Measurement of true specific heats of bituminous coals of different rank,and of a high-temperature coke,in the temperature range 30–350 °C

The true specific heats of four bituminous coals from the Ruhr district with volatile matter ranging between 9 and 37 wt% and of a high-temperature coke were measured with an adiabatic metal calorimeter. The precision of the equipment was checked by measurements on pure nickel, with the result that the experimental values were found to differ by <1% from the adequately accurate average values given in the literature. The results of this study and the scrutiny of all obtainable systematic specific heat measurements of bituminous coals lead to the conclusion that, contrary to the opinion given in the literature, the specific heats do not increase rectilinearly with the volatile matter. The relation of the specific heats of the coals investigated to volatile matter and temperature can be explained by the chemical composition and structure of the coals. The specific heats calculated from the distribution of the elements carbon, hydrogen and oxygen in the structural groups of bituminous coals correspond well to the experimental values. The procedure is more promising than that using only the results of immediate analysis without taking into account chemical composition and structure. In the initial stage of pyrolysis, variations from a monotonic increase are observed and semiquantitative considerations on this are given. The effect of temperature on the specific heats of coals is compared with that on other physical quantities and explained.     

Improved electrochemical performance of trivalent-chrome coating on Al 6063 alloy via urea and thiourea addition

This project aims at improving the electrochemical performance of trivalent-chrome coating through urea and thiourea addition. The electrochemical behaviors of coatings formed with different concentrations of urea and thiourea were investigated in 3.5 wt.% NaCl solution at 25 °C, using potentiodynamic polarization curves and EIS. The corrosion resistance of coatings is improved greatly by adding a small amount of inhibitors, whereas the excessive addition deteriorates the corrosion resistance. Thiourea addition presents better effect than urea. To explain the EIS results of the coatings, a simple equivalent circuit was designed. The EIS parameters were obtained by fitting the EIS plots. The results of the polarization curves and EIS show that the inhibitor-containing coatings present better corrosion resistance than the coating without inhibitor. The morphology and composition and valence state of the conversion coatings were examined by SEM and EDS and XPS, respectively. The results indicated that the trivalent chromium coating was developed on Al 6063 alloy, urea and thiourea inhibitors were also deposited on the substrates, respectively. A noticeable chemical shift was also observed.     

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.     

Differential scanning calorimetry studies on coal. 2. Hydrogenation of coals

Results of exothermic heats involved during hydrogenation of twenty U.S. raw coals of varying rank at 5 · 6 MPa (gauge) and temperatures up to 570 °C are reported. The heat evolved during hydrogenation up to 570 °C decreases with increase in coal rank. A part of the total heat released during hydrogenation of coals appears to be due to the exothermic reaction between H₂ and surface carbon-oxygen complexes removed during the reaction. The transition temperature, that is the temperature corresponding to the onset of exotherms, is markedly dependent on coal rank. A sharp increase in the transition temperature occurs for coals having a carbon content, on a dry-ash-free basis, in the 75–80% range. Demineralization of coals lower in rank than HVA bituminous decreases the heat of hydrogenation; in the case of higher-rank coals, exothermic heats increase upon demineralization. The presence of pyrite has a beneficial catalytic effect on coal hydrogenation.     

Coal carbonization with addition of hydrochloric acid as a way of improving coke quality

The possibility to expand the base of raw materials for carbonization using more lower grade and noncoking coals has been examined. Plastometric indices of the parent coals (C=79.0-87.2 wt% daf): x, shrinkage of coal and y, thickness of the thermoplastic layer were determined. It was established that the addition of 1 M HCl to coal increases the thickness of the thermoplastic layer (y) of gaseous (C=82.7 wt% daf) and rich (C=87.2 wt% daf) coals by 15-20% and the strength of the solid carbonized residue from 64 up to 84% and from 92 up to 94 %, respectively. A comparative evaluation of gaseous (C=82.2-82.7 wt% daf) coal according the degree of its restorativity is given. The strength of the coke is obtained from untreated gaseous coal and with HCl additive in the temperature region of carbonization of 450-800 °C. It is established that the greatest increase of coke strength takes place in the temperature region of 550-750 °C. Data of X-ray diffraction show that structural changes take place at coal carbonization.     

The effect of coal properties on the viscosity of coal-water slurries

Studies on coal-water slurries (CWSs) have been conducted for many years to replace fuel oil. In this research project, the effect of coal properties on CWSs have been investigated using two Turkish coals of different ranks and a Siberian bituminous coal. Physical, chemical and surface properties of coal samples were determined. Furthermore, adsorption tests were carried out in order to put forward the effect of additive adsorption on the viscosity of CWSs. Viscosity measurements were realized for CWSs of various solid ratios by weight that were prepared using coal samples having mean particle sizes of 19, 35 and 50 μm.     

