In this paper,the effect of height of the riser,Hriser,on gas-solid flow and combustion characteristics in the coal-fired circulating fluidized bed(CFB)was numerically studied by using computational particle fluid dynamics(CPFD),with Hriser of 10 m,20 m and 30 m respectively and under low solid circulation rates(Gs).The results show that Hriser directly impacts the axial particle and gas concentration distributions.As the axial bed height increases,the particle concentration gradually decreases.When Hriser is larger,the particle density and the concentrations of various gas species remain nearly unchanged after the axial bed height exceeds 15 m,indicating that Hriser of 15 m can provide sufficient residence time for the fuel to burn out.It was also found that as Hriser increases,the temperature of the lower bed is closer to that in the upper bed,and the entire temperature distribution is more uniform.However,after the axial bed height exceeds 15 m,Hriser barely influences the temperature uniformity.When the height exceeds 15 m,the concentrations of various gas species remain basically stable,indicating that the riser of more than 15 m.The overall distribution trends of the concentrations of various gas species in the 20 m and 30 m risers are close,Based on the simulation results,when designing CFB combustion test system,Hriser should be kept above 15 m to ensure the consistence between combustion in experimental system with that in a practical CFB boiler.
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