Cofiring biomass with fossil fuels is emerging as a viable option for promoting the use of low quality renewable biomass fuels including energy crops. In the Current work, dairy biomass (DB) is evaluated as a cofiring fuel with coal in a small scale 29 kW(t) boiler burner facility. Two types of coal (Texas lignite, TXL and Wyoming Powder River Basin coal, WYO) and two forms of partially composted DB fuels were investigated (low ash separated solids LA-PC-SepSol-DB and high ash soil Surface HA-PC- SoilSurf-DB). Proximate and ultimate analyses performed on both coals and both DBs reveal the following: higher heating value (HHV) of 28,460-29,590 kJ/kg for dry ash free (DAF) coals and 21,450 kJ/kg for DB: nitrogen loading of 0.36 and 0.48 kg/GJ for WYO and TXL, respectively and 1.50 and 2.67 kg/GJ for the LA-PC-SepSol-DB and the HA-PC-SoilSurf-DB respectively; Sulfur loading of 0.15 and 0.42 kg/GJ WYO and TXL, respectively and 0.33 and 0.43 kg/GJ for the LA-PC-SepSol-DB and the HA-PC-SoilSurf-DB respectively; ash loading from 3.10 to 8.02 kg/GJ for the coals and from 11.57 to 139 kg/GJ for the DB fuels. The cofiring experiments were performed with 90:10 and 80:20 and 100:00 (mass%) coal:DB blend (96:4, 92:8, 100:00 - % on heat basis), The results revealed that the blend burns more completely in the boiler, due to the earlier release of biomass volatiles and higher amount of volatile matter. Results were obtained for burnt fraction, NOx and CO emission. Pure TXL produced 1505 ppm of CO at an equivalence ratio of 1.1. An 80:20 blend of TXL:LA-PC-SepSol-DB produced 4084 ppm of CO at the same equivalence ratio. The NOx emissions for equivalence ratio varying from 0.9 to 1.2 ranged from 0.4 to 0.13 kg/GJ for pure TXL coal. The corresponding NOx emissions are 0.8-0.10 kg/GJ for pure WYO coal. For 80:20 TXL:LA-SepS-DB blend they ranged from 0.375 to 0.05 kg/GJ over the same range. In general, the blends produced less NOx than pure coal under rich conditions even though the DB contained more nitrogen. This result is probably due to the fuel bound nitrogen in dairy biomass is mostly in the form of urea which reduces NOx to N-2. (C) 2009 Elsevier Ltd. All rights reserved.