1,721,126 research outputs found
Reduced-order condensed-phase kinetic models for polyethylene, polypropylene and polystyrene thermochemical recycling
No full textThermochemical recycling of plastic waste (PW) into chemicals and energy vectors requires coupling particle and reactor-scale simulations to accurate condensed phase pyrolysis mechanisms for each constituent. This work proposes a methodology to derive reduced-order condensed-phase kinetic models from validated semi-detailed kinetic mechanisms. Two types of kinetic models are obtained for polyethylene (PE), polypropylene (PP) and polystyrene (PS): reduced semi-detailed models and multi-step fully lumped ones. These families offer different compromises between accuracy and computational cost. The former employ 50-100 gas + liquid species and describe both the radical degradation and the detailed carbon distribution of the products. Conversely, the latter involves 5-10 species per polymer tracking only the main petroleum cuts. The kinetic mechanisms are complemented by the definition of thermochemical properties of gas, liquid, and solid-phase species, accounting for phase-transitions through pseudo-chemical reactions. Model validations are performed by comparison with experimental data and the original semi-detailed mechanisms in terms of mass loss, heat fluxes and product distribution profiles. The resulting CHEMKIN-like condensed-phase models are attached as Supplementary Material and as a GitHub repository. Extending the proposed approach to other polymers and coupling it with existing subsets in the CRECK kinetic framework (e.g., biomass, PVC, PET) offers a powerful tool to model thermochemical recycling of PW and biomass/PW mixtures
Special issue of thermo-chemical conversion of biomass
Full text linkThe rapid depletion of fossil fuels and the increasing of carbon
emissions drive us to exploit the renewable energy. Biomass, the only
carbon-contained renewable energy resource, has aroused more and
more attentions due to its natural abundance and carbon neutrality.
Thermo-chemical processes, such as gasification and pyrolysis, are
regarded as a promising technology for the utilization of biomass since it
can rapidly convert biomass into liquid fuels, biochar and gases. Those
can be directly or further used as energy, fuel, and chemical products,
which have already been applied in various industries. However, the
thermo-chemical conversion of biomass still remains to be further
explored in many aspects, such as deep mechanism, novel processes and
catalyst materials, as well as new modeling and analyzing methods, etc.
The above-mentioned has been the motivation for the publication of the
current special issue “Thermo-chemical Conversion of Biomass”. This
Special Issue aims at reporting the latest advances in biomass to biofuel
via thermo-chemical conversion technology including original research
papers and reviews (mini-reviews).
Many submissions were received for this Special Issue. All sub
missions have been subjected to the usual screening within the editorial
office and/or subsequent rigorous peer-review process. Upon the
completion of the whole editorial process, a total of 19 papers (including
7 review articles and 12 research articles) have been finally accepted for
publication in this Special Issue, as listed in Table 1.
The Special Issue compiles a list of review and original research ar
ticles describing the latest fundamental research and technology
development in thermo-chemical conversion of biomass, with the
concise synopsis of each paper given below
Experimental and numerical study of pollutant emissions from a domestic condensing boiler fed with natural gas enriched with H2
No full textHydrogen is recognized as a promising resource for decarbonizing not only the industrial sector, but also the domestic heating systems. Through the partial substitution of natural gas with hydrogen, domestic combustion- based conversion systems can potentially offer improved efficiency, reduced carbon emissions, and cleaner combustion, i.e., lower levels of particulate matter. However, hydrogen exhibits properties that are significantly different from natural gas: (i) because of its higher laminar flame speed, hydrogen is more susceptible to flashback, which may pose significant concerns from the safety point of view; (ii) because of its higher adiabatic temperature, NOx emissions are expected to increase. Thus, experimental and numerical investigations are needed to better understand how the addition of hydrogen to the fuel mixture modifies the combustion process and how to mitigate/control the higher propensity to flashback and NOx formation within domestic devices. In this study, we investigated experimentally and numerically the performances and the emissions of a domestic condensing boiler with a stainless steel coil heat exchanger equipped with a perforated cylindrical burner fed with mixtures of H2 2-enriched natural gas and air, at several power levels (15, 24, and 30 kW), in a wide range of dilution ratios (from 1.16 to 1.4). 3D numerical simulations, including a detailed kinetic mechanism and conjugate heat transfer between the gaseous phase and the burner plate, were carried out with satisfactory agreement with the experimental data. The experimental results demonstrated the ability of the investigated device to properly work with fuel mixtures including up to 35% (molar basis) of hydrogen. The numerical simulations were repeated by considering pure hydrogen as a fuel in more diluted conditions (with dilution ratios from 1.4 to 2) and the same heat exchanger with a modified perforated burner to prevent the occurrence of flashback phenomena. The numerical results suggested the possibility to (partially) replace natural gas with hydrogen in domestic boilers with minimal modifications to existing perforated burners
A Predictive Physico-chemical Model of Biochar Oxidation
