Stuart Day

Methane capacities of Bowen Basin coals related to coal properties

Abstract To help in assessing coalbed methane resources, methane adsorption capacities of Permian coals from the Bowen Basin of Queensland were investigated and related to other coal properties. Maximum methane capacities of moisture-equilibrated coals, normalized to a pressure of 5 MPa and 30°C, showed a continuously increasing (and reasonably linear) trend with increasing rank over the range 80–92 wt% total carbon. Such a linear trend was not observed for methane adsorption calculated on a dry basis. Methane adsorption capacity decreased with increasing temperature by ∼0.12 mL g−1K−1. Methane capacity also decreased as moisture content increased, by ∼4.2 mL g−1 coal for each 1 wt% increase in moisture. Coal surface area (CO2, Dubinin-Radushkevich) showed reasonable correspondence with methane capacity, although not precise enough to provide a reliable estimate of capacity. Comparison of Langmuir adsorption isotherms for nitrogen and carbon dioxide on Bowen Basin coals with the corresponding methane isotherms showed that, with a knowledge of the methane isotherm alone, nitrogen and carbon dioxide isotherms could be reliably constructed. Volumetric and gravimetric methane isotherms measured on the same coal were identical, confirming the accuracy of the procedures and calculations used.

Moisture-Induced Swelling of Coal

The gas-induced swelling behavior of coal is important when considering CO2 sequestration into coal seams or enhanced coalbed methane applications, but coals may also swell in the presence of moisture, or shrink on drying. In this paper we examine the moisture-swelling properties of coals from Australia and elsewhere. Results on the moisture uptake and corresponding swelling measurements are presented for 15 coals of various ranks (sub-bituminous and bituminous) at 22°C and atmospheric pressure. Measurements were made by exposing sample blocks of coal (nominally 30 × 10 × 10 mm) to relative humidities ranging from 0% to 97%. A selection of coals was also fully saturated in water. Moisture uptake at 97% relative humidity (RH) ranged from about 2.5% to more than 16% db. Maximum linear strain associated with the moisture sorption (measured at 97% RH) varied from about 0.2% to 1.3%, with lower rank coals showing the most swelling. In all cases, swelling was greater in the direction perpendicular to the bedding plane. These results correspond to volumetric swelling of about 0.5% to around 5%. Although exhibiting significant expansion, all of the samples returned to their original dimensions upon drying. Volumetric moisture sorption and the amount of swelling induced were found to be strongly correlated by a single linear expression that held for all of the coals examined. It was further found that the volume of the water adsorbed was linearly related to the pore space within the coal, however, at 97% relative humidity, only about 60% of the available pore space is occupied by water. Exposure to liquid water allowed the pores to completely fill; although for the lowest rank coals, the volume of water absorbed appeared to be slightly more than the corresponding pore volume. Despite the additional water uptake, immersion in water did not produce further swelling beyond that induced at 97% relative humidity.

Dehydroxylation kinetics of kaolinite and montmorillonite from Queensland Tertiary oil shale deposits

Water released during clay dehydroxylation was measured using a small reactor coupled to a mass spectrometer. Dehydroxylation of Rundle montmorillonite and Nagoorin kaolinite occurs over the temperature ranges 250–700 °C and 280–600 °C, respectively. The low temperature of montmorillonite decomposition suggests iron substitution for aluminium in the crystal lattice (a nontronite structure). The montmorillonite dehydroxylation kinetics best fit a diffusion-controlled reaction in a sphere (a D(3) mechanism) to about 80% reaction. Kaolinite dehydroxylation is best represented by second-order kinetics (to at least 90% of reaction). Activation energies for the dehydroxylation reactions were determined as 145 ± 15 and 99 ± 8 kJ mol−1 for kaolinite and montmorillonite, respectively.

A comparison of three methods for the quantification of greenhouse gas emissions from spontaneous combustion in open-cut coal mines

Greenhouse gas emissions from spontaneous combustion in coal mines are currently excluded from national inventories by the United Nations Framework Convention on Climate Change because there are no robust methods available to quantify the emissions. This article reports on investigations of three approaches being pursued with the aim of developing methods which are sufficiently robust to enable these emissions to be included in inventories. The first method was based on the use of airborne thermal infrared photography coupled with chamber measurements of emissions from hot mine spoil pile surfaces. In the second method, crosswind traverses of the plume using an instrumented vehicle were used to estimate the emission fluxes. The third method was based on inverse atmospheric modelling using stationary CO2 monitors. All three methods showed considerable scatter in their estimates but also showed appreciable overlap. While the three methods used in this study have shown convergence, there is still considerable uncertainty associated with any single approach.

Factors controlling CO 2 absorption in Australian coals

Publisher Summary Carbon dioxide sequestration into coal seams is considered as a potential method for reducing atmospheric carbon dioxide emissions and for enhancing coal bed methane recovery. It was recognized early that for carbon dioxide to be sequestered into coal seams, the carbon dioxide would be in a supercritical state (greater than 31°C, 7.3 MPa). Although coals have long been known to absorb carbon dioxide strongly at pressures below 5 MPa, relatively little work has been done measuring the effect of supercritical carbon dioxide on coals. Some studies on the adsorption of gases onto activated carbons have indicated that, at high pressures, where the gas density approaches the density of the adsorbed phase, the excess (or Gibbs) absorption is more useful and more accurately determinable than the absolute adsorption. The excess adsorption for carbon dioxide on dry activated carbon at 45°C reaches a maximum at about 8 MPa and then decreases linearly with increasing gas density (not pressure), reaching zero when the gas density is the same as the adsorbed phase density.

