Monitoring of the aqueous phase from hydrothermal liquefaction using GC-MS and qNMR

Abstract

Hydrothermal liquefaction (HTL) is a promising technique used for production of biocrude. Along with the biocrude, there is also produced large amounts of aqueous waste containing high concentrations of both soluble organics and nutrients. This thesis focus on the aqueous phase in the HTL process since little research is done on the aqueous phase. Several analytical techniques were utilized for monitoring the product of the aqueous phase. Feedstocks of corn stover (CS), wheat straw (WS) and sugar kelp (SK) were pretreated and processed under alkaline (KOH-catalyzed) conditions, while sewage sludge (SS) and cattle manure (CM) were processed without a catalyst present. All feedstocks were processed in a continuous flow pilot scale HTL reactor at 350 ºC and 220 bar. The samples were analyzed by 1H and 13C - nuclear magnetic resonance (NMR) spectroscopy, gas chromatography mass spectrometry (GC-MS), in addition to obtaining data from elemental analysis, Karl Fischer analysis and ash content. The GC-MS analyses showed that during the first 40-50 minutes a relatively stable composition of compounds was established in the HTL process. The GC-MS analysis provided chromatographic separation of compounds identified through MS fragmentation and library connected to the instrument, while qNMR analysis gave quantitative results of compounds of the aqueous sample characterized by unique peaks. The thesis work has focused mainly on products in the aqueous phase. The CS and WS experiments among other compounds, gave alcohols as products while the CM and SS experiments among other compounds, gave N-containing products like pyrazine and pyridine. Acetic acid was found as the major component in all aqueous phases. Elemental analyses showed that the WS biocrude had the highest oxygen content and lowest higher heating value (HHV) for all examined feedstocks, and also the lowest sulphur and nitrogen content. The biocrudes of the other three examined feedstocks (CS, SS and CM) had oxygen content in the range 13.7 – 16.5%, and calculated HHV in the range 33.1-34.1.