Yield and Characteristics of Tar Formed during Biomass Fluidised-Bed Gasification

Abstract

In this study, the behavior of gasification tar formed was investigated in a 100 kWth bubbling fluidized-bed gasifier using rubber woodchip as fuel. Tar yield and characteristics as well as the composition of product gas were monitored and correlated with respective gas residence time, which were varied by means of gas sampling at different positions, i.e. four along the height of the gasifier and one after the cyclone. At selected experimental condition (ER=0.33), the significant concentrations of CO and H2 were formed at 4.8 s, after which the increased concentrations of CO and H2 at longer residence times may be attributed to the tar reforming reactions and thermal cracking. The total gravimetric tar increased from 4.5 to 10.5 g/Nm3, when the residence time was increased from 2.5 to 9.8 s. The increasing gravimetric tar yield as the residence time increased inside the gasifier suggests that more tar was formed as it was exposed to high temperature. The concentrations of all tar major species also increased following the trend of gravimetric tar yield, but the ratio of smaller molecular weight materials increased at extended residence time. This therefore suggests that longer residence time would be preferable as it would allow more time for the reaction of tar vapor to crack into smaller molecular weights and to form more product gases.