Effect of temperature,catalyst and charge gas on the mean chemical structures of the products from hydrogenation of Liddell coal

Liddell coal (New South Wales, Australia) has been hydrogenated at 400, 425 and 450 °C with excess tetralin as vehicle and nitrogen or hydrogen as charge gas for 4 h at reaction temperature. In some experiments a nickel-molybdenum catalyst was used. The structures of the liquid and solid products were investigated by nuclear magnetic resonance spectroscopy, gel permeation chromatography and combustion analysis. Increasing the hydrogenation temperature from 400 to 450 °C decreases the yield of liquid products but increases conversion. At higher temperatures the liquid products are smaller in molecular size and molecular weight and contain a greater proportion of aromatic carbon and hydrogen; the solid residues also contain a greater proportion of aromatic carbon. The changes in variation of yield and structure with temperature are independent of the presence of catalyst under nitrogen and the nature of the charge gas. However, as the reaction system is capable of absorbing more hydrogen than can be supplied by the tetralin, the products from reactions with hydrogen as charge gas contain more hydrogen, some in hydroaromatic groups. Catalyst has little, if any, role in dissolution of the coal when a nitrogen atmosphere is used. When nitrogen is used as charge gas, reactions of coal-derived liquids with the catalyst do not alter the hydrogen, carbon or molecular size distributions in the products. The results show that the changes in composition of the liquid and solid products with increase in hydrogenation temperature are due to pyrolytic reactions and not to increased hydrogenation of aromatic rings.     

Structural aspects of sub-bituminous coal deduced from solvation studies. 1. Anthracene-oil solvents

In order to study coal structure indirectly and the role of hydrogen donors, an investigation of the major parameters involved in the solvation of a Wyoming sub-bituminous coal has been made. This study utilized the catalytic and non-catalytic hydrogenation of anthracene oil and coal-solvent slurries. Indirect evidence concerning major structural units in the coal was obtained and the net contribution of coal to liquefaction products was estimated. The significance of each parameter to the degradation of coal molecules was also estimated. Data support the concept that coal liquefaction follows a solid → asphaltene → resin → oil route. This stepwise dissociation of the solid is directly related to the breaking of CO, CN, and to a lesser degree CC bonds, resulting in the formation of free radicals of relatively low molecular weight. These free radicals are stabilized by hydrogen transfer from hydroaromatic solvent molecules. A lack of significant quantities of high-molecular-weight hydrocarbons derived from coal solvation implies the prevalence of small molecular units in the coal structure.     

Coal flash pyrolysis: 2. Polymethylene compounds in low temperature flash pyrolysis tars

Pyrolysis of coals at low temperatures (< 600 °C) produces tars containing the precursors of the low molecular weight aliphatic hydrocarbons, such as ethylene and propylene, observed on flash pyrolysis of the coals at higher temperatures (700–800 °C). This is shown by further pyrolysis of these low temperature tars at high temperatures. Various methods, including isolation by h.p.l.c. were used to confirm the presence of straight chain paraffin and olefin pairs (C₁₄C₂₆ and above) in the low temperature tars. Pyrolysis of pure paraffins and olefins in this molecular weight range at temperatures > 700 °C produce ethylene, propylene and other cracking products similar to those obtained on flash pyrolysis of coal.     

Characterization of pyrolysis products of harbolite and Avgamasya asphaltites: comparison with solvent extracts

The products of pyrolysis at 525 and 840 °C of two asphaltites from South-Eastern Turkey have been analysed and compared with the bitumen obtained by solvent extraction. The yield of oil product is reasonably similar for all three treatments, with gas (hydrogen, ethene, C₁C₄ alkanes and hydrogen sulphide) being liberated during pyrolysis. Greater percentages of alkanes with shorter chain lengths (along with some alkenes), and of pentane-soluble aromatic oils with reduced molecular masses, are generated during pyrolysis, at the expense of asphaltenes. The extra alkanes are generated partly by the cracking of aromatic side-chains and also from kerogen. Pyrolysis reduces the number of sulphur linkages in the oil, but nitrogen- and oxygen-containing structures are liberated from kerogen during heating.     

