Char Oxidation Research Articles

Phase-field model of char oxidation in ablative thermal protection system materials

We develop a phase field model of char oxidation during atmospheric entry in the ablation zone of the Phenolic Impregnated Carbon Ablator (PICA). The phase field model is coupled to a model of heat transport, to capture the interaction between the oxidation and temperature, and it is based on our previous model of carbon fiber oxidation. We explore the char oxidation behavior using three 2D examples. First, we simulate oxidation with an imposed temperature gradient for char structures with increasing amounts of initial porosity. The oxidation time decreases with increasing porosity and the oxidation rate fluctuates as pores are opened to oxygen transport. Next, we predict the decrease in temperature due to the endothermic oxidation reactions in thermally-insulated systems with and without transport of oxygen through the top boundary. In both cases, the temperature decreases until oxidation stops; it stops due to the decrease in temperature in the case open to oxygen transport and due to lack of oxygen in the case closed to oxygen. Finally, we explore the oxidation behavior under different heat fluxes. The oxidation reactions compensate for the heat flux until the char is fully consumed and then the temperature increases. Our results demonstrate the importance of considering the interactions between the oxidation reactions and the temperature when modeling char oxidation.

Insight into the fate of nitrogen during char thermal conversion and the influence mechanism of potassium: A theoretical research

Nitrogen oxides (NOₓ) are primary pollutants produced during biomass combustion. During the devolatilization stage, char nitrogen (char(N)) is formed. In the subsequent char combustion stage, char(N) can decompose directly into NOx precursors or engage in heterogeneous reactions with O2 or NO to form NO and N2. Nonetheless, a comprehensive understanding of the reaction mechanisms and competitiveness of char(N) migration, especially the influence of the alkali metal potassium (K) present in biomass, remains incomplete. Building on the Zigzag char(N) models, the present study delves into the migration reactions of char(N), assessing their competitive dynamics through the integration of density functional theory, electronic structure analysis, and conventional transition state theory. Furthermore, it examines the impact of K on char(N) conversion. The competitiveness of the heterogeneous reactions follows the sequence: heterogeneous reduction of NO to N2 > heterogeneous oxidation of char(N) to NO > decomposition of char(N) to NOx precursors. Moreover, the formation of HCN is more favorable than NH₃ production. The successive conversion from char(N) to NO and then to N2 is the predominant migration route for char(N), with NO generation as the pivotal step. The less preferred char(N) migration route involves decomposition to NH3/HCN, followed by oxidation to NOx within the main combustion zone, which cannot be mitigated by char. K can accelerate NO generation and sustain the primacy of the heterogeneous NO reduction, consequently enhancing the oxidation-reduction process of char(N). As a result, K plays a constructive role in managing NOx emissions during the thermal conversion of char.

Catalytic torrefaction effect on waste wood boards for sustainable biochar production and environmental remediation

Wood boards used in construction are generally treated with toxic chemicals, making them unsuitable for further use and causing environmental pollution. This study evaluates the possibility of using catalytic torrefaction as a pretreatment to improve wood pyrolysis and combustion for greener biochar production. Waste beech boards were impregnated with different K2CO3 solutions (0–0.012 M), then torrefied between 5 and 60 min at 275 °C. The ICP-AES showed that the board's surface held more potassium than the core. Torrefaction coupled with potassium decreased the C–O and –OH stretches. Thermogravimetric analysis of torrefied wood showed that the board's internal heating degraded the core more than the surface. The exothermic reactions made potassium's catalytic action more efficient in the core. Interactions between the potassium content and torrefaction duration decreased the pyrolysis' maximum devolatilization temperature. During combustion, potassium decreased the ignition temperature by up to 9% and 3% at the surface and core, respectively, while the torrefaction increased it. The catalytic torrefaction significantly decreased the devolatilization peak during combustion, thus making the wood's combustion similar to that of coal, having only the char oxidation step. These findings highlight the advantages and challenges of waste wood's catalytic-torrefaction for biochar production to reduce environmental pollution.

