# Numerical Study and Hydrodynamic Calculation of the Feasibility of Retrofitting Tangentially Fired Boilers into Slag-Tap Boilers

^{1}

^{2}

^{3}

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## Abstract

**:**

## 1. Introduction

_{x}generation in slag-tap boilers using a numerical method. In their model, the slag film was simplified, and the focus was on heat and mass transfer in the furnace. Ni et al. [17] proved the hydrodynamic and slagging safety of liquid slag-tap boilers when burning high-alkali coal through experimental methods. However, the water-cold wall in their study was once-through; the natural circulation form of the water-cold wall was not discussed. Jing et al. [18] obtained the flue gas characteristics of slag-tap boilers that burn different fuels using the actual measurement method. Their results are of reference significance in numerical simulation.

## 2. Numerical Model Description

#### 2.1. Retrofit Scheme

#### 2.2. Numerical Model

#### 2.3. Mesh Generation

#### 2.4. Boundary Conditions

#### 2.5. Simulated Working Conditions

## 3. Hydrodynamic Calculation Methods

#### 3.1. Calculation Principle

_{in,i}represents the flow from the component i at the upper level (kg s

^{−1}), G

_{out,i}represents the flow into the component i at the lower level (kg s

^{−1}), and G

_{s}represents the mass source term (kg s

^{−1}), that is, the flow exchanged between the component and the external environment. When the flow is leaving the component, the value of G

_{s}is positive; otherwise, it is negative.

^{−1}m

^{−1}), $\overline{\rho}$ represents the average fluid density in the component (kg m

^{−3}), g represents the acceleration of gravity (m s

^{−2}), and h represents the height from the inlet of the component to the outlet (m).

^{0}refers to the flow result in the previous iteration of the calculated component (kg s

^{−1}).

_{ave}is the tube wall average temperature (K), t

_{w}is the fluid temperature (K), μ(r) is the heat diversion coefficient at the radius r, β is the ratio of the outer diameter of the tube to the inner diameter, q

_{o}is the heat flux accepted by the tube section (W m

^{−2}), α

_{2}is the convective heat transfer coefficient (W m

^{−2}K

^{−1}), δ is the wall thickness (mm), and λ is the thermal conductivity of the metal (W m

^{−1}K

^{−1}).

#### 3.2. Division of Steam–Water System

## 4. Results and Discussion

#### 4.1. Model Validation

#### 4.2. Comparison before and after Retrofit

^{3}, while the average flue gas velocity here reaches 22.96 m/s. Therefore, high-temperature flue gas fills the entire combustion chamber quickly. This is consistent with the results in the literature [15]. Given the relatively concentrated combustion area in the slag-tap boiler, the maximum combustion temperature is high, reaching 2306.8 K. Different from the tangentially fired boiler, the flue gas temperature remains high near the water-cold wall in the combustion chamber of the slag-tap boiler. In addition, the temperature near the ash hopper is also high. This allows the ash particles adhering to the wall to keep flowing down and exit the boiler in a molten state [35].

_{x}in slag-tap boilers is a significant issue that requires attention. In coal-fired boilers, the focus is primarily on thermal NO

_{x}and fuel NO

_{x}[37], which can be calculated by activating the corresponding models in Fluent. Figure 13 presents a comparison of NO

_{x}concentration distribution between the tangentially fired boiler and the slag-tap boiler. The NO

_{x}emission from the slag-tap boiler is considerably higher than that from the tangentially fired boiler. The average outlet NO

_{x}concentration of the slag-tap boiler is 380.6 mg m

^{−3}, while for the tangentially fired boiler, it is only 53.1 mg m

^{−3}. This stark difference arises due to the higher combustion temperature and larger high-temperature region in the slag-tap boiler. The elevated temperature promotes the generation of thermal NO

_{x}[38]. Obviously, it is necessary to consider the optimization of the de–NO

_{x}system when implementing the retrofit scheme in practice.

#### 4.3. Combustion Characteristic of the Slag-Tap Boiler under Varying Boiler Loads

_{x}concentration in the boiler continuously decreases, as shown in Figure 16. When the coal consumption is at 50% of BMCR, the outlet NO

_{x}concentration significantly drops to 285.1 mg/m

^{−3}. However, it remains much higher than that of the tangentially fired boiler.

