Coal Pyrolysis Research Articles

Response of nano-pores and heterogeneity of tar-rich coal to microwave pyrolysis

ABSTRACT In-situ pyrolysis of tar-rich coal can alleviate the external dependence of oil and gas in China, and achieve efficient and safe exploitation of tar-rich coal. In fact, the nano-pores and heterogeneity of tar-rich coal undergo significant changes during pyrolysis, but there is currently limited research, which seriously restricts the efficient utilization of tar-rich coal. In this study, the pore and heterogeneity characteristics of tar-rich coal were studied by microwave pyrolysis experiment, gas adsorption results and multifractal theory, and the pyrolysis evolution mechanism of pores of tar-rich coal was proposed. The results show that microwave pyrolysis significantly promoted the development of nano-pores in tar-rich coal. The nano-pore size distribution of tar-rich coal after pyrolysis showed obvious multifractal characteristics, and the influence of sparse area on nano-pore size distribution was more obvious, while the nano-pore distribution heterogeneity increases initially and subsequently drops with temperature. Moreover, the development of nano-pores in tar-rich coal occurred in three stages. Stage I, the total pore volume changed little, increasing only from 0.06851 to 0.09463 ml/g, and thermal cracking is the main reason for pore development. Stage II, numerous pores were produced as a result of the pyrolysis products’ generation, and total pore volume grew rapidly increased to 0.331 ml/g. Stage III, liquid hydrocarbons and precipitated volatile products blocked nano-pores, so that the total PV began to decrease. The total pore volume was only 0.19967 ml/g at 1000°C. This study provides guidance for the investigation of pore evolution during pyrolysis and enriches the theory of in-situ pyrolysis of tar-rich coal.

Use of In-Situ ESR Measurements for Mechanistic Studies of Free Radical Non-Catalytic Thermal Reactions of Various Unconventional Oil Resources and Biomass

The exhaustion of conventional light oils necessitates the shift towards unconventional sources such as biomass, heavy oil, oil shale, and coal. Non-catalytic thermal cracking by a free radical mechanism is at the heart of the upgrading, prior to refining into valuable products. However, thermal pyrolysis is hindered by the formation of asphaltenes, precursors to coke, limiting cracking, causing equipment fouling, and reducing product stability. Free radicals are inherently present in heavy fractions and are generated during thermal processes. This makes these reactive intermediates central to understanding these mechanisms and limiting coking. Electron spin resonance (ESR) spectroscopy facilitates such mechanistic studies. Over the past decade, there has been no review of using in-situ ESR for studying thermal processes. This work begins with a brief description of free radicals’ chain reactions during thermal reactions and the wealth of information ESR provides. We then critically review the literature that uses ESR for mechanistic studies in thermal pyrolysis of biomass, heavy oil, shales, and coal. We conclude that limited literature exist, and more investigations are necessary. The key findings from existing literature are summarized to know the current state of knowledge. We also explicitly highlight the research gaps.

Exploring the catalytic conversion of aromatic model compounds of coal pyrolysis over Ca(OH)2

The distribution of pyrolysis products from aromatic model compounds in coal catalyzed by Ca(OH)2 was investigated at the molecular level. The composition and relative abundance of the pyrolysis products from coal were analyzed using Py-GC/MS. The rapid pyrolysis products of coal at 600 °C consisted of phenols (15.94 %), non-phenolic oxygenated compounds (25.31 %), aliphatics (49.03 %), aromatic compounds (21.74 %), and other compounds (0.03 %). Six representative aromatic model compounds (2-methoxy-4-methylphenol, p-cresol, 2,4-dimethylphenol, o-cresol, guaiacol, and catechol) were selected. The pyrolysis process of model compounds was primarily the cleavage of C-O and C-C bonds, which resulted in the formation of methoxy and methyl radicals. The results revealed that Ca(OH)2 undergoes acid-base reactions with -OH, thereby increasing the stability of the model compounds. Notably, the impact of Ca(OH)2 on the composition and distribution of pyrolysis products was significantly more pronounced in aromatic compounds containing both -OCH3 and -OH compared to those containing solely -OH. The formation pathways of pyrolysis products involving guaiacol and Ca(OH)2 were elucidated through density functional theory (DFT) calculations, demonstrating that Ca(OH)2 could facilitate more free radicals release and the conversion of model compounds. This study contributes to the understanding of the transformation of aromatic compounds during coal pyrolysis at the molecular level.

