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Fast pyrolysis is a promising thermochemical technology that breaks down renewable and abundant lignocellulosic biomass into a primary liquid product (bio-oil) in seconds. The bio-oil can then be potentially catalytically upgraded into transportation fuels and multiple commodity chemicals. Hemicellulose is one of the three major components of lignocellulosic biomass and is characterized as a group of cell wall polysaccharides that are neither cellulose nor pectin. The composition and structural features of hemicellulose (mixture of different heterogeneous polysaccharides) and different specific hemicellulose polysaccharides are reviewed. Particular focus is then given to reviewing the status of hemicellulose pyrolysis in terms of experimental investigations, reaction mechanisms, and kinetic modeling. For each aspect, recent results, challenges, and future prospects are addressed. [ABSTRACT FROM AUTHOR]

Sodium ions, one of the natural inorganic constituents in lignocellulosic biomass, significantly alter pyrolysis behavior and resulting chemical speciation. Here, experiments were conducted using a micropyrolyzer to investigate the catalytic effects of NaCl on fast pyrolysis of glucose-based carbohydrates (glucose, cellobiose, maltohexaose, and cellulose), and on a major product of cellulose pyrolysis, levoglucosan (LVG). A mechanistic model that addressed the significant catalytic effects of NaCl on the product distribution was developed. The model incorporated interactions of Na+ with cellulosic chains and low molecular weight species, reactions mediated by Na+ including dehydration, cyclic/Grob fragmentation, ring-opening/closing, isomerization, and char formation, and a degradation network of LVG in the presence of Na+. Rate coefficients of elementary steps were specified based on Arrhenius parameters. The mechanistic model for cellulose included 768 reactions of 222 species, which included 252 reactions of 150 species comprising the mechanistic model of glucose decomposition in the presence of NaCl. © 2015 American Institute of Chemical Engineers AIChE J, 62: 766-777, 2016 [ABSTRACT FROM AUTHOR]

A mechanistic model considering the significant catalytic effects of Na+ on fast pyrolysis of glucose-based carbohydrates was developed in Part 1 of this study. A computational framework based on continuous distribution kinetics and mass action kinetics was constructed to solve the mechanistic model. Agreement between model yields of various pyrolysis products with experimental data from fast pyrolysis of glucose-based carbohydrates dosed with NaCl ranging from 0-0.34 mmol/g at 500 °C validated the model and demonstrated the robustness and extendibility of the mechanistic model. The model was able to capture the yields of major and minor products as well as their trends across NaCl concentrations. Modeling results showed that Na+ accelerated the rate of decomposition and reduced the time for complete thermoconversion of carbohydrates. The sharp reduction in the yield of levoglucosan (LVG) from fast pyrolysis of cellulose in the presence of NaCl was mainly caused by reduced decomposition of cellulose chains via end-chain initiation and depropagation due to Na+ favoring competing dehydration reactions. Analysis of the contributions of reaction pathways showed that the decomposition of LVG made a minor contribution to its yield reduction and contributed less than 0.5% to the final yield of glycolaldehyde from fast pyrolysis of glucose-based carbohydrates in the presence of NaCl. © 2015 American Institute of Chemical Engineers AIChE J, 62: 778-791, 2016 [ABSTRACT FROM AUTHOR]

Abstract: A detailed, mechanistic model for high-density polyethylene pyrolysis was created based on the modeling framework developed in our previous work and was used to study the time evolution of low molecular weight products formed. Specifically, the role that unzipping, backbiting, and random scission reaction pathways play in the evolution of low molecular weight species was probed. The model tracked 151 species and included over 11,000 reactions. Rate parameters were adapted from our previous work, literature values, and regression against experimental data. The model results were found to be in excellent agreement with experimental data for the evolution of condensable low molecular weight products. The time evolution curves of specific low molecular weight products indicated that the random scission pathway was important for all species, while the backbiting pathway played a complementary role. Net rate analysis was used to further elucidate the competition between the pathways. Net rate analysis of end-chain radicals showed that the unzipping pathway was not competitive with the other pathways, as expected based on experimental yields of ethylene. The random scission pathway was found to be controlling, with the backbiting pathway playing a more minor role for product formation. By comparing the net rates for formation of specific mid-chain radicals via intramolecular hydrogen shift reactions, the contribution of the backbiting pathway was shown to vary, with radicals formed via the most facile x,x +4-intramolecular hydrogen transfer reactions being favored. [Copyright &y& Elsevier]

