Hydrothermal liquefaction of woody biomass process plant design

Hydrothermal liquefaction (HTL) is an alternative to other biomass-to-fuel technologies that require feed drying. However up to this day, no industrial-scale HTL unit is in operation. In this work, a process design of the full-scale unit in the paper and pulp industry is investigated, with woody residues as the primary feedstock.
The optimal set of HTL conditions was first identified using a small-scale autoclave. It was shown that keeping the reaction mixture at 400°C for 0 – 10 minutes results in an optimum between the quality and quantity of the main product called biocrude. Additionally, keeping the pH of the aqueous byproduct at 5 ± 0,5 assures plausible yields of char byproduct, regardless of the additive type. Further autoclave testing was dedicated to the process design. It was concluded that the wood can be pretreated at 210°C, which may enhance the deoxygenation of biocrude. Using quartz capillaries, sufficient phase split between the biocrude and aqueous phase was observed at 160°C, while the full miscibility was estimated at 240°C onwards. A considerable increase of biocrude yield while reducing the additives intake by more than 50% was demonstrated during the aqueous phase recycle testing.
The study on plant operability has proven that batch-wise processing is not viable due to its practicality issues and significant material costs for given reaction conditions. Hence, the conceptual design of the continuous plant is proposed. Based on experimentally obtained input, the mass and energy balances of the concept were elaborated and estimated that 5,4 GJ per ton of produced biocrude is required to run the process (base case). The energetic return of investment (EROI) value was estimated to ~ 6 when the energy content of the biocrude was considered. Even when the most unlikely cases were applied in the sensitivity studies, the possible lowest obtained EROI value was 4,7, meaning that the process is energetically feasible in its operational window.
Detailed model of supercritical heat exchanger (HX) was developed for sizing and cost estimation purposes. It was shown that an external thermal fluid is required to keep both process streams in the tube side of the HX. Increasing the process pressure by 20 bar was found beneficial for the unit sizing with potential material cost reduction by 28%. The modelling section has concluded that installing a separate reactor unit is redundant since the wood travels through the HX for a sufficiently long time to convert. The HX unit can integrate 3,87 MW of process heat per 1 MW of externally supplied heat. Implementing the model results into the mass and energy balances results in the energy consumption of 6,2 GJ per ton of biocrude, which is 15% higher compared to the base case, but still yielding very promising EROI of 5,2.
Lastly, the preliminary cost analysis suggests the CAPEX to be in the ballpark range of 130 M€ per plant, with the HX unit covering more than 57% of the capital investment. The OPEX was estimated at around 19 M€ per year. Based on the economic analysis, the final biocrude production cost is 321 € per ton. Considering current oil prices, the project payback time foreseen at 7-9 years.