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.     

Tar evolution from coal and model polymers: Theory and experiments

While tar is a major component produced in coal pyrolysis, there is little agreement on how to quantitatively describe the steps in its evolution (formation of tar molecules, transport, evaporation and repolymerization). Because coal is insoluble, heterogeneous and complex, it is very difficult to verify proposed models. In an attempt to overcome some of these difficulties, polymers with ethylene bridges between naphthalene rings have been used to model tar release in pyrolysis of softening coals. A theory has been developed that combines random cleavage of weak bonds in the polymer (to produce metaplast) with transport of depolymerization fragments by vaporization and diffusion. The theoretical predictions for various temperatures, heating rates and pressures compare favourably with tar yields measured in a heated grid apparatus and molecular weight distribution determined by field ionization mass spectrometry. The predicted trends agree with the results on coal tars produced under the same conditions.     

Chemically-bound n-alkyl groups in coal

Tars produced from the flash pyrolysis of three Australian coals and the liquid products obtained from the subsequent catalytic hydrotreatment of these tars have been analysed by high resolution gas chromatography. Yields of n-alkanes and, in the case of the tars, n-alkenes have been determined. For the subbituminous coal the total yields on a dry, ash-free coal basis increased from 2.4 wt% in the tar to a maximum of 5.8 wt% in the hydrotreated product, and for a brown coal from 0.6 wt% in the tar to 2.5 wt% in the hydrotreated product. Thus, in both cases, a substantial proportion of the n-alkanes appeared only after extensive hydrotreatment of the tar, and did not consist simply of hydrogenated n-alkenes. The results imply that a major fraction of the n-alkyl groups in the parent coal are chemically bound to other structures and are released and vaporized as such during pyrolysis. Subsequent hydrogenation of the tar liberates n-alkanes by cleavage of carbon-carbon bonds. Results for the bituminous coal are complicated by high gas formation in the hydrotreatment reactor, but suggest that some n-alkyl groups are also chemically bound.     

Flash pyrolysis of an Italian low rank coal

Flash pyrolysis of low rank Italian coal (Sulcis coal) has been studied in a fluidized bed pyrolyser using temperatures between 460 and 900°C. The maximum yield of oils (tars) was obtained at about 600°C. Yields of C₁C₃ hydrocarbons increased with increasing temperature, reaching 6% at 900°C. Fractionation of tars showed that the composition was strongly dependent on pyrolysis temperature. By the behaviour of the composition of tars on temperature, a possible reaction mechanism is suggested.     

中、低温煤焦油加氢生产清洁燃料油技术

介绍了中、低温煤焦油的性质及特点,阐述了国内外中、低温煤焦油加氢技术研发现状和发展趋势,煤焦油加氢技术研究结果说明,中、低温煤焦油通过沸腾床加氢工艺饱和了含氧化合物和单、双烯烃等杂质,简化了后续固定床加氢流程,沸腾床-固定床组合加氢技术为中、低温煤焦油加氢合理选择。中、低温煤焦油采用适宜的加氢技术可以生产清洁石脑油和清洁车用燃料油或调和组分,是改善环境、综合利用煤炭资源,提高企业经济效益的有效途径之一。     

Molecular weight distributions of tars produced by flash pyrolysis of coals

The molecular weight distributions of tars produced during flash pyrolysis of coal have been determined by a combination of gel permeation chromatography and vapour phase osmometry. Small particles (62–88 μm) of two high-volatile and one low-volatile bituminous coal, and a lignite have been pyrolysed at heating rates of ≈1000 K s^?1 at temperatures from 600 to 1300 K in a heated wire mesh apparatus. The molecular weight distributions range from ≈100 to 4000 and peak in the range from 250 to 750 in all cases. The evidence gathered on a softening bituminous coal clearly indicates a selective evaporation of light fractions of the metaplast.     

Upgrading of flash pyrolysis tars to synthetic crude oil: 4. Hydrotreatment with iron catalyst in a slurry-phase reactor

Flash pyrolysis tars, produced from Millmerran (Queensland subbituminous) and Piercefield (New South Wales bituminous) coals, have been hydrogenated in a slurry-phase bubble-column reactor using red mud and sulphur as catalyst. Oils of low coking propensity and high volatility were produced from both coal tars. The slurry-phase reactor successfully overcame the severe coking problems previously encountered when hydrogenating flash pyrolysis tars in fixed-bed reactors, completely eliminating coke formation. For Millmerran coal tar the effect of reaction temperature on reactor performance was investigated in the range 420–460 °C. The major effects of increasing temperature were to reduce the coking propensity of the product oil and to increase its volatility. Product oils from both tars were high in aromatics and heteroatoms, and the distillate fractions derived from these oils would require further refining to produce finished products (i.e., gasoline and diesel). It should be possible to achieve this using conventional refining technology.     

