Char Structure Research Articles

Evolution of char structure during non-isothermal low temperature pyrolysis of ZhunDong coal by microwave heating: A comparative study with conventional heating

In this paper, the evolution of char structure during the non-isothermal low temperature pyrolysis of ZhunDong coal under microwave (MW) heating was investigated and compared with that under conventional (CV) heating. The results show that from room temperature to 200 °C, the volatile content in coal decreases from 35.6% to 24.0% by MW-heating although the temperature is lower. Meanwhile, FTIR analysis showed that the normalized value of aliphatic CH of coal increases to 3.38, far exceeding the increment of that under CV-pyrolysis. The carboxyl functional group decreases to 0.7. The normalized value of aromatic CH significantly declines to 0.62. Notably, it rises sharply to 1.22 from 200 °C to 400 °C. These results indicated that MW-pyrolysis promote the breaking of aliphatic side chains below 400 °C. However, the release of functional groups and the evolution of carbon microcrystals of coal under MW-heating is slower than that under CV-heating from 400 °C to 700 °C, which should be related to the directional movement of oxygen free radicals under MW-field. But above 700 °C, the pyrolysis rate of coal under MW-heating accelerates. CV-pyrolysis is more conducive to pore development than MW-pyrolysis. The SBET of CV-char at 600 °C is 7.0 cm2 g-1, while that of MW-char is 3.5 cm2 g-1. The combustion performance of MW-char is inferior to that of CV-char.

CO2 Gasification Kinetics and Structural Characteristics of Tri-High Coal Char Prepared at Elevated Temperature.

The combined effects of surface area, pore structure, degree of graphitization, and number of carbon functional groups on the gasification kinetics of tri-high coal char prepared at an elevated temperature were studied by a thermogravimetric analyzer and various characterization methods [scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) theory, X-ray diffraction (XRD), Raman spectroscopy, and Fourier transfer infrared spectroscopy]. In the kinetic analysis, the CO2 gasification of high-ash coal was not adequately described by the random pore model because ash influenced the char structure during pyrolysis and gasification. Meanwhile, the SEM and BET results indicated the promotion of micropore formation and mesopore expansion of the coal char during pyrolysis. The XRD, Raman, and FTIR results evidenced a significant increase of large aromatic groups during the pyrolysis process, attributable to the cracking of aliphatic groups and the polycondensation of the cracking residues. Overall, the porous structure and aromatic groups of coal char developed during the pyrolysis process improved the CO2 gasification kinetics of the tri-high coal char.

Pyrolytic Transformation of Indigenous Biomass Wastes into Biochar: An Insight into Char Structure and Physicochemical Characteristics

Pyrolytic Transformation of Indigenous Biomass Wastes into Biochar: An Insight into Char Structure and Physicochemical Characteristics

Pyrolytic Transformation of Indigenous Biomass Wastes into Biochar: An Insight into Char Structure and Physicochemical Characteristics

Pyrolytic Transformation of Indigenous Biomass Wastes into Biochar: An Insight into Char Structure and Physicochemical Characteristics

Improved pyrolysis behavior of ammonium polyphosphate-melamine-expandable (APP-MEL-EG) intumescent fire retardant coating system using ceria and dolomite as additives for I-beam steel application

This study describes the effects of ceria (CeO2) and dolomite [CaMg(CO3)2] additives on the pyrolysis behavior and fire resistive property of conventional intumescent flame retardant (IFR) coating system for I-beam steel substrate called ammonium polyphosphate-melamine-expandable graphite (APP-MEL-EG) system. The fire resistance of various formulations was evaluated using the standard vertical Bunsen burner fire test. Thermogravimetric analysis (TGA) was used to understand the degradation of coating formulations. Observations by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) demonstrated that significant amounts of additives favored the formation of homogeneous compacted char structures, which were predominantly composed of carbon (C), phosphorus (P) and oxygen (O). These three main components of the char were also found to be in various binding combinations with other lighter elements like nitrogen (N) and hydrogen (H) as illustrated by the attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy results. X-ray photoelectron spectroscopy (XPS) further suggest that polyethylene([(CH2–C2H2–CH2)n−]) free radicals were abundant on the char surface for the two best formulations and the binding energy of this radical promoted the formation of aromatic carbon chains that enhanced the char's thermal stability. This means that the selection of appropriate additives and combinations of flame-retardant ingredients could significantly change the morphology of the char layer and improve its thermal stability during fire exposure.

