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The carbon content in the fly ash from most Chinese circulating fluidized bed (CFB) boilers is much higher than expected, which directly influences the combustion efficiency. In the present paper, carbon burnout was investigated in both field tests and laboratory experiments. The effect of coal property, operation condition, gas−solid mixing, char deactivation, residence time, and cyclone performance are analyzed seriatim based on a large amount of experimental results. A coal index is proposed to describe the coal rank, having a strong effect on the char burnout. Bad gas−solid mixing in the furnace is another important reason of the higher carbon content in the fly ash. Some chars in the fly ash are deactivated during combustion of large coal particles and have very low carbon reactivity. Several suggestions are made about design, operation, and modification to reduce the carbon content in the fly ash.

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There is a large range of time and length scales of turbulent fluctuations in an entrained flow coal gasifier. To figure out the turbulent effects on coal gasification processes and the controlling mechanisms of different regions in an entrained flow coal gasifier, the characteristic time and length scales for all major processes are estimated. On the basis of the comparison between the length/time scales of turbulent mixing in all ranges and the characteristic time of homo-and heterogeneous reactions, turbulent effects on reactions and coal gasification processes in different regions in an entrained coal gasifier are studied. In the flame region, there is a strong coupling effect between macro-scale turbulent fluctuation and heterogeneous reactions. The combustion of the volatile is strongly affected by the fluctuation of micro-scale Kolmogorov scales because the diffusion boundary layer of the particles is destroyed by this kind of fluctuation. In the nonflame region, the heterogeneous char gasification reactions are not affected by turbulent fluctuations; however, the coupling effect between turbulent mixing and gas-phase reactions should not be omitted.