The effects of oxygen on the yields of the thermal decomposition products of catechol under pyrolysis and fuel-rich oxidation conditions

In order to investigate the effects of oxygen on the distribution of thermal decomposition products from complex solid fuels, pyrolysis and fuel-rich oxidation experiments have been performed in an isothermal laminar-flow reactor, using the model fuel catechol (ortho-dihydroxybenzene), a phenol-type compound representative of structural entities in coal, wood, and biomass. The gas-phase catechol pyrolysis experiments are conducted at a residence time of 0.3 s, over a temperature range of 500-1000 °C, and at oxygen ratios ranging from 0 (pure pyrolysis) to 0.92 (near stoichiometric oxidation). The pyrolysis products are analyzed by nondispersive infrared analysis and by gas chromatography with flame-ionization and mass spectrometric detection. In addition to an abundance of polycyclic aromatic hydrocarbons, catechol pyrolysis and fuel-rich oxidation produce a range of C₁-C₅ light hydrocarbons as well as single-ring aromatics. Quantification of the products reveals that the major products are CO, acetylene, 1,3-butadiene, phenol, benzene, vinylacetylene, ethylene, methane, cyclopentadiene, styrene, and phenylacetylene; minor products are ethane, propyne, propadiene, propylene and toluene. Under oxidative conditions, CO₂ is also produced. At temperatures <850 °C, increases in oxygen concentration bring about increases in catechol conversion and yields of C₁-C₅ and single-ring aromatic products—in accordance with increased rates of pyrolytic reactions, due to the enhanced free-radical pool. At temperatures >850 °C, catechol conversion is complete, and increases in oxygen bring about drastic decreases in the yields of virtually all hydrocarbon products, as oxidative destruction reactions dominate. Reactions responsible for the formation of the C₁-C₅ and single-ring aromatic products from catechol, under pyrolytic and oxidative conditions, are discussed.     

The preparation and pyrolysis of O- and C-benzylated Illinois No. 6 coal

Illinois No. 6 coal was modified by selective benzylation reactions and the O- and C-benzyl, benzyl-d₇ and benzyl-1-¹³C products were pyrolysed in a wire screen reactor at 500–1000 °C. The char, tar and gas yields were measured and the distribution of the isotopic labels in the lower molecular weight gaseous products was determined. The results suggest that the modified coals are more reactive than the natural coal. First, the modifications increase the concentration of reactive radicals and of hydrogen donor groups. Second, the exchange patterns also suggest that the energetically more favourable reactions occur reversibly and that radical addition and recombination reactions compete favourably with fragmentation and radical substitution reactions. Third, the non-random distribution of the isotopic labels in the products indicates that the reactions in the coal particle are kinetically controlled even at temperatures near 850 °C. Fourth, the distribution of the labels in the water and ethene implies that non-radical processes contribute. Thus, theories of pyrolysis that are based exclusively on radical processes may be seriously misleading.     

The effects of oxygen on the yields of polycyclic aromatic hydrocarbons formed during the pyrolysis and fuel-rich oxidation of catechol

To better understand the effects of oxygen on the formation and destruction of polycyclic aromatic hydrocarbons (PAH) during the burning of complex solid fuels, we have performed pyrolysis and fuel-rich oxidation experiments in an isothermal laminar-flow reactor, using the model fuel catechol (ortho-dihydroxybenzene), a phenol-type compound representative of structural entities in coal, wood, and biomass. The catechol pyrolysis experiments are conducted at a fixed residence time of 0.3 s, at nine temperatures spanning the range of 500-1000 °C, and under varying oxygen ratios ranging from 0 (pure pyrolysis) to 0.92 (near stoichiometric oxidation). The PAH products, ranging in size from two to nine fused aromatic rings, have been analyzed by gas chromatography with flame-ionization and mass spectrometric detection, and by high-pressure liquid chromatography with diode-array ultraviolet-visible absorbance detection. The quantified PAH products fall into six structural classes: benzenoid PAH, indene benzologues, fluoranthene benzologues, cyclopenta-fused PAH, ethynyl-substituted PAH, and methyl-substituted PAH. A comparison of product yields from pyrolysis and fuel-rich oxidation of catechol reveals that at temperatures <800 °C, where only two-ring PAH are produced in significant quantities, increases in oxygen concentration bring about increases in yields of the two-ring aromatics indene and naphthalene. At temperatures >800 °C, increases in oxygen concentration bring about dramatic decreases in the yields of all PAH products, due to oxidative destruction reactions. The smaller-ring-number PAH are produced in higher abundance under all conditions studied, and the oxygen-induced decreases in the yields of PAH are increasingly more pronounced as the PAH ring number is increased. These observations regarding PAH ring number, from the fuel-rich oxidation experiments with catechol, fully support our finding from catechol pyrolysis in the absence of oxygen: that PAH formation and growth occur by successive ring-buildup reactions involving the C₁-C₅ and single-ring aromatic products of catechol’s thermal decomposition. The yield/temperature data reported here represent one of the most extensive quantifications of the effects of oxygen on PAH produced during the pyrolysis of any fuel.     

