Query: Molecular dynamics of biomass pyrolysis

title: Diffusion of Biomass Pyrolysis Products in H ZSM 5 by Molecular Dynamics Simulations
abstract: Diffusion of biomass pyrolysis vapors and their upgraded products is an essential catalytic property of zeolites during catalytic fast pyrolysis and likely plays a critical role in the selectivity of these catalysts. Characterizing the diffusivities of representative biofuel molecules is critical to understand shape selectivity and interpret product distribution. Yet, experimental measurements on the diffusivities of oxygenated biofuel molecules at pyrolysis temperatures are very limited in the literature. As an alternative approach, we conducted MD simulations to measure the diffusion coefficients of several selected molecules that are representative of biomass pyrolysis vapors, namely water, methanol, glycolaldehyde, and toluene in H ZSM 5 zeolite. The results show the diffusion coefficients calculated via MD simulations are consistent with available NMR measurements at room  The effect of molecular weight and molecular critical diameter on the diffusivity among the chosen model compounds i.

title: Reactive Molecular Dynamics Simulations of Biomass Pyrolysis and Combustion under Various Oxidative and Humidity Environments
abstract: Biomass, as a renewable carbon neutral energy source with abundant reserves, is a good candidate for future energy supplies. In this paper, a simplified biomass model composed of cellulose, hemicellulose, and lignin, described by a carefully selected reactive force field (ReaxFF) is investigated using molecular dynamics (MD) simulations. The pyrolysis and combustion processes of the biomass under different temperatures and oxidative and humidity conditions, are studied. We find that the individual products from the pyrolysis of the three biomass components are similar, including H2O, H2, CO, CO2, and small organic molecules. The calculated activation energies for C C bond dissociation are 34.53, 26.08, and 16.23 kJ mol 1, respectively, for cellulose, hemicellulose, and lignin, consistent with the trend in experiments. Interestingly, light tar (C5 13) production reaches a maximum under intermediate temperatures, which could be further explored to optimize the production of light tar as liquid fuels. Compa.

title: Initial reaction mechanisms of cellulose pyrolysis revealed by ReaxFF molecular dynamics
abstract: Mechanism investigation of cellulose pyrolysis is remarkably useful for efficient utilization of biomass. In this paper, a new methodology rooted in the first GPU enabled ReaxFF MD simulation program (GMD Reax) and the unique cheminformatics based reaction analysis tool (VARxMD) was employed to investigate the initial reaction mechanism of cellulose pyrolysis. Both the overall spectrum product evolution and underlying detailed chemical reactions of cellulose pyrolysis have been revealed. A reaction scheme of cellulose pyrolysis with detailed reaction pathways for major pyrolyzates has been obtained that is not readily accessible by experiments. The simulated evolution tendencies of the major pyrolysis products (glycolaldehyde, levoglucosan and water) with temperature at 500 1400 K agrees well with the Py GC/MS experimental observations at 673 1073 K. Compared with the large temperature discrepancy imposed by the widely used simulation strategy of artificially increased temperature in ReaxFF MD, the very close temperature range between the simulations and experiments suggests that cellulose is a good model system to validate the ReaxFF force field in predicting the behavior and chemistry events in pyrolysis of complex molecular systems. The computational approach of large model simulation facilitated by efficient computation of GMD Reax, and chemical reaction analysis capability of VARxMD can shed new light on the detailed chemical mechanisms of pyrolysis for cellulose and other biomass.

title: High temperature pyrolysis modeling of a thermally thick biomass particle based on an MD derived tar cracking model
abstract: Biomass pyrolysis in the thermally thick regime is an important thermochemical phenomenon encountered in many different types of reactors. In this paper, a particle resolved algorithm for thermally thick biomass particle during high temperature pyrolysis is established by using reactive molecular dynamics (MD) and computational fluid dynamics (CFD) methods. The temperature gradient inside the particle is computed with a heat transfer equation, and a multiphase flow algorithm is used to simulate the advection/diffusion both inside and outside the particle. Besides, to simulate the influence of intraparticle temperature gradient on the primary pyrolysis yields, a multistep kinetic scheme is used. Moreover, a new tar decomposition model is developed by reactive molecular dynamic simulations where every primary tar species in the multistep kinetic scheme cracks under high temperature. The integrated pyrolysis model is evaluated against a pyrolysis experiment of a centimeter sized beech wood particle at 800 to 1050 degC. The simulation results show a remarkable improvement in both light gas and tar yields compared with a simplified tar cracking model. Meanwhile, the MD tar cracking model also gives a more reasonable prediction of the species yield history, which avoids the appearance of unrealistically high peak values at the initial stage of pyrolysis. Based on the new results, the different roles of secondary tar cracking inside and outside the particle is studied. Finally, the model is also used to assess the influence of tar residence time and several other factors impacting the pyrolysis.

