Synergetic combustion behavior of aluminum and coal addition in hybrid iron-methane-air premixed flames

Abstract

This study investigated the combustion behaviors of pure iron and mixed particles, particularly iron–aluminum and iron–coal mixtures, doped into methane (CH4)–air premixed flames. The mechanically mixed particles were prepared with a weight ratio of 1:1. Thermogravimetric analysis revealed that the Fe particles and the Fe–coal mixture underwent oxidation in similar regions of relatively low temperatures; the Fe‒Al mixture underwent a multistage oxidation process. A conical CH4–air premixed flame—with the CH4–air equivalence ratio maintained at the stoichiometric value—was doped with micron-sized solid fuels at various feed rates. Increasing the particle feed rate appeared to alter the flame front characteristics. The interdependency between solid fuels and the CH4–air premixed flame was investigated with respect to flame temperatures, gas emissions, and metal oxide products. Particle microexplosions occurred in the Fe–coal combustion. Regarding the mechanism underlying the microexplosions, we hypothesized that the bubbles inside the Fe particles may have contained dissolved O2, N2, and CO; the dissolved CO may have generated iron carbonyl (Fe(CO)5). Coalescence, repeated bubbling, and bubble expansion processes led to the expansion of iron oxides with hollow shells. The rapid increase in inner pressure and explosive internal combustion caused by the ripening and flammability of the (Fe(CO)5)/O2 bubbles engendered the microexplosions. CO was added to the Fe flame to validate this hypothesis.