Molecular characterization of biochar from five Brazilian agricultural residues obtained at different charring temperatures

Important parameters that influence biochar properties include charring temperature and biomass type. We characterized the molecular properties of biochars from five agricultural residues with pyrolysis gas chromatography mass/spectrometry (pyrolysis-GC/MS) in comparison with atomic H/C and N/C ratios. Feedstocks included chicken manure, eucalyptus sawdust, coffee husk, sugarcane bagasse and pine bark. Biochars produced at three different temperatures (Tchar) were analyzed, including 350, 450 and 750 °C, as well as the uncharred materials. The optimum temperature during analysis with pyrolysis-GC/MS (Tpy) was examined. Tpy 600 °C gave the best results for all Tchar by showing a larger diversity of pyrolysis products compared to Tpy 700 °C and 800 °C; Tpy 600 °C was therefore used for qualitative and quantitative comparison of the samples. Charring temperature was the dominant factor that determined the chemical composition of the biochar pyrolysates. Uncharred feedstocks had the largest contribution from carbohydrates, lignin phenols and long chain n-alkanes, all of which rapidly decreased with charring; biochars produced at Tchar 350 and 450 °C showed the largest contribution from phenols, mid-chain n-alkanes, benzofurans, indenes, biphenyls and PAHs, from which the benzofurans, indenes, biphenyls and PAHs were particular abundant in samples produced at Tchar 450 °C; pyrolysates of biochars produced at Tchar 750 °C were characterized by branched aliphatics, short chain n-alkanes/n-alkenes and low molecular weight (LMW) benzenes. Factor analysis showed that the variation of products differed largely within some chemical groups. For the N-containing compounds, caffeine, C16 alkylnitrile and diketopiperazines were associated with uncharred materials, benzonitriles and quinolines were associated with Tchar 350–450 °C. Another part of the variation of N-containing compounds was associated with chicken manure, and to a lesser extend also coffee husk, independently of Tchar. For all five agricultural residues, the highest chemical diversity was found for biochar produced at Tchar 350 °C. As the charring temperature increased, the diversity of pyrolysis products diminished.