Evolution Of Carbon Structure Research Articles

Insights into pyrolysis product characteristics and carbon structure evolution of bituminous coal under high-temperature thermal shock

The current investigation aims to reveal the pyrolysis characteristics and particle microstructure evolution of bituminous coal under the high-temperature thermal shock. In the experimental temperatures, the purification of carbon matrix in the coal char is a successive process linear on temperature. In contrast, the graphitization process of carbon structure existed a transition temperature (1600 ℃). Specifically, the range of 1000–1500 ℃ is critical for the transformation of amorphous carbon to disordered graphite, but the growth of graphite microcrystals was minimally affected. While the range of 1600–1900 ℃ is critical for the formation of the graphite-like carbon structure. It is characterized by the release of defective structures such as oxygen-containing functional groups, and a significant increase in crystal size. Although beneficial for the subsequent study of carbon materials, the ordered graphite transformation led to the deactivation of char. The pyrolysis behavior study under thermal shock insight into the thermal behavior of coal char in high-temperature furnaces, as well as a new idea for the clean utilization of coal resources.

Precise Control of Ultramicropores of Novel Carbons Molecule Sieves Derived from Coffee Bean for Efficient Sieving Propylene from Propane.

The development of granular carbon materials with outstanding selectivity for the separation of alkenes and alkanes is highly desirable in the petrochemical industry but remains a significant challenge due to closely similar molecular sizes and physical properties of adsorbates. Herein, we report a facile approach of using natural biomass to prepare novel granular carbon molecule sieves with a molecular recognition accuracy of 0.44 Å and propose a new three-region model for the pore size distribution of amorphous porous carbons. Coffee bean-based granule carbon molecular sieves (CFGCs) were prepared with precise micropore regulation with subangstrom accuracy and characterized using molecular probes to reveal the evolution of carbon structure during preparation. The CFGC-0.09-750 demonstrates exceptional selectivity adsorption toward C3H6 while excluding C3H8, with an uptake ratio of 106.75 and a C3H6 uptake of 1.88 mmol/g at 298 K and 100 kPa, showcasing its immense potential in industrial applications for separating C3H6 and C3H8. The novel three-region model established in this work can clearly and reasonably elucidate why the samples CFGCs can screen propylene from propane at the subangstrom level. This study provides important guidance for the development of new carbon molecular sieves with subangstrom accuracy in molecular recognition and separation capacity.

Effect mechanism of phosphorous-containing additives on carbon structure evolution and biochar stability enhancement

The regulation of the pyrolysis process is a key step in increasing the carbon sequestration capacity of biochar. The effect of K3PO4 addition on the yield, chemical composition, characteristic functional groups, macromolecular skeleton, graphite crystallites, and stability of biochar was studied in this paper using two-dimensional infrared correlation spectroscopy (2D-PCIS), X-ray photoelectron spectroscopy, Raman spectrum, and other characterization methods combined with thermal/chemical oxidation analysis. It is discovered that adding K3PO4 may effectively minimize the graphitization temperature range and increase biochar's yield, aromaticity, H/C ratio, and proportion of refractory/recalcitrant organic carbon. The 2D-PCIS and Raman analysis revealed that K3PO4 mostly promoted the dehydrogenation and polycondensation process of the aromatic rings in the char precursor, transforming the amorphous carbon structure of biochar into an ordered turbostratic microcrystalline structure. K3PO4 enhanced biochar stability mostly at medium-high temperatures (350 ~ 750℃) by stimulating the transformation of unstable structures of biochar to stable carbon-containing structures or by inhibiting the interaction of its active sites with oxidants through the mineralization process. A 20% phosphorus addition increased biochar's refractory index (R50) by roughly 11%, and it also boosted biochar's oxidation resistance (H2O2 or K2CrO4) efficiency, reducing carbon oxidation loss by up to 7.31%. However, at higher temperatures (> 750 ℃), the doping of phosphorus atoms into the carbon skeleton degraded the biochar structure's stability. The results of this study suggest that using exogenous phosphorus-containing additives is an efficient way to improve the stability of biochar.Graphical abstract

