Heavy Tar Research Articles

Production characteristics in initial pyrolysis of lignin based on reactive molecular dynamics simulations

Abstract Carbonization, direct liquefaction, and gasification, which are based on pyrolysis reactions, have been proposed as efficient technologies for energy recovery from biomass resources. However, modeling pyrolysis reactions of biomass with complex molecular structures is challenging, and numerical analysis methods that consider product compositions have not yet been established. In this study, molecular dynamics simulations were performed using LAMMPS, with the force field parameterized using ReaxFF. A molecular model based on Adler softwood lignin was created. The atomic trajectories were analyzed to identify the products of the pyrolysis process, and changes over time in inorganic gas, organic gas (C1-C4), light tar (C5-C14), heavy tar (C15-C39), and char (C40+) during the initial pyrolysis of lignin were investigated. The distribution of compositions formed and the formation behavior of typical low-molecular-weight components (such as formaldehyde and methanol) and inorganic gases from lignin were evaluated. The characteristics of the products in the initial pyrolysis of lignin, as analyzed from the molecular dynamics simulations, were based on the functional groups in the molecular model of lignin. Furthermore, similarities with the product compositions identified in experiments were also confirmed.

ReaxFF simulations on the transformation pathway of nitrogen elements in the heavy tar under oxy-coal combustion

Heavy tar is a crucial intermediate product during coal combustion. To explore the transformation pathway of N atoms in heavy tar under oxy-coal combustion, a comprehensive molecular model of heavy tar with typical N-containing functional groups was constructed. Different temperatures and chemical equivalence ratios were set for the oxy-coal combustion. The ReaxFF was employed to study various products' distribution and molecular numbers. The reaction network among different precursors and NOx was extracted, and the NO to N2 conversion mechanism was summarized. The results indicated that, like char combustion, the proportion of heavy tar gradually declined, the proportion of light tar and organic gas first rose and then gradually declined, and the proportion of inorganic gas continuously rose during heavy tar combustion. As the temperature increased, the proportion of cyanide precursors decreased, while the proportion of amine precursors and NOx increased. The oxidation of N-containing intermediates became more intense as the O2 content rose, but this oxidation effect was inhibited, and the NOx generation was reduced as the O2 content further increased. NO could bond with NHi, HNO, CN, and activate NO, decomposing to produce N2O, and N2O reacted with H radical to produce N2.

Catalytic Fast Pyrolysis of Tar-Rich Coal with a Bauxite Residue for Upgrading High-Value Products: Product Distribution and Kinetic Analysis.

Fast pyrolysis technology can reduce the secondary reactions, improve the volatile product yield, and reduce the semicoke yield. Still, the high proportion of heavy tar components affects the development of fast pyrolysis industrialization. Therefore, this paper put forward a catalytic upgrading method of coal based on the solid waste bauxite residue (BR) as a catalyst. This study investigated the impact of varying particle sizes of pulverized coal and the addition of the BR catalyst on the product distribution and kinetics of coal fast pyrolysis. The results found that the tar yield was the highest at 600 °C when the particle size of pulverized coal was 75-150 μm, which was 19.44%. In the range of 550-650 °C, the relative content of benzene and toluene xylene (BTX) in liquid products increased with the temperature. With the increase of the proportion of the BR catalyst, the yield of semicoke in coal pyrolysis products increased, the yield of the gas phase also increased, and the yield of the liquid phase decreased.