Mini-Review on Char Catalysts for Tar Reforming during Biomass Gasification: The Importance of Char Structure

With the depletion of fossil fuel reserves and the increasing concern about greenhouse gas emission, the need to develop technologies for using renewable and sustainable energy resources is becomin...

Effects of pelletizing conditions on the structure of rice straw-pellet pyrolysis char

The pyrolysis char structure affects gasification reactivity. In this study, the effects of pelletizing conditions on the structural characteristics of straw-biomass-pellet fast pyrolysis char were investigated. Agricultural rice straw was used to prepare cylindrical pellets having a diameter of 8 mm. Pyrolysis at 600 °C was carried out in a fixed-bed reactor. The porosity, pore structure, and carbon crystalline structure of the pyrolysis char were analyzed using the Brunauer-Emmett-Teller (BET) method and Raman spectroscopy. The results demonstrated yield of pellet pyrolysis char is significantly greater than that of raw biomass, with more mesopores and fewer macropores present in the pellet pyrolysis char. Additionally, the char yield of the pellets made of <88 μm straw particles was higher than that of the pellets with other particle size ranges. The pellet char had the largest specific surface area and showed greater carbon disorder when the biomass particle size was 150–250 μm. Moreover, the effects of pelletizing pressure on char yield, porosity, and carbon structure were insignificant.

Flame retardant effect and mechanism of nanosized NiO as synergist in PLA/APP/CSi-MCA composites

Abstract Intumescent flame retardants (IFRs) are widely used in preparing functional polymer composites , but it is still a challenge to improve their flame retardant efficiency. Herein nanosized nickel oxide (NiO) as synergist was incorporated into poly( l -lactic acid)/ammonium polyphosphate/silicone-containing macromolecular charring agent (PLA/APP/CSi-MCA) composites with the aim of improving their flame retardancy. According to limited oxygen index (LOI), vertical burning (UL-94) and cone calorimetry test results, the obtained PLA composites showed a LOI value as high as 33.0%, V0 ratting in UL-94 and 63.1% reduction for PHRR with the incorporation of 1 wt% NiO. The flame retardancy mechanism was also discussed by analyzing residual char structure and the composition of the liquid degradation products. It was found that the carbon layer contained two types of chars: APP and CSi-MCA could form char 1 via esterification and cross-linking reaction, while the PLA matrix could produce char 2 via the catalyzing carbonization of CSi-MCA and NiO to generate MWCNTs. The combination of char 1 and char 2 with better barrier effect resulted in the improvement on flame retardancy of PLA composites.

Investigations on the emissivity of burning coal char particles: Influence of particle temperature and composition of reaction atmosphere

Radiative properties of burning char particles are one of the key parameters for heat transfer in pulverized fuel boilers. Among the parameters which are suspected to change emissivity of burning char surfaces, particle temperature and the reaction atmosphere (air-fired vs. oxy-fuel) are two factors needing investigation. An experimental campaign is presented which studies the effect of both factors in the spectral range from 1.25 µm to 5.5 µm. An in-flight spectrometer test rig measuring thermal radiation emitted by single coal particles was employed. Particle temperature was adjusted by varying the oxygen content in the oxy-fuel atmosphere between 15 and 40%. Average char particle temperatures increase from 1952 to 2582 K with increasing oxygen content. The emissivity drops from ε = 0.6 to ε = 0.26 (λ = 2.4–5.5 µm) and ε = 0.33 to ε = 0.24 (λ = 1.25–2.25 µm) with increasing temperature. The influence of the reaction atmosphere was investigated by performing experiments in a N2/O2 atmosphere and comparing to the results to data from an oxy-fuel atmosphere, both containing 20% of oxygen. The difference εoxy-fuel-εN2/O2 is in the range of 0.1 and 0.2 for λ = 1.25–2.25 µm and λ = 2.4–5.5 µm. Effects of burnout and char structure are discussed to evaluate the results.

