Mills with recovery boilers with excess capacity can potentially mix other biofuels into the black liquor to produce more bio-electricity. This work is a laboratory study of the effect on combustion of mixing other biofuels with black liquor. The four fuels mixed with softwood black liquor were bark, wood chips, peat and biosludge (which some mills already burn with black liquor). Droplets of the mixed fuels were burned at 1100 °C and 3% 02 in a droplet furnace. Video and on-line gas analyzers for CO, C02 and NO were used to measure swelling, combustion times, carbon evolution (C02+CO), and NO formation tendency. We found changes in swelling, char burning and NO formation, but the results indicate that mixing biofuels with black liquor may be a reasonable method of producing additional bio-electricity.