Product distribution and carbon structure evolution characteristics of semicoke from isothermal pyrolysis of Naomaohu coal

Isothermal pyrolysis of Naomaohu coal was carried out at a temperature ranging from 500 to 750 0C using a fixed-bed reactor. The product distributions of gas and tar were measured using gas chromatography and gas chromatography-mass spectrometry, respectively. In addition, the effect of temperature on carbon structure evolution characteristics of semicokes were also investigated. The results showed that the gas yield increased dramatically from 61.50 to 168.43 mL g−1 when the temperature increased from 500 to 750 0C. CO2 and C2-C3 gases were mainly produced during the primary pyrolysis of lower energy chemical bonds at a lower temperature. The tar content ranges from 18.6 to 20.5 wt%, with the highest at 550 0C. At higher temperature, the branched hydrocarbon and phenols can react with each other to produce more polycyclic aromatic hydrocarbon. The infrared absorption peak area associated with the functional groups of O-H, C-H, CO and C-O initially increased with temperature from 500 to 550 0C before sharply decreasing as the temperature increased from 550 to 750 0C. At a higher pyrolysis temperature, the carbon content of the semicoke and the degree of graphite crystalline structure increased. Semicoke produced at higher pyrolysis temperatures exhibited better crystalline behavior and more pores and cracks.

Carbon structural evolution and interfacial reaction mechanism investigation of the green anode in kneading and baking via ReaxFF-MD and force-biased Monte Carlo

ABSTRACT Baking is the most costly and critical process in anode preparation, and the study from the microscopic scale is conducive to the optimisation of anode baking process. In this research, a hybrid simulation method of reactive force field molecular dynamics (ReaxFF-MD) and force-biased Monte Carlo (fbMC) was proposed to understand the interaction mechanism between coal tar pitch (CTP) and calcined petroleum coke (CPC) during kneading and baking processes, and to explore the optimal ratio of CTP from the molecular scale. Results showed that the CTP content of 14 wt.% was beneficial for anode production. The mechanism of anode coking behaviour at high temperatures was also analyzed. The carbonisation of CTP during high-temperature baking broke the carbon rings and formed straight chains, exposing active carbon atoms that can bond with the remaining layers. As the high-temperature simulation progresses, the carbon chains at the bonding sites became increasingly cyclized and aromatised, which made the layered structure of the baked block similar to that of graphite. The generation of various volatile hydrocarbons during the baking process was observed, namely CH4, C2H4, C2H2, C2H6, and C3H6. This work provides theoretical basis for process optimisation.

Using the Conditional Process Analysis Model to Characterize the Evolution of Carbon Structure in Taxodium ascendens Biochar with Varied Pyrolysis Temperature and Holding Time

Temperature determines biochar structure during pyrolysis. However, differences in holding time and feedstock types may affect this relationship. The conditional process analysis model was used in this paper to investigate the potential to affect this mechanism. The branch and leaf parts of Taxodium ascendens were separately pyrolyzed at 350, 450, 650, and 750 °C, and kept for 0.5, 1, and 2 h at each target temperature. We measured the fixed carbon and ash contents and the elemental composition (C, H, O and N) of the raw materials and their char samples. After plotting a Van Krevelen (VK) diagram to determine the aromatization of chars, the changes in the functional groups were analyzed using Fourier transform infrared (FTIR), Raman, and X-ray photoelectron spectroscopy (XPS). The results revealed that pyrolysis at temperatures between 450 and 750 °C accounted for the aromatization of biochar because the atomic H/C ratio of branch-based chars (BC) decreased from 0.53–0.59 to 0.15–0.18, and the ratio of leaf-based chars (LC) decreased from 0.56–0.68 to 0.20–0.22; the atomic O/C ratio of BC decreased from 0.22–0.27 to 0.08–0.11, while that of LC decreased from 0.26–0.28 to 0.18–0.21. Moreover, the average contents of N (1.89%) and ash (13%) in LC were evidently greater than that in BC (N:0.62%; Ash: 4%). Therefore, BC was superior to LC in terms of the stability of biochar. In addition, the increasing ID/IG and ID/I(DR+GL) ratios in BC and LC indicated an increasing amount of the amorphous aromatic carbon structure with medium-sized (2~6 rings) fused benzene rings. According to the CPA analysis, an extension of the holding time significantly enhanced the increase in aromatic structures of LC with temperature. But this extension slightly reduced the growth in aromatic structures of BC. All indicate that holding time and feedstock types (branch or leaf feedstock) could significantly affect the variation in biochar aromatic structure with respect to temperature.

