Reaction Kinetics for Hydrothermal Liquefaction of Biomass

Abstract

Hydrothermal liquefaction (HTL) involves the conversion of biomass into a renewable crude oil in subcritical water. Co-products of the process include solid, aqueous and gas phase products. In order for the process to be upgraded to industrial scale the products from HTL need to be characterised. Various sources of biomass contain different fractions of carbohydrate, lignin, lipid, protein and inorganic material. At different reaction conditions, including temperature and time, variations in product fractions and their compositions have been observed. The principal objective of this work was to develop a model to predict the trends in HTL product fractions for various biomass compositions at different reaction times and temperatures. This may allow the optimum reaction conditions to produce a maximum amount of crude to be identified for a variety of feedstocks. In order to develop a kinetic model for the HTL of different biomass feedstocks, model polymer and monomer compounds to represent the organic constituents of biomass were first reacted alone and the trends in product fractions observed. The HTL experiments were conducted at reaction temperatures of 250, 300 and 350°C over reaction times 0 to 60 minutes. The crude produced was analysed via thermogravimetric analysis and gas-chromatography mass-spectrometry to determine the variations in crude from different sources of biomass. Reactions with polymer and monomer model compounds alone allowed the conversion pathways in HTL to be identified. Mixtures of polymer model compounds were also reacted to determine the effects of interactions between the organic fractions of biomass on product distribution and crude composition. The final step in the model development involved building a kinetic model from HTL experiments with microalgae, sewage sludge and pine wood biomass using the reaction pathways developed for model compounds. Results from experiments with polymer model compounds showed that the lipid produced the highest crude yield, followed by protein, carbohydrate and lignin. Monomer compounds resulted in lower crude yields than polymer compounds, except in the case of lignin. HTL of the intermediate reaction products, by monomers, resulted in the same compounds identified in the crude. It was concluded from experiments with mixtures that the interactions between the organic constituents of biomass result in variations in yields of product fractions compared to those when polymers were reacted alone by 0 to 35%. These variations depend on reactant composition, reaction time and temperature. The compounds identified in the crude produced from a given mixture of polymers were the same as the compounds produced from the individual polymers. In experiments with biomass, product fractions differed compared to what was expected from model compounds by up to 42% due to interactions between the organic constituents of biomass and the presence of inorganic compounds. From the results of this study, a kinetic model to describe the HTL reactions for microalgae, sewage sludge and pine wood was produced. The model could predict product yields with less than 15% error. The results will allow suitable reaction conditions and biomass feedstocks to be identified for production of crude from HTL at industrial scale.