Behaviors and Mechanism of Volatiles Producing during Coal Pyrolysis with Calcium Oxide Loading: Insights from Model Compounds Analysis

Calcium is a highly effective catalyst for controlling the tar compounds produced during the pyrolysis of low-rank coal, forming valuable end products. However, the exact catalytic mechanism of calcium in coal pyrolysis remains unclear due to the complex nature of coal. In this study, the pyrolysis process and tar composition of lignite, both with and without the addition of calcium, were investigated across three distinct temperature intervals. The addition of calcium oxide increased the hydrocarbon content of tar by 16.39% in the low-temperature range and 26.53% in the intermediate-temperature range. Four model compounds containing different coal-related functional groups were selected to investigate the effects of calcium on pyrolysis performance and tar composition using thermogravimetric analysis, fixed bed reactor experiments, gas chromatography/mass spectrometry, in situ Fourier transform infrared spectroscopy, X-ray diffractometer, and X-ray photoelectron spectroscopy. The addition of calcium did not affect the thermal decomposition of polystyrene or polyethylene terephthalate; however, the primary pyrolysates underwent secondary reactions on the surface of calcium, which affected the composition of the liquid pyrolysis product. Meanwhile, calcium interacted with the carboxyl and phenolic hydroxyl groups in trimeric acid and phenolic resin to form carboxylates and phenates, which affected the thermal reaction and distribution of liquid products. The influence of calcium on the transformation mechanism of coal-related functional groups provides a theoretical basis for understanding the mechanism of calcium-catalyzed coal pyrolysis.

Kinetic analysis of coal oxidative pyrolysis before and after immersion effect

The oxidative pyrolysis kinetics of coal before and after immersion effect was studied by thermogravimetric experiments. Non-isothermal thermogravimetric analysis data were analyzed at four heating rates of 5, 10, 20 and 30 K/min. Furthermore, temperature-programmed experiments and Fourier-transform infrared (FTIR) spectroscopy were employed to understand the oxidation activity and chemical structure of raw coal and soaked coal. After coal immersion effect, the increase in the content of active functional groups, the increase in the concentration of gaseous products, and the decrease in crossing point temperature during oxidation process all indicate that soaked coal is easier to oxidize and spontaneously combust. Lastly, four model-free methods such as Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), Friedman and Kissinger methods are employed to calculate the activation energy (Eα). Kinetic analysis reveals that Eα value obtained by the former three methods significantly depends on the conversion of coal oxidative pyrolysis process, and the average Eα value of soaked coal obtained by four methods is lower than that of raw coal. For the kinetic analysis of coal oxygen adsorption and combustion stages, it is more reliable to adopt the FWO and KAS methods in turn. This research helps to better understand the mechanism of enhanced oxidation activity of soaked coal and optimizes the calculation method of kinetic parameters in different oxidation stages of coal.

Base-acid tandem catalytic upgrading of coal pyrolysis volatiles: the effects of alkaline earth oxides and modified HZSM-5 zeolites

Catalytic upgrading of coal pyrolysis volatiles is a promising technology to bolster the production of value-added chemicals from coal. To optimize product distribution and mitigate catalyst deactivation, this work investigated tandem catalysts combining the upper alkaline earth oxides (CaO or MgO) with the lower modified HZSM-5 (mesopore creation and Ga loading). The pre-cracking of volatiles over CaO or MgO lowered the average molecule size of tar components. The diffusion properties and aromatization of these pre-cracking-derived intermediates were improved, facilitating the secondary upgrading over HZSM-5-based catalysts, promoting the formation of aromatics. The experimental results showed that tandem CaO-HZSM-5 catalysts presented better synergy to improve the tar components as compared to MgO-HZSM-5, considering the formation of aromatics in tar. In this context, the mesopore and metal Ga were introduced into HZSM-5 to further upgrade the tar quality. The mesoporous structure for HZSM-5-meso and Ga/HZSM-5-meso promoted the conversion of aliphatic hydrocarbons into aromatics. Simultaneous introduction of mesopores and Ga synergistically increased the aromatic content in tar to 41.6% and reduced the carbon deposition to 0.88wt.%. In summary, this work elucidated the mechanism of tandem CaO-modified HZSM-5 catalysis for the upgrading of volatiles from coal pyrolysis.

