Unravelling the mechanisms of microwave pyrolysis of biomass

•Hemicellulose decomposition can be initiated at 130 °C using microwave heating.•Cellulose decomposition can be initiated at 180 °C using microwave heating.•Low microwave heating rates produce the same effect as conventional pyrolysis.•Dielectric loss factor is two orders of magnitude higher at high heating rates.•Hydrolysis of hemicellulose and cellulose is proposed at high heating rates. This study uses empirical observations and mass transfer simulations to establish a new mechanism for microwave pyrolysis of biomass. Experiments were conducted on cellulose and hemicellulose, using microwave equipment that could vary the observed heating rate. No microwave-absorbing additives were used. At high heating rates it is shown categorically that microwave pyrolysis can significantly reduce the pyrolysis temperatures for hemicellulose and cellulose, but when microwave heating is used to deliver a low heating rate the pyrolysis behaviour is identical to that obtained with conventional heating. Dielectric properties are shown to vary by over an order of magnitude depending on the heating rate. The implications of heating rate on mass transfer and phase behaviour are developed and discussed within the paper, with liquid-phase water identified as a key driver for the observed differences in the microwave pyrolysis process. This is the first study in microwave pyrolysis that is able to reconcile microwave heating phenomena against simple and well-understood mass transfer and phase equilibria effects. As a result, a number of processing strategies have emerged with the potential to use microwave heating to enable more selective pyrolysis and bio-oils with more targeted quality than has been possible with conventional approaches.