A Study of Thermal and Catalytic Pyrolysis of Centimeter-scale Biomass Particles

Abstract

Finding renewable alternatives to fossil fuels is one of the most important challenges of the twenty-first century. In this context, biomass is an important source of renewable energy. This thesis concerns the pyrolysis of centimeter-scale biomass particles, a process that is a key step in the thermochemical conversion of biomass resources to biofuels.

Biomass particles commonly have irregular shapes and sizes, which are critical parameters affecting heat and mass transfer rates during the pyrolysis process. To investigate the pyrolysis behavior of biomass particles of various shapes and sizes, a mathematical model was introduced and numerically solved to simulate the complex physical and chemical reactions during biomass pyrolysis. Prolate and oblate ellipsoids were chosen to represent the biomass particles of arbitrary and irregular geometries. Numerical simulations were validated against relevant literature and experimental data. The effect of the particle shapes and sizes on the mass loss, center temperature, pyrolysis duration, and the product yields were investigated.

In this study, the bio-oil directly produced from the pyrolysis of centimeter-scale biomass particles contains a large number of oxygenated compounds and does not have any hydrocarbon compounds. In order to improve the bio-oil quality, both the pre-treatment of the biomass and post-treatment of the pyrolysis vapors were investigated respectively in a fixed-bed furnace reactor.

Torrefaction is the thermal treatment of biomass particle before they are pyrolyzed. To investigate the effect of torrefaction temperature and time on wood pyrolysis, centimeter-scale pine wood particles were first torrefied at 225 °C, 250 °C, 275 °C, and 300 °C for 15 min, 25 min, and 35 min. Then the torrefied wood particles were used as the feedstock for pyrolysis. Pyrolysis yields of liquid, gas, and char were calculated and found to be affected by the torrefaction conditions. The temperature profiles in the center of the particles were measured and, both the endothermic and exothermic reactions were observed and explained. Chemical analysis of bio-oil obtained from torrefied wood showed that no hydrocarbon species were detected after torrefaction treatment.

Zeolite cracking is a post-treatment technique of the pyrolysis vapor in a high-temperature catalyst environment. Pine wood particles were pyrolyzed in a vertical tube furnace at 500 °C followed by the upgradation of pyrolysis vapors using zeolite ZSM-5 at catalyst temperatures of 400-600 °C (steps of 50 °C). The catalyst was later regenerated to recover its acidity and activity. Pyrolysis oil collected before and after catalysis was characterized by measuring the yield, the water content, and the chemical composition of its organic content. The pyrolysis oil before catalysis was homogeneous and highly oxygenated and neither aromatic nor polycyclic aromatic hydrocarbons (PAH) were detected. Upon catalytic treatment, the yield of bio-oil was markedly reduced from 56.32% to 43.69% (catalyst temperature: 400 °C), and it decreased with increasing catalyst temperature. The change of water content was very small for different catalytic pyrolysis cases. Chemical analysis of the bio-oil showed that aromatic hydrocarbons and PAH were formed in significant amounts upon catalytic treatment. The content of acids and ketones was reduced after catalysis. The overall effect of the usage of regenerated catalyst on the pyrolysis products was not significant in the current study.