Pyrolysis of Khat waste vs. Coal: Experimental and Aspen plus analysis

This study explores the potential of Khat waste as a biofuel through a detailed analysis of its physical properties and pyrolytic behavior, employing both laboratory and simulation techniques. Biomass, including Khat waste, is a valuable renewable energy source with potential applications in reducing waste and CO2 emissions, especially in rural areas of developing countries like Ethiopia. The physical properties of the Khat waste were thoroughly examined, including moisture content, ash, volatile matter (VM), fixed carbon (FC), and elemental composition. The apparent density of the pyrolyzed khat waste was found to be 0.9688 g/cm³. The TGA results showed a moisture loss of 6.7 %, volatile matter of 4.7 %, fixed carbon of 6.78 %, and ash content of 5.55 %. Notably, Khat waste exhibited unique thermal behavior with a downward shift in TG curves above 220 °C. DTGA identified three decomposition peaks: moisture evaporation at 100 °C, hemicellulose breakdown at 210 °C, and cellulose degradation at higher temperatures. Fourier Transform Infrared Spectroscopy (FTIR) provided insights into the functional groups present in Khat waste, including alkene functional groups and O-H bonds, which are crucial for biofuel properties. Simulation using Aspen Plus software modeled the pyrolysis process, highlighting how varying heating rates affect volatile matter and fixed carbon content. Increasing heating rates decreased volatile matter and moisture but increased fixed carbon content. These findings offer valuable insights for optimizing Khat waste as a biofuel, emphasizing the need for tailored pyrolysis conditions to enhance biofuel production efficiency.

Preparation and growth mechanism of high-quality multilayer graphene from simulated coal pyrolysis gas via chemical vapor deposition

Chemical vapor deposition (CVD) of graphene from coal pyrolysis gas provides new ways for both large-scale graphene preparation and high value-added utilization of coal. However, green, efficient and continuous preparation of high-quality multilayer graphene with both good uniformity and ideal structure characteristic remains a great challenge. Herein, we first investigated the influence of main carbonaceous species (CH4, C2H6, C3H8, CO2, and CO) in coal pyrolysis gas on the quality of CVD graphene products. The experimental results indicated that CO2 and CO have little effect on the growth of graphene, while methane as carbon source is conducive to obtain ideal graphene materials, propane and ethane could also influence structural defects and layer number of graphene products remarkably. On this basis, we designed a mixed gas of methane, ethane, and propane with optimized ratio as simulated coal pyrolysis gas, and successfully prepared high-quality multilayer graphene products on nickel foam from the as-designed simulated coal pyrolysis gas by CVD method. Notably, with a CH4:C2H6:C3H8 ratio of 2:1:1, the graphene products prepared from simulated coal pyrolysis gas outperformed that of raw coal pyrolysis gas in terms of physical structure, layer number, and quality uniformity. The corresponding I2D/IG value reached 0.97. The graphene growth process and mechanism were investigated and discussed macroscopically and microscopically. This work is of great significance for the intrinsic understanding of CVD growth of graphene from coal pyrolysis gas, and also inspires an efficient and environmentally friendly avenue for high-throughput, uniform production of high-quality graphene materials.

Insight into evolution characteristics of pyrolysis products of HLH and HL coal with Py-VUVPI-MS and DFT

To achieve a comprehensive understanding of the influence of chemical structure on the evolution characteristics of coal pyrolysis products, pyrolysis reaction of two kinds of low rank coal (Huolinhe and Huangling coal) were investigated with pyrolysis-vacuum ultraviolet photoionization mass spectrometry (Py-VUVPI-MS). The soft ionized mass spectral detection can provide evolved information of original pyrolysis product. The chemical structure of coal samples was characterized by solid-state 13C-NMR, indicating HLH coal with more branched chain and longer aliphatic chain structure. The bond dissociation enthalpies (BDE) of β-Cal-Cal and β-Cal-O within model compounds were obtained with density functional theory. The difference of peak temperature with maximum evolution was collected to investigate the effect of the chemical environment on evolution behavior of pyrolysis products. The results reveal that branched and long aliphatic side-chains can reduces BDE of β-Cal-Cal and β-Cal-O, resulting in the lower peak temperature of pyrolysis products derived from HLH coal. Substituent groups (such as alkyl and hydroxyl groups) attached on the aromatic rings can reduce peak temperatures. Moreover, the effects of the different substituted position on the aromatic ring of the methyl group presented on differences of the BDE. An increase in aromatic ring size correlates with a certain degree of reduction in BDE for β-Cal-Cal; consequently, peak temperature of pyrolysis products with larger aromatic rings is lower. The pyrolysis behavior of coal were discussed based on the experimental observations and theoretical calculation, which are beneficial to understand the reaction route and mechanism of coal pyrolysis.

