A comparison between high airflow drying and adsorption assisted drying for the dewatering of fine coal

Abstract

Development in the fossil fuel sector should be an on-going process to promote sustainability, while reducing the environmental footprint. It is important to improve operations in the mining sector to ensure that coal is being used sensibly to extend the lifespan thereof. It is generally accepted that coal will remain an industry player in the foreseeable future as South Africa relies primarily on this cheap and abundant fossil fuel for electricity generation, whilst local petrochemical and metallurgical industries also require substantial coal resources. One part of the mining industry that undeniably needs attention is the considerable wastage of valuable fine and ultra-fine coal. Even after dewatering, these coal fines carry around 15-30%wt moisture and is subsequently discarded in an effort to supply a coal product with specified moisture requirements. In South Africa, this practise has led to the accumulation of approximately 1 billion tons of discarded fines and an estimated 53 million tons are added annually. Studies have shown that coal fines, when upgraded, have similar calorific values compared to the coarser fraction of coal. Therefore, effective removal of the mineral matter and high moisture content from these coal fines, would directly increase the heating value thereof. While coarser coal can easily be dewatered, fine and ultra-fine coal tend to retain a large percentage of water. Beneficiating and dewatering these fines to a valuable resource that can supplement the saleable coal fraction, will not only increase revenue, but reduce environmental problems as well. Conventional mechanical dewatering technologies prove to deliver poor dewatering results and effective thermal drying technologies are too costly to warrant the upgrading of this discarded fraction of coal. The industry critically requires feasible, practical and economically viable dewatering technologies for the fine and ultra-fine coal to recover this valuable wasted resource and to improve current operations in the coal mining sector. The aim of this thesis is to propose possible dewatering technologies focussed specifically on the dewatering of the finer particle size distributions. High airflow drying and adsorption assisted drying were investigated. Laboratory scale equipment made it possible to examine these techniques while finding the best operating conditions. With these, finding the key advantages and limitations of each technique could be determined to make an informed decision regarding the most suitable option for implementation on a larger scale. This thesis contributes to the field of fine coal dewatering and the following key findings resulted from this study: • When compared to high airflow drying, adsorption assisted drying resulted in lower energy consumption. An added benefit is that sorbent material could be regenerated to its original state; showing no degradation. The regenerated sorbent could be reused to obtain similar drying rates and final coal product moisture targets. • The loaded sorbent material could successfully be regenerated with air at ambient conditions instead of applying thermal techniques. It required less than 10 minutes to dry the sorbent material with airflow in a packed bed. Leaving the sorbent material in atmospheric conditions also indicated that the sorbent material could reach its initial inherent moisture content. The possibility of regenerating the sorbent material ensures that adsorption assisted drying is an energy positive and financially viable option for implementation on a larger scale. • It was proven that moisture transport occurred in the liquid phase, requiring direct contact between the coal and sorbent particles to initiate and sustain the movement of moisture. The moisture transport mechanism could be described by determining the capillary resistance against liquid flow. The moisture transport mechanism indicated that increasing sorbent surface area available for contact led to a decrease in capillary resistance, which allowed for added liquid moisture flow. These findings led to the conclusion that dewatering of coal fines by means of adsorption assisted drying is feasible and proved to be a practical approach for handling and the dewatering thereof. This approach is specifically beneficial for drying the finer fraction as transport of moisture is increased with optimised contact between the large surface area of the sorbent material and coal particles. Therefore, adsorption assisted drying, relying on optimised contact between the wet coal fines and sorbent material, proved to be specifically beneficial for drying the finer coal fraction.