Comparative analysis of pyrolysis and combustion of bisphenol A polycarbonate and poly(ether ether ketone) using two-dimensional modeling: A relation between thermal transport and the physical structure of the intumescent char

A quantitative understanding of the thermal transport within a growing intumescent char layer remains largely unsolved. The development of improved techniques to analyze charring and intumescent materials is necessary to investigate the physics of heat transfer within the char. To aid in this endeavor, a systematic methodology to parameterize comprehensive pyrolysis models for charring and intumescent materials is presented. Thermogravimetric analysis, differential scanning calorimetry and microscale combustion calorimetry were conducted on 4–7 mg samples to analyze the kinetics and thermodynamics of thermal decomposition and determine heats of complete combustion of gaseous pyrolyzates. A multi-step reaction mechanism, consisting of sequential first-order reactions, was constructed to capture the physical transformations and chemical reactions observed in the milligram-scale experiments. 0.07 m diameter disk-shaped samples were gasified in the Controlled Atmosphere Pyrolysis Apparatus II to characterize the thermal transport within the condensed phase material and evolving char layer. ThermaKin2Ds was employed to interpret the experimental data using inverse analysis techniques. Bisphenol A polycarbonate and poly(ether ether ketone), widely used intumescent materials, were analyzed within this study. The resulting two-dimensional models of bisphenol A polycarbonate and poly(ether ether ketone) were capable of predicting the experimental gasification mass loss rates with a mean error of 17.8% and 16.9%, respectively. An analysis of the char pore structure was also conducted from which quantitative relationships were subsequently developed between relevant thermal transport quantities and physical descriptors of the char's physical structure. Two prominent linear correlations were discovered: the char's effective thermal conductivity increased as a function of increasing volume-weighted mean pore diameter and the product of density and thermal conductivity increased with an increasing char porosity based on image analysis. Finally, a brief sensitivity analysis of the polycarbonate and poly(ether ether ketone) burning rates was conducted to determine which key material properties were responsible for the observed differences in flammability.

On the evolution of pore microstructure during coal char activation with steam/CO2 mixtures

The present work develops a novel mathematical model for coal char activation with CO2, steam, and a CO2/steam mixture. The main assumption is that CO2 and steam react with distinct active sites, considered by attributing a unique char structure effect to each gasifying agent; the CO2-char reaction increases the pore length, whereas the steam-char reaction increases the pore radius. Additionally, pore overlapping effect was taken into account and the radial pore growth was computed by means of a population balance equation. Model predictions of both the pore size distribution and specific surface area changes were compared with experimental data, resulting in an outstanding fit. This supports the model main assumption, which reconciles discrepancies between various authors regarding the overall reaction rate during activation of coal chars with CO2/steam mixtures. Finally, the maximum specific surface area occurs under chemically-controlled conditions, where all particle zones reach their maximum surface areas simultaneously, while for activation with diffusional limitations each particle zone attains its maximum specific surface area at a different time.

In-situ decoupling effect of H2O on the whole process of coal gasification in MFBRA and TG-FTIR-MS

O2/H2O oxy-fuel combustion, as one of the most promising combustion technologies for CO2 reduction, has become a hot spot around the world. In oxyfuel combustion, H2O gasification has an important effect on the overall reaction process. This paper used Chinese Shenhua coal as raw material to decouple the effect of H2O on the whole process of raw coal-H2O gasification reaction. At 800 °C in 15 vol.% H2O, the changes of gas-liquid-solid phase product concentration and the development of char structure in the coal-H2O gasification were investigated. The MFBRA was used to analyze the rapid coal-H2O gasification reaction, while the slow-heating-rate gasification of coal was analyzed by TG-FTIR-MS. The results show that in the rapid single-particle reaction at 800 °C, H2O promotes the whole devolatilization of coal, with a certain inhibitory effect on the secondary gas-phase reaction, especially for the H2 release. H2O increases the oxygen-containing functional groups on the char surface and increases the structural defects in char. H2O would increase the −COO- groups, and promotes the consumption of CC on the surface. H2O has an obvious etching effect on the char surface. For slow-heating-rate gasification of coal, the reaction has obvious secondary weightlessness, and H2O promotes initial devolatilization, but has an inhibitory effect on the secondary one. At >700 °C, the H2O gasification is more pronounced than that of a single-particle rapid reaction. H2O reduces the activation energy of the initial stage and increases the reaction activation energy of the secondary one during devolatilization. H2O can promote the consumption of aromatic liquid-phase products such as toluene and phenol during devolatilization of pulverized coal under the slow-heating condition.

