Investigation of Biomasses and Chars Obtained from Pyrolysis of Different Biomasses with Solid-State 13C and 23Na Nuclear Magnetic Resonance Spectroscopy

Abstract

A number of biomass samples (reed, pine pellets, Douglas fir wood chips, wheat straw, peach stones, and olive residue), pretreated biomass samples (leached wheat straw, leached peach stones, and leached olive residue), as well as their chars obtained by pyrolysis using different heating rates (5, 10, and 20 degrees C/min) at the end temperature of 800 degrees C were characterized with C-13 and Na-13 nuclear magnetic resonance (NMR) spectroscopy. In the case of the biomass samples, C-13 solid-state cross-polarization magic-angle spinning (CPMAS) NMR was used, resulting in spectra indicating the various groups of carbons present in the biomass samples. No evidence was found that the leaching process could affect the carbon composition of the biomass samples. In the char samples, only aromatic groups of carbon were left, shown by C-13 solid-state single-pulse MAS NMR. According to the results, the origin of biomass, the leaching procedure, and the different heating rates do not affect the char carbon composition. Therefore, it could be assumed that the carbon structure does not affect the reactivity of the char. Na-23 single-pulse solid-state MAS NMR was applied to investigate the sodium environment in the biomass and char samples. Despite experimental difficulties, we were able to observe definite differences in line width and shifts of the studied compounds. This proves that leaching affects the fuel properties, i.e., sodium chemistry in a biomass sample, and therefore, it could be assumed that the char reactivity is affected. Higher magnetic fields and more sophisticated techniques of resolution enhancement multiple-quantum magic-angle spinning (MQ-MAS) or double rotation (DOR) may provide more detailed local information regarding the sodium environment in biomasses and their char samples.