Australian Black Coals Research Articles

A Study of Sewage Sludge Co-Combustion with Australian Black Coal and Shiitake Substrate

Co-combustion technology can be a gateway to sewage sludge valorization and net CO2 reduction. In this study, combustion characteristics of sewage sludge, Australian black coal, shiitake substrate, and their blends were analyzed via thermogravimetric analysis (TGA) coupled with Fourier transform infrared spectroscopy. The ignition temperature, burnout temperature, flammability index (C), and combustion characteristics index (S) of the fuels and their respective blends were estimated. Kinetic parameters were also estimated using the Coats-Redfern method. The results showed that the oxidation of the blends had two distinct stages. Synergistic effects existed for all the blends, with negative ones occurring at temperatures between 300 and 500 °C and positive ones during the char oxidation period. In the first oxidation stage, both C and S indexes increased with sludge addition to the coal. However, they decreased with sludge addition in the final oxidation stage. The catalytic effect of the sludge and the shiitake was pronounced in the final oxidation stage and it resulted in a decrease of activation energy. As for the pollutant emissions, the results showed that NOx and SO2 emissions decreased for 25 wt.% sludge addition to the coal. For the sludge-shiitake blends, NOx and SO2 emissions decreased with increasing shiitake addition. The single-pellet combustion results showed that ignition delay time reduced with increasing sludge/coal ratio but increased with increasing sludge/shiitake ratio. The volatile combustion duration decreased with the addition of sludge and total combustion time decreased sharply with increasing sludge ratio.

Numerical Study of Pulverized Coal MILD Combustion in a Self-Recuperative Furnace

A numerical study of pulverized coal combustion under Moderate or Intense Low oxygen Dilution (MILD) combustion conditions is presented in a parallel jet self-recuperative MILD combustion furnace. The Reynolds-Averaged Navier–Stokes equations, in a three-dimensional axisymmetric furnace domain, were solved using the Eddy Dissipation Concept model for the turbulence–chemistry interaction. The main aim of this study is to understand the influence of devolatilization models on the prediction accuracy of pulverized coal combustion under MILD combustion conditions. In particular, three devolatilization models are analyzed: a conventional single-rate model, a two-competing-rates model, and an advanced chemical percolation devolatilization (CPD) model based upon structural networks of the coal with a global kinetics mechanism. In addition, a new simplified numerical model is developed for Australian black and/or hard coal and optimized for the MILD combustion conditions. The modeling cases are validated with the experimental results of MILD combustion of high volatile Australian brown coal from Kingston using N2 as a carrier gas and low volatile Australian black coal from the Bowen basin using CO2 as a carrier gas in a MILD combustion furnace. From the comparison, the advanced CPD devolatilization model with a three-step global kinetic mechanism gives, as expected, the best agreement with the experimental measurements for the Kingston brown coal case, while all the models tested provide similar results for the Bowen basin black coal case. The heterogeneous reactions on the char burnout rate for pulverized coal combustion under MILD combustion conditions are presented and discussed. The characteristics of three NO formation mechanisms, namely, thermal-NO, prompt-NO, and fuel-NO, together with the NO reduction mechanism, named NO-reburning, are evaluated using the postprocessing modeling approach. The results reveal that the sum of contributions from the thermal-NO and prompt-NO routes only accounts for less than 3.8% to the total NO emissions, while the fuel-NO route dominates the NO emissions under MILD combustion conditions. It is also found that the total NO emission is reduced by up to 47% for the brown coal case and up to 39% for the black coal case via the reburning mechanisms.

Stranded assets, externalities and carbon risk in the Australian coal industry: The case for contraction in a carbon-constrained world

Plans to triple or even quadruple Australian black coal production on 2010 levels by 2030 continue to inform the policy goals of Australian state and federal governments. However, only three years after many of these plans were either formally or informally approved, a number of them are no longer financially viable, and face mounting domestic and international opposition based on three main arguments: (a) the aggregated costs of subsidies, externalities and foreign-ownership in the Australian coal industry suggest that the economic benefits to Australia are not as great as the industry and its political backers claim; (b) although Australian black coal production is not likely to peak until the 2040s or later, world coal production is likely to peak around 2030, indicating that a transition to low or zero carbon energy sources in those countries currently dependent on coal is becoming an increasingly high priority from an energy security perspective; (c) because coal is one of the main contributors to anthropogenic climate change and Australia has some of the largest untapped reserves, most of that resource constitutes ‘unburnable carbon’. These three arguments provide a compelling case for the contraction of the Australian coal industry over the next twenty years.