Effects of Pressure on the Characteristics of Bituminous Coal Pyrolysis Char Formed in a Pressurized Drop Tube Furnace

Abstract: To investigate pyrolysis characteristics of Shenhua bituminous coal under pressurized conditions, the effects of pressure (0-1.2 MPa) on the physicochemical structure and combustion reactivity of pyrolysis char samples prepared at 1073 and 1273 K were studied in a pressurized drop tube furnace (PDTF). The low temperature nitrogen adsorption test was used to characterize the physical structure of char samples. The results showed that, the increase of pyrolysis pressure is conducive to formation of mesopores, and the specific surface area is larger at 1273K. Raman and XPS analysis were used to characterize the chemical structure of char samples. The results showed that, the increase of pressure can promote the transformation of smaller aromatic structure into larger aromatic structure, and the graphitization degree are more obvious at 1273K. With the increase of pressure, the proportion of C=C/C-C bond in char structure increases and the proportion of C-H bond decreases. It indicates that the unst...

Effect of occurrence modes of nickel and vanadium on gasification characteristics of petroleum coke

This study investigates the effect of the occurrence modes of nickel (Ni) and vanadium (V) on char structure and reactivity during the steam gasification of petroleum coke (petcoke) in a fixed-bed reactor. Sequential extraction is applied to obtain the occurrence modes of Ni and V. Results indicates that Ni and V associated with the exchangeable form strongly promotes gasification reactivity and increases the methane yield by accelerating aromatic structure decomposition. The carbonates restrain pore structure development by increasing the graphitization degree to lower reactivity and gas yields. The exchangeable form and Fe-Mn oxides are beneficial to the micropore structure development. In comparison with continental petcoke, marine facies form highly disordered structures because high Ni and V contents can promote the pore structure formation. The carbonates strongly lower the reactivity of char and are unfavorable for the heat value of syngas, which should be selectively removed before gasification.

POSS-functionalized graphene oxide hybrids with improved dispersive and smoke-suppressive properties for epoxy flame-retardant application

Functionalized graphene oxide sheets (FGO) with improved dispersive and smoke-suppressive properties were synthesized by covalently grafting octa (propyl glycidyl ether) polyhedral oligomeric silsesquioxane on graphene oxide sheets (GO) with γ-aminopropyl triethoxysilane as a chemical bridge. The good dispersion of FGO sheets in the epoxy resin (EP) matrix endowed EP composites with stable thermal resistance, enhanced tensile and smoke-suppressive properties. Cone calorimeter tests indicated that the addition of 0.7 wt% FGO sheets to EP composites reduced the peak of heat release rate, total heat release, and total smoke release by 49.7%, 34.3%, and 41.5%, respectively. Two important effects that originated from FGO promoted this improvement; that is, well-dispersed FGO sheets exhibited a tortuous effect by lengthening the heat path and inhibiting heat diffusion in the EP matrix, and the increase in char residue and the reduction in gas volatiles confirmed the barrier effect of FGO sheets by forming protective char structures on the surface of the matrix, which restrained smoke release. This method provided a feasible concept for effectively enhancing the flame retardancy of EP composites by combining the characteristics of graphene and polyhedral oligosilsesquioxane through the construction of effective interfacial interactions.

Physicochemical structure and reactivity of char from torrefied rice husk: Effects of inorganic species and torrefaction temperature

This study aims to evaluate the effects of inorganic species (K) and torrefaction temperature on the physicochemical structure and reactivity of char from pyrolysis of torrefied rice husk in a laboratory-scale fluidized-bed apparatus. Washed rice husk (WRH) and washed rice husk with impregnating of potassium (0.1K-WRH) were torrefied at the different temperatures of 230–270 °C. The physicochemical structure of char samples were determined by elemental composition, pore structure (N2 adsorption-desorption) and carbon microstructure (Raman spectroscopy). Furthermore, the char reactivity in air was analyzed using thermogravimetric analyzer (TGA). The results indicated that the changes in elemental composition, pore structure and carbon microstructure of char samples caused by the combined effects of torrefaction process and the presence of K have the important role on the char reactivity. The dimensionless index (DI) accounted for the physicochemical structure was defined, which exhibited a strongly linear relationship between the oxidation activity index (S) of char samples. In general, both of the torrefaction temperature and the inorganic species (K) contained in rice husk samples have the important effects on the physicochemical structure and reactivity of char samples.