Insights into heavy components evolution in the condensed volatiles from amino acids pyrolysis

Heavy components, comprised of molecules with a substantial molecular mass, are essential constituents within volatiles. Within these molecules, nitrogen-containing compounds have a tendency to undergo transformation into coke during thermal treatment, exerting a substantial impact on the efficient and secure utilization of biomass. Three amino acids- alanine with an aliphatic structure, phenylalanine with additional aromatic features, and tryptophan with an extra heterocyclic ring were selected as model compounds, individually subjected to pyrolysis within the temperature range of 450 to 650 °C. The evolution of heavy components and the associated reaction mechanisms were comprehensively explored through extensive characterization and network analysis. The results showed that alanine transformed into heavy components mainly through dehydration followed by dissociation of CH2 and CH. In contrast, phenylalanine and tryptophan tended to undergo fragmentation into C8H7N· and C12H10N2· derivatives, subsequently recombining to form heavy molecules. As the temperature increased, heavy components in alanine-derived volatiles monotonously decreased due to elimination reactions and the breaking of C–C bonds. Conversely, heavy components in phenylalanine and tryptophan initially increased and then decreased due to the enhanced cracked fragments, along with the competitive relationship between aromatization and polymerization reactions. For all three amino acids, there is a positive correlation among the average mass of heavy components, tar yield, and the intensity of permanent free radicals in the char. This implies that we may rapidly predict the evolutionary patterns of heavy components and tar through free radical detection. HCN dissociation is mathematically identified as a pivotal reaction in heavy component evolution, irrespective of amino acid type, possibly unveiling an important pathway for NOx formation. This study shows substantial theoretical significance for the eco-friendly and efficient conversion of nitrogen-rich biomass.

Co-pyrolysis behaviors of biomass and polymer plastics by using reactive molecular dynamics simulation

Co-pyrolysis of oxygen-rich biomass and hydrogen-rich plastic polymer is a promising approach for waste management, which has been investigated by using reactive molecular dynamics simulation (ReaxFF MD) in this work. The blend models containing multicomponent mixtures of polymer (polyethylene, polypropylene, polystyrene) and biomass (cellulose, hemicellulose and lignin) were constructed for the first time, which is close to the real utilization of waste materials. The simulation results show that the degree of positive synergistic effect between biomass and polymer increases with temperature, where the polymer promotes biomass pyrolysis and delays its recombination. Co-pyrolysis greatly affects the product yields through cross reactions of hydrogen and hydroxyl transfer, leading to more oxygen-containing bio-oils produced with the aid of polymers providing hydrogen. Produced from heavy tar at first and light char at late pyrolysis period, char precursors are generated more and faster in the co-pyrolysis system with more biomass because oxygen functional groups provide the initial cross-linking sites. As compared to pyrolysis of only binary components, the observed qualitative synergistic results in co-pyrolysis of biomass-polymer mixtures provide important insights into the detailed reactions for co-pyrolysis process, which can complement experimental observations to modulate the nature and yield of the desired bio-oils and bio-chars.

Catalytic pyrolysis of poplar sawdust over biochar of varied origin: Impact of volatile-char interactions

Volatile-char interaction is almost unavoidable in pyrolysis, which is part of the process shaping composition of bio-oil/gases and properties of char. In this study, the influence of the char from sawdust/spinach/rice/spirulina/pig manure on pyrolysis of poplar sawdust was investigated at 500 °C, aiming to understand the impact of the char of same or varied origin on formation of volatiles/gases and change of the char catalysts themselves from the volatile-char interaction. The results suggested that the sawdust-, spinach- or rice-char catalyzed cracking of volatiles, reducing formation of heavy tar, decreasing production of bio-oil (28.6–35.5 % vs 49.4%) while enhancing gases generation (36.9–44.0 % vs 22.4%). The spirulina- and manure-char enhanced secondary condensation of volatiles, producing more heavy tar, more bio-oil, and additional oxygen-rich carbonaceous deposit with highly aliphatic nature. This reduced thermal stability, the C/O ratio of the char catalyst, and also led to coverage of char surface, especially for the manure-char catalyst. The in-situ IR techniques showed that manure-char catalyzed formation of abundant ketones and esters/lactones in intermediates, which further involved in polymerization. In comparison, the sawdust-char catalyst catalyzed cracking and aromatization of the volatiles of same origin, producing carbon- and hydrogen-rich carbonaceous solid, enhancing aromatic nature of the char catalyst, which was actually a real process for evolution of structure of biochar in pyrolysis of the sawdust.

