The mechanisms governing the behaviour of limestone for in-furnace desulphurisation during oxy-fuel combustion have been investigated in this paper. Apart from the use of pure limestone, the mixtures of limestone with a brown coal (i.e., lignite) at different mass ratios have also been examined to address the influence of ash on the behaviour of limestone. The combustion experiments were conducted in a drop-tube furnace at a furnace temperature of 1273 K and a reactor length of 1200 mm for a nominal particle residence time of approximately 1.0 s. Pure air and an O-2/CO2 mixture at a volume ratio of 27: 73 were tested. SO2 at a concentration of 1000 ppm was also doped into these two gases to examine its competition with Si and Al in the ash on the interaction with limestone. A high CO2 partial pressure has been confirmed to slow down the calcination of limestone in O-2/CO2. However, mixing limestone with coal enhanced the limestone calcination by increasing the particle temperature through radiative heat transfer from flame. With a large quantity of limestone such as the 18% tested here, direct sulphation is a major route contributing to sulphur capture during oxy-fuel combustion. The importance of direct sulphation is exceeded by indirect sulphation via an initial calcination of limestone when medium and low quantities of limestone are added to coal. The Si and Al in coal undermine the calcination and sulphation of limestone in O-2/CO2 because of the preferential formation of Ca aluminosilicate eutectics. These eutectics tend to melt and deposit on the unreacted limestone surface, thus providing extra resistance to the diffusion of gas through the limestone particles. Increase of SO2 content in flue gas favours the indirect sulphation of calcium over the interaction of calcium with ash, thereby increasing the extent of desulphurisation during oxy-fuel combustion. (C) 2012 Elsevier Ltd. All rights reserved.