Study on thermal decomposition and enrichment quality of coal from Mogoin gol deposit in Mongolia

The main purpose of this study is to investigate the thermal stability and the mechanism of thermal decomposition of Mogoin gol coal, the possibility of liquefaction by pyrolysis and thermolysis, and the possibility of enriching by heavy liquids to reduce the mineral content of coal and improve its quality. Under this purpose, The Mogoin gol coal was characterized by proximate and ultimate analysis, thermogravimetry, and investigated its thermal decomposition (thermolysis and pyrolysis). Thermogravimetric analysis was performed using Japanese HITACHI TG/DTA7300 instrument and pyrolysis investigation was carried out at different heating temperatures 200–700 °C with constant heating rate 20 °C/min for 80 min. On the basis of proximate and elemental analysis results, it has been indicated that the Mogoin gol coal is high-rank coking coal. The pyrolysis of Mogoin gol coal was studied by SNOL furnace at different heating temperatures and obtained from pyrolysis products such as hard residue, tar, pyrolytic water, and gas. From pyrolysis, the yield of pyrolysis tar (6.28%) was highest at 700 °C. The experiment of thermal decomposition (thermolysis) was carried out in air closed autoclave at 350–450 °C and using hydrogen donor solvent (tetraline) with different mass ratios of coal and solvent (1:1.75; 1:1.5). In the thermolysis experiment, the yield of liquid product is highest with the coal: solvent ratio of 1:1.5 at 450 °C.

Co-evolution with pore and molecular structure during tar-rich coal pyrolysis

In-situ pyrolysis of tar-rich coal is a potentially green and low-carbon development technology. However, there are restrictions on the transport of tar and gas products due to the evolution of pore structure during pyrolysis. Hence, revealing the evolution of pore structure during pyrolysis is helpful to understand this restraining behavior. In this work, both the open and closed mesopores inside coal with thermal treatment were investigated by low-temperature nitrogen adsorption and small angle X-ray scattering, and the molecular structure under corresponding conditions was characterized using Fourier transform infrared spectroscopy, 13C nuclear magnetic resonance, and X-ray diffraction. The results showed that the evolution of open mesopores with temperature exhibited a trend of decreasing and subsequently increasing, which was consistent with closed mesopores. These evolutions were affected by the adjustment of molecular structure during pyrolysis, and the effect was specifically divided in two stages. In the first stage (<500 °C), molecular structures such as aliphatic and oxygen were continuously removed affected by the decomposition reaction, resulting in a drastic reduction of the mesopores. In the second stage (>500 °C), molecular structure dominated by aromatic structures were continuously condensed under the influence of polycondensation reaction, leading to a gradual increase in mesopores.

Formation and evolution of coke of pyrolytic volatiles of low-rank coal on the carbon-based catalysts

The catalytic upgrading of coal pyrolysis volatiles is an effective technique to achieve clean and efficient conversion of low-rank coal. However, a major problem in this process is catalyst deactivation caused by coke deposition. This study investigated the catalytic cracking effect of activated carbon and metal-modified activated carbon on coal pyrolysis volatiles. It also explained the physical and chemical properties of coke and its formation mechanism related to volatile cracking. The results indicate that phenolic oils, pitch, oxygen-containing compounds, and polycyclic aromatics are linearly related to the coke deposited on the surface of carbon-based catalysts. These compounds act as precursors and are easily transformed into coke via cracking, deoxidation, crosslinking, and polycondensation reactions when the cracking rate of volatiles does not match the hydrogen supply rate. The micro-morphology of coke is primarily composed of amorphous carbon, which is mainly found in pores ranging from 0.50 to 5.00 nm and on the outer surface of carbon-based catalysts. The chemical structure of coke primarily consists of large aromatic clusters containing at least six benzene rings and oxygen-containing cross-linking compounds with a high degree of graphitization and a low number of carbon structural defects. This results in an increase in radical concentration of carbon-based catalysts, as well as a decrease in combustion reactivity. The outcomes will aid in controlling coke deposition during catalytic cracking of pyrolytic volatiles of low-rank coal.