Low-Temperature-Solid Combustion Technology of Biomass for Pollution Reduction: Potentials and Necessary Fundamentals.

The high temperature of solid combustion in modern combustors leads to serious pollution in the combustion of biomass and solid wastes. The broad demands of relatively small-scale combustion of solid fuels and the imposition of increasingly strict emission limits require more economical methods for pollution reduction. Based on literature and our own work, low-temperature-solid combustion technology, which applies low temperature for solids but normal temperature for gas during combustion, was introduced in this paper. The potential of this technology in pollution reduction was analyzed, and necessary fundamentals for equipment design/operation were discussed. It showed that, for straws, more than 60% of deposit and particle emission can be reduced and 100% of ash can be recycled when the solid temperature is <600 °C. Fundamentals on rates of inorganic release, char oxidation, phase transformation in the condensed phase, and NOx/SOx release are necessary for better application of the technology.

TG-MS analysis and kinetic study of co-combustion of ca-rich oil shale with biomass in air and oxy-like conditions

In this study, the combustion behavior of ca-rich oil shale, spruce biomass, and their blends under air and oxyfuel environments of 21/79 % and 30/70 % O2/CO2 like conditions were investigated. Non-isothermal thermogravimetric (TG) experiments coupled with a quadrupole mass spectrometer (MS) were conducted to study individual fuels and their blends at 1:0, 4:1, 3:2, 2:3, 1:4, and 0:1 (0, 20, 40, 60, 80 and 100 wt.% Biomass) under three different heating rates 10, 30 and 50°C/min. Co-combustion synergistic along with the kinetic analysis by the isoconversional Friedman method has been carried out to evaluate the combustion process.The results show that the addition of biomass enhanced combustion performance and reduced burnout temperatures. Under oxy conditions, the ignition temperatures stabilized with a biomass ratio > 40 %. Ash content was reduced with biomass addition and when switching from air to oxyfuel combustion, the temperature of blends’ carbonate decomposition was stable at ∼ 720 °C. A positive synergistic effect in the devolatilization and the combustion of light hydrocarbons occurred at higher biomass ratios. Yet, the char oxidation peaks were below zero indicating a negative effect under 21 % of inlet O2. With increasing the heating rate, the negative synergistic effect was weakened for the three combustion atmospheres. SO2 emissions were reduced with increasing biomass ratio and increased under oxy mode along with H2O release. The activation energy (E) was lower in oxy conditions than in air mode for individual fuel and their blends, compared to the OS sample, the reduction was higher with increasing BM ratios and for the BM fuel. Overall, this study determines the appropriate conditions for the co-combustion of oil shale and spruce biomass under air and oxy modes for future carbon-negative capture applications in industrial oil shale combustion boilers.

Understanding the effect of char oxidation on wood temperature profiles for varying heating and oxygen conditions

The use of mass timber framing as a sustainable material, particularly in high-rise buildings, requires detailed structural fire performance calculations. Thermal models describing only the solid phase are cost-effective alternatives to provide information to structural behavior models. Their accuracy depends on an adequate description of drying, pyrolysis, charring and eventually flaming phenomena. While in recent years there have been considerable contributions to the development of such models, there are still open questions. This work proposes a thermal model which incorporates char oxidation, describing both the kinetic- and diffusion-controlled regimes. The model was used to replicate two sets of experimental results which used standard fire calorimeters to study the ignition of thick wood specimens within a range of incident heat fluxes and oxygen concentrations, respectively. The model yields adequate temperature predictions in the early heating stages, but fails to replicate the behavior at later stages, when the effect of the surface combustion is noticeable. In terms of mass loss rates, a poorer performance is observed. To change from one oxidation regime to another, a Damköhler number is proposed, based on char oxidation reaction rates. It is found that for compartment fire conditions, char oxidation will mostly occur develop under diffusion-controlled conditions.

