•Combustion of dodecane in the presence of hydrogen, nitrogen, and oxygen nanobubbles was investigated using ReaxFF molecular dynamics.•Nanobubbles can alter the activation energy and influence the ...production of intermediates and radicals.•In nanobubbled samples, heat absorption by NB atoms contrasts with pure samples, altering thermal decomposition dynamics.•While increasing the density raises the activation energy, it also boosts the frequency of collisions, thereby increasing fuel consumption.
Blended fuels, created by adding methanol to ammonia or incorporating nanobubbles such as a hydrogen nanobubbles into existing fuels, offer significant opportunities for decarbonization of combustion devices. Nanobubbles (NBs), defined as gaseous cavities smaller than 1 μm in diameter, possess unique characteristics such as long-term stability, the capacity for free radical generation, and a high surface-to-volume ratio. These attributes make them potential candidates for various industrial applications, including water purification, medical engineering, and the energy and power sector. However, the fundamental understanding of the effects of NBs on chemical processes such as combustion cannot be easily captured through experimental techniques. Alternatively, reactive molecular approaches can provide a clear understanding of the impact of NBs on the combustion process. In this study, ReaxFF molecular dynamics simulations were utilized for a comparative study of the combustion of pure dodecane and hydrogen, nitrogen, and oxygen nanobubbled samples. The findings reveal that nitrogen NBs lower the activation energy to 52.89 kcal/mol for samples with a density of 0.17 g/mL by boosting the production of intermediates and radicals, which in turn increases dodecane consumption. In contrast, oxygen NBs increase the activation energy to 66.93 kcal/mol for samples with the same density, reducing dodecane consumption. At 2000 K, C2H4 is the primary product in pure, hydrogen, and oxygen nanobubbled samples, while water is the most prevalent in nitrogen nanobubbled sample at a density of 0.017 g/mL. Moreover, a temperature increase significantly enhances the thermal decomposition of the fuel in pure sample compared to nanobubbled samples. Additionally, as the density of hydrogen nanobubbled samples increases, there is a corresponding rise in fuel consumption, and water emerges as the primary product in samples with a density of 0.25 g/mL at 2000 K.
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•Dehydrogenation and cyclization were the two emblematic reactions in combustion.•The cyclization of long carbon chains is a main pathway for PAHs formation.•Ethylene dehydrogenates ...with the assistance of OH, HO2, H, and O2.•The oxygenated intermediates formed by ethanol inhibit cyclization reactions.
To gain detailed atomic-level insights into the reaction mechanisms associated with the oxy-fuel combustion of ethylene and ethanol, ReaxFF reactive force field molecular dynamics simulations were performed to analyze the behavior of their high-temperature pyrolysis and oxidation. Results showed that the main reaction of pure ethylene involves the formation of long carbon chains by C–C polymerization in an oxygen-free environment. The polycondensation-cyclization of long carbon chains significantly contributes to the formation of polycyclic aromatic hydrocarbons (PAHs). The addition of ethanol enriches the initial reaction pathways of ethylene. For instance, ethanol participates in the following reactions: C2H4 + C2H6O → C4H10O, C2H6O + C2H4 → C2H5 + C2H5O, and C2H6O + C2H4 → C2H5 + H2O + C2H3. Ethanol inhibits the growth of long carbon chains, with a more pronounced effect at higher ethanol concentrations. In an oxygen atmosphere, the decay rate of ethylene decreases significantly, and its main reaction pathway involves dehydrogenation and oxidation with the assistance of OH, HO2, H, and O2. The final products are released in the form of CO, CO2, and H2O. In addition, the formation of soot particles is mainly divided into three steps: (i) ethylene aggregates to form long carbon chains, (ii) rearrangement and cyclization of long carbon chains, and (iii) growth of PAHs.
