Biohydrogen - AbsoluteAstronomy.com

Biohydrogen is defined as hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

produced biologically, most commonly by algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

and bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

. Biohydrogen is a potential biofuel

Biofuel

Biofuel is a type of fuel whose energy is derived from biological carbon fixation. Biofuels include fuels derived from biomass conversion, as well as solid biomass, liquid fuels and various biogases...

obtainable from both cultivation and waste organic materials.

Introduction

Currently, there is a huge demand of the chemical hydrogen. There is no
log on the production volume and use of hydrogen world-wide. However the
estimated consumption of hydrogen is expected to reach 900 billion cubic
meters in 2011

Refineries are large-volume producers and consumers of hydrogen. Today
96% of all hydrogen is derived from fossil fuels, with 48% from natural gas

Natural gas

Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

,
30% from hydrocarbon

Hydrocarbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

s, 18% from coal and about 4% from electrolysis.
Oil-sands processing, gas-to-liquids and coal gasification

Coal gasification

Coal gasification is the process of producing coal gas, a type of syngas–a mixture of carbon monoxide , hydrogen , carbon dioxide and water vapour –from coal...

projects that are
ongoing, require a huge amount of hydrogen and is expected to boost the
requirement significantly within the next few years. Environmental regulations
implemented in most countries, increase the hydrogen requirement at
refineries for gas-line and diesel desulfurization

An important future application of hydrogen could be as an alternative for
fossil fuels, once the oil deposits are depleted. This application is however
dependent on the development of storage techniques to enable proper
storage, distribution and combustion of hydrogen. If the cost of hydrogen
production, distribution, and end-user technologies decreases, hydrogen as a
fuel could be entering the market in 2020.

Industrial fermentation of hydrogen, or whole-cell catalysis, requires a limited
amount of energy, since fission of water is achieved with whole cell catalysis, to
lower the activation energy. This allows hydrogen to be produced from
any organic material that can be derived through whole cell catalysis
since this process does not depend on the energy of substrate.

Algaeic biohydrogen

In 1939 a German researcher named Hans Gaffron

Hans Gaffron

Dr. Hans Gaffron is born in Lima, Peru, on May 17, 1902, as the son of the German physician Eduard Gaffron and his wife Hedwig von Gevekot....

, while working at the University of Chicago, observed that the algae he was studying, Chlamydomonas reinhardtii
Chlamydomonas reinhardtii
Chlamydomonas reinhardtii is a single celled green alga about 10 micrometres in diameter that swims with two flagella. They have a cell wall made of hydroxyproline-rich glycoproteins, a large cup-shaped chloroplast, a large pyrenoid, and an "eyespot" that senses light.Although widely distributed...

(a green-algae), would sometimes switch from the production of oxygen to the production of hydrogen. Gaffron never discovered the cause for this change and for many years other scientists failed in their attempts at its discovery. In the late 1990s professor Anastasios Melis

Anastasios Melis

Anastasios Melis is a biologist at the University of California, Berkeley who is researching the possibility of creating hydrogen from algae. Hydrogen power is considered one of the key ways of producing electricity without continuing to use up fossil fuels...

a researcher at the University of California at Berkeley discovered that if the algae culture medium is deprived of sulfur it will switch from the production of oxygen (normal photosynthesis), to the production of hydrogen. He found that the enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

responsible for this reaction is hydrogenase

Hydrogenase

A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....

, but that the hydrogenase lost this function in the presence of oxygen. Melis found that depleting the amount of sulfur available to the algae interrupted its internal oxygen flow, allowing the hydrogenase an environment in which it can react, causing the algae to produce hydrogen. Chlamydomonas moewusii is also a good strain for the production of hydrogen. Scientists at the U.S. Department of Energy’s Argonne National Laboratory are currently trying to find a way to take the part of the hydrogenase enzyme that creates the hydrogen gas and introduce it into the photosynthesis process. The result would be a large amount of hydrogen gas, possibly on par with the amount of oxygen created.

