Compositions, structures, and confined pyrolysis of the alga Chlorella and its nonhydrolyzable fractions

Abstract

Nonhydrolyzable organic carbon (NHC) fractions were isolated from commercial Chlorella using trifluoroacetic acid (TFA), sodium periodate (SP), ball mill (MES), and ball mill-sodium periodate (BSP) methods. The structures and relationships of the fractions with oil and gas production potentials (OGP) were investigated by solid-state 13C Cross Polarization/Total Sideband Suppression (CP/TOSS) NMR spectroscopy techniques and by confined pyrolysis. The results demonstrated that the TFA and SP fractions exhibited the highest polymethylene carbon contents and the lowest proportions of polar materials. Both bulk Chlorella and the NHC fractions had considerable oil and gas generation potential. The hydrogen indices evaluated by Rock-Eval analysis were highly significantly correlated with the OGP values estimated for the NHC fractions from the NMR methods. In addition, the oil and gas generated from the Chlorella sample and its NHC fractions were dominated with primary pyrolysis products. The contents of total n-alkanes (n-C6–33) of the pyrolysates increased with increasing pyrolysis times. The n-alkanes of the TFA, SP, and BSP fraction pyrolysates were dominated by mid chain-length components (n-C15, n-C17, and n-C23). Moreover, the SP fraction exhibited high NHC yield and OGP value, and low contents of nitrogen and oxygen. This study significantly improves our understanding of the bio-oil generation potential of this green alga.