Evolution Of Char Structure Research Articles

Rapid pyrolysis of cellulose: Revealing the role of volatile matter and char structure evolution

The complexity of the cellulose structure hinders its energy conversion and utilization. Pyrolysis is not only an independent thermochemical conversion technology but also a necessary stage in the gasification process. The real-time capture of volatile components in cellulose pyrolysis is difficult, and the evolution of char structures is lacking, which is extremely challenging. In this work, elemental analysis, industrial analysis, thermogravimetric analysis, infrared spectroscopy (FTIR), Raman spectroscopy (Raman), and X-ray diffraction spectroscopy (XRD) were used to characterize the raw materials and products. Combined with rapid pyrolysis experiments, the pathways of cellulose pyrolysis volatiles and char structure evolution were explored. Dehydrated sugars are the initial products formed below 350 ℃, which quickly dehydrate to form furan compounds at 400 ℃ and then break CC bonds at 600 ℃ to generate fatty aldehydes, ketones, and carboxylic acids. Biochar generated at low temperatures (<300°C) has strong lipid solubility. An increase in pyrolysis temperature leads to a continuous decrease in the content of H and O in biochar, which destroys its functional groups and physical structure, and ultimately tends to form graphite-type char with aromatic structures. Based on the correlation between the volatile matter release characteristics and the evolution of the semichar structure, a cellulose pyrolysis pathway was established, providing theoretical support for the high-value utilization of cellulose.

Decoupling study on the influence of the interaction between biomass hydrochar and coal during co-pyrolysis on the char structure evolution

Co-pyrolysis of biomass hydrochar (biomass processed by hydrothermal carbonization, HTC) and coal, is a viable method for the clean and high-efficiency co-utilization of conventional and renewable resources. Interaction between biomass hydrochar and coal during co-pyrolysis directly impacted the physiochemical structure evolution of char and its downstream application. However, interaction mechanism hasn't been systematically revealed till now. Therefore, this study conducted a decoupling investigation on co-pyrolysis of rice straw hydrochar (RSH) prepared at different HTC conditions and bituminous coal (BC) using a staged fixed-bed reactor. The results demonstrated that RSH volatiles increased the number of deposits and cracks formed on the surface of co-pyrolysis decoupled BC char. Additionally, volatiles from RSH prepared at higher HTC temperatures led to the decrease of surface deposits, surface functional groups (such as –OH, C–H, and C–O) and graphitization degree of BC char. However, volatiles prepared at different solid-liquid ratios had a negligible impact on physicochemical structure of BC char. At HTC temperature of 200 °C, volatiles from BC led to increased surface functional groups (such as –OH and CC) and graphitization degree of co-pyrolysis decoupled RSH char. Furthermore, volatiles from BC showed more noticeable effect on graphitization process of RSH char prepared at higher solid-liquid ratio.

Evolution of char structure during cobalt‑calcium catalyzed coal hydrogasification and its impact on gas-solid hydrodynamics of gasifier

This study aims to cognize the evolution of char structure during Co-Ca catalyzed coal hydrogasification and its influence on gas-solid hydrodynamics of gasifier. Co-Ca catalyzed coal hydrogasification was carried out in a pressurized bubbling fluidized bed at 1123 K and 3 MPa. The results showed that the initial 5 min of coal hydrogasification can be considered as coal hydropyrolysis period. Particle size increased during coal hydropyrolysis, reduced when carbon conversion lies below 69% and then remained basically stable during char hydrogasification. The apparent particle density decreased progressively. Micropores were formed in abundance during coal hydropyrolysis and then decreased gradually during char hydrogasification. Whereas mesopores kept increasing and the macropores firstly increased then collapsed with carbon conversion. The simulation results of gasifier suggested that more particles aggregated at the bottom of the gasifier during coal hydropyrolysis, and the gas-solid flow state changed increasingly from bubbling to turbulent during char hydrogasification.

