Hong Tang

With the rapid development of renewable energy sources in power generation, utility boilers need to perform load regulation over a wide range to maintain the stability of the power supply system. Preheating combustion technology is a potential approach to achieve wide load range operation, improve combustion stability, and lower NO_x emissions from utility boilers. Preheating combustion devices (PCDs) were designed and installed in the reduction zone of a boiler. These devices preheated the coal at an excess air ratio ranging from 0.35 to 0.7 to generate high-temperature gas and char, which effectively reduced NO_x formation in the furnace. Numerical studies were conducted to evaluate the combustion performance and nitrogen oxides emissions of a 330 MW utility boiler retrofitted with PCDs at different loads. The simulations were conducted over a load range of 20% to 100% of the rated load, corresponding to an electrical power of 66 MW to 330 MW. The preheated combustion device’s previous experimental data served as the boundary conditions of the preheated product nozzles. The simulation results demonstrated that the retrofitted boiler could operate stably from 20% to 100% of the rated load, maintaining acceptable combustion efficiency and lower NO_x emissions. The combustion efficiency gradually dropped with decreasing boiler load, reaching a minimum value of 95.6%. As the load declined, the size of the imaginary tangent circle of the boiler shrank, while the ignition distance increased. Additionally, the variation in NO_x concentration with load was complex. The NO_x concentration at the furnace outlet was between 102.7 and 220.3 mg/m³, and the preheated products effectively reduced the nitrogen oxides produced by combustion in the furnace at all loads.

To reduce the impact of renewable energy generation on power grid stability, preheating combustion technology is introduced to maintain coal-fired boiler efficiency at low loads. A 330 MW coal-fired boiler is retrofitted with preheating combustion devices to improve combustion performance and lower NO_x emissions. The device is installed in the reduction zone between the furnace burnout zone and the burner zone. The combustion characteristics of the boiler with and without these devices are examined at 50% rated load. Numerical simulations are conducted to analyze the effects of preheating coal input and burner arrangement on temperature and species distribution within the boiler. Results show that increasing preheating coal input from 0 to 30 t/h enhances NO_x reduction due to a higher flow rate of preheated products. At a preheating coal input of 20 t/h, the combustion efficiency reaches 96.9%. The NO_x concentration at the furnace exit rises from 122.4 to 171.3 mg/Nm³ as the height of the burner arrangement increases. The middle three-layer burner arrangement achieves a uniform temperature distribution and a peak combustion efficiency of 97.6%. The bottom and middle three-layer burner arrangements are recommended for efficient and clean combustion. Compared to the original boiler, the retrofitted boiler’s combustion efficiency increases from 96.3% to a maximum of 97.6%, while the NO_x concentration at the furnace outlet drops from 168.1 to 93.2 mg/Nm³, showing that installing preheating combustion devices promotes efficient and clean combustion.

With the proportion of renewable energy power in the electricity market gradually increasing, coal-fired power is transforming from primary to basic power, with it providing peak and frequency shaving. However, most current methods for peaking below 50% load have been applied industrially, sacrificing the efficiency of the unit. This is not in line with the goals of energy conservation and emission reduction. Therefore, this study proposes a new preheating-based peaking method. This study experimentally and simulatively explores the flow characteristics, pyrolysis gas law, and NO_x emission characteristics of a preheating burner at 40–100% load. The results show that the burner has a significant preheating effect, producing high-temperature char and large amounts of pyrolysis gas. As the load decreases, the burner exit temperature increases, whereas the airflow stiffness decreases. There is little variation in the pyrolysis gas concentration between 40% and 100% loads. The NO_x concentration at the burner outlet increases and the reduction efficiency decreases with decreasing load. At 40% load, NO_x emissions are 91.53 mg/Nm³ and the reduction efficiency reaches 95.9%. Therefore, preheating is an economical, stable, and low-NO_x-emission-peaking method. This study provides theoretical guidance for the application of preheating burners for the 40–100% load peaking of coal-fired units.

Under the current requirements of environmental protection, carbon peaking and carbon neutrality goals, the capacity of the new energy interconnection network is increasing day by day. The thermal power unit requires the ability of rapid load adjustment to meet the requirements of real-time power balance. However, when the unit is in a variable load condition, the denitrification system is inefficient or even inoperable. The nitrogen oxide emission is large, which makes it difficult to meet environmental protection requirements. In this study, a new method for predicting reductive products in the preheating process is proposed, introducing the gasification mechanism into the pulverized coal preheating process. The effects of air coefficient on combustion temperature, velocity, O2, CO2, CO, CH4, H2, volatile matter and NOx emission are investigated by numerical simulation. It is demonstrated that the gasification mechanism is applicable to predict the reduction products and NOx during preheating. Internal combustion burners can stabilize the precipitation of volatile matter, CH4, CO and H2. The burner outlet temperature and flow rate decrease with decreasing air coefficient. The local maximum concentration of volatile matter and CH4 are 8.3 vol% and 2.54 vol% respectively. The CO concentration increases with decreasing air coefficient under the effect of CO2 gasification reaction. When the air coefficient is 0.33, the NOx concentration at the burner outlet is 2.8 mg/Nm3 (@6% O2).