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•Vitrinite pyrolysis process was studied by TG-MS and ReaxFF MD.•Hydrogen-rich gas is produced from aliphatic and partially aromatic moieties of vitrinite.•Transferable hydrogen was ...derived from active sites in vitrinite and gas-phase alkyl radicals.•Gas-phase hydrogen transfer cycle was involved in the formation of hydrogen-rich gases.•Transferable hydrogens have higher collision probability with other moieties.
Understanding the evolution mechanism of hydrogen-rich coke oven gas in vitrinite pyrolysis is crucial for improving the clean utilization of coal. This work focused on the vitrinite extracted from Fangezhuang coal as the sample. Its three-dimensional molecular model was constructed based on elemental analysis, IR spectroscopy, 13C NMR, and density functional theory. ReaxFF molecular dynamics simulation, combined with TG-MS, was used to study the vitrinite pyrolysis process and its mechanism of hydrogen-rich gas evolution. Experimental results indicated that H2 and CH4, the main hydrogen-rich gases, originated from the aliphatic and partially aromatic moieties of vitrinite, and required the participation of a large number of transferable hydrogen atoms. The reaction network confirmed that the transferable hydrogen atoms included not only the easily-dissociated hydrogen atoms in vitrinite but also the active hydrogen atoms in the gas-phase hydrocarbon radicals. In particular, the latter significantly increased the diffusivity of condensed hydrogen atoms and the collision probability with other components through the gas-phase hydrogen transfer cycle, which favors the generation of gas products. This work provided theoretical support for optimizing the production of coke and coke oven gas in synergy, as well as improving the quality of hydrogen-rich gas in the industry.
Ammonia is gradually becoming a hot research topic as an important hydrogen energy carrier and carbon-free fuel. In this study, we performed detailed numerical simulations on the combustion mechanism ...of DME/NH3 mixtures through reactive force field molecular dynamics simulations and density functional theory calculations. The results showed that the system temperature is positively correlated with the reaction rate and NOx production, and ammonia is consumed significantly earlier than DME. The ammonia molecules were always consumed before the DME molecules. An increase in temperature from 2500 K to 3500 K resulted in a significant decrease in the time for complete consumption of the DME molecules, from 450 ps to 68 ps. Additionally, in the initial reaction process between DME and ammonia, C2H6O => CH3O + CH3 serves as the dominant reaction of DME. Ammonia is mainly consumed by oxidation via reactions NH3 + HO2 => NH2 + H2O2 and NH3 + O2 => NH2 + H2O. Eventually, density functional theory utilized to further explain the intrinsic mechanism of the initial reaction pathways and the molecular active reaction sites. Overall, the results of this work would provide theoretical basis for exploring the chemical reaction kinetic mechanism of DME/NH3 blended fuels.
•System temperature is positively correlated with both the reaction rate and NOx production.•Ammonia is consumed earlier than DME in mixture systems.•The dominant reaction for DME is C2H6O => CH3O + CH3.•Ammonia is mainly consumed via reactions NH3 + HO2 => NH2H2O2 and NH3 + O2 => NH2 + H2O.
In order to investigate the effect of oxygen concentration on ammonia combustion, the effect of oxygen concentration on ammonia combustion and its mechanism were systematically investigated by using ...the ReaxFF molecular dynamics simulations and density functional theory (DFT) calculations. The results show that different oxygen concentrations significantly affect the concentration of reactants, free radicals, intermediates and products in the ammonia combustion process. High oxygen concentration significantly increases the concentration of free radicals, which effectively promotes the ammonia reaction. However, more free radicals such as ·H and ·OH are produced in the early stage of the reactions, which react with ·NH2 radicals. It promotes the production of the intermediates HNO and indirectly inhibits the production of the intermediates N2H4 and N2H2. It promotes the production of NOX. The results of the DFT calculations show that the relative Gibbs free energy barrier of the reaction of ·OH with NH3 is low at 1.00 kcal/mol, so the increase in ·OH concentration greatly facilitates the reaction of NH3. These results not only identify the intrinsic effect of oxygen concentration on ammonia combustion and its mechanisms, but also provide some theoretical guidance for the study of efficient and clean ammonia utilization.
