Experimental Study of Biomass Pyrolysis in a Fluidized Bed Reactor: Effect of Biomass Blending and In-situ Catalysis

Abstract

Lignocellulosic biomass has been recognized worldwide as a renewable resource of energy for the production of liquid biofuels due to its low cost and abundance. Several routes to convert biomass to biofuels have been studied extensively, but they are limited by the multiple conversion steps and high cost of processing required, which makes it difficult to implement them on a large scale. Fast pyrolysis is a promising technology to convert biomass to hydrocarbons. Fast pyrolysis has several advantages such as short residence time, moderate temperature (400 to 600 °C) and its ability to convert biomass to liquid fuel in a single step with high yield. However, some of the properties of the liquid product from fast pyrolysis such as high acidity, high oxygen content, high viscosity and low heating value are impediments to the development of this technology. High acidity renders the liquid to be corrosive, while high viscosity and low stability make it unattractive as a fuel and practical application of bio-oil suffers due to these properties. A brief review of the background information of this study, including the factors affecting pyrolysis, properties of bio-oil and various upgrading techniques, is summarized in Chapter 2.

A fluidized bed reactor based experimental setup was designed and fabricated for this study which had an overall goal of investigating different techniques to improve the physical and chemical properties of bio-oil obtained from fast pyrolysis of biomass. This goal was achieved by carrying out two specific objectives. The first objective was to identify the optimum residence time and process temperature for fast pyrolysis of southern pine, followed by evaluating the effect of blended biomass feedstocks (southern pine and switchgrass) to understand their impact on the quality of bio-oil obtained from fast pyrolysis (Chapter 3). From this study, it was observed that blending switchgrass with southern pine improved the pH and viscosity of the bio-oil, but there was no significant effect on the heating value of the bio-oil. However, bio-oil produced with greater amounts of switchgrass than pine resulted in a clearly phase separated bio-oil.

In the second objective, basic catalysts (MgO, CaO) and acidic catalyst (ZSM-5) were incorporated in the fluidized bed reactor as in-situ catalysts to upgrade pyrolysis vapors (Chapter 4). The performance of each catalyst was evaluated based on the quality of bio-oil obtained, when compared to control experiments using inert bed material (quartz sand). CaO was found to be effective in reducing the acidic components in bio-oil, resulting in improvements to the total acid number (88.9 to 46.6) and pH (2.39 to 3.98) of the bio-oil produced. While the catalytic activity of MgO was observed to be minimal, ZSM-5 proved to be the most effective in deoxygenating bio-oil although it had a disadvantage of significantly reduced liquid yield. The stability of the bio-oils produced was compared by performing accelerated aging tests and it was observed that bio-oil produced through in-situ catalysis by CaO was the most stable when compared to the control and other bio-oils tested in this study.