Experimental study on combustion characteristics of coal/NH3 in a flat flame burner

Abstract

To meet the “peak carbon and carbon neutrality” targets, the gradual reduction of coal-fired electricity generation through the co-firing of coal and NH3 has emerged as a new pathway to achieve carbon reduction. However, the co-firing characteristics of coal and NH3 are not yet clear. Therefore, the coal/NH3 co-firing experiments were conducted based on a flat-flame burner to investigate the influence of ammonia blending ratio (E(NH3), 0−100%) and coal/NH3 injection methods (pre-mixed, non-pre-mixed) on combustion characteristics. Camera, flue gas analyzer, and thermocouples were used to observe flame morphology, gas composition, temperature distribution along the centerline heights above the burner (HAB), and measure the unburned carbon content in fly ash. The results showed that during the initial stage of combustion, the competition for O2 between coal and NH3 was more pronounced. The NH3 combustion in the combustion zone created a rich water vapor atmosphere. Under low E(NH3) conditions in pre-mixed combustion, CO reacted with OH, resulting in a decrease in CO concentration. On the other hand, under high E(NH3) conditions, NH3 preferentially reacted with O2, leading to an incomplete combustion of a significant amount of carbon. Additionally, the rich water vapor atmosphere promoted coal gasification reactions, resulting in a significant increase in CO concentration in the combustion reduction zone, reaching a maximum of 19773.05 mg/Nm3. However, this process altered the pore structure of the coke, increased the specific surface area of the coke, accelerated the combustion process of coal powder, and reduced the residual carbon content in fly ash from 13.90% (pure coal combustion) to 13.44% (E(NH3)=80%) under pre-mixed conditions. An early injection of NH3 reduced the preheating effect of NH3 combustion on coal powder and decreased the combustion intensity in the flame reaction zone. The addition of NH3 significantly increased the NOx emissions, and with increasing E(NH3), there was an initial increase followed by a decrease in NOx, with the peak concentration of NO occurring earlier. The unburned NH3 content and oxygen content were the main factors influencing the formation of N2O and NO2. Increasing the residence time of coal/NH3 combustion, reducing the unburned NH3 content, and creating a reducing atmosphere were effective ways to reduce NOx emissions. Under high E(NH3) conditions in pre-mixed combustion (E(NH3)≥80%, HAB=100 mm) and non-pre-mixed conditions, the CO2 percentage showed a decreasing trend with increasing E(NH3). Pre-mixed conditions with an E(NH3) range of 40% to 60% were more favorable for achieving low nitrogen, low carbon, and high-efficiency combustion of coal/NH3. The co-firing of coal powder with NH3 exhibited mutual promotion and restriction effects, requiring some effective measures based on specific circumstances to harness the promotion effect of coal/NH3 co-firing.

Keywords

• co-firing ammonia with coal
• combustion characteristics
• ammonia blending ratio
• coal/nh3 mixing method
• flue gas components