Abstract
Carbon dioxide emissions are considered a major environmental threat. To enable power production from carbon-containing fuels, carbon capture is required. Oxy-fuel combustion technology facilitates carbon capture by increasing the carbon dioxide concentration in flue gas. This study reports the results of calcium rich oil shale combustion in a 60 kWth circulating fluidized bed (CFB) combustor. The focus was on the composition of the formed flue gas and ash during air and oxy-fuel combustion. The fuel was typical Estonian oil shale characterized by high volatile and ash contents. No additional bed material was used in the CFB; the formed ash was enough for the purpose. Two modes of oxy-fuel combustion were investigated and compared with combustion in air. When N2 in the oxidizer was replaced with CO2, the CFB temperatures decreased by up to 100 °C. When oil shale was fired in the CFB with increased O2 content in CO2, the temperatures in the furnace were similar to combustion in air. In air mode, the emissions of SO2 and NOx were low (<14 and 141 mg/Nm3 @ 6% O2, respectively). Pollutant concentrations in the flue gas during oxy-fuel operations remained low (for OXY30 SO2 < 14 and NOx 130 mg/Nm3 @ 6% O2 and for OXY21 SO2 23 and NOx 156 mg/Nm3 @ 6% O2). Analyses of the collected ash samples showed a decreased extent of carbonate minerals decomposition during both oxy-fuel experiments. This results in decreased carbon dioxide emissions. The outcomes show that oxy-fuel CFB combustion of the oil shale ensures sulfur binding and decreases CO2 production.
Highlights
Carbon dioxide emissions continue to be a major environmental concern of solid fuel combustion.Carbon capture and storage is required in order to meet future targets for greenhouse gas emissions.The typical technologies related to CO2 capture are pre-combustion, post-combustion and oxy-fuel combustion [1]
Higher CO2 partial pressure and lower temperature in the system inhibited decomposition of CaCO3, but the Ca/S molar ratio in the oil shale was as high as 8.0. It means that there was a lot of excess calcium in the system and even if the sulfur binding rate was inhibited as noted in reference [23], it was still enough to bind more than 99% of the sulfur
The oxy-fuel ashes contained a considerable amount of carbonates ((CO2 )mineral ) indicating decreased decomposition of carbonate minerals
Summary
Carbon dioxide emissions continue to be a major environmental concern of solid fuel combustion. Theoretical calculations [20,21,22] and a previous study in a lab-scale fluidized bed batch reactor [23] revealed that carbonate minerals do not fully decompose under oxy-fuel combustion environment and the sulfur binding rate decreases, similar to the experiments of Li et al [24]. Nitrogen content in the Estonian oil shale is low, below 0.1% in dry fuel Due to this and the low temperatures in CFB combustors, no problems with excess nitrogen oxide formation were recorded from the utility boilers [10]. To investigate possible changes in the flue gas and produced ash when applying oxy-fuel combustion on Estonian oil shale, combustion experiments in a 60 kWth. CFB combustor were performed using air and mixed gases to simulate possible oxy-fuel combustion environments.
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