Effect of pyrolysis temperature on char structure and chemical speciation of alkali and alkaline earth metallic species in biochar

Abstract

To investigate the effect of pyrolysis temperature on char structure and chemical speciation of alkali and alkaline earth metallic (AAEM) species in biochar, experiments were carried out in an entrained flow reactor with nitrogen to reduce volatile–char interactions. Pyrolysis was conducted at temperatures ranging from 500°C to 900°C. Raman, Fourier transform infrared and X-ray photoelectron spectroscopies were used to identify the char structure and functional groups. The assessment of the chemical speciation of AAEM species in the biochar was carried out by chemical fractionation analysis and inductively coupled plasma mass spectrometry. Pyrolysis between 500 and 900°C results in ring condensation, which produces large aromatic ring systems containing six or more fused benzene rings. In addition, C–O and C–C linkages form through thermal polycondensation and cyclization to produce stable aromatic rings, which shows CC stretching bands in the FTIR. Conjugated aromatic carbonyl/carboxyl CO groups disappear at temperatures higher than 700°C, and the relative concentration of carbonyl oxygen from quinines increases gradually with increasing temperature. More than 50% of K and Na in the sawdust and chars prepared at 500–900°C exists as ions, while most of the Ca (65%) and Mg (60%) are bound in organic compounds. The release of K and Na occurs mainly in the form of inorganic salts and hydrated ions, while volatilization is the main release mechanism for Ca and Mg in the organic compounds.