Comparative Performance Assessment of a U-Shaped Recuperative Radiant Tube under Conventional and MILD Combustion Modes

Abstract

Moderate and intense low-oxygen dilution (MILD) combustion, characterized by low reaction rate, uniform heat flux, and significantly reduced pollutants, is a promising technology which is widely used in various industries. This paper presents experimental research on the successful application of the MILD combustion mode to a U-shaped recuperative radiant tube (RRT) with a nominal power capacity of 7 kW in order to optimize the performances of the RRT. The combustion characteristics and the effects of air-staged proportion, one of the key operating parameters, on the performance of RRT are investigated. Tube wall temperature uniformity and NOx emission are improved obviously when the air-staged proportion decreases from 3:7 to 1:9 in the conventional combustion mode. It is shown that there are some limitations to further performance improvements. Accordingly, the MILD combustion mode is successfully introduced and applied by varying burner configuration and operational processes, and the key parameters (e.g., recirculation ratio KA and flue gas temperature Tf) to establish the MILD combustion mode are analyzed quantitatively. It is concluded that to establish the MILD combustion mode, KA as a fundamental key parameter must first meet a critical value and the values of KA and Tf then are acquired jointly. With the increase of KA, the qualified flue gas temperature Tf required to implement the MILD combustion decreases accordingly, and the fifth-order polynomial relationship between Tf and KA with adjusted R2 of .97 is determined based on the experimental data to accurately judge the establishment of the MILD combustion mode. Thus the application range of the MILD combustion mode, the superiority of which is reflected in significant enhancement of performance, can be successfully expanded to U-shaped RRTs, providing the theoretical basis for the industrial production of such high-quality heating items.