NOx generation from the combustion of Australian brown and subbituminous coals

The formation of NO_x during the combustion of pulverized brown and subbituminous coals from Victoria and Queensland respectively was investigated in an entrainment reactor. As no NO₂ was detected, all the NO_x was present in the form of NO. The brown coals exhibited a significantly greater potential for NO emission under fuel-lean conditions than did the subbituminous coal, even though the latter coal had a higher nitrogen content. However, under fuel-rich conditions the conversion of coal nitrogen to NO for the subbituminous coal was higher than for the brown coals. The differences in conversion efficiency may have been related in part to the nature and reactivity of the volatile nitrogen species. Reactivity differences between the chars produced from the brown and subbituminous coals may also have accounted for different extents of removal of NO. There was a significant reduction in the amount of NO emitted when brown coal was added to a combustion gas stream containing an appreciable quantity of NO before coal injection.     

Pyrolysis—gas chromatography of Australian coals. 2. Bituminous coals

Pyrolysis—gas chromatography (Py—g.c.) was used to characterize quantitatively a series of high- to low-volatile bituminous Permian Australian coals. The levels of n-alkanes, n-alkenes and triterpenoids released by pyrolysis all decrease as a function of increasing rank and thus, the coal samples can be classified into three distinct groups. Carbon Preference Indices (CPI's) for alkanes and alkene/alkane ratios also decrease as a function of rank. The triterpenoids have exclusively the hopane skeleton. The hopane isomeric distributions exemplify the geological maturity of bituminous coals relative to brown coal (lignite). A significant correlation has been established between the level of n-alkanes and n-alkenes released under Py-g.c. conditions and the predicted oil yield by pyrolysis of these coals. Further development and application of the techniqueshould enable much to be learnt relating to the quality and yield of flash pyrolysis tars as well as the original coal macromolecular structure.     

Reactivity of hydrolysed coals in liquefaction

A hvA bituminous, a subbituminous and a lignite coal have been hydrolysed by 20–30% aqueous caustic solution at 100–300 °C and total pressure from ambient to 8.3 MPa (1200 psi). Reactivity of these pretreated coals toward liquefaction has been examined. The conversion to benzene-soluble material (BS) and oil increases, and the preasphaltene and char residue decreases after pretreatment. Improvement in the conversion to the BS fraction is only marginal for the pretreated bituminous coal, but substantial for the low-rank coals. For the subbituminous coal, the liquefaction reactivity (conversion to BS) increases with the severity of hydrolysis pretreatment. Analyses of chemical compositions, ¹H n.m.r. nuclei distributions and hydroxyl concentrations of the acid-insoluble hydrolysis coal extracts indicate that both O and S are enriched in the extracts with half of the oxygen atoms being in hydroxyl forms. The hydroxyl concentrations of the extracts (acid-insoluble) are ≈2 to 3 times higher than their parent coals. Coal activation by this alkali pretreatment is explained by the hydrolytic attacks on ether C–O linkages, and the removal of some constituents rich in oxygen functional groups which are responsible for poor liquefaction behaviour.     

Ignition behaviour of different rank coals in an entrained flow reactor

An experimental study to determine the temperature and mechanism of coal ignition was carried out by using an entrained flow reactor (EFR) at relatively high coal feed rates (0.5 g min⁻¹). Seven coals ranging in rank from subbituminous to semianthracite, were tested and the evolved gases (O₂, CO, CO₂, NO) were measured continuously. The ignition temperature was evaluated from the gas evolution profiles, and it was found to be inversely correlated to the reactivity of the coal, as reflected by the increasing values of the ignition temperature in the sequence: subbituminous, high volatile bituminous, low volatile bituminous and semianthracite coals. The mechanism of ignition varied from a heterogeneous mechanism for subbituminous, low volatile bituminous and semianthracite coals, to a homogeneous mechanism for high volatile bituminous coals. A thermogravimetric analyser (TGA) was also used to evaluate coal ignition behaviour. Both methods, TGA and EFR, were in agreement as regards the mechanism of coal ignition. From the SEM micrographs of the coal particles retrieved from the cyclone, it was possible to observe the external appearance of the particles before, during and after ignition. The micrographs confirmed the mechanism deduced from the gas profiles.     

