Search

Pyrolysis of polyolefins has been proposed as a potential resource recovery strategy by converting macromolecules into valuable fuels and chemicals. Due to variations in possible backbone structures, chain-length distributions, and arrangements of pendant groups, their decomposition behavior via pyrolysis can be complex. In the present work, a review of historical data and empirical models for two distinct polyolefins, polyethylene (PE) and polypropylene (PP), is provided followed by a comparison to recent mechanistic models. The characteristic sigmoidal behavior of linear polymer decomposition is captured with global, lumped-species, and mechanistic models of high-density polyethylene. The PE model was extended to simulate PP using the same reaction families and reaction family parameters, but with distinct rate coefficients that accounted for the difference in the structure of PP with its pendant methyl groups compared to PE as manifested through heats of reaction embedded in the Evans–Polanyi relationship, Ea= E0 + γ×ΔHreacn. The change in structure and its associated kinetic parameters resulted in no sigmoidal conversion, consistent with experimental reports for atactic PP. This suggests that mechanistic modeling could be an important complement to global model studies to understand when other effects are at play in the pyrolytic decomposition of polymers such as PP.

A kinetic Monte Carlo model of polyurethane polymerization which explicitly tracks the polymer sequences is developed and shared. This model is benchmarked against theoretical and experimental polyurethane data and used to investigate the effect on oligomer distributions of unequal reactivity of the first and second isocyanate to react. The reverse reactions using thermodynamic consistency are then added to the framework, and analogous to the addition polymerization concept of ceiling temperature, equilibrium chain length distributions at various temperatures are calculated. For a mixture of three monomers AA, BB, and CC, where BB and CC do not react with one another, are present in stoichiometric proportions, and have different enthalpies of reaction with AA, an odd-even effect emerges. Odd length chains are more likely than even length chains for temperatures at which BB and CC have significantly different equilibrium conversions. The concept of ceiling temperature that is typically cited for addition polymers is extended here to provide a measure of conditions under which depolymerization for recycling is favored.

Conventionally, syngas produced by steam reforming is used to convert methane into methanol and other valuable products. In this work, methane reforming in a glow electrical discharge is studied under highly non-thermal conditions. The gas temperature at the core of the discharge depends on gas pressure and discharge current, and ranges between 1500 and 2000 K. The underlying chemistry and important reaction pathways of non-thermal plasma methane reforming are unravelled using a batch reactor coupled with a mass spectrometer and gas chromatography system. Rate of production analysis supports the hypothesis of predominantly methane dissociation reactions into methyl free radicals recombine to form ethane, with a selectivity of more than 94% on carbon basis. Ethylene and acetylene are identified as secondary products with a C-based selectivity of less than 5.2%. This plasma approach can provide a global selectivity of 63% for methane reforming directly to ethane which is higher than reported values for other electrical discharges of typically 30–40%. The lowest energy cost achievable in the glow discharge for ethane formation is estimated to be 8.8 MJ/mol which is highly competitive with a barrier discharge performance of 17.4 MJ/mol. Moreover, glow discharge has an advantage of a negligible soot production which is a major drawback of many plasma-assisted processes reported. However, a comparison between investment costs and the gross margin indicates that methane reforming using glow discharge is economically profitable only for very long time scales due to the low profit margins. To make plasma assisted gas reforming feasible for industrial scale additional optimization of the process is required.

Valorization of pyrolytic lignin to fuels and chemicals is still poorly understood due to its ill-defined structure and the complexity of the decomposition chemistry. To shed some light on the dominant reaction pathways of lignin thermolysis, novel experimental and first-principles based calculations of its building blocks have been carried out. Pyrolysis chemistry of hydroxycinnamic acids is investigated in this work using a unique Py-GC × GC-FID/TOF-MS coupled with a customized GC to detect water and gases, to gain an understanding of the role of the branching ratios in lignin and its linkages with hemicellulose. Mean residence times of cinnamic and ferulic acids were estimated to be 12 and 21 s at 573 K, based on time-resolved experiments. Cinnamic acid undergoes a CO2 elimination reaction at temperatures higher than 873 K without an intermediate liquid phase. At temperatures as low as 573 K, –OH and –OCH3 substituted cinnamic acids underwent decarboxylation despite bearing similar BDEs for Cβ–Cγ scission. At these temperatures, p-coumaric and ferulic acids were converted into 4-vinylphenol and 4-vinylguaiacol by 40 wt% and 30 wt%, respectively. On the other hand, sinapinic acid converted nearly by 80 wt% at temperatures below its boiling point of 676 K. In conjunction with novel quantum chemical calculations, it could be ruled out that decarboxylation was not occurring via concerted unimolecular reactions at low temperatures. Instead, water-catalyzed reactions of hydroxycinnamic acids seem to be the primary cause for the CO2 elimination in the intermediate liquid phase via a 6-centered transition state.

Fast pyrolysis of lignocellulosic biomass is considered to be a promising thermochemical route for the production of drop-in fuels and valuable chemicals. During the past decades, a comprehensive understanding of feedstock structure, fast pyrolysis kinetics, product distribution, and transport effects that govern the process has allowed to design better pyrolysis reactors and/or catalysts. A variety of lignocellulosic biomass feedstocks, like corn stover, pinewood, poplar, and model compounds like glucose, xylan, monolignols have been utilized to study the thermal decomposition at or close to fast pyrolysis conditions. Significant progress has been made in understanding the kinetics by developing unique setups such as droptube, PHASR, and micropyrolyzer reactors in combination with the use of advanced analytical techniques such as comprehensive gas and liquid chromatography (GC, LC) with time-of-flight mass spectrometer (TOF-MS). This has led to initial intrinsic kinetic models for biomass and its main components, namely cellulose, hemicellulose, and lignin, validated using experimental setups where the effects of heat and mass transfer on the performance of the process, expressed using Biot and pyrolysis numbers, are adequately negligible. Yet, not all aspects of fast pyrolysis kinetics of biomass components are equally well understood. The use of time-resolved or multiplexed experimental techniques can further improve our understanding of reaction intermediates and their corresponding kinetic mechanisms. The novel experimental data combined with first principles based multiscale models can reshape biomass pyrolysis models and transform biomass fast pyrolysis to a more selective and energy efficient process.