Structure Characterization and CO2 Gasification Kinetics of Tri-High Coal-Chars Derived from High-Temperature Pyrolysis.

The pore structures and chemical composition features of two kinds of tri-high coal and their char samples prepared at a 750 °C temperature were analyzed. The results showed that the pyrolysis process has a great influence on the pore structure and the chemical composition of the char, and the influence is highly related to the coal ranks. The gasification kinetics of the two chars in pure CO2 atmosphere was also studied. The results indicated that the classical random pore model (RPM) cannot be used to explain the gasification kinetics throughout the char gasification. A modified RPM, considering the inhibitory effect of ash on the gasification kinetics, was adopted to estimate the kinetics, and the kinetic constants and the corresponding activation energies were calculated. It was observed that it was necessary to include the effect of ash on the variations of char structures during the char gasification to get an accurate description of reaction rate versus carbon conversion throughout the gasification of the tri-high coal chars.

Flame Retardant Composite Foam Modified by Silylated Nanocellulose and Tris(2-chloropropyl) Phosphate

Improving flame retardancy is one of the most crucial issues to use polymeric materials for building construction. Most of the flame retardant materials containing halogen atoms delay fire spread, but produce harmful gases during combustion. Hence, we designed and fabricated a composite foam by using a green nanomaterial. Silylated and nanofibrillated cellulose (Si-NFC) was added to polyurethane foam (PUF) containing tris(2-chloropropyl) phosphate (TCPP) in order to reduce the emission of smoke during combustion. Thermal characteristics of the composite foams were investigated through thermogravimetric analysis, limiting oxygen index (LOI), and cone calorimeter tests. The LOI of the Si- NFC embedded composite was increased from 19.3 % to 24.6 %. In addition, the Si-NFC led to an improvement in the thermal stability of the composites by reducing the peak release of heat and smoke. Chemical structure of the residual char was analyzed by using energy-dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy.

Simultaneous NO/CO2 removal performance of biochar/limestone in calcium looping process

A novel simultaneous NO/CO2 removal system using biochar and calcined limestone in the calcium looping process was proposed. Coconut shell char and calcined limestone were added into a carbonator in the calcium looping process as the NO reductant and CO2 sorbent, respectively. The simultaneous NO/CO2 removal performance of coconut shell char/calcined limestone in the calcium looping process was investigated in a bubbling fluidized bed reactor. NO and CO2 in flue gases are effectively and simultaneously removed by coconut shell char/calcined limestone in the presence of O2. O2 plays an important role in NO removal by coconut shell char. The calcined limestone shows a strong catalytic effect on NO reduction by CO generated by the reaction of coconut shell char and O2. The calcined limestone supports active sites for NO reduction by CO. High CO concentrations and high carbonation temperatures have positive effects on NO reduction by CO with calcined limestone catalysis. However, the catalytic effect of calcined limestone is weakened by its carbonation, which is promoted by the high temperature and additional CO2 produced by the oxidation of char. The simultaneous NO removal and CO2 capture efficiencies can reach above 97% and 80%, respectively. The porous structure of coconut shell char is an important factor in enhancing NO reduction with calcined limestone catalysis in the presence of O2.

Highly Effective Flame-Retardant Rigid Polyurethane Foams: Fabrication and Applications in Inhibition of Coal Combustion.

The extemporaneous combustion of coal remains a major threat to safety in coal mines because such fire accidents result in casualties and significant property loss, as well as serious environmental pollution. This work proposed the fabrication of flame-retardant rigid polyurethane foam (RPUF) containing expandable graphite as char expander/sealant with melamine phosphosphate and 2-carboxyethyl (phenyl)phosphinic acid as char inducer and radical trapping agents. The as-prepared RPUF successfully inhibited coal combustion by forming thermally stable high graphitic content expandable intumescent char sealing over the coal. The RPUF achieved UL-94 V-0 rating in addition to significant reductions in peak heat release, total heat release, and CO and CO2 yields. The external and the internal residual char structure was studied by X-ray photoelectron spectra, Raman spectroscopy, and real-time Fourier transform infrared spectra techniques, and a flame-retardant mode of action has been proposed. This work provides important insight into a facile fabrication of highly efficient and economical flame-retardant RPUF to inhibit the spontaneous combustion of coal.