Sampling and characterization of high-molecular-weight polyaromatic tar compounds formed in the pressurized fluidized-bed gasification of biomass

Abstract

Sampling and analysis methods for high-molecular-weight hydrocarbons (heavy tar compounds, MW > 200) formed in pressurized fluidized-bed gasification were developed for analysing the tar content of fuel gas produced from sawdust, wood chips, wood residues (bark, forest residue chips), straw, wood, wood-coal mixtures, etc. The best sampling method tested was based on controlled condensation at 150°C. The tar compounds were condensed on the walls of a cooling tube and a quartz fibre filter, from which they were dissolved by dichloromethane. Low concentrations of some heavy material able to pass through in the vapour phase were collected by impinger bottles at the end of the sampling equipment. Gravimetric analysis at room temperature gave the total amount of heavy tar. Two gas-chromatographic methods were developed fairly successfully to match the result of the gravimetric analysis. The high-temperature g.c. method detected ∼ 20 wt% more of the tar compounds than the general g.c. method. The other analytical methods were gel permeation chromatography (g.p.c.) and pyrolysis-gas chromatography-atomic emission detector (Py-g.c.-a.e.d.). The results attained with g.p.c. were semiquantitative, since the mass responses detected for some calibration compounds were too widespread. The aim of the Py-g.c.-a.e.d. tests was to determine the carbon/hydrogen ratio of the pyrolysis products. Most of the carbon was bound in the pyrolysis residue, which was too small in amount to analyse.