_2}^{b}exp\left(-\mathrm{133,947}/RT\right)$$

(4)

$${\dot{\omega }}_{{NH}_3\to N_2}=1.8\times {10}^8{c}_{{NH}_3}{c}_{NO}exp\left(-\mathrm{113,018}/RT\right)$$

(5)

$${\dot{\omega }}_{char\to NO}=\eta {\dot{\omega }}_{char}{Y}_{Nchar}{M}_{NO}/M_N$$

(6)

In Equations (2)~(6),${\dot{\omega }}_{HCN\to NO}$ is the reaction rate of HCN conversion to NO; ${\dot{\omega }}_{HCN\to N_2}$ is the reaction rate of HCN conversion to N₂; ${\dot{\omega }}_{{NH}_3\to NO}$ is the reaction rate of NH₃ conversion to NO; ${\dot{\omega }}_{{NH}_3\to N_2}$ is the reaction rate of NH₃ conversion to N₂;${\dot{\omega }}_{char\to NO}$ is the reaction rate of char nitrogen conversion to NO;$c_{HCN}$,$c_{{NH}_3}$ and $c_{NO}$ represent the molar concentrations of HCN, NH₃ and NO, respectively; ${\dot{\omega }}_{char}$ is the combustion rate of the char; $Y_{Nchar}$ represents the mass fraction of nitrogen in the char; M_NO and M_N are the molecular weights of NO and N, respectively; $\eta$ is the frequency factor for the conversion of nitrogen in the char to NO; and exponent b in Equation (2) and Equation (4) is related to the concentration of O₂.

3.4. Slagging Model

The slagging model in this study primarily comprises the particle transport model and the particle adhesion model. The particle adhesion model assumes that ash particles transported by the gas stream collide with the heat transfer surfaces inside the furnace. The adhesion likelihood of these particles to the surface is determined based on the particle’s inherent viscosity and the wall temperature, which are used to calculate the deposition rate.

The particle sticking probability is expressed as:

$$p_{stick}=p_i\left(T_{ps}\right)+\left(1-p_i\left(T_{ps}\right)\right)p_s\left(T_s\right)$$

(7)

where p_i(T_ps) is the deposition probability of the sticky particle at temperature T_ps on the surface; p_s(T_s) represents the probability of the non-sticky particle adhering to the surface at temperature T_s.

$$p=\left\{\begin{array}{c}{\mu }_{ref}/\mu , \mu >{\mu }_{ref}\\ 1, \mu \le {\mu }_{ref}\end{array}\right.$$

(8)

The particle viscosity is the key factor influencing adhesion probability. A large body of research uses critical viscosity to determine whether a particle will adhere to the wall [36,37,38,39,40,41,42]. As shown in Equation (8), when the particle viscosity μ falls below or matches the critical viscosity μ_ref, it exhibits complete viscosity with a unity adhesion probability. Conversely, if the particle viscosity μ surpasses the critical viscosity μ_ref, the adhesion probability equals the ratio of μ_ref to μ. Particle viscosity μ is related to the composition of the ash, and

Table 2. Oxides in coal ash.

Oxide	SiO2	Fe2O3	Al2O3	CaO	MgO	TiO2	K2O	Na2O	SO3
wt. %	54.53	6.3	21.69	5.3	2.18	0.86	1.53	1.17	4.59

presents the mass fractions of oxides in coal ash. The ash particles’ viscosity is computed by means of the modified Kalmanovitch model according to particle temperature and composition [37]. The critical viscosity μ_ref is taken as 10⁵ Pa·s, which has been proven to provide good prediction results [37,42].

3.5. Operating Conditions of Simulated Cases

A total of 6 cases were set up to study the influences of burner yaw (i.e., horizontal) and tilt angle on the NOx formation and slagging characteristics under the BMCR operating condition.

Table 3. Burner yaw and tilt angles in each case.

