Chemical Structures of Swine-Manure Chars Produced under Different Carbonization Conditions Investigated by Advanced Solid-State 13C Nuclear Magnetic Resonance (NMR) Spectroscopy

Two types of swine-manure chars, hydrothermally produced hydrochar and slow-pyrolysis pyrochar, and their raw swine-manure solid were characterized using advanced solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy. In comparison to raw swine-manure solid, both hydrochars and pyrochar displayed significantly different structural features, with lower alkyl carbons, NCH, OCH3, O-alkyl, and COO/N-C=O groups but higher aromatic/olefinic and aromatic C-O groups. The chemical structures of four hydrochars varied with different processing conditions. In comparison to the hydrochar with only water wash (HTC-swine W), washing hydrochar with acetone (HTC-swine A) removed the soluble intermediates deposited on the hydrochar, as shown by the decrease of O-alkyl (primarily carbohydrates), corresponding increase of aromatic/olefinic carbons and complete removal of OCH3 groups. With citric acid prewash and acetone wash (HTC-AW-swine A), aromatic C-O and aromatics/olefinics were increased and alkyls were decreased, with O-alkyls totally removed in comparison to just acetone wash (HTC-swine A). Citric acid catalysis and acetone wash (HTC-AC-swine A) increased aromatic C-O and non-protonated aromatics/olefinics, decreased alkyls further, and reduced protonated aromatics/olefinics compared to citric acid prewash and acetone wash (HTC-AW-swine A). The ratios of non-protonated to protonated aromatic/olefinic carbons for HTC-swine W, HTC-swine A, and HTC-AW-swine A hydrochars were quite similar but enhanced for HTC-AC-swine A hydrochar. Obviously, citric acid catalysis and acetone wash (HTC-AC-swine A) provided deeper carbonization than other hydrothermal processes. Hydrothermal carbonization (HTC) processes were associated with the hydrolysis and subsequent decomposition of major biopolymer components in swine manure. The increase of aromaticity during HTC was likely due to condensation polymerization of the intermediates from the degradation of carbohydrates. Pyrochar produced from slow pyrolysis was structurally different from HTC hydrochars. The dominant component of pyrochar was aromatics, whereas that of hydrochars was alkyl moieties. The aromatic cluster size of pyrochar was larger than those of hydrochars. Slow pyrolysis at 620 degrees C provided deeper carbonization than HTC processes.