Infrared absorption characteristics of hydroxyl groups in coal tars

Tar evolution was observed over a temperature range of 150–600 °C for four coals: Pittsburgh bituminous, Illinois No. 6, Rawhide subbituminous, and Texas lignite. Isolation of the evolved tars in a nitrogen matrix at 15 °K produced better resolved infrared spectra than those in a coal matrix, thus enhancing structural characterization of the tar molecules. Two distinct hydroxyl functional groups in the tar molecules free of hydrogen bonding were identified for the first time without interference from H₂O absorptions. These absorptions at 3626.5 and 3580.9 cm⁻¹ have been assigned to phenolic hydroxyls. It is suggested that carboxylic and aliphatic hydroxyl groups do not survive the vaporization process. Tars from Illinois No. 6 were found to contain the largest amount of phenolic hydroxyl ; Pittsburgh No. 8 tar contains approximately half of that for Illinois No. 6 while Rawhide and Texas lignite contain much less phenolic than either of the other coals.     

Effect of catalysts in the quality of syngas and by-products obtained by co-gasification of coal and wastes. 1. Tars and nitrogen compounds abatement

The aim of this work is to analyse the possibility of using co-gasification technology to process coal mixed with wastes to take profit of its energy content and at the same time to minimize the environmental impact associated with the use of wastes and to diminish the costs of flue gas treatment. The addition to coal of different types of materials, like: pine based waste, petcoke and polyethylene (PE), was not found to give rise to any operational problems, regarding both the feeding system and gasification process and led to higher energy conversions, however, the gas presented higher tars and hydrocarbons content. Several catalysts were tested, such as, dolomite, olivine, nickel and magnesium oxides, zinc oxides and cobalt and molybdenum oxides. Catalyst action was analysed in tars release and also in ammonia compounds reduction. The presence of catalysts allowed increasing hydrogen release, whilst there was a decrease in hydrocarbons and tars contents. A nickel-magnesium oxide was the catalyst that led to the highest reduction in hydrocarbons and tars. This catalyst also led to the lowest NH₃ content in the fuel gas produced, due to the catalyst efficiency in NH₃ destruction.     

Properties of coal tar characterized by slight pyrolysis

The behavior of coal tars with different levels of pyrolysis in thermal fractionation is compared in the laboratory. The properties and group composition of the nonboiling residues (medium-temperature pitch) obtained by the distillation of coal tars at different temperatures, in comparable conditions, is investigated. The results indicate that the distillation of tars characterized by slight pyrolysis is accompanied by vigorous destruction processes, also affecting the α₁ fraction (t = 335–350°C), with subsequent reactions among the destruction products, leading to active increase in the α₁ and α₂ fractions (t ≥ 350°C). This is a significant difference between tars characterized by light pyrolysis and considerable pyrolysis.     

Carbonylation studies with Illinois No. 6 bituminous coal

Sodium metal and carbon dioxide were used to carbonylate Illinois No. 6 coal in a 1 l, high-pressure, autoclave in anhydrous tetrahydrofuran from 150 to 350 °C. Solubility of the reaction products in tetrahydrofuran and water, and the sodium uptake in the insoluble residue were determined. The reduction and subsequent carbonylation of coal produced a limited amount of solubility in these solvents, and the uptake of sodium in the insoluble residue reached a maximum of 6.29% at 250 °C with no carbon dioxide added. This corresponded to a ratio of sodium atoms per 1000 carbon atoms of 62.9, a 69-fold increase over the unreacted dried coal. Total acidity and carboxyl group content were calculated for the insoluble residues and unreacted bituminous coal. The per cent increase of carboxyl groups reached a maximum of 1.37 meq g^?1 at 2000 °C which represents a 174% increase over the unreacted coal.     

