Combustion Behavior of Algal Biochars Obtained at Different Pyrolysis Heating Rates

Abstract

In this work, the combustion performance of Chlorella vulgaris (CV), Dunaliella salina (DS), and Haematococcus pluvialis (HP) algal biochars was analyzed based on the multicomponent method. The biochars were obtained via nonisothermal pyrolysis of raw algal biomasses at three different heating rates (i.e., 30, 40, and 50 °C/min), and biochar combustion was performed from 200 to 700 °C at a heating rate of 5 °C/min. The complex oxidative reaction of algal biochar was resolved into combined reactions of multiple pseudo-components based on the peak deconvolution method using a bi-Gaussian model. The activation energies (Ea) for each pseudo-component (PC) of all biochar samples were calculated by the Coats-Redfern isoconversional method and four kinetic models (i.e., diffusion, nucleation, order-based, and shrinking core models). The results showed that the highest Ea values were predicted by the diffusion model. Except that the Ea for the first PC of CV biochar decreased by 16.45%, the Ea values for all other biochar samples generally increased with increasing the pyrolysis heating rate. Moreover, when the diffusion model was used, the Ea for the second PC of CV biochar increased by 50.87%, that for the first PC of DS biochar increased by 16.85%, and those for the first and third PCs of HP biochar increased by 4.66 and 11.66%, respectively. In addition, the combustibility index (Sn) was evaluated based on the ignition and burnout temperatures as well as the mean and maximum weight loss rates. Generally, the combustion performance of all biochar samples was good at a low temperature but deteriorated toward a high temperature. As the pyrolysis heating rate increases, an overall increase in the combustion quality was also seen for the second PC of CV biochar and the first PCs of DS and HP biochars because their Sn increased from 2.70 × 10-15 to 3.07 × 10-15 °C-5, 2.53 × 10-13 to 3.88 × 10-13 °C-5, and 3.00 × 10-13 to 3.26 × 10-13 °C-5, respectively.