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Highlights • Simplified coal compositions method is used for simulation of high moisture coal. • Char-steam gasification process is discussed in this work. • Effects of water content and primary air volume on coal combustion are studied. • Boiler performance is more affected by water content than primary air volume. • Raising primary air volume to maintain drying capacity is not recommended. Abstract Effects of elevated water content in lignite (WCL) and the resultant increased primary air volume (PAV) on coal combustion and boiler performance were numerically studied in a 660 MW wall-tangentially lignite-fired boiler. A special method for considering WCL was adopted in this work, which was proved to be more suitable for the simulation of high moisture lignite. Seven cases were simulated with a previously validated numerical model, and distribution of combustion temperature, heat flux, gas species and residual char particles were analyzed. Results show that with the simultaneous increases of WCL and PAV, coal combustion process deteriorates much at first and then remains at a low level, which is the compromise result of monotonous deterioration of combustion performance caused by WCL and parabolic variation of combustion behavior caused by PAV. Besides, it was found that effects of WCL on overall boiler performance overweigh that of PAV. These findings reveal the influence of operating conditions on boiler performance and point out that PAV should not be increased in practical boiler operation to prevent boiler performance from further degradation. Besides, the suggested method of simplified coal compositions can be useful for the future numerical investigation of high water coal. [ABSTRACT FROM AUTHOR]

Abstract To study the effects of momentum ratio and velocity difference between primary and secondary air flow on boiler performance, simulation work was conducted on a lignite-fired boiler within a three dimension computational fluid dynamics model. Within the model, 8 cases were simulated where momentum ratio and velocity difference vary among cases. The distributions of combustion temperature, radiative heat flux, gas species concentration and residual particles' concentration were analyzed. Results show that momentum ratio of primary and secondary air flow significantly determines coal combustion behavior and boiler performance, and a reasonably small momentum ratio is beneficial for boiler performance. The velocity difference between primary and secondary air also affects coal combustion performance, and a relatively large velocity difference is favorable in boiler operation. These findings reveal the effects of momentum ratio and velocity difference on boiler performance, and can be helpful in guiding the actual operation of lignite-fired boiler to avoid the unfavorable boiler performance degradation. Highlights • A 3D CFD model is established on the basis of a lignite-fired boiler. • Effects of Momentum ratio on boiler performance are investigated. • Effects of flow velocity difference on coal combustion behavior are revealed. • Suggestions are given to prevent performance degradation in actual operation. [ABSTRACT FROM AUTHOR]

Based on a previously validated 660 MW tangentially-fired boiler model, effects of equivalent ring diameter (ERD) and ring centroid offset distance (RCOD) of the proposed burner with annularly combined multiple airflows (ACMA) on boiler performance are investigated. Results show that both ERD and RCOD have great influences on ACMA boiler performance. Small ERD enhances the mutual entrainments of combined airflows, but the interweave of combined airflows hinders flue gas to get into their interior. Although large ERD weakens airflows' mutual entrainments, flue gas can enter the interior of combined airflows to avoid the large pressure difference from the fireside to the backfire sides, so airflow deflection is alleviated. For coal combustion process, small ERD only promotes the intense combustion inside each ACMA burner, while increased ERD makes the intense coal combustion process more continuous at the burner gaps. The increased RCOD adversely affects both the aerodynamics and coal combustion performance, resulting in large airflow deflections and deteriorated coal combustion process. For the investigated boiler, ERD of 3.6 m and RCOD of 5.0 m are recommended. These findings deepen the understanding of ACMA burners and, together with future experimental research results, can provide guidance for its application in practical boilers. [ABSTRACT FROM AUTHOR]

