Char Density Research Articles

Novel insight into calcium-catalyzed steam gasification behaviors of lignite

Lignite is a low-rank coal with abundant reserves, and direct combustion causes serious environmental pollution. Calcium-catalyzed steam gasification is a cleaner technology that can convert lignite into high-value fuels and chemicals. However, the detailed structural evolution of calcium species during lignite steam gasification remains inadequately understood. The present work addresses this issue by subjecting the calcium-catalyzed steam gasification process of lignite to a detailed analysis, focusing on both the pyrolysis and gasification stages. The transitions in the chemical structures of the lignite and calcium catalysts during the pyrolysis stage were investigated in situ by Fourier Transform Infrared Spectroscopy. The morphological and chemical characterization of chars was performed using SEM-EDS, Raman spectroscopy, XPS, and N2 adsorption/desorption. The catalytic performance and gas composition were investigated by TGA and a fixed-bed reactor during the gasification stage. The results verify that the calcium catalyst reduced the initial gasification temperature of lignite by 150 °C, resulting in a 13.91% increase in the H2 concentration of the synthesis gas and an improvement of the H2/CO ratio to 20.96. The active Ca-C site and intermediate Ca-O-C formed during the co-pyrolysis stage increase the defect density of char, and the transformation of organic calcium species catalyzes the gasification of lignite. The enhanced concentration of H2 in the syngas can be attributed to the catalytic effect of CaO on the water–gas shift reaction during the gasification process. These results are expected to guide efforts in supporting catalytic conversion processes and promoting the efficient utilization of low-rank coal.

Firebrand burning under wind: an experimental study

Background Spot fires play a significant role in the rapid spread of wildland and wildland–urban interface fires. Aims This paper presents an experimental and modelling study on the flaming and smouldering burning of wood firebrands under forced convection. Methods The firebrand burning experiments were conducted with different wind speeds and firebrand sizes. Key results The burning rate of firebrands under forced convection is quantified by wood pyrolysis rate, char oxidation rate and a convective term. The firebrand projected area is correlated with firebrand diameter, char density, wind speed, and flaming or smouldering burning. A surface temperature model is derived in terms of condensed-phase energy conservation. We finally establish a simplified firebrand transport model based on the burning rate, projected area and surface temperature of firebrands. Conclusion The mass loss due to wood pyrolysis is much greater than that due to char oxidation in self-sustaining burning. The burning rate is proportional to U1/2, where U is wind speed. The projected area for flaming firebrands decreases more rapidly than that for smouldering ones. The firebrand surface temperature is mainly determined by radiation. Implications Knowledge about firebrand burning characteristics is essential for predicting the flight distance and trajectory in firebrand transport.

A computational approach to heat transfer and ablation in space capsule insulation

In this article, the heat transfers and ablation phenomena of thermal insulations utilized in manned space capsules are investigated. In this regard, by collecting and solving the equations related to ablation insulations, a computer program has been developed that is able to predict the thermal response of these insulations under various operating conditions. This approach considers mass and heat transfer equations in two-dimensional solid bodies. To solve these equations, finite volume method and implicit formulation for time dependence have been used. The reaction equation, written in the form of Arrhenius, is solved using the Runge-Kutta method, and the density and the flux of the gas produced at each step are obtained. Furthermore, a practical model with low computational requirements and execution time and execution time is presented to consider the regression rate based on determining the identifier for destroyed cells, charred cells, and virgin material. The research results demonstrate that increasing the thickness of the layers, the heat of decomposition and ablation, the intensity of the reaction, and reducing the thermal diffusion coefficient, density of char, and temperature of ablation enhance the efficiency of thermal insulation. Validation of the model with experimental results in silica-phenolic insulation reveals a good agreement between simulation and experimental observations. Additionally, it was shown that for a carbon-phenolic insulation with a thickness of 5 mm in the specified space capsule under the given conditions, the maximum recession rate is 1.9 mm at a maximum temperature of 1000 K and 4.9 mm at a maximum temperature of 1300 K.

