1,721,002 research outputs found
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
Yield, composition and active surface area of char from biomass pyrolysis
No full textBiomass materials represent promising carriers for both heat, energy and chemicals production. Nevertheless, several aspects must be intensively investigated and understood, leading to a better design and optimization of industrial combustors, gasifiers and pyrolyzers. The first objective of this work is to update the POLIMI multistep kinetic mechanism of biomass pyrolysis, focusing on the prediction of both yield and composition of the solid residue (biochar). To this end, a large set of literature experimental data was collected and organized into a database, which was further used to finely tune and validate the proposed kinetic mechanism. Then a method to estimate the biochar active surface area is introduced, deriving from the biochar composition predictions. Keeping the previous agreements with the rate of biomass pyrolysis, formation and distribution of gas and tar products, the novelty of this work is the additional potential to predict the evolution of biochar yield and composition in a wide range of operative conditions, predicting also some important surface features. The model describes the solid residue as a mixture of pure carbon together with lumped metaplastic compounds, which represent the whole range of oxygenated and hydrogenated groups bonded to the carbonaceous matrix. These metaplastic species are released to the gas phase with their own kinetics and describe both mass loss and elemental composition change of the biochar. These are relevant topics because a comprehensive evaluation of biochar composition and its structural characteristics is crucial for an accurate description of the successive oxidation and gasification processes
Differences in formation and oxidation of Colombian coal chars in air and oxy-fuel atmospheres
No full textThe increasing interest towards more efficient and clean technologies, specially paying attention to CO2 neutral processes is encouraging the investigation of coal thermochemical conversion under oxy-fuel atmospheres (i.e. without N2). The process offers many advantages such as easy separation of the CO2 produced and low NOX/SOX emissions. While coal conversion in air is already well understood, full understanding of the influences of CO2-rich atmospheres is still required. A series of experiments in thermogravimetric analyser, drop-tube reactor and flat-flame burner were performed using a mid-range bituminous coal (Colombian coal) to understand the differences in the chars obtained after the pyrolysis step. Comparing the pyrolysis in N2 and CO2 atmospheres, significant differences were observed in the resulting chemical (composition) and physical properties of the chars, whereas mass loss was very similar for short residence times (< 130 ms). Afterwards, the chars obtained were submitted to oxidation and gasification, under several different operating conditions, in order to evaluate the difference in reactivity of these chars. Chars obtained under CO2 atmospheres revealed a lower reactivity, despite their higher surface area. These aspects cannot be explained and captured by a model which does not focus on some important details. In this paper, the results obtained in these experiments are summarized and discussed on the point of view of the POLIMI modelling approach for thermochemical conversion of solids. This model offers several advantages, such as being flexible to improvements, requiring simple experimental data of the fuel and offers an all-in-one solution for describing the kinetics of the whole process. The developments accounted for a wide range of experimental data, which allowed its calibration for several fuels, mostly in air combustion. It was first developed to describe the pyrolysis step, and later char oxidation/gasification was included in a simplified approach. The detailed mechanism of homogeneous gas phase reactions of the volatiles is also coupled. In order to extend the predictive capabilities of the model for oxy-fuel conditions, dedicated experiments must be considered for future improvements. In this work these missing effects are discussed, identifying the main necessary improvements, allowing this model to be extended and applied also for the designing of reactors that use oxy-fuel technologies
Can small polyaromatics describe their larger counterparts for local reactions? A computational study on the H-abstraction reaction by an H-atom from polyaromatics
No full textHydrogen abstraction is one of the crucial initial key steps in the combustion of polycyclic aromatic hydrocarbons. For an accurate theoretical prediction of heterogeneous combustion processes, larger systems need to be treated as compared to pure gas phase reactions. We address here the question on how transferable activation and reaction energies computed for small molecular models are to larger polyaromatics. The approximate transferability of energy contributions is a key assumption for multiscale modeling approaches. To identify efficient levels of accuracy, we start with accurate coupled-cluster and density functional theory (DFT) calculations for different sizes of polyaromatics. More approximate methods as the reactive force-field ReaxFF and the extended semi-empirical tight binding (xTB) methods are then benchmarked against these data sets in terms of reaction energies and equilibrium geometries. Furthermore, we analyze the role of bond-breaking and relaxation energies, vibrational contributions, and post-Hartree-Fock correlation corrections on the reaction, and for the activation energies, we analyze the validity of the Bell-Evans-Polanyi and Hammond principles. First, we find good transferability for this process and that the predictivity of small models at high theoretical levels is way superior than any approximate method can deliver. Second, ReaxFF can serve as a qualitative exploration method, whereas GFN2-xTB in combination with GFN1-xTB appears as a favorable tool to bridge between DFT and ReaxFF so that we propose a multimethod scheme with employing ReaxFF, GFN1/ GFN2-xTB, DFT, and coupled cluster to cope effectively with such a complex reactive system
