Chemistry of carbonization

Considerable progress towards understanding the chemistry of carbonization has been achieved through the efforts of a large number of investigators. Advanced analytical techniques such as: chromatographic methods, thermal analysis, mass spectroscopy, and electron and nuclear magnetic resonance, have enhanced our ability to characterize carbonaceous materials. Structural studies using X-ray have demonstrated the relationships of starting structure to final graphitizability. Reaction studies, using model compounds have shown the importance of molecular rearrangement and dehydrogenative polymerization on the overall carbonization process.     

Carbonization of pitch: Compatibility of components in carbonization

The carbonization of four kinds of solvent-refined-coal (SRC) pitches was investigated in order to determine the properties required for the formation of needle coke. Although the pitches were free from Ql materials, two of them gave needle cokes, whereas the other two gave mosaic cokes. The BS fractions of all pitches formed needle cokes, and all Bl fractions were infusible and isotropic. A combination of suitable BS and Bl fractions gave a needle coke, whereas another mixture formed a mosaic coke, indicating that the BS and the Bl fractions can be compatible to yield a needle coke. Cocarbonization of the BS fractions from other pitch sources with the Bl from the SRC pitches was further studied to evaluate the compatibility, which has been discussed from the structural viewpoint. Trials to improve the properties were proposed, based on the structural information.     

Influence of the quinoline-insoluble matter in pitch on carbonization behaviour and structure of pitch coke

The influence of the quinoline-insoluble matter in pitch on the carbonization behaviour and especially on the pitch-coke properties is of basic interest both for the production of pitch coke and the manufacture of carbon electrodes. A more isotropic coke is produced by increasing the amount of quinoline-insolubles. The coefficient of thermal expansion (CTE) by volume measured directly at the coke particle is strongly influenced by the type of quinoline-insoluble. A larger amount of the primary type of quinoline-insolubles causes a large increase of the CTE (volume) from 3.2 to 17.4 × 10^?6K^?1, whereas the secondary type influences the CTE only slightly. This strong influence of quinoline-insolubles on the structure of pitch-binder coke results in a remarkable increase of mechanical strength of carbon artefacts.     

Primary carbonization of an anisotropic ‘mesophase’ pitch compared to conventional isotropic pitch

The evolution during carbonization treatments of a 100% anisotropic pitch (pitch C) was compared to that of Ashland 240 (100% γ resins). The anisotropic pitch C results from a gas-sparge preparation leading to a composition of 93% β resins (QS-TI) and 7% γ resins (QS). It is made of a major component (QS-TI), in which droplets (100-300 nm in size) partially toluene soluble are distributed. The physicochemical, textural and microtextural evolutions of the two pitches were studied. During pitch C primary carbonization, anisotropic droplets grow by coalescence, then decompose into Brooks and Taylor mesophase spheres suspended in isotropic drops. These drops develop at the expense of the anisotropic matrix by a continuous regeneration of the small anisotropic droplets which feed the isotropic drops by diffusion process. Then inside these drops, mesophase spheres grow then coalesce and the behaviour of a conventional pitch is restored. These various molecular associations are due to absence of chemical events below 450 °C, leading to the global mass spectrum being constant. At 500 °C the material is homogeneously anisotropic though plastic, the metastable system is destroyed and the evolution of conventional pitches is recovered, i.e. above 550 °C macropores develop up to solidification at 600 °C (semi-coke stage).     

Release of carcinogenic hydrocarbons in the carbonization of coal pitch

The carbonization of industrial coal-pitch samples is studied, with particular attention to the liberation and conversion of benz[a]pyrene and other carcinogenic polycyclic aromatic hydrocarbons. The hydrocarbon composition of pitch sublimates and the benz[a]pyrene concentration in the smokestack gases are determined during regular and high-speed carbonization. The maximum benz[a]pyrene content in the smokestack gases is seen at 750–800°C, regardless of its content in the initial pitch. The benz[a]pyrene content basically determines the carcinogenic hazards of pitch processing.     

Influence of pitch additions on coal carbonization: Characterization of pitches

The technique of etching polished coke surfaces and examination by SEM was used to compare the abilities of a series of pitches to modify the carbon texture of cokes prepared from two low-rank coals. Cokes prepared from the pitches were similarly examined and a numerical texture index, the magnitude of which increased with increasing content of the larger textural components, was found to provide a useful measure of the ability of the pitches to modify the coke carbon texture.     

Emission of carcinogenic polyaromatic hydrocarbons in the carbonization of coal pitch

The composition of polyaromatic hydrocarbons liberated at different carbonization rates of coal pitch in an inert atmosphere is studied. The benz[a]pyrene equivalent is calculated. It is shown that reducing the carbonization rate increases the emissions of carcinogenic polyaromatic hydrocarbons.     