Dissolution of subbituminous coal in tetrahydroquinoline

Two different samples of Wyodak subbituminous coal from the Powder River Basin in Wyoming were liquefied in a two dm³ batch reactor using 1,2,3,4 tetrahydroquinoline, THQ, as a solvent. Sufficient sample was produced to determine product boiling ranges by distillation and to measure THQ distribution in the product. Product distillation showed that even at cyclohexane conversions greater than 50%, net distillate yields produced using THQ as a solvent were negative. In some cases, high boiling, coal-derived resid yields were greater than the dry coal charged to the reactor. These observations have been attributed to THQ losses resulting from dimerization of the THQ and reactions between THQ and coal derived components.     

煤焦油中脱除喹啉不溶物技术的研究进展

煤焦油是一种制备高附加值碳材料的理想原料,然而其深加工利用受到内部高含量喹啉不溶物(QI)的影响。为此,本文介绍了煤焦油的组成和性质,简述了煤焦油中QI的组成、来源和其对煤焦油加工利用的危害,综合对比分析了煤焦油净化分离常用的几种方法。热过滤法分离效率虽然满足工业需求,但工业成本较高;重力沉降法操作简单,但分离效率低;离心分离法分离效率高,但处理量小,难以满足工业化应用;蒸馏处理法分离效率高,但原料利用率低;溶剂萃取法容易操作,但溶剂需求量大;改质法效果明显,但成本较高;静电分离法能量消耗低,但静电分离最佳条件变动性较大;联合处理法分离效率较高,应用前景广阔,可以作为一种净化分离的科学有效方法,并可以作为一种净化分离技术的发展趋势。     

Correlation of tar yields from rapid pyrolysis with coal type and pressure

A new correlation independently relating maximum tar yield from rapid pyrolysis (heating rate ≳ 100 Ks^{− 1}) to coal type and to total pressure is presented. Coal type effects are treated using only information on the elemental composition of the unpyrolysed coal. Pressure effects are correlated via empirical parameters obtained from best-fit analyses of existing data. The correlation is tested against experimental data for coals from the USA, France and W. Germany representing a wide range of rank, and for pressures from ‘Vacuum’ to 9 MPa. Satisfying results are obtained, with predictions of tar yield generally within ± 5 wt% (of dmmf coal) of observed values at all pressures for most coals. The correlation is most suited to predicting maximum tar yields under reaction conditions affording minimal opportunity for extra-particle secondary reactions of newly formed pyrolysis tars. It is also useful for predicting probable upper bounds on pyrolysis tar yields under other operating conditions.     

Pyrolysis of a lignite in an entrained flow reactor: 2. Effect of metal cations on decarboxylation and tar yield

The role of exchangeable cations in the release of carboxylic functional groups and tars from a Montana lignite has been studied. Pyrolysis was carried out in an entrained flow reactor using a dilute stream of coal particles, and high heating and cooling rates. Direct determination of the carboxylic functional groups on the coal and char samples by ion exchange with barium acetate was employed to study both the amount and kinetics of decarboxylation. These results indicate that the presence of metal cations decreases the extent of decarboxylation at temperatures of 973, 1073 and 1173 K when compared with the behaviour of an acid washed lignite. It was also possible to determine first order activation energy values using this direct measure of decarboxylation. The activation energies determined for decarboxylation were 100 and 97 kJ mol⁻¹ for the raw and acid washed samples, respectively. Furthermore, it is reported that the presence of metal cations on the coal resulted in a decrease of tar release of 70–94%. Analysis of the tars by Fourier transform infrared spectroscopy indicated that the tars released during pyrolysis were more aliphatic when metal cations were present.     

Hydrogen transfer from tetralin and decalin to high-temperature coal tars. Relation with carbon deposit formation

Studies of pyrolysis of a coal charge containing various quantities of water from 0.8 to 15 wt.% in a two-stage reactor were performed. The rate of formation of solid carbon deposits was followed and some properties of high-temperature tars were investigated. These tars differ in elemental composition and hexane solubility. Hydrogen acceptor abilities were evaluated by reaction with tetralin at 320°C and with decalin at 400°C. Hydrogen transfer from decalin to several model organic compounds was also studied. The rate of carbon deposit formation and hydrogen acceptor abilities of tars are different for various quantities of water in the starting coal charge. The relation between the hydrogen transfer properties of tars expressed as the ratio of decalin hydrogen transfer (DHT) to tetralin hydrogen transfer (THT) and the rate of carbon deposit formation seems to indicate the importance of hydrogen transfer during secondary pyrolysis.     