Recent history of sediment metal contamination in Lake Macquarie, Australia, and an assessment of ash handling procedure effectiveness in mitigating metal contamination from coal-fired power stations

This study assessed historical changes in metal concentrations in sediments of southern Lake Macquarie resulting from the activities of coal-fired power stations, using a multi-proxy approach which combines 210Pb, 137Cs and metal concentrations in sediment cores. Metal concentrations in the lake were on average, Zn: 67mg/kg, Cu: 15mg/kg, As: 8mg/kg, Se: 2mg/kg, Cd: 1.5mg/kg, Pb: 8mg/kg with a maximum of Zn: 280 mg/kg, Cu: 80 mg/kg, As: 21 mg/kg, Se: 5 mg/kg, Cd: 4 mg/kg, Pb: 48 mg/kg. The ratios of measured concentrations in sediment cores to their sediment guidelines were Cd 1.8, As 1.0, Cu 0.5, Pb 0.2 and Zn 0.2, with the highest concern being for cadmium. Of special interest was assessment of the effects of changes in ash handling procedures by the Vales Point power station on the metal concentrations in the sediments. Comparing sediment layers before and after ash handling procedures were implemented, zinc concentrations have decreased 10%, arsenic 37%, selenium 20%, cadmium 38% and lead 14%. An analysis of contaminant depth profiles showed that, after implementation of new ash handling procedures in 1995, selenium and cadmium, the main contaminants in Australian black coal had decreased significantly in this estuary.

Influence of selected coal contaminants on graphitic carbon electro-oxidation for application to the direct carbon fuel cell

A novel method examining the fundamental electrochemical behaviour of carbon is outlined here involving the use of a half cell set-up and solid sacrificial anode. Using this method, electrochemical oxidation of graphite is assessed using selective contamination of a graphite electrode with major coal contaminants identified in selected Australian black coals using X-ray diffraction. Contaminants identified include anatase, alumina, pyrite, quartz, kaolin and montmorillonite. From the systematic introduction of these contaminants it is shown that clay materials, such as kaolin and montmorillonite, act catalytically to increase the rate of graphite oxidation. Metal oxides and sulfides such as anatase, alumina and pyrite give a limited increase in the normalised current, whereas quartz gives a significant decrease in performance. This demonstrates a clear effect of the solid phase interaction of these contaminants on the electrochemical oxidation of graphite since the same effect is not observed when the contaminants are added instead to the molten carbonate electrolyte.

Compositional Variations in Pilot Gasifier and Laboratory-Produced Slags and their Impacts on Slag Viscosity and Coal Assessment

The flow behaviour of coal mineral matter at high temperatures is an important parameter for coal use in entrained-flow gasification technologies. Recently, gasification performance data was obtained from a series of pilot-scale gasification tests on a suite of well-characterised Australian black coals. Evaluation of the results of the pilot tests and the detailed laboratory investigations provided the opportunity for evaluation of the practical applicability of different laboratory and modelling techniques for coal assessment in terms of mineral matter behaviour in entrained flow gasification. A series of viscosity measurements was made over the range 1200–1600uC using slags produced in a pilot scale gasifier at temperatures between 1200 and 1700 uC, and laboratory-produced slags. These data were compared with viscosity predictions based on an empirical model developed from an extensive database of slag viscosity measurements. Major differences between predicted and measured viscosities were investigated and, where appropriate, related to slag composition and microstructure. There were some significant differences (in some cases up to 100% of the viscosity values) in the viscosity behaviour of laboratory-prepared slags and those produced during the pilot-scale gasification test runs. These differences were attributable to differences between the composition of the laboratory-produced slags and those tapped from the pilot scale gasifier. The major source of these compositional variations appears to be a result of partitioning of mineral matter components into fly ash and slag in the gasifier, and the possible subsequent interaction of this slag with slag already present on the wall of the gasifier. These observations have implications for the manner in which coal mineral matter is assessed for its likely behaviour, and ultimate suitability for use, in entrained flow gasification systems. In order to improve the reliability of coal slag assessment procedures, test procedures should include preliminary modelling based on expected coal ash and slag compositions, viscosity measurements of laboratory-produced slags, and analyses of ash and slag compositions where possible to ascertain the degree of compositional partitioning and its impact on slag behaviour. Ongoing work is required to better understand the nature of mineral matter transformations under gasification conditions and the impact of this on coal and gasifier performance. f 2011 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association All rights reserved. A R T I C L E I N F O Article history: Received 8 November 2010; Received in revised form 21 March 2011; Accepted 24 March 2011

