Two-dimensional computational fluid dynamics simulation of nitrogen and sulfur oxides emissions in a circulating fluidized bed combustor

Based on the previously established two-dimensional computational fluid dynamics (CFD) model which described processes of coal devolatilization, volatile combustion and char combustion in circulating fluidized bed (CFB) combustors, nitrogen and sulfur oxides emissions are numerically simulated and investigated in the present paper. First of all, a more accurate heat transfer model was established by applying energy conservation equations to gas and solid phases separately, rather than one conservation equation of the mixture enthalpy in our previous model. Interphase heat transfer mechanism was considered as well as bed-to-wall heat transfer. For the constant wall temperature boundary condition, proportional heat sinks were adopted in the furnace to compensate the missing heat transfer surfaces from 3-D cylinder riser to 2-D planar model. Results of temperature distributions agreed well with the experimental data.Secondly, processes related to nitrogen and sulfur oxides emissions are included. Results from our previous studies showed that, no NH3 was detected during pyrolysis of this bituminous coal by TG-FTIR (Thermogravimetry coupled with Fourier Transform Infrared) analysis. So it was assumed that fuel N and S partially released to the volatile as HCN and SO2, and partially retained in the char. HCN converted to NO and N2O quickly through homogenous reactions. Char N and S converted to NO and SO2 during char combustion. NOx was reduced to N2 by char carbon or CO. SO2 was retained by CaO calcined from CaCO3. By converting reaction rate expressions to suitable forms for Eulerian–Eulerian modeling, sulfation reaction rates from two different literatures were compared. Performances of SO2 emission were evaluated for conditions with/without considering sulfur self-retention. Distributions of gas components in the furnace were predicted and the outlet gas concentrations were validated by the experimental data. Distributions of certain reaction rates in the riser were also illustrated.

► Coal combustion in CFB is successfully simulated by a comprehensive CFD model.
► Two separate energy equations can predict local high temperature regions.
► The 70 s simulation achieves a steady state for gas compositions.
► Predictions of N and S dioxides agree well with experiments.
► Secondary air has important impacts on distributions of temperature, NO and SO2.