Case	Yaw Angle 1	Tilt Angle 2	Remarks on Imaginary Tangential Circle Diameter (ITCD)
1	0°, 0°	0°	ITCDs are 548 mm and 1032 mm
2	+47′, +1°28′	0°	ITCDs are 274 mm and 516 mm
3	−47′, −1°28′	0°	ITCDs are 822 mm and 1548 mm
4	−1°34′, −2°57′	0°	ITCDs are 1096 mm and 2064 mm
5	0°, 0°	−15°	ITCDs are 548 mm and 1032 mm
6	0°, 0°	−25°	ITCDs are 548 mm and 1032 mm

¹: the angles listed here refer to the deviations of the primary air injection angle into the furnace between the base case and the other cases, where a “+” indicates a counterclockwise deviation. ²: “−” stands for tilting downward.

provides the specific parameters for each case. For case 1, the base case, the coal feed rate injected into the boiler was 40.54 kg/s, the excess air coefficient was 1.2, and the proportions of primary air, secondary air, SOFA, and perimeter air were 11:22:12:5. The temperature of the primary air was set at 348 K, while the temperature of the secondary air reached 610 K. For the outlet, a pressure-outlet boundary condition was specified. As shown in Table 3, in cases 2–4, the burner yaw angles were adjusted so that ITCDs were 0.5, 1.5, and 2 times that of case 1, respectively. The burner was tilted downward by 15° and 25° in cases 5 and 6.

3.6. Solution Procedure

The simulation process is illustrated in Figure 4. The coal combustion simulation employs a Eulerian–Lagrange framework, considering interactions between gas and particle phases. The detailed formulations of the governing equations implemented in the numerical solution can be referred to in References [25,30]. A hybrid initialization is performed based on prescribed boundary conditions to obtain the gas-phase flow and temperature fields. Then, the particle trajectories are calculated by solving particle motion equations. Particles without wall interaction will escape the computational domain. For particles experiencing wall impingement, a deposition model evaluates deposition probability. Other walls may still trap the rebounded particles in their subsequent movement. On the particle trajectories, interphase mass/momentum transfer mechanisms are modeled through de-volatilization and heterogeneous char combustion. These interactions are represented by applying source terms to the gas-phase governing equations. The coupled system undergoes iterative solutions until convergence. A convergence criterion specifies the absolute residual value of 10⁻⁴ for pressure, momentum and scalar equations, with 10⁻⁶ applied to the energy equation. Additionally, temperature variations at monitoring points should not exceed 10 K. Once a converged solution is obtained, NO concentration distribution is determined using a post-processing approach.

3.7. Model Validation

To assess the validity of the NO formation model, field testing was conducted under case 1 with the boiler firing coal 2. NO and CO emissions at the exit of the furnace were measured. Model predictions were compared against experimental data, as presented in

Table 4. Comparison between predicted and measured NO and CO emission.

	NO Emission (mg/m3, 6%O2)	CO Emission (ppm)
Measured	219	908.8
Predicted	235	945

. Results demonstrate close agreement between simulated values and measurements.

To validate the deposition model, the simulation methodology presented herein was applied to replicate experiments reported by Zhou et al. [43]. Zhou et al. measured gas temperature distributions of a 100 kW downward-fired oxy-fuel combustion furnace. Ash particle deposition rates were also determined utilizing a probe positioned at the furnace outlet. As illustrated in Figure 5, our numerical predictions demonstrate agreement with Zhou’s simulation results across the combustion chamber. The simulated temperatures maintain relative deviations within ±20% from experimental measurements in the mid-lower furnace region.

Table 5. Comparison between predicted and reported deposition rates [43].