•NH3 combustion with different O2 numbers was studied by ReaxFF MD simulations.•The main chain reactions of NH3 combustion were studied by DFT calculations.•The effect of O2 concentration on NH3 combustion was studied by DFT calculations.•High oxygen concentration promotes the NH3 combustion.•High oxygen concentration promotes the production of NOX of NH3 combustion.
•Adding H2-NH3 can reduce carbon emissions and soot formation in C4H10 flames.•ReaxFF MD is used to analyze A1 mechanism in H2-NH3 blended C4H10 combustion.•Adding H2 has opposite effects on PAHs ...formation in n-C4H10 and i-C4H10 flames.•Adding H2-NH3 reduces soot peak average primary particle diameter in C4H10 flames.
This study employed extinction method and thermophoretic sampling techniques to explore the characteristics of soot formation in C4H10 combustion with H2-NH3 addition. The impact of H2-NH3 addition on the formation of PAHs and soot was analyzed using ReaxFF MD. It is found that the adding H2-NH3 leads to a decrease of fvmax, with the inhibitory effect on soot formation being stronger for i-C4H10 compared to n-C4H10. Adding H2-NH3 inhibits the growth of soot particles, reducing the peak average primary particle diameter of soot particles. The chemical effect of H2 promotes soot formation in n-C4H10 combustion but inhibits it in i-C4H10 combustion. However, both NH3 and H2-NH3 addition chemically inhibit soot formation in C4H10 combustion. The A1 pathways reveal that the C–C bond cleavage in n-C4H10 is significantly stronger than C–H bond cleavage, while it is exactly the opposite in i-C4H10. Adding H2 promotes the reaction equilibrium CH3+H2↔CH4+H towards the forward reaction, facilitating C–C bond cleavage and A1 formation in n-C4H10 but inhibiting C–H bond cleavage and A1 formation in i-C4H10. However, adding NH3 results in the cleavage of N–H bonds, producing a large amount of H. This shifts the reaction equilibrium CH3+H2↔CH4+H towards the reverse reaction, ultimately suppressing A1 formation.
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•The influence of CaO hydration on CO2 adsorption performance was studied.•CaO surface can be easily passivated by water just through absorption and dissociation.•Hydration failed to ...promote the CO2 adsorption rate of CaO surface.•CO2 adsorption can accelerate the diffusion of H protons.•H diffusion through transition HO2 state changes the lattice structure.
Hydration has been regarded as an effective method to improve the cyclic activity of CaO during the Calcium looping CO2 capture process, but its mechanism remains unclear. In this work, the hydration reaction of CaO surface and its underlying mechanism was studied by ReaxFF molecular dynamics simulation combing with TGA experiments. The effect of hydration on the initial CO2 adsorption rate showed a trend of promoting at low H2O degree and inhibiting at high H2O concentration. The CaO surface was easily passivated, and it can maintain its crystal structure and begin to distort at about 1.0 H2O monolayer contents. Hydration failed to promote the CO2 adsorption rate, inversely the CO2 adsorption process accelerated the diffusion of H ions inward the CaO lattice·H2O dissociation produced two hydroxyl groups: direct OwH and indirect OsH. Atomic density profiles showed that the direct hydroxyl pairs were always, and indirect OsH groups can consume surface oxygen active sites, reducing the initial rapid CO2 adsorption activity. On the pre-hydrated CaO surface, CO2 molecules were more likely to bind to solid oxygen active sites than OH groups. The H free radical diffused inside the lattice in the form of transition HO2 state, promoting the slow CO2 adsorption amount.
