Primary Coke Research Articles

Investigation into the coking-related key reaction steps in dry reforming of methane over NiMgOx catalyst

With pre-reduced NiMgOx catalyst, the structure evolution, catalytic performance, surface species and coking-related reaction steps are investigated in methane dry reforming (DRM) at selected temperatures. Satisfactory conversion of CH4 and CO2, H2/CO ratio, and anti-coke stability are achieved in long-term (78 h) reaction at 650 °C as a proper temperature. The characterizations evidence co-existence of metallic Ni0 and solid solution phases in the working catalysts. Combined catalytic evaluation and coke analysis reveal that, at 650 °C, endothermic CO2-free CH4 decomposition shows a high initial rate for H2 production but severe coke deposition for rapid deactivation, serving as the primary coking process. In contrast, exothermic CO disproportionation exhibits a lower reaction rate and a higher coke graphitization degree. The in-situ DRIFT spectra demonstrate that carbonates and bicarbonates serve as the primary adsorbates during CO2 dissociation process, whereas formate intermediates dominate in DRM reactions. Such evolution of the surface species verifies facile dissociation of both CH4 and CO2 in well-matched rates, promoting coke-resistance DRM reaction over NiMgOx catalyst.

The effect of coke and soot formation on the bio-oil combustion at high temperatures

Bio-oil shows promising potential as an alternative fuel source. However, the formation of coke and soot during the combustion process hinders its use. This study investigates the formation mechanisms of primary coke, secondary coke, and soot during the initial stage of bio-oil combustion. The relationship between the physicochemical properties and the combustion characteristics of coke and soot is further analyzed to mitigate their adverse effect. It indicates that heavy aromatic compound polymerization plays a dominant role in primary coke formation, while graphitization and aromatic ring growth initially increase and then decrease with increasing temperature. Primary coke obtained at 1000 °C exhibits the highest degree of graphitization but small microscopic size, resulting in poor combustibility. Secondary coke is generated by the recombination reaction and its yield increases with temperature. The intensified reaction results in increased graphitization and decreased microscopic size, further leading to reduced combustibility due to its condensed structure. Soot generated above 1100 °C has low graphitization and combustibility due to short residence time. Based on the analysis, the temperature range around the feeding port is recommended to be controlled in 800–900 °C as it produces less coke and soot with better combustibility.

Hydrogen distribution in primary coke oventar and its fractions

The aim of this work is to determine the hydrogen distribution in primary coke oven tar and its fractions. The hydrogen distribution in the primary coke oven tar of «ShubarkolKomir» JSC, its distillate fractions and dis-tillation residue have been determined by the methods of elemental analysis, IR and PMR spectroscopy. The atomic ratio of C: H in the primary coke oven tar is 0.79. All fractions of the tar contain a large amount of al-kyl-substituted aromatic compounds, phenols and other substances with alkyl groups. The initial tarcharac-terized by a high content of hydrogen in the α-and β-positions to the aromatic ring, 29% and 34% respec-tively, which indicates a large number of alkyl substituents in the aromatic rings and near double bonds. The total amount of aliphatic and aromatic hydrogen in the tar is 79% and 21% respectively. Olefinic hydrogen is presented in the initial tar in an amount of 8%. It is possible to make a choice of techniques for further processing (hydrogenation, coking, thermal cracking) to obtain products with high added value on the basis of determination of the elemental composition, quantitative distribution of hydrogen in the primary coke oven tar and its fractions by the using of above mentioned physical and chemical methods.

Effects of the Gas-/Liquid-Phase Interactions on the Evolution of Bio-oil during Its Thermal Treatment

To clarify the interactions among bio-oil components during bio-oil pyrolysis, the gas- and liquid-phase interactions were decoupled and investigated, respectively. In this study, the bio-oil was separated into the aromatic-rich fraction and the aromatic-poor fraction, and these two fractions were pyrolyzed in a quartz basket with a baffle at the bottom to eliminate the liquid-phase interactions. The results showed that the gas-phase interactions promoted the coke formation via the cross-polymerization between the light compounds and aromatic compounds during bio-oil pyrolysis at 300–400 °C and the tar yields and content of the aromatics in the tar decreased correspondingly. At 500–800 °C, more large molecules were generated as a result of the intensified gas-phase interactions; thus, both the tar and coke yields were increased as well as the aromatics with more than three rings in the tar. Additionally, the increasing percentage of the product yields was higher at higher temperatures. The liquid-phase interactions promoted the formation of the non-evaporable large molecules, which affected the subsequent gas-phase reactions, leading to the increase in the heavy tar yields and decrease in the light tar yields. Meanwhile, the liquid-phase interactions promoted the formation of the primary coke, inhibited the formation of the secondary coke, and promoted the formation of the O-containing functional groups in the coke.

