The present invention relates to the preparation and burning of biomass-derived fuels such as wood waste or peat. Wood waste or "hog fuel" from the forest industries is of particular interest. Normally the entire stream of this material must be finely ground if it is to be successfully burned in an air suspension burner without any fossil fuel support. It has now been discovered that a bimodally sized fuel can be used. Only about 10-20% of the total heat energy is provided from a portion ground to a size less than about 100 .mu.m. This serves as an ignition component for a principal fuel which may be of much larger size. The usual hog fuel pile contains both bark and wood. Of these two materials, bark is much more friable and easily ground to fine particle size than wood. A major reduction in grinding energy is achieved by selecting the more friable material to be ground to fine size as the ignition fuel. The more resistant material is used as the principal fuel. When using a bimodal system, best results are obtained when the amount of ignition fuel sent to the burner is maintained constant. Load swings are accommodated by varying only the principal fuel component.