Hydrothermal liquefaction of biosolids

Abstract

Hydrothermal liquefaction (HTL) is a promising thermochemical conversion process to convert biosolids into renewable crude oil. HTL process can be achieved at temperatures between 200 to 350°C, pressures between 50 to 250 bar, and residence time between 1 and 60 minutes. The HTL produces four phases: renewable crude oil, aqueous, gaseous and solid phases. For the process to be upgraded to an industrial scale, it is needed to gain a better understanding of the HTL of biosolids. However, there is limited information to validate the effects of the interactions between the biosolid content under HTL reaction conditions on the yield and the composition of the produced renewable crude oil. The primary objective of this research is to provide a better understanding of the HTL of biosolids, which was achieved through the following detailed objectives. The first objective is to quantify the variability in the biosolids composition to determine the chemical compositions of biosolids. The second objective is to understand how this variable biosolids feedstock behaves through HTL, especially to measure the effects of organic compounds of biosolids: lipids, proteins, carbohydrates, and lignins on the HTL yields. The third objective is to provide a new understating of the characterisation of HTL products from biosolids by identifying the effects of biosolid components and the HTL conditions on both the distributions of the HTL products’ yields and on the qualities of renewable crude oil. The fourth objective is to assess the use of biosolids with dominant organic fraction via different reaction temperatures and residence times on the composition and fractions of the produced renewable crude oil. From the results of the experiments, biosolids have different characters that affect the yield and quality of renewable crude oil. Applying a Van Krevelen diagram to compare biosolids with other biomass indicated that only some biosolids samples have similar characteristics to that of biomass. The difference in the characteristic of the organic content of biosolid samples could depend on several reasons, such as the sources of the biosolids and the treatment process. The effects of the biosolids’ composition on the HTL yield show that lipids and proteins have positive impacts on the renewable crude oil yield, while carbohydrates and insoluble lignin led to an increase in the solid residue. The renewable crude oil contained a high amount of high-boiling point materials in comparison with low-boiling point materials for all biosolids samples used in this study. The effect of the operating conditions, such as temperature was significant. The renewable crude yield usually increases with an increase in temperature until a specific temperature is reached, at which point the renewable crude yield starts to decrease. Various residence times also affected renewable crude oil yields significantly. The optimal residence times depended on the biosolids content and temperature. The HTL of biosolids with different organic fractions resulted in different renewable crude oil compositions, which contained a complex mixture of >300 major compounds that were identified using Gas chromatography-mass spectroscopy analyser. The predominant components identified from the lipid, protein, carbohydrate and lignin constituents were cyclic terpanes and terpenes, along with nitrogenous, oxygenated, and phenolic components. Based on the boiling point of the produced compounds, high gasoline and naphtha-like and high diesel-like yields were produced from biosolid samples with high lipid and protein content, while the kerosene-like best yield was generated from a high lipid sample. A significant gas oil-like yield was produced from the high lipid and carbohydrate biosolid samples, while a high yield of wax, lubricating oil and vacuum gas oil-like contents were generated from the high lignin sample. In summary, the results of the outcomes of this work and the methods used to analyse the chemical compositions of biosolids can form a significant facet of future industrial development of HTL of biosolids, particularly in commercial plants design and management. Finally, it is hoped that the methods presented here, especially the methods used to analyse the chemical compositions of biosolids and the outcomes of this work, especially regarding the composition of the produced renewable crude oil, can form a significant facet of future industrial development of the HTL of biosolids.