	Measured Value [43]	Predicted Value [43]	Predicted Value in This Work
Deposition rate (kg/s)	1.167 × 10−7	1.667 × 10−7	2.067 × 10−7

quantitatively compares the deposition rate predictions. The current study’s prediction of 2.067 × 10⁻⁷ kg/s aligns reasonably with the experimental value of 1.167 × 10⁻⁷ kg/s and Zhou’s model prediction of 1.667 × 10⁻⁷ kg/s. These results demonstrate the model’s engineering applicability for predicting ash deposition trends in utility boilers.

4. Results and Discussions

4.1. Temperature Field

Temperature is a key factor influencing NO emission and slagging in the furnace. Figure 6 illustrates the temperature profile across the vertical cross-section of the furnace under the base case, as well as the temperature distributions observed on horizontal cross-sections corresponding to the B-layer burner, D-layer burner, and second SOFA levels. The pulverized coal mixed with air jets enters the main combustion zone through the burners arranged at the four corners, establishing a tangential combustion pattern. In the tangential combustion region, the temperature reaches a maximum of approximately 2000 K. However, the temperature at the center is relatively lower. For example, the central temperature in the horizontal plane of the B-layer burner is around 1250 K. As the height increases, the temperature profile above the SOFA region exhibits higher temperatures at the center, with temperatures decreasing towards the periphery.

The variation in the average temperature of the horizontal cross-sections along the vertical direction of the furnace under different cases is shown in Figure 7. For all the cases examined, the average temperature of horizontal planes increases vertically along the furnace height, plateaus in the combustion zone and reaches its peak just before the SOFA region. Upon mixing with SOFA, the average flue gas temperature drops rapidly and decreases rapidly again after entering the platen superheater arranged at the top of the furnace.

It can be shown that the yaw angle of the burner has a minor impact on the temperature field by comparing cases 1~4 in Figure 7. When the yaw angle is adjusted so that the ITCD is 0.5 times that of the base case, the mean temperature within the primary combustion zone is relatively high, which indicates that the flame center inside the furnace has dropped. This is evident in Figure 8, where the high-temperature region of case 2 is the lowest among cases 1~4. Conversely, when adjusting the yaw angle of the burner so that the ITCD is 1.5 times and 2 times that of the base case, it is found that the actual tangential circle diameters (ATCDs) are similar to the base case, resulting in temperature distribution relatively close to the base case.

Comparing cases 1, 5, and 6 in Figure 7 and Figure 8 reveals significant temperature field variations induced by burner tilt angles. Figure 7 demonstrates that downward tilting burners by 15° and 25° increases average flue gas temperatures in the ash hopper region compared to the base case. A 15° downward tilt results in lower average temperatures in the main combustion zone than the base case, but temperatures rise rapidly before the SOFA zone to match base case levels. Figure 8 illustrates that a 15° downward tilt also raises the flame center. Conversely, with a 25° downward tilt, the average temperature of flue gas in the main combustion zone is higher than the base case, as shown in Figure 7. However, the flue gas temperatures drop rapidly in the SOFA zone and are lower than the base case, indicating near-complete pulverized coal burnout within the main combustion zone. As shown in Figure 8, a 25° downward tilt leads to an enlarged elevated-temperature area in the primary combustion zone due to intensified fuel oxidation. However, the enhanced post-SOFA mixing cooling effects reduce the high-temperature zone above the SOFA relative to case 1. Therefore, the vertical position of the flame center in the furnace of case 6 is the lowest among cases 1~6.

4.2. NO Emission

Figure 9 shows the concentration of NO (@6% O₂) at the exit of the furnace under different operating conditions. It reveals that as the ITCD increases, there is a slight reduction in the concentration of NO at the furnace exit. Increasing ITCD to twice the base case reduced NO emissions by 1.5%, while reducing ITCD to 0.5 times the base case increased NO emissions by 3%. However, the concentration of NO at the furnace exit rises significantly with the downward tilt angle of the burner. When the burner’s downward tilt angle is 15° and 25°, NO emissions increase by 8% and 19%, respectively. Combined with the research findings of Tan et al. [29], who reported a similar increase in NO concentration with the upward tilt angle of the burner, it is recommended to avoid excessive adjustments to the burner’s tilt angle for optimal NO control.

