Release Behavior of Tar and Alkali and Alkaline Earth Metals during Biomass Steam Gasification

We developed a drop-tube reactor with a separation unit containing a quartz glass filter that is capable of fractionating alkali and alkaline earth metals (AAEMs) released as solid particle, vapor, and combined with water-soluble and water-insoluble tar. The ratio of released AAEMs in pulverized wood chips that had been steam-gasified was examined by changing the temperatures of the reactor and the separation unit independently. We found that approximately 63-80% of the released AAEMs were combined with tar. In addition, as the gasification temperature increased, the yields of the AAEMs combined with water-soluble and water-insoluble tars remained nearly constant, even though the amount of recovered water-soluble tar decreased as a result of secondary decomposition. This finding implies that AAEMs released by the secondary decomposition of water-soluble tar are fine particles that cannot be collected in a quartz glass filter. Additionally, yields of condensed AAEMs substantially increased as the gasification temperature increased, implying that the evolution of AAEMs from char was enhanced at high gasification temperatures. Evaluation of the water-soluble and water-insoluble tar using a Fourier transform infrared spectrometer (FTIR) and an elemental analyzer indicated that AAEMs likely combine with the OH groups of carboxylic acids and phenols in water-soluble tar. This finding infers that the AAEMs recovered from water-insoluble tar are present in aromatic compounds that are derivatives of benzene, xylene, furfural, and naphthalene in the product gas.