Experimental Investigation of the Combustion of Bituminous Coal in Air and O2/CO2 Mixtures: 1. Particle Imaging of the Combustion of Coal and Char
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Combustion of a low-volatile bituminous coal in air versus two O2/CO2 mixtures (21/79 and 27/73, v/v) was conducted at two furnace temperatures of 800 and 1000 °C in a lab-scale drop tube furnace (DTF). Through in situ photographic observation and measurement of overall coal burnout rate, CO emission profile, and unburnt char properties, a variety of distinct phenomena relating to oxy-fuel combustion has been revealed. Consistent with the literature, the significant thermal effect of CO2 due to its large product of Cpρ (specific heat capacity and density) relative to that of N2 retarded volatile ignition in the two O2/CO2 mixtures. As a result, the volatiles released in O2/CO2 remained as a thick protective sheath on char surface for a relatively long duration, which mainly converted into CO through partial oxidation in 21% O2/79% CO2.Increasing the O2 fraction to 27% in CO2 triggered the ignition/oxidation of the unburnt volatiles once their concentrations were critically accumulated on char surface in a relatively low position in the DTF. Char oxidation behavior in the late stages of the DTF was also greatly changed under oxy-fuel conditions. Due to an insufficient O2 in char particle vicinity, the partial oxidation and even gasification of char to CO were favored during oxy-firing, which yielded less enthalpy heat and hence lowered char particle temperature substantially. Char consumption rate was, however, affected little or even slightly increased. A detailed mathematical modeling is required to quantitatively clarify the oxidation behavior of coal char in the presence of the abundant CO2 in the DTF.
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