Effect of Ca–Fe composite catalysts on pyrolysis performance of demineralised anthracite and its char structure transformation

AbstractWhile catalytic pyrolysis with catalyst addition has been widely studied with a primary focus on optimising gas, tar and char yields, studies exploring the evolution of char structure during coal pyrolysis remain scarce. This knowledge gap potentially affects subsequent char gasification processes during coal conversion within gasifiers. This study presents demineralised anthracite (TM) prepared by HCl–HF acid washing. The pyrolysis performance of TM while adding Ca, Fe and Ca–Fe composite catalysts was investigated through thermogravimetric analysis. The char structure, morphology and functional groups in the obtained char samples were clarified through X‐ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy, respectively. The results indicated that de‐ashing by acid washing promoted the formation of C–O functional groups in coal due to the decomposition of C=O or a combination of oxygen with unsaturated carbon, or both. The TM sample exhibited improved thermal decomposition upon adding either Ca or Fe catalysts, which became more pronounced after adding the Ca–Fe composite catalyst under the same conditions. While the addition of Ca–Fe composite catalysts inhibited the crystalline carbon formation and char graphitisation degree compared to their individual additions, it led to enhanced decomposition of phenol C–O, thereby facilitating the thermal decomposition of TM.

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.

Char structure evolution and behaviors of sodium species during catalytic gasification of sodium-rich direct coal liquefaction residue under CO2 atmosphere

In this work, to better understand catalytic gasification process of direct coal liquefaction residue rich in sodium species, char structure evolution and behaviors of sodium species during gasification under CO2 atmosphere were investigated in detail by N2 adsorption and desorption, FT-IR, XRD, SEM, and Raman analyses. The results show that sodium species developed pore structure of direct coal liquefaction residue during gasification, especially expanded mesoporous structures which increased from 0.05 to 0.16 cm3/g at maximum. With the increase of gasification time, different crystalline compounds were formed in chars. Most of the mineral matters identified by XRD were calcium-containing ones, whereas no obvious sodium-containing crystalline compounds were found. This was because that most of sodium species volatilized at high temperature and the crystalline forms of sodium-containing compounds had defects. Compared with sodium species, calcium species were more prone to react with aluminosilicates, which happened to make sodium species remain active during gasification process. The ratio of (GR + VL + VR)/D rose initially and then decreased, which could be explained as the dissociation of the large aromatic and the rearrangement of small aromatic rings into large aromatic structures. Moreover, release ratio of sodium species was closely related with gasification time and 49.8% of them released in the initial stage of gasification process (within 15 min). Compared with that of direct coal liquefaction residue reloaded with water-soluble sodium species, the release ratio of sodium species in the original direct coal liquefaction residue was on a lower level (85.2% versus 89.7%).

Effects of AAEMs on the char heterogeneous structure evolution during Zhundong coal pyrolysis: Insights from micro-Raman spectroscopy

The effects of alkali and alkaline earth metallic species (AAEMs) on the heterogeneous evolution of coal/char structure during pyrolysis are rarely investigated. In this study, micro-Raman mapping technology was developed to characterize the effects of AAEMs on the distribution and heterogeneous evolution of the chemical structure of coal/char particles on micro-scale during pyrolysis. The pyrolysis chars from raw coal, leached coal and loaded coal samples were prepared by using a thermogravimetric analyzer, and more than 300 Raman spectra for each sample were analyzed by micro-Raman mapping technology. The results indicate that the water-soluble AAEMs inhibit the release of volatiles during the whole pyrolysis process. The ion-exchangeable AAEMs promote the release of volatiles at the early stage of pyrolysis and then inhibit the release. For integral char structure evolution, water-soluble and ion-exchangeable AAEMs enhance the average relative amount of small aromatic rings obviously at the devolatilization stage and aromatization polymerization stage, respectively. Besides, both water-soluble and ion-exchangeable AAEMs firstly decrease the average relative amount of active structures obviously and then slightly. For heterogeneous evolution, at the devolatilization stage, the water-soluble AAEMs promote the crack of polyolefin and substitutional groups from active components, and the transformation of these species to small aromatic rings. Meanwhile, water-soluble AAEMs promote the breakdown of the cross-linked structures of the large aromatic ring system from inert components and decrease the heterogeneity of the chemical structure of char particles obviously. Besides, the ion-exchangeable AAEMs promote the breakdown of chemical bonds between aromatic rings and the release of the active components. At the aromatization polymerization stage, the ion-exchangeable AAEMs inhibit the condensation of small aromatic rings and increase the heterogeneity of the chemical structure of char particles. In the later stage of pyrolysis, char chemical structure tends to be stable and AAEMs have little effect on the chemical structure of char particles.