Thermal degradation of polymers. II. Mass spectrometric thermal analysis of phenol-formaldehyde polycondensates

The degradation of phenol–formaldehyde polycondensates has been investigated by mass spectrometric thermal analysis to 800°C. The thermal oxidative mechanism proposed by Conley and co-workers has been confirmed. Direct analysis of products at frequent intervals of a temperature-programmed heating cycle demonstrates the existence of postcuring, general degradation, and char-forming stages. Activation energies for formation of each product and for degradation of the polymer have been determined. Phenolic homologs, products of thermal scission of methylene–phenyl bonds, show high activation energies; oxidation products have activation energies comparable to or slightly higher than that for oxidation of methylene bridges to carbonyl groups. The tarry high molecular weight products found on pyrolysis at atmospheric pressure are not formed in high vacuum because recombination reactions of phenols and formaldehyde are minimized. Otherwise, no major change in product composition, as compared to the high heating rate pyrolysis–gas chromatographic investigation of Jackson and Conley, was observed.     

Effects of thermal pretreatment in helium on the pyrolysis behaviour of Loy Yang brown coal

A wire-mesh reactor capable of multi-step heating/holding and minimising secondary reactions of volatiles was used to investigate the effects of thermal pretreatment in inert gas on the subsequent rapid pyrolysis behaviour of Loy Yang brown coal. Our results indicate that the presence of small amounts (<10 wt%) of moisture in brown coal has little influence on the tar and char yields from the pyrolysis of brown coal at 1000 K s⁻¹. While the hydrogen bonds between the moisture and the O-containing functional groups in the brown coal have little effects on its pyrolysis behaviour, the hydrogen bonds among the O-containing functional groups tend to induce cross-linking reactions to reduce the tar yields. Preheating the brown coal at >250 °C leads to reduced tar and increased char yields. However, the characterisation of tars using UV-fluorescence spectroscopy indicates that significant decreases in the release of larger aromatic ring systems are only observed after preheating at >380 °C for 30 min. The presence of ion-exchangeable cations (e.g. Ca²⁺) in the brown coal tends to stabilise the carboxylate groups and only preheating at >350 °C would result in changes in pyrolysis yields during the subsequent pyrolysis at 1000 K s⁻¹. These results may be explained by considering the formation of cross-links involving peripheral groups at low preheating temperatures and the formation of cross-links involving aromatic ring systems at elevated temperatures.     

Study on the structure and pyrolysis characteristics of Chinese western coals

The structure and pyrolysis characteristics of three inertinite-rich Chinese western coals were researched and compared with one relative vitrinite-rich Chinese middle coal by means of XRD, TG-DTG and fixed-bed reactor. The results show that the atomic ratio O/C, aromaticity factor, even ring condensation number and ring condensation index increase and atomic ratio H/C decreases with increasing inertinite content in coal; inertinite contains more aromatic ring structure than that of vitrinite; the crystallite structure order of coal char increases slightly with increasing heat treatment temperature. The higher inertinite content in coal is, the lower pyrolysis reactivity of coal is at lower temperature, and yet they have obvious second pyrolysis reactivity in higher temperature. The pyrolysis reaction in primarily devolatilisation phase that comes mainly from the decomposition of containing hydrogen function groups and the secondary devolatilisation at high temperature is mainly the decomposition of stable containing oxygen function groups in coal matrix with higher inertinite.     

