Harry J. Hurst

Ash and slag qualities of Australian bituminous coals for use in slagging gasifiers

The suitability of Australian bituminous coals from an ash and slagging viewpoint, has been examined for use in entrained-flow slagging gasifiers which form the basis for more efficient power generation technologies based on integrated gasification-combined cycle (IGCC). Several coal deposits in both NSW and Queensland appear suitable for slag tapping without the addition of any flux. Many more thermal coals are shown to require a limestone flux addition of <3% CaCO3 by weight of coal and there could well be opportunities to reduce costs of flux addition by blending with other coals with lower ash fusion temperatures. Coal blending to yield a SiO2/Al2O3 ratio of 1.6–2.0 can minimise limestone flux requirements and avoid some limitations which arise from slag crystallisation. Limestone flux requirements can also be reduced by slag tapping at 1500°C rather than 1400°C, but this should be balanced against increased operating costs and losses in cold gas efficiency at higher gasification temperatures. Australian export coals with very low iron contents, after limestone flux addition, appear to have the potential advantage that slag viscosities (and hence slag tapping performance) are essentially independent of variability in coal ash composition. This should lower gasifier operating and maintenance costs, offsetting the costs of flux addition. Ash and slag characteristics and possible strategies for optimum coal use in entrained-flow slagging gasifiers are discussed.

Siderite decomposition in retorting atmospheres

Abstract This work extends the characterization of siderites found in Tertiary Australian oil shales by investigating their decomposition in atmospheres resembling retorting conditions. The decomposition of mineral siderite samples and selected oil shale samples with high siderite contents, representing the four types of siderites most commonly found, were studied by thermogravimetry in controlled carbon dioxide-nitrogen-water vapour atmospheres. The decomposition temperature of each of the siderite types increased with increasing partial pressure of carbon dioxide, and decreased with increasing partial pressure of water vapour. Both effects were non-linear: larger temperature changes were observed at lower partial pressures. This study makes it possible to predict the amount of siderite decomposition under typical retorting conditions and the consequences for both the retorting and combustion stages during oil shale processing.

Kinetics of dehydroxylation, in nitrogen and water vapour, of kaolinite and smectite from Australian Tertiary oil shales☆

Abstract Dehydroxylation of kaolinite and smectite is significant in oil shale processing. The possibility of shifting these endothermic reactions from the retort to the combustor by increasing the water vapour pressure was confirmed and the kinetics of dehydroxylation was determined by thermogravimetry. For kaolinite, non-isothermal and isothermal dehydroxylation in dry N 2 was second-order, with activation energies of 163 and 208 kJ mol −1 and pre-exponential factors of 2.0 × 10 12 and 3.0 × 10 13 s −1 respectively. Increasing water vapour pressure markedly increased the dehydroxylation temperature. Isothermal data were fitted to a first-order model at water vapour pressures 5 kPa, but Arrhenius plots showed large changes in the kinetic parameters. Non-isothermal results increasingly deviated from second-order with increasing water vapour pressure, but this was successfully accounted for by the Altorfer model. For Rundle smectite, non-isothermal kinetic data for dehydroxylation in dry N 2 were best fitted by the Ginstling-Brounshtein diffusion model, with an activation energy of 83 kJ mol −1 and a pre-exponential factor of 1.0 × 10 7 min −1 .

Mineral reactions in the processing of Australian Tertiary oil shales

Abstract Mineralogical, mineral characterization and thermogravimetric studies were made of raw and processed shale samples from the most prospective sections of the Rundle, Stuart, Condor, Duaringa, Nagoorin South, Nagoorin, Yaamba and Lowmead oil shale deposits. The effects of processing on minerals were determined in sufficient detail to establish the relevance of the various minerals and their reactions in oil shale processing. Minerals identified as the most significant in processing were smectite, kaolinite, siderite-type minerals and pyrite. Smectite and siderite-type minerals are characteristic of the oil shales of eastern Queensland. Smectites were found to be comparable in the various deposits and to be members of the montmorillonite/nontronite series with unusually low thermal stability. Four types of siderite (siderite, manganoan siderite, magnesian siderite and high magnesian siderite) were found, with progressively increasing decomposition temperatures in the retort. The most important mineral reactions occurring in the preheater, retort and combustor have been identified. Endothermic dehydroxylation and decomposition reactions of smectite, kaolinite, siderite minerals and pyrite influence heat requirements and gaseous products from retorting and combustion. The results indicate the possibility of transferring some of these reactions from the retort to the combustor by control of partial pressures of vapour phases in the retort. A sound basis has been established for future studies of the effects of specific minerals on oil coking reactivity and oil yields in retorting.

