An improved gasifier and method for production of combustible gases from biomass fuels are disclosed in which a massive refractory layer has a first inclined fuel supporting surface or ramp. Massive refractory is provided above and opposing the first fuel supporting surface within a primary gasification chamber. Another mass of refractory cooperates with the first inclined fuel supporting surface of the massive refractory ramp to provide an air seal upon fuel delivery and thereby avoid the disadvantageous introduction of air into the pyrolysis zone of the gasifier. During gasifier operation, massive refractory heats to high temperature.; Biomass fuel delivered to a carbonization zone releases volatile gases by virtue of heat radiation from the massive refractory, and as a consequence of the reflection of heat generated in the combustion zone from massive refractory surfaces within the primary chamber. Additionally, the gasifier includes a lower refractory layer having a second inclined surface or ramp. In a preferred embodiment, the second surface has a plurality of air inlet holes therethrough to provide air distribution only in a specific and concentrated combustion zone located below the carbonization zone. The lower refractory layer second inclined surface is appropriately inclined at an angle of inclination or steepness greater than the first fuel supporting surface of the massive refractory layer to provide for continuous and smooth flow of biomass fuel through respective zones of the gasification chamber.