Influence of carrier gas flow and heating rates in fixed bed hydropyrolysis of coal

Fixed bed hydropyrolysis experiments on a UK bituminous coal (82% dmmf C) at 580–650 °C and pressures up to 300 bar have indicated that tar yields depend strongly on the velocity of the hydrogen carrier gas relative to the static coal particles. Tar yields increase with increasing pressure provided that the superficial gas velocity is not reduced. Otherwise, tar yields can actually decrease because the beneficial hydrocracking reactions that occur are no longer sufficient to counter the increased char formation resulting from the slower rates of intra-particle diffusion and devolatilization of tar molecules. While raising the heating rate from 1 to 20 °C s⁻¹ had little effect on overall conversions, hydrocarbon gas yields increased significantly at the expense of tar. Moreover, the higher heating rate gave more aromatic tars, and the available evidence strongly suggests that the primary volatiles are hydrocracked before escaping from the coal particles as well as in the vapour phase.     

‘Solubility parameters’ in coal and coal liquefaction products

‘Solubility parameter’ spectra have been used in polymer research to determine the characteristics of cross-linked polymer systems. Cross-linked systems are not soluble in any solvent. Instead, solvent is imbibed by the cross-linked polymer and causes it to swell. The ‘solubility parameter’ of the solvent which causes maximum swelling is identified as that of the cross-linked material. Coal can be thought of as having some characteristics of a cross-linked system^1,2. That is, when immersed in a solvent with which it interacts, it will swell. In addition, coal contains extractable material. If coal is regarded as a ‘multipolymer’, this extractable matter can be thought of as the uncross-linked portion of the coal. Swelling spectra have been taken for untreated coal and coal from which some extractable matter has been removed, which partly suppresses swelling. This extractable matter can be thought of as similar to the uncross-linked coal molecule. Its structure can therefore be used to model the coal matrix itself, to determine the coal structure without using destructive chemical methods to break the coal apart. Dissolution spectra for both the coal extract and the coal liquefaction products from the PAMCO and Synthoil processes were taken. A set of mixed solvents with effective ‘solubility parameters’ ranging from 14.3 to 47.9 MPa^1/2 (7.0 to 23.4 hildebrands) was used. The behaviours of the coal extract and coal liquid products show striking similarities, leading us to believe that molecules similar to those found in liquefaction products already exist in the virgin coal and that hydrogenation products reflect the properties of the starting material.     

Co-pyrolysis as a method of upgrading some products of coal processing

Co-pyrolysis was investigated as a method of upgrading various products resulting from coal processing. Co-pyrolysis of vacuum residue (VR) with coal extraction products as well as with primary tars from flash pyrolysis leads to a considerably enhanced yield of liquid products. It has been established that superheated steam and increased outgassing rate, favour the yield of liquid products. The proportion of the ingredients in the mixture as well as the quality of the VR also have a definite effect. The excess yield of liquid products in co-pyrolysis of coal extraction products was 8–23 wt%, depending on operating conditions and the composition of the mixture. The flash co-pyrolysis of primary tars yielded a 1.5–15.9 wt% surplus of liquid products depending on the mixture composition. Products originating from co-pyrolysis of these raw materials with VR are characterized by relatively high atomic hydrogen to carbon ratio, usually not less than 1.5 and the total abscence of asphaltenes. Generally, co-pyrolysis of VR with various products of coal processing is comparable with hydrogenation in the light of good yields of liquid products.     

Analysis of carboxyl group in coal and coal aromaticity by Fourier transform infrared (FT-IR) spectrometry

Thirty-five SS coals as well as two Australian lignites were analyzed by Fourier transform infrared (FT-IR) spectrometry using the KBr tablet method; the SS coals are a series of standard coals produced in various countries. A selected zone 1800-1500 cm⁻¹ of the FT-IR spectrum obtained was deconvoluted, and the peak area for COOH group in coal was obtained. The content of COOH group in coal was calculated from the FT-IR analysis with the aid of the molar absorption coefficient, which had been derived using various aromatic carboxylic acids. On the other hand, the conventional chemical analysis (ion-exchange method) for the determination of COOH group in coal was carried out. When the 37 coals were tested, the COOH contents obtained from the FT-IR analysis were in good agreement with those from the chemical analysis. The zones of 3100-ca. 2990 cm⁻¹ and ca. 2990-2750 cm⁻¹ are assigned to aromatic CH and aliphatic CH, respectively. The ratio of peak area for the two peaks are appreciably correlated to the fraction of aromaticity (f_a) obtained from the solid-state ¹³C NMR analysis. In addition, the ash content could be roughly estimated from the crystal water peak in the FT-IR spectrum.