Effect of biomass variability on catalytic fast pyrolysis to produce renewable transportation fuels

Abstract

The paradigm shift in recent years towards the use of renewable sources of energy to fuel economies around the world is primarily due to the limited availability of conventional sources such as fossil fuels, coal and methane. Global concern about climate change and air pollution related to the use of these fuels has also created the need to identify and utilize carbon-neutral sources of energy. Lignocellulosic biomass is one such carbon-neutral source of energy, which could be converted through catalytic fast pyrolysis (CFP) to produce energy in the form of syngas, hydrocarbon fuels and chemicals that can significantly reduce our dependence on crude oil and greenhouse gas emissions. However, economical conversion of biomass to produce fuels and chemicals of consistent quality is affected due to the innate variability in different types of biomass, specifically due to changes in its moisture content, bulk and particle density, carbohydrate content (cellulose and hemicellulose), lignin content and ash composition (alkali and alkaline earth metals or AAEMs) among other factors. This dissertation is an effort to understand some of the sources of variability in biomass and its ultimate impact on the downstream conversion process to produce renewable transportation fuels. A brief introduction to the background information of this study, including the motivation to pursue renewable bio-based resources and the rationale behind this work is discussed in Chapter 1. The effect of variability in the ash composition of biomass on the primary breakdown of its constituents (cellulose, hemicellulose and lignin) and its subsequent influence on catalytic fast pyrolysis is elaborated in Chapter 2. In order to understand the individual influence of different iii AAEMs, biomass was doped with various levels of these metals and was subsequently converted to various products of pyrolysis in a micro-reactor. From this study, Mg was revealed to be relatively inert, while Ca, K and Na showed a stronger catalytic activity by influencing the pathways of thermal degradation of biomass. CFP product distribution was also influenced due to the presence of higher levels of Ca, K and Na in the biomass, resulting in changes in the selectivity of the products towards the formation of undesirable side-products (thermally-derived char and non-condensable gases) at the expense of the yield of aromatic hydrocarbons. During biomass pyrolysis, the fate of various AAEMs after pyrolysis has been studied extensively and these metal species have been reported to volatilize and accumulate on the surface of the catalyst. Chapter 3 discusses the influence of these individual AAEMs on the functionality of the CFP catalyst during pyrolysis. ZSM-5 catalyst was deactivated by different levels of Ca, K and Na and the resulting changes in the properties of the catalyst are reported. Changes in the surface area and acidity of the catalyst due to deactivation by the individual AAEMs were correlated to the observed loss in activity when the catalysts were used in CFP experiments. Higher levels of deactivation (2, 5 wt.% of K or Na) were observed to render the catalyst completely inactive and resemble an inert material during CFP. Lignin, one of the major components of biomass, varies in composition between different biomass species. Chapter 4 discusses the effect of thermal pretreatment (torrefaction) on lignin as well as the resulting structural changes and its influence on the product distribution from pyrolysis. Organosolv lignin extracted from woody biomass (pine) and herbaceous biomass (switchgrass) was torrefied at different temperatures (150 ºC – 225 ºC), and the torrefied lignins were iv characterized to study the changes in the structure, which revealed the polycondensation and de-methoxylation of the aromatic units of lignin. Significant changes to the product distributions and selectivity from non-catalytic pyrolysis as well as CFP experiments are also reported in this chapter, which revealed that torrefaction could be detrimental to achieving a higher aromatic hydrocarbon yield from lignin. Finally, an overall summary and directions for future research in this field are presented in Chapter 5.