Ignition and volatile combustion behaviors of a single lignite particle in a fluidized bed under O2/H2O condition

Abstract

O2/H2O combustion, as a new evolution of oxy-fuel combustion, has gradually gained more attention recently for carbon capture in a coal-fired power plant. The physical and chemical properties of steam e.g. reactivity, thermal capacity, diffusivity, can affect the coal combustion process. In this work, the ignition and volatile combustion characteristics of a single lignite particle were first investigated in a fluidized bed combustor under O2/H2O atmosphere. The flame and particle temperatures were measured by a calibrated two-color pyrometry and pre-buried thermocouple, respectively. Results indicated that the volatile flame became smaller and brighter as the oxygen concentration increased. The ignition delay time of particle in dense phase was shorter than that in dilute phase due to its higher heat transfer coefficient. Also, the volatile flame was completely separated from particles (defined as off-flame) in dense phase while the flame lay on the particle surface (defined as on-flame) in dilute phase. The self-heating of fuel particles by on-flame in dilute phase was more obvious than that in dense phase, leading to earlier char combustion. At low oxygen concentration, the flame in the H2O atmosphere was darker than that in the N2 atmosphere because the heat capacity of H2O is higher than that of N2. With the increase of oxygen concentration, the flame temperature in the O2/H2O atmosphere was dramatically enhanced rather than that in the O2/N2 atmosphere, where the diffusion rate of oxygen in O2/N2 atmosphere became the dominant factor.