Computational investigation of hydrodynamics, coal combustion and NOx emissions in a tangentially fired pulverized coal boiler at various loads

Highlights

• Decreasing boiler loads impair air–fuel mixing and steady furnace combustion.

• High-temperature zone and flame move toward side walls with decreased boiler loads.

• O₂ and NOx contents increase, whereas CO content decreases with reduced load.

• A decreased burner tilt angle favors heat transfer and NOx emissions.

Abstract

This work presents a computational investigation of hydrodynamics, coal combustion and NOx emissions in a tangentially fired pulverized coal boiler at different loads (630, 440 and 300 MW; relative loads of 100%, 70% and 48%) to clarify the effect of load change on the furnace processes. A computational fluids dynamics model was established; the flow field, temperature profile, species concentration and NOx emissions were predicted numerically; and the influence of burner tilt angles was evaluated. Simulation results indicate that a decrease in boiler load decreases the gas velocity, attenuates the airflow rotations, and increases the tangent circle size. The high-temperature zone and flame moved toward the side walls. Such behaviors impair air–fuel mixing, heat transfer and steady combustion in the furnace. In terms of species concentrations, a decrease in boiler load increased the O₂ content, decreased the CO content, and decreased the char burnout rates only slightly. A change in boiler load from 630 to 440 and 300 MW increased the NOx emissions from 202 to 234 and 247 mg/m³, respectively. Burner tilt angles are important in coal combustion and NOx emissions. A burner angle of –15° favors heat transfer and low NOx emissions (<185 mg/m³) for the current tangentially fired boiler.

Introduction

As the most widely used combustion systems, tangentially fired pulverized coal boilers have been used extensively in coal-fired power plants, owing to their favorable coal flexibility, good flame stability, high combustion efficiency, and easy operation (Liu, Fan, & Wu, 2017; Park et al., 2013; Sankara, Santhosh Kumara, & Balasubramanian, 2019). During their history of ∼50 years, improvements in tangentially fired pulverized coal boilers have been targeted with respect to their efficiency and the environment (Beer, 2000).

In recent years, the increase in intermittent renewable energy has driven coal-fired power plants to undertake deep peak-load regulation, and boilers are operated frequently under variable load conditions (Gu, Dong, Chen, Wang, & Li, 2016; Wang et al., 2018). During the load change, fuel and air that are introduced into the boilers are adjusted; the hydrodynamics, temperature and oxygen distribution in the furnaces vary; and the combustion rates and char burnout change. These changes further affect NOx emissions from the boilers. In the new market situation of deep peak-load regulation, a better understanding of the hydrodynamics, coal combustion and NOx formation in the boilers at different loads, especially for intermediate and low loads, is of utmost importance.

Because full-scale experiments are restricted by expense, computational fluid dynamics (CFD) simulations have been used to investigate tangentially fired pulverized coal boilers (Belosevic, Sijercic, Oka, & Tucakovic, 2006; Choi & Kim, 2009; Fan, Qian, Ma, Sun, & Cen, 2001; Tan et al., 2017; Wu, Fan, Liu, & Bian, 2019; Zha, Li, Wang, & Che, 2017). Several studies have been devoted to the CFD modeling of gas flow, heat transfer, coal combustion and NOx emissions at variable/low loads. For example, Shi, Li, Yang, and Duan (2019) analyzed flow and heat transfer in a 1000-MW tangentially fired pulverized coal boiler at various loads. At a 50% boiler load, the furnace-temperature change was irregular, and the heat transfer deteriorated in the water-cooled wall region. Belosevic, Sijercic, Tucakovic, and Crnomarkovic (2008) developed a comprehensive CFD model to predict complex turbulent reactive flows in a tangentially fired boiler. The influence of burner configuration and boiler load on the combustion processes was evaluated. Al-Abbas, Naser, and Hussein (2013) presented a CFD study for brown coal combustion in a 550 MW tangentially fired furnace under different operating conditions. The effects of load change on velocity, temperature, species concentrations and char consumption were predicted. Based on their improved CFD model, Belosevic, Tomanovi, Crnomarkovic, and Milicevic (2019) indicated that the operating conditions at low loads affect the flow/temperature fields, flame geometry, combustion reactions and species concentrations. Yuan et al. (2019) presented a detailed CFD simulation of the variable load combustion in a 660-MW supercritical swirling opposed boiler. NOx emissions were lowest at a boiler load of 30%, a primary air ratio of 0.2, and a pulverized coal size of 50 μm. A reduction in boiler load decreased the combustion stability, and the NOx emissions increased.

The above research shows that the use of CFD in tangentially fired pulverized coal boilers provides extensive information on the combustion characteristics and boiler performance. The temperature distributions, species concentrations and heat flux under different conditions were obtained numerically. Problems in variable load combustion, such as heat transfer deterioration and a decrease in combustion stability, could be predicted. The combustion performance and NOx emissions may be improved by optimizing the operating conditions. However, compared with full-load combustion simulations, limited CFD simulations exist regarding variable load combustions. The influence of load change on the hydrodynamics, heat transfer, coal combustion and NOx emissions is not fully understood. A comprehensive CFD model for tangentially fired pulverized coal boilers at various loads is needed.

This work focused on CFD investigations of hydrodynamics, coal combustion and NOx emissions in a 630-MW tangentially fired pulverized coal boiler at various loads (630, 440 and 300 MW; a relative load of 100%, 70% and 48%), and aimed to clarify the effects of load change on the furnace processes. To achieve this aim, a comprehensive CFD model, including gas–particle flow, heat transfer, coal combustion and NOx formation, was established. Based on the grid independence and model validation, the flow field, temperature profile, species concentration and NOx emission characteristics were predicted numerically for different boiler loads. The effect of burner angle on the furnace processes was evaluated. Such a comprehensive simulation study is expected to provide useful guidelines for tangentially fired pulverized coal boilers that are operated under different conditions.