Objective  To establish a set of method suitable for quality evaluation of biochar and to explore the main factors influencing biochar quality.

Method  Toally 24 biochars (Y1−Y24) were prepared from eight biomass materials under three different temperatures. Sixteen quality indexes of biochars were measured, and the data were converted using the subordinate function. SPSS 19.0 was used for factor analysis. Factor loading matrix was obtained using the biquadratic maximum rotation method. The comprehensive evaluation scores were calculated. Correlation analysis and factor analysis were performed for the 16 indexes, and the comprehensive evaluation system of biochar quality was established based on factor analysis. The biochars were ranked using the comprehensive evaluation scores.

Result  Through factor analysis of 16 quality indexes of 24 biochars, we extracted five common factors with eigenvalues above one and their cumulative variance contribution was 77.910%. The first common factor consisted of carbon content, cation exchange capacity (CEC) and pH, with variance contribution of 31.090%. The second common factor consisted of specific surface area and pore volume, with variance contribution of 19.878%. The third common factor consisted of hydrogen atom content with variance contribution of 12.819%. The fourth common factor consisted of phosphorus and potassium contents with variance contribution of 7.479%. The fifth common factor consisted of maximum adsorption capacity of NH₄⁺-N with variance contribution of 6.643%.

Conclusion  Factor analysis is a good statistical method for evaluating biochar quality. The key factors affecting biochar quality include chemical characteristic factors (C content, C/N ratio, C/H ratio, pH and CEC), physical characteristic factors (specific surface area and pore volume), active energy factor (H atom content), nutrition factor (P and K contents) and maximum adsorption capacity of NH₄⁺-N.