Confined pyrolysis-driven one-dimensional carbon structure evolution from polyacrylonitrile fiber and its microwave absorption performance

The evolution of one-dimensional carbon structures from polyacrylonitrile (PAN) fiber precursors is investigated based on confined pyrolysis strategy, which is employed to convert electrospun PAN fibers into carbon in a closed system. Carbon tube with ultra-large diameter derived from silica-coated organic PAN precursor fiber is observed and its low-performance of microwave absorption is revealed. To enhance microwave absorption, cobalt acetylacetonate is introduced as a magnetic resource into PAN, facilitating one-dimensional carbon structure evolution and yielding magnetic/carbon nanofibers. The mechanism resulting in such one-dimensional carbon structure evolution is discussed. The microwave absorption performances of the prepared magnetic carbon nanofibers are analyzed based on their complex permittivity and permeability. Remarkably, the magnetic carbon nanofibers exhibit strong microwave absorption intensity of −42 dB at only 20 wt% filler content. Further simulations are conducted based on radar-absorbing structure with prepared magnetic carbon nanofibers-coated laminate and wing models by using CST software. The laminated composite structure appears the maximum RCS reduction value of 18.70 dBsm at 6 GHz with 3.0 mm thickness in the frequency range of 2–18 GHz. In the corresponding wing structure, highly efficient electromagnetic wave absorption is observed and the RCS reduction can achieve −14.12 dBsm at 10 GHz with 3.5 mm thickness.

In-situ Raman spectroscopy study on coal pyrolysis and subsequent char low temperature oxidation and gasification

Thermal conversion is pivotal for clean and efficient coal utilization. Evolution of solid structure profoundly affects the gasification mechanism. However, in-situ study on coal structure evolution during gasification is limited. In this study, in-situ Raman spectroscopy was used to investigate the carbon structure evolution during thermal conversion. Results revealed two stages in coal pyrolysis: polycondensation and aromatic ring fusion. During polycondensation, numerous side chain groups break, while aromatic compounds condensed and rings fused in the aromatic ring fusion stage, increasing molecular size. In char gasification, larger particles required more time to achieve stable shrinkage, thus delaying coal char graphitization process. Low temperature oxidation led to decreased order due to oxygen-containing structure formation, evolving into a graphite-like structure as disordered carbon diminished. In addition, oxygen-containing groups acted as active oxidation sites. The insights into real-time structure evolution enhance understanding of the particle behavior during thermal conversion of coal.

Carbon structure evolution of coal slime under different pyrolysis methods: Quantitative analysis of carbon structure and finite-element simulation calculations

Microwave pyrolysis (MWP) has exhibited several advantages compared to conventional pyrolysis (CP). However, comprehensive analysis of the changes in the carbon structure of coal-char from various aspects has not been systematically reported. This paper investigated the carbon structure properties of coal slime (CS), and the temperature distribution within MWP-char was analyzed using finite-element simulation. The results demonstrated that MWP-char displays lower H/C and O/C molar ratios than CP-char as the pyrolysis temperature increased. This finding suggests that microwaves contributed considerably to the deoxygenation and dehydrogenation reactions of CS and was verified by Fourier Transform Infrared (FTIR) spectroscopy results. Owing to the selective excitation of polar functional groups by microwave energy, a “local hot spot” was generated inside the sample, and finite-element simulation verified this finding. The “local hot spot” in the sample promoted the formation of carbon defects in char, stimulated the transformation of small aromatic rings to large ones, and enhanced the condensation reaction of CS. The use of microwaves reduced the thickness and increased the diameter of the aromatic layer of CS-char, which increased the aromaticity, dielectric constant, and dielectric loss of MWP-char. A correlation was observed among the aromaticity of char, degree of carbon defects, and dielectric constant. This study provides effective theoretical and experimental basis for the efficient conversion of microwave-enhanced CS into carbon-based materials and high-value fuels.

