Burning
 
Self-heating Rates of Australian and New Zealand Low Rank Hard Coals
B Beamish, A Coope, University of Queensland, Brisbane; M Barakat and J St George University of Auckland, New Zealand
Self-heating rates have been determined for New Zealand and Australian subbituminous coals using an adiabatic oven.  New Zealand subbituminous coals yield self-heating rate indices in excess of 14 ºC/h.  There is a distinct rank relationship in the samples tested, which shows that as rank decreases the self-heating rate increases.  Strip samples taken from the Callide Coalfield of Queensland indicate that the freshly exposed subbituminous coal has a self-heating rate index (R70) in excess of 5 ºC/h.  Intraseam variation is also present, which appears to relate to the presence of sideritic layers in the coal. The difference in self-heating propensity between the New Zealand and Australian coals can be attributed to the greater inertinite content of the Australian coals, which tends to inhibit the self-heating process.

Mathematical Modelling of Propane Combustion in a Fluidised-bed Gasifier
Davide Ross, Hong Ming Yan and Dong-ke Zhang CRC Clean Power from Lignite Melbourne
A mathematical model for a bubbling fluidised-bed coal gasifier was developed for simulating the performance of a laboratory-scale gasifier with feeds of both Yallourn char and propane by incorporating propane decomposition and combustion reactions and reaction kinetics.  Model predictions of the in-bed axial gas concentration profiles for O2, CO, CO2, CH4, and C3H8 compared well, except for the minor gas species of both C2H4 and C3H6, to the experimental data at operating bed temperatures of 850oC and 950oC, respectively.  In contrast, the predicted gas species show a poor agreement with the experimental data, particularly for the carbon oxide species at 750oC. Most importantly, the addition of propane to simulate the volatile matter released from coal devolatilisation process results in an increase in the proportion of oxygen consumed by homogeneous combustion.  This leads to an increase in an availability of char for char gasification reactions, subsequently showing an increase in carbon conversion due to gasification over the sole gasification of char.  This further demonstrates the importance of incorporating homogenous combustion into the model for correctly predicting the gasifier performance, particularly for coal with high volatile content.  However, the information regarding the measured temperature profiles of gases in the fluidised-bed coal gasifier has been neither measured in the experiments nor found in the literature, and this leads to some difficulties for the verification of the predicted temperature profiles from the model.

Thermally Induced Change in the Macromolecular Structure of Coal
H Kumagai, K Tanabe and T Chiba Hokkaido University, Japan
The effects of heating rate on the change in associated molecular structure of slightly-coking coal, Witbank coal, was investigated.  Spin-spin relaxation phenomenon of the coal during heat treatment was observed using in-situ pulsed 1H-NMR.  The fractional intensity and spin-spin relaxation time for mobile component has changed with rapid heating treatment up to 644K.  The results indicate that change in the associated macromolecular structure of coal has strongly affected with heating rate.

CAMD Study on the Change in the Pore Size Distribution of Brown Coal along with a Progress of Moisture Release 
H Kumagai Hokkaido University, Japan; and K Nakamura Osaka Gas Co Ltd, Japan
A computer-aided molecular design (CAMD) method was utilized to evaluate the change in the physical and chemical characteristics of brown coal along with a progress of moisture release.  The results indicated that the removal of water molecules results in a drastic change in the density of cell which contain coal model molecules and water molecules at the final stage of moisture removal.  Size distribution of micropore stuffed with water molecules shifted slightly toward a smaller radius up to 80% of moisture removal.  Then, the distribution became broader and mean pore radius increased from extent of moisture removal of 80%.  This increase in mean pore radius well corresponded to the change in the density of cell.  The results appeared to represent the characteristics of the brown coal, at least change in the density and pore size distribution along with a progress of moisture release.