褐煤经四氢化萘处理后的结构及热解-气化特性分析

在小型反应釜上采用非极性有机溶剂四氢化萘(tetrahydronaphthalene, THN)对内蒙古锡林郭勒褐煤进行脱水,获取了不同温度下的脱水试样,运用傅里叶红外光谱(Fourier transform infrared spectroscopy,FT-IR)分析技术对比研究了在不同脱水温度下煤样中有机官能团的变化,通过FT-IR谱图的分峰拟合计算,对脱水煤样的化学结构变化特征进行半定量分析,并结合热重(thermal gravity analysis/differential thermal gravity,TG/DTG)和实验室固定床反应器(fixed bed reactor)考察了不同脱水温度下煤样的热解气化特性和热解气相产物分布规律,并对脱水煤样在最大失重速率区间的动力学参数进行了计算。试验结果显示:四氢化萘溶剂对褐煤的脱水提质是有效的,150~200℃时C O开始分解,而此时芳香环上的C C仍然相对稳定。随着脱水温度的升高,芳香类氢含量先减小后增大,脂肪类氢含量先增大后减小,芳香度和芳香碳在脱水温度范围内逐渐增大。煤样的脱水温度升高,热解气相产物H₂、CH₄、CO累积产率增大,CO₂累积产率减小;脱水煤样热解活化能随着脱水温度的升高而升高,进而导致脱水温度较高煤样热解失重率降低。     

Infrared study of the evolution of kerogens of different origins during catagenesis and pyrolysis

The absorption coefficient has been measured for the various bands present in the infrared spectra of series of natural kerogens and of shallow kerogens pyrolysed at increasing temperatures, in order to investigate the effect of both natural evolution and heat treatment on the chemical composition, with consideration of the effect of the parent organic matter. The sequence of infrared spectra offers a quantitative picture of the main chemical modifications taking place: removal of carboxyl and carbonyl groups, and of saturated hydrocarbon groups, formation and subsequent removal of aromatic CH which follow the same evolution as the free radicals detected by e.s.r., evolution of hydroxyls and other COR functions. Catagenesis leads to residual kerogens with a chemical composition independent of the parent material; on the contrary pyrolysis of more oxygenated kerogens leads to products with a higher concentration of oxygen, in the form of COR functions. The reorganization of the carbonaceous residue into clusters of oriented carbonaceous particles is influenced by oxygenated functions, particularly by COR groups.     

Hydrogenation of Liddell coal. Yields and mean chemical structures of the products

The products from the hydrogenation of an Australian medium-volatile bituminous coal (Liddell) in batch autoclaves have been investigated. Tetralin was used as a vehicle and Cyanamid HDS-3A as catalyst. The influences of temperature (315–400 °C), hydrogen pressure (3.4–17.2 MPa) and reaction time (0–4 h) on the yields of pre-asphaltene, asphaltene, oil and pitch were studied. The chemical compositions of these materials were investigated by nuclear magnetic resonance and infrared spectrometry, and high-pressure liquid chromatography. Higher temperatures (400 °C) and pressures (17.2 MPa) favour the formation of products with lower average molecular size, lower aromatic carbon and aromatic proton contents and smaller average aromatic fused-ring number. N.m.r. evidence is presented which shows that increasing the temperature from 370 °C to 400 °C or pressure to 17.2 MPa assists reactions which bring about hydrogenation and cleavage of aryl rings. Longer reaction times (4 h) promote reactions by which the oxygen content of the product is decreased and by which polymethylene becomes cleaved from other functional groups. The results show that asphaltenes are true intermediates in the formation of oil from coal.     

