Development and technical-economic assessment of an integrated process for the production of biobutanol from rice straw

The economy of the European Union is moving towards a circular and resource-efficient model. Within this framework, second-generation biorefineries can play a key role in valorizing lignocellulosic residues into high value-added chemical products. Unlike first-generation biorefineries, the processing of these residues requires preliminary stages for their breakdown (pretreatment and hydrolysis), which increase the cost of industrial implementation. Among the available biotechnological routes, the acetone– butanol–ethanol (ABE) fermentation stands out for the production of biobutanol, a compound of great interest as a chemical intermediate. However, its deployment presents additional challenges such as the inhibition of solventogenic bacteria at butanol concentrations as low as 10 g L⁻¹, which limits its yield and, moreover, generates diluted broths that make subsequent separation more expensive. The general objective of this doctoral thesis is the development of an integrated bioprocess for the valorization of rice straw into biobutanol, combining advanced bioreactor configurations with low-energyconsumption purification systems. This objective is complemented by a techno-economic analysis (TEA) to assess its feasibility. The selection of rice straw as raw material is based on its wide availability in the Albufera Natural Park (Valencia, Spain), where its current management represents an environmental problem. Rice straw was subjected to alkaline pretreatment (solids 5% w/v, NaOH 0.75% w/v, 134 °C, 40 min) followed by saccharification (solids 8% w/v, Cellic® CTec2 20 FPU g⁻¹, 50 °C, 48 h) according to the conditions previously established by the research group. In the first study, fermentations with immobilized cells of C. acetobutylicum DSM 792 or C. beijerinckii DSM 6422 on granular activated carbon (GAC) were evaluated in continuous stirred-tank reactors (CSTR). The use of GAC allowed operation at dilution rates (D) higher than the washout threshold (D>0.30 h⁻¹) observed in suspended cultures. With the C. beijerinckii strain, a higher performance was achieved (butanol productivity of 2.29 g L⁻¹ h⁻¹ at D=0.60 h⁻¹), being 2.1 times higher than the maximum value obtained with the C. acetobutylicum strain. In addition, a hybrid gas stripping and pervaporation system operating under moderate vacuum was developed. Starting from the fermentation broth (7.1 g L⁻¹ of butanol), this system produced a purified effluent containing 291 g L⁻¹ of butanol. The energy demand was only 20.6 MJ kg⁻¹, lower than that required by distillation. Based on the results of the first study, a novel packed-bed reactor (PBR) configuration with GAC using the C. beijerinckii strain was evaluated. The PBR achieved butanol productivities of up to 4.44 g L⁻¹ h⁻¹ under a D of 1.20 h⁻¹, although sugar consumption was low (~40%). Alternatively, a configuration of two PBRs in series with in-line gas stripping and effluent recirculation was developed, which increased sugar conversion up to ~69% while maintaining high butanol productivities (2.8 g L⁻¹ h⁻¹). In the following study, bioelectrochemical systems were evaluated as potential tools for the metabolic control of the C. beijerinckii strain. The coculture with the bacterium Geobacter sulfurreducens showed the greatest effect among the systems studied, as it redirected carbon and electron fluxes towards more oxidized and industrially relevant products (mainly acetone). Specifically, the ABE yield (0.492 g g⁻¹) doubled compared to that obtained in pure cultures (0.240 g g⁻¹). In the fourth study, the experimental results obtained in this doctoral thesis were integrated into the biorefinery model proposed by the National Renewable Energy Laboratory (NREL) to assess, through TEA, the economic feasibility of this process in comparison with the market price of petrochemical-based butanol ($1.1–1.4 kg⁻¹). A first deployment was proposed to cover the annual rice straw production in the Albufera area (90,000 tons year⁻¹). Different pretreatment scenarios coupled to a simultaneous saccharification and fermentation (SSF) scheme and to the proposed purification system were evaluated, together with the remaining necessary stages (enzyme production, effluent treatment, energy generation, and auxiliary systems). The intensification of the alkaline pretreatment through its combination with steam explosion at high solid loadings was identified as a promising option to reduce costs compared to sulfuric acid explosion. Using a system of four PBRs in series with gas stripping between stages reduced the reactor volume and energy demand compared to the SSF scheme. Processing 270,000 tons year⁻¹ of rice straw and considering 30% investment subsidies along with biofuel credits of $0.4 kg⁻¹, the market price could reach a competitive value ($1.15 kg⁻¹).