Numerical simulation of an externally heated fixed bed reactor for coal pyrolysis based on porous media

A cutting-edge three-dimensional transient model utilizing porous media has been developed to accurately simulate the pyrolysis process of externally heated coal. This innovative model comprehensively considers the evaporation and condensation of water, as well as the release of volatiles. In addition, it meticulously examines the change in coal seam porosity and the interphase heat transfer between pyrolysis gas and solid coal seam. The model's precision has been appropriately confirmed through validation against the central temperature evolution inside the experimental reactor. Notably, this model systematically illustrates various aspects, including the evolution of coal layer temperature, evaporation and moisture density, changes in coal layer density, porosity distribution, volatile release, and interphase heat transfer. The obtained results reveal that the heat absorption by moisture phase change causes the coal seam temperature to relatively stabilize at around the water boiling point for a period, thus delaying the initiation of the coal pyrolysis reaction. The pyrolysis gas released by center coal seam pyrolysis tends to flow radially toward the heating wall before flowing out of the reactor. Additionally, the change in coal seam porosity increases the heat transfer rate by an average of 1.5 °C/min during the rapid heating stage. The analysis also highlights the significant occurrence of interphase heat transfer throughout the pyrolysis process and elucidates its mechanism in various stages. Ultimately, this work offers essential theoretical guidance for the design, optimization, and scaling of subsequent externally heated fixed-bed coal pyrolysis reactors.

Investigation on Nitrogen Chemistry Modeling in Coal Combustion Process with CPD Calculation

ABSTRACT This paper presents a model that combines the CPD model to predict coal pyrolysis and a nitrogen chemistry model to calculate NOx emissions from coal combustion process. The model saves the need to obtain the pyrolysis components of coal by experimental means while ensuring the accuracy of the calculation results to a certain extent. The model is validated by comparison with experimental results from the literature on coal pyrolysis cases and coal combustion cases with non-staging, fuel staging, as well as air staging. In general, the modeling results for the three combustion cases are satisfactory, except for the deviation of the results for NO in the case of some small air ratios at air staging combustion, which leads to some limitations of the model. While according to the reported studies, in the actual process, the small air ratio values are not in the optimal conditions for air staging combustion, making the model of some practical significance. In the future, more work is needed in solving the calculations of coal combustion processes at low air ratios to further refine the model.

Chlorine evolution and char characteristics during pyrolysis upgrading of Xinjiang high chlorine coal

Xinjiang ShaErHu (SEH) coal, characterized by a high chlorine concentration exceeding 1 %, can cause severe chlorine corrosion when directly burned. To utilize this high-chlorine coal in an effective and clean way, specific pretreatments such as low-temperature pyrolysis upgrading are required. In this paper, the changes in tar, pyrolysis gas, combustion characteristics, and chlorine evolution features were analyzed during the low-temperature pyrolysis of SEH coal. The effects of increasing temperature (350 °C, 450 °C, and 550 °C) were studied. The results show that the proportion of aromatic compounds released consistently increases with temperature, reaching a peak of 80 % at 550 °C in the tar. It is noteworthy that chlorine is hardly detected in tar. The percentage of chlorine released in the pyrolysis gas remains relatively stable as the temperature rises from 350 °C to 550 °C, accounting for approximately 17.5 % of the total chlorine. The percentages of water-soluble chlorine in raw coal, char-350 °C, char-450 °C and char-550 °C are 74.3 %, 80.4 %, 82.2 %, and 55.1 % respectively. The majority of the chlorine in the char produced at 350 °C and 450 °C is inorganic, making up 68.9 % and 69.4 %, respectively. In contrast, char generated at 550 °C has 58.4 % chlorine that is primarily organic. About 40 % of the inorganic chlorine is converted to organic chlorine when the pyrolysis temperature is raised to 550 °C. This conversion likely occurs as a result of the inorganic chlorine ions combining with carbon to form C-Cl bonds. In order to effectively prevent high temperature corrosion, high-chlorine coal can be treated by pyrolysis at temperatures between 350 and 450 °C, followed by water washing and burning. This research provides a theoretical foundation for the clean and efficient use of high-chlorine coal.

