MILD combustion of natural gas using low preheating temperature air in an industrial furnace

Moderate or intense low oxygen dilution (MILD) combustion is a promising strategy for reducing pollutant emissions and enhancing thermal efficiency. For industrial scale furnaces, the MILD combustion characteristics using lowly preheated air still needs further investigation. In this paper, an experimental study is presented on natural gas MILD combustion in a 0.3MW industrial furnace using low temperature air (130°C). First, the performances of MILD combustion burner are evaluated with comparison to conventional burner in terms of flow dynamics, temperature distribution and species formation. Subsequently, the transition process during the switching operation from conventional combustion to MILD combustion is examined, regarding the final emission as well as the furnace wall temperature variation. In addition, the impacts of switching temperature on the stability of the transition process are analyzed. The results show that, the present burner succeeds in establishing MILD combustion using low temperature air (130°C) even with a swirling fuel nozzle, resulting in quite uniform temperature distribution and over 80% reduction of NO emission as compared to conventional combustion. However, the lowly preheated air suppresses the combustion rate in the upstream furnace, leading to a backward movement of the high temperature reacting region under MILD operation. Nevertheless, the average furnace temperature is improved as compared to conventional combustion. More importantly, higher switching temperature is helpful for stabilizing the transition process with easier established MILD combustion mode as well as reducing CO emission during transition process.