Full text linkPyrolysis of solid fuels forms a solid carbon-rich fuel, also called char, whose physico-chemical description is rather complex. Heterogeneous oxidation reactions take place during thermochemical conversion of char. The present work proposes a predictive detailed kinetic model, opening a new path for a deeper understanding of the char conversion process. This model considers porosity, surface area, density of surface sites, and their evolution along the conversion process. The chemical aspects of char oxidation are modeled assuming a carbonaceous bulk structure, surrounded by a variety of surface sites which represent the chemical functionalities typically present in such materials. The heterogeneous chemical reactions and their kinetic parameters are defined based on previous studies in the literature and by analogy to homogeneous gas-phase reactions of aromatic species. A mathematical framework is proposed to couple physical and chemical descriptions of the oxidation process. Although the proposed model benefits from experimental information, it is able to comprehensively describe the conversion rate of a broad range of carbonaceous materials such as carbon nanotubes, graphite, and chars only on the basis of their elemental composition. The proposed model represents a first step in exploring the explicit and coupled treatment given to the physical and chemical evolution of the fuel throughout its conversion, allowing us to consistently describe the particle evolution, opening a path for reliable models to manage the chemistry of char conversion
DropletSMOKE++: A comprehensive multiphase CFD framework for the evaporation of multidimensional fuel droplets
Full text linkThis paper aims at presenting the DropletSMOKE++ solver, a comprehensive multidimensional computational framework for the evaporation of fuel droplets, under the influence of a gravity field and an external fluid flow. The Volume Of Fluid (VOF) methodology is adopted to dynamically track the interface, coupled with the solution of energy and species equations. The evaporation rate is directly evaluated based on the vapor concentration gradient at the phase boundary, with no need of semi-empirical evaporation sub-models. The strong surface tension forces often prevent to model small droplets evaporation, because of the presence of parasitic currents. In this work we by-pass the problem, eliminating surface tension and introducing a centripetal force toward the center of the droplet. This expedient represents a major novelty of this work, which allows to numerically hang a droplet on a fiber in normal gravity conditions without modeling surface tension. Parasitic currents are completely suppressed, allowing to accurately model the evaporation process whatever the droplet size. DropletSMOKE++ shows an excellent agreement with the experimental data in a wide range of operating conditions, for various fuels and initial droplet diameters, both in natural and forced convection. The comparison with the same cases modeled in microgravity conditions highlights the impact of an external fluid flow on the evaporation mechanism, especially at high pressures. Non-ideal thermodynamics for phase-equilibrium is included to correctly capture evaporation rates at high pressures, otherwise not well predicted by an ideal gas assumption. Finally, the presence of flow circulation in the liquid phase is discussed, as well as its influence on the internal temperature field. DropletSMOKE++ will be released as an open-source code, open to contributions from the scientific community
Electronic structure-based rate rules for H: ipso addition-elimination reactions on mono-aromatic hydrocarbons with single and double OH/CH3/OCH3/CHO/C2H5substituents: a systematic theoretical investigation
No full textThe recent interest in bio-oils combustion and the key role of mono-aromatic hydrocarbons (MAHs) in existing kinetic frameworks, both in terms of poly-aromatic hydrocarbons growth and surrogate fuels formulation, motivates the current systematic theoretical investigation of one of the relevant reaction classes in MAHs pyrolysis and oxidation: ipso substitution by hydrogen. State-of-the-art theoretical methods and protocols implemented in automatized computational routines allowed to investigate 14 different potential energy surfaces involving MAHs with hydroxy and methyl single (phenol and toluene) and double (o-,m-,p-C6H4(OH)2, o-,m-,p-CH3C6H4OH, and o-,m-,p-C6H4(CH3)2) substituents, providing rate constants for direct implementation in existing kinetic models. The accuracy of the adopted theoretical method was validated by comparison of the computed rate constants with the available literature data. Systematic trends in energy barriers, pre-exponential factors, and temperature dependence of the Arrhenius parameters were found, encouraging the formulation of rate rules for H ipso substitutions on MAHs. The rules here proposed allow to extrapolate from a reference system the necessary activation energy and pre-exponential factor corrections for a large number of reactions from a limited set of electronic structure calculations. We were able to estimate rate constants for other 63 H ipso addition-elimination reactions on di-substituted MAHs, reporting in total 75 rate constants for H ipso substitution reactions o-,m-,p-R′C6H4R + H → C6H5R + R′, with R,R′ = OH/CH3/OCH3/CHO/C2H5, in the 300-2000 K range. Additional calculations performed for validation showed that the proposed rate rules are in excellent agreement with the rate constants calculated using the full computational protocol in the 500-2000 K range, generally with errors below 20%, increasing up to 40% in a few cases. The main results of this work are the successful application of automatized electronic structure calculations for the derivation of accurate rate constants for H ipso substitution reactions on MAHs, and an efficient and innovative approach for rate rules formulation for this reaction class
Interface-resolved simulation of the evaporation and combustion of a fuel droplet suspended in normal gravity