The effect of CO 2 saturation on mechanical properties of Australian black coal using acoustic emission

Acoustic emission (AE) methods are now widely used for damage evaluation. For a better understanding of the damage mechanics of materials such as rocks, AE has been used to monitor stresses which induce crack closure, crack initiation and crack damage. In the present study, an AE system was used to study the damage behaviour of some Australian black coal samples subjected to uniaxial compression. Several samples were left in a container filled with 100% carbon dioxide (CO 2) at a certain pressure for 72 h prior to testing. The results were compared with samples which had only been exposed to the atmosphere to see if CO 2 had any adverse effect on the strength of coal. Strain gauges were installed on the samples and the measured axial and volumetric strains were studied in conjunction with the AE counts. The AE method was successfully used for detecting the onset of crack initiation and the crack damage stress threshold of the black coal samples. Of the coal samples examined, crack initiation and crack closure of the samples subjected to saturation with CO 2 occurred at stress corresponding to a higher percentage of the peak strength when compared to the samples which had only been exposed to atmospheric conditions. However, crack damage occurred at a higher percentage of peak strength and the average peak strength showed a higher value for samples in atmospheric condition when compared to CO 2 saturated samples. The results show that sorption of CO 2 can cause a reduction in strength of the black coal samples when tested under uniaxial compression. As the coal samples were highly inhomogeneous more tests are required in order to be able to confirm whether the adsorption of CO 2 will cause strength reduction in coal and to identify the actual underlying mechanisms.

Mechanical Properties of Reconstituted Australian Black Coal

Coal is usually highly heterogeneous. Great variation in properties can exist among samples obtained even at close proximity within the same seam or within the same core sample. This makes it difficult to establish a correlation between uniaxial compressive strength (UCS) and point load index for coal. To overcome this problem, a method for making reconstituted samples for laboratory tests was developed. Samples were made by compacting particles of crushed coal mixed with cement and water. These samples were allowed to cure for four days. UCS and point load tests were performed to measure the geomechanical properties of the reconstituted coal. After four days curing, the average UCS was found to be approximately 4MPa. This technical note outlines some experimental results and correlations that were developed to predict the mechanical properties of the reconstituted black coal samples. By reconstituting the samples from crushed coal, it is hoped that the samples will retain the important mechanical and physicochemical properties of coal, including the swelling, fluid transport, and gas sorption properties of coal. The aim is to be able to produce samples that are homogeneous with properties that are highly reproducible, and the reconstituted coal samples can be used for a number of research areas related to coal, including the long-term safe storage of CO2 in coal seams.

Hydrodynamic Performance of a Novel Design of Pressurized Fluidized Bed Combustor

A bench-scale fluidized bed combustor with a novel fluidizing gas injection manifold was successfully built for characterization of Australian black coals under PFBC conditions. Instead of the usual horizontal distributor plate to support the bed and distribute the fluidizing gas, the fluidizing gas was injected horizontally through 8 radial ports in the cylindrical wall of the combustor. To verify satisfactory hydrodynamic performance with the novel gas injection manifold, the fluidization was directly investigated by measuring differential pressure fluctuations under both ambient and PFBC conditions. In addition, a Perspex cold model was built to simulate the hydrodynamics of the hot bed in the PFBC facility. Under PFBC conditions, the bed operated in a stable bubbling regime and the solids were well mixed. The bubbles in the bed were effectively cloudless and no gas backmixing or slugging occurred; so the gas flow in the bed could be modeled by assuming two phases with plug flow through each phase. The ratio of Umf for the simulated bed to Umf for the hot PFBC bed matched the conditions proposed by Glicksman’s scaling laws. The bubbles rose along the bed with axial and lateral movements, and erupted from the bed surface evenly and randomly at different locations. Two patterns of particle movement were observed in the cold model bed: a circular pattern near the top section and a rising and falling pattern dominating in the lower section.

The sintering temperature of ash, agglomeration, and defluidisation in a bench scale PFBC

Five Australian black coals were studied in a bench scale pressurised fluidised bed combustor (PFBC) to investigate the agglomeration propensity. It was found that coals with higher proportions of calcium aluminosilicate showed higher propensity for agglomeration and defluidisation. The pressure-drop sintering technique can predict the agglomeration propensity for coals. Samples with a sintering temperature lower than the operating temperature of the PFBC showed agglomeration. The laboratory ash can be a good representative of the PFBC ash when studying agglomeration and defluidisation.