Formation and chemical structure of preasphaltenes in short residence time coal hydrogenolysis

The formation and chemical structure of preasphaltenes in short residence time coal hydrogenolysis were investigated. In short residence time coal hydrogenolysis, preasphaltenes are the major product. The maximum yield for this parametric study was obtained under reaction conditions of 500 °C and 21 s. The formation of preasphaltenes reached the maximum value in the initial stage of the liquefaction reaction. As the liquefaction reaction continued, the deoxygenation of preasphaltenes proceeded. However, the decrease in aromatic atoms bound to the hydroxy, methoxy and oxygen atoms of the diphenyl ether group (Arz.sbnd;O) is small, and the ArO functionality still remains abundant in preasphaltenes. Preasphaltene-I is characterized by carbon aromaticity (f_a) of 0.6–0.7, aromatic rings of from 1 to 3–5 per condensed aromatic ring system, 55–70% substitution of aromatic ring carbons and C_2–3 aliphatic substituents. The molecular weight ranges from 500 to 650, and is not much different from that of the asphaltenes. The f_a values based on the Brown-Ladner method and on solid state CP/MAS ¹³C n.m.r. spectra data agree closely.     

A literature survey and an experimental study of coal devolatilization at mild and severe conditions: influences of heating rate, temperature, and reactor type on products yield and composition

Coal devolatilization studies to maximize the yield of condensable products by operating at elevated temperatures and heating rates have been published. The objectives of this study were to investigate the influences of relatively mild operating conditions (e.g. relatively low temperature and pressure) on product quality, by comparing devolatilized products obtained at various temperatures and heating rates. Fixed bed, fluid bed, and entrained flow reactor units were used to obtain pyrolysis products. In addition, literature data on tar yields in various reactor units at a range of temperatures and residence times were surveyed and compared with experimental data. The liquids were characterized by a number of techniques, including field ionization mass spectroscopy (f.i.m.s.), sequential elution solvent chromatography (s.e.s.c.) and elemental analysis. The results demonstrate that the quality and yield of liquids obtained at a rapid heating rate are functions of peak pyrolysis temperature. It was shown that at a rapid heating rate, the yields of heavier polyfunctional groups (i.e. hydrocarbons with greater mean molecular weight) are greater than those obtained in the fixed bed slow heating rate reactor. The liquids generated at a slow heating rate are of lower molecular weight, viscosity, and sulphur content, and of higher H/C atomic ratios compared with the liquids obtained in a rapid heating rate unit. The effect of increasing the maximum pyrolysis temperature (at a constant slow heating rate) was to increase the yield of light gases (mainly H) at the expense of char hydrogen content and char reactivity. The tar yield is not markedly influenced when the peak devolatilization temperature is increased at a relatively slow heating rate. However, the quality (as defined by the H/C (atomic) ratio) of the liquids, and the reactivity (in air) of char, was reduced when the peak pyrolysis temperature was increased. At a rapid heating rate, the primary products, which have many structural characteristics of the parent coal, are devolatilized. The quality of the liquids obtained at a rapid heating rate is, therefore, determined by the devolatilized primary coal fragments evolved at the devolatilization temperature. In a slow heating rate fixed bed unit, however, the primary coal fragments undergo additional cracking reactions which involve stabilization of free radicals by donatable hydrogen. This leads to the formation of low molecular weight hydrocarbons of relatively higher quality. In-situ (both intraparticle or extraparticle) stabilization of reactive coal fragments by donatable hydrogen may lead to a significant improvement in the overall quality of the pyrolysis liquids in a fixed bed system in which time-temperature history is conducive for such reactions.     

Design of laboratory reactors for the measurement of catalytic carbon and coal gasification kinetics

A great deal of conflicting experimental kinetics results have appeared for catalytic carbon and coal gasification by water and carbon dioxide. The reason for this can in large part be attributed to inhibition of these reactions by their products and the influence of this product inhibition on the measured kinetics in different laboratory reactor systems. The measurement of differential rates and the determination of true kinetic values requires the use of feed streams with an excess of water and hydrogen for the catalytic CH₂O reaction and an excess of carbon dioxide and carbon monoxide for the catalytic CCO₂ reaction. Recommendations are put forward for the design of laboratory reactors for the measurement of catalytic carbon and coal gasification kinetics.     