Petrographic features and carbon structural evolution of a series of high-temperature treated anthracites

Clean energies such as solar, wind, and geothermal power have developed rapidly in recent years and they will probably become the dominant energy source in the future. Thus, coal utilization will have to shift from traditional combustion for energy to coal being a source for raw carbon materials. In this paper, anthracite from the Jinzhushan mine located in central Hunan, China, was treated up to 2900 °C to examine its chemical, petrographic, and carbon structural evolution. Carbon content increased rapidly from 90.2 to 98.8% (daf basis) when treated up to 1700 °C; beyond this temperature the carbon content increased gradually to 100% (daf) at 2900 °C. Altered macerals (vitrinite and inertinite) with an amorphous carbon structure were prevalent in samples treated to ≤1500 °C. Graphitized components were identified in the anthracite heated to ≥1700 °C, as confirmed by micro-Raman spectroscopy and HRTEM. Based on D1 band FWHM versus R1 plots, the carbon structural evolution of anthracite between room temperature and 1700 °C was classified as carbonization, whereas the graphitization process occurred between 1700 and 2900 °C. The nanometric polyaromatic rings retained short-range order during carbonization; the condensed aromatic rings remained randomly oriented at the submicron scale. During graphitization, the aligned structural units were able to undergo condensation reactions and de-wrinkle with temperature increase, leading to a marked increase in crystallite parameters La and Lc. Most vitrinite and inertinite in the anthracite transformed into graphitic components in samples treated to 2600 and 2900 °C, and La showed a significant increase whereas Lc only showed a slight increase between 2600 and 2900 °C. This study will provide insights for optimizing temperature control strategies during preparation of anthracite-based carbon materials in the future.

HRTEM analysis of carbon structure evolution during the formation of metallurgical coke and impacts on coke quality

The coke quality influences the performance of the blast furnace, and the coal properties and coke structure play important roles in affecting the ultimate coke quality. The evolution of the physical and chemical structures during the plastic layer formation is generally accompanied by the modification of the carbon structure, which influences the strength and reactivity of coke, e.g., mechanical strength (micum indices-M40/M25, M10) and hot strength (coke strength after reaction with CO2-CSR and coke reactivity index-CRI). This paper reports the evolution and transformation of carbon structures during the coking processes of six Australian coking coals, aiming to draw correlations between the carbon structures of coke and coal properties as well as coke strength. Results indicate that cokes contain various proportions of turbostratic structures and graphitized structures derived from the transformation of carbon structures in terms of coal properties. After the chemical structural transformation of coals during the coking process, the heteroatoms are removed, and the carbon structures of the cokes consist of more ordered stacked structures. The cross-links and oxygen-containing functional groups in low-rank coals evolved to form more tortuous stacked structures. Cokes made from higher-rank coals formed fringes with longer average lengths and lower tortuosity in a more tightly stacked structure, leading to the formation of anisotropic structures. The carbon structure of coke was found to be closely correlated with the cold strength parameters.

Dynamic structure transformation of char precursors during co-pyrolysis of coal and HDPE by using ReaxFF MD simulation and experiments

Understanding the effects of co-pyrolysis synergies between low-rank coal and high-density polyethylene (HDPE) on the carbon structure evolution in char and the relationship between char structure and chemical bonds evolution is very needed. A combination of ReaxFF MD and experimental approaches provides an opportunity for fully understanding the dynamic profiles of char structures and coking trends during co-pyrolysis process of coal and HDPE. In this work, consistent conclusions were obtained from fixed-bed experiments and ReaxFF MD simulations in product distribution during both individual coal pyrolysis and co-pyrolysis at different temperatures. The ReaxFF MD simulation results reveal that adding HDPE partially inhibited the coking process of coal char. The RDF results indicated higher order degree and aromaticity exist in coal/HDPE(7:3)-Char when compared to those in coal-Char, which agrees well with the findings from XRD and FTIR approaches. By analyzing the evolution of C=C, C=O, C-O, C(sp2)-C(sp3), C(sp3)-C(sp3) and C(sp2)-H bonds in C40+ fragments during secondary simulations of co-pyrolytic char, it was observed that the structural order and aromaticity of co-pyrolytic char increase with temperature, and the oxygen-containing bonds are the initial recombination sites for char formation. These findings provide rich theoretical information to complement the understanding of char structure in co-fed systems and the interaction between coal and HDPE blend.