Abstract: A detailed mechanistic model for polystyrene pyrolysis was created that built on a modeling framework developed in our previous work and was used to probe three competing pathways to dimer formation: benzyl radical addition, 1,3-hydrogen shift, and 7,3-hydrogen shift, based on recent literature reports. To incorporate the chemistry involved in the 7,3-hydrogen shift pathway, the 1,7- and 7,3-hydrogen shift reaction families were added to the model. The updated version of the model tracks 75 species and over 3500 reactions. Rate parameters for all families were specified based on our previous work, more recent literature reports, and regression against limited experimental data. The model was able to accurately predict the experimental results for polystyrene pyrolysis for different reactor configurations for a temperature range of 100°C and two orders of magnitude of initial molecular weight for experimental data collected in our own lab and from Bouster and coworkers and Bockhorn and coworkers. The results from our model were studied using net rate analysis to gain insight into the competitiveness of the various reaction pathways to dimer formation. The net rate analysis demonstrated that 7,3-hydrogen shift is the dominant reaction pathway to dimer formation at the temperatures studied. Benzyl radical addition becomes a more competitive reaction pathway as the temperature increases, which is caused predominantly by an increase in the benzyl radical concentration with increasing temperature. Overall, it is quantitatively shown that both 7,3-hydrogen shift and benzyl radical addition are important pathways for dimer formation, with their relative competitiveness influenced by temperature. [Copyright &y& Elsevier]

Biomass pyrolysis—a sustainable way to produce renewable fuels and chemicals: The cover image depicts a sustainable world where green fuels and chemicals will be produced from renewable, abundant, and cheap biomass by using a variety of technologies (e.g., biological, thermochemical, and catalytic). Hemicellulose, one of the three major components of biomass, receives scarce attention compared to cellulose and lignin; however, it deserves much more. Herein, this contribution highlights the progress in characterizing the composition and structural features of hemicellulose and particularly focuses on salient experimental investigations, reaction mechanisms, and kinetic modeling of hemicellulose pyrolysis through a critical literature analysis. Challenges and future prospects are also discussed. More details can be found in the Review by X. Zhou, W. Li, R. Mabon, L. J. Broadbelt from Northwestern University and Exxon Mobil (DOI: 10.1002/ente.201600327). [ABSTRACT FROM AUTHOR]

A correction to the article "A Critical Review on Hemicellulose Pyrolysis" is presented.

Bio-oil produced from biomass fast pyrolysis often requires catalytic upgrading to remove oxygen and acidic species over zeolite catalysts. The elementary reactions in the mechanism for this process involve carbenium and oxonium ions. In order to develop a detailed kinetic model for the catalytic upgrading of biomass, rate constants are required for these elementary reactions. The parameters in the Arrhenius equation can be related to thermodynamic properties through structure-reactivity relationships, such as the Evans-Polanyi relationship. For this relationship, enthalpies of formation of each species are required, which can be reasonably estimated using group additivity. However, the literature previously lacked group additivity values for oxygenates, oxonium ions, and oxygen-containing carbenium ions. In this work, 71 group additivity values for these types of groups were regressed, 65 of which had not been reported previously and six of which were newly estimated based on regression in the context of the 65 new groups. Heats of formation based on atomization enthalpy calculations for a set of reference molecules and isodesmic reactions for a small set of larger species for which experimental data was available were used to demonstrate the accuracy of the Gaussian-4 quantum mechanical method in estimating enthalpies of formation for species involving the moieties of interest. Isodesmic reactions for a total of 195 species were constructed from the reference molecules to calculate enthalpies of formation that were used to regress the group additivity values. The results showed an average deviation of 1.95 kcal/mol between the values calculated from Gaussian-4 and isodesmic reactions versus those calculated from the group additivity values that were newly regressed. Importantly, the new groups enhance the database for group additivity values, especially those involving oxonium ions. [ABSTRACT FROM AUTHOR]