Quantitative study on cross-linking reactions of oxygen groups during liquefaction of lignite by a new model system

Cross-linking reactions (CLR) of oxygen groups during liquefaction of lignite were quantitatively studied by a new model system. Chinese Yitai lignite (YT) was first oxidized by nitric acid at 70 °C and about 98% of the oxidized sample could be dissolved in tetrahydrofuran (THF) at room temperature. Then benzyl alcohol, PhCH₂OH (BA), as a model compound was added into the oxidized coal, also acted as solvent in the subsequent liquefaction. Temperature-programmed reactions (TPR) at liquefaction conditions under hydrogen atmosphere were performed to evaluate the CLR by quantitative analysis of THF-insoluble solid products (THFI) after reaction. Extensive CLR were observed even under high pressure of H₂ at 200-400 °C, and more than 51.7% and 81.2% of the THFS fraction was converted into the THFI at 300 °C with tetralin (TET) and BA as solvent, respectively. The THFI fraction was almost solely caused by the CLR, which makes it possible to quantitatively study the CLR by analyzing the amount of the cross-linked solid products (CSP). The pyrolysis behaviours of CSP and oxidized coal were examined by TG. Other model compounds containing oxygen-functional groups (alcohol, phenol, carboxyl, carbonyl and ether groups) can also be used in this model system to study CLR of oxygen groups in low-rank coals.     

The effect of hydrogen pressure and aromatic structure on methane yields from the hydropyrolysis of aromatics

The chemistry of the formation of methane from the hydrogasification of naphthalene, substituted naphthalenes and toluene has been investigated using a flow tube. Temperatures were varied between 650–1050 °C (depending on the aromatic) and pressures ranged over 0.5–2 MPa. The results show that methane yields increase with increasing hydrogen pressure. For naphthalene the methane yield increases linearly with temperature for a given pressure. Methyl substituents are lost from aromatic rings, in a reaction which is insensitive to hydrogen pressure, to form 1 mole of methane and the parent aromatic. At these hydrogen pressures the phenolic group in 1-naphthol is removed predominately as H₂O to form naphthalene and the methane yields from this species parallel those from naphthalene. Analyses of the condensed products demonstrate that increased hydrogen pressure results in a reduction in the amounts of high molecular weight condensation products resulting in increased yields of methane.     

Free radical chemistry of coal liquefaction: role of molecular hydrogen

Model compounds containing the types of carboncarbon bonds thought to be present in coal were pyrolyzed in the presence of tetralin and molecular hydrogen at 450 °C. The relative rates of conversion of the model structures are predictable from the bond dissociation energies of the compounds. Conversion of dibenzyl in the presence of both tetralin and molecular hydrogen or in the presence of hydrogen alone proceeds along two parallel reaction paths. Toluene is produced by a thermal cracking reaction in which the rate-controlling step is the thermal cleavage of the β-bond in dibenzyl. Benzene and ethylbenzene are produced by a hydrocracking reaction. The rate of the hydrocracking reaction is directly proportional to the hydrogen pressure. The strong bond in diphenyl is hydrocracked in a system containing both molecular hydrogen and a source of free radicals. These studies on model coal structures offer firm evidence that molecular hydrogen can participate directly in free radical reactions under coal liquefaction conditions. Under some conditions molecular hydrogen can compete with a good donor solvent to stabilize the thermally produced free radicals. Molecular hydrogen can also promote some hydrocracking reactions in coal liquefaction that do not occur to an appreciable extent in the presence of only donor.     

Catalytic pyrolysis of polyvinylchloride in the presence of metal chloride

The pyrolysis of polyvinylchloride (PVC) was studied in a quartz reactor with and without the addition of metal chlorides as catalysts. The results indicated that the addition of metal chlorides such us ZnCl₂ or BaCl₂ decreased the temperature of dehydrochlorination of PVC. The introduction of H₂ in pyrolysis enhanced the degradation of PVC at 400°C or higher and decreased the amount of solid residue. Gas chromatography/mass spectroscopy analysis indicated that Zn and Ba chlorides reduced the formation of high molecular weight products. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2464–2471, 2000