#### 4.4. Hydrodynamic Characteristic of the Slag-Tap Boiler

## 5. Conclusions

- The maximum temperature in the slag-tap boiler is higher than that in the tangentially fired boiler, with values of 2095.8 and 2306.8 K, respectively. Moreover, the filling degree of high temperature flue gas is higher in the combustion chamber of the slag-tap boiler. The average temperature in the combustion chamber is 2080.3 K, ensuring that the slag can be discharged in a molten state.
- When the boiler load is decreased, the temperature level in the furnace drops obviously. When the coal consumption is halved from the BMCR condition, the maximum temperature in the furnace decreases from 2306.8 to 2220.3 K. However, the temperature distribution in the combustion chamber remains relatively uniform, which would not affect the discharge of slag.
- The slag-tap boiler exhibits reliable hydrodynamic characteristics. Under both calculated conditions, the fluid flow rate in the water-cold wall is positively correlated with the heat flux. The maximum wall temperatures under the two working conditions are 653.9 and 590.6 K, respectively, both within the safe range of the tube wall material.
- Based on the results obtained in this study, the proposed retrofit scheme demonstrates robust performance in terms of slagging and hydrodynamic safety. This retrofit approach provides a practical solution for effectively burning high-alkali coal by retrofitting tangentially fired boilers into slag-tap boilers.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 3.**Temperature along the center line of the combustion chamber with the three generated meshes.

**Figure 13.**NO

_{x}concentration distributions in the (

**a**) tangentially fired boiler and (

**b**) slag-tap boiler.

**Figure 14.**Flow field under the working conditions with (

**a**) 100%, (

**b**) 75%, and (

**c**) 50% coal consumption relative to BMCR conditions.

**Figure 15.**Temperature field under the working conditions with (

**a**) 100%, (

**b**) 75%, and (

**c**) 50% coal consumption relative to BMCR conditions.

**Figure 16.**NO

_{x}concentration distribution under the working conditions with (

**a**) 100%, (

**b**) 75%, and (

**c**) 50% coal consumption relative to BMCR conditions.

**Figure 17.**Wall heat flux under the working conditions with 100% ((

**a**) Rear view, (

**b**) Left view, (

**c**) Front view) and 75% ((

**d**) Rear view, (

**e**) Left view, (

**f**) Front view) coal consumption relative to BMCR conditions.

**Figure 18.**Flow distribution of water-cold wall of cooling chamber under BMCR conditions and the condition of 75% coal consumption relative to BMCR conditions.

Parameter | Outlet Flow of Superheater (t h ^{−1}) | Superheated Steam Temperature (K) | Superheated Steam Pressure (MPa) | Feed Water Temperature (K) | Exhaust Gas Temperature (K) | Outlet Excess Air Ratio | Boiler Thermal Efficiency (%) |
---|---|---|---|---|---|---|---|

Value | 1025 | 814.0 | 17.5 | 555.0 | 409.0 | 1.25 | 93.24 |

Proximate Analysis (%) | Elemental Analysis (%) | Q_{net,ar}(MJ kg ^{−1}) | ||||||
---|---|---|---|---|---|---|---|---|

M_{ar} | A_{ar} | V_{daf} | C_{ar} | H_{ar} | O_{ar} | N_{ar} | S_{ar} | |

17.25 | 14.72 | 32.04 | 54.31 | 2.78 | 9.84 | 0.56 | 0.54 | 19.937 |

Proximate Analysis (%) | Elemental Analysis (%) | Q_{net,ar}(MJ kg ^{−1}) | ||||||
---|---|---|---|---|---|---|---|---|

M_{ar} | A_{ar} | V_{daf} | C_{ar} | H_{ar} | O_{ar} | N_{ar} | S_{ar} | |

22.60 | 11.02 | 45.42 | 50.46 | 3.32 | 11.46 | 0.67 | 0.47 | 18.800 |

Deformation Temperature (K) | Softening Temperature (K) | Hemisphere Temperature (K) | Flow Temperature (K) |
---|---|---|---|

1413 | 1433 | 1453 | 1463 |

Working Condition | Boiler Type | Fuel Type | Excess Air Ratio | Coal Consumption (t h^{−1}) |
---|---|---|---|---|

1 | Tangentially fired boiler | Design coal | 1.25 | 143.3 |

2 | Slag-tap boiler | Naomaohu coal | 150.4 | |

3 | Slag-tap boiler | Naomaohu coal | 112.8 | |

4 | Slag-tap boiler | Naomaohu coal | 75.2 |

Wall | Number | Fluid Temperature (K) | Average Tube Wall Temperature (K) | Front Point Wall Temperature (K) | Fin Center Temperature (K) |
---|---|---|---|---|---|

