Technological strategies for managing sugarcane vinasse in two-stage biodigestion plants: Energetic and economic aspects

Abstract

Anaerobic digestion is recognizably the most efficient approach (on both energetic and environmental bases) to manage sugarcane vinasse in ethanol-producing plants. In particular, separating the fermentation from methanogenesis (two-stage schemes) step improves the process by providing higher stability to methanogens. Based on experimental data, this study compared the techno-economics of different operating strategies for the two-stage biodigestion of sugarcane vinasse: biohydrogen exploitation, non-recovery of biohydrogen and removal of sulfate (fermentation) coupled to two alkalinization approaches in methanogenesis (NaHCO3 dosing or NaOH dosing + liquid phase recirculation). Potential biogas uses included the production of electricity, purified biohydrogen or biomethane. Removing sulfate during the fermentation increased the energy yield of biodigestion by over 30% (peaking at 914 TJ when producing biomethane), a 5-fold higher level than exploiting biohydrogen (contribution of 25.3 TJ). However, the non-optimized dosing of chemicals still represents the main economic bottleneck of the process, nullifying the aforementioned energy gains. Replacing NaHCO3 dosing in the sulfidogenic-fermentative reactors by recycling the bicarbonate produced during methanogenesis coupled to minimizing the inputs of NaOH in the methanogenic units provided the best economic performance: internal rates of return of 21.6% (electricity production) and 28.9% (biomethane production towards diesel replacement). The highly efficient production of electricity using the combined cycle represents an attractive alternative for exploiting the vinasse-derived biogas in a short-term time horizon, whilst maximizing the economic exploitation of biomethane was found to directly depend on the replacement of diesel. Overall, exploiting the biogas produced from vinasse is profitable; however, the maximization of gains depends more on a rational use of chemicals than on more efficient methane evolution rates.