Effects of solvent and iron-compounds on the liquefaction of brown coal

Hydrogen-donor solvents such as hydrophenanthrene are the most effective aromatic solvents for the liquefaction of brown coal. The hydrogen-donating ability of the solvent is more important for brown coals than for bituminous coals, because the thermal decomposition and subsequent recombination of the structure of the brown coals occurs rapidly. Three-ring aromatic hydrocarbons are more effective solvents than two-ring aromatics, and polar compounds are less effective solvents with brown coals than with bituminous coals. The thermal treatment of brown coal, accompanied by carbon dioxide evolution at temperatures > 300°C, in the presence of hydrogen-donating solvent did not affect the subsequent liquefaction reaction. However, thermal treatment in the absence of solvent strongly suppressed the liquefaction reaction, suggesting that the carbonization reaction occurred after the decarboxylation reaction in the absence of hydrogen donor. To study the effect of various iron compounds, brown coal and its THF-soluble fraction were hydrogenated at 450°C in the presence of ferrocene or iron oxide. The conversion of coal and the yield of degradation products are increased by the addition of the iron compounds, particularly ferrocene, and the yield of carbonaceous materials is decreased.     

Application of XPS to coal characterization

The use of X-ray photoelectron spectroscopy to probe the chemistry of coal surfaces is reviewed and its application to the functional group composition of bulk coals is discussed. The surface composition of a range of 19 coals (anthracite to brown coal), ground under heptane, was measured and compared with the results of bulk analysis. A good correlation was obtained for oxygen, with the bituminous and higher-rank coals showing surface enrichment in oxygen. The surface bulk correlation was less good for sulphur, nitrogen and chlorine and was poor for silicon, aluminium and iron. Silicon and aluminium are enriched at the surface while iron is surface deplected. These effects are either due to different particlesize distributions of the mineral and organic phases or to the mechanism of fracture in heptane preferentially exposing specific components of the coal. Oxidation and carbonization of a bituminous coal were also investigated. Oxidation was seen to occur initially via the exterior surface, producing a distribution of carbon—oxygen groups. Singly-bonded species predominate at all temperatures, stable carboxyl groups forming in significant proportions only at temperatures > 250 °C. Carbonization was seen to result in the formation of ether linkages by condensation of hydroxyl groups.     

Measurement of volatile matter in Victorian brown coal

The effects of using one-stage and two-stage heating, using air-dried coal and coal oven-dried under nitrogen, and using final temperatures of 850°C and 900°C on the measured volatile matter for twelve Victorian brown coals have been examined by the use of a statistically controlled experiment. The repeatability of the two-stage method was found to be significantly better than that of the one-stage method (0.7% abs. compared to 0.9% abs.). The volatile matter of coals at the front of the furnace was found to be significantly different from that of coals at the back of the furnace for the one stage method but not for the two-stage method. A two-stage heating method (7 min at 400°C and then 7 min at 900°C) using oven-dried coal is recommended for the routine determination of volatile matter in brown coal.     

Changes in state of combination of inorganic constituents during carbonization of Victorian brown coal

The inorganic constituents of low-rank Victorian brown coal, which are mainly present as inherent inorganic combinations attached to the coal molecule, are different from the minerals present in higher-rank coals. Changes in the state of combination of the inorganics in the chars of these coals have been studied by determining the minerals formed when chars are prepared at various temperatures. It is shown that when brown coals containing inorganic carboxylates are carbonized, the reactive functional groups which are present will begin to decompose below 400 °C and their decomposition is completed by 600 °C. The inorganic elements released will form mineral combinations depending on the carbonizing temperature and the reduction potential of the metal. Sodium carbonate, calcium oxide, magnesium oxide (periclase) and iron oxide (magnetite) are usually the chief minerals that may be formed. Mineral constituents present in the original coal may also change during carbonization. As the temperature increases, hydrated oxides of iron, clay and aluminium will lose water of crystallization and become dehydrated, forming magnetite, dried clay and alumina. Pyrite will lose up to half its sulphur at 400–450 °C. If the carbonization temperature is over 600 °C, metallic iron may be formed from the magnetite, and sodium chloride (and later sodium carbonate) will volatilize. Quartz remains unaltered.     

