Tar Cracking Reactor Design for Biomass Gasification Processes

In this thesis, experimental studies on biomass tars thermal cracking, in the temperature range of 1000 – 1200 °C and gas residence time of 4– 10 s, were carried out in three tar cracking reactor configurations, namely continuously stirred tank reactor (CSTR), packed bed reactor and plug flow reactor (PFR), to investigate the feasibility of reducing tar concentration below 150 mg/Nm3 after tar cracking. Tars were sampled with SPA methods and analyzed in GC/MS for quantification and characterization based on the number of aromatic rings in the tar components.
It was observed that in CSTR configuration, the high degree of gas axial dispersion was significantly impacting the tar cracking extent, which was observed to be as high as 1100 mg/Nm3 at 1200°C and residence time of 8.3 s. Using alumina packing material with average diameter of 6 mm, lower gas axial dispersion was achieved and tar concentration was measured to be 575 mg/Nm3 at 1200 °C and shorter gas residence time (4.1 s). In PFR configuration from an initial tar concentration of 8500 mg/Nm3, tar concentration was measured to be 100 mg/Nm3 at 1160 °C and gas residence time of 8.1 s. Increasing the initial tar concentration to 20000 mg/Nm3, similar results were obtained at 1150°C.
It was also observed that the main products of tar cracking were carbon, hydrogen gas, CO and CO2. Furthermore, the carbon formed after tar cracking significantly decreased at 1160°C, promoting formation of carbon monoxide, carbon dioxide and hydrogen gas.
From the experiments, the apparent activation energy for biomass tar cracking was calculated to be 239 kJ/mol. A kinetic model based on the apparent kinetics of the lumped components and degree of axial dispersion based on the dimensionless Bodenstein (Bo) number, was developed to design a tar cracking unit for a 4 ton/h biomass gasification process.