Abstract: A vertical, countercurrent, updraft reactor is provided which can be advantageously used to produce synthesis gas by the partial oxidation of a solid fuel, especially a wood particle fuel. The reactor has a substantially cylindrical, vertical shell. The reactor has a fuel inlet opening in the upper portion of the reactor and an ash outlet opening in the lower portion of the reactor. Also in the lower portion of the reactor is a synthesis gas outlet opening. A heating zone is created within the reactor by internal heating zone walls which terminate at a lower-most edge. A grate is located below the heating zone. The outer perimeter of the grate and the lower-most edge of the heating zone walls form an annular slit which allows ash to gravitate out of the heating zone and into the lower-most portion of the reactor. The grate is reciprocal so that the annular slit can be periodically narrowed and widened.