Adsorption of methane on dry coal at elevated pressure

Methane equilibrium adsorption isotherms were determined on Illinois No.6 (Herrin seam), Oklahoma Hartshorne, Pennsylvania Pittsburgh, and Virginia Pocahontas No.3 United States coal seams, using a volumetric method and an equation of state for methane: the amount of methane adsorbed on crushed and dried coal was measured as a function of pressure. Isotherms were measured at 30 °C for the Illinois coal and at 0, 30 and 50 °C for the others. Most measurements were made to 150 atm pressure, but a few to 240 atm. The data were correlated by the Langmuir and the Polanyi adsorption models. The methane—coal system, uncorrected for adsorbate density, adhered well to the Langmuir model up to 150 atm pressure, but Polanyi behaviour could not be demonstrated satisfactorily, with or without a correction for adsorbate density. Monolayer volumes at 30 °C from the Langmuir equation were 28, 24, 19 and 20cm³ (STP)/g coal respectively for the four coals studied (the range for several investigators was 13 to 39 cm³/g and for the Langmuir constant b 0.03 to 0.23 atm^?1). Isosteric heats of adsorption at zero coverage and 30 °C were 4.2, 2.4 and 5.3 kcal/mol for the Hartshorne, Pittsburgh and Pocahontas coals, which indicate that the adsorption is physical. No effect of particle size on equilibrium adsorption was observed in the U.S. mesh range 6–325.     

Effectiveness of gases in desulphurization of coal

The effect of air, steam and hydrogen on the desulphurization of 10 U.S. high-volatile bituminous coals was investigated. Air treatment was most effective at 450 °C where an average of 38% total sulphur, comprising 51% of the inorganic sulphur and 20% of the organic sulphur, was removed. With steam at 600 °C, 61% of the total sulphur, 87% of inorganic and 25% of organic was lost. Hydrogen was not effective below 850 °C, but at 900 °C 86% of the total sulphur was dispelled, i.e. 94% of the inorganic and 76% of the organic sulphur. Without oxidative pretreatment the sulphur was much more difficult to remove; after oxidative pretreatment at 300 °C for 10 min followed by treatment with hydrogen at 900 °C, as much sulphur was removed in 4 min as in 60 min without the pretreatment. With raw coal, heating under nitrogen ‘cooked-in’ or fixed some of the sulphur making it more difficult to remove with hydrogen; whereas following oxidative pretreatment, heating for up to 1 h did not lessen the reduction of sulphur with hydrogen. For temperature-swelling coals with large quantities of organic sulphur, heating at 300 °C in air followed by reduction with hydrogen at 900 °C appears to permit rapid discharge (3–10 min) of the organic as well as the inorganic sulphur, to produce a smokeless product with a CV (per unit of product) similar to the fuel value of the untreated coal.     

Ambient-pressure thermogravimetric characterization of four different coals and their chars

Ambient-pressure thermogravimetric characterization of four different coals and their chars was performed to obtain fundamental information on pyrolysis and coal and char reactivity for these materials. Using a Perkin-Elmer TGS-1 thermobalance, weight loss as a function of temperature was systematically determined for each coal heated in helium at 40 and 160 °C/min under various experimental conditions, and for its derived char heated in air over a temperature range of 20 to 1000 °C. The results indicate that the temperature of maximum rate of devolatilization increases with increasing heating rate for all four coals. However, heating rate does not have a significant effect on the ultimate yield of total volatiles upon heating in helium to 1000 °C; furthermore, coupled with previous data⁹ for identical coal samples, this conclusion extends over a wide range of heating rate from 0.7 to 1.5 × 10⁴ °C/s. Using the temperature of maximum rate of devolatilization as an indication of relative reactivity, the devolatilization reactivity differences among the four coals tested that were suggested by this criterion are not large. For combustion in air, the overall coal/char reactivity sequence as determined by comparison of sample ignition temperature is: N. Dakota lignite coal ≈ Montana lignite coal > North Dakota lignite char > III. No. 6 bituminous coal ≈ Pittsburgh Seam bituminous coal > Montana lignite char > III. No. 6 bituminous char > Pittsburgh Seam bituminous char. The reactivity differences are significantly larger than those for devolatilization. The reactivity results obtained suggest that coal type appears to be the most important determinant of coal and char reactivity in air. The weight loss data were fitted to a distributed-activation-energy model for coal pyrolysis; the kinetic parameters so computed are consistent with the view that coal pyrolysis involves numerous parallel first-order organic decomposition reactions.     

Conversion of brown coals in aqueous and toluene-containing media under supercritical conditions in the presence of catalysts

The conversion of brown coals from the Borodino and Kangalas deposits in an aqueous medium and in a mixture of toluene with water was studied under supercritical conditions over the temperature range of 375–550°C and at pressures from 22 to 40 MPa. It was found that the methanation, hydrolysis, and oxidation reactions of brown coals with the predominant formation of gaseous products (methane, carbon dioxide, and hydrogen) prevailed in an aqueous medium. Liquid substances were formed in an insignificant amount. In the toluene solvent under supercritical conditions at 440°C, the addition of a small water amount (15%) stimulated the degradation of coals with the predominant formation of liquid products and moderate gas formation. The use of calcium oxide and sodium hydroxide as catalysts increased the yields of liquid products. It was noted that the reactivity of Kangalas coal in this process was higher than that of Borodino coal.     

