Probing into Volatile Combustion Flame and Particulate Formation Behavior During the Coal and Ammonia Co-firing Process

Co-firing ammonia in coal-fired utility boilers is a promising de-carbon technical route for power stations, yet currently, there is still no information on how co-firing ammonia would affect the release and conversion of volatiles. Here, coal pellets were burned with/without ammonia co-firing on a flat-flame burner facility in both fuel-lean and -rich conditions. Detailed information on time-resolved evolution of volatile flame, size evolution of soot particles in flame, and changes in their physiochemical structures was obtained. It was observed that co-firing ammonia promoted devolatilization of coal and release of volatiles, leading to an earlier ignition moment in both fuel-lean and -rich conditions. In the flame, massive soot particles were formed from volatiles, and co-firing ammonia affected the conversion of volatiles into soot and changed the flame radiation properties. Interestingly, both the number density of all soot and size of primary soot particles increased after co-firing ammonia in fuel-lean conditions (by 2.5 times and ∼10 nm, respectively), while they decreased in fuel-rich conditions. In fuel-lean conditions, co-firing ammonia promoted inception and surface growth of soot due to competitive consumption of O2 and increased flame temperature, while in fuel-rich conditions, these effects were offset by partial consumption of soot precursors by forming nitrogen-containing species. Furthermore, when ammonia was co-fired, fringe length, tortuosity, and especially inter-fringe spacing of soot increased slightly, indicating that particles formed in co-firing flames might show higher oxidation reactivity than those formed without ammonia co-firing.