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

Abstract 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 (T char ) were analyzed, including 350, 450 and 750 °C, as well as the uncharred materials. The optimum temperature during analysis with pyrolysis-GC/MS (T py ) was examined. T py 600 °C gave the best results for all T char by showing a larger diversity of pyrolysis products compared to T py 700 °C and 800 °C; T py 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 T char 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 T char 450 °C; pyrolysates of biochars produced at T char 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, C 16 alkylnitrile and diketopiperazines were associated with uncharred materials, benzonitriles and quinolines were associated with T char 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 T char . For all five agricultural residues, the highest chemical diversity was found for biochar produced at T char 350 °C. As the charring temperature increased, the diversity of pyrolysis products diminished.