Oxidative pyrolysis of spirulina: Impacts of oxygen on bio-oil and property of biochar

Externally added oxygen could react with volatiles in oxidative pyrolysis, which might also react with biochar and tailor property of biochar at varied temperatures via distinct ways. This was investigated herein through oxidative pyrolysis of spirulina in a mixture of N2/O2 (11 vol%O2) at 300–600°C. The results suggested that the added oxygen oxidized light volatiles and heavy tar, increasing production of bio-oil while diminishing formation of heavy π-conjugated organics. Oxidation of biochar occurred above 300°C, forming additional oxygen-containing organics but showing little impact on mass yield until 600°C. The excessive oxidation reactions formed oxygen-rich biochar at 300 or 400°C, while the oxygen-containing organics started to be cracked at 500 or 600°C, producing the biochar of fragmented surface. The additional oxygenated organics formed reduced thermal stability of the biochar but improved the overall combustion performance of the resulting biochar at 400°C via lowering ignition temperature and burnout temperature, due to the presence of abundant oxygenated aliphatic structures. The characterization with in-situ IR technique showed that the additional functionalities generated were mainly in the form of -OH, adsorbed CO2, CO, and C-O-C via oxidation of the aliphatic structures, which was less thermally stable than the C-N groups formed through dehydrogenation of amino acid in the oxidative pyrolysis.

Pyrolysis characteristics and product distribution of low-rank coal with heat-carrying particles adopting TG-FTIR and a novel self-mixing down tube reactor

A novel self-mixing down tube (SDT) pyrolyzer has been developed, which can separate the heat carrying particles preheated in the fluidized bed from the coal, and the two can be mixed through a specific structure to instantly heat the coal particles to a predetermined temperature. The present study investigated the pyrolysis behavior of low-rank coal with hot silica sand heat-carrying particles adopting TG-FTIR. Identifies optimal conditions for maximum tar production in a laboratory-scale SDT using a significantly larger range of coal feed rates (4 kg/h). An increase in the degree of coal metamorphism increased the initial temperature and the yield of char. The tar yield from Huolinhe lignite coal (6.56 %) and Daliuta bituminous coal (7.48 %) showing a trend of first increased and then decreased with elevating the operating temperature and optimal temperature at 520 °C, respectively. The effect of coal particle size on tar yield mainly comes from the different degrees of secondary cracking caused by contact with solid heat-carrying particles as well as the differences in the content of macerals. GC‒MS spectra of tar and deposited carbon on silica sand indicated that heavy tar can easily deposit on the surface of sand and form char through polymerization reactions.

Pyrolytic Modification of Heavy Coal Tar by Multi-Polymer Blending: Preparation of Ordered Carbonaceous Mesophase.

In order to achieve the high-value utilization of heavy tar for the production of enhanced-performance graphite foam carbon, the carbon mesophase was ready from the heavy component of low-temperature coal tar, and the coal tar was modified by styrene-butadiene-styrene (SBS), polyethylene (PE) and ethylene-vinyl-acetate (EVA) copolymers. The order degree of the carbonite mesophase was analyzed using a polarizing microscope test, Fourier transform infrared spectroscopy and X-ray diffraction to screen out the most suitable copolymer type and addition amount. Furthermore, the mechanism of modification by this copolymer was analyzed. The results showed that adding SBS, PE and EVA to coal tar would affect the order of carbonaceous mesophase; however, at an addition rate of 10.0 wt.%, the linear-structure SBS copolymer with a styrene/butadiene ratio (S/B) of 30/70 exhibited the optimal degree of ordering in the carbonaceous mesophase. Its foam carbon prepared by polymer modification is the only one that forms a graphitized structure, with d002 of 0.3430 nm, and the maximum values of Lc and La are 3.54 nm and 2.22 nm, respectively. This is because, under elevated pressure and high-temperature conditions, SBS underwent chain scission, releasing a more significant number of methyl and other free radicals that interacted with the coal tar constituents. As a result, it reduced the affinity density of heavy coal tar molecules, enhanced fluidity, promoted the stacking of condensed aromatic hydrocarbons and increased the content of soluble carbonaceous mesophase, ultimately leading to a more favorable alignment of the carbonaceous mesophase.