Modification of the carbonization properties of coal-tar pitch by supercritical fluid extraction

The carbonization properties of coal-tar pitch were modified by supercritical fluid (SCF) extraction. Pitches extracted with supercritical toluene (SCFE pitch) contained none of the quinoline insoluble (QI) matter responsible for anisotropic structures with small unit sizes. The size of anisotropic structures from SCFE pitches was closely related to the β-resin (toluene insoluble and quinoline soluble fraction) content. Anisotropic structures from blended pitches prepared to have the same β-resin content as SCFE pitches (by blending toluene soluble (TS) and β-resin fractions obtained through a conventional liquid solvent extraction) were smaller. Extraction of the β-resin fraction with supercritical toluene could be interpreted by the co-solvent effects of the TS fraction dissolved in the extract phase. The high concentration of TS fraction in the extract phase enhanced the solubility of the β-resin fraction into this phase. In the raffinate phase, the heavier β-resin components, which are unsuitable for the development of mesophase structures, coagulated and formed unextractable QI matter through reduction in the concentration of the TS fraction. Since the TS concentration in both the phases depends on pressure and the ratio of the amounts of supercritical toluene and pitch, the control of the β-resin content in the SCFE pitches is possible through the adjustment of these two parameters.     

Effects of fullerene addition on the carbonization of synthetic naphthalene isotropic pitch

The toluene soluble fraction of fullerene soot, consisting of C₆₀ and C₇₀ and other fullerenes, was co-carbonized with synthesized isotropic pitch derived from naphthalene. Mixtures of fullerene and pitch gave carbons in higher yield than expected from their single carbonizations at fullerene contents <30 wt%. The fullerenes suppressed the expansion of the pitch during carbonizations, and changed the optical textures of resultant carbons. At levels of addition of fullerenes <30 wt%, no fullerenes could be detected in resultant carbons by spectroscopy, but were detected as spheres of ca. 10–20 nm diameter in the carbons by TEM. It is considered that fullerenes remove hydrogen from the naphthenic structures of the pitch and so alter carbonization characteristics. Hydrogenation breaks the spheroidal fullerene framework.     

Oxidation and carbonization in the preparation of carbon fibres from petroleum pitch

Conclusions -- Gaseous and resinous products are evolved in the thermo-oxidation of spun pitch fibres. The main components of the gas are H₂O, CO, and CO₂. The effect of rate of temperature elevation, velocity of the air stream, and the duration and temperature of thermo-oxidation on the amount of evolved oxidation products, and oxygen content and sample weight gain have been investigated.-- The kinetics of evolution of H₂, CO, CH₄, and CO₂ has been studied in the carbonization of spun and oxidized pitch fibres. It has been shown that during temperature elevation CO₂ is initially evolved, and then CO and CH₄. H₂ is evolved at temperatures of 540°C and higher.Bashkirskii State University. Translated from Khimicheskie Volokna, No. 3, pp. 37–38, May–June, 1991.     

沥青球在流化床反应器中氧化不熔化及后续炭化行为的研究

用流化床反应器对沥青球进行了氧化不熔化处理,考察了不同温度下沥青球的氧化行为及其后续炭化行为。采用热重分析仪、傅里叶变换红外光谱仪、元素分析仪及扫描显微镜对氧化沥青球和炭化球进行了表征。结果表明:流化床反应器强化了沥青球氧化不熔化的传热和传质过程,在0.21 m/s的临界流态化气速下,以0.5℃/min快速升温至300℃进行沥青球的不熔化处理,得到具有良好表面形貌和球形度的氧化沥青球及其炭化球,极大改善了沥青球制备过程中氧化不熔化的耗时、耗能问题。     

Transformation of doped Fe in pitch sphere in carbonization and activation processes

Ferrocene was added into the starting pitch in order to produce the pitch-based spherical activated carbon with mesopore (PSAC-M) forms of doped Fe in different stages—stabilization, carbonization, and activation—during the preparation process of the PSAC-M being analyzed using Mössbauer spectroscopy (MES), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Ferrocene was oxidized into -Fe₂O₃ at the stabilization step. The major portion of -Fe₂O₃ was reduced into -Fe during carbonization at 900°C, which was subsequently transformed into Fe₃C by 1200°C. No obvious sintering or agglomeration of iron particles occurred during carbonization process even when the carbonization temperature was as high as 1200°C. The activation re-oxidized -Fe into γ-Fe₂O₃, which aggregated and grew up along with the activation time and iron content.     

Analysis of the carbonization and formation of coal tar pitch mesophase under dynamic conditions

Thermokinetic analysis of three pitch samples was carried out: coal tar pitch obtained from light coke oven tar (P), mesophase pitch after 10.5 h (MP1), and mesophase pitch after 12 h (MP2) thermopreparation at 410 °C. The process was realized in a continuous system with a 10 kg mass being charged to the reactor. It was demonstrated using Kissinger’s law that the temperature criterion, the first-order thermokinetics and the calculated Arrhenius law parameters fulfill the isokinetic effect when the classical routes of thermokinetic analysis of the samples prepared under dynamic conditions (at three heating rates) are used, which makes the qualitative interpretation of differences between these samples difficult. An alternative solution was proposed using the relative rate of thermal decomposition. The temperature ranges of the chemical reactions leading to the formation of mesophase structures, as well as the temperature ranges of the coking processes of the Fixed Carbon phase, were determined.     