Full text linkAn interface-resolved simulation of the combustion of a fuel droplet suspended in normal gravity is presented in this work, followed by an extensive analysis on the physical aspects involved. The modeling is based on DropletSMOKE++, a multiphase solver developed for the modeling of droplet vaporization and combustion in convective conditions. A wide range of phenomena can be described by the model, including the interface advection, the phase-change, the combustion chemistry, non-ideal thermodynamics and multicomponent mixtures. To our knowledge, this is the most detailed simulation performed on this configuration, providing a useful theoretical and numerical support for the experimental activity on this field. A recent experimental work is used as a reference, in which a methanol droplet is suspended on a quartz fiber and ignited at different oxygen concentrations. The numerical analysis offers a detailed insight into the physics of the problem and a satisfactory agreement with the experiments in terms of diameter decay, radial temperature profiles and sensitivity to the oxygen concentration. The vaporization rate is affected by the thermal conduction from the fiber, due to the high temperatures involved. Moreover, the fiber perturbs the flame itself, providing quenching at its surface. The combustion physics is compared to the one predicted at zero-gravity, evidencing a lower standoff-ratio, a higher flame temperature and an intense internal circulation. The distribution of the species around the droplet shows (i) a local accumulation of intermediate oxidation products at the fiber surface and (ii) water absorption in the liquid phase, affecting the vaporization rate
Theoretical kinetics of HO2 + C5H5: A missing piece in cyclopentadienyl radical oxidation reactions
No full textThe resonantly-stabilized cyclopentadienyl radical (C5H5) is a key species in the combustion and molecular growth kinetics of mono and poly-aromatic hydrocarbons (M/PAHs). At intermediate-to-low temperatures, the C5H5 reaction with the hydroperoxyl radical (HO2) strongly impacts the competition between oxidation to smaller products and growth to PAHs, precursors of soot. However, literature estimates for the HO2 + C5H5 reaction rate are inaccurate and inconsistent with recent theoretical calculations, thus generating discrepancies in global combustion kinetic models. In this work, we perform state-of-the-art theoretical calculations for the HO2 + C5H5 reaction including variable reaction coordinate transition state theory for barrierless channels, accurate thermochemistry, and multi-well master equation (ME) simulations. Contrary to previous studies, we predict that OH + 1,3-C5H5O is the main reaction channel. The new rate constants are introduced in two literature kinetic models exploiting our recently developed ME based lumping methodology and used to perform kinetic simulations of experimental data of MAHs oxidation. It is found that the resonantly-stabilized 1,3-C5H5O radical is the main C5H5O isomer, accumulating in relevant concentration in the system, and that the adopted lumping procedure is fully consistent with results obtained with detailed kinetics. The reactivity of C5H5O with OH and O-2 radicals is included in the kinetic mechanisms based on analogy rules. As a result, C5H5O mostly reacts with O-2 producing smaller C-3/C-4 species and large amounts of C5H4O, suggesting that further investigations of the reactivity of both C5H5O and C5H4O with oxygenated radicals is necessary. Overall, this work presents new reliable rate constants for the HO2 + C5H5 reaction and provides indications for future investigations of relevant reactions in the sub-mechanisms of cyclopentadiene and MAH oxidation
Improvements in the simulation of liquid fuel combustion in a low-temperature fluidized bed
No full textTheoretical study of sensitive reactions in phenol decomposition
Full text linkThe reactivity of phenol is of utmost importance in combustion systems. In fact, phenol is the simplest phenolic compound, abundant in bio-oils derived from biomass fast pyrolysis and therefore included as a reference component in bio-oil surrogate mixtures. Phenol is also relevant to the mechanism of oxidation of benzene, a building block in the growth of polycyclic aromatic hydrocarbons (PAHs), precursors of soot formation. Hence, in a modular and hierarchical approach to combustion chemistry, the knowledge of the pyrolysis and combustion kinetics of phenol is essential to characterize the reactivity and the combustion properties of bio-oils and mono aromatic hydrocarbons (MAHs), as well as to improve the understanding of PAHs and soot formation. Although the reaction pathways of phenol decomposition are well defined in the literature, the rate constants still require more accurate assessment, and a validation of the reaction mechanism of phenol pyrolysis against the full set of experimental data available is still missing. In this work, we compute with the ab initio transition state theory based master equation (AI-TST-ME) method the rate constants of the main reaction pathways of phenol decomposition, also relevant to benzene oxidation. In particular, we investigate phenol molecular decomposition to C5H6 + CO and its competition with the O-H bond fission and H-atom abstraction reactions by H to form the phenoxy radical (C6H5O). We also investigate the successive decomposition of C6H5O to C5H5 + CO and the H-atom abstraction reaction on C5H6 by H, which plays a pivotal role in controlling the H concentration in phenol pyrolysis and combustion. The calculated rate constants are found to be in good agreement with experimental values. The CRECK kinetic model is updated with the new rate constants and validated against the available experimental data of phenol pyrolysis providing, to our knowledge, the first comprehensive validation of phenol decomposition kinetics. However, discrepancies are still present in the profiles of products formed from secondary reactivity. Our analysis suggests that further investigation of the reactivity of C5H6 is required, providing guidelines for a more accurate characterization of the decomposition to smaller species
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