Char structure evolution during molten salt pyrolysis of biomass: Effect of temperature

Pyrolysis of wheat straw in Li2CO3-K2CO3 binary salt was conducted in a fixed-bed reactor, the char structure evolution under different pyrolysis temperatures was analyzed. The results showed that the biochar yield decreased with temperature increasing, the addition of Li2CO3-K2CO3 salt enhanced heat transfer and promoted charring reactions to form more char. At 450–550 °C, the changes in biochar structure from molten salt pyrolysis showed similar trend with those from conventional pyrolysis, the C content increased and the H and O contents decreased, the aromatization degree of biochar increased, the surface functionalities on char surface all decreased. At 600–700 °C, the effect of temperature on biochar structure from molten salt pyrolysis showed significantly different trend. The increase of temperature further decreased H content, but increased O content and decreased C content, the molar ratio of H/C decreased while that of O/C increased. The etching of K salt consumed more C and retained more O in char structure, thus caused insignificant increase in aromatization level of biochar and retaining more CO/COC and –OH functionalities. More porous structures were formed due to the enhanced activation effect of molten salt at higher temperature. The BET surface area of 700MBC could achieve 568.20 m2/g. A functionalized mesoporous biochar with enriched O-containing groups was thus obtained from molten salt pyrolysis of biomass.

Study on the Effect of Pyrolysis Conditions on the Combustion Behavior and Char Structure Evolution.

Pyrolysis polygeneration technology has been widely applied due to its wide adaptability to coal types and mild reaction conditions. In this paper, the in situ process test combining pyrolysis and combustion of Pingshuo coal was carried out using a thermogravimetric analyzer and a fluidized-bed reactor. The influence of pyrolysis conditions on the combustion behavior and char structure was studied. The reduction of the pyrolysis temperature and the increase in pyrolysis residence time resulted in a lower burnout temperature of char, thereby improving the combustion characteristics of char. Furthermore, the maximum CO2 release rate reached 2.8 mg/(g·s) during the combustion process. The decrease in the pyrolysis temperature was conducive to the increase of CO2 release mass, suggesting a more complete burning of char. The char combustion process was mainly controlled by the chemical reaction according to the reaction kinetics study. The apparent rate constant of the combustion process increased gradually with the decrease in the pyrolysis temperature of char, and the rate of the combustion reaction increased, which contributed to the enhancement of the combustion performance of the char within the range of 850–950 °C. Meanwhile, the crystal structure inside the char changed significantly after different pyrolysis conditions. However, it had little influence on the molar content of carbon structure, which suggested that the amount of CO2 released was mainly caused by the different resistances of oxygen diffusion to the reaction interface.

Pyrolysis of the food waste collected from catering and households under different temperatures: Assessing the evolution of char structure and bio-oil composition

The accumulation of food waste brings several challenges to the economy and environment, its pyrolysis technology has received extensive attention. The present investigation mainly focused on the evolution of char structure and bio-oil composition during the pyrolysis process of the typical food waste, which was characterized by FT-IR, Raman, UV, and GC/MS. The experimental results show that the bio-oil has the highest yield at 400–600 ℃ and remains basically unchanged (around 40%), and decreases apparently at 700 ℃ due to the secondary thermal cracking and reforming. With the increase of temperature, the cracking of macromolecular organic matter in the food waste becomes more intense, resulting in the yield of bio-char gradually decreasing from 34.90% to 23.54% in the range of 400–700 ℃. The ratio of small aromatic ring systems to large aromatic systems (I(VR+VL+GR)/ID) in the bio-chars increases apparently with temperature, and the amounts of aromatic rings in bio-oil also increase gradually. The content and relative percentage of N-compounds are the largest in bio-oil, and the content of it reaches the maximum value at 500 ℃, while the changes in its relative percentage are slightly different, which decreases from 33.57% to 24.15% at 400–600 ℃, and then increases apparent to 40.63% at 700 ℃. Hydrocarbons are also one of the main components in bio-oil, their relative percentage decreases from 17.70% to 2.40% in the range of 400–700 ℃. In addition, Carboxylic acid, alcohols, esters, ketones, saccharides, phenols, and other substances are also contained in bio-oil, and its content has different changing trends with temperature.

