Effect of bluff body addition in fuel-rich stream on reaction behaviours of large-proportion semicoke rich/lean blended combustion

Fuel-rich/lean combustion is considered as a novel combustion technology due to its enhanced ignition performance and low NO_x emissions. To further improve the poor performance of large-proportion semicoke and bituminous coal blended combustion and reduce NO_x emissions, under a bias concentration ratio (BCR) of 2:1, an addition of bluff bodies at the fuel-rich side is proposed to numerically and experimentally investigate the turbulence-chemistry interaction behaviours of the fuel-rich/lean combustion regime in a 0.3 kW pilot-scale bias combustion furnace for various bluff body blockage ratios (BR = 0%, 11% and 21%). The results show that, increasing the blockage ratio evidently shortens the ignition delay time and burnout time of bituminous coal and semicoke, and the promotive effect of triangular bluff-body (BR = 21%) is greater. The turbulent Damköhler number Da_t maximums decrease from 194 to 113 with increasing BR, denoting the regime is traditional combustion mode controlled by mixing. On the fuel-rich side, a local homogeneous MILD combustion zone (Da_t <10) extends with increasing BR. For the heterogeneous combustion, the char-O₂ reaction determined by diffusion/kinetics and the char gasification reaction controlled by kinetics both show a kinetically controlled trend with increasing BR. Moreover, with increasing BR, a decreasing in the NO concentration in the main reaction zone can be attributed to an enhanced recirculation rate K_v. There is a strong correlation between the maximum NO and the local homogeneous/heterogeneous Damkhöler number on the fuel-rich/lean side in the plane of maximum K_v. Increasing BR can evidently decrease the maximum NO concentration on the fuel-rich side due to the enhanced local MILD combustion regime, and thus reducing NOx exit emissions. Considering temperature, ignition, burnout, turbulence-chemistry interactions and NO_x emissions, a triangular-shaped bluff body on the fuel-rich side (BR = 21%) is recommended rather than a cubic bluff body in the fuel-rich/lean co-combustion of semicoke.