Combustion Mechanism of Rich-Lean Flame Burner Controlled Boundary Zone

Abstract

Rich-lean flame burners are widely used for supplying domestic hot water and heating in Japan. These burners exhaust low NOx and CO emissions, and conventionally consist of a layered structure of lean flame with two sides of rich flame. Top-runner regulation applied for the domestic hot water generator of fuel gas, is to run to achieve the CO2 reduction two years later in Japan. Not only low NOx and CO emissions, but also higher efficiency, are required for the next generation of domestic hot water generators. Kurachi et al. confirmed experimentally that a new concept, a unique burner with air supplied from the boundary zone between the rich and lean premixed gas nozzles, produces lower NOx and CO emissions (1, 2, 3). Numerous experimental and numerical simulation studies of conventional rich-lean flame burners have been reported, and the mechanism of the complex field mixed with the rich and lean premixed gas has been clarified (4). But the characteristics of the new concept burner have only been investigated experimentally. In this study, a two-dimensional numerical simulation of the new burner was executed to clarify the mechanism of the lower NOx and CO emissions compared to the conventional burner (mesh; 25,000, chemical reaction; GRI-mech II, laminar flow). Heat input was 6.5kW (half of a full load). A conventional burner, without an air supply from the boundary zone, was also calculated to compare with the new concept burner. In a conventional burner, the reaction ratio R178 (N+NO = N2+O), which is a part of the Zeldovich mechanism, is dominant at the area downstream of the rich flame. This area is almost same as the maximum temperature area of the burned gas. The maximum temperature of the new concept burner (1,923K) is approximately 50K lower than that of the conventional burner, successfully maintaining stable combustion. Because of its lower maximum temperature, the amount of NOx emission from the new concept burner is approximately 40% of that from the conventional burner. With the air supply from the boundary zone, the concentration of CO in the flue gas also is decreased by approximately 1/3. In particular, the formation of thermal NOx in the lean flame zone is suppressed by lowering the flame temperature. The amount of CO emission from the rich flame zone is also decreased due to the promotion of complete combustion with the air supply from the boundary zone. As a result, these characteristics are in relatively good agreement with the experimental results, and the dominancy of the new concept burner is also clarified.