Co-combustion characteristics and kinetics of microalgae chlorella vulgaris and coal through TGA

Abstract

The combustion characteristics and kinetics of microalgae (Chlorella vulgaris) and sub-bituminous coal blends (CCBs) are studied by a thermogravimetric analyzer (TGA), and those of pure Chlorella vulgaris (C. vulgaris) and coal were also taken respectively as control groups. The microalgae to coal blending ratio (MCR) is 3/7, 5/5 and 7/3. The results showed that three stages were observed during the combustion of CCBs. And the main combustion of CCBs was occurred at the second stage ranged from 254.6 ~ 389.4°C to 698.7 ~ 741.0°C. Both of the ignition temperature (Ti) and the final temperature detected when stabilization of samples mass occurred (Tf) were decreased as the C. vulgaris content increased in the CCB. The maximum combustion rate (Rmax) of C. vulgaris was maximum. The average reaction rate (Rv) was firstly decreased, and then increased as the content of C. vulgaris in CCBs increasing. With the increasing content of C. vulgaris, both of the ignition index (Di) and the comprehensive combustion characteristic index for the blends (SM) were increased. Some deviations from their expected characteristics indicate interaction. As the heating rate (β) increases, Ti, the peak temperature (Tp), the reaction rate at the peaks (Rp), Rv and Tf were all increased significantly, while the residual mass (Mr) was first increased, and then decreased. For CCBs, the activation energy (E) was the first decreased, and then increased, and the minimum E was obtained as MCR = 5/5. Among all the samples, E of pure coal was the minimum one. Finally, kinetic triplets were determined by the Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), and master-plots method, they are respectively E = 62.90, 108.99, 85.28, 92.27, 104.98 kJ/mol, the reaction order (n) = 1.4, 4.1, 2.7, 3.2, 4 and the frequency factor (A) = 6.38 × 105, 1.05 × 106, 2.29 × 104, 8.73 × 104, 2.93 × 106 min−1 for the coal, blends with MCR of 3/7, 5/5, 7/3 and C. vulgaris combustion at β = 20°C/min.