Imidazole ionic liquid effects on coal swelling behavior and pyrolysis characteristics: Experiment and molecular simulation

In view of low-rank coal conventional swelling solvent large pollution and the clean efficient utilization of coal, the effects of imidazole ionic liquid (IL) pretreatment with different anion groups as green solvent on coal thermal expansion and pyrolysis performance were investigated. IL interacts with coal molecules, destroying the hydrogen bonds, resulting coal expansion. Different anion groups have different modification effects, [Bmim]Cl has the best swelling effect, which is 17.37 % higher than raw coal, and the best modification time is 24 h. In addition, IL modification has a catalytic effect on coal pyrolysis, which can significantly reduce the maximum weight loss temperature and increase tar content. Coal solvent pretreatment provides a good theoretical basis for application of swelling and pyrolysis. The reduction of thermochemical reaction temperature not only reduces the content of heavy groups in pyrolysis products, but also provides new ideas for carbon emission reduction.

Kinetic characteristics and product distribution of tar-rich coal in-situ underground pyrolysis: Influence of heterogeneous coal seam

Tar-rich coal contains more hydrogen-rich structures such as aliphatic side chains and bridge bonds, which is a good raw material for coal to oil. In-situ underground pyrolysis of tar-rich coal has significant advantages such as environmental friendliness and high safety factor; it is an efficient, clean coal utilization technology. Due to the influence of geological structure and other factors in the formation process, underground coal seam was often heterogeneous, generally accompanied by coal gangue interlayers. Due to the coal gangue being relatively dense and has poor thermal conductivity, it is also rich in metal oxides, it would inevitably affect the pyrolysis characteristics and product distribution during the pyrolysis process of heterogeneous tar-rich coal seam. The pyrolysis characteristics of heterogeneous tar-rich coal seam with different positions and layers of coal gangue interlayer were investigated by the thermogravimetric analyzer, and related kinetic parameters were obtained by the distributed activation energy model (DAEM). Based on the fixed bed tube furnace reactor, the pyrolysis experiments of heterogeneous tar-rich coal seam with different positions and layers of coal gangue interlayer were also carried out, and the pyrolysis products were characterized and analyzed. The thermogravimetric results showed that in the pyrolysis process of heterogeneous tar-rich coal seam, the initial pyrolysis temperature and the temperature corresponding to the maximum weight loss rate shifted to the higher temperature range, and the pyrolysis characteristic index decreased significantly. Under the same conditions, the corresponding pyrolysis characteristic indexes of the upper coal gangue interlayer and 4 layers of coal gangue interlayer were lower, which were 11.69 (10−8·%·min−1·℃−3) and 11.65 (10−8·%·min−1·℃−3), respectively. Compared with homogeneous tar-rich coal seam, pyrolysis activation energies of heterogeneous coal seam generally showed the decreasing trend. Under the conditions of the upper part containing coal gangue interlayer and the upper-and-lower parts containing coal gangue interlayer, the pyrolysis activation energies were higher due to the hindrance of the cover layer of coal gangue to the volatile release, which were 555.88 kJ·mol−1 and 546.66 kJ·mol−1 respectively. The results of fixed bed pyrolysis experiments showed that compared with homogeneous tar-rich coal seam, the tar yield obtained was reduced, the degree of tar lightening was significantly improved, and the proportion of lighter tar was up to 70.00 wt%. The contents of aromatic hydrocarbons and phenolic compounds in tar increased, the contents of aliphatic hydrocarbons and oxygenated compounds decreased significantly, and the maximum reductions were nearly 4.50 wt% and 6.50 wt%, respectively. The gas yield increased, the proportions of H2 and CO2 in the gas increased, and the proportions of CO and CH4 decreased to varying degrees. The tar yields were lower under the conditions of upper gangue and 4 layers gangue, which were 6.53 wt% and 5.81 wt%, respectively, and the degrees of tar lightening were also more significant. From the relationship between pyrolysis characteristic parameters and tar properties of heterogeneous tar-rich coal seam pyrolysis, it was found that the existence of coal gangue interlayer was not conducive to the tar production, but it could promote the lightening of tar and improve the tar quality. Exploring the kinetic characteristics and pyrolysis product distribution of heterogeneous tar-rich coal seam could provide theoretical guidance for the site selection of in-situ underground pyrolysis engineering and the well arrangement.

