The combustion of coal in pure oxygen and recycled flue gas (oxy-fuel combustion) has been proposed as an appealing option to capture CO₂ generated from fossil fuel based power plants. With a different combustion medium, the combustion behaviour in oxy-fuel combustion is expected to be different from conventional air combustion. A fundamental knowledge of coal particle devolatilization and ignition is necessary to recognize the impact of replacing N₂with CO₂ during oxy-fuel combustion. The objective of this study is to compare the ignition behaviour of single coal particles in simulated air (O₂/N₂) and oxy-fuel (O₂/CO₂) environments. This study involves online monitoring of the radiant emission from coal particles using a photomultiplier tube (PMT) when they burn individually in an entrained flow reactor. The time for volatile release and combustion and char combustion were obtained from the PMT monitored emission history for a particle. Individual particles of a bituminous coal were fed into a simulated air and oxy-fuel oxidizing environment generated by a flat flame at 1550 K and 1800 K with oxygen concentrations in the ambient ranging from 10% to 50% by volume. The volatile and char combustion times were longer in the presence of CO₂with the difference being significant at lower gas temperature and oxygen levels. Also, the intensity traces obtained showed varied patterns (with two distinct peaks, overlapping peaks and a single peak) with flame temperatures and also with oxygen levels. An existing single particle flame sheet model has been extended to simulate the ignition of a single coal particle in air and oxy-fuel environments. The flame sheet model assumes the volatiles burn in a shell surrounding the particle within a boundary layer called a flame sheet, where the particle heating is enhanced by the energy feed back from the flame sheet. The differences in volatile and char burning times between air and oxy-fuel conditions were consistent with trends predicted by the model. This is attributed to the differences in the thermo physical properties of N₂and CO₂.
35th International Technical Conference on Clean Coal & Fuel Systems. Proceedings of the 35th International Technical Conference on Clean Coal & Fuel Systems (Clearwater, FL 6-10 June, 2010)