Infrared Spectrum and Principal Component Analysis of Heavy Tar Cut by Different Fractions from Tar-Rich Coal.

The composition and content of heavy tar vary significantly depending on the pyrolysis conditions and separation methods. This study aimed to effectively identify the main components and content of heavy coal tar and provide a theoretical basis for its subsequent utilization. To achieve this, simulated distillation and infrared spectrum analysis of heavy coal tar were conducted with a focus on understanding the impact of simulated distillation on the composition and structure of tar. The results showed that the fraction content in the tar underwent significant changes after simulated distillation at different temperatures. Specifically, the content of light oil decreased from 4.3 to 0.1%, while the asphalt content increased from 77.6 to 90.6%. Infrared spectrum and peak fitting revealed that the distilled coal tars exhibited similar characteristic peaks in regions associated with hydroxyl, aliphatic hydrocarbon, oxygen-containing functional group, and aromatic hydrocarbon structure. Based on the infrared spectrum of heavy coal tar, principal component analysis was conducted on different fractions. When using two principal components, the cumulative value reached 96.93%. It was found that PC1 displayed strong peak signals around 749 and 687 cm-1, while PC2 exhibited strong peak signals near 2356 and 1143 cm-1.

Purification of Quinoline Insolubles in Heavy Coal Tar and Preparation of Meso-Carbon Microbeads by Catalytic Polycondensation.

The quinoline-insoluble (QI) matter in coal tar and coal tar pitch is an important factor affecting the properties of subsequent carbon materials. In this paper, catalytic polycondensation was used to remove QI from heavy coal tar, and meso-carbon microbeads could be formed during the purification process. The results showed that AlCl3 had superior catalytic performance to CuCl2, and the content of QI and heavy components, including pitch, in the coal tar was lower after AlCl3 catalytic polycondensation. Under the condition of catalytic polycondensation (AlCl3 0.9 g, temperature 200 °C, and time 9 h), AlCl3 could reduce the QI content in heavy coal tar. The formed small particles could be filtered and removed, and good carbon materials could be obtained under the condition of catalytic polycondensation (AlCl3 0.9 g, temperature 260 °C, and time 3 h).

Experimental study on hydrogen production from heavy tar in biomass gasification furnace catalyzed by carbon-based catalysts

High boiling point heavy tar compounds such as pyrene and naphthalene are key factors causing corrosion and blockage in the pipelines of biomass thermal utilization equipment, and catalytic reforming represents an effective means to address heavy tar issues. Biomass carbon, characterized by its cost-effectiveness, high efficiency, and environmental friendliness, is envisioned as a catalyst carrier with extensive application prospects. In this study, the tar produced by a biomass fixed-bed gasifier was investigated, with an initial analysis of the pyrolysis process and definition of its different stages conducted. It was discovered that the condensation and secondary cracking stages are the predominant phases for the formation of heavy tars, including polycyclic aromatic hydrocarbons (PAHs). Carbon-based carriers were prepared using apricot shell as the raw material, and biomass carbon-based catalysts were synthesized via impregnation methods. The heavy tar extracted from the original tar through distillation was utilized in the hydrogen production tests of steam catalytic reforming with biomass carbon-based catalysts. The investigation into the impact of temperature, steam-to-tar mass ratio (S/T), and tar-to-catalyst mass ratio (T/C) on the catalytic reforming of tar has been conducted. In considering a variety of parameters, including hydrogen production rate, tar conversion rate, and the volume of hydrogen generated, it was found that under the conditions of 800 °C, S/T = 3, and T/C = 2, the Ni/C catalyst yielded the highest amount of hydrogen (91.52 g H2/kg tar) with a tar conversion rate of 93.30 %. The Ni-Co/C composite catalyst exhibited stronger catalytic activity and superior deactivation resistance compared to the monometallic catalysts. For the latter, steam regeneration methods effectively restored the catalytic activity of the carbon-based catalysts, enhancing the hydrogen yield rates of Ni/C and Co/C catalysts by 2.74 and 1.55 times respectively.

