Coal is a highly hygroscopic and porous material. In situ normally it is saturated with water and acts like an aquifer. Water in coal influences many aspects of coal technology, like bulk flow properties, abrasiveness during grinding, spontaneous heating and combustion rates. Water in coal, however, does not only influence coal technology. Major implications that arise out of the moisture content are its diluents effect on calorific value and high shipment costs. Furthermore, coal needs to be dewatered in order to avoid difficulties during the transportation, e.g. freezing, to maintain high pulverizer capacity and to improve the handling for special applications, such as coke, chemicals or briquettes. In general, 5 different forms of moisture in coal are recognized, that are bulk, capillary, physical or chemical sorbed[1].

charcoal processing

These are:

  • interior adsorbed water in micro pores,
  • surface adsorbed water in forms of layer adjacent to coal molecules,
  • capillary water in small cleats,
  • inter-particle water between some particles,
  • adhesion water as a layer around the surface of coal agglomerates.

The first to forms of water, defined as inherent moisture, can only be removed by special techniques, normally thermal drying. Capillary water can be removed by a filter cycle depending on time and coal cleat aperture size. Contrary to the first two forms the last two forms of water, defined as surface moisture, can be technically removed with vacuum filters or centrifuges. The different forms of water associated with coal are displayed in Figure 1.

Coal is a relatively poor absorber of mr regarding the organic compounds. Effects observed are more due to the fact, that water in the this owns di-electric properties and is a good absorber of the energy. When water is exposed to mr, which is an alternating electric field, the molecules adjust themselves in the direction of the field with the same frequency. This fast movement generates intermolecular friction, which leads to rising temperatures. If the exposing time is sufficient the water will vaporize and expand. This process creates internal pressures in the coal particles in a first step, generating fissures, and in the second step the moisture evaporates. In principle all minerals within the coal matrix are heated up according to their ability to absorb mr as well. With exception of Pyrite, however, most of the minerals are not significantly affected by mr. Beside the moisture reduction the grindability of coal increases as a result of the induced internal pressures. Research projects document up to 50% reduction of the relative work index for the milling process. In addition to this, the project proves that low rank coals are more sensitive to mr than high rank coals [7]. Coal is often accompanied by Sulphides. Pyrite for example causes problems in the boiler systems by generating sulphuric acid during combustion in power plants. Additionally, emission control for the flue gas is asked for. Therefore, low sulphur coals are mined and mixed with high sulphur coals to avoid negative effects on one hand and simultaneously been able to utilise/mine high sulphur coal deposits. Microwaves can increase the mine able reserves of high sulphur content coals, if Magnetite is added and the coal is processed with mr and magnetic separation [8]. 629 When compared to other techniques, a general advantage of mr heating is the way energy is transferred into the coal matrix. Normally the heat energy supplied by steam or hot fumes penetrates the sample from the surface to the core known as convective drying. Contrary to that mr is able to penetrate the this completely and transmit the energy direct to di-electric components defined as volumetric drying. Due to this effect energy can be saved, because the coal agglomerates are not heated up completely, “only” the water is heated and vaporized.


[1] Unsworth J.F., Fowler C.S., Heard A, Weldon V.L.: Moisture in coal 1. Differentiation between forms of moisture by n.m.r. and microwave attenuation techniques. 1988, J Fuel, vol. 67, 1111–1119

[2] Lester E., Kingham S.: The effect of microwave pre-heating on five different coals. 2004, J Fuel, vol. 83, 1941–1947

[3] Uslu T., Atalay U.: Microwave heating of coal for enhanced magnetic removal of pyrite. 2003, J Fuel Processing Technology, vol. 85, 21–29