Fractionation of coal by use of high temperature solvent extraction technique and characterization of the fractions

The authors have recently presented a new coal fractionation method that can separate a bituminous coal into several fractions, just like petroleum distillation, without decomposing coal. In this paper this method was applied to two bituminous coals and a brown coal. Sequential solvent extraction at different temperatures lower than 350 °C successfully separated the two bituminous coals into several fractions having different molecular mass compounds. Since all the extracted fractions are almost free from mineral matter, and some fractions were found to be fusible like a synthesized pitch when heated, the possibility of producing high performance carbon materials from the coal fractions was investigated. On the other hand, fractions obtained from the brown coal by the sequential solvent extraction were very close in both chemical composition and molecular mass, although the sequential extraction could greatly suppress the decomposition of the brown coal below 350 °C. The difference in the extraction behavior between the bituminous coals and the brown coal were attributed to the difference in their chemical structure.     

Effects of hydrogen pressure on hydrogasification of Taiheiyo (Japan) coal

The non-isothermal hydrogasification of Taiheiyo coal is studied at hydrogen pressures up to 5 MPa and temperatures of 900 °C using a high-pressure thermobalance and tubular reactor. Gaseous products are analysed and liquid products obtained from the mass balance. Rates of formation of methane increased with temperature to two maxima, at 550 °C and at 750 °C. Corrections to rate are necessary because of appreciable weight losses. In the temperature range 650–800 °C the activation energy of methane formation is ≈ 115 kJ mol^?1. Below 55 °C, the pressure dependence of reaction is 0.3, becoming first order at higher temperatures. Rates of formation of methane and ethane indicate a similar mechanism of formation. Rates of formation of liquid hydrocarbons maximize at ≈ 450 °C and increase with hydrogen pressure.     

Effect of rank,temperatures and inherent minerals on nitrogen emissions during coal pyrolysis in a fixed bed reactor

Pyrolysis of 11 coals with carbon contents of 77–93 wt.% (daf) and corresponding demineralized samples has been studied in a fixed bed quartz reactor with a heating rate of 20 K/min to examine rank, demineralization, temperature and inherent mineral species dependences of nitrogen distribution. Nitrogen mass balances fall within 92.5–104.6%. The results indicate that the chars derived from the coals with higher rank show larger nitrogen retention. Demineralization suppresses volatile nitrogen emission during coal pyrolysis, especially for low rank coals. Coal-N conversion to tar-N reaches the asymptotic values at 600 °C. HCN yields are lower than NH₃ yields during coal pyrolysis. The trends in HCN and NH₃ emissions are very similar and the yields reach the asymptotic value at about 1200 °C. N₂ starts emitting at 600 °C, and as the temperature increases the conversion increases linearly with a corresponding reverse change of char-N. With the catalysts added, N₂ formation is prompted with the sequence of Fe>Ca>K>Ti≫Na≫Si≈Al, meanwhile, char-N decreases correspondingly. Fe, Ca, K, Na, Si and Al increase coal-N conversion to NH₃ with the sequence of Fe>Ca>K≈Na≫Si≈Al in the pyrolysis. Na addition prompts HCN formation; however, the presence of Ti and Ca decrease the HCN yields with small value. The other catalysts have no notable influence on HCN emission in the pyrolysis. Demineralization and Ti addition increase coal-N conversion to tar-N slightly whereas K, Ca, Mg, Na, Si and Al additions decrease tar-N yield weakly, other catalysts hardly influence tar nitrogen emission. N₂ emits mainly from char-N with slight contribution of volatile nitrogen. The mechanism of different N-containing species formation and catalysts influence in the pyrolysis is also discussed in the paper.     

Flash hydrogenation of coal 2. Yield structure for Illinois No. 6 coal at 100 atm

Hydrocarbon yields are mapped for a mine-mouth sample of Illinois No. 6 coal in 100 atm of flowing hydrogen, brought to reaction temperature at 650 °C/s. The influence of reac tion temperature was explored from 620 to 980 °C with a vapour-product residence time of 0.6 s. The effect of increasing residence time was explored at 700 °C. The only light products observed in more than trace amounts (above 1%) were methane, ethane, propane, and BTX (benzene, toluene, and xylene). Carbon balances show little if any heavier material in the product at temperatures beyond 850 °C at 0.6 s vapour-residence time or beyond a residence time of 3 s at 700 °C.     

