A kinetic model for the hydrothermal liquefaction of microalgae, sewage sludge and pine wood with product characterisation of renewable crude

Abstract

Hydrothermal liquefaction (HTL) of biomass is an emerging technology that is being developed to produce renewable crude oil in water at sub-critical conditions. Different feedstocks and reaction conditions result in different product fractions of renewable crude and co-products of solid, aqueous and gas phase products. Biomass considered as feedstocks for HTL include microalgae, sewage sludge and lignocelluloses. Each of these biomass sources contains varying amounts of lipid, carbohydrate, protein and lignin organic fractions as well as some inorganic components. To develop a bulk kinetic model to predict the yields of crude, solid, aqueous and gas phase products, HTL experiments were conducted at reaction temperatures of 250, 300 and 350 °C over reaction times of 0–60 min with Tetraselmis sp. microalgae, sewage sludge and Radiata pine. The crude was analysed via gas-chromatography mass-spectrometry to identify variations in the compounds in the crude produced from different types of biomass. The highest crude yield was produced from algae at up to 30%, followed by up to 25% from sludge and up to 10% from pine. A reaction temperature of 300 or 350 °C was preferable for maximum crude yield and increasing reaction time over 5 min was seen to cause minimum variation in crude yield for most cases. The variation in product distribution is strongly dependent on both the organic and inorganic content of the biomass feedstock. A unified bulk kinetic model for prediction of crude yield from a wide range of biomass was developed. Predictions showed up to 15% variation from measurements illustrating that further experimental data from HTL of a wider range of feedstocks are required to refine the model and build up model rigour.