A computational fluid dynamics based study of the combustion characteristics of coal blends in pulverised coal-fired furnace

Coal blends are commonly used in pulverised fuel fired power plants. Past experience has shown that some coals exhibit synergistic effects when co-fired with other coals. Despite the recent progress in the field, there is still a general lack of understanding about how and why such synergistic effects take place. It is, therefore, imperative to develop reliable techniques to predict the combustion characteristics of coal blends. In the present paper, a commercially available computational fluid dynamics (CFD) software package, known as Fluent, is applied to simulate the combustion of binary coal blends of Australian black coals in a pilot-scale furnace. The modelling was mainly concerned with employing the two-mixture-fraction-pdf approach to separately track the individual components of each blend. Additionally, another approach, which takes the blend as a single coal with the weighted average properties of the components, was also used and compared. The main combustion characteristics of coal blends, such as ignition, burnout and NOx emissions were studied and corresponding predictions were validated against measurements in a pilot-scale furnace. The comparisons indicated that the CFD model with the two mixture fraction approach can successfully predict the non-additive (synergistic) combustion behaviour of coal blends, providing an effective tool for full-scale applications. The single coal approach presents nearly additive prediction of coal bends, and is only applicable to the blends of similar coals.

An experimental study of the effect of coal blending on ash deposition

In this study, deposition experiments with well characterized samples of Australian black coals and blends were conducted in a laminar drop-tube furnace to assess the blends behavior and their potential to form ash deposits. Comparison of the results between the blends and single coals shows that the blends behavior was not additive in nature. Some blends developed a thicker ash deposit layer with a maximum thickness higher than 700 μm compared to a thickness of 300 μm for the source coals, whereas, other blends had a lower potential to form ash deposits. This non-additive behavior of coal blends results from the interaction between ash particles within the deposit layer. Therefore, the coal blends performance may not be the same as that of the source coals with same bulk composition. Three techniques were used to explain the trends of the deposition experiments. The use of slag viscosity measurements and a slagging index based on the ash bulk properties, were not successful. Thermomechanical analysis (TMA) test was conducted as another technique that represents the thermal behavior of the ash deposits. The (TMA) penetration curves showed a reasonable matching trend with the deposit thickness obtained in the deposition experiments.

Char Attrition from Australian Black Coals in Pressurized Fluidized Bed Combustion

Char attrition is the main mechanism for the fine char generation in PFBC. The char attrition rates were significantly different for each of the two coal chars tested. Specific char attrition rate increased with the surface porosity of burning char particles. A coal with higher ratio of telocollinite/inertinite contents formed chars in PFBC with larger pores or higher porosity during devolatilization, which resulted in a higher specific attrition rate.

On the Effects of High Pressure and Heating Rate during Coal Pyrolysis on Char Gasification Reactivity

Effects of pyrolysis pressure on char reactivity remain a poorly understood aspect of the coal gasification process. In an attempt to address this problem, effects of pyrolysis pressure on char structure and reactivity are being investigated. In this paper, chemical reactivities to O2, CO2, and H2O of chars made from three Australian black coals were measured in a pressurized thermogravimetric analyzer, under conditions where chemical processes alone controlled reaction rates. These chars were prepared at a range of pyrolysis pressures, using pressurized flow reactors and an atmospheric pressure tube furnace, to ascertain any effects of devolatilization pressure and heating rate on the chemical reactivity of the resultant coal chars. It was found that while the apparent (as measured) reaction rate can be affected by pyrolysis pressure, the rate normalized to the char surface area (intrinsic rate) is much less affected, because of large effects of pyrolysis pressure on char micropore surface area. This fin...

