A study of the factors influencing the physical properties of coke for blast furnace practice

The behaviour of cokong coals heated through the plastic swelling range has been investigated. Uniform heating of the samples was obtained by the dielectric method. Earlier work (l) along similar lines had suggested that the rate of swelling could be expressed as a function of the rate of gas evolution minus the rate of escape of gas through the plastic envelope and that the latter would be related to the permeability of the envelope. The quantitative analysis of the results was handicapped because no independent estimate of the volumetric rate of evolution of volatile matter could be made. A satisfactory method for making this determination has now been developed. The expression previously used relating rate of swelling pressure and rate of gas evolution has been modified to dv/dt = R Where dv/dt = volume rate of expansion (cm3/min.) R = volume rate of gas evolution (cm3/min.) Pd = applied pressure (atm.) r = permeability coefficient, A = cross sectional area (cm2) = effective thickness (mm.) Rates of expansion and rates of gas evolution have been measured for 4 coals over a range of applied pressures. Variables such as particle size grading, rate of heating and mass of sample have also been examined. Permeability coefficients, calculated from the above expression, decrease rapidly with increasing pressure. Values reached a constant (minimum) at much the same applied pressure for the coals examined, although the minima varied considerably from coal to coal. Particle size grading, within limits, did not affect the results. Rate of heating did but in the opposite sense to what was expected. Similarly the mass of the sample unexpectedly had a marked effect on the calculated permeability coefficient. The only possible explanation for this seems to be that plastic coal has dilatant properties, under increasing rates of shear the structure becomes less "dense" and permeability increases. Proof of this hypothesis will not be easy but the fact that the permeability coefficient increases with increase in the rate of swelling is of considerable significance. It implies a self compensating swelling mechanism which must have considerable bearing on the pressures developed by coal in the coke oven. There the thickness (mass) and the rate of temperature increase of the swelling layer are inversely proportional to one another and a more thorough investigation of these two interrelated variables seems desirable