Comparative study on flash pyrolysis characteristics of microalgal and lignocellulosic biomass in entrained-flow reactor

Entrained-flow fast pyrolysis of Chlorella vulgaris microalgae (MA) and palm kernel shell (PKS) was investigated in the temperature range of 600-900 °C. The effects of pyrolysis temperature and biomass type on the yields and composition of pyrolysis products were systematically investigated and the possible reaction pathways during MA and PKS biomass pyrolysis were discussed. The highest bio-oil yields of 60.22 wt% and 73.74 wt% were obtained during MA and PKS pyrolysis at 800 °C and 600 °C, respectively. The results showed that MA bio-oil was rich in hydrocarbons and nitrogen-containing compounds, while PKS bio-oil mostly contained phenols, esters, and hydrocarbons. High concentration of nitrogen-containing compounds in bio-oil from MA was attributed to its high-protein nature. Nitrogen-containing compounds such as indole, imidazole, and pyridine were formed as the result of primary decomposition of basic amino acids such as tryptophan and histidine in MA. The formation of single-ring aromatics (such as styrene) during MA pyrolysis was the result of phenylalanine amino acid decomposition. The higher content of phenols in PKS was attributed to decomposition of lignin in its structure. PAHs in PKS bio-oil were formed during secondary gas phase reactions as a result of direct combination of aromatic rings. The concentration of H2 and CO in bio-gas generally increased with temperature, while CO2 concentration showed an opposite trend. Bio-char analysis showed that at higher pyrolysis temperatures the concentration of functional groups decreased in bio-char, while the formation of large-size aromatic rings and amorphous carbon was enhanced. Scanning electronic microscopy (SEM) images showed that sphericity and surface smoothness of bio-chars increased with temperature, indicating softening of biomass particles under fast heating rates.