Microwave Drying Effect on Pyrolysis Characteristics and Kinetics of Microalgae

Abstract

Microalgae are one of the most potential biomass energy sources. An efficient drying method is important to development and utilization of microalgae. Microwave drying is receiving increasing attention because it is a rapid, high-efficiency, and economical method compared to conventional drying. Pyrolysis characteristics of microalgae (C. vulgaris) after conventional drying (drying at 105 °C for 20 h) and microwave drying (the microwave drying time of 20, 30, and 40 min) were investigated. The pyrolysis experiment of microalgae was carried out at the heating rates of 10, 20, and 40 °C·min−1 in a thermogravimetric analyzer (TGA). And the bio-char after pyrolysis of C. vulgaris under different heating powers (conventional power of 2500 W and microwave power of 600, 1000, 1500, and 2250 W) were analyzed. Results show that comprehensive pyrolysis characteristic index (S) of C. vulgaris after microwave drying was higher than conventional drying; however, energy consumption and activation energy (E) after microwave drying were lower. For microwave drying, as microwave drying time increases, ignition temperature (Ti), final temperature detected as mass stabilization (Tf), reaction rate at the second peaks (Rp2), residual mass (Mr), and energy consumption increased, while average reaction rate (Rv) decreased. As the heating rate (β) increased, the Ti, Tf, Rp2, Rv, and S of C. vulgaris increased, while Mr decreased, and E firstly decreased and then increased. And except for microwave power of 600 W, as microwave power increased, the volatile content and the fixed carbon content of C. vulgaris bio-char decreased, and the ash was increased.