title: Depolymerization Pathways for Branching Lignin Spirodienone Units Revealed with ab Initio Steered Molecular Dynamics.
abstract: Lignocellulosic biomass is an abundant, rich source of aromatic compounds, but direct utilization of raw lignin has been hampered by both the high heterogeneity and variability of linking bonds in this biopolymer. Ab initio steered molecular dynamics (AISMD) has emerged both as a fruitful direct computational screening approach to identify products that occur through mechanical depolymerization (i.e. in sonication or ball milling) and as a sampling approach. By varying the direction of force and sampling over 750 AISMD trajectories, we identify numerous possible pathways through which lignin depolymerization may occur in pyrolysis or through catalytic depolymerization as well. Here, we present eight unique major depolymerization pathways discovered via AISMD for the recently characterized spirodienone lignin branching linkage that may comprise around 10% weight of all lignin in some softwoods. We extract representative trajectories from AISMD and carry out reaction pathway analysis to identify energetically favorable pathways for lignin depolymerization. Importantly, we identify dynamical effects that could not be observed through more traditional calculations of bond dissociation energies. Such effects include thermodynamically favorable recovery of aromaticity in the dienone ring that leads to near barrierless subsequent ether cleavage and hydrogen bonding effects that stabilize newly formed radicals. Some of the most stable spirodienone fragments that reside at most 1 eV above the reactant structure are formed with only 2 eV barriers for C C bond cleavage, suggesting key targets for catalyst design to drive targeted depolymerization of lignin.

title: SIMULATION OF MOLECULAR DYNAMICS IN CELLULOSE PYROLYSIS
abstract: The parameters of optimized structure of cellulose molecular which is main component of biomass are obtained by PM3 method.The pyrolysis process of single chain of cellulose(the degree of polymerization is 9) is simulated under the condition of different force filed by molecular dynamic method.The energy change,pyrolysis temperature and the cracked group of simulation process is obtained in different force fields.The simulated result quite matches the experimental data of pyrolysis of cellulose molecular by Amber force field.The process of pyrolysis of cellulose molecular is simulated in detail with the help of Amber force field and quantum mechanics.The main pyrolysis temperature range,bond break sequence and the first cracked group of cellulose molecule pyrolysis are got and the first production of pyrolysis of cellulose molecule are analyzed.

title: Water assisted liquefaction of lignocellulose biomass by ReaxFF based molecular dynamic simulations
abstract: ReaxFF based molecular dynamics (MD) simulation provides opportunities for fundamentally understanding pyrolysis of lignocellulose biomass through precisely controlled reaction conditions and monitoring of reaction evolution processes. Despite demonstration of simulating the pyrolysis process of dry lignocellulose, MD investigation of this process assisted by water is yet to be performed. This is important considering that hydrothermal liquefaction could be promising to concentrate the energy of biocrude yielded from lignocellulose biomass. In this paper, roles of water on the pyrolysis process of the lignocellulose were investigated by ReaxFF MD simulation. In the simulation, both dry cellulose and lignin systems as well as their systems containing 33% and 66% water by weight were studied at a temperature range of 1250 2000 K at a time scale of 6 ns. Products were characterized by studying their phases, H/C, O/C ratios, and their higher heating values (HHV) that are used to evaluate their value as fuels. Time evolutions of water and other chemical products were investigated to determine the role of water in the reactions and to reveal the reaction mechanism. Compared with dry systems, pyrolysis of the cellulose in presence of water shows several interesting trends, including enhanced breakdown of the cellulose polymer, increased oxygenation of the products, and shift of the final products from char to oil. In contrast, lignin remains largely unaffected by water, and simulation has reproduced experimental results of lignin char formation at elevated temperature in liquefaction. Moreover, it is found that the temperature plays an important role in the reactions. As temperature increases the water's oxygenating effects in the cellulose is decreased. These theoretical results provide solid evidence for unveiling the reaction mechanism of biomass pyrolysis, offering useful guidance for processing wet biomass to liquid fuels.