It would take about 25,000 square kilometres to be sufficient to displace gasoline use in the US. To put this in perspective, this area represents approximately 10% of the area devoted to growing soya

Soybean

The soybean or soya bean is a species of legume native to East Asia, widely grown for its edible bean which has numerous uses...

in the US. The US Department of Energy has targeted a selling price of $2.60 / kg as a goal for making renewable hydrogen economically viable. 1 kg is approximately the energy equivalent to a gallon of gasoline. To achieve this, the efficiency of light-to-hydrogen conversion must reach 10% while current efficiency is only 1% and selling price is estimated at $13.53 / kg. According to the DOE cost estimate, for a refueling station to supply 100 cars per day, it would need 300 kg. With current technology, a 300 kg per day stand-alone system will require 110,000 m² of pond area, 0.2 g/l

Litre

pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...

cell concentration, a truncated antennae mutant and 10 cm pond depth. Areas of research to increase efficiency include developing oxygen-tolerant FeFe-hydrogenase

Hydrogenase

A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....

s and increased hydrogen production rates through improved electron transfer.

Process requirements

If hydrogen by fermentation is to be introduced as an industry, the fermentation process
will be dependent on organic acids as substrate for photo-fermentation.
The organic acids are necessary for high hydrogen production rates.

The organic acids can be derived from any organic material source such as sewage

Sewage

Sewage is water-carried waste, in solution or suspension, that is intended to be removed from a community. Also known as wastewater, it is more than 99% water and is characterized by volume or rate of flow, physical condition, chemical constituents and the bacteriological organisms that it contains...

waste waters
or agricultural wastes. The most important organic acids are acetic acid

Acetic acid

Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

(HAc), butyric acid

Butyric acid

Butyric acid , also known under the systematic name butanoic acid, is a carboxylic acid with the structural formula CH3CH2CH2-COOH. Salts and esters of butyric acid are known as butyrates or butanoates...

(HBc) and propionic
acid (HPc). A huge advantage is that production of hydrogen by fermentation does not require glucose

Glucose

Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

as substrate.

The fermentation of hydrogen has to be a continuous fermentation process, in order sustain high production rates, since the amount of time for the fermentation to enter high production rates are in days.

Fermentation

Several strategies for the production of hydrogen by fermentation in lab-scale
have been found in literature. However no strategies for industrial-scale productions
have been found. In order to define a industrial-scale production, the
information from lab-scale experiments has been scaled to an industrial-size
production on a theoretical basis.
In general, the method of hydrogen fermentation is referred to in three main
categories. The first category is dark-fermentation

Dark fermentation

Dark fermentation is the fermentative conversion of organic substrate to biohydrogen. It is a complex process manifested by diverse group of bacteria by a series of biochemical reactions involving three steps similar to anaerobic conversion...

, which is fermentation
which does not involve light. The second category is photo-fermentation,
which is fermentation which requires light as the source of energy. The third
is combined-fermentation, which refers to the two fermentations combined.

Dark-fermentation

There are several bacteria with a potential for hydrogen production. The Gram-positive

Gram-positive

Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...

bacteria of the Clostridium
Clostridium
Clostridium is a genus of Gram-positive bacteria, belonging to the Firmicutes. They are obligate anaerobes capable of producing endospores. Individual cells are rod-shaped, which gives them their name, from the Greek kloster or spindle...

genus, is promising because it has a natural high hydrogen production rate. In addition, it is fast growing and capable of forming spores, which make the bacteria easy to handle in industrial application.

Species of the Clostridium genus allow hydrogen production in mixed cultures, under mesophilic or thermophilic conditions within a pH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

range of 5.0 to 6.5. Dark-fermentation with mixed cultures seems promising since a mixed bacterial environment within the fermenter, allows cooperation of different species to efficiently degrade and convert organic waste materials into hydrogen, accompanied by the formation of organic acids.

For the fermentation to be sustainable in industrial-scale, we need to be able to control the bacterial environment inside the fermenter. If the fermentation process is feed with sugar waste, we have a risk, that the feed will contain micro-organisms, which could change the bacterial environment inside the fermenter. A way to prevent harmful micro-organisms from gaining control of the bacterial environment inside the fermenter could be through addition of probiotics which favors or promotes the intended bacterial environment and prevents harmful micro-organisms from gaining control of the fermenter.

The dilution rate has to ensure that the amount of biomass inside the fermenter is stable and that the organic acids are removed properly with the outlet stream.
The organic acids are toxic to the bacteria and huge amounts will interrupted the fermentation process. This fermentation of hydrogen is accompanied production of carbon-dioxide which can be separated from hydrogen with a passive separation process.

The fermentation will convert some of the sugar waste into biomass

Biomass

Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....

instead of hydrogen.
The biomass is however a carbohydrate-rich by-product which can be fed back into the fermenter, to ensure that the process is sustainable. Fermentation of hydrogen by dark-fermentation is restricted by incomplete degradation of organic material, into organic acids and this is why we need the photo-fermentation.

