Autothermal oxidative pyrolysis of biomass feedstocks over noble metal catalysts to liquid products.

Abstract

Two thermal processing technologies have emerged for processing biomass into renewable liquid products: pyrolysis and gasification/Fischer-Tropsch processing. The work presented here will demonstrate oxidative pyrolysis of biomass as an alternative process to avoid the intrinsic disadvantages of traditional pyrolysis. Additionally, work has been conducted to examine the processing of biomass derived synthesis gas to condensable products, which involves mitigating new challenges when compared with the processing of conventional coal-based feedstocks during gasification/Fischer- Tropsch. The research group of Professor Lanny D. Schmidt has pioneered autothermal partial oxidation of a variety of gas and liquid feedstocks on noble metal catalysts to synthesis gas with high selectivity, char-free operation, and on millisecond timescales at temperatures of 600 - 1000 ◦C. More recently, cellulose has been shown to decompose on the catalyst surface to also produce high selectivities to synthesis gas. Chapter 2 discusses the discovery of an intermediate liquid phase during the autothermal processing of cellulose particles over rhodium-based catalysts. Volatilization of 300 μm cellulose particles on a 700 ◦C catalytic surface were filmed using a high speed camera capable of 1000 frames per second. The cellulose particles decomposed through an intermediate liquid, which boiled to gaseous species that convected into the catalyst bed. The high heat transfer rates made possible by the intimate contact of the boiling liquid and the hot surface allowed rapid reactions without leaving char residues. This unique insight allows new processes to be designed that exploit this type of cellulose thermal decomposition. Experiments were conducted to investigate the extension of catalytic partial oxidation over noble metal catalysts to convert biomass to liquid pyrolysis products, termed ‘oxidative pyrolysis’. Model compounds were chosen to represent the lignin fraction of lignocellulosic biomass to more easily and accurately study the proposed system. Chapter 3 discusses the autothermal oxidative pyrolysis of monoaromatics over noble metal catalysts. Benzene, toluene, ethylbenzene, cumene, and styrene were independently studied over five noble metal-based catalysts (Pt, Rh, Rh/ -Al2O3, Rh-Ce, and Rh-Ce/ -Al2O3) while varying the carbon-to-oxygen feed ratio. Aromatic rings were observed to be very stable in the reactor system, while homogeneous reactions of the alkyl groups of ethylbenzene and cumene were prevalent. Chapter 4 addresses the oxidative pyrolysis of microcrystalline cellulose particles as a model for lignocellulosic biomass to yield liquid products. Cellulose was demonstrated to autothermally convert to combustion, partial oxidation, and pyrolysis products without char formation. The effects of support geometry, catalyst metal, and hydrogen addition on product selectivities were studied. Platinum-coated alumina spheres maximized the yield of pyrolysis products by favoring combustion chemistry and minimizing reforming activities, as compared with rhodium-based catalysts. Up to 60 % carbon selectivity to pyrolysis products could be achieved on a Pt catalyst with hydrogen addition. As mentioned, previous research in the Schmidt group has shown high selectivities to synthesis gas by autothermal reforming of cellulose particles. However, utilizing this biomass-derived syngas, as opposed to traditional coal-based syngas, is not well studied. Biomass-derived synthesis gas presents a new set of inorganic impurities that may affect catalyst performance during Fischer-Tropsch processing. Small quantities (ppm) of typical biomass inorganics (Na, K, Li, and Ca) were loaded onto -Al2O3- supported Co-Re powder catalysts to study the effect on product selectivities (Chapter 5). The inorganic impurities were found to affect the reduction of Co and increase CO2 and C5+ selectivities, which were largely attributed to electronic effects. Chapter 6 proposes future research utilizing gas chromatography and mass spectrometry to identify and quantify specific components within liquid pyrolysis products, generally termed ‘pyrolysis oil’. This work will build on the research presented in Chapter 4: the demonstration of oxidative pyrolysis of cellulose to produce up to 50 % carbon selectivity to pyrolysis products. Further characterization of the pyrolysis oil will involve pH and water fraction measurements. Preliminary work shows the presence of several acids, alcohols, phenols, pyrans, among other small oxygenated species in the pyrolysis oil. Levoglucosan was identified as being the largest carbon-based fraction of the oil, up to 11 wt% under certain conditions. Additional experiments extending oxidative pyrolysis to process polymer feedstocks are also proposed