Addressing global climate change will require increasing sustainable energy usage. Cofiring biomass fuels with coal for electrical power generation is an efficient, cost effective method of CO2 mitigation. This work is an experimental investigation of the flame structure and nitrogen chemistry differences occurring between coal, biomass and cofiring flames. A pilot-scale facility was fired with a dual-feed low-NOx burner capable of independently conveying 2 separate fuels unblended to the burner. Spatially detailed gas species measurements were made for 8 flames, including a coal, straw, finely ground straw, wood, and 4 straw/coal cofiring flames. Particle samples were also obtained from 5 of the flames. Intermittent flamelets were frequently observed in the flames. Viewing the substructure of the flame as individual flamelets provides critical insight for the interpretation of the data. The biomass and cofiring flames show larger flame volumes due to increased primary momentum, increased volatile yields, and differences in fuel particle characteristics (size and shape). The straw and cofiring flames also include secondary flame structures. The secondary flames result from delayed reaction of the straw “knees" due to differences in fuel characteristics. Biomass fuel-N was shown to evolve primarily through NH3, while the coal showed roughly equal amounts of NH3 and HCN. Due to increases in the flame volume and greater NH3 release within these larger fuel-rich regions, as well as lower fuel-N content, effluent concentrations of NO for the biomass and cofiring flames are lower than the coal flame. In-flame reduction of NO corresponds spatially to the presence of NH3, suggesting advanced reburning. Lower fuel-N contents are thought to increase the overall NO production efficiency, but this effect is uncertain for this work due to differences in flame structure and fuel-N chemistry. A mixing model based on intermittent flamelet behavior is included. The model uses dual-delta functions (DDF) to represent lean and rich eddies passing through a sampling volume. Both the beta-pdf and the DDF model were fit to data obtained in this study and compared. The beta-pdf model was unable to capture intermittent behavior. The DDF model was able to represent intermittent behavior, but produced physically unrealistic results.