Solvent extraction of superfine pulverized Coal. Part 4. Carbon skeleton characteristics

To better understand the molecular structure of superfine pulverized coal and promote its practical utilization, the NMR (nuclear magnetic resonance) technology is adopted to study the structural information of all the products from the solvent extraction process, including raw coal, residue, and extract. The whole set of NMR spectra is resolved into different individual curves with specific linewidths, shapes, and intensities. According to the chemical shift, the carbon skeleton characteristics like the aromaticity, cluster size, and other structural lattice parameters are analyzed in detail. More importantly, the influences of coal maturity, coal size, and solvent on the carbon structural evolution during the extraction process are focused on. The results show that the aromatic ring stacking and aromatic ring exposure and fragmentation occur competitively during the superfine comminution, promoting the structural evolution of the coal. In addition, aliphatic carbon transfers more from raw coal into the extract than aromatic carbon in the whole extraction process, which is accompanied by the disappearance of carbonyl groups. Furthermore, the reduction of particle size can promote the extraction effect of the extractant on the coal powder, and reduce the proportion of aliphatic carbon in the residue. The work introduces the correlations among the raw coal, residue and extract from a carbon skeleton point of view, providing a reference for elucidating the solvent extraction mechanisms. The results shed light on better understanding the coal structure and its further molecular modeling.

The effect of vacancy size on the oxidation process of graphene by CO2: A ReaxFF molecular dynamics study

The gasification process of graphene has been studied extensively, but the kinetic behavior of its surface vacancies under CO2 atmosphere is not clear enough. Correctly understanding its behavior can help people to better understand the evolution of carbon structure during gasification from a microscopic perspective. In this study, ReaxFF-MD was used to investigate the entire gasification process of the defective graphene in the CO2 atmosphere, including the generation of some gases. When the vacancy existed, the stable structure of the surface would be destroyed, resulting in the formation of the 5-member ring and 7-member ring vacancies. Meanwhile, the carbon bond near these vacancies would extend or break, which was used to capture the O atom of CO2 molecule and generate CO gas molecule. With the increase of the aperture of the vacancy, the number of active sites on the defective surface increased, which not only improved the CO2 adsorption reaction rate but also promoted the expansion of the vacancy size. Our study will have important implications for clarifying the gasification process of CO2 gas molecules on the carbon structure of vacancies of different sizes, deepening the understanding of the gasification mechanism from the atomic scale. By revealing the interaction mechanism between the defective graphene and CO2, it provides a theoretical basis for the industrial graphene application process.

Investigation on the Mineral Catalytic Graphitization of Anthracite during Series High Temperature Treatment

Graphite can be artificially converted from anthracites under high temperatures; however, the exact mechanism through which inorganic minerals contribute to the graphitization process is still unknown. In light of this, several selected minerals in different amounts were added to demineralized anthracite coal. The anthracite–mineral mixtures were subjected to artificial graphitization experiments under temperatures ranging from 1700 to 2900 °C in the laboratory. The obtained series of coal-based graphites with various levels of graphitization were characterized by X-ray diffraction (XRD), and the derived structural parameters, such as d002 and FWHM (002), La, and Lc were used to compare the carbon structural evolution during the high temperature treatment and mineral catalytic graphitization. Moreover, the amorphous carbon of anthracite is eventually transformed into the highly ordered crystalline carbon of coal-based graphite. The five added minerals show interesting structural variation during the graphitization process, in which pyrite is decomposed into iron (Fe), illite, quartz, and kaolinite, which can react with disordered carbon in organic matter to form moissanite (SiC), while dolomite seems to react with sulfur to form oldhamite (CaS). At temperatures less than 2300 °C, the minerals could significantly enhance the catalytic effect. There is a clear difference in the catalytic effect of different minerals on graphitization. Kaolinite exhibits the strongest catalytic effect. The minerals dolomite, illite, and quartz only show a certain degree of catalysis. Pyrite, however, only has a limited effect on improving the degree of graphitization at a temperature of 1700 °C. However, once the temperature exceeds 2300 °C, the dominant factor controlling the graphitization of anthracite appears to be the temperature. According to the growth pattern at microcrystalline sizes (La and Lc), the minerals’ catalytic effects can be classified into three groups. The first group includes minerals that preferentially promote La growth, such as pyrite, illite, and quartz. The second group includes minerals that preferentially promote Lc growth, such as dolomite. Finally, kaolinite is in a separate group that promotes microcrystal growth in both the lateral and vertical directions simultaneously. The mechanisms of the minerals’ catalytic graphitization are discussed in this paper. The promotion role of minerals in the artificial graphitization process may help to optimize the graphitization process and reduce the process cost in the future.

