Sustainability of mild combustion of hydrogen-containing hybrid fuels

Nowadays a process of sensitization on the fundamental problems related to energetic sources and their environmental impact is observed. In this context, the so-called flameless or mild combustion technology offers great advantages in terms of thermal efficiency and pollution emissions with respect to conventional burner-stabilized firing. The achievement of mild conditions requires to heat up the combustion chamber above a threshold temperature and to design the air and fuel jet nozzles so as to obtain dilution factor and jet velocity values larger than their threshold values. However, while it is well known that mild combustion is effective in reducing significantly NOx emissions and design guidelines are available for standard fuels such as methane, few quantitative information are available for other fuels. This is a limitation to the extension of mild combustion technology to non-conventional fuels, such as industrial by-products and gasification fuels. Consequently, the main aim of this work has been to demonstrate how experimental results obtained in a laboratory-scale burner can be used to identify the main design parameters for burning non-conventional fuels with the flameless technology. In particular, the case of an industrial by-product, the so-called coke oven gas (CH4/H2 40/60% by volume), has been considered. It has been found that hydrogen-containing industrial by-products allow sustaining efficient mild combustion conditions. With respect to the well-established flameless combustion of methane, hydrogen-containing fuels require a larger jet velocity and it has been evidenced the possibility to reduce combustion air preheating without flameless extinction, in order to operate with lower average furnace temperatures. Furthermore, it has been demonstrated the ability of hydrogen to lead to completion the hydrocarbon oxidation also in very diluted conditions. These findings, together with the suitability of mild conditions for soot depression, suggest the possibility to perform a hydrogen-assisted flameless combustion of low-calorific dirty fuels.