Alkaline pretreatment to enhance one-stage CH4 and two-stage H2/CH4 production from sunflower stalks: Mass, energy and economical balances

• Alkaline pretreatment increased by 29% methane production from sunflower stalks.
• Same total energy was produced in one-stage CH4 and two-stage H2/CH4 bioprocesses.
• High solid loading and heat recovery are required for positive energy balance.

Biological production of second generation biofuels such as biohydrogen (H2) or methane (CH4) represents a promising alternative to fossils fuels. Alkaline pretreatments of lignocellulosic biomass are known to enhance the accessibility and the bioconversion of hollocelluloses during anaerobic digestion and dark fermentation processes. In the present study, four different configurations were investigated: one-stage CH4 continuous and two-stage H2 batch/CH4 continuous process with and without alkaline pretreatment of sunflower stalks (55 °C, 24 h, 4 g NaOH/100 g TS). The results showed that two stage H2/CH4 (150 ± 3.5 mL CH4 g−1 VS) did not improve methane yields compared to one stage CH4 (152 ± 4 mL CH4 g−1 VS). Although alkaline pretreatment was shown to be inefficient in improving the H2 yields in the two-stage H2/CH4 process, an increase in methane yields by 26% and 29% were observed with one-stage CH4 and two-stage H2/CH4 production compared to one-stage CH4 process without alkaline pretreatment, respectively. Chemical analysis of the solid digestate showed that hemicelluloses were the most preferred substrates compared to cellulose whereas lignin remained undegraded in all four studied configurations. Finally, energy balance showed that a positive energy balance and economic sustainability can be achieved when the alkaline pretreatment is applied at a high substrate concentration and/or when heat is recovered at a maximum efficiency during the pretreatment step.