Formation and emission characteristics of PAHs during pyrolysis and combustion of coal and biomass

The emission of polycyclic aromatic hydrocarbons (PAHs) poses a significant risk to the ecological environment. Therefore, it is of great importance to investigate the formation characteristics of PAHs during the coal and biomass utilization. The formation of PAHs is primarily attributed to the incomplete combustion of fuels. In this work, the organic components of raw bituminous coal (BC), wheat straw (WS), and corn straw (CS) were analyzed and the formation characteristics of PAHs were explored using a fixed-bed pyrolysis system. The emission characteristics of particulate matter (PM) and PAHs were investigated in a drop-tube furnace. The results indicate that BC contains a higher proportion of aliphatic hydrocarbons, and the oxygen-containing functional groups of WS and CS are more complex. The content of PAHs in the extracts of raw BC, WS, and CS is 24.16 µg/g, 0.39 µg/g, and 0.41 µg/g, respectively. During the pyrolysis, the PAHs formation characteristics of coal are comparable to biomass. The content of PAHs in pyrolysis gas is significantly higher than that in the char. With the increase of temperature, the PAHs content in pyrolysis gas increases and then decreases, reaching a maximum value at 1000℃. At the low temperatures, the primary source of PAHs is the decomposition of the organic component of fuels. At the high temperatures, the rupture of the basic backbone and the condensation polymerization reaction are the important factors influencing PAHs emissions. The maximum PAHs emissions of BC, WS and CS are 4196.94 µg/g, 4426.78 µg/g, and 5479.50 µg/g, respectively. During the combustion, with the increase of oxygen content, the emissions of PM and PAHs gradually decrease. The proportion of low-ring PAHs increases, while the proportion of high-ring PAHs decreases. Furthermore, PM1-2.5 accounts for 52.34–84.37 % of total PM emissions. PAHs are more easily enriched in these fine particles with the proportion of 53.71–71.09 % of the total PAHs emissions.

Study on coal pyrolysis characteristics by combining different pyrolysis reactors

The pyrolysis process of Shendong coal (SD) was first studied by combining the characteristics of thermal gravimetric (TG), pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and Gray-King assay (G-K). The results show that the order of coke yields is G-K (76.35% (mass))>TG (73.11% (mass))>Py (70.03% (mass)). G-K coke yield caused by condensation reaction and secondary reaction accounts for 3.08% (mass) and 3.24% (mass), respectively. Compared with slow pyrolysis, fast pyrolysis has stronger fracture ability to coal molecules and can obtain more O-compounds, mono-ring aromatics and aliphatics. Especially, the content of phenolics increases significantly from 15.49% to 35.17%, but the content of multi-ring aromatics decreases from 23.13% to 2.36%. By comparing the compositions of Py primary tar and G-K final tar, it is found that secondary reactions occurred during G-K pyrolysis process include the cleavage of alkane and esters, condensation of mono-ring aromatics with low carbon alkene, ring opening, isomerization of tri-ring aromatics, hydrogenation of aromatics and acids.

Mechanism of thermal gravimetric difference between simulated and experimental of coal group components

Understanding the molecular mechanism underlying coal's thermal gravimetric behavior is crucial for advancing efficient and clean coal utilization technology, a task that remains challenging to address experimentally. ReaxFF molecular dynamic simulation has been widely applied across various systems and materials, particularly excelling in simulating combustion and pyrolysis processes with complex coal models. It is necessary to establish more connections between simulation and experiment, and to analyze and discuss the differences between them. This will lay the foundation for the reliability of simulation results in revealing experimental phenomena. In this study, coal was divided into four components using a full component classification method: the dense medium component, loose medium component, heavy component, and light component. The connections of the simulated and experimental thermal gravimetric behaviors of the coal group component skeletons were conducted, and founding the inconsistencies in the later stages of the curves. To understand these discrepancies, the test environments were analyzed, identifying the behavior of tar free radicals generated during pyrolysis as a key factor influencing the thermo-gravimetric curve. To mitigate the influence of tar free radicals and reduce the observed differences, a correction coefficient was introduced into the calculation formula for the yield of non-volatile products, achieving an improved alignment between the simulated and experimental thermo-gravimetric curves. This study significantly advances the application of ReaxFF molecular dynamics simulation in elucidating the mechanisms of coal pyrolysis.