Thin films (~115 μm thick) of milled wood lignin from hybrid poplar and acid-washed hybrid poplar were pyrolyzed at 500 °C and ~55 °C/s at five pressures (4, 250, 500, 750, and 1000 mbar) to determine the impact of secondary liquid intermediate reactions on the product distribution. For both milled wood lignin extracted from poplar and acid-washed hybrid poplar wood, pressure had a significant effect on the product distribution for thin film pyrolysis between 4 and 1000 mbar. For lignin, lowering the pressure from 1000 mbar to 4 mbar reduced the char yield from 36 to 23% and enhanced production of large cluster pyrolytic lignin. However, the pressure did not dramatically impact the gas yield (CO2, CO, methane, H2, ethane, propane, and butane), nor did it significantly impact the release of monomeric phenolic compounds. ICR-MS shows limited changes in the composition of lignin oligomers. The increase in the production of large lignin oligomers observed by UV fluorescence and the reduction of char yield with vacuum confirm the importance of oligomeric combination reactions to form large polyaromatic structures in the liquid intermediate. For hybrid poplar, lowering the pressure from 1000 mbar to 4 mbar decreased the char yield from 19 to 7% and enhanced production of heavy sugars (cellobiosan and cellotriosan). ICR-MS results clearly show the importance of dehydration reactions in the liquid intermediate. Lowering the pressure also enhanced production of CO, CO2, and methane due to heterogeneous catalysis by residual alkali and alkaline earth metals in the solid wood matrix. However, it also decreased production of levoglucosan from 10 to 6.1 wt %. The yields of levoglucosan and cellobiosan obtained for hybrid poplar were higher and lower, respectively, compared with those expected if the pyrolysis products were the result of the additive contribution of hybrid poplar constituents. This result could be explained by the tendency of lignin liquid intermediate to bubble vigorously, contributing in this way to the removal of cellulose oligomers from the liquid intermediate. [ABSTRACT FROM AUTHOR]

Review: 86 refs. [ABSTRACT FROM AUTHOR]

A computationalframework based on continuous distribution kineticswas constructed to solve the mechanistic model that was developedfor fast pyrolysis of glucose-based carbohydrates in the first partof this study [Zhou et al. Ind. Eng. Chem. Res.2014, 53. DOI 10.1021/ie502259w]. Comparing modeling results with experimental yieldsfrom fast pyrolysis over a wide range of reaction conditions validatesthe model. Agreement between model yields of final pyrolysis productswith experimental data of fast pyrolysis of cellulose at temperaturesranging from 400 to 600 °C and maltohexaose, cellobiose, andglucose at 500 °C showed that the mechanistic model was robustand extendable. In comparison to our previous model [Vinu,R.; Broadbelt, L. J. Energy Environ. Sci.2012, 5, 9808–9826], the mechanistic model presentedin this work incorporating new findings from experiments and theoreticalcalculations showed enhanced performance in capturing experimentalyields of major products such as levoglucosan-pyranose, char, H2O, CO2, CO, and especially glycolaldehyde and 5-hydroxymethylfurfural.The model was also able to well match the yields of pyrolysis productsthat our previous model did not include, such as levoglucosan-furanose,methyl glyoxal, and minor products with yields of less than 1 wt %like levoglucosenone, acetone, dihydroxyacetone, and propenal. Themechanistic model showed its versatility in providing insights thatwere difficult to obtain from experiments, including a time scaleof 4–5 s for complete thermoconversion of cellulose at 500°C. Analysis of the contributions of competing reaction pathwaysshowed that decomposition of cellulosic chains played a more importantrole in the formation of levoglucosan and glycolaldehyde than in thatof other pyrolysis products. [ABSTRACT FROM AUTHOR]

