Correlations between different fuel property indicators and carbonization degree of oxidatively torrefied microalgal biomass

The correlation analysis of fuel property indicators is essential to explore the variation and principle of the torrefied biochar evaluation. This study adopts numerous indicators concerning torrefaction parameters to assess and analyze the relationship between different fuel properties. Good linear distributions are obtained between most indicators, but the contact angle is insufficient for fuel property prediction. The solid yield and enhancement factor are important indicators for fuel performance analysis. Moreover, carbonization indexes of fixed carbon, carbonization index, elemental carbon proportion, and graphitization degree are the main focus to reflect the elemental carbon transformation and densification degree variation of the torrefaction process. The four carbonization indexes mentioned above can accurately reflect the changes in different fuel properties and maintain a good linear relationship. In addition, a strong linear relationship is observed between different carbonization indexes, and the variations of torrefaction severity index and contact angle are also correlated to the changes in the fuel property of the torrefied biochar.