High-calorific biohydrogen production under high pressure: Ca²⁺ addition, theoretical prediction, and continuous operation

The removal of CO₂ (called “upgrading”) is essential in bio-H₂ production processes including dark fermentation (DF), which is energy-intensive and adds economic burden. Here, it was attempted to generate high calorific bio-H₂ (high H₂-content) directly from the fermenter under high pressure in the presence of Ca²⁺. High pressure was automatically generated by holding the produced gas at a certain limit. Batch test results showed that Ca²⁺ addition was helpful to keep the pH by precipitating with dissolved CO₂, resulting in the gradual increase of H₂ content to 93 % at 17.5 bar (vs. 77 % at 11.2 bar in the control). Then, the theoretical approaches, based on balance and equilibrium equations, were made to investigate the effects of Ca²⁺ addition at different dosages (0–1 g CaCl₂/L) under different pressures during continuous operation. It was found that the H₂ content of 90 % at the permissible pressure (≤10 bar) can be only achieved at 1 g CaCl₂/L addition. Lastly, continuous operation of DF was conducted in an up-flow anaerobic sludge blanket reactor fed with glucose (10 g COD/L) with 1 g CaCl₂/L addition. The H₂ content in bio-H₂ gradually increased with the pressure increase, from 49.5 % at 1 bar to 84.9 % at 9 bar. No distinct changes in morphological and physico-chemical properties were observed in the granules exposed at different pressures. Although there was a drop in H₂ production due to the increase in dissolved H₂ portion and metabolic shift at high pressure, the decrease in upgrading cost made high-pressure DF economically feasible.