Use of photogrammetry to determine spruce and OSB char bulk density

AbstractThis paper deals with the bulk density determination of char, which is an important input parameter for numerical simulations of pyrolysis. The aim was to develop a time and cost‐effective, non‐destructive, and repeatable method capable to determine the bulk density of highly porous, fragile wood and engineered wood samples of various shapes, namely char of spruce and oriented strand board. Char samples were prepared in a cone calorimeter under various heat flux exposures. Photogrammetry was chosen for this purpose. A detailed photogrammetric measurement procedure consisting of sample preparation, sample measurement and software reconstruction was designed and validated. It was found that the bulk density of char depends on the virgin material and the way char was formed. The spruce and oriented strand board char bulk density value is provided to be used in pyrolysis modeling and compared to literature.

Flame heights and charring on a particle board – An experimental study

Vertically oriented particle-board samples were exposed to external venting flames to study the fire spread and charring behaviour along a timber façade. Variation in flame height and the height, volume, area, density, and depth of the char layer were studied to determine the impact of heat-release rate and experiment duration. There was a peak flame height after which the flame returned to steady height approximately equal to the value before the ignition of the particle board and flame heights with inert panels. Flames did not spread to the top of the panel with increased experiment duration. Char height and area were found to increase with heat-release rate but were not affected significantly by experiment duration. Char depth and volume increased with both experiment duration and heat-release rates. Char density decreased with increased experiment duration and heat-release rate.

Preparation of boron nitride hybrid containing phosphorus and silicon and its effect on flame retardant, smoke suppression, and thermal conductivity properties of styrene‐butadiene rubber

AbstractIn this study, a nanofiller (M‐DOPO‐BN) was synthesized by surface modification of hexagonal boron nitride with vinyltrimethoxysilane (KH‐171) and 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO), and compounded with aluminum diethyl phosphinate (ADP) to study their effects on flame retardant, smoke suppression and thermal conductivity properties of styrene‐butadiene rubber (SBR). The results showed that the composites with an ADP content of 20 parts per hundred of rubber (phr) have good flame retardant properties, but the total smoke production increased significantly. The flame retardant properties of SBR composites compounded with 3 phr M‐DOPO‐BN and 17 phr ADP were not only further improved, but also the peak carbon monoxide production rate (pCOP) and peak carbon dioxide production rate (pCO2P) were reduced by 37.0% and 67.5%, respectively, compared with pure samples, showing a good smoke inhibition effect. In addition, the thermal conductivity of the composites have also been improved to a certain extent. The variable temperature infrared and residual char analysis of SBR composites showed that the reasons for its flame retardancy and smoke suppression are that M‐DOPO‐BN can catalyze the formation of carbon, and the silica and boron nitride generated by decomposition during combustion can improve the density of residual char.

POSS‐modified ammonium polyphosphate for improving flame retardant of epoxy resins

AbstractAn innovative phosphorus‐silicone flame retardant (KAPP‐OGPOSS) was prepared by grafting octa (propyl glycidyl ether) polyhedral oligomeric silsesquioxane (OGPOSS) on ammonium polyphosphate (APP) through silane coupling agent (KH‐550). The chemical structure, morphology, flame retardancy, and mechanism of the obtained materials were comprehensively investigated. TG results demonstrated that EP with 15 wt% KAPP‐OGPOSS produced a char residue of 42.6% at 700°C, which is three times that of pure EP. The flame retardant tests showed that the limiting oxygen index value of EP/15 wt% KAPP‐OGPOSS was 30.6% (67.7% higher than that of neat EP) and its UL‐94 grade reached to V‐0 rating. Cone calorimeter testing indicated that the incorporation of 15 wt% KAPP‐OGPOSS to EP significantly reduced heat, smoke, CO, and CO2 release by 79%, 83%, 80%, and 82.5%, respectively. The modification of APP with OGPOSS was also beneficial for the improvement of the char yield and density of char. A flame retardant mechanism was revealed according to the investigation into the char residue. This modified approach provided an innovative yet practical means of enhancing the flame retardant properties of EP.

Investigating the morphological and genetic divergence of arctic char (Salvelinus alpinus) populations in lakes of arctic Alaska.

