Evidence of Self-Gasification during the Microwave-Induced Pyrolysis of Coffee Hulls

Pellets of coffee hulls were pyrolyzed at different temperatures using microwave and electrical heating. A comparison of the gas composition obtained by both methods suggests that the different mechanisms of heating that take place in the microwave, in comparison to conventional heating, give rise to the formation of “microplasmas”, which induce self-gasification of the char that is being formed. This hypothesis was corroborated by subjecting the char to reaction with CO2 at different temperatures using both methods of heating. The results showed that, whereas the transition in the reaction mechanism controlling the Boudouard reaction (i.e., chemical or diffusional control) takes place at about 800 °C in conventional heating, in the case of microwave heating the temperature is much lower and the reaction never proceeds under pure chemical control, the differences between microwave and conventional heating being quite significant even at low temperatures.