Chemical nature of carbonaceous materials from biomass by hydrothermal carbonization and low temperature conversion

Efforts are conducted worldwide to accelerate conversion of biomass into coal by chemical means in view of environmental, technological and socio-economic issues. With respect to the challenge of climate change, dehydration of carbohydrates can contribute to a negative atmospheric carbon dioxide balance by transferring carbon from the atmospheric cycle to the geological cycle. Besides conventional charcoal formation by partial oxidation of dry wood at temperatures above 600 °C, the rediscovered solvo-thermal process of high pressure hydrothermal carbonization (HTC) and the thermo-catalytic low temperature conversion (LTC) process at atmospheric pressure are potential techniques for enhanced carbonization. The present investigation at laboratory stage compares three different experimental arrangements: Thermogravimetry (TG) simulating a LTC process at micro scale, vertical tube reactor (LTC) and a micro high pressure autoclave (HTC) using hemicellulose, cellulose, lignin and spent grains for production of carbon-enriched substances at temperatures between 180 °C and 400 °C. Results of mass balance and concentration of carbon in the solid products are compared at the same operating temperature. Below T = 260 °C materials produced by HTC treatment show a higher degree of carbonization. However, a temperature increase of 40 °C in LTC reactors compared to HTC gives rise to the same amount of carbonization. Up to 240 °C little difference in functional groups of substrates and corresponding conversion products is shown in infrared spectra. Solid-state ¹³C-NMR analysis reveals that increased sp²-hybridization of C-atoms can only be seen in LTC-treatment at 400 °C. Consequences may be drawn in view of fertilizing soil additive and green coal for incineration.