Effects of heating rate on the evolution of bio-oil during its pyrolysis

Bio-oil from the fast pyrolysis of biomass can be converted to solid carbon materials, chemicals and syngas by various thermochemical conversion methods. As a first step in all of these processes, bio-oil undergoes drastic components changes due to its exposure to the elevated temperature. Understanding the effects of heating rate on bio-oil transformation during its pyrolysis is therefore crucial for effective utilization of bio-oil. In this study, a bio-oil sample produced from the fast pyrolysis of rice husk at 500 °C was pyrolyzed in a fixed-bed reactor at temperatures between 300 and 800 °C at three different heating rates: fast (≈200 °C/s), medium (≈20 °C/s), and slow (≈0.33 °C/s). In addition to the quantification of coke and tar yields, the tar was characterized with an ultraviolet (UV) fluorescence spectroscopy, a gas chromatography/mass spectrometer (GC/MS) and a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). Our results indicate that slow heating rates promote polymerization of bio-oil components, particularly at low temperatures (<500 °C), resulting in higher primary coke yields than that of the fast heating rates. Decomposition reaction was found to be pronounced at fast heating rates, causing decreases in the tar yields and abundance of light compounds. The increases in the yields of the secondary coke, the formations of more condensed aromatic structures and macromolecules (m/z > 500) were also promoted at fast heating rates via the more intense secondary reactions.

Ni/SBA-15 catalysts for hydrogen production by ethanol steam reforming: Effect of nickel precursor

The effect of nickel precursor on Ni/SBA-15 catalysts was studied in ethanol steam reforming (ESR) for hydrogen production. These catalysts were prepared via incipient-wetness impregnation method using nickel nitrate and nickel citrate precursors, respectively (denoted as Ni/SBA-15(N) and Ni/SBA-15(C), respectively), and characterized by various techniques including H2-TPR, XRD, TEM and TG. It was found that the use of nickel citrate precursor, compared to nickel nitrate precursor, could greatly strengthen the NiO-support interaction and promote the homogeneous distribution of nickel species, to obtain the small nickel particles with high dispersion. After a 25 h time-on-stream test, much lower coke deposition was formed over Ni/SBA-15(C) than Ni/SBA-15(N). Moreover, NiCx species had be found over the used Ni/SBA-15(C), in which the carbon could be removed easily at lower temperature to exposure the active Ni sites; While carbon nanofibers with regular graphite-structure were the primary coke species over the spent Ni/SBA-15(N), which was difficultly remove and thus covered the active Ni sites easily. Due to these, Ni/SBA-15(C) displayed the higher catalytic activities and better stabilities in ESR than Ni/SBA-15(N). In summary, nickel citrate is an excellent precursor for the preparation of Ni/SBA-15 catalysts with high dispersion and strong interaction.

Monitoring subsurface coal fires in Jharia coalfield using observations of land subsidence from differential interferometric synthetic aperture radar (DInSAR)

Coal fires in the Jharia coalfield pose a serious threat to India’s vital resource of primary coking coal and the regional environment. In order to undertake effective preventative measures, it is critical to detect the occurrence of subsurface coal fires and to monitor the extent of the existing ones. In this study, Differential Interferometric Synthetic Aperature Radar (DInSAR) technique has been utilized to monitor subsurface coal fires in the Jharia coalfield. Results showed that majority of the coal fire-related subsidence were concentrated on the eastern and western boundaries of the coalfield. The magnitude of subsidence observed was classified into high (10–27.8 mm), low (0–10 mm) and upliftment (−10–0 mm). The results were strongly supported by in situ observations and satellite-based thermal imagery analysis. Major subsidence was observed in the areas with repeated sightings of coal fire. Further, the study highlighted on the capability of the methodology for predicting potential coal fire zones on the basis of land surface subsidence only. The results from this study have major implications for demarcating the hazardous coal fire areas as well as effective implementation of public safety measures.

