Recently, biodiesel has become more attractive since it is made from renewable resources and also for the fact that the resources of fossil fuels are diminishing day by day. This study compares combustion of B5, B10, B20, B50, B80 and B100 with petroleum diesel over wide input air flows at two energy levels in an experimental boiler. The comparison is made in terms of combustion efficiency and flue gas emissions (CO, CO(2), NO(x), and SO(2)) and influence of air flow at two energy levels 219 kJ/h and 249 kJ/h is studied. The findings show that at higher level energy diesel efficiency was a little higher than that of biodiesel, but at lower level biodiesels are efficient than diesel. Except B10, Biodiesel and other blends emitted less pollutant CO, SO(2) and CO(2) than diesel. B10 emitted lower CO(2) and NO(x), but emitted higher SO(2) than diesel. Despite studies reporting an increase in the NO(x) level resulting from burning of biodiesel over conventional petroleum diesel fuels in engines, our findings indicated at the second energy level a reduction in the NO(x) level in the flue gases resulting from burning of biodiesel. (C) 2011 Elsevier Ltd. All rights reserved.