Systematic evaluation and kinetic modeling of low heating rate sulfur release in various atmospheres
No full textIn the present work, a systematic experimental and numerical study of sulfur release from coal in varying atmospheres at low heating rates (LHR) is presented. To this aim, two bituminous coals were investigated, Colombian hard coal (K1) with typical sulfur content, and American high-sulfur coal (U2), with elevated sulfur content. Mass loss and release of target volatile species - H2S, COS, SO2- were tracked using a TG-MS. The samples were heated at 10 Kmin−1 under different atmospheres: argon, CO2, synthetic air (21 vol% O2/79 vol% Ar) and oxy-fuel (21 vol% O2/79 vol% CO2). The role of the different atmospheres in the sulfur release was elucidated as well as the fate of the volatile species in the gas-phase. The advantage of investigating the release at LHR is that heat and mass transfer effects can be neglected, as the experimental conditions allow the process to remain in the kinetic regime. The successive increase in atmosphere complexity allowed to individuate the chemical paths leading to the formation of SOX and its precursors in each of the conversion steps: devolatilization, char conversion as well as the coupling to gas-phase reactions. The experiments were further analyzed with a kinetic model for the solid-phase of coal conversion, coupled with a detailed gas-phase kinetic mechanism. The solid-phase kinetic model was modified accounting for the particularities of the fuels, for the effects of oxy-fuel atmosphere. A small number of kinetic parameters was adjusted for improved predictions of the release rate and the yields of sulfur species
A predictive model of biochar formation and characterization
No full textBiomass is increasingly being recognized as a promising carrier for both heat, energy and chemicals production. However, several aspects still require intense research activity towards a better design and optimization of industrial combustors, gasifiers and pyrolyzer. The objective of this work is to update the CRECK kinetic mechanism of biomass pyrolysis, allowing a better prediction of both yield and composition of the solid residue (biochar). Moreover, further model modifications allow to better describe the variability of hemicellulose in different biomass. To this end, a large set of literature experimental data is collected and organized into a database, which is used to further tune and validate the proposed kinetic mechanism. Although the kinetic model maintains the previous agreement in respect of the rate of biomass pyrolysis, formation and distribution of gas and tar products, the novelty of this work is the greater attention to the predictions of biochar yield and composition, in a wide range of operative conditions. The model describes the solid residue as a mixture of pure carbon together with lumped metaplastic compounds, which represent the whole range of oxygenated and hydrogenated groups bonded to the carbonaceous matrix. These metaplastic species are released to the gas phase with their own kinetics and describe the change of both mass loss and elemental composition of the biochar. These comprehensive predictions of biochar composition are crucial for an accurate description of the successive oxidation and gasification processes
Experimental and modeling assessment of sulfur release from coal under low and high heating rates
No full textCoal combustion releases elevated amounts of pollutants to the atmosphere including SO X . During the pyrolysis step, sulfur present in the coal is released to the gas phase as many different chemical species such as H 2 S, COS, SO 2 , CS 2 , thiols and larger tars, also called SO X precursors, as they form SO X during combustion. Understanding the sulfur release process is crucial to the development of reliable kinetic models, which support the design of improved reactors for cleaner coal conversion processes. Sulfur release from two bituminous coals, Colombian hard coal (K1) and American high sulfur coal (U2), were studied in the present work. Low heating rate (LHR) experiments were performed in a thermogravimetric analyzer coupled with mass spectrometry (TG-MS), allowing to track the mass loss and the evolution of many volatile species (CO, CO 2 , CH 4 , SO 2 , H 2 S, COS, HCl and H 2 O). High heating rate (HHR) experiments were performed in an entrained flow reactor (drop-tube reactor - DTR), coupled with MS and nondispersive infrared sensor (NDIR). HHR experiments were complemented with CFD simulation of the multidimentional reacting flow field. A kinetic model of coal pyrolysis is employed to reproduce the experiments allowing a comprehensive assessment of the process. The suitability of this model is confirmed for LHR. The combination of HHR experiments with CFD simulations and kinetic modeling revealed the complexity of sulfur chemistry in coal combustion and allowed to better understand of the individual phenomena resulting in the formation of the different SO X precursors. LHR and HHR operating conditions lead to different distribution of sulfur species released, highly-dependent on the gas-phase temperature and residence time. Higher retention of total sulfur in char is observed at LHR (63%) when compared to HHR (37-44%), at 1273 K. These data support the development of reliable models with improved predictability