RELEVANCE OF CARBONATE MINERALS IN THE PROCESSING OF AUSTRALIAN TERTIARY OIL SHALES

Abstract The Tertiary oil shale deposits of eastern Queensland, Australia, comprise an important future source of alternative liquid fuels. Carbonate minerals are important in relation to selection of processing conditions and in limiting sulphur dioxide emissions. This paper characterizes such minerals from the Stuart, Rundle, Condor, Nagoorin, Nagoorin South, Duaringa, Yaamba and Lowmead deposits. Composite and stratigraphically selected samples were mineralogically analysed using chemical analyses, X-ray diffractometry, thermogravimetry and scanning electron microprobe analysis. Carbonate minerals significant in processing include calcite, magnesian calcite, siderite and magnesian, calcian and manganoan siderites which occur in different amounts in the various deposits. Dolomite and ferroan dolomite were sporadically observed. Calcite and magnesian calcite are decomposed in the combustor and reduce sulphur dioxide emissions by reaction to form CaSO 4 in the combusted solids. Siderite type minerals are fully or partially decomposed depending upon siderite type and process conditions in the retort. The iron oxides produced react with hydrogen sulphide in the retort gases. Calcium replacing iron in the siderite also reacts with sulphur dioxide to form CaSO 4 and hence limits sulphur dioxide emissions from the combustor.

Dissolution of lime into synthetic coal ash slags

Abstract One of the alternate processes presently being investigated to produce electrical power from coal is Integrated Gasification Combined Cycle (IGCC). The ash, which remains when the coal is gasified in this process, is removed by tapping the molten ash at 1400–1500°C. To ensure that the coal ash is molten at 1400–1500°C, the melting temperature of the coal ash may need to be reduced by addition of a flux, usually limestone, which is added with the coal to the gasifier. The rate of dissolution of the flux is uncertain. This paper reports the investigation of the rate of lime dissolution into synthetic coal ashes, consisting of SiO 2 , Al 2 O 3 and CaO. Results previously reported have shown that the free dissolution of fine particles (50–200 μ m) is mass-transfer controlled [S.M. Wang, T.F. Wall, J.A. Lucas, L. Elliott, A.C. Beath, Experimental studies and computer simulation of dissolution of lime particles into coal ash slags. Australian Symposium on Combustion and the Fourth Flame Days, Univ. of Adelaide, South Australia, November 9–10, 1995]. To investigate forced dissolution, a high-temperature viscometer was used to rotate a cylinder of lime in the molten slag for a given period. At temperatures between 1450°C and 1650°C, reaction products of 3CaO·SiO 2 /3CaO·Al 2 O 3 , 2CaO·SiO 2 /3CaO·Al 2 O 3 form around the lime cylinder. The concentration gradient involved in the mass transfer and the diffusion coefficient are currently being investigated.

Comments on the use of molybdenum components for slag viscosity measurements

The interest in coal gasification for power generation has led to the increased use of high temperature viscosity measurements for the characterisation of slag properties. This communication reports on the slag reactivity with molybdenum crucibles for a large range of slag compositions.

High pressure high temperature electron spin resonance study of oil shale retorting

Abstract Previous studies of high pressure retorting under hydrogen have reported large increases in the Fischer assay oil yield for carbonaceous shales. A high temperature, high pressure, electron spin resonance cavity has been constructed and used to measure spin concentrations of the radicals during pyrolysis. The results are independent of the gas and do not show the expected reduction in spin concentration for hydrogen due to hydrogen capping; this can be explained by the failure of steady state electron spin resonance measurements to detect rapid radical formation and decay processes.