One way to reduce tar is by oxidative and thermal cracking by means of partial combustion of the producer gas in the gas reformer, an apparatus stage subsequent to the woody biomass gasifier. During the partial combustion process of the producer gas, inverse diffusion flame is formed as oxidizer is supplied to the hot producer gas. Cracking and polymerization of tar occur simultaneously at the proximity of the inverse diffusion flame. Polymerized tar grows into soot passing through polycyclic aromatic hydrocarbons (PAH). Several growth mechanisms of PAH have been proposed, which are common to the primary PAH growth followed by abstraction of a hydrogen-atom from the reacting hydrocarbon by a hydrogen radical. We can, therefore, point to the possibility of suppression of the soot formation by controlling the hydrogen concentration at the proximity of the inverse diffusion flame. In the present study, hydrogen concentration at the proximity of the inverse diffusion flame has been controlled by the small amount of hydrogen addition to the oxidizer. Soot formation is suppressed by the chemical effect of a small amount of additional hydrogen (approximately 0.5 % in the total enthalpy of the model producer gas), rather than by a slight change in oxygen concentration and the flow velocity of the oxidizer.
