Q.P. Campbell

An Analysis of the Slow Compression Breakage of Coal Using Microfocus X-Ray Computed Tomography

The degradation of coal and the production of coal fines during handling and transport is a serious problem in processes that depend on closely sized large particles. To minimize the production of fines, a fundamental understanding of coal breakage is required, so, to clarify the complex nature of coal breakage, a series of experiments was conducted to determine the influence of the internal physical coal structures on compression breakage characteristics. The structures investigated are the cleat and layered structure of coal and the mineral inclusions. Samples of uniform size and shape were prepared from a large block of South African Waterberg coal. The samples were analyzed nondestructively using microfocus x-ray computed tomography then wrapped in cling film and mechanically compressed while the pressure applied was measured. The virtual three-dimensional volume tomograms of the initial sample and the progeny were compared and the changes qualitatively analyzed. Conclusions were drawn as to where the fatal cracks initialized and how the cracks propagated. Particle-size distributions were done to quantify the extent of breakage versus the breakage strength of the sample. It was found that, of all the internal structures, the inherent crack distribution has the biggest influence on breakage and breakage patterns of coal.

Fine Coal Dewatering Using High Airflow

Fluidized bed drying is currently receiving much attention as a dewatering option after the beneficiation of fine coal. Apart from concerns about safety and combustion, the operating costs can be high if very high gas or air temperatures are used. The aim of this study was to investigate the removal of moisture from fine coal by using air at a lower temperature (25°C − 40°C) within a controlled environment by lowering the relative humidity of air. It was firstly proven that fluidization has a major advantage over normal static beds when allowed to reach equilibrium. Hereafter, several parameters that influence the dewatering rate and final moisture content were tested, from which it was concluded that the relative humidity of air acts as the largest driving force for dewatering. It was, therefore, shown that this is a viable technology for the dewatering of fine coal.

Validation of Using a Modified BET Model to Predict the Moisture Adsorption Behavior of Bituminous Coal

A good correlation exists between the amount of moisture adsorbed and the oxygen content of medium-rank B vitrinite-rich and medium-rank C inertinite-rich bituminous coal. Total mass moisture adsorbed per gram of coal determined from the modified BET model provides a better approximation than the BET model. The relationship between the modified BET moisture monolayer coverage and the area of surface oxygen groups suggests that moisture is adsorbed at these sites. Oxygen content is a good indicator of the amount of moisture adsorbed on bituminous coal. Open hysteresis loops indicate that moisture is retained on medium-rank C inertinite-rich bituminous coals.

Mineralogical, chemical, and petrographic properties of selected South African power stations’ feed coals and their corresponding density separated fractions using float-sink and reflux classification methods

ABSTRACT Three South African feed coal samples for the combustion process were beneficiated to produce carbon-rich and mineral-rich fractions. The mineralogical, petrographical, and chemical properties of these feed coals and their density separated fractions were investigated using XRD, XRF, QEMSCAN, Electron microprobe, and petrography analyses. This work was conducted with the goal of better understanding the processes and operational problems which could possibly occur during coal utilization, with particular focus on the included and excluded mineral matter transformational behavior at elevated temperatures. The conventional float-sink and reflux classification methods used were shown to successfully eliminate liberated minerals and produced maceral-rich float fractions (98%) macerals. The main differences between the feed coals were related to the mode of occurrence of mineral matter. An integration of these different analytical techniques allowed for better determination of the concentrations of mineral matter responsible for industrial ash-related problems. In this paper, we propose that blends of the different density fractions will reduce or minimize clinker and slag formation as well as the abrasive nature of the clinkers or slags. Possible blends to minimise clinker and slag formation include the float and sink fractions of the feed coals in varying proportions based on the specific mineralogical, petrographical and chemical data.

