BIOLOGICAL PRODUCTION OF HYDROGEN

Biomass belongs to most prospective renewable resources. Its energetic use including hydrogen production is versatile. Its content in biomass (6-6.5% mass fraction) is lower compared to natural gas (25% mass fraction) but equal to the content of hydrogen in coal (5% mass fraction).

Dry Biomass

Dry biomass is a name for wooden or dry plant waste. If can be further processed by burning and gasification.

Thermochemical processes

Thermochemical processes include steam reformation of biomass. This two-step process consists of pyrolysis, where gas products are generated (methane, hydrogen, carbon monoxide) and the   second step where high-temperatures (600°–1.000°C) are used.  During the second step a series of chemical reactions, the residual solids and methane are transformed into hydrogen and carbon dioxide using water stem and the total yield of hydrogen is further increased by transforming carbon dioxide into hydrogen and carbon monoxide. The chemicals used in the process can be reused within each cycle, creating a closed loop that consumes only water and produces hydrogen and oxygen. Materials that can be processed by this method range from a general waste, food industries waste, farm waste to coal. The process can then vary based on the raw materials used, temperature or type of catalysts.

High-moisture content biomass

Compared to dry biomass, high-moisture content biomass is due to economic reasons unsuitable for traditional thermochemical processes. Instead, it undergoes biotechnological processes catalysed by microorganisms in water environment under low temperature and pressure. These biological processes usually use algae or anaerobic bacteria that are found in environment with no atmospheric oxygen. The effect of microorganisms then differs based on the feedstock and process conditions used.

An overview of the most common methods of hydrogen production using biotechnological processes:

Direct photolysis

Direct photolysis uses sunlight and enzymes produced by microorganisms to split water into oxygen and hydrogen. The process uses the photosynthetic microalgae system to harness solar energy and convert it into a chemical energy needed for water splitting. These processes are possible only under anaerobic conditions where the oxygen content is a maximum of 0.1 % as the enzymes are highly sensitive to free oxygen presence. The entry substance of direct photolysis is just water which is free and easy to access. The disadvantage of direct photolysis is the low efficiency of 5%, which can be increased under laboratory conditions up to 15%. Another option is indirect photolysis, a more complex method consisting of several steps: biomass production through photosynthesis, concentration of biomass, anaerobic fermentation and acetate conversion (acetic acid salts). Indirect photolysis uses cyanobacteria.

Fermentation

Fermentation is a conversion of substances using microorganism enzymes due to metabolic activity. Organic substances (carbohydrates) are converted into low-energy compounds (ethanol, carbon dioxide). The most suitable resources are potatoes and sugarcane. There are two main types of fermentation. The first one is hydrogen fermentation (dark fermentation) and the second one is photofermentation.[1]

Dark Fermentation

Dark fermentation is carried out by obligate anaerobes and facultative anaerobes in the absence of light and oxygen. Organic compounds are the primary source of energy and hydrogen. Different types of bacteria use the proton reduction to hydrogen for storing the electrons produced during oxidation of organic compounds. Theoretical yield from 1 mol glucose is shown in the following equation. The maximum amount of direct hydrogen yield is 4 mol and 206kJ of energy is released. Another producing 2 mols of acetate are produced which can be further used for gaining another 4 mols of H₂.

C₆H₁₂O₆ + 4H₂O → 2CH₃COO^- +2CO₃^2- + 6H⁺ + 4H₂

Photofermentation

 

Similarly to dark fermentation photofermentation leads to production of hydrogen and CO₂ using bacteria and organic matter. The difference is that the processes are carried out using sunlight. One of the groups of microorganisms with the ability of photofermentation is purple non-sulphur bacteria that, under anaerobic conditions, use simple organic acids. The process is described by the following equation: [2]

The advantage of using bacteria is their ability to adapt the metabolic process. Meaning they can be used in various conditions. Both types of fermentation are combined with increasing economic profit, and the acetate by-product of dark fermentation is used for photofermentation. It is a two-step bioproduction of hydrogen. In the first step, hydrogen is produced from organic matter via hydrogen fermentation, and in the second step, biogas is resourced, or hydrogen is produced via photofermentation. It is also possible to produce energy by burning residual biomass. [3]

 

The efficacy of the process is influenced by materials and technology used. On its own, fermentation has low efficiency (around 10%) but using a combination of fermentations; the efficiency can reach up to 40%. Energy demands of the process depend on the amount of heat needed to warm up the entry materials and can be relatively high. A small amount of NOx and CO emissions are released into the atmosphere during the two-step fermentation process. However, due to its low concentrations, the emissions should not significantly affect the environment. There is great potential in fermentation as it is constantly under development, and extensive research is focusing on the genetic modification of microorganisms to increase the efficiency of the process. The greatest asset of fermentation is contributing to waste management as the waste production worldwide keeps increasing.