Fastpyrolysis of lignocellulosic biomass, utilizing moderate temperaturesranging from 400 to 600 °C, produces a primary liquid product(pyrolytic bio-oil), which is potentially compatible with existingpetroleum-based infrastructure and can be catalytically upgraded tofuels and chemicals. In this work, experiments were conducted witha micropyrolyzer coupled to a gas chromatography–mass spectrometry/flameionization detector system to investigate fast pyrolysis of neat celluloseand other glucose-based carbohydrates. A detailed mechanistic modelbuilding on our previous work was developed for fast pyrolysis ofneat glucose-based carbohydrates by integrating updated findings obtainedthrough experiments and theoretical calculations. The model describedthe decomposition of cellulosic polymer chains, reactions of intermediates,and formation of a range of low molecular weight compounds at themechanistic level and specified each elementary reaction step in termsof Arrhenius parameters. The mechanistic model for fast pyrolysisof neat cellulose included 342 reactions of 103 species, which included96 reactions of 67 species comprising the mechanistic model of neatglucose decomposition. [ABSTRACT FROM AUTHOR]

Abstract: Conventional continuum mechanistic models for polymer degradation typically involve thousands of coupled differential-algebraic equations, requiring an efficient solver to solve the complex set of stiff model equations. This can be overcome by formulating the problem in terms of a stochastic simulation procedure, requiring only iterative operations to solve the model. The present work describes the detailed mechanistic modeling of pyrolysis of poly(styrene peroxide) (PSP) using kinetic Monte Carlo (KMC) simulation to predict the product yields and gain a better understanding of the product evolution pathways. The traditionally accepted mechanism of PSP pyrolysis proposed by Mayo and Miller, which involves the key reaction steps of peroxide bond fission, alkoxy radical recombination and disproportionation, and end chain β-scission, was initially tested using the KMC model to predict the peroxide concentration profile and the product yields. This model was only qualitatively able to predict the major products, benzaldehyde and formaldehyde, while the formation of minor products like α-hydroxy acetophenone, phenyl glycol, and phenyl glyoxal was not captured at all. Hence, a new mechanism that also incorporated hydrogen abstraction and β-scission was proposed and implemented in KMC. The final model tracked 949 reactions of 83 species. The rate coefficients for all the reaction steps were based on the existing literature reports, and hence no parameter estimation was done to fit the model against the experimental data. The revised model was quantitatively able to predict all the products of PSP pyrolysis, which was attributed to the stabilization of the alkoxy radicals by hydrogen abstraction, and the subsequent generation of additional alkoxy radicals by β-scission. KMC allowed the dominant pathways for the formation of minor products and dimers to be identified explicitly. Finally, the implications of this study in understanding the effect of trace peroxide bonds on poly(styrene) pyrolysis are outlined. [Copyright &y& Elsevier]

• Some of the highest experimental yields (>70 wt. %) of glycolaldehyde achieved and reported to date. • Good agreement between model and experimental results. • Dominant fragmentation pathways elucidated in hydrothermal pyrolysis of glucose. • Preferred operating conditions and feedstock sugar identified toward maximum glycolaldehyde production. Pyrolysis of glucose and glucose-based carbohydrates has been shown to produce a range of chemical species that can be used directly as fuels and chemicals or as feedstocks to further chemical transformations. It is known that process operating conditions such as temperature, heating rate, reactor configuration, moisture content, and pretreatment method can have a significant effect on the observed product distribution. Pyrolysis of carbohydrates in the presence of water at high concentrations can significantly alter the product spectrum and favor the production of specific products at unusually high yields. In this work, we show that pyrolysis of aqueous glucose solutions at high temperatures can result in highly selective production of glycolaldehyde, a C2 hydrocarbon with an aldehyde and hydroxyl functionality, toward direct applications in the food industry or as a chemical building block toward value-added products. A glucose pyrolysis model that was developed previously was expanded to capture the pyrolysis kinetics of glucose at hydrothermal conditions, accurately reproducing the observed product yields for a range of temperatures and feedstock compositions. Dominant reaction families were identified and interrogated to quantify the effect of hydrothermal conditions on the predicted kinetics. High yields of glycolaldehyde were achieved experimentally, with the maximum values observed at moderate temperatures and pure glucose feed. Elevated temperatures and increasing fructose concentrations negatively affected observed glycolaldehyde yields, resulting in increased production of undesired decomposition products in the C1-C3 range. The results of this study include preferred operating conditions toward maximizing the yield of glycolaldehyde as described by a predictive kinetic model that explicitly accounts for the major reactions comprising a complex network taking place and the effect of the operating conditions on their relative contributions to glucose conversion and product yields. [ABSTRACT FROM AUTHOR]