Acidic Functional Groups in Brown Coal
L Clemow CRC for Clean Power from Lignite, Monash University, Melbourne; W Jackson Monash University, Melbourne ; R Sakurovs CSIRO Division of Energy, Sydney and
D Allardice Allardice Consulting, Victoria
The heterogeneous nature of brown coal has led to lengthy but unresolved discussions as to its structure.  To obtain some concrete information, we have been studying the acidic oxygen functional groups by aqueous and non-aqueous titrations.  These results are of practical importance in determining coal-water interactions which underpin the hydrophilic nature of the coal and could lead to improved understanding of drying processes which are critical to the development of more efficient power generation technologies.  A number of methods for determining acidic oxygen in brown coals were examined.  It was found that the best techniques for measuring the acidic oxygen functionality were barium exchange based aqueous titrations modified from well known methods.  Results obtained using these techniques and their implications for explaining the structure of brown coal will be discussed.

Percolative Fragmentation of Char particles During Gasification
B Feng and S Bhatia University of Queensland, Brisbane;
Three Australian cola chars were observed under an optical microscope after gasified at various temperatures (450 – 600 C) in air to various conversions.  Many fragments were observed at low conversions (55%) during gasification even under chemical control.  The fragments appear earlier when diffusion becomes important.  These results indicate percolative fragmentation.  This is supported by the percolations behavior of the variations of the electrical resistivity with the conversions, ie the electrical resistivity initially increase only slightly with the conversion, until at a certain conversion it rises sharply.  The percolations models satisfactorily predicted the behavior of the electrical resistivity.

Effect of Heat Treatment of Reactivity and Structure of an Australian Semi-anthracite
B Feng and S Bhatia University of Queensland, Brisbane
J C. Barry The University of Queensland Brisbane
An Australian coal was observed under a high resolution transmission electron microscopy (HRTEM) after heat treatment at various temperatures (950 – 1150 °C) for various times (2 minutes to 12 hours). While the internal structure of pure carbon in coal char is highly disordered, the carbon near the iron particles shows graphite structure. The clay in coal char also slightly enhanced the ordering of carbon at the interface of carbon/clay. The results suggest catalytic graphitization with iron being the major catalyst. This is supported by the observation that the fraction of organized carbon by x-ray diffraction (XRD) increases with the heat treatment temperature and heating time for the raw coal char, while it does not change significantly for the ash-free coal char. The electrical resistivity (ER) shows the same behavior, i.e the ER for the raw coal char decreases with heat treatment temperature and heating time,  while the ER for the ash-free coal char does not change significantly, suggesting the occurrence of catalytic graphitization.

Hydrogen Chemisorption and Desorption at Carbon Sites
Y Otake University of Queensland, Brisbane; and R Jenkins University of Vermont, USA
Edge carbon atoms responsible for the existence of oxygen surface complexes on a microporous carbon have been studied further by hydrogen chemisorption experiments.  Results indicate that hydrogen can be
chemisorbed exclusively at newly created edge carbon atoms that are generated by thermal decomposition of a specific type of CO-evolving surface complex.

Leachability of Fly Ash
S McEvoy, J Killingley and L Dale  CSIRO Energy Technology, Sydney
The leachability of elements from fly ash has historically been determined using batch methods such as the Synthetic Ground Water Leaching Protocol (SGLP) or Toxicity Characteristic Leaching Procedure (TCLP) USEPA SW-846 Method 1311. These batch methods extract the fly ash with a single fixed volume of solution in a well mixed but closed system. While these methods often provide useful information as to the degree of leachability of many major and trace elements, they fail to compensate for temporal changes in conditions as fly ash is leached. The effects of changes in pH, the variation in relative solubilities and the subsequent inhibition of release of trace elements associated with phosphate and sulphate compounds have a considerable impact on composition of leachates from a dynamic open system. Additionally, the batch leaching methods, which require an extraction with a liquid to solid ratio (L:S) of 20:1, provide little indication of the true concentrations or volumes required to leach many elements.  Column leaching work in our laboratories in association with the Black Coal CRC and ACARP has indicated that leaching of many major and trace elements from fly ash is complete with as little as 0.5 to 2 L:S.
 
Other environmentally significant elements are delayed in their leaching. In some columns, for reach their maximum levels after 5 or more L:S volumes. These elements are not well characterised by the batch methods due to their poor leachability into the strong common ion environment of the batch methods.
 
This work describes some of the observations and data resulting from a comparative column / batch method study and offers the column technique as an alternative method that could provide a strong predictive tool for the long term leachability of many elements from fly ash dams associated with power stations.
 
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