Low temperature oxidation of coals of different rank and different sulphur content

Five coals of different rank and of different content of sulphur were subjected to oxidation by peroxyacetic acid (PAA), 5% nitric acid, by oxygen in 0.5N Na₂CO₃ aqueous solution and the air oxidation for 7 days at 125 °C. The processes of oxidation were carried out for coal samples demineralised by the Radmacher method, and additionally for the pyrite-free coal samples. Proximate, elemental and spectral analyses of the initial coals and the products obtained from them were made. The most effective oxidising agents were 5% HNO₃ and PAA. As a result of oxidation, a significant part of the organic components of coal is converted into acid soluble products. Depending on the oxidising agent, the loss of sulphur in the solid oxidation products was different and the highest for the coal samples oxidised with HNO₃ and PAA. Formation of oxidised sulphur species (SO and -SO₂) was detected by IR spectroscopy. FTIR data also provide useful information on evolution of the molecular structure of different rank coals on oxidation, in particular in the carbonyl and aliphatic ranges (1800-1500 and 3500-2800 cm⁻¹).     

Investigation of an iron-based additive on coal pyrolysis and char oxidation at high heating rates

Iron-based catalysts have been shown to enhance coal pyrolysis and char oxidation at low to moderate temperatures and heating rates (< 1250 K and 1–1000 K/s). Such catalytic activity has not been demonstrated at high heating rates and temperatures approaching pulverized coal combustion applications. The effect of an iron-based additive on coal pyrolysis and char combustion was studied in a flat-flame burner system at high particle heating rates using a Kentucky bituminous coal. Pyrolysis and char reactivity of two treated coals with different catalyst loadings were studied and compared with the untreated coal. The total volatiles yield for the treated coals increased between 14 and 18% (absolute) on a dry ash-free basis compared to the untreated coal in experiments conducted at 1300 K. A first-order char oxidation model was used to compare the apparent char reactivities of the treated and untreated coals measured at 1500 and 1700 K. An increase in apparent char reactivity was observed for both treated samples.     

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.     

The effects of poly(ethylene glycol) on the low-temperature oxidation reaction of coal as monitored using in situ series diffuse reflectance FTIR

In situ series diffuse reflectance FTIR was used to study the effects of poly(ethylene glycol) as a potential chemical additive inhibiting coal oxidation process at low temperatures. Two coals with different volatile content and, different ash percentages were examined following treatment with 5 wt% poly(ethylene glycol) 200. The surfaces of samples both with and without the additive were analyzed at temperature up to 200 °C in air using in situ diffuse reflectance FTIR. The results showed that poly(ethylene glycol) 200 is capable of inhibiting the oxidation of aliphatic moieties such as methyl and methylene groups, and also reducing the quantity of surface hydroxyl groups through reactions that form more stable ether linkages, thus improving the thermal stability of the coal. A mechanism by which the additive interacts with the coal surfaces is proposed.     

Importance of the textural characteristics of inert additives in the reduction of coal thermoplastic properties

Seven carbonaceous materials of different origin were chosen in order to study the influence of their porous structure on the modification of the thermoplastic properties of a bituminous coal. The materials included were: two non-coking coals, a petroleum coke, coke fines, two residues from tyre recycling and a bituminous residue. The materials were heat-treated to 900 °C to prevent any chemical interaction between the volatiles evolved during co-carbonization. The thermoplastic properties of blends that contained 10 wt.% of additive were measured by means of the Gieseler test. Microporosity was measured by CO₂ adsorption at 273 K, whereas meso and macroporosity were determined by means of mercury porosimetry. The results of the porous structure assessment are discussed in relation to the modification of coal plasticity.     

Hydrothermal dewatering of lower rank coals. 2. Effects of coal characteristics for a range of Australian and international coals

A range of lower rank coals from Australia, Indonesia, and the USA have been dried under hydrothermal dewatering conditions at 320 °C for 30 min in a 500 ml autoclave using a 1:3 dry coal/water mixture. The hydrothermally dewatered (HTD) products were characterised by elemental analysis (both organic and inorganic components), volatile matter determinations, moisture holding capacity, calorific value and mercury porosimetry. The total organic carbon (TOC) and the concentrations of inorganics in the waste waters were also determined. The coals fell into two broad groups, the lower rank Australian brown coals with gross calorific values in the range 10-16 MJ/kg (afm) and the higher rank Indonesian and US coals with gross calorific values in the range 18-25.5 MJ/kg (afm). Rank was the major factor influencing the properties of the HTD products but lithotype was also important. TOC values of the waste water from drying of the low rank Australian coals were much higher than those of the waste water from the higher rank coals. Important variations in the amount of leached calcium, magnesium, and iron were noted.     

