A novel method to improve stability of MILD combustion in a highly heat-extracted furnace

Abstract

Abstract In industrial boilers, a large amount of combustion heat is typically extracted from water-cooled metallic walls or water tubes. This feature is unfavourable for establishing a relatively uniform high-temperature environment in the furnace, which is essential for sustaining stable moderate or intense low-oxygen dilution (MILD) combustion. Hence, MILD combustion is yet to be applied to industry boilers. To address this issue, this study proposes a novel method to improve the stability of MILD combustion by inserting an internal recirculation device (IRD) into the combustion chamber. computational fluid dynamics modelling is performed after being validated by our previous experiments. Then, the dependence of combustion behaviour and stability on the heat extraction ratio (qex) is investigated for both traditional and MILD combustion. The results indicate that traditional combustion can be maintained at qex\xa0=\xa088%, while MILD combustion occurs only at qex\xa0≤\xa057% in the absence of an IRD. With IRD, however, MILD combustion can proceed stably even at qex\xa0=\xa088%. In addition, as qex increases, the NO emission decreases monotonously, while the CO emission decreases first and then increases, in both traditional and MILD combustion. MILD combustion in the presence of an IRD produces ultralow NO emissions, even lesser than 5% of that produced in traditional combustion. Moreover, in general, as the thermal conductivity (λ) of the IRD material increases, both the NO and CO emissions decrease, while the maximal qex decreases gradually. Hence, it is suggested that while using the proposed method for industrial boilers, IRD properties, such as thickness and λ, should be considered carefully to ensure a high thermal utilisation efficiency and low emissions.