Release characteristics of alkali and alkaline earth metals in nascent char during rapid pyrolysis

The release law of alkali and alkaline earth metals (AAEMs) during coal pyrolysis is crucial for controlling scaling, corrosion and slagging in the utilization of high AAEMs coal. To approximate entrained-flow bed gasification process, a free-falling reactor was used for coal pyrolysis to study the release characteristics of AAEMs in nascent char. Effects of pyrolysis temperature, residence time and atmosphere were investigated. The cumulative release of Na, Ca and Mg at 1100 °C was 68.0%, 47.5% and 69.8%, respectively. As pyrolysis temperature increased from 900 °C to 1100 °C, the release of Na, Ca and Mg increased by 7.0 %, 3.5% and 5.3%, respectively. The cumulative release of AAEMs increased with the pyrolysis residence time, but the release rate decreased significantly in middle and late stages. The promoted thermal decomposition of functional groups by CO2 resulted in a lower char yield. CO2 promoted Na release in early stage and Ca and Mg release in late stage. Furthermore, release of AAEMs was associated with char structure evolution in rapid pyrolysis. The presence of CO2, high temperature and prolonged residence time led to enhanced particle fragmentation, increased surface cracks, and intensified thermal decomposition of functional groups, resulting in the migration of AAEMs in carbon matrix to char surface and the release of AAEMs organically bound to carbon matrix.

Insight into the fast pyrolysis of lignin: Unraveling the role of volatile evolving and char structural evolution

Unraveling the role of volatile evolving and char structural evolution, as well as the pyrolysis reaction pathway, is crucial for lignin effective valorization by a pyrolysis-based biorefinery approach. The fast pyrolysis of lignin at 200–800 °C was conducted to explore volatile release characteristics and their link with char structure evolution. The release of simple substituted aryl compounds below 350 °C was aided by the cleavage of β-O-4 and α-O-4. The C-O, C–C, and carboxyl groups of the depolymerized free radical fragments were cleaved further, providing stable mono-phenol (350 °C → 500 °C). Secondary reactions and pyrolytic macromolecular compounds resulted in the synthesis of polycyclic aromatic hydrocarbons (>700 °C). A cubic polynomial was developed to correlate the aromaticity with the atom ratio of H/C. The ring condensation degree and H/C demonstrated an excellent positive linear correlation. Additionally, the aromatization started at 350 °C, and small aromatic rings were changed into more ordered rings (≥6 rings). The fused-ring structures of char had progressed in the following order: 1 × 2, 2 × 3, 4 × 4, and 4 × 5, corresponding to 350 °C, 500 °C, 600 °C, and 800 °C, respectively. High temperatures contributed to the formation of aromatic monomers. Lignin pyrolysis pathway was established based on the correlation between volatile release characteristics and char structural evolution.