Polymorphism facilitates coexistence of divergent morphs (e.g., phenotypes) of the same species by minimizing intraspecific competition, especially when resources are limiting. Arctic char (Salvelinus sp.) are a Holarctic fish often forming morphologically, and sometimes genetically, divergent morphs. In this study, we assessed the morphological and genetic diversity and divergence of 263 individuals from seven populations of arctic char with varying length‐frequency distributions across two distinct groups of lakes in northern Alaska. Despite close geographic proximity, each lake group occurs on landscapes with different glacial ages and surface water connectivity, and thus was likely colonized by fishes at different times. Across lakes, a continuum of physical (e.g., lake area, maximum depth) and biological characteristics (e.g., primary productivity, fish density) exists, likely contributing to characteristics of present‐day char populations. Although some lakes exhibit bimodal size distributions, using model‐based clustering of morphometric traits corrected for allometry, we did not detect morphological differences within and across char populations. Genomic analyses using 15,934 SNPs obtained from genotyping by sequencing demonstrated differences among lake groups related to historical biogeography, but within lake groups and within individual lakes, genetic differentiation was not related to total body length. We used PERMANOVA to identify environmental and biological factors related to observed char size structure. Significant predictors included water transparency (i.e., a primary productivity proxy), char density (fish·ha‐1), and lake group. Larger char occurred in lakes with greater primary production and lower char densities, suggesting less intraspecific competition and resource limitation. Thus, char populations in more productive and connected lakes may prove more stable to environmental changes, relative to food‐limited and closed lakes, if lake productivity increases concomitantly. Our findings provide some of the first descriptions of genomic characteristics of char populations in arctic Alaska, and offer important consideration for the persistence of these populations for subsistence and conservation.

Numerical modeling of a batch fluidized-bed gasifier: Interaction of chemical reaction, particle morphology development and hydrodynamics

The present study aims to analyze the interaction of chemical reactions, particle morphology change and hydrodynamics in fluidized-bed gasifiers. The char gasification in a batch fluidized-bed gasifier was modeled. The two-fluid model approach was coupled with an extended version of the uniform conversion model accounting for inert ash fraction. Model assumptions were confirmed via measurement of char density, porosity and particle size distribution at different stages of conversion in a lab-scale fluidized-bed gasifier. The CFD model correctly predicts the effects of particle structure change on chemical reactions, approved by a first of its kind comprehensive validation against unsteady experiments. Based on the validated CFD model, the interaction of hydrodynamics and local particle density variations due to chemical reactions is analyzed, and the effect of the initial bed height is studied.

The morphology evolution of char particles during conversion processes

The change in a particle’s morphology affects both its trajectory and its carbon consumption rate. For that reason, the present work studies the evolution of particle’s morphology during the char conversion process. Particle-resolved transient CFD calculations were performed to estimate changes in particle density and volume. The complex physical and chemical processes taking place in the vicinity and the interior of the char particle were studied and discussed in detail for different process conditions and reaction regimes. The char conversion study confirmed the theoretical correlation between the char diameter and the char density exponent. New expressions were discovered for both exponents α and β, depending on the effectiveness factor η and the stage of carbon conversion XC. In addition, the Random Pore Model, which describes the development of the specific surface area of the particle, was extended to include an empirical correlation γ, making it valid for different reaction regimes and operating conditions.

Numerical Simulation of the Burning Process in a King-Size Cigarette Based on Experimentally Derived Reaction Kinetics