To study how to prevent the boiler performance degradation under large primary air ratio (PAR) condition, a three dimensional computational fluid dynamics model was established on the basis of a 660 MW wall-fired brown coal boiler. Accuracy of simulation models was established by carrying out a mesh independence test and a comparison with real-life data. Then it was used to investigate the effects of proposed measures on boiler performance. The distribution profiles of combustion temperature, heat flux, CO mass fraction and unburnt char particles were selected to analyze the boiler performance under different cases. Results show that boiler performance deteriorates evidently when PAR is increased to 0.425, and the overall heat flux decreases by 37.0 MW. The three proposed measures can indeed improve the boiler performance that has been deteriorated when PAR is increased, and the overall heat flux was increased by 6.42 MW, 6.59 MW and 15.53 MW respectively. Among the proposed measures, the method of closing part of secondary air nozzles is recommended in regular boiler operation, due to its operability and convenience. The findings of this research and recommended measures can be used as guidelines in the actual operation in brown coal power plant. [ABSTRACT FROM AUTHOR]

In previous work, a novel burner arrangement scheme with annularly combined multiple airflows (ACMA) was proposed to improve tangentially fired boilers' performance. Its superiority over conventional wall-tangentially fired boilers was previously confirmed. Based on this, this work futher explores the influences of the number of burners and air nozzles (NBAN) in each wall-mounted ACMA burner on the boiler performance through three cases with different NBANs. Parameters related to in-furnace aerodynamics, combustion and heat transfer processes, and NOx transformation are analyzed. Results reveal that NBAN significantly affects the in-furnace aerodynamics, combustion behavior and heat transfer characteristics. With NBAN increasing from 4 to 6, the more scattered airflows effectively reduce flue gas peak velocity, thus reducing flue gas swirling intensity and the actual tangent circle diameter. Meanwhile, combustion performance and heat transfer uniformity with six burners and air nozzles are improved. However, when NBAN increases to 8, the coal combustion deteriorates slightly and the heat transfer uniformity decreases. Thus, six burners and six air nozzles in each ACMA burner are ideal to obtain the most desirable overall boiler performance. These findings deepen the understanding of ACMA burner, which can guide its future application in utility boilers. • Burner layout scheme with annularly combined multiple airflows (ACMA) is optimized. • Number of burner and air nozzle (NBAN) in ACMA burner affects boiler performances. • The increased NBAN can improve aerodynamics and avoid problems on water-tube walls. • Combustion process improves first and then deteriorates with the increase of NBAN. • Flow, combustion and heat transfer performances are optimal with NBAN value of 6. [ABSTRACT FROM AUTHOR]

• Effects of MCL and IFMC on coal combustion process and NOx emission are studied. • Simplified coal composition method is used to better study effects of MCL and IFMC. • Coal combustion process deteriorates and NOx emission increase with MCL increasing. • An increased portion of water vapor in IFMC decrease the final NOx emission. • Optimal drying degree of coal should be find out in practical boiler operation. Effects of moisture content in lignite (MCL) and its input form (IFMC), i.e. liquid water or water vapor, on coal combustion behavior and NOx transformation characteristics were studied in this paper. For this, 6 cases were numerically investigated within a previously validated 660 MW lignite-fired pulverized boiler model, where coal properties were defined by simplified coal compositions method. Among cases, MCL and IFMC were artificially altered while the relative contents of other coal components remain unchanged. Results show that when MCL is increased as H 2 O(l), coal combustion process deteriorates heavily and thus results in decreases in combustion temperature and fuel utilization rate. Meanwhile, the weakened in-furnace reductive atmosphere leads to a monotonic increase of NOx emission with liquid MCL increasing. Besides, IFMC is found to greatly affect NOx emission, and an increased proportion in water vapor tends to significantly lower the final NOx emission. Based on this, the operating cost of pre-drying system is correlated to that of the post-furnace denitrification process. Considering the potential widespread use of high moisture lignite in future power plants, these findings can be of great theoretical help in guiding the practical operation of lignite boilers. [ABSTRACT FROM AUTHOR]