An approach for the improved measurement of pyrolysis products

The composition of a fuel and the combustion environment are factors that impact pyrolysis processes. The pyrolysis of a natural polymer, timber, and the synthetic polymers, polymethylmethacrylate (PMMA) and polystyrene (PS), was performed at a constant mass loss rate (MLR) using a feedback control loop utilising the Fire Propagation Apparatus (FPA) under inert and oxidative environments. The controlled MLR (2 g s−1 m−2) enabled a representative means of monitoring emissions via a combination of spectroscopic techniques. Reported yields were calculated utilising adapted Fourier-transform infrared spectroscopy (FT-IR) data based upon the species detected via gas chromatography-mass spectrometry (GC-MS). The speciation offered by the GC-MS was used to adjust recorded FT-IR yields, facilitating an insight into the composition of pyrolysis emissions. The presence of molecular oxygen was not found to drastically influence the decomposition behaviours of the investigated synthetic polymers with the yields of primary monomers being relatively unaffected by the atmospheric environment. However, a difference was noted when using timber, as the generated char was visually different under the oxidative atmospheres. The presence of char oxidation was confirmed as a lower applied heat flux was required to sustain the desired mass loss rate. We conclude that the proposed combined analytical approach shows great potential for tracking and quantifying effluent emissions.

Combustion and flue gas evolution characteristics of rice husk in various mixtures and layers with Al2O3 particles and char prepared with different pyrolysis parameters

During pyrolysis and combustion experiments, rice husk and Al2O3 were combined in two ways, namely blending (rice husk was blended with Al2O3) and covering (rice husk was covered by Al2O3) modes, respectively. Rice husk biomass (RHB) char was prepared under different pyrolysis conditions. The resulting combustion characteristics and corresponding gaseous evolution of the biochar were compared. The maximum combustion rate decreased as particles accumulated, causing a shift in the thermo-gravimetric curve to higher temperature ranges. The combustion reaction was hindered in the covering mode. The combustion reactivity of the prepared char decreased as the char preparation time increased. During the char oxidation process, the release amounts of H2O and CO2 from char combustion increased first and then decreased, while the release amounts of CO, CH4, and organic components containing C=O gradually decreased with increasing char preparation time. Char prepared with the covering mode exhibited higher burn-out rates and combustion indices but lower activation energies required for combustion reaction. Additionally, the covering mode delayed the timely release of gases from the biomass heating in air, and the quantity of combustion gas released from the char produced at covering mode was greater than that released from the char produced at blending mode. The results obtained can improve the understanding of stacked biomass particles combustion process.

Investigation on axial evolution of particle group and conversion of a single coal particle in an impinging entrained-flow gasifier

Based on a bench-scale impinging entrained-flow gasifier and visualization system, the conversion of particle group and a single coal particle in the impinging-flow zone below the burner plane is investigated in-situ. The axial evolution of particle group including particle size, particle concentration, velocity field and spatial distribution is characterized with image processing algorithm and statistical method. The particle fragmentation, single particle conversion process and volatile oxidation behavior are discussed. The results indicate that the Sauter mean diameter (SMD) is distributed in 560–720 µm in all statistical regions, and the particle concentration is distributed in 20,000–360,000. With the axial downward movement of the particle group, the SMD, particle concentration and spatial distribution frequency show an overall decreasing trend. The proportion of particles ranging in the velocity interval of 0–4 m/s increases in the three impinging-flow zones, which are 85.76 %, 92.33 % and 97.88 %, respectively. The fragmentation mode of a single coal particle can be classified into central fragmentation and peripheral fragmentation. The occurrence probabilities of the two modes in the upper impinging-flow zone are more than one-third, indicating that the particles tend to be fragmented near the burner plane. The conversion of a single coal particle can be divided into heat up and pyrolysis, volatile oxidation, volatile/char oxidation and char oxidation. In the upper impinging-flow zone, less than 30 % of the particles are in volatile oxidation stage, while about 50 % of the particles are in char oxidation stage. With the axial downward movement of the particle group, the proportion of particles in char oxidation stage decreases, while the proportion of particles in volatile oxidation stage shows an opposite trend. A macroscopic description of ignition and propagation of particle volatile flame is proposed. The propagation of volatile flame between singles particles can be extended to the propagation of volatile flame within particle groups.