Summary A comprehensive two-dimensional (2D) mathematical model has been proposed to simulate the burning process of a king-size cigarette. The characteristics of this model are including: 1) the use of kinetic models for the evaporation of water, the pyrolysis of tobacco and the oxidation of char, 2) the application of mathematical relationships between the release amounts of certain products (i.e., “tar” and CO) and different reaction variables (i.e., temperatures and oxygen concentrations), 3) the introduction of mass, heat and momentum transports, 4) the consideration of filtration effects of the cigarette filter on “tar”. These characteristics were expressed in a set of coupled equations that can be solved numerically by FLUENT. The information about the char density field, temperature field, flow velocity field, “tar” and CO density fields and the filtration efficiency could be obtained from the model. This model was validated by comparing the predictions with experimental data on puff number, the temperatures at specific locations, the filtration efficiency and the yields of “tar” and CO under different puff intensities. The calculated results show a good agreement with the experimental data. The predicted puff number was 7.3, and the experimental puff number was 6.8. The standard root mean square error (NRMSE) between the experimental and the predicted temperatures at specific locations is < 18%. The predicted filtration efficiency for “tar” was 46.1%, and the experimentally determined filtration efficiency for nicotine was 44.5%. The maximum relative deviations of the yields of “tar” and CO under different puff intensities were 8.9% and 10.6%, respectively.

Electrostatic-Interaction-Driven Assembly of Binary Hybrids towards Fire-Safe Epoxy Resin Nanocomposites.

Manganese dioxide (MnO2), as a promising green material, has recently attracted considerable attention of researchers from various fields. In this work, a facile method was introduced to prepare binary hybrids by fabricating three-dimensional (3D) zinc hydroxystannate (ZHS) cubes on two-dimensional (2D) MnO2 nanosheets towards excellent flame retardancy and toxic effluent elimination of epoxy (EP) resin. Microstructural analysis confirmed that the morphologies and structures of MnO2@ZHS binary hybrids were well characterized, implying the successful synthesis. Additionally, the morphological characterization indicated that MnO2@ZHS binary hybrids could achieve satisfactory interfacial interaction with the EP matrix and be well dispersed in nanocomposites. Cone calorimeter test suggested that MnO2@ZHS binary hybrids effectively suppressed the peak of heat release rate and total heat release of EP nanocomposites, performing better than MnO2 or ZHS alone. Condensed-phase analysis revealed that MnO2@ZHS binary hybrids could promote the char density and graphitization degree of char residues and thereby successfully retard the permeation of oxygen and flammable gases. Moreover, through the analysis of gas phase, it can be concluded that MnO2@ZHS binary hybrids could efficiently suppress the production of toxic gases during the degradation of EP nanocomposites. This work implies that the construction of 2D/3D binary hybrids with an interfacial interaction is an effective way to fabricate high-performance flame retardants for EP.

Effects of FGR and Changeable Combustion Parameters and Coal/Char Properties on the Formation of Ultrafine PMs during Pulverized Coal Char Combustion under Various O2/N2 and O2/CO2 Atmospheres

ABSTRACTAiming at the wide adoption of flue gas recirculation (FGR) in coal power plants for NOx removal and CO2 capture, and the imperfect removal of particulate matters (PMs), especially ultrafine PMs which constitute most of the particles discharged into atmosphere, consequently causing atmospheric haze and diseases, the effects of FGR and combustion parameters, and coal/char properties on PMs formation were therefore studied under different O2/N2 and O2/CO2 atmospheres using a self-developed kinetic model. Modeling results showed that the inclusion vaporization rate and ratio had a dominant effect on PMs number density and size respectively. At the same level of oxygen content, smaller ultrafine PMs were generated under O2/CO2 than those generated under O2/N2. Increasing oxygen content facilitated the formation of larger and fewer PMs. The PMs size in 27O2/73N2 shows the biggest, followed by that in 36O2/64CO2, 27O2/73CO2, 21O2/79N2, and 21O2/79CO2 in turn. Compared to without FGR, FGR decreased the size of ultrafine PMs and increased the number, thus hindering PMs removal. With increasing FGR ratio, which had the most significant effect, the ultrafine PMs size decreased due to dilution effect. With increasing dust removal efficiency, the ultrafine PMs size decreased initially and then increased, peaking close to 100% of dust removal efficiency. The effect of recirculated particle size was negligible. Sensitivity analyses revealed that furnace gas temperature, inclusion size, and char density were the most influential factors on inclusion vaporization and PMs formation. The effects of CO/CO2 ratio, char load, coal ash content, and inclusion content were positive, while the effects of char density and ash content, inclusion size, and moisture were negative. High furnace temperature increased vaporization, but significantly decreased the PMs size. The number density of ultrafine PMs followed mass conservation. These results provide meaningful guidelines for practical applications.