Through a previously validated numerical model, seven combustion scenarios were simulated to study the effects of primary air ratio (PAR) on coal utilization mode and NOx formation characteristics of a wall-fired lignite boiler. Distribution profiles of combustion temperature, concentration of gas components, mole fractions of NO and volatile N -intermediates were discussed. Results show that the effects of PAR on coal utilization mode, i.e. char-O 2 oxidation and char-H 2 O gasification process, differ in two separate ranges. When PAR is slightly increased from 0.35 to 0.41, char-O 2 oxidation deteriorates in the lower furnace region while char-H 2 O gasification is enhanced. However, when PAR is largely increased from 0.44 to 0.53, char-O 2 oxidation behavior gets re-improved but char-H 2 O gasification becomes weaker. Besides, NOx emission at the furnace exit rises monotonously with PAR increasing. Variation of coal utilization mode is attributed to the changes of mixing process between coal particles and secondary air, while the increase of NOx emission is ascribed to the increased initial oxygen around coal particles. The results thoroughly reveal the influence of air distribution between primary and secondary air on coal utilization process and NOx-related characteristics, which can be used as theoretical guidance for the practical operation of lignite-fired boilers. • Effects of primary air ratio on coal utilization mode and NOx emission are studied. • Importance of char-H 2 O gasification is represented by molar ratio of CO to H 2. • Primary air ratio has parabolic effects on char-O 2 oxidation and char-H 2 O reaction. • NOx formation and emission are significantly affected by primary air ratio. • Increase of NOx formation is due to elevated initial oxygen around coal particles. [ABSTRACT FROM AUTHOR]

A novel burner arrangement scheme with annularly combined multiple airflows (ACMA) was previously proposed for wall-tangentially fired boiler (WTFB), and its superiority over traditional WTFB had been proved. However, coal-fired boilers are often required to play the role of peak-shaving. To this, low load performances of ACMA boiler were investigated in this work to test whether ACMA can still work well under low boiler thermal load (BTL) conditions. Results show that ACMA boiler performance is still better than that of traditional WTFB under low BTL condition, with smaller airflow deflection, improved coal combustion behavior and alleviated thermal deviation on suspended heating surfaces. Besides, NOx emission characteristic of ACMA boiler under low BTL is more desirable. Furthermore, the location of non-service burners has significant effects on coal combustion and heat transfer processes under 440 MW load conditions, and closing the upper burner group is the optimal choice in terms of the comprehensive boiler performance. These findings deepen the understanding of ACMA boiler and provide theoretical and practical guidance in its application in power plants. • Low load performance of ACMA boiler is studied and compared with traditional boiler. • Advantages of ACMA boiler can be maintained under low thermal load conditions. • NOx emission from ACMA is lower than traditional boiler under low load condition. • In-service burner location affects ACMA boiler performance under low load condition. [ABSTRACT FROM AUTHOR]

To improve the performance of tangentially-fired pulverized boiler, a new burner arrangement scheme with annularly combined multiple airflows is proposed and its performance is thoroughly investigated with a previously validated 660 MW lignite boiler model. Results show that compared with conventional wall-tangentially fired boiler, the in-furnace aerodynamic field under the proposed burner arrangement scheme is obviously improved, embodying as reduced airflow deflections, increased furnace fullness and alleviated phenomenon of airflow scouring furnace walls. Besides, coal combustion behavior is also improved, resulting in increased combustion temperature and enhanced heat transfer process in the main burners' region. Furthermore, the reduced flue gas swirling intensity mitigates the deviations of flue gas velocity and temperature in the upper furnace region, which finally lowers the unevenness of heat absorption on the suspended heating surfaces. With the improvement of the abovementioned aspects, the safety and economy of tangentially-fired boilers can be largely increased. Considering the increasingly enlarged boiler capacity and the widespread use of low-rank coal, the proposed burner arrangement scheme can be of great use in retrofitting the existing boilers and designing new ones. • A novel burner arrangement scheme for tangentially fired coal boiler is proposed. • Airflow deflection and its scouring on furnace walls are alleviated. • Coal combustion and heat transfer processes in main burners region are promoted. • Flue gas deviation and uneven heat absorption in the upper furnace are reduced. • Mechanisms of how the proposed burner arrangement scheme work are presented. [ABSTRACT FROM AUTHOR]