The Technology of Biohydrogen

Abstract

Hydrogen produced by microalgae and bacteria is biohydrogen. There are currently no practical biohydrogen production processes. However, several concepts have promise for near- to long-term process development.

The conversion of CO to H₂, the microbial shift reaction, operates at ambient temperatures in a single-stage process, compared to the two-stage, high-temperature, chemical catalyst processes currently used. Process development is just beginning, but this concept appears promising for near- to mid-term practical applications.

H₂ yields from dark fermentations of organic wastes are typically less than 20% (on a heating value basis) compared to CH₄ fermentations. Higher yields might be possible at elevated temperatures, with nutrient limitations, and through metabolic engineering of the bacteria. Photofermentations, the conversion of organic substrates to H₂ by nitrogen-fixing photosynthetic bacteria, achieve high H₂ yields, but low solar conversion efficiencies. The inefficiency of the nitrogenase enzyme suggests that biohydrogen processes must be based on reversible hydrogenases. Even then, dark fermentations of wastes to H₂ would be preferable to light-driven processes, in part due to the high cost of photobioreactors. The production of H₂ from organic wastes is of mid-term potential but limited by resources and competition from other processes.

Larger-scale biohydrogen production requires biophotolysis processes—H₂ production from water and sunlight. Photobioreactor costs and solar conversion efficiencies are main challenges in the development of practical processes. Direct biophotolysis couples the reductant produced by photosynthesis directly to hydrogenase, producing O₂ and H₂ simultaneously, while indirect processes separate these basically incompatible reactions through intermediate CO₂ fixation. Direct, but not indirect, biophotolysis processes require hydrogenase activity in the presence of high O₂ levels, something not known to occur. Hybrid indirect processes using both algae and photosynthetic bacteria have been proposed and even tested outdoors, but are complex and inefficient. The simplest indirect process would use the same algal cells for both CO₂ fixation/O₂ evolution and H₂ production, at separate times or even in different reactors. The H₂ reactions would take place both in the dark and light. Light-driven H₂ evolution requires suppression of the O₂ evolution process. Biophotolysis process must achieve the highest possible solar conversion efficiencies, which will require development of algal strains with reduced light-harvesting (“antenna”) pigment content.

A preliminary economic analysis of a large-scale, two-stage, indirect process suggests that biohydrogen costs close to $10/GJ, lower than for PV-electrolysis processes. This assumed use of very large, low-cost (<$10/m²) open ponds for the first (CO₂ fixation) stage operating at a 10% solar conversion efficiency, and much smaller (approximately 10% of the total area) photobioreactors (<$150/m²) for the second H₂ production stage. Single-stage biophotolysis systems require low-cost (<$50/m²) photobioreactors, but may be suitable for much smaller-scale (perhaps even roof-top) applications.

The development of practical biohydrogen production systems will require focusing on the more promising alternatives and critical R&D issues and could be accelerated through international basic and applied R&D collaborations.