Development and application of an efficient chemical reactor network model for oxy-fuel combustion
No full textThe excessive computational cost of computational fluid dynamics (CFD) simulations of complex reactors is still a barrier to investigations using detailed chemical kinetics. Chemical rector network modeling is a promising tool for affordable numerical investigations of novel combustor technologies, such as oxy-fuel combustion, using directly coupled detailed chemical kinetics. In this work, a novel chemical reactor network solver, NetSMOKE, which is part of the OpenSMOKE++ suite, is presented and discussed. The numerical model employed, together with the relative solution method, is explained, also exploiting a combined sequential-modular and equation-oriented approach for solving the global system of equations. The novel solver was employed to build a chemical reactor network that represents the Technische Universität Darmstadt oxy-fuel burner using a reduced number of ideal reactors with directly coupled detailed kinetic models for the first time. On the basis of previously available CFD simulations and measurements, the complex flow is accurately characterized and discretized into macrozones, facilitating the development of a simplified reactor network. Carbon monoxide emissions were analyzed in detail, supported by sensitivity analysis with respect to the reactor temperatures. The sensitivity analysis revealed that the post-flame zone is crucial for the overall CO emission. Therefore, on the basis of the sensitivity analysis, an iterative approach for refining the reactor network model is developed. The increase in the number of ideal reactors in targeted areas of the system allows for significant improvements with respect to CO predictions. The chemical reactor network provides good agreement with the experimental data, requiring only a limited increase in the overall computational cost. The presented tool offers a computationally efficient strategy to investigate and predict the behavior of complex reactors, including the emission of pollutants in combustion devices, allowing for the employment of state-of-the-art, detailed chemical kinetic mechanisms available from the literature
An experimental and numerical study on the combustion of lignites from different geographic origins
No full textCoal combustion involves multi-scale, multi-phase and multi-component aspects, in a process where both transport phenomena and reaction kinetics must be considered. The aim of the work is to investigate the differences between the combustion characteristics of Turkish (Soma lignite, Tunçbilek lignite, Afşin-Elbistan lignite) and German (Rhenish lignite) lignites. Combustion characteristics of these lignites were investigated experimentally and numerically. Experiments were conducted using a high temperature (1000 °C) and high heating rate (~104 °C/s) drop tube furnace (DTF), along with a thermogravimetric analyzer (TGA) at non-isothermal conditions (5, 10, 15, 20 °C/min). Both experimental trials were done in a dry air environment and atmospheric pressure. Additionally, DTF and TGA are the experimental setups used to validate the numerical model used in this work. The numerical part of the study includes the computational fluid dynamic analysis of DTF and the predictive multi-step kinetic model analysis of the fuel particle. TGA experiments showed that fuel ratio has an effect on the ignition times. Moreover, maximum reaction rates obtained by TGA experiments were inversely proportional to the ash contents of the fuels used. High heating rate DTF experiments showed similar combustion behaviours with TGA experiments. According to DTF experiments, RL has the highest reactivity (RL: 7.8 s−1) among all fuels (AEL: 5.3, SL: 4.7, TL: 2.9 s−1). In comparison to experimental data, PoliMi model predictions on high-temperature volatile yields are satisfactory with 5–7% errors. PoliMi model overpredicted the devolatilization rates whereas char oxidation rate predictions seem to be lower than the experimental results
Calibration and validation of a comprehensive kinetic model of coal conversion in inert, air and oxy-fuel conditions using data from multiple test rigs
No full textThis work presents detailed information on pyrolysis and char oxidation for a high-volatile Colombian bituminous coal. The investigation includes experiments at low and high particle heating rates, performed in a thermogravimetric analyzer (TGA), a drop-tube reactor (DTR), a flat-flame burner (FFB) and a fluidized-bed reactor (FBR). The TGA and DTR data were used when developing and calibrating the kinetic model for the conversion of coal in air and oxy-fuel atmospheres, while the FFB and FBR data were used to validate the resulting mechanism. The proposed model is an updated version of the CRECK-S-C model from the Politecnico di Milano (PoliMi), consisting of a fuel characterization step, coupled with a multi-step kinetic mechanism based on reference coals. Both the devolatilization and heterogeneous char reactions are accounted for and interconnected seamlessly. Key reactions were introduced and the existing reactions were calibrated to account for the particularities of this fuel and the effects of the abundant CO2 concentration in the reactors. The importance of successive gas-phase reactions was observed and a gas-phase kinetic model was coupled to properly simulate such conditions. The resulting model is applied to simulate and systematically evaluate the experimental findings, highlighting the model's features and limitations
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