Understanding the product distribution from biomass fast pyrolysis
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Fast pyrolysis of biomass is an attractive route to transform solid biomass into a liquid bio-oil, which has been envisioned as a renewable substitute for crude oil. However, lack of fundamental understanding of the pyrolysis process poses a significant challenge in developing cost-effective pyrolysis based technologies for producing transportation fuels. The fundamental knowledge of pyrolysis pathways, product distribution and underlying mechanisms will have a direct and significant impact on the reactor design, strategic operation and kinetic modeling of the pyrolysis process. However, this knowledge has remained obscure due to the complexity of the pyrolysis process and lack of well established analytical methodologies.
The present work provides a systematic approach to study pyrolysis, where many factors that affect the pyrolysis process are decoupled and their effect is systematically studied. The study employs a combination of analytical techniques such as Gas Chromatography - Mass Spectrometry, Gas analysis, Liquid Chromatography - Mass Spectrometry, Capillary Electrophoresis, Ion Chromatography and Gel Permeation Chromatography to identify and quantify the pyrolysis products and establish the mass balance.
Pyrolysis involves a complex scheme of reactions consisting of several primary and subsequent secondary reactions. Disassociating primary and secondary reactions is often challenging because of the typical residence time of pyrolysis vapors in the traditional pyrolysis reactors. However, mechanistic understanding of the pyrolysis pathways needs information of the primary pyrolysis products, prior to complex series of secondary reactions. This was achieved by employing a system consisting of a micro-pyrolyzer which had vapor residence time of only a few milliseconds, directly coupled with the analytical equipment. The problem was further simplified by considering the pyrolysis of each individual component of biomass (hemicellulose, cellulose and lignin) one at a time. Influence of minerals and reaction temperature on the primary pyrolysis products was also studied. Secondary reactions, which become important in industrial-scale pyrolysis systems were studied by comparing the cellulose pyrolysis product distribution from micro-pyrolyzer and a bench scale fluidized bed reactor system.
The study provides fundamental insights on the pyrolysis pathways of hemicellulose, cellulose and lignin. It shows that the organic components of biomass are fragmented completely into monomeric compounds during pyrolysis. These monomeric compounds re-oligomerize to produce heavy oligomeric compounds and aerosols. It also provides the understanding of the effect of parameters such as presence of minerals and temperature on the resulting product distribution. This knowledge can help tailor the pyrolysis process in order to obtain bio-oil with desired composition. The pyrolysis product distribution data reported in this dissertation can also be used as a basis to build descriptive pyrolysis models that can predict yield of specific chemical compounds present in bio-oil. Further, it also serves as a basis for distinguishing secondary reactions from the primary ones, which are important consideration in the industrial-scale systems.