Ca-Si-Al interactions induced char structure and active site evolution during gasification–A comparative study based on different gasifying agent

Catalytic steam gasification, as a promising technology for producing hydrogen-enriched syngas, has been a hotspot in recent studies. This study aims to analyze the role of calcium-silicon-aluminum (Ca-Si-Al) interactions in char gasification and distinguish the difference of the interaction in different gasifying agent. The gasification reactivity of coal char was closely linked with the physiochemical structure and the mineral interactions. It was found that whether in CO2 or H2O, the calcium oxide (CaO) has function to promote the generation of mesopores, which is beneficial to the diffusion of reactant and product gases. The scanning electron microscopy (SEM) images showed that no SiO2 particles agglomerated in coal char containing silicon-aluminum ((Si-Al)-coal), which was a good reason why Si-Al inhibited gasification severely; the well dispersed SiO2 particles have no function to adsorb the gasifying agent molecular to transfer oxygen. New minerals including Ca2Al2SiO7, Ca2SiO4 and CaSiO3 were formed in CO2 and H2O, while CaAl2Si2O8 only was detected in H2O. These new minerals exert a positive effect on active site in CO2 but negative effect in H2O. While in H2O, compared with active sites, the mesopores dominate the reaction more. Overall, the interactions catalyze gasification to a moderate degree whether in CO2 or H2O.

Structure evolution of lignite char in step pyrolysis and its combustion reactivity

Step pyrolysis was conducted in a laboratory-scale fixed bed reactor for char preparation. Characterization techniques such as volume solvent swelling, thermal gravimetric analysis (TGA), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were applied to reveal char structure evolution during step pyrolysis. Combustion reactivities of the prepared chars were examined at a thermal gravimetric analyzer. Newly formed bond structures in char such as Cal-Cal and Car-Cal/O have been identified at the 1st stage of step pyrolysis, which evidences the occurrence of cross-linking reactions. As a result, step pyrolysis has higher char yield than direct pyrolysis. The char yield from step pyrolysis increases as the 1st stage pyrolysis temperature increases and reaches a maximum at 773 K. An index was put forward to assess the extent of cross-linking reactions during pyrolysis, which is well correlated with char production. The char from step pyrolysis has relatively high graphitization degree than from direct pyrolysis, and thus has lower combustion reactivity. Step pyrolysis can suppress sulfur and nitrogen release in comparison with direct pyrolysis owing to the formation of more stable sulfur and nitrogen species in char. The difference in nitrogen release varies within 8 percentage points and for sulfur release, it is within 5 percentage points under the examined experimental conditions.

Influence of Interaction between Biomass Inorganic Components and Volatiles on Corncob Pyrolysis and Char's Structure Evolution

Influence of Interaction between Biomass Inorganic Components and Volatiles on Corncob Pyrolysis and Char's Structure Evolution

Evolution of char structure during the pyrolysis of biomass pellet: Further understanding on the effects of chars two phases

The understanding of the evolution of char is critical to optimizing the thermochemical process of biomass. In this study, sawdust powder and pellet chars are prepared at various pyrolysis temperatures and separated into two phases by microwave-assisted extraction. More detailed information on the chemical composition of the chars is gained by characterization of two separated phases using different analytical methods. The results show that the extraction residues of powder and pellet chars have similar stable radical concentration at the same temperature. However, there are more macromolecules (>200 Da) in the extractable components of the pellet chars having steric hindrance effect, resulting in more stable radicals of pellet char than that of powder char. The extractable components of pellet chars contain more aromatic structures than that of powder chars from pyrolysis at same conditions. The reactions involved of these aromatic structures promoted by these radicals, such as the volatile-char interaction, are enhanced with increasing temperatures, leading to the formation of recombined compounds, such as lipid compounds.

Study of Electrochemical Catalytic Coal Gasification: Gasification Characteristics and Char Structure Evolution.

Electrochemical catalytic coal gasification experiments with Fuxin (FX) coal under a CO2 atmosphere were conducted to evaluate the effects of power and temperature on coal gasification and char structure evolution during electrochemical catalytic gasification (ECG). When the power was 400 W, with temperature increasing from 800 to 1000 °C, the CO content in the gas products increased by 8.16%, the H2 content increased by 8.39%, and the CH4 concentration in the gas products initially increased and then decreased. When the temperature is 900 °C, with power increasing from 0 to 400 W, the CO content in the gas products increased by 58.27%, the H2 content increased by 81.33%, and the CH4 concentration in the gas products increased from 1.31 to 2.37%. The gasification reactivity and the concentration of combustible gas generated during ECG were higher than those during common coal gasification. Thermal electrons play important roles in ECG. These electrons could promote ring opening reactions and aromatic compound cracking and inhibit aromatization reactions while increasing the number of oxygen-containing functional groups in char, consequently enhancing the char gasification reactivity.