Mineral-induced catalytic mechanism of sodium-calcium binary catalyst during coal char gasification

Binary catalyst composed of alkali metal and alkaline earth metal was widely used to overcome the shortcoming of single-metal catalyst. However, the synergistic mechanism between components remains unclear. In present work, sodium and calcium were loaded into the coal char individually or together. TGA gasification was used to investigate catalytic effect. In order to evaluate catalytic mechanism, Raman spectra of gasification residue, Raman spectra of quenched slag and phase composition at thermodynamic equilibrium were obtained. Loading of sodium or calcium significantly increased gasification rate of coal char and catalytic effect of sodium was better than calcium. Above 900 °C, catalytic effect of sodium-calcium binary catalyst was better than that of single catalyst, and gasification residue of binary catalyst loaded char had more calcium silicate and less sodium silicate than gasification residue of single-metal catalyst loaded char. It was speculated that calcium reacted first with silica in coal to inhibit sodium inactivation. Raman spectra of quenched slag and theoretical calculation showed that silica preferentially reacted with calcium oxide to form calcium silicate in calcium oxide-sodium carbonate-silica system, which proved above speculation. Furthermore, the significant catalytic activity of binary catalysts inhibited the graphitization evolution of carbon structure during char gasification, which is conducive to the improvement of gasification rate.

Boosting sodium storage performance of hard carbons by regulating oxygen functionalities of the cross-linked asphalt precursor

Pre-oxidation treatment is a key process to produce carbon anodes for sodium ion batteries from pitch because it governs the orientation of carbon layers during subsequent carbonization. However, the relationship between the oxygen functionalities in the oxidized pitch and the evolution of carbon structure during carbonization remains unclear. Herein, cross-linked asphalts with differentiated oxygen functionalities such as C–O and C(O)–O (ester group and anhydride), are synthesized to explore their effects on the evolution of carbon structure. The planar cross-linked structures constructed by thermally stable C–O fail to prevent the sliding of carbon layer, leading the formation of short-range order carbon structures with lager lateral size. In contrast, the three-dimensional cross-linked structures constructed by C(O)–O effectively prevent the rearrangement of asphalt at early pyrolysis. Owing to the gas generated by C(O)–O removal during subsequent carbonization, the as-formed more disorder carbon structures with small microcrystalline sizes can generate more closed pores and ultra-micropores, which enhances the low-voltage plateau capacity from 96.06 to 132.2 mAh g−1. The insights for the differentiated oxygen functionalities in regulating the carbon structural conversion pave the way for further development of pitch-based hard carbons with higher performance and their practical applications in SIBs.

Effect of Evolution of Carbon Structure during Torrefaction in Woody Biomass on Thermal Degradation.

