Catalytic Upgrading of Biofuel Pyrolysis

Abstract

Anthropogenic climate change is driving the need for renewable and carbon free/neutral technologies to offset and replace traditional fossil fuels. One category of viable fossil fuel alternatives is biobased fuels – hydrocarbons in liquid, solid or gaseous form derived from dedicated crops, agricultural or municipal waste, or animal byproducts. These hydrocarbon fuels benefit from the high energy density of their chemical bonds, potentially reaching levels close to fossil fuels. Additionally, biobased fuels can be used in the existing combustion-focused infrastructure in transportation and power generation. Biobased fuels are carbon neutral (when properly managed) and can be used to sequester atmospheric carbon by generating solid biochars composed of stable graphitic carbon. While there are numerous routes to generate biobased fuels, this work focuses on thermochemical conversion (specifically pyrolysis) of lignocellulosic biomass. Pyrolysis utilizes high temperatures under atmospheric pressure and anoxic conditions to devolatilize biomass and generate bio-oils, gases, and carbonized chars. The lack of oxygen prevents the biomass from combusting. Lignocellulosic biomass represents an underutilized resource. The United States generates hundreds of millions of tons of crop residues per year, which are typically left to decompose and return carbon (largely in the form of carbon dioxide) back to the atmosphere. The majority of this carbon is recycled between plants and atmosphere, and by converting these crop residues into biofuels, that carbon can be stored and used before returning to the carbon cycle. The generation and use of liquid bio-oils derived from thermochemical conversion of lignocellulosic biomass has not seriously challenged the dominance of fossil fuels. These bio-oils are hampered by the formation of tar compounds that impart high acidity, viscosity, and instability, which typically require substantial upgrading. The added cost of further refining and upgrading the bio-oils into biofuels has prevented lignocellulosic biofuels from widespread adoption. Part of this upgrading cost involves the use of rare or expensive catalysts which require recovery and recycling. This work successfully reduced the formation of tar compounds during pyrolysis by utilizing in situ and ex situ catalysts to promote devolatilization and remove oxygenated functional groups. The catalysts used fall into two groups: transition metals and clay minerals. Both have demonstrated their effectiveness as catalysts for various thermochemical processes and benefit from widespread abundance translating to low costs. Because of their availability, these catalysts can be used once and do not require recharging or recycling. Additionally, this work has demonstrated the potential for the solid biochar to act as a water remediation tool. Transition metals and clay minerals used in low quantities (<5% of biomass sample by weight) increased dehydrogenation and deoxygenation, and improved the oxygen/carbon ratios of bio-oils. Specifically, the use of in situ zinc reduced the overall bio-oil oxygen concentration without sacrificing yield. Ex situ manganese promoted high quantities of hydrogen and carbon dioxide in the non-condensable gas while retaining alcohol functional groups in the oil. Copper demonstrated an ability to promote devolatilization at earlier temperatures and reduced high molecular weight compounds. The clay montmorillonite promoted alkene (unsaturated) compounds and benzene derivatives in the bio-oil, and generated high quantities of H2 gas. Attapulgite and illite clays reduced the fatty acid content and oxygen content of the bio-oils. The transition metal and clay mineral catalysts have a varied impact biomass devolatilization, and their use can be tailored to the desired outcomes, depending on the specific issues of the biomass.