The researchers studied how to burn coal more cleanly and efficiently by using a special burner system in a pilot-scale furnace. They found that by using a swirling low-NOx burner and adjusting the oxygen levels, they could achieve stable combustion with low emissions. The coal burned better with oxy-fuel combustion compared to regular air-fuel combustion, even though the flame temperature was lower. By using this method, they were able to reduce NOx emissions by 30-50%. Overall, the study showed that oxy-fuel combustion with a low-NOx burner system is an effective way to burn coal cleanly and efficiently.