Front wall | Gf1 | 629.47 | 636.64 | 638.72 | 636.11 |

Gf2 | 633.93 | 647.02 | 651.84 | 647.88 | |

Gf3 | 633.39 | 643.06 | 649.02 | 647.14 | |

Gf4 | 632.76 | 642.7 | 649.15 | 647.3 | |

Gf5 | 632.6 | 641.19 | 646.78 | 645.18 | |

Gf6 | 632.47 | 638.42 | 642.28 | 641.17 | |

Left wall | Gl1 | 624.52 | 627.85 | 628.91 | 627.74 |

Gl2 | 628.63 | 643.79 | 648.63 | 643.61 | |

Gl3 | 633.49 | 643.63 | 649.63 | 647.59 | |

Gl4 | 633.03 | 643.13 | 649.67 | 647.78 | |

Gl5 | 632.69 | 641.36 | 647.04 | 645.43 | |

Gl6 | 632.6 | 638.59 | 642.52 | 641.41 | |

Rear wall | Gre1 | 629.01 | 635.95 | 638.03 | 635.54 |

Gre2 | 633.4 | 649.11 | 653.87 | 648.53 | |

Gre3 | 633.46 | 643.18 | 649.13 | 647.24 | |

Gre4 | 633.02 | 643 | 649.45 | 647.58 | |

Gre5 | 632.86 | 646.31 | 653.1 | 646.45 | |

Right wall | Gr1 | 624.79 | 628.3 | 629.35 | 628.09 |

Gr2 | 629.34 | 645.27 | 650.09 | 644.66 | |

Gr3 | 633.37 | 643.4 | 649.41 | 647.42 | |

Gr4 | 632.74 | 642.82 | 649.37 | 647.49 | |

Gr5 | 632.58 | 641.27 | 646.95 | 645.34 | |

Gr6 | 632.46 | 638.46 | 642.39 | 641.27 |

**Table 7.**Wall temperature distribution under the condition of 75% coal consumption relative to BMCR conditions.

Wall | Number | Fluid Temperature (K) | Average Tube Wall Temperature (K) | Front Point Wall Temperature (K) | Fin Center Temperature (K) |
---|---|---|---|---|---|

Front wall | Gf1 | 573.19 | 578.17 | 580.17 | 578.62 |

Gf2 | 572.98 | 578.83 | 584.58 | 583.31 | |

Gf3 | 572.47 | 579.27 | 585.98 | 584.56 | |

Gf4 | 571.84 | 579.11 | 586.29 | 584.81 | |

Gf5 | 571.56 | 577.85 | 584.08 | 582.8 | |

Gf6 | 571.42 | 575.77 | 580.08 | 579.19 | |

Left wall | Gl1 | 573.2 | 575.91 | 576.92 | 576.04 |

Gl2 | 572.98 | 578.92 | 584.75 | 583.44 | |

Gl3 | 572.45 | 579.35 | 586.15 | 584.71 | |

Gl4 | 571.82 | 579.2 | 586.48 | 584.98 | |

Gl5 | 571.55 | 577.94 | 584.25 | 582.95 | |

Gl6 | 571.42 | 575.84 | 580.2 | 579.3 | |

Rear wall | Gre1 | 573.25 | 578.39 | 580.38 | 578.75 |

Gre2 | 573.05 | 578.9 | 584.65 | 583.39 | |

Gre3 | 572.58 | 579.38 | 586.08 | 584.67 | |

Gre4 | 572.11 | 579.38 | 586.57 | 585.08 | |

Gre5 | 571.89 | 584.19 | 590.57 | 584.49 | |

Right wall | Gr1 | 573.2 | 575.91 | 576.92 | 576.04 |

Gr2 | 572.98 | 578.92 | 584.75 | 583.44 | |

Gr3 | 572.45 | 579.35 | 586.15 | 584.71 | |

Gr4 | 571.82 | 579.2 | 586.48 | 584.98 | |

Gr5 | 571.55 | 577.94 | 584.25 | 582.95 | |

Gr6 | 571.42 | 575.84 | 580.2 | 579.3 |

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**MDPI and ACS Style**

Guo, Q.; Yang, J.; Zhao, Y.; Du, J.; Da, Y.; Che, D.
Numerical Study and Hydrodynamic Calculation of the Feasibility of Retrofitting Tangentially Fired Boilers into Slag-Tap Boilers. *Processes* **2023**, *11*, 3442.
https://doi.org/10.3390/pr11123442

**AMA Style**

Guo Q, Yang J, Zhao Y, Du J, Da Y, Che D.
Numerical Study and Hydrodynamic Calculation of the Feasibility of Retrofitting Tangentially Fired Boilers into Slag-Tap Boilers. *Processes*. 2023; 11(12):3442.
https://doi.org/10.3390/pr11123442

**Chicago/Turabian Style**

Guo, Qianxin, Jiahui Yang, Yonggang Zhao, Jiajun Du, Yaodong Da, and Defu Che.
2023. "Numerical Study and Hydrodynamic Calculation of the Feasibility of Retrofitting Tangentially Fired Boilers into Slag-Tap Boilers" *Processes* 11, no. 12: 3442.
https://doi.org/10.3390/pr11123442