Liquefaction of subbituminous coals with a basic nitrogenous model process solvent

Liquefaction reactions in a tubing-bomb reactor have been carried out as a function of coal, coal sampling source, reaction time, atmosphere, temperature, coal pre-treatment, SRC post-treatment and process solvent. Pyridine as well as toluene conversions ranging from 70 to > 90 wt% involving both eastern bituminous and western subbituminous coals are obtained. 1,2,3,4-Tetrahydroquinoline (THQ) has been extensively used as a process solvent under optimized liquefaction conditions of 2:1 solvent: coal, 7.5 MPa H₂, 691 K and 30 min reaction time. Comparisons of THQ with other model process solvents such as methylnaphthalene and tetralin are described. Liquefaction product yield for conversion of subbituminous coal is markedly decreased when surface water is removed from the coal by drying in vacuo at room temperature prior to liquefaction. The effect of mixing THQ with Wilsonville hydrogenated process solvent in the liquefaction of Wyodak and Indiana V coals is described.     

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.     

Gasification reactivity of Chilean coals

Gasification reactivities of raw and acid-washed coal chars obtained from the three most important coal-bearing regions in Chile have been determined in 0.1 MPa of oxygen using a thermobalance. Oxygen chemisorption capacities of the demineralized chars were also measured gravimetrically at 373 K in 0.1 MPa of oxygen. The subbituminous coals of Catamutun and Peckett are more reactive than the bituminous coals of Lota and Trongol due to the catalytic effect of their inorganic constituents. However, in the absence of catalytically significant mineral matter, coal rank is not an important parameter of char reactivity. The reactivity of chars based on carbon active surface areas, estimated from gravimetric chemisorption measurements, agreed very well with the previously reported value based on active surface areas obtained in a volumetric system. These results supply additional evidence that active surface area is the fundamental parameter that can explain most of the observed differences in the kinetic behaviour of coal chars and carbons in general.     

An FTIR study of australian coals: Characterization of oxygen functional groups

The FTIR data in the oxygen-hydrogen stretching and carbonyl stretching regions for 28 Australian coals and their liquefaction products have been investigated. It was found that for the higher rank coals and asphaltenes, the absorbance at 3200 cm⁻¹ correlates with the atomic O/C values and the acidic oxygen contents as measured by a non-aqueous titration technique. The technique of least-squares curve fitting was applied to deconvolute the carbonyl stretching region. The results showed that for the higher rank coals there were relationships between the amount of carbonyl functional groups present, as measured from the band areas, and the coal O/C values. Most brown coals with a high oxygen content (O/C > 0.26) contain less guest material and have a lower concentration of carbonyl groups both as acids and esters than brown coals with a lower oxygen content. They have a higher non-acidic oxygen content, principally as ethers.     

Cd(II) sorption onto chemically modified Australian coals

Cadmium (II) adsorption onto chemically modified Australian coals has been studied as a function of p[H⁺] at 25 °C. The low rank Collie and Loy Yang coals and the bituminous Norwich Park and Mount Arthur coals were modified with hydrogen peroxide. While both treated bituminous coals showed little affinity for Cd(II) the adsorption properties of the two low rank coals were drastically modified by treatment with hydrogen peroxide. NICA-Donnan model calculations for the treated Collie coal indicated an increase in functionality (ca. 5-fold) which was reflected in the important increase in Cd(II) adsorption obtained from batch adsorption experiments. On the other hand, the modified Loy Yang coal underwent a ca. 6.25 decrease in surface functionality and decomposition of the coal structure itself. These changes were reflected in the DRIFT spectra in bands characteristic of the oxygen-containing functional groups. The Cd(II) adsorption capacity of this modified coal was consequently substantially decreased. The two low rank coals were also treated with ammonia at p[H⁺] 7.5 and 9.5. This treatment did not, however, induce any substantial change in the adsorption of Cd(II). DRIFT spectra showed that the predominant functional groups present in both Collie and Loy Yang coal remained essentially unchanged after treatment with aqueous ammonia.     

Interactions between coking coals in blends

The thermoplastic behaviour of 78 binary blends of Australian coking coals was measured using proton magnetic resonance thermal analysis. Their measured extents of fusion were compared with the values calculated from measurements made on the component coals assuming additivity. Significant differences between calculated and measured results were found for most blends, though only at temperatures between 400 and 520 °C: the coals interacted at these temperatures in a way that affected their fluidity. Both positive and negative differences were observed. The magnitude of the differences increased both with increasing differences in rank between the coals and differences in fluidity between the coals. A statistical study of the differences showed that material that became fluid in coal at temperatures below about 360 °C did not appear to contribute to the interactions, which suggests that fluid material derived from liptinite plays a much smaller role in interactions than fluid material derived from vitrinite or inertinite. Additionally, the study indicated that the less fusible material in a blend slightly reduced the extent to which the associated more fusible material fused. It was not acting as an inert diluent.Fifteen blends of six Argonne premium coals were examined to see if the relationships found for Australian coals between the magnitude of the interaction and coal properties could be generalised. In most cases the agreement was good. However, at some temperatures, blends of Upper Freeport coal with lower rank coals were far less fluid than expected, suggesting that the fluidity of Upper Freeport coal is especially sensitive to these low rank coals.The general effect of interactions between coals of different ranks was to narrow the thermoplastic temperature range of the blend without reducing the maximum fluidity, in effect making the thermoplastic profile of the blend resemble the profile expected from an individual coal of the same average rank as the blend. The interactions are attributed to transfer of plasticising volatile material between the coals.     