Molecular insights into the role of char-bound hydrogen in char-NO reactions under high pressures: A reactive molecular dynamics simulation study

In the present work, the role of char-bound hydrogen in char-NO reaction under pressurized oxy-fuel combustion (POFC) conditions was studied using the reactive molecular dynamics (ReaxFF MD) simulations. The effects of char-bound hydrogen on the chemisorption of NO, formation of N2, kinetics of char-NO reaction, and structure of char at different pressures and temperatures were investigated. The results show that the char-bound hydrogen hindered the chemisorption of NO, and inhibited the generation of CN. The role of char-bound hydrogen in the chemisorption of NO weakened with the increase in pressure. Considerable amount of H2O was generated during the reaction of NO with H-containing char, which proves that char-bound hydrogen provided another channel for the conversion of NO to N2, especially at high pressures. The activation energies of the chemisorption of NO and the formation of N2 during the reaction of NO with H-containing char were determined to be 174 and 126 kJ/mol, respectively. The ReaxFF MD simulation results were in good agreement with previous experimental and DFT values. The char-bound hydrogen increased the activation energies of the chemisorption of NO and the formation of N2 by 40% and 29%, respectively. The role of char-bound hydrogen in char gasification and char structure were also examined. The char-bound hydrogen reduced the consumption of char, and CO was the main product from char gasification. The results of the O/C and N/C ratios of char indicate that char-bound hydrogen had little effect on the desorption of CO. By analyzing the dynamic evolution of char structure, it is found that char-bound hydrogen inhibited N-down chemisorption of NO.

Char structural evolution characteristics and its correlation with reactivity during the heterogeneous NO reduction in a micro fluidized bed reaction analyzer: The influence of reaction atmosphere

Experiments were conducted on chars prepared from R- and H- form sawdust (SD) and Shenmu coal (SM) to investigate NO reduction in different atmospheres, and physicochemical structures of chars after the reaction were analyzed. The effects of atmospheres on NO reduction reactivities and structural evolution characteristics of different chars were discussed. The NO reduction reactivity of SD chars was always better than that of SM chars, and there were some differences in the specific reactivity of four chars under different atmospheres. When NO, CO2 and O2 were sequentially involved, the char ordering degree tended to increase, which also had a clear positive impact on the pore development. With the participation of CO2 and O2 in the reaction, some N-6 and N-X in R SD char were converted into N-5 and N-Q, while N-6, N-5 and N-X in the other three chars were converted into N-Q.

Temporal and spatial evolution of biochar chemical structure during biomass pellet pyrolysis from the insights of micro-Raman spectroscopy

Biomass pellet is a popular fuel, and pyrolytic char can be a promised carbon material, further understanding char structural evolution can promote its utilization. To deeply investigate the evolution of char chemical structure and its heterogeneity during biomass pellet pyrolysis process, a new method was developed to fragment the char in space, and the chemical structures of the fragmented sections were analyzed by a micro-Raman spectroscopy (equipped with a 532 nm laser). The first-order Raman spectra (800–1800 cm−1) of varied positions of sawdust pellet pyrolytic char were curve-fitted with 10 Gaussian bands to reveal the temporal and spatial evolutions. The results indicate that pyrolysis of sawdust pellet can be divided into two stages: i) the decomposition of raw biomass (temperature heterogeneity in pellet remains); ii) the condensation reactions of aromatic ring systems (temperature in pellet becomes uniform). The axial and radial heterogeneity of char chemical structure is mainly caused by heat transfer when the whole pellet temperature is uniform during heating. While mass transfer, in accordance with volatile diffusion, can eliminate the heterogeneity of char chemical structure by volatile-char interaction. The combined effects of heat and mass transfer lead to a heterogeneity in pellet char chemical structure.