Figure 10 depicts the concentration profile of NO in the furnace across various cases. Comparing cases 1~4, it becomes evident that the NO distribution remains almost consistent across these cases. The NO concentration reaches its peak within the main combustion zone, while in the SOFA region, air mixing leads to a decrease in NO concentration due to dilution. Above the SOFA region, the combustion of remaining volatile matter and char contributes to higher NO concentration. Figure 11 demonstrates that the reaction rates of thermal NO and fuel NO experience a slight decline as the ITCD increases. This phenomenon can be ascribed to the reduction in the mean temperature of the main combustion region, as depicted in Figure 7.

When the burner is tilted downwards by 15°, the mean temperature of flue gas is lower compared to the base case in the main combustion zone. Consequently, the reaction rate of thermal NO decreases, as shown in Figure 11. However, due to the combustion atmosphere, the production rate of fuel NO increases. When the burner is tilted downwards, the O₂ concentration level is higher than the base case in the main combustion zone. As the O₂ concentration increases, more HCN is converted to NO, leading to higher fuel NO formation. When the burner’s downward tilt angle is 25°, the generation rates of thermal NO and fuel NO increase compared to the base case. As illustrated in Figure 7, the average temperature in the main combustion zone is relatively high, promoting the NO formation rate. Meanwhile, compared to the base case, the O₂ concentration in the main combustion zone is also higher, and the rate of fuel NO generation increases, resulting in the highest NO emissions among cases 1~6.

4.3. Slagging Characteristics

Figure 12 compares the slagging distribution on the front wall, rear wall, side wall, and platen superheater in the furnace across various operational scenarios. It reveals that the slagging distribution remains relatively similar across cases 1~6. As depicted in Figure 7, from the upper portion of the ash-collector to the base of the platen superheater, the average flue gas temperature exceeds 1450 K Therefore, the particles exhibit low viscosity in this region, making them prone to sticking and forming slag upon colliding with the walls. Specifically, the maximum mean temperature of the furnace is reached in the region between the main combustion zone and the SOFA zone. Consequently, the slagging problem is more serious above the main combustion zone. The narrowing of flue gas passages between the platen superheaters leads to an increased collision frequency between particles and the superheaters, further exacerbating the slagging situation in this area. Notably, the particle deposition rate at the lower part of the platen superheaters is higher. Interestingly, tilting the burner downwards by 25° results in a moderate reduction in slagging at the upper part of the platen superheaters.

Figure 13 quantitatively compares particle deposition rates on the front wall, rear wall, side wall, and platen superheaters under different cases. The visualization simultaneously presents normalized percentages of each component to total particle deposition. Adjusting the yaw angle of the burners reveals a slight decrease in the total particle deposition rate as the ITCD decreases. Increasing ITCD to twice the base case increased the particle deposition rate by 1.3%, while reducing ITCD to 0.5 times the base case reduced the particle deposition rate by 2%. However, the deposition rate on the platen superheater remains almost unchanged. Tilting the burner downwards by 15° effectively reduces the total particle deposition rate by approximately 7%. However, a further reduction in the burner tilt angle to 25° results in a slight increase in the total particle deposition rate. This finding suggests that the particle deposition rate does not exhibit a monotonic relationship with the downward tilt angle of the burner. A downward tilt of approximately 15° appears to be an optimal operating condition to minimize slagging.

4.4. Research Implications for Burner Adjustment Strategies in Boiler Operation

This study demonstrates that NO emissions control and furnace slagging mitigation can be achieved through coordinated adjustments of burner yaw angles (regulating ITCD) and downward tilt angles. Enlarging ITCD through yaw angle adjustment marginally reduces NO emissions but slightly increases the particle deposition rates. Downward tilting elevates NOx formation, while simultaneously reducing particle deposition rates. Notably, increasing the tilt angle from 15° to 25° amplifies particle deposition on water-cooled walls due to elevated combustion zone temperatures, despite reduced deposition on the platen superheater, resulting in a net increase in total particle deposition rate compared to the 15° configuration.