Predicting the yield of coke-plant products on the basis of petrographic analysis

Mathematical models are developed for predicting the yield of coke and the primary coking byproducts from mixed batch on the basis of petrographic data. Coefficients are calculated for conversion from laboratory yields to industrial yields.

Reactions of ethylidene on a model chromia surface: 1,1-dichloroethane on stoichiometric α-Cr2O3 (10[formula omitted]2)

The reaction of CH3CHCl2 over the nearly-stoichiometric α-Cr2O3 (101¯2) surface produces an ethylidene intermediate that yields primarily gas phase CH2══CH2 and surface chlorine adatoms; however, trace amounts of HC≡CH, CH3CH3, H2 and CH3CH══CHCH3 are also observed. A rate-limiting intramolecular isomerization (2,1-hydrogen shift) in the surface ethylidene species produces gas phase CH2══CH2. The chlorine freed from the dissociation of CH3CHCl2 binds at the five-coordinate surface Cr3+ sites on the stoichiometric surface, completing the octahedral coordination sphere, and inhibits the surface chemistry by simple site blocking. No surface carbon deposition is observed from the thermal reaction of 1,1-dichloroethane under the conditions of this study, demonstrating that the ethylidene intermediate is not a primary coke forming intermediate over (101¯2) facets of α-Cr2O3 under the conditions of this study.

Reactions of ethyl groups on a model chromia surface: Ethyl chloride on stoichiometric α-Cr 2O 3 (1 0 [formula omitted] 2)

The reaction of CH 3CH 2Cl over the nearly-stoichiometric α-Cr 2O 3 (1 0 1 ¯ 2) surface yields gas phase CH 2 CH 2, CH 3CH 3, H 2 and surface chlorine adatoms. The decomposition reaction is initiated via C–Cl bond cleavage to give a surface ethyl (CH 3CH 2–) intermediate. A rate-limiting β-hydride elimination from the surface ethyl species produces gas phase CH 2 CH 2 and surface hydrogen atoms. Two parallel competing reactions form CH 3CH 3, via α-hydride addition to remaining surface ethyl species (reductive elimination), and H 2, via the combination of two surface hydrogen atoms. The chlorine freed from the dissociation of CH 3CH 2Cl binds at the five-coordinate surface Cr 3+ sites on the stoichiometric surface and inhibits the surface chemistry via simple site blocking. No surface carbon deposition is observed from the thermal reaction of ethyl chloride, suggesting that ethyl intermediates are not primary coke forming intermediates in the dehydrogenation of ethane over (1 0 1 ¯ 2) facets of α-Cr 2O 3.

Heptane Reforming over Pt–Re/Al2O3: Reaction Network, Kinetics, and Apparent Selective Catalyst Deactivation

This is a continuation of our previous work aimed at developing a fundamental, a priori model for predicting how a reforming catalyst deactivates in a fixed-bed reactor due to coke formation. Specifically, we construct a lumped reaction network for n-heptane reforming on a bifunctional Pt–Re/Al2O3 catalyst using both differential and integral rate data. The data are obtained from a vibrational microbalance and a multioutlet fixed-bed reactor with n-heptane or reaction intermediates as the feed. The network, including a five-membered naphthene lump that is the primary coke precursor, has a minimum number of reactions and corresponding adjustable rate parameters, almost all of which are individually determined by targeted experiments. The resulting kinetic model, coupled with a previously developed catalyst coking kinetic model, correctly predicts the long-term spatiotemporal behavior of product composition and coke-on-catalyst for a fixed-bed reformer. The results strongly suggest the need to partition the reforming reactions into two subgroups: those that are significantly affected by coke and those that are not. The approach taken here, which provides both fundamental insights and a quantitative basis for improving catalysts and processes, can also be extended to other catalytic systems.