The role of desorption of moisture from coal in its spontaneous heating

As a part of investigations on the influence of humidity on the initial stage of the spontaneous heating of coal, the effect of desorption of moisture from the coal was studied. Laboratory experiments with different coals were carried out under conditions where desorption of water from the coals by air was certain to take place. Thermal changes were measured by a specially designed calorimeter in isothermal conditions, mostly at 30 °C. The results showed that in each case there occurred a cooling of the coal, indicating that the rate of heat loss from a coal due to this process was greater than the rate of heat release due to oxidation of the coal. The data suggest that moisture desorption acts as an inhibitor to the spontaneous heating of coal. For a particular coal, the rate of heat loss increases with the increase in the equilibrium humidity deficiency of the air. The effects of rank, particle size and weathering on the process are also discussed.     

Effect of NaOH treatment on surface properties of coal

Surface properties of sub-bituminous Wyodak and bituminous Illinois coals were examined using sorption and immersion techniques, to elucidate the effects of aqueous NaOH treatment. Sorption of non-polar gases such as CO₂ and N₂ decreased significantly after the alkali treatment for the Wyodak coal; however, similar tests with the Illinois coal did not produce such dramatic results. Water sorption and heats of immersion into liquid water increased considerably for both coals following the NaOH treatment. It is concluded that treatment with aqueous NaOH affects the chemical and physical structure of the coal surface.     

Factors affecting the rate and extent of heat release during the immersion of powdered coals in alkanols

The heats of immersion of both bituminous coals (Virginia-C and Kentucky-Blend) and subbituminous coals (Wyodak) in alkanols have been determined micro-calorimetrically. Studies have been made of the effects on immersional heat release of varying outgassing conditions and prior extraction of the coal samples with methanol. Investigations of the heat release and the rate of heat release across the range of simple alkanols containing from one to ten carbon atoms show considerable variation in behaviour, with the butanol isomers revealing particularly striking differences. The observed values of the heats of immersion are interpreted in terms of limited alcohol accessibility to the pore structure of the coals.     

Vaporization behavior of boron from standard coals in the early stage of combustion

Boron-containing compounds have been listed as one of environmentally hazardous substances in Japan since 2001, and known to condense in coal fly ash particles during coal combustion and coal fly ash formation in coal-fired electric power stations. So far, the authors have revealed that the speciation of boron-containing compounds in coal fly ash particles is mostly a calcium orthoborate or pyroborate. In this research, the speciation of boron compounds in standard coals and their char generated by laboratory-scale combustion test has been investigated by using a microwave-assisted acid digestion method and a Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS-NMR) in order to reveal the vaporization behavior of boron in standard coals during combustion at relatively low temperature. Three isolated peaks are observed in ¹¹B MAS-NMR spectra of standard coals, and all of them are attributed to four-oxygen-coordinated boron atom. Around 50% of boron vaporizes even though heating condition is 200 °C and O₂ = 25%, and the percentage of vaporization reaches higher value than 80% at 400 °C and O₂ = 25%. The remaining boron contents in ash components are relatively small, and it suggests that most of boron in standard coals exist with relatively volatile carbon contents, and they volatilize in the very early stage of coal combustion.     

Enhancement in hydrophobicity of low rank coal by surfactants — A critical overview

The flotation of fine (− 0.5 mm) low rank or oxidized coal is difficult to achieve with the common coal flotation collectors like kerosene, fuel oil or diesel oil (oily collector). The presence of small amounts of oxygen is enough to cause oxidation. The oxidation of coals starts with the physical adsorption of oxygen on the surface to form an oxycomplex followed by chemical adsorption of oxygen to form polar phenolic-OH, carbonyls, phenols and peroxide type oxygenated moieties by the rupture of cyclic rings. The addition of promoter, surfactant or oxygenated functional groups to the collector molecule markedly enhances the flotation of lower rank and oxidized coals due to the hydrogen bonding with the polar part of the coal surface and the reagent. The performance of these reagents is compared with that of oily collectors, namely kerosene, dodecane, nonylbenzene and polar part of the surfactant having an oxygen atom. The mode of addition of non-ionic surfactant with oily collector also has a major role in the flotation response. The addition of non-ionic surfactant after the oily collector has shown a positive effect on yield and grade.     

