Process Parameters of Hydrogen Production by Biological Fermentation
2019.12.21
(1) Temperature
Temperature influences the metabolic rate of hydrogen production by fermentative bacteria. There are considerable differences in the optimal hydrogen-producing temperatures among different hydrogen-producing bacteria. Research results show that most fermentative hydrogen-producing bacteria are mesophilic; to date, there have been no reports of hydrogen-producing bacteria active at ambient temperature, and reports of thermophilic fermentative hydrogen-producing bacteria are also rare. Satisfactory hydrogen production can be achieved at a maximum temperature of 55 °C.
(2) pH Value
Solution pH is one of the key parameters affecting biological fermentative hydrogen production. This is because pH affects microbial metabolism, directly influencing the activity of hydrogenase inside hydrogen-producing microbial cells, cellular redox potential, the types and forms of metabolic products, and substrate availability. Most studies indicate that hydrogen-producing microorganisms generally achieve high hydrogen production efficiency under weakly acidic conditions.
pH directly determines metabolic products: at higher pH, fermentation metabolites are dominated by acids; at lower pH, ketones and alcohols are the main products. The optimal pH for hydrogen production in ethanol-type fermentation is 4.2–4.5, while that for butyrate-type fermentation is 6.0–6.5.
(3) Substrates
Substrates significantly influence the efficiency of biological fermentative hydrogen production. Substrates commonly used in theoretical research include glucose, sucrose, starch, cellulose, and other simple-structured carbohydrates. However, hydrogen production from organic wastes as substrates is far more complex, as substrate composition varies greatly depending on the source of wastewater.
For hydrogen production from organic wastes via biological fermentation, complex waste streams must first be pretreated to make organic matter accessible or readily utilizable by hydrogen-producing microorganisms. Five typical pretreatment methods are available: ultrasonic treatment, acid treatment, sterilization, freeze-thaw treatment, and addition of methanogen inhibitors. Studies show that freeze-thaw and acid treatments yield the best hydrogen production performance, followed by sterilization.
Inorganic nutrients in substrates are essential for the growth of fermentative hydrogen-producing bacteria and directly affect the progress of hydrogen production. For example, Fe, as a key component of the active centers of intracellular enzymes, maintains the stability of biological macromolecules and cell structures. Hydrogenase activity decreases with iron depletion, and iron is also an important constituent of ferredoxin.
(4) Hydraulic Retention Time
For continuous fermentative hydrogen production systems, hydraulic retention time (HRT) becomes a critical factor. HRT varies with the type of fermentative hydrogen production reactor, and typically ranges from 2 to 24 hours.