Life cycle and techno-economic assessment of acid washing as a pretreatment in the production of microalgal hydrochar through hydrothermal carbonization

The increasing demand for energy and material inputs in the agroindustrial sector has intensified pressure on natural resources, highlighting the need for sustainable and low-environmental-impact production pathways. In this context, this study assesses the effects of acid pretreatment of microalgal biomass prior to hydrothermal carbonization (HTC) on hydrochar production, considering technical, economic, and environmental aspects. Thus, three scenarios were evaluated: (i) direct HTC (SC1); (ii) HTC with acid pretreatment (SC2); and (iii) HTC with acid pretreatment and acid recovery and recirculation (SC3). Hydrochar production was modeled using Aspen Plus®, economic feasibility was assessed through techno-economic analysis, and environmental impacts were quantified via life cycle assessment using SimaPro®. In SC1, the mass yield was 418.08 kg.h⁻¹ and the minimum selling price (MSP) was USD 2.83 kg⁻¹. In contrast, SC2 increased the higher heating value (HHV) of the hydrochar, but reduced hydrochar production to 264.45 kg.h⁻¹, raised the MSP to USD 9.92 kg⁻¹ (90.9% of operating costs attributed to HCl consumption), and resulted in higher environmental impacts in 17 out of 18 midpoint categories evaluated. SC3 maintained the hydrochar HHV and mass yield observed in SC2 while reducing HCl consumption, resulting in an intermediate performance, with an MSP of USD 7.41 kg⁻¹. The acid recovery and recirculation proved to be an effective approach to reduce costs and environmental impacts, although it was still not sufficient to outperform SC2. Overall, SC1 showed superior economic and environmental performance, whereas SC2 and SC3 improved hydrochar quality at the expense of higher costs and environmental impacts.