Methods of Increasing the Calorific Value of Fine Coal Waste

Abstract

The research and testing conducted so far on the application of fine coal waste as a basic and additional fuel in power boilers and furnaces suggest the possibility and purposefulness of improving their quality as well as the chance to increase economic and ecological effects by their producers and end-users. Energy properties of fine-grained coal waste as a fuel result from the contents and quality of the occurring coal maceral, deciding about their energy-generating properties. The presence of water and mineral components in fuel lowers the thermal effect as the thermal energy is absorbed in the wide temperature range into specific heat and the heat of vaporization. The simplest way to increase the calorific value of fine coal waste (coal sludge, floatation tailings), is to lower their water content (humidity) and/or ash content. In the case of fine coal waste materials from current production, their humidity is considerably influenced by the used dewatering installations for water-sludge suspensions from radial and lamella thickeners. In the practice of our mining industry, filtration presses proved more effective than band and plate filters. The degree of hydration of coal sludge and floatation tailings recovered from the shut-down settling ponds depends on the degree of their gravitational dewatering (time of deposition and settling pond construction), and the method of winning and storing of the excavated material (aeration, atmospheric drying). That is why the water content range of the fine coal waste, finding its way to the power stations, is very wide, from 14 to 45%. A significant improvement of fine coal waste quality has been achieved through the implementation of their granulation, especially with the use of a quicklime binder. The obtained granulated product is resistant to transport and storage conditions, it does not degrade in changing weather conditions and has better energy properties due to lower water content as well as the content of active calcium compounds, binding sulfur oxides during their combustion process. The granulated sludge can be used as an independent fuel and as a component of fuel mixtures made in mines, by wholesalers (in wholesale stores), and by the users of such fuels. Particularly interesting economic effects can be obtained by substituting sludge-water pulp with sludge granulated product. In the case of the fluidized bed under analysis, there was achieved a unitary increase of combusted fuel net calorific value by approximately 2,000 kJ/kg. Moreover, the method of feeding the granulated product to the fluidized bed is much simpler and less energy-consuming. The essential quality improvement of fine coal waste can be obtained first of all by lowering their mineral components’ content. The simplest solution is a selective separation of flows that are richest in coal from water-sludge circuit; as for the storage yards, it is a selective deposit mining. The richest coal concentrates from coal sludge, current production, and storage yards, are obtained by means of wet production technologies through grain classification and separating a grain fraction over 30-60 "u00b5m. In the events of applying dewatering vibration sieves with fabric filtration diaphragms as well as arch sieves and centrifugal dewatering sieves with properly selected slots, there were obtained the richest coal concentrates with net calorific values in the range of 16 to 22 MJ/kg. To increase the effectiveness of grain fractioning process on the sieves, the water-sludge suspension is de-silted and thickened in hydrocyclones before the process. In the deposition methods of water-sludge suspensions in settling ponds used so far, the possibility of gravitational enrichment of water-sludge suspensions with coal grains have not been used. Through water-sludge suspensions’ flow channeling in settling ponds, there can be obtained rich and very rich-in-coal deposit areas as well as more effective deposit water cleaning. Flotation tailings’ cleaning calls for different methods of coal concentrates’ separation from mineral fraction than described, by way of example, for cleaning coal sludge. In this case, most methods are based on specific gravity differences of coal grains and mineral components and their differences in surface properties of particular grains contained in fine coal waste. Most often, the cleaning installations are constructed for processing flotation tailings from storage yards and partly from current production by using hydrocyclones, heavy liquid, and flotation process technologies. These technologies make it possible to obtain the richest coal concentrates. Based on the conducted analysis of properties, composition and conditions of generating fine coal waste it is evident that fine-grained fractions rich in coal can be separated by means of specific, simple changes in technological flow charts used so far. By using simple methods of grain and densimetric classification, there can be cleaned and separated high calorific value fine-grained coal concentrates. Channeling water-sludge suspension flows in settling ponds can lead to zoning of ash-poorest fine-grained waste which means the location of the richest coal concentrates.