Experimental Validation of a Solid-Phase Model for Wood Ignition and Burning

An unsteady one-dimensional model for the solid phase is applied to simulate the spontaneous ignition and burning of thick wood samples with grain orientation parallel or perpendicular to incident heat fluxes in the range 18-40kW/m2, that is, in the absence of flame. The description of heat and mass transfer processes, at constant gas pressure, is combined with global volumetric rates of wood decomposition and char oxidation. Surface regression occurs for a limit value of char density while a critical surface temperature describes ignition. Good quantitative predictions are obtained for the ignition times and the surface temperature and mass loss rate profiles during burning. Conversion always consists of three main stages. The first short transients correspond to the formation of a relatively thin charred surface layer and glowing ignition. The second much longer stage represents a pseudo-steady state burning, where the rates of advancement of the decomposition and oxidation zones are approximately const...

Char particle burning behavior: Experimental investigation of char structure evolution during pulverized fuel conversion

The present work provides experimental results on structural changes of burning char particles derived in an entrained flow reactor. Changes in the char particle diameter, apparent and true density, specific surface area, and porosity are studied at different residence times using a bituminous coal with a moderate ash content of 11.8wt%. Results show the significance of mineral matter on structural properties of samples extracted along the combustion. The apparent density of a sample composed of mineral and organic matter increases progressively with proceeding char conversion, whereas the isolated char density decreases. The mean particle diameter decreases during conversion, indicating regime II conditions, with pore diffusion being the rate-controlling step. The decrease in char density and diameter follow well-known power law expressions, where the exponents are shown to be dependent on the effectiveness factor, and, thus the reaction regime. However, an ideal burning behavior in regime III, where the diffusion of oxidant through the boundary layer is rate-controlling, is not observed. Instead, large openings in the char structure enable the penetration of oxidant into the particle interior, in particular for highly porous cenospherical char particles. Hence, it is recommended to consider large openings and voids in the char structure and their changes, as well as mineral matter effects in advanced char combustion models.

High temperature gasification of high heating-rate chars using a flat-flame reactor

The increasing interest in gasification and oxy-fuel combustion of biomass has heightened the need for a detailed understanding of char gasification in industrially relevant environments (i.e., high temperature and high-heating rate). Despite innumerable studies previously conducted on gasification of biomass, very few have focused on such conditions. Consequently, in this study the high-temperature gasification behaviors of biomass-derived chars were investigated using non-intrusive techniques. Two biomass chars produced at a heating rate of approximately 104K/s were subjected to two gasification environments and one oxidation environment in an entrained flow reactor equipped with an optical particle-sizing pyrometer. A coal char produced from a common U.S. low sulfur subbituminous coal was also studied for comparison. Both char and surrounding gas temperatures were precisely measured along the centerline of the furnace. Despite differences in the physical and chemical properties of the biomass chars, they exhibited rather similar reaction temperatures under all investigated conditions. On the other hand, a slightly lower particle temperature was observed in the case of coal char gasification, suggesting a higher gasification reactivity for the coal char. A comprehensive numerical model was applied to aid the understanding of the conversion of the investigated chars under gasification atmospheres. In addition, a sensitivity analysis was performed on the influence of four parameters (gas temperature, char diameter, char density, and steam concentration) on the carbon conversion rate. The results demonstrate that the gas temperature is the most important single variable influencing the gasification rate.

Computational modeling of oxy-coal combustion with intrinsic heterogeneous char reaction models

This paper simulated single char particle conversion and oxy-coal combustion in furnaces using modified intrinsic char reaction models that accounted for CO2 gasification. Firstly, modified char reaction models were exhibited, which encompassed intrinsic reaction mechanism, optimized ratio of primary char oxidation products, specific surface area and char density submodel, and effectiveness factor for reactants' diffusion. Then, simulations for single char particle conversion were conducted to explore the sensitivities of model parameters, and also to analyze the effects of conversion fraction on reaction rate and reactants' diffusion; the results showed that the pre-exponential factor, tortuosity factor, and roughness factor had the largest effect on reaction rate, meanwhile, oxidation rate and O2 diffusion were not greatly affected by conversion fraction until a high value (above 0.9), whereas gasification rate and CO2 diffusion were hardly influenced. Finally, the availability of the modified model was validated by several CFD calculations (maximal deviation of burnout <2% compared with experimental data), and a correlation that related reactivity with coal type was proposed through linear regression of experimental data. Besides, the study of the effects of CO2 gasification on overall temperature and char burnout revealed that the reaction temperature was crucial.

