Modelling of low temperature oxidation of coal dumps.

[摘要] Storage and waste dumps from coal mining tend tospontaneously combust. This is mainly as a result of theoxidation process which is accelerated by theavailability of oxygen and the exothermic nature of theoxidation process. In cases of poor ventilation the heataccumulation within the bed is thought to lead to thespontaneous combustion of coal.The work in this dissertation aims to investigate thechange in oxygen concentration in a bed of coal and alsomeasure the rate of oxidation (oxygen absorption) in aclosed reactor under isothermal conditions. Drying rateof coal under nitrogen was also looked into.An analysis of the oxygen concentration profile in athree metre 20 cm ID plastic column filled up with coalhas been carried out. As the coal ages (becomes oxidised)its reactivity towards oxygen decreases and changes inthe oxygen concentration profile are noticed.Experiments have been carried out up to 8 months and fromthe results obtained, a simple pseudo-steady-state modelhas been developed to describe the diffusion of oxygeninto a reacting coal bed. The findings could prove usefulin trying to find a solution to coal and waste dump fire control.The second experiment is a simple isothermal oxygenabsorption experiment in which the rate of absorption ofoxygen on a given coal sample is measured at differentinitial concentrations of oxygen. The initialconcentration of oxygen is varied over a fairly widerange in order to determine the dependence of the rate ofoxidation on the oxygen concentration. The rate- limitingstep in low temperature oxidation of coal is found to bethe absorption of oxygen.Moisture also plays a role in coal oxidation. Dryingexperiments were also carried out so as to quantify andinvestigate the rate of loss of moisture. Models havebeen developed which try to explain tile mechanismsinvolved in the drying process. The modelling suggestthat the bound water model is more appropriate to thetype of behaviour exhibited during the drying process.

[发布机构] University of the Witwatersrand