Single-coal-particle combustion in O2/N2 and O2/CO2 environments

A fundamental investigation has been conducted on the combustion of single particles of a bituminous coal (45–53, 75–90, and 150–180 μm), of a lignite coal (45–53 and 75–90 μm), and of spherical and monodisperse synthetic chars (43 μm) at increasing O2 mole fractions in either N2 or CO2 balance gases. The synthetic particles were used to facilitate the observation of combustion phenomena with minimum distractions from particle-to-particle variabilities. The laboratory setup consisted of a drop-tube furnace operated at temperatures of 1400 and 1600 K. A calibrated three-color pyrometer, interfaced with the furnace, recorded luminous particle temperature–time profiles. Experimental results revealed that coal particles burned at higher mean temperatures and shorter combustion times in O2/N2 than in O2/CO2 environments at analogous oxygen mole fractions. In the case of the bituminous coal used herein and for the experimental combustion conditions tested, measured volatile and char temperatures as in air (21% O2) were attained with an oxygen content in the CO2 mixtures in the neighborhood of 30%. Bituminous coal volatile and char burnout times comparable to those in air (21% O2) were attained with oxygen content in the CO2 mixtures in the range of 30–35%. In the case of the lignite coal burned, the corresponding differences in oxygen mole fractions, which result in similar particle temperatures and burnout times in the two different gas mixtures, were less pronounced.