The separation of organic acids from biomass in the outlet stream can be done with a settler tank in the outlet stream, where the sludge (biomass) is pumped back into the fermenter to increase the rate of hydrogen production.

Photo-fermentation

Photofermentation

Photofermentation is the fermentative conversion of organic substrate to biohydrogen manifested by a diverse group of photosynthetic bacteria by a series of biochemical reactions involving three steps similar to anaerobic conversion...

refers to the method of fermentation where light is required as the source of energy.
This fermentation relies on photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

to maintain the cellular energy levels. Fermentation by photosynthesis
compared to other fermentations has the advantage of light as the source of energy instead of sugar.
Sugars are usually available in limited quantities.

All plants, algae

Algae

and some bacteria are capable of utilizing light as the source
of energy. Cyanobacteria is frequently mentioned capable
of hydrogen production by photosynthesis. However the purple non-sulphur
(PNS) bacteria genus Rhodobacter
Rhodobacter
In taxonomy, Rhodobacter is a genus of the Rhodobacteraceae.The most famous species of Rhodobacter is Rhodobacter sphaeroides, which is commonly used to express proteins.-External links:...

, holds significant promise for the production
of hydrogen by fermentation.

Studies have shown that Rhodobacter sphaeroides
Rhodobacter sphaeroides
Rhodobacter sphaeroides is a kind of purple bacteria; a group of bacteria that can obtain energy through photosynthesis. Its best growth conditions are anaerobic phototrophy and aerobic chemoheterotrophy in the absence of light. R. sphaeroides is also able to fix nitrogen...

is highly capable of hydrogen
production while feeding on organic acids, consuming 98% to 99% of the organic acids during hydrogen production.

As for the dark-fermentation the separation of biomass can be done with a settler tank in the outlet stream, where the sludge (biomass) is pumped back into the fermenter to increase the rate of hydrogen production .

Currently there is limited experience with photo-fermentation at industrial-scale.
Photo-fermentation require light in the ultra-violet (UV) range up to 400 nm.
The distribution of light within the industrial scale photo-fermenter
has to be designed to prevent self-shading inside the fermenter and
to ensure sustainable hydrogen production.

A method to ensure proper light distribution and limit self-shading within
the fermenter, could be to distribute the light with an optic fiber where light is transferred into the fermenter and
distributed from within the fermenter. Photo-fermentation with Rhodobacter sphaeroides require mesophilic conditions.
The optic fiber will transfer light and thus heat into the fermenter,
but the heat transferred is limited.

The design with an ultra-violet light-source has a huge advantage to other fermentations
since ultra-violet light has the potential to eliminate foreign micro-organisms and to prevent contamination.
This will limit the need of cleaning procedures. However the production rates with photo-fermentation
is not as high as with dark-fermentation.

Combined fermentation

Combining dark- and photo-fermentation has shown to be the most efficient
method to produce hydrogen through fermentation. The combined fermentation
allows the organic acids produced during dark-fermentation of waste
materials, to be used as substrate in the photo-fermentation process.

For industrial fermentation of hydrogen to be economical feasible, by-products
of the fermentation process has to be minimized.
Combined fermentation has the unique advantage of allowing reuse of the otherwise
useless chemical, organic acids, through photosynthesis.

As the method for hydrogen production, this method currently holds significant
promise.

Metabolic processes

The metabolic process

Metabolism

Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

for hydrogen production are dependent on the reduction of the metabolite ferredoxin

Ferredoxin

Ferredoxins are iron-sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co...

.

4H⁺ + ferredoxin(ox) → ferredoxin(red) + 2 H₂

For this process to run, ferredoxin has to be recycled through oxidation. The
recycling process is dependent on the transfer of electrons from nicotinamide adenine dinucleotide

Nicotinamide adenine dinucleotide

Nicotinamide adenine dinucleotide, abbreviated NAD, is a coenzyme found in all living cells. The compound is a dinucleotide, since it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an adenine base and the other nicotinamide.In metabolism, NAD is involved...

(NADH) to ferredoxin.

2 ferredoxin(red) + 2 NADH → 2 ferredoxin(ox) + H₂

The enzymes that catalyse this recycling process are referred to as hydrogen-forming enzymes
and have complex metalloclusters in their active site and require several maturation proteins
to attain their active form. The hydrogen-forming enzymes are inactivated by molecular oxygen
and must be separated from oxygen, to produce hydrogen.