Study on the effect of cyclic catalytic pyrolysis on sludge pyrolysis products

Pyrolysis technology has become a hot issue in the industry of harmless treatment of sludge in recent years. The present ex-situ catalysis process suffers from the defects of unidirectional flow of pyrolysis steam, easy to form coke buildup on the catalyst surface, and poor heating value of the output heavy tar, which is difficult to be reutilized. This paper designed a cyclic catalytic cracking by pyrolytic steam process to optimize the above defects: under the optimal experimental conditions (pyrolysis starting temperature was 500 ℃, final temperature was 800 ℃, iron-based carbon catalyst dosage was 4 g, and motor speed was 1050 r/min), this process generated more pyrolysis gas and light tar. The increase in pyrolysis temperature had the most significant effect on pyrolysis gas yield, the dosage of iron-based carbon catalyst had the most significant effect on pyrolysis gas composition, and the increase in motor speed had the most significant effect on tar cracking. The cyclic catalytic cracking by the pyrolytic steam process can increase the residence time of pyrolysis steam in the reaction unit and promote the secondary cracking, catalytic, and reforming effects of tar.

Three-dimensional porous carbon materials as catalysts for upgrading coal pyrolysis volatiles to boost light tar

The large proportion of heavy components in volatiles from coal pyrolysis is liable to entrain dust and form coke, resulting in the difficulty of separating oil from dust, which thus causes the blocking of technical pipelines. Carbon-based catalysts (BC and BHC) and metal-modified carbon-based catalysts (Fe-BC, Ni-BC, Fe-BHC and Ni-BHC) were prepared for the upgrading of volatiles from coal pyrolysis. This three-dimensional interconnected macroporous carbon material is primarily derived from the pyrolysis of the blend of a biochar and a caking coal. The results show that the metal-modified carbon-based catalysts aid in activating small molecules, such as CH4, CO2, and H2O, increasing in-situ hydrogen donation while cracking polycyclic aromatics, macromolecular oxygen-containing compounds, and N, S-containing heterocyclic compounds. This thus increases the selective catalysis of benzene, naphthalene, biphenyl, and anthracene and improves tar quality. The unactivated Fe-BC and Ni-BC catalysts exhibit high selectivity for light oil with a boiling point below 170 °C. In contrast, the activated Fe-BHC and Ni-BHC catalysts have a stronger ability to crack heavy tar due to the synergistic effect of metal and carbon defect structures. However, the micropores within the catalysts prevent mass transfer of macromolecular compounds, drastically lowering the tar yield with coke formation. Overall, the unactivated metal modified catalyst with three-dimensional porous structure allows for the synergistic effect of dust removal and upgrading of volatiles while maintaining the tar yield, especially the Fe-BC catalyst with a lower price.

Developing Additives Based on Russian Coal for the Thermal Hydrocracking of Heavy Tar 4. X-ray Diffraction Study of Coal Composition

Developing Additives Based on Russian Coal for the Thermal Hydrocracking of Heavy Tar 4. X-ray Diffraction Study of Coal Composition

Catalytic Effect of Reactive Extraneous Mineral Composites on Char and Tar Distribution during Pyrolysis of Highveld Partially Oxidized Fine-Coal Reject and Its Beneficiated Residues.