Structure in cokes from coals of different rank

A range of bituminous coals has been carbonized to 1273 K. Polished surfaces of the solid products, carbons or cokes, are examined for optical texture by optical microscopy. Fracture surfaces of the carbons are examined by scanning electron microscopy (SEM). The carbon from the lowest rank coal (NCB Code No. 702) is isotropic and fracture surfaces are featureless. Carbons from coals of ranks 602, 502 are optically isotropic but fracture surfaces are granular (size 0.1–0.2 μm), indicating small growth units of mesophase. In the carbon/coke from a 401 coal, the anisotropic optical texture and grain size are both ≈0.5–10 μm diameter. Coke from a coking coal (301a, 301b) has a layered structure extending in units of at least 20 μm diameter with sub-structures ~ 1.5 μm within the layers, indicating perhaps that the bedding anisotropy of these coals is not totally lost in the fluid phase of carbonization. The carbons from the higher rank coals have the bedding anisotropy of the parent coal. The combined techniques of optical microscopy and SEM (both before and after etching of the fracture surfaces of coke in chromic acid solution) reveal useful detail of structure in carbons/cokes and of the mechanism of carbonization of coking coals.     

Principal component analysis of upgraded Western Canadian coals

Three Western Canadian coals with different ranks have been upgraded by evaporative drying in several atmospheres at temperatures up to 500°C. Data from proximate and ultimate analyses, along with equilibrium moistures and heating values, have been evaluated by principal component analysis. The results indicate that significant changes in the products occur above 400°C for lignite and above 300°C for subbituminous B and high-volatile C bituminous coals. No significant differences are observed when the data for the upgraded coals are compared with published data for naturally occurring Western Canadian and US coals of the same rank.     

Comparison of Australasian tertiary coals based on resolution-enhanced solid-state 13C n.m.r. spectra

¹³C solid-state nuclear magnetic resonance spectroscopy was used to characterize 32 low-rank coals from New Zealand and Austrlia. A combination of high magnetic field (4.7 T) and resolution ennhancement was used to extract spectral details beyond those seen in published spectra of coals of similar rank. Signal heights were used to characterize organic functional distributions. The spectra showed close similarities between Australian brown coals and low-rank New Zealand subbituminous coals, particularly those mined in the North Island. The spectra of New Zealand lignites all showed stronger signals from cellulose, methoxyl groups and phenols. Almost all of the New Zealand coals showed a relatively strong signal from polymethylene chains, compared with the Australian brown coals. This led to a prediction of higher alkene yields from pyrolysis of the New Zealand coals. Variations in phenolic substitution patterns were attributed to variations in the relative proportions of tannins and lignins in the depositional environments.     

Reactions of coals of different ranks with CO---H2 mixtures with and without sodium aluminate

Well-characterized coals of different H/C atomic ratio and rank were reacted at 365°C with CO, H₂ and CO---H₂ mixtures in water in the presence and absence of sodium aluminate. The optimum H₂/CO ratio for conversion was found to vary with the type of coal. It was <1 : 1 for low-rank and subbituminous coals, whereas the conversion of bituminous coals either did not vary with H₂/CO ratio or reached a maximum at a higher H₂/CO ratio (2 : 1). Even for a bituminous coal there was no advantage in reducing the water/coal ratio below 2 : 1 in NaAlO₂-catalysed reactions. The conversions increased with increasing H/C atomic ratio of the coal and decreasing rank, with or without NaAlO₂. Asphaltene yields increased with increasing coal rank and increasing proportion of CO in the H₂---CO reacting gases. The mechanistic implications of these results are briefly discussed.     

Conversion of non-coking coals to coking coals by thermal hydrogenation

Coking properties are observed in four non-coking coals, a lignite, a subbituminous coal, a semianthracite and an oxidized bituminous coal which had been treated by partial thermal hydrogenation. The effects of temperature, reaction time and hydrogen pressure on liquid and solid product yields are examined. Microscopic examination of the hydrogenated solid residues shows that they all contain structures somewhat spherical in shape which are associated with mesophase development. The dilatation, plastic character and free swelling index of the hydrogenated solid products were considerably better than those of the original coals. Dilatation residues produced from hydrogenated solids exhibited anisotropic structures.     

