Efficiency of Different Heat Exchange Mechanisms for Ignition of Coal–Water Compositions

Abstract

An optimal self-ignition of different fuels requires certain conditions according to the fuel properties. It is clear that the main factor is the surrounding temperature, but in fact, the chosen mechanism of heat transfer has a definitive influence on the ignition delay time. We have shown that ignition of the fuel composition containing 35–40 wt % water has very different dependencies of ignition delay versus temperature when radiative and convective heating are used. The estimation of a generalized activation energy of the ignition process at radiative heating shows that it is 3 times higher than that at convective heating. Together with this, the changes of the fuel composition up to 5 wt % do not change the activation energy more than 30%. The water evaporation decreases the efficiency of convective heat transfer and makes the growth of the furnace temperature almost useless for fuel ignition.