INVESTIGATION OF PROLONGED DEACTIVATION-REGENERATION REGIMES ON THE DEHYDROGENATION ACTIVITY OF PLATINUM/ALUMINA CATALYST

The various types of coke found deposited on a deactivated Pt/Al2O3 reforming catalyst are described. They are claSsified into major types: oxidiZable (primary) coke, reducible (secondary) coke and non oxidizable and non reducible (tertiary) coke. Primary coke is generally non toxic to catalytic hydrocarbon conversion as its removal restores initial catalyst activity. The secondary and tertiary forms are, however, toxic to this reaction. A methodology for the prolongation of reforming catalyst life is evident from these results. In the new method, secondary coke is also removed from the catalyst surface subsequent to primary coke removal. Results of the application of this methodology using accelerated aging of Pt/Al2O3 catalyst is presented. The overall catalyst life was extended for up to 52 deactivation-regeneration cycles from the 23 previously reported. Catalyst life was not correlated with the oxidizable and reducible coke content. These profiles were found to be aperiodic and exhibited a nonlinear dynamic behavior. In contrast, catalyst life per cycle of deactivation-regeneration correlate with the rate of oxidizable coke deposition. The model coking reaction investigated was the dehydrogenation of cyclohexane to benzene in nitrogen carrier at 430°C on a commercial Pt/Al2O3 catalyst using a Berty CSTR.

A Fundamental Kinetic Model for the Catalytic Cracking of Alkanes on a USY Zeolite in the Presence of Coke Formation

The catalytic cracking of alkanes in the presence of deactivation by coke formation is presented. Elementary reactions such as protonation, deprotonation, hydride transfer, isomerization, β scission, and protolytic scission are explicitly accounted for. A distinction is made between the formation of primary coke by irreversible adsorption of hydrocarbons on catalytic sites and the formation of coke by further growth on said primary coke. Termination of the coke growth occurs when the coke molecules reach the dimensions of the zeolite pores. Primary coke molecules are formed out of the reaction of an alkene with a carbenium ion on the catalyst surface. The degree of coverage of the catalyst surface with carbenium ions is obtained from the pseudo-steady-state approximation. The deactivation effect of coke on each elementary reaction is modeled with empirical exponential deactivation functions. These functions are expressed as a function of the amount of primary formed coke, this being a measure for the amou...

Kinetics of asphaltene hydroconversion: 1. Thermal hydrocracking of a coal residue

An asphaltenic residue from deasphalting a synthetic crude obtained by direct liquefaction of a Spanish subbituminous coal was submitted to several upgrading routes. This paper reports the results from thermal hydroprocessing. Solubility-based lumped kinetics with parallel reactions for oils, gases and coke formation are proposed. Conversion data fit second-order kinetics for asphaltene conversion and for oil, gas and coke formation. Coke formation was inhibited except under the harshest reaction conditions. Two pathways for coke formation are proposed: one as primary reaction product after an induction a period and the other as secondary product from a sequence of polymerization steps. The presence of hydrogen strongly inhibits the primary coke formation. Structural analyses show higher aromaticity of the oils from products obtained at 475°C than at 450°C, which supports the proposed condensation mechanism for coke production at high temperatures.

Developpement de la texture poreuse des cokes de houille au cours de la gazeiffication par la vapeur d'eau

By measuring adsorption, densities, mercury penetration under pressure and small angle X-ray scattering, the changes in the porous texture of two cokes produced from a high and a low volatile coal during gasification with steam at 900°C were followed. A study was also made of the considerable differences in the initial porosity of the cokes, caused by the preliminary treatment of the raw material which was first finely ground and subsequently briquetted. Maximum adsorption properties imply the presence in the primary coke not only of a network of micropores, which are almost totally responsible for the measured internal surface, but also of macropores, which are needed as means of access. A sufficiently dense distribution of the macropores makes a uniform gasification in the whole bulk of the sample possible. In addition, it appears that the pores of intermediate size have an unfavorable effect, since they impede diffusion and cause the consumption of carbon to increase. An optimal primary texture is characterized by a ‘bimodal’ pore size distribution. Cokes from high volatile coals are to a certain extent representing this ideal type, while cokes from low-volatile coals not submitted to a preliminary treatment do not yield products with good adsorption properties.