Investigation on Pyrolysis of Low Lipid Microalgae Chlorella vulgaris and Dunaliella salina

Chlorella vulgaris and Dunaliella salina are two kinds of microalgae, which are widely distributed in China. Thermal decomposition of low-lipid C. vulgaris and D. salina were performed using thermogravimetric analysis. The effect of heating rates on pyrolytic characteristics was investigated, and thermal decomposition kinetics was determined as well. Furthermore, pyrolysis experiments were carried out on a fixed-bed reactor. The gas, char, and tar yields were analyzed, and the mass balance was from 88.4 to 96.8%. C. vulgaris had higher H₂ yields and lower CH₄ yields than D. salina during pyrolysis. The theoretical calorific value of the pyrolytic gas of D. salina was higher than that of C. vulgaris because D. salina had a higher amount of high heating value components, such as C₂H₆, C₂H₄, and C₂H₂. The biochar from microalgae had a smaller Brunauer–Emmett–Teller surface area than the char from pyrolysis of lignocellulosic biomass. Highest yields of pyrolytic oil were 49.2 and 55.4% (water-free basis) for C. vulgaris and D. salina at 500 °C, respectively. The characteristics of bio-oil from microalgae pyrolysis, including water content, density, acidity, and heating value, were investigated as well as the chemical composition at different pyrolysis temperatures. The microalgae pyrolytic oil was found to have significant levels of alkanes, alkenes, alkines, and esters and be particularly high in nitrogenous compounds. In comparison to the bio-oils from common lignocellulosic biomass, the microalgae oil had lower oxygen and water contents, a lower total acid number, and a higher heating value.