At the forefront: evidence of the applicability of using environmental DNA to quantify the abundance of fish populations in natural lentic waters with additional sampling considerations

Environmental DNA (eDNA) sampling has proven to be a valuable tool for detecting species in aquatic ecosystems. Within this rapidly evolving field, a promising application is the ability to obtain quantitative estimates of relative species abundance based on eDNA concentration rather than traditionally labor-intensive methods. We investigated the relationship between eDNA concentration and Arctic char (Salvelinus alpinus) abundance in five well-studied natural lakes; additionally, we examined the effects of different temporal (e.g., season) and spatial (e.g., depth) scales on eDNA concentration. Concentrations of eDNA were linearly correlated with char population estimates ([Formula: see text] = 0.78) and exponentially correlated with char densities ([Formula: see text] = 0.96 by area; 0.82 by volume). Across lakes, eDNA concentrations were greater and more homogeneous in the water column during mixis; however, when stratified, eDNA concentrations were greater in the hypolimnion. Overall, our findings demonstrate that eDNA techniques can produce effective estimates of relative fish abundance in natural lakes. These findings can guide future studies to improve and expand eDNA methods while informing research and management using rapid and minimally invasive sampling.

Multi-fluid Modeling Biomass Fast Pyrolysis in the Fluidized-Bed Reactor Including Particle Shrinkage Effects

The fast pyrolysis of biomass in a bubbling fluidized-bed reactor was simulated with the multi-fluid model combing the variable particle density and diameter model based on the mass conservation at the particle scale. Different particle shrinkage effects on the reactor performance were investigated through changing the apparent density of char species. The detailed distributions of particle density and diameter in the reactor and entrained out of the system were revealed. The results demonstrate that the reactor performance, including the particle density and diameter distribution, entrainment behavior, biochar composition and yield, and biomass conversion, is dramatically affected by the particle shrinkage effects. The average particle density increases, while the average particle diameter decreases, with the increase of the char density, which means more intense shrinkage. A weaker shrinkage effect leads to stronger entrainment behavior, larger biochar yield and mass fraction of biomass in biochar, and ...

Effect of high-temperature pyrolysis on the structure and properties of coal and petroleum coke

In this study, for the purpose of substituting part of petroleum coke with low ash anthracite to prepare carbon anode used in aluminum electrolysis, the effects of high-temperature pyrolysis on the structure and properties of coal char and petroleum coke were studied at the temperature range of 1000°C–1600°C. Results showed that the carbon crystallite structure of coal char and petroleum coke became more ordered with the increase of the temperature. However, the graphitization degree of coal char was lower than that of petroleum coke at the same pyrolysis temperature. The Brunauer-Emmett-Teller (BET) surface area of coal char decreased with the increase of temperature, whereas the BET surface area of petroleum coke decreased first and then increased. The increase of pyrolysis temperature generally inhibited the gasification reactivity of coal char, whereas the gasification reactivity of petroleum coke was inhibited first and then promoted. Moreover, the increase of pyrolysis temperature led to the rapid decrease of powder resistivity of coal char and petroleum coke. In particular, the powder resistivity of coal char was significantly higher than that of petroleum coke at the same pyrolysis temperature. Additionally, the real density of coal char and petroleum coke showed a different trend with the increase of temperature. Specifically, the real density of the former decreased, whereas that of the latter increased. The causes of the above observations were also discussed. Overall, this study provides further understanding about the differences between coal char and petroleum coke, which will facilitate the preparation of carbon anode from the mixture of coal char and petroleum coke.