Biomass cofiring impacts on flame structure and emissions

The impacts of cofiring biomass and coal on flame structure and NO emissions are investigated in the context of a swirl-stabilized, pilot-scale burner with straw and coal fired independently. The comparatively low energy density of biomass generally leads to higher transport air requirements per unit energy, increasing the momentum of biomass streams relative to an energy equivalent coal stream in burner feeds. Increasing the primary momentum in this manner alters the flow field and stoichiometry patterns of the burner. Detailed species concentration measurements as well as particle sampling were employed to investigate the flame structures of both high and low straw primary air flowrates. Large straw particles penetrate the internal recirculation zone at the high primary air flowrate, elongating the flame structure by forming fuel-rich eddies. The knees (relatively dense sections of straw) of the straw penetrated much further into the reactor, forming a secondary combustion zone. The NO emission was seen to decrease as the straw primary air flowrate increased because of increased numbers of fuel-rich eddies providing more reducing zone, where the fuel nitrogen from the large particles was released. It is also shown that the fuel-rich eddies served as reburning and/or advanced reburning centers, reducing the effluent NO emission further.