A method and portable apparatus is described for the conversion of cellulose and other blomass waste materials through a pyrolysis and partial combustion sequence in a downdraft gasifier to produce a gas which can be immediately utilized to fuel an internal combustion engine in a generator set (genset). More specifically, the heat from the combustion of part of the cellulosic or other waste input is used to pyrolyze the remainder of the input to produce a mixture of permanent fuel gases. Particulates are removed (water scrubbers, filters) from the gas mixture which can then be used directly as a major part of the fuel to operate the internal combustion engine in the genset. All movement into, through, and out of the gasifier and purification train is controlled by the vacuum associated with the intake of the internal combustion engine, thereby ensuring a steady production of electricity.