Hydrothermal Carbonisation of Novel Biomasses

Abstract

Renewable sources of thermal and chemical energy are needed in order to satisfy the world’s ever growing energy needs while limiting the rise of global temperatures below 2 °C above pre-industrial levels. Plant biomass is a vast resource which if harnessed properly could help revolutionise the global energy economy. Hydrothermal Carbonisation (HTC) is a technology wherein biomass exposed to subcritical water at 180 °C – 260 °C is thermochemically converted into an energy-dense “hydrochar” with strong thermal and elemental similarities to fossil coal. However, key aspects of the HTC reaction remain unknown, especially with regard to the degradation of the key biochemical macromolecules hemicellulose, cellulose, lignin and protein. In this thesis, three novel biomasses, Australian saltbush, hemp and macroalgae, were subjected to HTC and the hydrochars analysed for fuel properties and biochemical composition. The breakdown of the key macromolecules was then described using kinetic modelling to build a mechanistic model of the overall conversion of biomass to hydrochar. Each of the three biomasses underwent profound chemical changes during the HTC reaction, resulting in much lower oxygen content, and much higher carbon content. This caused the energy content of the hydrochars to rise to levels that rivaled or even exceeded those observed in fossil lignites. In addition, the ash content of the biomass was reduced, although certain reaction conditions at higher intensities saw the reabsorbence of inorganic elements back into the char. A twin-pathway mechanistic model was adapted and developed from the literature to describe the overall HTC process and the formation of two different kinds of hydrochar: Mechanism 1, involving solid phase conversions that yield “primary char” derived directly from undissolved and partially converted starting material; and Mechanism 2, a two-step pathway that involves the degradation of the feedstock into dissolved intermediates, and the subsequent repolymerisation of those intermediates into “secondary char”. Using this model as a framework of the backdrop of the HTC reaction, the kinetics of the degradation step in each macromolecule where then analysed in detail. There were numerous broad similarities in the behaviour of the key macromolecules between the different biomasses, in spite of their different origins. The degradation of polysaccharides was determined mainly by the degree of crystallinity; non-crystalline hemicellulose degraded very quickly in every biomass in a pseudo-first order reaction, often being completely eliminated from the feedstock within minutes. On the other hand, highly crystalline cellulose was more recalcitrant, and the reaction orders and rates of degradation of cellulose varied considerably across the three biomasses, although it was consistently slower than its non-crystalline counterparts. Variations in the degree of crystallinity in both cellulose and hemicellulose appeared to result in dramatic differences in the degradation kinetics. Lignin was found to be partially susceptible to HTC degradation, with the majority being dissolved with similar kinetics to hemicellulose, and the remainder being inert. The mechanism of the degradation of protein in macroalgae was opaque and difficult to model, with proteins possibly undergoing Maillard reactions with carbohydrates. It is hoped that the methods presented here, especially regarding the biochemical analyses of the hydrochars, can form a major facet of future research and industrial development of HTC.