Structural modification of coal-tar pitch fractions during mild oxidation—relevance to carbonization behavior

Modified pitches with softening points of about 175 °C were prepared by air-blowing at 300 °C of coal-tar pitches from different commercial coke-oven tars. The modifications induced by the mild oxidation were monitored using solvent and extrographic fractionation, elemental analysis, ¹H NMR and HPLC. Optical microscopy was used to follow the effect of air-blowing on carbonization behaviour. Low molecular weight cata-condensed PAHs and those with basic nitrogen and hydroxylic functionalities present in extrographic fractions F2 and F4, respectively, are preferentially polymerized pitch constituents. In contrast, peri-condensed PAHs in extrographic fractions F2 and F3, are practically unreactive under the oxidation conditions used. The mild oxidation enhances the tendency of quinoline insoluble (QI) particles to form aggregates in an early stage of thermal treatment, modifying the mode of mesophase development and leading to a non-homogeneous optical texture. The enhanced propensity of QI to aggregation is discussed in terms of structural peculiarities of the parent pitch and possible oxidative polymerization reactions.     

高温煤焦油沥青黏结剂碳化固结作用在炭质型材中的应用与研究进展

高温煤焦油沥青（high temperature coal tar pitch,HTCTP）具有优良的润湿性和黏结性,可用作黏结剂。HTCTP黏结剂能与炭质颗粒物料产生良好的固结作用,因此在不同类型炭质型材的制备中获得广泛的应用与研究,HTCTP高温过程的黏结性能及碳化固结作用效果决定了炭质型材的机械强度和理化性能。本文综述了HTCTP作黏结剂制备炭质型材的一般工艺过程和相关研究进展,梳理了不同应用领域对HTCTP碳化固结作用的共性机制和个性特点,总结了HTCTP的性能影响因素及碳化固结作用机理。通过分析HTCTP不同组分在碳化固结过程中的作用及转化过程,揭示HTCTP碳化固结作用与炭质型材机械强度的关联机制及影响碳化固结强度的关键因素,提出强化HTCTP碳化固结作用的措施,提升HTCTP黏结剂在炭质型材制备中的应用效果。     

Modifying carbonization properties of pitches. 1. Conversion of benzene-insoluble matter of coal-tar pitch into graphitizable carbon

Reactivities of benzene-insoluble/quinoline-soluble matter (QS—BI) and quinoline insolubles (QI) of coal-tar pitch in reductive alkylation and hydrogenation with the aid of alkali metals were investigated, to obtain basic information on the possibility of changing their graphitizability and the orientation of their cokes. Alkylation increased the solubility in benzene of both components; however only the soluble fraction of QS—BI was fusible. In contrast, the soluble fractions produced by hydrogenation from both QS—BI and QI were fusible giving a flow-pattern structure and graphitizable carbon. Fusibility of pitches in the early stage of carbonization is discussed from the viewpoints of graphitization and orientation of cokes obtained by subsequent heat treatment.     

High temperature 1H-NMR study of coal and pitch at the early stages of carbonization

The temperature dependence of the ¹H-NMR absorption in coals and pitches have been obtained using a pulsed Fourier transform NMR spectrometer with a high temperature probe. The samples as received were heated in the high temperature probe and NMR spectra were obtained simultaneously. With increasing temperature, the value of the line width at half-height (ΔH_1/2) of a brown coal decreases and then increases rapidly. On the other hand, tar pitch and the γ component of a coal indicate that the values of ΔH_1/2 remain small over a wide temperature range. It is found that there is an excellent relation between the temperature dependence of ΔH_1/2 and the optical texture of the mesophase.     

Molten storage and carbonization characteristics of premium grade coal-tar pitch: Influence of crude tar type and distillation conditions

A growing trend towards the transport and use of molten rather than solidified pitch has caused tar distillers to examine the relative stability of pitches while held in storage tanks. The scope of laboratory storage-stability tests can be extended to higher temperatures as a method of studying the earliest stages of pitch polymerization/carbonization. Some indications of the ultimate performance of the pitch in the end-use process can be gained. Crude tar has a strong influence on the ultimate properties and performance of a pitch. Seven tars have been made into 120 °C Mettler softening-point pitches by two distillation methods to study their subsequent storage and carbonization behaviour. The behaviour exhibited by each pitch allows a judgement on overall ‘reactivity’ and on the sensitivity to thermal conditions in the distillation of each tar. Two distillation schemes were adopted: batch distillation followed by heat treatment; and batch distillation at a pressure which was progressively reduced. The results show that time at elevated temperature has a major effect on the properties and performance of the resulting pitch. Vacuum distillation allows lower processing temperatures. Larger amounts of secondary quinoline-insolubles were formed under the ‘distillation with heat treatment’ scheme. Complete wetting of coke by pitch took place at lower temperatures for the vacuum-distilled pitches. Heat-treated pitches increased more rapidly in softening point and lost more mass when thermally soaked. Distillation under vacuum conditions resulted in significantly modified pitch carbonization characteristics.