A burner for fuel in pellet or granular form in which the fuel has a high ash content, for example grass or hay pellets in which the ash content is 5% or more. The burner has a combustion chamber with a grate at its lower end, into which fuel and air are fed, the combustion chamber having a hot gas outlet for exhausting combustion gases. The grate is provided with a rotary member having upstanding vanes which is adapted to continually move ash or clinker on the grate through outlet apertures spaced around the grate. The grate is a generally planar, horizontal surface which is imperforate apart from the outlet apertures; air inlet ports are arranged in a circumferential wall surrounding the grate.