Oxyfuel technology: Oil shale desulfurization behavior during staged combustion

A high sulfur content in Jordanian oil shale is considered one of the biggest challenges preventing its full utilization. To resolve this issue, direct limestone injection during staged combustion has been investigated for the simultaneous control of NO and SO2 emissions. Staged El-Lajjun oil shale combustion under oxyfuel conditions and conventional air-firing conditions was performed at 1200 °C in a 20-kW vertical reactor. Limestone was mixed with oil shale before combustion. Three different molar ratios of Ca/S, three different burner oxygen ratios (0.75, 0.85 and 0.95) and different positions of the burnout oxidant probe were tested in both firing conditions to determine the optimum conditions for the lowest SO2 and NO emissions. The SO2 emissions were significantly lowered by adding limestone during staged air-firing and OF27 (27% O2/73% CO2 environment) combustion. The desulfurization efficiencies ranged from 32 to 61% and from 25 to 60% during the staged air-firing and staged OF27 combustion, respectively. The staging level, the Ca/S molar ratio and the position of the secondary oxidant are important parameters that affect the desulfurization efficiency. Higher desulfurization efficiencies were obtained during OF27 combustion compared with air-firing at burner oxygen ratios of 0.85 and 0.95. At a burner oxygen ratio of 0.75, the opposite trend was observed. The SO2 emissions during staged combustion were significantly higher than those during unstaged combustion in air and during OF27 combustion. This was due to high competition among many reactions, which minimized the desulfurization efficiency. The maximum desulfurization efficiency during staged OF27 combustion was 60% with λ = 0.85 and Ca/S = 2 at 1 m from the burner. During air-firing combustion, the maximum desulfurization efficiency was 61% with λ = 0.85 and Ca/S = 3 at 1.5 m from the burner. Based on the current and previous studies, the authors recommend that oxyfuel technology be used for oil shale combustion so that CO2, SO2 and NOx emissions can be simultaneously controlled.