Numerical research on ammonia-coal co-firing in a 1050 MW coal-fired utility boiler under ultra-low load: Effects of ammonia ratio and air staging condition

Abstract

Co-firing ammonia (NH3) with coal for thermal power generation is a promising strategy for achieving global carbon neutrality. However, the drawbacks of NH3 combustion (e.g., narrow flammability limits, low theoretical flame temperature, high NOX emissions, etc.) can negatively impact the performance of coal-fired boilers, particularly for large-scale ones operating under ultra-low loads. As such, this study was conducted to investigate the ultra-low-load combustion characteristics of a 1050 MW coal-fired boiler when employing NH3-coal co-firing technology. Three-dimensional numerical simulations of ten combustion scenarios were performed to examine the effects of the NH3 co-firing ratio (ranging from 0% to 80%, by calorie) and air staging condition (Separated Over Fire Air (SOFA) ratio ranging from 4% to 24%) on temperature, gas species distributions, and emissions. Results show that as the NH3 co-firing ratio increases to 80%, the in-furnace average gas temperature decreases by 100 ∼ 200 K. The oxidation of NH3 takes precedence over the reduction of NO with NH3, leading to a surge in NO emissions· NH3 slip can be prevented at ultra-low loads due to the rapid consumption of NH3 near coal burners. Meanwhile, CO2 emissions reduce to 20% of that observed in pure coal firing. The increased H2O content in flue gas promotes coal burnout through the water–gas reaction, while the high H2O emissions may potentially cause low-temperature corrosion. At a SOFA ratio of 16%, a relatively large high-temperature region is achieved in the furnace, indicating more stable combustion under ultra-low loads. Excessively small or large SOFA ratios can reduce combustion intensity in the primary combustion zone while contributing to lower NO emissions.