## 4 Conclusions  
The oxy-combustion characteristics of Australian coal in a 0.3 MW\({}_{\text{th}}\) furnace were investigated. The influence of various oxygen flow rates and recirculated flue gas (RFG) on heating performance and pollutant emissions are examined in O\({}_{2}\)/RFG environments. The general conclusions drawn from results of this study are as follows. With increases in the secondary RFG flow rate, the temperatures (\(T_{w1}\)-\(T_{w5}\)) in the radiative section decreased, while the temperatures (\(T_{g1}\)-\(T_{g5}\)) in the convective section increased. When firing Australian coal in the oxy-combustion mode at an oxygen supply of 38.5 Nm\({}^{3}\)/h, with various RFG flow rates, the CO\({}_{2}\) concentration could be stably maintained at about 94%. High CO\({}_{2}\) concentrations in the flue gas of up to 94.8% can be achieved. It is noteworthy that the high CO\({}_{2}\) concentration in the flue gas suggests great potential for reducing CO\({}_{2}\) emissions through carbon capture and storage, thus mitigating global climate change. Furthermore, a higher O\({}_{2}\) supply rate led to higher O\({}_{2}\) and NO concentrations in the flue gas, while it lowered CO, SO\({}_{2}\), and CO\({}_{2}\) concentrations in the flue gas when firing Australian coal. With increases in the secondary RFG flow rate, the NO emissions decreased while SO\({}_{2}\) emissions increased. Compared to air combustion, a significant reduction in NO emissions of as high as 74% could be achieved under oxy-combustion conditions. For oxy-coal combustion, an appropriate amount of RFG is required, which not only reduces the flame temperature, but also enhances the mixing of fuel and oxidant in the furnace, resulting in more complete combustion. High CO\({}_{2}\) concentration in the flue gas of up to 94.8% can be achieved when the residual oxygen concentration in the flue gas is controlled in the range of 3-5 vol %. The energy requirement for CO\({}_{2}\) compression is reduced in all CO\({}_{2}\) sequestration options as the purity of the CO\({}_{2}\) increases. It is suggested that 95% should be a reasonable CO\({}_{2}\) purity [56]. Thus, in this study, the high CO\({}_{2}\) concentration in the flue gas around 95% suggests that it is possible and beneficial to achieve easy carbon capture and storage. While this is true, however, in view of the research results, one realizes that using oxy-fuel combustion requires more capex (oxygen plant, flue gas recirculation) and produces a lower power output (oxygen plant and gas compressor power demand) than a conventional coal plant. In addition, CCS requires storage sites for CO\({}_{2}\), which are not available in many locations. As such, there are still no commercial oxy-fuel plants in commercial operation, and one of the largest demo plants was the 30 MWth plant at Callide in Australia. The oxy-fuel combustion process is still in the development stage, and a lot of research is still needed to fully clarify the consequences of its implementation in power plants. Furthermore, it would be better to handle economic aspects of operational cost with a comparison with conventional combustion. In the present study, coal was directly burned under oxy-fuel combustion conditions to achieve high CO2 concentrations, which is beneficial to carbon capture and storage (CCS). It is recognized that coal gasification is a process that reduces the CO2 emission and emerges as a clean coal technology. Accordingly, coal can be gasified in a fuel-rich (hypoxic) combustion mode, and then the resulting synthesized gases can be properly burned under oxy-fuel combustion [57]. The coal gasification process is controlled by several operating parameters, including temperature, pressure, reaction time, coal rank, porosity, and choice of catalyst. The reaction temperature is one of the parameters that has the most influence on the gasification reaction. For example, the carbon conversion and gasification rate increased with increases in the reaction temperature.