The catalytic effect of iron oxide phases on the conversion of cellulose-derived chars in diluted O2 and CO2

The conversion of biomass-derived char is substantially influenced by its metal content. One of the main catalytically active metallic elements in biomass is Fe, which occurs in various mineral forms. For the implementation of catalytic effects into char conversion models, investigations on the role of mineral type and loading are required. In this work, the catalytic effect of an Fe sulfate loading series on the oxidation and gasification of an inherently mineral-free cellulose-derived char was analysed. Characterisation focused on the Fe phases present in the char identifying the transformation from FeSO4 to γ-Fe2O3 during doping, and further to ε-Fe2O3 and α-Fe2O3 upon char oxidation as well as to FeO and γ-Fe upon char gasification. Very high loading-dependent activities of ε-Fe2O3 and FeO were quantified by kinetic modelling. These iron oxides strongly catalyse char conversion, lowering the activation energy by up to 14% and 18%, respectively, relative to the mineral-free char.

Unfolding of Lignin Structure Using Size-Exclusion Fractionation.

The heterogeneous and recalcitrant structure of lignin hinders its practical application. Here, we describe how new approaches to lignin characterization can reveal structural details that could ultimately lead to its more efficient utilization. A suite of methods, which enabled mass balance closure, the evaluation of structural features, and an accurate molecular weight (MW) determination, were employed and revealed unexpected structural features of the five alkali lignin fractions obtained with preparative size-exclusion chromatography (SEC). A thermal carbon analysis (TCA) provided quantitative temperature profiles based on sequential carbon evolution, including the final oxidation of char. The TCA results, supported with thermal desorption/pyrolysis gas chromatography-mass spectrometry (TD-Py-GC-MS) and 31P NMR spectroscopy, revealed the unfolding of the lignin structure as a result of the SEC fractionation, due to the disruption of the interactions between the high- and low-MW components. The "unraveled" lignin revealed poorly accessible hydroxyl groups and showed an altered thermal behavior. The fractionated lignin produced significantly less char upon pyrolysis, 2 vs. 47%. It also featured a higher occurrence of low-MW thermal evolution products, particularly guaiacol carbonyls, and more than double the number of OH groups accessible for phosphitylation. These observations indicate pronounced alterations in the lignin intermolecular association following size-exclusion fractionation, which may be used for more efficient lignin processing in biorefineries.

A computational fluid dynamics model to estimate local quantities in firebrand char oxidation

Firebrand burning is a complex phenomenon that is influenced by several parameters which are difficult to fully explore experimentally. Computational fluid dynamics models capable of predicting local quantities are essential for accurate prediction of char oxidation in firebrands. This article presents a computational fluid dynamics model to estimate firebrand mass loss, diameter change, and surface temperature during char oxidation. The model was validated using previously conducted wind tunnel experiments. These experiments were conducted for firebrands of two different aspect ratios, which were arranged in three different configurations (single, horizontal array, and vertical array), and for four different wind speeds (0.5, 1, 1.5, and 2 m/s). The computational fluid dynamics results were compared with a previous 1 D model. In all the test cases, the computational fluid dynamics model predicted the physical phenomena with significantly improved accuracy compared to a 1 D model. The char oxidation model presented in this article can be coupled with other models to study firebrand generation and trajectory, biomass pyrolysis, fluidized bed reactors, and coal combustion.

Measurement of char oxidation rate of glue laminated timber of Japanese cedar and larch under forced air flow

Char oxidation during the post-fire cooling period, which generates substantial heat is one of the important factors in achieving the self-extinguishment required for wooden structural members in high-rise or large-scale buildings. Here, the char oxidation rates of Japanese cedar glue laminated timber were measured at various heating intensities using a cone calorimeter. The critical conditions for stopping of char oxidation were a heating intensity of 6.0 kW/m2 and surface temperature of 250.0 °C, indicating that Japanese cedar continues to char oxidation at lower temperatures than larch. An equation relating the mass loss rate to the surface temperature of larch was developed for Japanese cedar. The effect of a forced airflow over the surface of the specimen on the char oxidation was also investigated. When a forced airflow of 1.1 m/s was applied to specimen surface, the mass loss rate increased compared with that without a forced airflow; however, the surface temperatures were similar to or slightly greater than those without a forced airflow.