Konovalov correlation for molar heats of combustion of organic compounds: its adoption for coal

Konovalov's correlation for molar heats of combustion of organic compounds has been suitably adopted for coal. Studies of a number of coal models, supported by experimental data on actual coals of varying rank, have revealed that ‘x’ the structure-related factor remains virtually constant at about 0.43 MJ kg⁻1 for all high volatile coals (C≤81%) beyond which it decreases almost linearly with increasing rank. A logical derivative of the correlation is also presented to give calorific values of coal on dry basis, from lignite to anthracite. Accuracy of the new correlation has been found to be as good as those of previous correlations, SD being about ±0.16, whereas its potential accuracy has been found to be as low as 0.05 MJ kg⁻1. Such high accuracy has not been hitherto reported for any other correlation. Significance of this vis-à-vis the values of ‘x’ and hence the structural pattern of coal is also discussed.     

A thermal investigation on coals from Assam (India)

A thermal characterization of two coal samples from Ledo and Tikak collieries of Makum coalfield, Assam, India using XRD, FT-IR, and TGA was reported in this paper. The coal samples were heated for 20, 40 and 60 min in a 1000-watt heater (temperature ∼ 250 °C) in presence of air and characterized by XRD and FT-IR spectroscopy. Both the coals contain amorphous and crystalline phases. The raw coals also contain very small peaks due to quartz, calcite, gypsum, pyrite, and chlorite. The XRD patterns were found to change upon heating. In the coals heated for 20 and 40 min, it was observed that both amorphous and crystalline parts are common in them; crystalline part being the major one in the 40 min heated samples. The XRD patterns of the samples heated for 60 min indicate the presence of major quantities of α-quartz, hematite, and chlorite in them. They also show some new peaks, which are assigned to be kaolinite, illite, magnetite and very small in comparison to the amorphous portion in raw coals. α-quartz was found to be most stable crystalline phase of silica in the coals. The crystallinity % (X-ray) of the coals heat-treated for different times was determined and found to be increasing with time of heating. The FT-IR spectra of raw and heat-treated coal samples at 250 °C were also recorded and compared. The spectra were observed to be almost similar and it was observed that few functional groups disappear on heating at 250 °C. The same coal samples were also characterized by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) techniques. On heat treatment in air atmosphere up to 800 °C, 20–27% weight loss occurs due to removal of various volatile materials. DTA results indicate the chemical reactivity of the coal sample initially at 80–110 °C due to loss of water, and two other major reactions at around 420 and 530 °C due to primary and secondary volatization.     

Trace element affinities in two high-Ge coals from China

The Lincang (Yunnan Province, Southwest China) and Wulantuga (Inner Mongolia, Northeast China) coal deposits are known because of the high-Ge content. These coals have also a high concentration of a number of other elements. To determine the mode of occurrence of the enriched elements in both coals, six density fractions from <1.43 to >2.8 g/cm³ were obtained from two representative samples using heavy-liquids. A number of peculiar geochemical patterns characterize these high-Ge coals. Thus, the results of the chemical analysis of these density fractions showed that both coals (very distant and of a different geological age) are highly enriched (compared with the usual worldwide coal concentration ranges) in Ge, As, Sb, W, Be, and Tl. This may be due to similar geochemistry of hydrothermal fluids influencing the Earth Crust in these regions of China. Moreover, Wulantuga coal (Early Cretaceous subbituminous coal) is also enriched in Ca, Mg, and Na, and Lincang coal (Neogene subbituminous coal) in K, Rb, Nb, Mo, Sn, Cs, and U. A group of elements consisting of Ge, W, B, Nb, and Sb mostly occur with an organic affinity in both coals. Additionally, Be, U, and Mo (and partially Mn and Zn) in Lincang, and Na and Mg in Wulantuga occur also with a major organic affinity. Both coals have sulfide-arsenide mineral assemblages (Fe, S, As, Sn, and Pb, and in addition to Tl, Ta, and Cs in the Lincang coal). The occurrence of Al, P, Li, Sc, Ti, V, Cr, and Zr in both coals, and Ba in Lincang, are associated with the mineral assemblage of silico-aluminates and minor heavy minerals. Furthermore, P, Na, Li, Sc, Ti, Ga, Rb, Zr, Cr, Ba, Th, and LREE (La, Ce, Pr, Nd, and Gd) in Lincang are associated with mineral assemblages of phosphates and minor heavy minerals. The two later mineral assemblages are derived from the occurrence of detrital minerals. Finally, the two coal samples have also the sulfate mineral assemblage (Ca and Sr) that probably occur as a consequence of a diagenetic oxidation and alteration of the coal seams. The enrichment of Ge in coal occurred when the organic matter was still reactive to trap Ge, but several features indicate that the enrichment was diagenetic.