Torrefaction is an effective method for upgrading biomass. Cedar torrefaction is carried out in a fixed bed reactor at the temperature of 200-300 °C. The structural parameters are obtained from elemental analysis and 13C nuclear magnetic resonance (NMR). Thermal degradation behavior of raw and torrefied cedar is monitored by thermogravimetry analysis. The results show that carbon structure varied during torrefaction has a significant effect on thermal degradation of cedar. Some unstable oxygen functional groups, such as C1 of hemicellulose, β-O-4 linked bonds, and amorphous C6 of cellulose, are decomposed at mild torrefaction of torrefied temperature ≤ 200 °C. The temperature of maximum weight loss rate increases from 348 °C of raw cedar to 373 °C of C-200. The amorphous cellulose is partly re-crystallized at moderate torrefaction of torrefied temperature 200-250 °C. The aromaticity of torrefied cedar increases from 0.45 of C-200 to 0.73 of C-250. The covalent bond in the side chain of aromatic rings in cedar was further broken during torrefaction at severe torrefaction of torrefied temperature 250-300 °C. The area percentage of DTG mainly signed at 387 °C of C-300. The proton aromatic carbon increases from 12.35% of C-250 to 21.69% of C-300. These results will further facilitate the utilization of biomass for replacing fossil fuel to drive carbon neutrality.

Investigation on the influence of inherent AAEMs on gasification reactivity of solid digestate char

Char reactivity usually determines the overall efficiency of the entire gasification process, while the presence of the alkali and alkaline earth metals (AAEMs) has a catalytic effect on char gasification. In this study, the influence of inherent AAEMs in the gasification behaviour of char was investigated. The char sample was prepared through the pyrolysis of solid digestate derived from anaerobic co-digestion of silage and dairy cattle slurry. The raw char and HCl-washed char were characterized by elemental analyzer, ICP-OES, SEM and XRD to explore their structural changes. HCl-char loses weight in the temperature range of 440–620 °C, and the weight loss percentage is significantly higher than that of char. Ash content of char is reduced by half after a pickling process. AAEMs are largely removed after char acid pickling, resulting in an increase in activation energy of the gasification reaction and a decrease in gasification reactivity, resulting in the gasification reaction time of HCl-char longer than char. Water-soluble AAEM and ion-exchange AAEM affect the evolution of carbon structure, which directly lead to char reactivity during the gasification reaction. As the carbon is consumed, the carbon microcrystalline structure of the residual carbon tends to be ordered, resulting in fewer active free carbon sites for the gasification reaction. Kinetic analysis showed that the loss and deactivation of AAEMs after char acid washing increased the average activation energy Ea by 115.76 KJ/mol compared with the original char.

Investigation on the mechanism of carbon structural evolution of Huolinhe lignite during different pretreatment processes

The depth understanding of the organic structure characteristics of coal and its evolution is beneficial to the clean and efficient coal directional conversion. This research probes the evolution path and mechanism of carbon structure of lignite during the hydrothermal transitions dewatering (HTD) and tetralin solvent thermal transitions dewatering (TTD). The structural evolution with these two transitions was explored using 13C NMR and XRD in an autoclave at different temperatures. The moisture content in lignite is effectively decreased during HTD and TTD processes, and this is mainly due to the removal of O-containing functional groups and the decrease of water holding capacity. The TTD process is achieved a higher dehydration and deoxidation for lignite compared with HTD process. This is possibly due to that the dehydrating energy of TTD process not only comes from the thermal energy, but also the tetralin is more likely to form hydrogen bonds with water. The aromaticity is increased during HTD and TTD processes, and this increase is possibly contributed by aromatization while the contribution of aromatic rings condensation was limited because the molar ratio of aromatic bridgehead carbon has changed little. Also, from the XRD data, the stacking of aromatic structural units are significantly promoted and are not obvious for the lateral extended size for crystal structural evolution during HTD and TTD processes. when the pretreatment temperature was at 310 °C, the aromaticity increase from 60.94% to 69.50% and 69.68% during HTD and TTD processes, respectively. The stacking height increase from 8.94 Å to 12.06 Å and 13.27 Å, and the stacking number increase from 2.49 to 3.51 and 3.86, the lateral sizes have changed little. The increase of aromaticity is finally manifested in aromatic units vertical stacking height and stacking layer number. These results indicate that the graphitization and ordering are promoted to some degree during HTD and TTD processes. Compared with the HTD, the TTD had the greater impact on the conversion of lignite carbon structure to high-rank coal. Those results are potentially useful to the clean and efficient utilization of lignite.