Diffusion models for gas production from coal: Determination of diffusion parameters

A method for the laboratory measurement of transient diffusivity parameters for methane in coal has been developed. Previously described Chromatographic techniques for measurement of single diffusivity parameters have been modified for the bi-disperse pore diffusion model. This model, which is necessary to describe methane diffusion and desorption in coal for both short and long times, requires the determination of three parameters to characterize a particular coal sample. The experimental technique uses a pulse of methane gas which is introduced into a helium stream flowing through a long tube containing the coal sample. The three desired parameters are determined by matching the experimental and theoretical elution curves. Further refinement of the technique accounts for tracer pulse duration thus allowing operation over a greater flow rate and sample size range. This technique has been applied to anthracite, bituminous and subbituminous coals. For all three coal types, the macropore diffusivity (≈10^?5 s) was about two orders of magnitude higher than the micropore diffusivity. The anthracite adsorbs more methane in the micropores than the other coals. These results showed the expected trends taking the known change of pore structure with rank into account.     

Liquefaction of partially dried and oxidized coals: 3. Liquefaction results

Samples of partially dried and oxidized Belle Ayr subbituminous coal were liquefied in a recycle donor solvent (SRC-ll heavy distillate) to observe the effect of coal pretreatment on conversion. Because subbituminous coals have moisture contents typically > 25%, it would appear useful to dry these coals prior to liquefaction; however, the drying of Belle Ayr coal, either in nitrogen or oxygen-containing gases, resulted in a significant decrease in yields of liquefied coal products. The liquefaction residues recovered from these runs were examined by optical microscopy and were found to contain high levels of coke. This coke appeared to have formed by polymerization of coal-derived liquid products.     

Carbonization of coals to produce anisotropic cokes: 3. Evaluation of low-rank bituminous and subbituminous coals by single and co-carbonizations with pitch

Single carbonizations and co-carbonizations of 17 low-rank bituminous and subbituminous coals have been studied to evaluate their suitability as sources of blast furnace coke in terms of pore-wall profile and anisotropic development within the cokes. Co-carbonizations suggest the possible use of low-rank coals which from single carbonizations would not have been considered suitable. To evaluate semi-quantitatively the coke quality, two structural characteristics of the cokes produced by single and co-carbonizations are graded on a scale of 1 to 5. Overall assessments for each coal are plotted against the atomic H/C and 0/C ratios of the original coals. Although there are a few exceptions, coals with similar assessments are located in the same region of the plot, indicating that, to a first approximation, the H/C and 0/C ratios are suitable indicators of the single and co-carbonization properties of a coal. The presence of cations in the coal appears to be an additional factor influencing the carbonization properties and may explain the exceptional behaviour of some coals. Removal of these cations by pretreatment of the coals improves the carbonization properties.     

Solubilization of coals by reductive alkylation

Anthracite, bituminous and subbituminous coal when treated with naphthalene anion in tetrahydrofuran added negative charges to form the corresponding coal anions. Alkylation of bituminous and subbituminous coal anion with ethyl iodide resulted in the addition of 16 and 14 ethyl groups per 100 carbon atoms. The alkylated coals were 88 and 45% soluble in benzene. The molecular weights of the benzene-soluble portions of the bituminous and subbituminous coal were respectively 2000 and 700. An attempt to add alkyl groups to anthracite anion was not successful.     

Thermogravimetric determination of the carbon dioxide reactivity of char from some New Zealand coals and its association with the inorganic geochemistry of the parent coal

Thermogravimetrically-determined carbon dioxide reactivities of chars formed from New Zealand coals, ranging in rank from lignite to high volatile bituminous, vary from 0.12 to 10.63 mg/h/mg on a dry, ash-free basis. The lowest rank subbituminous coal chars have similar reactivities to the lignite coal chars. Calcium content of the char shows the strongest correlation with reactivity, which increases as the calcium content increases. High calcium per se does not directly imply a high char reactivity. Organically-bound calcium catalyses the conversion of carbon to carbon monoxide in the presence of carbon dioxide, whereas calcium present as discrete minerals in the coal matrix, e.g., calcite, fails to significantly affect reactivity. Catalytic effects of magnesium, iron, sodium and phosphorous are not as obvious, but can be recognised for individual chars. The thermogravimetric technique provides a fast, reliable analysis that is able to distinguish char reactivity differences between coals, which may be due to any of the above effects.