The Influence of Bedding-Plane Orientation on the Degradation Characteristics of a South African Waterberg Coal

ABSTRACT A tomographic study of coal microstructure (coal bedding planes, pre-existing cracks, microlithotype boundaries, and mineral boundaries) was undertaken to explain its influence on coal degradation due to impact. A number of samples were cut from a large block of Waterberg run-of-mine coal. It was characterized using microfocus x-ray computed tomography before and after undergoing impact breakage in a single particle drop shatter test. The tomograms generated before and after impact were compared and the breakage characteristics determined. It was found that the internal structure of the coal samples influenced the breakage of coal, with new cracks initiating, terminating, or propagating along pre-existing cracks, microlithotype-, and mineral grain boundaries. The contribution of the physical structures to the breakage characteristics could not be predicted for individual particles, but the overall effect on a population of particles was fitted using a Rosin-Rammler distribution. The orientation of the bedding planes in relation to the impact surface also influenced the propagation of cracks through the samples. Newly formed cracks propagated deeper into a particle if the impact force was applied along the bedding planes.

Destoning of fine coal in a fluidized bed

Water scarcity is driving the development of dry coal beneficiation processes. A lot of research has gone into the development of dry dense medium fluidized bed technology (DMFB), especially in China. However, these processes focus mainly on +6 mm particles while little work has gone into the development of dry processes for -2 mm particles. This paper focusses on the possibility to remove mineral matter (high density particles) from valuable coal fines (-2+1 mm and -1+0.5 mm) by using a fluidized bed operated with and without added vibration, while adding dense media (magnetite, sand and fine coal discards). As a control the bed was also operated without any dense media. A fluidized bed column was designed and constructed by clamping several individual rings on top of one another. This design helped with the sampling of the coal in the bed. After a test run, each individual ring could be removed and the coal inside analysed for ash percentage and calorific value. The results clearly indicated that this process is viable by removing high ash value material in the bottom layer of the bed, leaving the rest of the bed to be significantly lower in ash value and as a result higher in the calorific value of the coal. It confirmed previous work by the authors that showed a negative overall performance of the bed when media was added, due to difficulty in separating the media from the fine coal particles. By vibration the bed, the sharpness of separation did increase slightly.

Coal product moisture control using stockpiles

The moisture content of product coal is a major factor influencing the efficiency of downstream coal utilization processes. Product stockpiles are often used as a control measure to regulate the moisture content of the coal by gravity drainage and evaporation. An understanding of the mechanisms of water migration and retention in coal stockpiles are required to optimise the management of these stockpiles. Apart from the process water carried over into the product after beneficiation, additional water due to rainfall can add to the total moisture contained in a stockpile. When the rain falls on the stockpile, it either runs off the surface or infiltrates the stockpile. The infiltrated water can evaporate from the surface (down to a certain depth), drain through a saturated toe, or remain within the stockpile to add to the total final moisture content. To study these mechanisms, laboratory scale experiments were designed. A drainage column was used to simulate the percolation of water in a stockpile, and the data verified that particle size, especially the -0.5 mm fraction, had the most significant influence on both the drainage rate and the water retained in the bed. The ratio between run-off water and infiltration water during rainstorms were also quantified, and it was shown that compaction of the bed had a major influence on infiltration. Evaporation from a coal bed surface was tested by measuring the mass loss from coal beds exposed to the atmosphere, while measuring weather conditions like temperature, relative humidity and wind speed. The average evaporation loss was about 0.8 L per m2 per day.

Drying of coal fines assisted by ceramic sorbents

It is proposed to introduce contact sorption drying as a method to reduce the moisture content in coal fines. The aim of this study was to investigate the possibility of drying fine and ultra-fine coal using porous ceramic as the moisture sorbent. The main focus of this report is to define how the air temperature, particle size variations and mixing ratio would influence the contact between coal and ceramic for effective moisture adsorption. Drying of coal fines assisted by ceramic sorbents proved to be a viable concept as the ceramic was able to not only reduce the moisture content of fine coal, but of ultra-fine coal as well. The larger surface area of smaller ceramics allowed for efficient contact and consequently higher dewatering rates. The addition of more ceramic resulted in better contact with the wet coal and reduced the operating time quite significantly. Improved contact between the coal and ceramic therefore proved to be the main driving force during adsorption drying.