Oxygenated fuels and nitro fuels are effective strategies for addressing incomplete combustion by increasing the oxygen-fuel ratio. A computational approach based on reactive molecular dynamics ...simulations reveals oxidation mechanisms of temperature-induced effects on a mixed fuel of methanol and nitromethane. The method enables the demonstration of the initial reaction scheme of a binary fuel mixture, even for complicated interplay and coupling reactions. The results showed that the first reaction step of nitromethane was homolysis in poor-oxygen conditions, mainly via CH3NO2 → CH3 + NO2 (Net flux = 183, ratio = 92.89%). Methanol undergoes dehydrogenation reaction with the assistance of active radicals (OH, HO2, CH3, NO, and NO2), and the participation rates of these active groups are 60%, 17.14%, 8.57%, 7.62%, and 6.67%, respectively. The decomposition of nitromethane provided NO2 and CH3 radicals and significantly increased the amount of OH and NO via a reaction of NO2 + H → HNO2 → OH + NO. By fragment analysis, the main C1 intermediates are formed by pyrolysis of methanol/nitromethane such as formaldehyde, hydroxymethyl radical, and formyl radical. The CH3OH and CH2O are relatively stable, and the dehydrogenation is mainly highly active groups such as OH and NO. In contrast, the dehydrogenation of CH2OH and CHO free radicals is completed by self-cleavage or with the help of O2, NO2, etc. Our findings shed light on the oxidation behaviors of methanol/nitromethane mixed fuel in combustion.
•The oxidation mechanism of mixed fuel is explored using the RMD simulation.•The initial reaction of methanol/nitromethane is analyzed.•The evolution of main intermediates is studied at different conditions.•The influence of temperature on products distribution is elaborated.
We combine density functional theory (DFT) and reactive force-field (ReaxFF) simulations to assess the stability and activity of unique catalytic sites at the interface between Pd clusters and a CeO2 ...support. ReaxFF-based Grand Canonical Monte Carlo (GC-MC) simulations provide insight into the oxide structure at the Pd/CeO2 interface. Surface models derived with GC-MC are employed in reactive molecular dynamics (RMD) simulations, which demonstrate that methane lightoff rapidly occurs when there is Pd mixing in the CeO2 lattice. DFT investigations, utilizing models inspired by GC-MC/RMD, demonstrate that Pd4+ states are stabilized in PdO x clusters partially embedded in the CeO2 lattice, and that such sites yield low methane activation barriers. The integrated DFT/ReaxFF methodology employed here demonstrates a combined quantum/classical workflow that can be extended to examine emergent behavior in other oxide-supported metal catalysts.
Understanding the co-pyrolysis behavior of sub-bituminous coal and lignin not only contributes to the efficient and clean use of coal but also enables low carbon emissions. The behavior and product ...evolution of pure coal, pure lignin, and co-pyrolysis systems under isothermal and non-isothermal conditions were explored using reactive force field molecular dynamics (ReaxFF MD) simulation. Through multiple isothermal simulation processes, the co-pyrolysis system showed an inhibition effect, promoting the generation of heavy compounds and inhibiting the generation of tar and gas. The inhibition effect at 2400 K was the most significant in the pyrolysis period. A similar phenomenon was found in the non-isothermal pyrolysis process at 2 K/ps. In addition, the H2 yield under non-isothermal conditions was lower than the calculated value. By distinguishing the same element from different sources, there were three types of H2 (Hcoal2, HcoalHlignin, and Hlignin2). Subsequently, the H atoms in lignin prefer to produce H2 than that in coal during co-pyrolysis. Finally, we further explored the evolution behavior of the configurations during the pyrolysis of non-isothermal coal and lignin systems. These findings enrich the theoretical information on the interaction between coal and lignin, providing the behavior of Hcoal and Hlignin atoms in the co-pyrolysis system.
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•The co-pyrolysis behavior of lignin and subbituminous coal was revealed.•All isothermal co-pyrolysis simulations inhibit gas and tar generation.•Non-isothermal simulation inhibited weight loss and gas generation, including H2.•The H atoms in lignin prefer to produce H2 than that in coal during co-pyrolysis.
