A furnace for burning dry or wet wood waste products such as hogged bark and the like is provided with a grating therein comprised of aligned rows of bricks resting on supporting cross beams, with at least some of the rows of bricks maintained a uniform distance from other rows of bricks by spacers disposed between such spaced-apart rows of bricks. The furnace is charged by turbulent air entering both above and below the grating, with a select portion of such air being pre-heated. A temperature gradient is established between an area immediately beneath the grating and the area above the grating in the range of 2200.degree. F. and can be controlled by selected initial placement of the bricks and spacers to achieve an optimum cross sectional area for flow of heated, turbulent air through the grating to produce a temperature for efficient heating, drying and burning of wood waste products in an essentially pollution-free manner.