Numerical simulations of flame spread in pine needle beds using simple thermal decomposition models

Computational fluid dynamics (CFD) models have increased in use for studying scenarios relevant to wildland fires, such as examination of the driving processes in flame spread in vegetative fuels. However, these tools utilize a complex set of submodels which require a large number of input parameters. Often the full set of fuel-specific parameters are not well-quantified and the user must rely upon the best available information. In this study, we examine the implications of using different simple models for thermal decomposition when simulating flame spread through a bed of dead pine needles in quiescent conditions. Model results using one set of common literature values are compared to those using data from milligram-scale characterizations of the fuel. An updated model for char oxidation is also implemented and tested. It was found that the literature values over-predicted mass loss rate by a factor of 2.4, while the fuel-specific values yielded predictions within the experimental uncertainty. A simple approach to decomposition modeling was also shown to be useful for investigating the role of bed structure on flame spread and the heat flux within the fuel bed.

Durability and fire resistance of compressed wheat-straw (triticum aestivum) panels subjected to real-world aging environments

This paper presents an investigation into the durability of cereal straw-building materials after fifty years of use in the Australian climate. Unfortunately, the performance and durability of natural materials such as cereal straw have historically been negatively preconceived compared to their synthetic counterparts. Investigation on their potential application is needed to understand the sustainability of both the agricultural and building and construction industries. Using a compressed wheat straw (triticum aestivum) insulation panel, the effect of half a century of real-world service was assessed at the stem and panel scales. The investigation at the stem scale indicated no change to composition or degradation to microstructure, which correlated to full retainment of the panels' functional properties. The mean stem breaking force for the aged straw was measured to be within +15% of the pristine straw. At the panel scale, the mechanical properties and insulative performance were found to be statistically unaffected by the real-world aging. The compression strength and thermal conductivity of the aged panel were within +7% of the pristine panel, while the flexural strength was subject to a larger difference of +35% due to manufacturing methodology changes over last fifty years. The panels resistance to fire was also found to be maintained with each panel showing nearly identical through-the-thickness penetration of the thermal, char (+4%) and char oxidation (−10%) fronts after two hours of heat exposure. The panels were further revealed to generate a very stable char phase which contributed to the excellent fire resistance.

Extension of the layer particle model for volumetric conversion reactions during char gasification

The so-called “layer model” or “interface-based model” is a simplified single particle model, originally developed for shorter computation time during computational fluid dynamics (CFD) simulations. A reactive biomass particle is assumed to consist of successive layers, in which drying, pyrolysis and char conversion occur sequentially. The interfaces between these layers are the reaction fronts. The model has already been validated for drying, pyrolysis and char oxidation. Layer models in the literature have commonly employed surface reactions at the reaction front to describe char conversion. In this work, the suitability of this surface reaction concept is assessed when gasifying biochar. It is shown that a particular layer model, already available, which originally employed surface reactions, was unable to adequately describe the mass loss during gasification of a biochar. In order to overcome this incapability, the model was extended to consider volumetric reactions in the char layer. The influence of intraparticle diffusion was considered through an effectiveness factor. The model is easily adaptable for different gas-solid kinetic rate laws, while still allowing for comparably fast solutions of the model equations. The extended model was validated using theoretical calculations and experimental measurements from literature. It was demonstrated that intraparticle diffusion can significantly slow down the biochar gasification process. A general guideline for when to employ volumetric reactions, rather than surface reactions, and when to consider intraparticle diffusion is provided based on the Thiele modulus as the criterion.

