(17e) Improvement of Biomass Gasification through Several Pretreatments

As one of the thermochemical conversions of biomass, gasification has been studied extensively. In order to achieve the efficient conversion, improvement of gasification rate and lowering of gasification temperature are required. We have developed several kinds of pretreatment methods for biomass so far. In this study, we examined the effect of these pretreatments on the gasification reactivity of biomass. One is a method for separating hemicellulose, cellulose and lignin from biomass, and for converting them into valuable chemicals. The method is basically a two-step process in which biomass treated in hot water at 180 °C was consecutively oxidized with 30%-H2O2 at 60 °C under ambient pressure. Through the oxidation, lignin fraction was converted to water-soluble compounds containing small molecular weight fatty acids such as HCOOH and CH3COOH. Since this water-soluble lignin was rich in aliphatic structure due to the cleavage of aromatic ring, its gasification rate was expected to be larger than that of the original lignin. From the viewpoint of the fully biomass conversion, we investigated the possibility to utilize the water-soluble compounds as a feedstock for the gasification by comparing the gasification rates between the oxidized lignin and organosolv lignin using the so-called temperature-programmed reaction (TPR) method in a steam flow. Organosolv lignin was gasified above 900 °C. On the other hand, the oxidized lignin started to be gasified at around as low as 600 °C. Even if once lignin was carbonized at 800 °C, gasification of the oxidized lignin char proceeded under a lower temperature than organosolv lignin char did. This presented the possibility to utilize the lignin fraction, which would be easily deactivated during pyrolysis under normal conditions, as a feedstock for the gasification. Another pretreatment is a method of loading biomass with inorganic compounds. Calcium hydroxide interacted with the functional groups of biomass, and there still remained the hydroxyl group after carbonization. On the contrary, iron chloride promoted dehydration reaction of biomass and volatilization of hydrogen chloride during pyrolysis. These two inorganic compounds brought about the exact opposite chemical structure of char, but both compounds showed the large catalytic effect for gasification. Thus it was clarified that these pretreatments contributed to the improvement of gasification reactivity of biomass.