Coke formation on recycled combusted Rundle oil shale

Results are presented on the coking reactivity of combusted Rundle oil shale towards various shale oil fractions and individual oil components. The quantity of coke formed was not greatly affected by the coking temperature in the range of 500–700 °C. However, the temperature at which the shale was combusted did have a significant effect: considerably more coke was deposited on shale combusted at 700 °C than on shale combusted at 900 °C. There was no apparent difference between the amount of coke formed on shale combusted under dry conditions and the amount deposited on shale which had been combusted in the presence of steam. Shale with an organic carbon content of about 3 wt%, corresponding to material which had been about 50 wt% combusted, showed little difference in coking activity to that of fully combusted shale. The amount of char produced by separate shale oil fractions increased with increasing boiling range. The amount of nitrogen incorporated into the coke also increased with boiling range but at a rate disproportionate to the nitrogen content of the original oil fraction. A range of pure compounds, which are typical oil shale components, were examined with respect to their coking reactivities on combusted Rundle shale. Trends of increasing coke formation as a function of chemical structure and boiling point were observed.

Removal of hydrogen sulphide by combusted Rundle oil shale: sulphidation of iron oxides

Abstract Oil shale from the Rundle deposit of Queensland, Australia, after retorting and combustion in air, contains haematite and magnetite. Studies of the reactions of hydrogen sulphide with this combusted shale, and with the individual iron oxides, showed the sulphidation kinetics to be first order with respect to hydrogen sulphide, with activation energies of 35 ± 1, 59 ± 3 and 66 ± 2 kJ mol −1 respectively for haematite, magnetite and combusted shale. Under dry conditions the shale trapped 36 mg of hydrogen sulphide per gram of combusted shale. With steam present, the iron oxides did not trap hydrogen sulphide and the combusted shale was also ineffective because, apart from negligible sulphidation of its iron oxide minerals, another sulphur-trapping mineral, calcium oxide, had effectively been removed by silication during combustion. The potential to control hydrogen sulphide (produced during steam retorting of shale) by recycled combusted Rundle shale is therefore poor, but a characteristic of this shale during pyrolysis is that most hydrogen sulphide is released above 500 °C, so that by minimizing retort temperatures, consistent with appropriate oil recovery, substantial control of hydrogen sulphide should be possible.

Chapter 1 – Spontaneous Combustion in Open-Cut Coal Mines: Australian Experience and Research

Spontaneous combustion results from self-heating caused mainly by low temperature oxidation of coal and other carbonaceous materials. In open-cut coal mines large quantities of carbonaceous waste material are disposed of in spoil piles within the mine site. Some of this material may be sufficiently reactive to begin to self-heat which can ultimately lead to spontaneous combustion in the spoil piles if not properly managed. Uncontrolled fires in spoil piles present a number of problems including safety hazards for mine personnel, the production of toxic gases, damage to rehabilitated land, and emission of greenhouse gases. Although a great deal of research into spontaneous combustion in coal has been conducted over many years, there has been comparatively little investigation of carbonaceous materials in spoil piles. In this chapter, some research aimed specifically at understanding self-heating and spontaneous combustion in spoil materials is reviewed, especially in the context of Australian open-cut coalmines. The principal conclusions of this work and resultant mine site management practices developed to minimize the occurrence of self-heating are discussed.

Fugitive greenhouse gas emissions in ventilation air from Australian underground coal mines

Seam gas released during coal mining is a substantial source of fugitive greenhouse gas emissions worldwide. Most of these emissions originate from underground mining; however, lack of robust global data means that the scale of emissions remains uncertain. Some coal-producing countries are attempting to improve the quality of emission estimates by using ventilation data to calculate emissions from individual mines. Legislation recently introduced in Australia now requires coal mine operators to estimate and report annual fugitive emissions. This article examines the methodology currently used at most Australian mines to meet these requirements. To assess the performance of this methodology, we compared the results of routine measurements of ventilation air flow rate and gas composition to those made with reference methods. The primary sources of uncertainty in the techniques are identified and their contributions to the uncertainty of the reported fugitive emission fluxes are estimated.

Effects of drying Rundle oil shale in simulated combustor flue gas

Rundle oil shale was dried under a variety of conditions in a small reactor coupled to a mass spectrometer. Changes in the mineralogy of the shale, the kinetics of water release, the oil yield, the chemical composition of extracted oil and flue gas composition were investigated. Water release on heating was found to conform to first-order kinetics for all grain sizes studied, and rates of release showed no apparent temperature dependence over the range 100–280 °C. The water evolved was predominantly from surface water and from dehydration of clays and gypsum. The finest fraction examined (< 75 μm) contained a disproportionate amount of gypsum (13 wt%); accounting for about half of the total in the shale. No oil yield loss occurred during drying to 300 °C in an anoxic atmosphere. However, with oxygen in the drying gas, the oil loss increased with oxygen level, temperature and the extent of drying. Oil yield loss during drying was quantitatively related to oxygen utilization and this relationship led to the production of ‘drying guides’, from which predictions could be made of oil yield loss for a given temperature and extent of drying. The composition of oil recovered from dried shale residues showed progressive increases in aromaticity, phenol and naphthalene contents with increasing oil yield loss. These changes indicated loss of aliphatics and increased oxygenation of the kerogen. Sulfur dioxide in the effluent drying gas increased with the temperature and oxygen content of the drying gas, and was estimated to be capable of producing condensate equivalent to 0.1 molar in acidity. Significant amounts of nitric oxide were released at drying temperatures above 200 °C, and there were indications of a maximum level at about 250 °C when oxygen was present in the drying gas. This is consistent with increased reduction of nitric oxide by char produced from the shale under severe drying conditions.