Theoretical analysis and experimental verification of diminishing the diffusion influence on determination of char oxidation kinetics by thermo-gravimetric analysis

Theoretical analysis and experimental verification of diminishing the diffusion influence on determination of char oxidation kinetics by thermo-gravimetric analysis

Studying the mechanism and impact of H2O-rich atmosphere on N oxidation during ammonia combustion and ammonia-coal co-combustion

Carbon-free fuel ammonia and coal co-combustion, an effective way to reduce thermal power CO2 emissions, has attracted more and more attention in recent years. Due to the hydrogen-rich property of ammonia, more H2O is generated during combustion, and the high H2O atmosphere significantly affects the NOx formation. Therefore, it is necessary to explore the oxidation path of N in the combustion of ammonia and its coupling coal in the H2O atmosphere. In this study, quantum chemistry theory and transition state theory are used to investigate the influence mechanism of H2O atmosphere on the oxidation of ammonia and fuel-N in the ammonia-coal co-combustion. The results show that the presence of OH in the H2O atmosphere promotes the formation of NH2 and NH, which is conducive to their oxidation to HNO and NO. The condition without OH is conducive to the oxidation of ammonia to HNO, and the presence of OH promotes the further oxidation of HNO to NO in the co-combustion of ammonia and coal. The H2O atmosphere reduces the energy barrier value of rate-determing step for ammonia and coal char oxidation during co-combustion, and increases the oxidation rate of ammonia-N and char-N. This work reveals the molecular mechanism by which H2O atmosphere promotes the combination of OH and O2 to generate HO2, and discovers the temperature conditions for HO2 formation. This study provides theoretical support for the development of N oxidation mechanism in the ammonia-coal co-combustion.

Biomass dust explosions: CFD simulations and venting experiments in a 1 m3 silo

This study presents CFD simulations of biomass dust explosions in a newly developed experimental 1 m3 silo apparatus with variable venting, designed and fabricated to operate similarly to the explosivity test standards. The aim of the study is to validate a CFD model under development and investigate its capability to capture the transient effects of a vented explosion. The model is based on OpenFOAM and solves the multiphase (gas-particle) flow using an Eulerian-Lagrangian approach in a two-way regime. It considers the detailed thermochemical conversion of biomass, including moisture evaporation, devolatilization, and char oxidation, along with the homogeneous combustion of gases, turbulence, and radiative heat transfer. The explosion is analyzed in all stages, i.e., dust cloud dispersion, ignition, closed explosion, and vented explosion. The results indicate excellent agreement between the CFD model and experimental tests throughout the sequence. Our findings highlight the critical role of particle size in dust cloud distribution and pre-ignition turbulence, which significantly influences flame dynamics and the explosion itself. This model shows great promise and encourages its application for future investigations of biomass dust explosions in larger-scale geometries, especially in venting situations that fall out of the scope of the NFPA 68 or EN 14491 standards, and to help design effective safety measures to prevent such incidents.

Increasing CO2 gasification rates of longan-seed char by a technique of char pre-oxidation

A simple and effective technique was presented to increase the longan-seed char gasification rate during CO2 activation by pre-oxidizing the char with air. The kinetics of char gasification was studied in a thermogravimetric analyzer (TGA) for the temperatures of 850–950 °C for char samples being oxidized in a tube furnace at the temperatures of 250, 280, and 300 °C. It was found that the gasification rate of the oxidized char was about twice higher than that of the non-oxidized char. The increase in the char reactivity for CO2 gasification is due to the increasing number of reaction sites caused by the disruption of functional group bonding created during the oxidation step. Additionally, the increase in the oxidation temperature gave rise to the increase in the char reactivity but the maximum temperature was limited by the ignition temperature of the longan-seed char. Four gasification kinetic models were tested against the experimental rate data and it was discovered that the modified volume reaction model (MVRM) provided the best description of the gasification conversion. The pre-exponential factor and the activation energy of the gasification reaction were determined from the Arrhenius equation. The estimated activation energy was found to be in the range of 156.87–218.98 kJ/mol and decreased with the increase of char oxidation temperature.