As a part of investigations on the influence of humidity on the initial stage of the spontaneous heating of coal, the effect of desorption of moisture from the coal was studied. Laboratory experiments with different coals were carried out under conditions where desorption of water from the coals by air was certain to take place. Thermal changes were measured by a specially designed calorimeter in isothermal conditions, mostly at 3O'C. The results showed that in each case there occurred a cooling of the coal, indicating that the rate of heat loss from a coal due to this process was greater than the rate of lieat release due to oxidation of the coal. The data suggest that moisture desorption acts as an inhibitor to the spontaneous heating of coal. For a particular coal, the rate of heat loss increases with the increase in the equilibrium humidity deficiency of the air. The effects of rank, particle size and weathering on the process are also discussed.
Investigations on the influence of moisture on the spontaneous heating of coal have been carried out both in field and laboratory over a long period and in several countries. The common conclusion is that damp conditions favour the self-ignition of coal in mines and storage. It has also been found that the moisture content is the most variable of all parameters of the mine atmosphere'.
Both air and coal are capable of holding certain amounts of moisture under given conditions of pressure, temperature and availability of water vapour. In an atmosphere of constant humidity, the water vapour pressures in the coal and air remain in equilibrium states. When the humidity of the atmosphere rises, the coal begins to sorb moisture from the surroundings until a new equilibrium is attained. Conversely, if the surroundings get drier, then the air picks up moisture from the coal. Studies on the thermal changes in coals during the first two conditions have been reported earlier2*3. A laboratory study of the thermal effect of desorption of water from coals by the surrounding atmosphere is described here.
Although previous investigators4-6 have studied this particular aspect of desorption of water vapour from the coal, the experimental conditions used appear to be rather extreme. The purpose of the present investigation was, therefore, to measure the heat changes during desorption of moisture from different coals in various humid atmospheres. The effects of rank, particle size and weathering on the process were also examined.
EXPERIMENTAL
The apparatus and its operational method are described in detail in an earlier publication'.
In brief, a specially designed calorimeter8 capable of measuring heat loss from the material in its cell was used. All experiments were carried out in an isothermal condition at 30Β°C with the exception of one test during which the temperature was 35*C. The equilibrium humidities of the moisture carrier