For the electric power generation by the woody biomass gasification, tar is incidentally formed at the same time. Tar means a compound of many kinds of aromatic hydrocarbons and causes some troubles, for example, clogging pipes when it is cooled and condensed before being supplied to the gas engine for electric power generation. One way for reducing tar is oxidative and thermal cracking by partial combustion of the producer gas in the gas reformer that is a stage subsequent to the biomass gasifier. During the partial combustion process of the producer gas, inverse diffusion flame is formed when oxidizer is supplied to producer gas. Cracking and polymerization of tar occur simultaneously at the proximity of the inverse diffusion flame. This polymerization of tar into soot is, however, a significant problem in the gas reformer. Experimental study was performed to clarify the effect of hydrogen concentration in the combustion region on soot formation and the growth of polycyclic aromatic hydrocarbons (PAHs) that is precursor of soot. In the present study, hydrogen concentration at the proximity of the inverse diffusion flame was controlled by the small amount of hydrogen addition to the oxidizer. The main results were as follows. Soot formation was suppressed by the small amount of hydrogen addition (approximately 0.5% to the total enthalpy of the producer gas). The suppression of soot formation was caused by higher concentration of hydrogen at the proximity of the combustion region since the aromatic radicals were neutralized before they could combine together or with acetylene. Carbon yield was increased with the increase in the amount of hydrogen added to the oxidizer as carbon content in the undetectable components by the integrated gas chromatograph such as the soot was decreased. In addition, the increase of carbon yield resulted mainly from the increase in carbon monoxide stemmed from reforming of high-boiling components such as soot.

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