title: Molecular properties of ultrafiltered dissolved organic matter and dissolved black carbon in headwater streams as determined by pyrolysis GC MS
abstract: This study aimed to assess the molecular properties of dissolved organic matter (DOM) and dissolved black carbon (DBC) using analytical pyrolysis (Py GCMS) The sample set was comprised of ultrafiltered DOM (UDOM) from North American headwater streams associated with Long Term Ecological Research network sites. Pyrolysis products for each UDOM sample were categorized as being sourced from non pyrogenic sources and DBC. Major non pyrogenic components of the headwater stream UDOM were comprised of phenolic compounds derived from lignin and chitin markers from microbial biomass, and their relative contributions indicated differences in organic matter dynamics of these ecosystems. The DBC pyrolyzates included benzene, PAHs and benzonitriles, which accounted for 12.5 4.5% of total quantified peak area (TPQA) and decreased in the order Alaskan boreal forest (19% Alaskan tundra (17% Appalachian deciduous forest (11% Colorado alpine tundra (9% Puerto Rican mountainous tropical rainforest (9% and Kansas tallgrass prairie (7% Pyrolysis products were compared to DBC content as determined by the benzenepolycarboxylic acid (BPCA) method. Although Py GCMS has quantitative limitations, this technique can detect weakly condensed and other DBC structures which fall outside of the BPCA analytical window.

title: High temperature molecular dynamics simulation of cellobiose and maltose
abstract: Thermochemical conversion of lignocellulosic biomass to renewable fuels and chemicals occurs through high temperature decomposition of the main structural components in plants, including cellulose, hemicellulose, and lignin. Cellulose and hemicellulose comprise mostly carbohydrates. Two disaccharides, maltose and cellobiose, are used as model compounds to explore differences in thermal stability due to the orientation of the glycosidic bond. First principles molecular dynamics and density functional theory have been used to probe the decomposition of these disaccharides during pyrolysis at 700 K. The results suggest that maltose, the a disaccharide, is less thermally stable. Dynamic bond length analysis for maltose indicates that several CC bonds and the CO bonds on the pyranose ring demonstrate signs of weakening, whereas no such scissile bonds were identified for cellobiose. The higher stability of the cellobiose is believed to originate from the persistence of low energy hydroxymethyl conformers throughout the simulation which enable strong inter ring hydrogen bonding. Thermogravimetric and mass spectroscopic experiments corroborate the enhanced thermal stability of cellobiose, wherein the onset of decomposition was observed at higher temperatures for cellobiose than for maltose.

title: Deciphering organic matter sources and ecological shifts in blue carbon ecosystems based on molecular fingerprinting.
abstract: Blue carbon ecosystems (BCE) play an essential role in the global carbon cycle by removing atmospheric carbon dioxide and storing it as organic carbon (OC) in biomass and sediments. However, organic matter (OM) deposition and degradation/preservation processes are poorly understood, especially on the long term and at molecular scales. We analysed sediment samples from six cores collected in tidal marshes, mangroves and seagrasses (up to 150 cm long cores spanning up to 10,000 yrs of OC accumulation) from Spencer Gulf (South Australia) by pyrolysis (Py GC MS and THM GC MS) and we compared the results with elemental and stable isotope data, to decipher OM provenance and to assess degradation/preservation dynamics. The results showed that: (1) the major biopolymers preserved were polysaccharides, polyphenolic moieties (lignin and tannin) and polymethylenic moieties (e.g. cutin, suberin, chlorophyll) with smaller apportions of proteins and resins; (2) the OM originates predominantly from vascular plant materials (in particular lignocellulose) that have been well preserved, even in some of the oldest sediments; (3) mangroves were found to be the most efficient OC sinks, partially explained by syringyl lignin preservation; (4) seagrasses were shown to store polysaccharide enriched OM; (5) large proportions of polycyclic aromatic hydrocarbons (PAHs) in surficial tidal marsh and mangrove sediments probably reflect pyrogenic OM from industrial combustion, and; (6) "ecosystem shifts" i.e. mangrove encroachment in tidal marsh and transition from seagrass to mangrove, were detected. Deposition environment and source vegetation control OC sequestration and there is no specific recalcitrant form of OM that is selectively preserved. For the first time, we demonstrate how analytical pyrolysis in combination with stable isotope analysis can be used to reconstruct (palaeo )ecological shifts between different BCE. This study improves our knowledge on OC accumulation dynamics and the response of BCE to environmental change, which can inform the implementation of strategies for climate change mitigation.