Grate firing is the most common way to burn bio-fuels in small-scale units. Different combustion modes are achieved depending on how fuel and primary air are introduced. In continuous systems fuel and air are usually fed in cross-current and counter-current flow. Here, combustion of wet biofuels is studied in a 31 MW reciprocating grate furnace (a cross-current how combustor), and additional experiments have been made in batch-fired pot furnaces. The fuel was forest waste with moisture content of approximately 50%. The combustion in a cross-current flow furnace is generally assumed to start by ignition on the surface of the bed, followed by a reaction front propagating from the surface down to the grate. Measurements and visual observations presented in this paper show, however, that in the case of wet fuels the ignition takes place close to the grate, followed by a reaction front propagating from the grate up to the surface of the bed. Hence, the progress of combustion in the bed is opposite to the expected one. (C) 2001 Elsevier Science Ltd. All rights reserved.