In this study, the South African partially oxidized fine-coal reject (FCR), which is associated with human health and environmental problems and sustains high disposal expenses, was subjected to density-separation, chemical fractionation, and demineralization experiments to isolate and evaluate the mode of occurrence of mineral-matter (MM) effects on the FCR pyrolysis. A unique composite of two reactive oxides (i.e., MgO and Fe2O3) and a hydrated oxide [i.e., Ca(OH)2] representing major extraneous coal-minerals were blended with either FCR, demineralized FCR, and its beneficiated samples to evaluate the yields of pyrolytic products and activation energy following a novel procedure. The properties of FCR samples and their pyrolytic products were determined by different analyses. Results indicate that the reactive oxides and a hydrated oxide composite addition increased the average activation energy (332.0-476.5 kJ/mol) for FCR due to the initial Ca(OH)2 decomposition and Fe2O3 reduction that took place under pyrolysis conditions. The FCR mineral-rich sink fractions achieved the highest carbon conversion (char yield = 78.8% and tar yield = 5.1%) compared to those of other samples (e.g., <1.9 g/cm3 float char yield = 87.3% and tar yield = 2.3%) evaluated due to higher proportions of calcite/dolomite/pyrite cleats and nonmineral inorganics (Ca, Mg, Na, and Fe) which catalyzed the pyrolysis reactions. On the other hand, CaCO3, CaMg(CO3)2, and metakaolinite formations in the char derived from a blend of reactive oxides and a hydrated oxide composite and FCR interfere with the pyrolysis reactions. Also, deoxygenation reactions were impeded by oxygen present in the reactive oxides and a hydrated oxide composite. The potent catalytic effects of cleat minerals and the extraneous minerals associated with cracking of heavy tars to lighter fractions open opportunities to further understand the mode of occurrence of MM present in FCR during utilization in global pyrolysis. This may reduce waste disposal costs, health-hazards, air-pollution, and FCR volumes and augments feed-coals.

Developing Additives Based on Russian Coal for the Thermal Hydrocracking of Heavy Tar 3. EPR, NMR, and IR Spectroscopy

Developing Additives Based on Russian Coal for the Thermal Hydrocracking of Heavy Tar 3. EPR, NMR, and IR Spectroscopy

Impacts of acidic or basic sites on poplar sawdust on pyrolytic products and pyrolysis kinetics

Pyrolysis of biomass typically involves intensive dehydration, cracking, secondary condensation, etc. It is known that these reactions could be accelerated or suppressed with externally added acidic/basic sites presenting on surface of a biomass feedstock, which is expected to impact evolution of pyrolytic products and pyrolysis kinetics. This was investigated herein by pyrolysis of the poplar sawdust loaded with H2SO4 or NaOH at 600 °C. The results indicated that H2SO4 presence enhanced charring of the volatiles including sugars, sugar-derivatives and some lignin-derivatives via dehydration, polymerization, etc. This enhanced biochar production while diminished bio-oil formation and abundance of most of the organics in bio-oil including carboxylic acids, heavy tar (i.e. π-conjugated aromatics), etc. Nevertheless, H2SO4 did not lead to substantial removal of hydrogen and oxygen in the biochar, but increased the activation energy (Ea) of the pyrolysis from 162.6 kJ/mol in pyrolysis of unloaded sawdust to 194.7 kJ/mol. In converse, NaOH promoted cracking/gasification of biochar and the volatiles including heavy tar, forming dominantly gases. NaOH presence also made structures of the semi-char (biochar precursor) more fragmented, reducing the Ea for pyrolysis to 135.7 kJ/mol. The in-situ IR characterization of the pyrolysis process showed that H2SO4 promoted removal of -OH via dehydration or transformation to CO and the further condensation of CO, but not dehydrogenation of -C-H. NaOH promoted mainly deoxygenation and cracking of the sugary structures but not dehydration of -OH to carbonyls.

Influence of the Preparation and Heat Treatment of Heavy Pyrolysis Tar on the Physicochemical Properties of Petroleum Pitch. 2. Group Chemical Composition

Influence of the Preparation and Heat Treatment of Heavy Pyrolysis Tar on the Physicochemical Properties of Petroleum Pitch. 2. Group Chemical Composition

Developing Additives Based on Russian Coal for the Thermal Hydrocracking of Heavy Tar. 2. Surface Morphology and Porous Structure

Developing Additives Based on Russian Coal for the Thermal Hydrocracking of Heavy Tar. 2. Surface Morphology and Porous Structure