Complex sentence: Firing with pure oxygen would result in too high a flame temperature, so it is mixed with recycled flue gas to carry fuel into the boiler, lower the flame temperature and ensure adequate convective heat transfer to all boiler areas.
Split into simpler sentences: Firing with pure oxygen would result in too high a flame temperature, so the mixture is further diluted by mixing with recycled flue gas. The recycled flue gas can also be used to carry fuel into the boiler and ensure adequate convective heat transfer to all boiler areas.