Carbon attrition from Australian black coals in pressurized fluidized bed combustion (PFBC)

Carbon attrition from Australian black coals in pressurized fluidized bed combustion (PFBC)

Coal topping in a fluidized bed system

A batch-fed bench-scale pressurised fluidised bed combustion (PFBC) facility was built to research the in-bed processes generating fine char particles during PFBC of Australian black coals. Elutriation of such particles is responsible for the combustion inefficiency and their combustion in the filter cake on high-temperature ceramic filters may contribute to the formation of ‘sticky ash’ and consequent cleaning difficulties. Measured carbon elutriation varied from coal to coal and the results agreed qualitatively with large-scale PFBC performance, allowing definition of satisfactory and unsatisfactory performance criteria. Carbon elutriation in PFBC was correlated tentatively with crucible swelling number although the range of values of the crucible swelling number needs to be extended. Carbon elutriation in PFBC did not correlate with volatile matter for the five Australian coals investigated in contrast with previous PFBC pilot plant studies on four Northern Hemisphere coals. The higher swelling coal produced a significantly larger average macro-pore diameter after devolatilisation in PFBC, which caused greater attrition of fine char from the particle surface during the char burnout stage.

Ignition temperature of coal carbonizates

A batch-fed bench-scale pressurised fluidised bed combustion (PFBC) facility was built to research the in-bed processes generating fine char particles during PFBC of Australian black coals. Elutriation of such particles is responsible for the combustion inefficiency and their combustion in the filter cake on high-temperature ceramic filters may contribute to the formation of ‘sticky ash’ and consequent cleaning difficulties. Measured carbon elutriation varied from coal to coal and the results agreed qualitatively with large-scale PFBC performance, allowing definition of satisfactory and unsatisfactory performance criteria. Carbon elutriation in PFBC was correlated tentatively with crucible swelling number although the range of values of the crucible swelling number needs to be extended. Carbon elutriation in PFBC did not correlate with volatile matter for the five Australian coals investigated in contrast with previous PFBC pilot plant studies on four Northern Hemisphere coals. The higher swelling coal produced a significantly larger average macro-pore diameter after devolatilisation in PFBC, which caused greater attrition of fine char from the particle surface during the char burnout stage.

In-bed char combustion of Australian black coals in pressurized fluidized bed combustion

In-bed char combustion of Australian black coals in pressurized fluidized bed combustion

Generation of fine chars from Australian black coals in pressurized fluidized bed combustion

Five Australian black coals were characterized and the most significant mechanisms for the in-bed generation of fine char particles were studied under pressurized fluidized bed combustion (PFBC) conditions of 1.6 MPa, 850°C, U = 0.9 m/s, 7% O2, and coal particle size 4 to 4.75 mm in a novel batch-fed, bench-scale pressurized fluidized bed of 1.3 mm silica sand particles. The generation of elutriable fine char particles in our PFBC tests was attributed to attrition, fragmentation, and combustion residues of in-bed char particles. Both primary fragmentation and secondary fragmentation mainly produced large particles, which remained in the bed, but also generated some elutriable small pieces, contributing to carbon elutriation from PFBC. However, attrition was the dominant contributor to the generation of elutriable fine char particles. The specific char attrition rates were approximately constant during the early stage of burnout (for larger char particles > ∼2 mm), because of the stable development of detachable asperities on the external surface of char particles, when combustion was controlled by the external mass transfer of oxygen. The specific attrition rates increased at the later stage of burnout, because of the more effective intraparticle diffusion of oxygen into the pores of smaller particles and the development of porous structure within the surface layer, giving a higher specific attrition rate.

Quantitative X-ray diffraction analysis and its application to various coals

A technique is presented to obtain the maximum structural information on carbonaceous materials from their X-ray scattering curves in the middle and high range of scattering angle. This technique involves a precise and systematic analysis of X-ray scattering curves. Four Australian black coals ranging in rank from semi-anthracite to HV bituminous are included in this study, and the results are compared with data in the literature. Based on qualitative observations, a simplified coal structure, in which only two types of carbon structures including crystalline carbon and amorphous carbon are considered, is suggested. The good agreement in intensity between experimental measurements and theoretical calculations demonstrates the validity of the simplified model. The quantitative analysis yields three structural parameters, viz., fraction of amorphous carbon ( x A), aromaticity ( f a) as well as crystallite size and its distribution ( L a, L c, d 002, p n). As expected, coal was found to contain a significant amount of highly disordered material, amorphous carbon, which gradually decreases during the coalification process. In agreement with the TEM observations, coal crystallites are found to be around 6 Å in diameter and piled up by 2–4 aromatic layers on average, with the high rank coal being more condensed in terms of the inter-layer spacing. However, our measurements suggest that the average crystallite height increases with coal rank from 7.5 Å for Coal1-DD to 13.75 Å for Coal5-YD. The measured aromaticity, which agrees with the results of NMR spectroscopy, shows a good relationship with the hydrogen content in coal.