Weight-loss characteristics, components of bio-oil and kinetics during pyrolysis from different types of microalgae

Abstract: As a kind of new-fashionable biomass material, microalgae has the advantages of large amount, fast-growing, high efficiency of photosynthesis, and less land for cultivating. Biomass fast pyrolysis technology is a promising technology to convert low-grade microalgae into high value-added advanced liquid bio-fuels and bio-chemicals. In this paper, a systematical comparison of components’ content, pyrolysis behaviors, kinetics, and evolved gas components, was carried out among 3 types of microalgae (Chlamydomonas reinhardtii, Chlorella vulgaris, and Microcystis aeruginosa) using thermogravimetric analyzer coupled with Fourier transform infrared spectrometry (TGA–FTIR), pyrolyzer coupled with gas chromatography/mass spectrometer (Py–GC/MS), and distributed activation energy model (DAEM). The result showed that: (1) Based on the TGA analysis, the thermal degradation process of microalgae was composed of 3 stages, namely drying stage (temperature range of 30-150 ℃), fast pyrolysis stage (temperature range of 150-550 ℃), and carbonization stage (temperature range of 550- 800?℃), and the maximum weight loss rate was observed for Microcystis aeruginosa with a value of 17.34 wt.%/min; the TG/DTG (thermogravimetry/differential thermogravimetry) curves moved to the side of high temperature and the weightlessness rate unit time (wt.%/min) gradually increased as the heating rate increased. (2) The FTIR analysis indicated that there were 6 strong infrared characteristic absorption bands from microalgae pyrolysis, which located at 3734 cm– 1 (-OH), 2?938 cm- 1 (-CH3), 2?360 cm- 1 (-C=O), 1?770 cm- 1 (C=O), 950 cm- 1 (P-O-P) and 667 cm- 1 (CO2). These characteristic functional groups represented the main evolved gas components were CH4, CO2, compounds containing C=O bond, compounds containing N-H and C-N bonds, in which the maximum content of CH4 was evolved from Chlamydomonas reinhardtii, and the maximum content of compounds containing C=O bond was evolved from Microcystis aeruginosa. (3) Based on DAEM analysis, as conversion rate increased, the activation energy values of 3 types of microalgae increased. The activation energy of Microcystis aeruginosa increased from 100 to 680 kJ/mol, the activation energy of Chlorella vulgaris increased from 40 to 265 kJ/mol, and the activation energy of Chlamydomonas reinhardtii increased from 20 to 250 kJ/mol. Among the 3 types of microalgae, Microcystis aeruginosa had the maximum value of activation energy. (4) Based on the Py-GC/MS analysis, the whole components in the bio-oil could be divided into the following categories, such as alkanes, olefins, benzene series, alcohols, ethers, aldehydes, ketones, nitriles, furans, indoles, and acids. Among them, long-chain alkanes, olefins, aldehydes and ketones, fatty acids and esters were mainly derived from pyrolysis of lipids and carbohydrate, while phenols, aromatics, amines and amides, heterocyclic compounds containing nitrogen were mainly derived from pyrolysis of protein. Chlorella vulgaris produced the maximum content of oxygenates compounds reaching up to 30.89%, while Microcystis aeruginosa produced the maximum contents of phenols, aromatic hydrocarbons, amines and amides, and other compounds containing nitrogen reaching up to 10.41%, 13.46%, 13.87% and 14.27%, respectively. In summary, this paper would be useful to supply scientific and basic data for industrial application of microalgae.