Simulation on hydrothermal liquefaction of pinewood to produce bio-crude in a zero-waste process scheme

Hydrothermal liquefaction (HTL) is one of the advanced biomass conversion technologies to produce bio-crude from wet lignocellulosic feedstocks. Hydrochar and water phase containing organics are always generated as by-products and their efficient re-use could be fundamental to decrease the whole energy consumption. Hydrogen producers like Fe are often added to the HTL reactor to maximize the bio-crude yields and quality. Furthermore, Fe can be easily recovered from the biochar at the end of the reaction and re-used, after a reduction treatment. In this work, the feasibility to produce bio-crude through HTL of pinewood in a continuous zero-waste process scheme is evaluated in an Aspen Plus® simulation. The zero- waste configuration was carried out using the water phase containing organics instead of distillate water in the HTL reactor and the hydrochar as renewable reductant of iron oxides. Experimental data were used to model the HTL (Ryield) while the iron oxide reduction and the combustion of the side streams were simulated in the Gibbs reactor (Rgibbs). Red mud, a waste stream of the Bayern process for aluminium production, containing 50 wt % of hematite (Fe2O3), was selected as low-cost iron source. The iron oxides reduction with hydrochar was performed at different temperature (400-1200 °C) to determine the optimal value (complete conversion to Fe). Based on the simulation results, hydrochar allows the complete red mud reduction at 780 °C but the separation of the carbon excess before Fe recirculation must be considered. The proposed continuous zero-waste HTL plant consumed a total energy of 5080 J s-1, mostly of it related to the HTL (4945 J s-1) and iron oxide reduction (640.2 J s-1). However, the combustion of both off gases and hydrochar excess can approximately provide the heat needed to plant (-5012 J s-1). Finally, the results demonstrated that the proposed HTL scheme might be a zero-waste process in terms of both mass and energy flows.