The volatilization behavior of chlorine in coal during its pyrolysis and CO2-gasification in a fluidized bed reactor

The volatilization behavior of chlorine in three Chinese bituminous coals during pyrolysis and CO₂-gasification in a fluidized bed reactor was investigated. The modes of occurrence of chlorine in raw coals and their char samples were determined using sequential chemical extraction method. The Cl volatility increases with increasing temperature. Below 600 °C the Cl volatility is different, depending on the coal type and the occurrence mode of Cl. Above 700 °C, the Cl volatilities for the three coals tested are all higher than 80%. About 41% of the chlorine in Lu-an coal and 73% of that in Yanzhou coal are organic forms, and most of them are covalently-bonded organic chlorine, which shows high volatile behavior even at low pyrolysis temperatures (below 500 °C), while the inorganic forms of chlorine in two coal samples are hardly volatilized even at low pyrolysis temperatures (below 400 °C). The restraining efficiency of addition of CaO on chlorine volatility is greatly dependent on pyrolysis temperature. The optimal restraining efficiency can be obtained at temperature range from 450 to 650 °C during pyrolysis of Lu-an coal. The volatile behavior of Cl is mainly dependent on temperature. Above 700 °C high volatility of Cl is obtained in both N₂ and CO₂ atmospheres.     

Absorption of iodine by coal and lignite

The vapor pressure of iodine over mixtures of iodine and various coals has been measured at temperatures of 65–95°C. Lignite and bituminous coals exhibit similar behavior in their absorption of iodine whereas the behavior of anthracite coal is different. A region of constant vapor pressure occurs in the reaction between iodine and the bituminous coals and lignite. Complex formation between the iodine and coal is postulated.     

Conversion of caking coal into non-caking coal by treatment with sulphur and sulphur dioxide

The effects on the caking properties of coals of reaction between the coals and S₈ and SO₂, have been studied. Caking coals (Akabira, Shinyubari, Zollverein, Indian Ridge, and Big Ben) lose their caking properties when treated with S₈ above 200 °C. For Shinyubari coal the crucible swelling number decreases from $\text{8}\text{1}\text{2}$ to 2 with treatment temperature of 235°C in which 5% of S is incorporated into the coal. The decaking of coal is attributed to thio-ether cross-linkages. Caking coals also lose completely their caking property when reacted with SO₂ at 170 °C. The decaking action of SO₂ is attributed to oxidation of coal in which ether cross-linkages are formed.     

Liquefaction of blended coal

The effect on liquefaction of the blending of two coals of different rank has been evaluated in a conventional autoclave experiment at ≈400 °C by the solvent-refined coal (SRC) method as well as by short-contact-time hydrogenation at temperatures up to 550 °C without solvent and using a specially designed cylindrical autoclave. Using the latter method, higher conversions of coal to gas and liquid, than those calculated by the additivity rule, were observed.     

New method of feeding coal: continuous extrusion of fully plastic coal

Continuous feeding of coal in a compressing screw extruder is described as a method of introducing coal into pressurized systems. The method utilizes the property of many bituminous coals of softening at temperatures from 350 to 400 ° C. Coal is then fed much in the manner of common thermoplastics, using screw extruders. Preliminary results show that coals can be extruded at rates of about 3.3 kg/MJ, similar to those for plastics.     

Effect of coal blending on particle agglomeration and defluidisation during spouted-bed combustion of low-rank coals

The possibility of blending coals to alleviate particle agglomeration and bed defluidisation during fluidised-bed combustion (FBC) of several low-rank coals was exploited. A laboratory scale spouted bed combustor was employed to fire coal blends from two lignites with a sub-bituminous coal at ratios of 50:50 and 90:10, at temperatures ranging 800°C. Experiments showed significant improvements in FBC operation with the coal blends compared to the raw lignites. No particle agglomeration and bed defluidisation were evident after 15 h of operation with the blends at 800°C. Chemical analyses indicated that the formation of low temperature eutectics was suppressed by calcium aluminosilicate phases from the sub-bituminous coal, rendering the surface of ash-coated particles dry and less sticky. This was identified as the key mechanism for the control of particle agglomeration and bed defluidisation in FBC, which led to extended combustion operation with the coal blends.     

Change in composition and porous structure of coal on thermal conditioning

Attention focuses here on methods of coal processing that require minimal quantities of water and yield products that may be effectively used as commercial and secondary raw materials. In the heat treatment of coals associated with semicoking, the accompanying physicochemical transformation of the coal significantly affects its potential for further processing. In semicoking, the filtration system within the coal pieces changes. The initial coal sample contains phytopores of equivalent diameter d_e up to 0.22 μm. More than 54% of these are pores smaller than 10 μm, mainly (65%) of slot and disk form. A small proportion (10%) of supercapillary cavities (d_e > 0.1 μm) is also observed. After heat treatment, the content of small pores is sharply reduced to 10% (450°C semicoke) and 6.6% (550°C semicoke)–that is, almost sixfold–while the content of supercapillary cavities is increased approximately fourfold (in 550°C semicoke).