Chemical characterization of char derived from slow pyrolysis of microalgal residue

•Pyrolysis temperature and holding time had obvious influence on the yield and nutrient properties of biochar, which was produced from a novel precursor–chlorella residue.•The resulting biochars contained the high concentrations of nitrogen (N) and inorganic elements.•The chlorella-derived biochar could be used as a high-N (>10%), rich-minerals and porous fertilizer.

In this study, the slow pyrolysis (10 °C/min) of chlorella-based residue was investigated for the production of biochar in a thermogravimetric system under the different temperatures of 300–700 °C and holding times of 0–60 min. To evaluate their potential for soil amendment, their nutrient properties were obtained by means of elemental analyzer (EA), inductively coupled plasma–optical emission spectrometer (ICP–OES), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy–energy dispersive spectrometer (SEM–EDS). The results showed that the dried microalgal biomass and its resulting biochars contained the high concentrations of N and inorganic elements, including P, Fe, Ca, K, and Mg. The C content of biochar was increased from 56.3% (300 °C) to a maximal value of 66.2% (500 °C), and then slightly declined to about 65% (700 °C). On the other hand, the contents of hydrogen (H) and nitrogen (N) showed a decreasing trend with temperature. Based on the findings, it suggests that the chlorella-based biochar could be used as a high-N (>10%), rich-minerals and porous fertilizer.