For boilers burning Zhundong coal with a high slagging propensity, combining reduced ITCD with a 15° downward tilt is recommended to prioritize slagging mitigation while leveraging SCR systems to neutralize NOx penalties. Conversely, high nitrogen-content coals necessitate ITCD enlargement with restricted tilt angles (0–15°) to balance NO formation and maintain acceptable slagging levels. During field implementation, fine-tuning should be performed by integrating actual boiler operating data (e.g., SCR inlet NO_x concentration), and regular monitoring of heating surface slagging conditions is required to validate the control effectiveness. Integration of deep air/fuel staging techniques to achieve simultaneous optimization of NO emissions control and furnace slagging mitigation could be investigated in future work based on the methods present in this study.

5. Conclusions

This study investigated NO emissions and slagging characteristics in a subcritical 330 MW corner tangentially coal-fired boiler burning Zhundong coal. Numerical simulations were conducted under various burner yaw angles and downward tilt angles, leading to the following conclusions:

(1)

Adjusting the burner’s yaw angle (modifying ITCD) results in negligible temperature changes. However, downward tilting induces significant variations in temperature distribution. At a 15° tilt, the actual tangential circle diameter (ATCD) expands, lowering average flue gas temperatures in the main combustion zone and post-SOFA mixing regions compared to the base case. At a 25° tilt, increased particle residence time and O₂ concentration enhance combustion completeness, yielding the highest main combustion zone temperatures but the lowest post-SOFA temperatures across all tested configurations due to enhanced cooling.

(2)

As the burner’s yaw angle is adjusted to decrease the imaginary tangential circle diameter (ITCD), the average flue gas temperature in the main combustion zone slightly increases, resulting in a higher thermal-NO and fuel-NO formation rate. Increasing ITCD to twice the base case reduced NO emissions by 1.5%, while reducing ITCD to 0.5 times the base case increased NO emissions by 3%.

(3)

Tilting the burner downward raises the O₂ concentration in the main combustion zone, increasing fuel NO formation. When the downward tilt angle is 15° and 25°, NO emissions increase by 8% and 19%, respectively.

(4)

The total particle deposition rate increases slightly as the ITCD increases by modifying the burner’s yaw angle. Increasing ITCD leads to an enlargement of ATCD, thereby elevating the frequency of particle impingement on water-cooled walls and increasing particle deposition rates. Increasing ITCD to twice the base case increased the particle deposition rate by 1.3%, while reducing ITCD to 0.5 times the base case reduced the particle deposition rate by 2%.

(5)

Adjusting the burner’s downward tilt angle effectively mitigates slagging. At a 15° downward tilt, reduced average flue gas temperatures in both the primary combustion zone and post-SOFA regions lower particle temperatures, thereby increasing particle viscosity and decreasing deposition rates. Conversely, while the 25° downward tilt achieves the lowest post-SOFA flue gas temperatures (reducing deposition on the platen superheater), the elevated primary zone temperature intensifies particle adhesion on water-cooled walls, resulting in an overall deposition rate marginally higher than that at 15°.

(6)

There is a compromise mechanism between NO emission control and furnace slagging mitigation by adjusting the burner’s yaw and downward tilt angles. Optimized burner configurations were identified through parametric analysis, where yaw angle adjustments achieving ITCDs of 548/1032 mm combined with a 15° downward tilt angle demonstrated optimal performance. This configuration increased NO emissions by 8% but reduced particle deposition by 7% compared to the base case, achieving optimal NO emission–slagging compromise. Future work could explore alternative combustion adjustments to enhance simultaneous NO emission and slagging control.