Efficient Microwave-Assisted Drying of Dairy Manure towards Biofuel Potential

The sustainability of thermochemical conversion processes is required to be enhanced by energy efficiency of drying of high-moisture biomass. This paper examines the microwave-assisted drying of dairy manure at different microwave power levels (400–800 W), temperatures (110–15 °C) and feedstock loads (10–50 g). Microwave power increased heating rate dramatically in the preheating phase, increasing the heating time of 503 s at 400 W to 162 s at 800 W (67.8% decreases) with no significant rise in total energy use. The sample temperature leveled at 95–100 °C during the drying process, which was the constant-rate evaporation stage. The rise in feedstock load, conversely, decelerated the heating and drying process because of lowered energy input per unit mass and a rise in internal transport resistance. The ratio of the power to mass was important in shaping the drying behavior. Apparently, volumetric microwave heating produces a high degree of internal vapor pressure that causes rapid mass loss at 700 W and 30 g and promotes the migration of moisture. But at high power (800 W) the surface became hard, and the transfer of mass was slowed down in the subsequent drying phase. A power of 700 W and a weight of 30 g gave the optimal drying performance.