Deciphering Biomass Fragmentation using Millisecond Microreactor Kinetics

Abstract

Pyrolytic conversion of lignocellulosic biomass utilizes high temperatures to thermally fragment biopolymers to volatile organic compounds. The complexity of the degradation process includes thousands of reactions through multiple phases occurring in less than a second. The underlying chemistry of lignocellulose decomposition has been studied for decades, and numerous conflicting mechanisms and kinetic models have been proposed. The fundamental science of biomass pyrolysis is still without detailed chemical kinetics and reaction models capable of describing the chemistry and transport in industrial reactors. The primary goal of this thesis was to develop mechanistic insights of biomass pyrolysis with the focus on fragmentation of cellulose using two novel microreactor systems, a. Quantitative Carbon Detector (QCD) b. Pulse Heated Analysis of Solid Reactions (PHASR). Current research of complex chemical systems, including biomass pyrolysis, requires analysis of large analyte mixtures (>100 compounds). Quantification of each carbon-containing analyte by existing methods (flame ionization detection) requires extensive identification and calibration. An integrated microreactor system called the Quantitative Carbon Detector (QCD) for use with current gas chromatography techniques for calibration-free quantitation of analyte mixtures was designed. Combined heating, catalytic combustion, methanation and gas co-reactant mixing within a single modular reactor fully converts all analytes to methane (>99.9%) within a thermodynamic operable regime. Residence time distribution of the QCD reveals negligible loss in chromatographic resolution consistent with fine separation of complex mixtures including pyrolysis products. The requirements are established for measuring the reaction kinetics of high temperature (>400 ˚C) biomass pyrolysis in the absence of heat and mass transfer limitations. Experimental techniques must heat and cool biomass samples sufficiently fast to elucidate the evolution of reaction products with time while also eliminating substantial reaction during the heating and cooling phases, preferably by measuring the temperature of the reacting biomass sample directly. These requirements were described with the PHASR (Pulsed-Heated Analysis of Solid Reactions) technique and demonstrated by measuring the time-resolved evolution of six major chemical products from Loblolly pine pyrolysis over a temperature range of 400 ˚C to 500 ˚C. Differential kinetics of loblolly pine pyrolysis were measured to determine the apparent activation energy for the formation of six major product compounds including levoglucosan, furfural and 2-methoxyphenol. Levoglucosan (LGA), a six-carbon oxygenate, is the most abundant primary product from cellulose pyrolysis with LGA yields reported over a wide range of 5−80 percent carbon (%C). In this study, the variation of the observed yield of LGA from cellulose pyrolysis was experimentally investigated. Cellulose pyrolysis experiments were conducted in two different reactors: the Frontier micropyrolyzer (2020-iS), and the pulse heated analysis of solid reactions (PHASR) system. The reactor configuration and experimental conditions including cellulose sample size were found to have a significant effect on the yield of LGA. Four different hypotheses were proposed and tested to evaluate the relationship of cellulose sample size and the observed LGA yield including (a) thermal promotion of LGA formation, (b) the crystallinity of cellulose samples, (c) secondary and vapor-phase reactions of LGA, and (d) the catalytic effect of melt-phase hydroxyl groups. Co-pyrolysis experiments of cellulose and fructose in the PHASR reactor presented indirect experimental evidence of previously postulated catalytic effects of hydroxyl groups in glycosidic bond cleavage for LGA formation in transport-limited reactor systems. PHASR experiments were performed to measure apparent kinetic parameters of cellulose fragmentation. The LGA formation step was decoupled from the initiation reactions by identifying cellobiosan as a chemical surrogate for cellulose pyrolysis intermediate melt phase. Kinetics of LGA formation step was measured using 13C1 cellobiosan samples to track the contribution of glucose monomer in cellobiosan. The activation energy Ea calculated from the slope of the Arrhenius plot was 26.9 ± 1.9 kcal/mol and the preexponential factor k0 calculated from the intercept was 4.2 × 107 sec-1. These kinetic parameters were found to be lower than the corresponding values for the previously proposed mechanisms of LGA formation calculated from DFT studies indicating a possibility of new, catalyzed mechanism of LGA formation.