Onset and propagation of smouldering in pine bark controlled by addition of inert solids

We investigated the smouldering onset and propagation in a biomass (pine bark) bed to mitigate or control the process. By the addition of sand, the bed porosity has been modified. Sand of different average particle size, in different arrangements and proportions (from 30 to 70%) has been used, modifying the bed porosity (between 0.44 and 0.67). Experiments have been carried out in a flow reactor, with controlled O2 content in the feed. The packed bed was large enough to allow the development of a reaction front, as expected in large scale applications. Pressure drop measurements revealed as a useful indication to monitor the smouldering onset and the front evolution. The process appears always controlled by the flux of O2, as expected for smouldering. Properly tuning the bed porosity with inert solids, allows to control the self-heating propagation, in view of its exploitation for low temperature heat production with simultaneous controlled conversion. The addition of sand supports a well-defined reaction front. The inert solid preserves the bed texture when the biomass is progressively consumed and allows to evenly distribute O2 to the biomass, thus regulating the heat dissipation and keeping the smouldering front confined.