The three main classes of hydrogen-forming enzymes are [FeFe]-hydrogenase,
[NiFe]-hydrogenase and nitrogenase

Nitrogenase

Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...

. These enzymes behave differently in
dark-fermentation with Clostridium and photo-fermentation with Rhodobacter.
The interplay of these enzymes are the key in hydrogen production by
fermentation.

Clostridium

The interplay of the hydrogen-forming enzymes in Clostridium is unique with little or no involvement of nitrogenase.
The hydrogen production in this bacteria is mostly due to [FeFe]-hydrogenase,
which activity is a hundred times higher than [NiFe]-hydrogenase and a thousand
times higher than nitrogenase. [FeFe]-hydrogenase has a Fe-Fe catalytic core with
a variety of electron donors and acceptors.

The enzyme [NiFe]-hydrogenase in Clostridium, catalyse a reversible oxidation of hydrogen.
[NiFe]-hydrogenase is responsible for hydrogen uptake, utilizing the electrons from
hydrogen for cellular maintenance.

In Clostridium, glucose is broken down into pyruvate and nicotinamide adenine
dicleotide (NADH). The formed pyruvate is then further converted to
acetyl-CoA and hydrogen by pyruvate ferredoxin oxidoreductase with the reduction
of ferredoxin. Acetyl-CoA is then converted to acetate, butyrate and
propionate.

Acetate fermentation processes are well understood and have a maximum
yield of 4 mol hydrogen pr. mol glucose. The yield of hydrogen from the
conversion of acetyl-CoA to butyrate, has half the yield as the conversion
to acetate. In mixed cultures of Clostridium the reaction is a combined production of
acetate, butyrate and propionate.
The organic acids which are the by-product of fermentation with Clostridium,
can be further processed as substrate for hydrogen production with Rhodobacter.

Rhodobacter

The purple non-sulphur bacteria Rhodobacter spharoids is able to produce
hydrogen from organic acids and ultra-violet light. The photo-system
required for hydrogen production in Rhodobacter (PS-I), differ from its
oxygenic photosystem (PS-II) due to the requirement of organic acids and
the inability to oxidize water.

In Rhodobacter, the hydrogen production is due to catalysis by nitrogenase.
The production of hydrogen by [FeFe]-hydrogenase is less than 10 times the
hydrogen uptake by [NiFe]-hydrogenase.

The interplay of hydrogenase and nitrogenase in this bacteria is responsible
for the production of hydrogen and require nitrogen-deficient conditions to
produce hydrogen.
The main photosynthetic membrane complex is PS-I which accounts for
most of the light-harvest. The photosynthetic membrane complex PS-II produces oxygen,
which inhibit hydrogen production and thus low partial pressures of oxygen most
be sustained during fermentation.

To attain high production rates of hydrogen, the hydrogen production by nitrogenase has
to exceed the hydrogen uptake by hydrogenase. The substrate is oxidized
through the tricarboxylic acids circle and the produced electrons are transferred
to the nitrogenase catalysed reduction of protons to hydrogen, through
the electron transport chain.

LED-fermenter

A cheap way to build a industrial-size photo-fermenter could be to build a fermenter with ultra-violet light emitting diodes (UV-LED) as light source. This design prevents self-shading within the fermenter, require limited energy to maintain photosynthesis and has very low installation costs. This design would also allow cheap models to be built for educational purpose.

Metabolic engineering

There is a huge potential for improving hydrogen yield by metabolic engineering. The bacteria Clostridium could be improved for hydrogen production by disabling the uptake hydrogenase, or disabling the oxygen system. This will make the hydrogen production robust and increase the
hydrogen yield in the dark-fermentation step.

The photo-fermentation step with Rhodobacter, is the step which is likely to gain the most from metabolic engineering

Metabolic engineering

Metabolic engineering is the practice of optimizing genetic and regulatory processes within cells to increase the cells' production of a certain substance. These processes are chemical networks that use a series of biochemical reactions and enzymes that allow cells to convert raw materials into...

. An option could be to disable the
uptake-hydrogenase or to disable the photosynthetic membrane system II (PS-II). Another improvement could be to decrease the expression of pigments, which shields of the photo-system.

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Introduction

Algaeic biohydrogen

Process requirements

Fermentation

Dark-fermentation

Photo-fermentation

Combined fermentation

Metabolic processes

Clostridium

Rhodobacter

LED-fermenter

Metabolic engineering

See also

External links