This work presents a novel numerical gas-solid reaction model which is based on the mass and the heat transport in the pellet. The initial nonporous pellet is transformed into a porous pellet upon the reaction. Due to this transformation, the increased surface of the pellet accompanied by an accelerated reaction rate leads to a periodic ignition-extinguishing behaviour of the system. These oscillations are discovered observing hydrochlorination of metallurgical-grade silicon (Si + 3HCl --> SiHCl3 + H-2), a process of importance in industry, and could be explained by the novel model.