Gasification of food waste, sewage sludge, biomass.
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Pyrolysis of biomass.
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Biological processes:
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1. anaerobic metabolism (dark fermentation);
2. photosynthetic hydrogen production (direct biophotolysis);
3. indirect hydrogen production;
4. photo-fermentation;
5. carbon monoxide metabolism, water–gas-shift reaction.
The most widely used process for producing hydrogen is steam reforming of natural gas which is the least expensive process and produces 97% of the hydrogen made.
The other two commercial processes are the gasification of coal (Chapter 4) and the partial oxidation of heavy oils. Clearly none of these processes are renewable and sustainable.
There are a number of renewable methods of producing hydrogen but to date these are all at the experimental stage (Kapdan and Kargi, 2006). The theme of the book is biofuels and the production of energy from biological materials and there- fore this section will concentrate on the biological production of hydrogen. As can be seen from the list there are a number of biological processes which result in hydrogen.
Production of hydrogen from biological material
Gasification of food waste, sewage sludge and biomass
If biomass is gasified at temperatures above 700°C, a mixture of gases and charcoal is produced. The gas produced contains mainly hydrogen, carbon monoxide, meth- ane, carbon dioxide and nitrogen. More hydrogen can be produced by a water-shift reaction where carbon monoxide is reacted with water to form carbon dioxide and hydrogen:
Biomass+ heat + steam → H2+ CO + CO2+ CH4 (5.5) + hydrocarbons + charcoal
Water-shift reaction
CO+ H2O= CO2+ H2 (5.6)
Using a fluidized bed gasifier with a catalyst it has been possible to obtain 60%
hydrogen production. The main problem with gasification is the formation of tar, which can be minimized by gasifier design, control and additives (Ni et al., 2006).
The hydrogen can be separated from the other gases by pressure swing adsorption.
Pyrolysis
Pyrolysis involves the heating of biomass at 300–500°C at 0.1–0.5 MPa in the absence of air. This produces liquid tars and oils, charcoal and gases including hydrogen, methane, carbon monoxide and carbon dioxide. The water-soluble pyrolysis oil can
Table 5.5. Microorganisms capable of producing hydrogen. (Adapted from Das and Veziroglu, 2001.)
Microbial type Example Comments
Green algae Scenedesmus obliquus Produce hydrogen from water using Chlamydomonas reinhardtii solar energy. Inhibited by O2 Cyanobacteria Anabena azollae Nitrogenase enzyme produces
(heterocysts) Nostoc muscorum hydrogen, the enzyme in heterocyst protected from O2 inhibition. Requires light.
Fixes nitrogen
Cyanobacteria Plectonema boryanum As above, but no protection of (non-heterocysts) Oscillotoria limnetica nitrogenase
Photosynthetic Rhodobacter sphaeroides Can use waste, require light bacteria Chlorobium limicola
Anaerobic bacteria Clostridium butylicum Functions in the dark, can use a (fermentation) Desulfovibrio vulgaris variety of substrates
be used for hydrogen production by catalytic steam reforming (water shift). Methane can be steam reformed to produce more hydrogen:
CH4+ H2O= CO + 3H2 (5.7)
The carbon monoxide can also be used to form hydrogen in a water-shift reaction:
CO+ H2O= CO2+ H2 (5.8)
Biological production of hydrogen
The biological production of hydrogen has been known since the early 1900s and the enzymes involved were discovered in the 1930s. Hydrogen production has been found in many prokaryotes, green microalgae, and a few eukaryotes as shown in Table 5.5 (Das and Veziroglu, 2001; Happe et al., 2002).
The production of hydrogen is due to the presence of two enzymes either nitro- genase or hydrogenase in the organism. Nitrogenase has the ability to use ATP and electrons to reduce substrates including protons to hydrogen gas and has been found in photoheterotrophic bacteria such as Rhodobacter sp.:
(5.9) Hydrogenases have been found in a large number of green microalgae such as Chlamy- domonas reinhardtii and Chlorococcum littorale, anaerobic bacteria such as Clost- ridium sp. and Cyanobacteria sp. Hydrogenases can be either uptake or reversible hydrogenases and can be divided into three classes based on their metal composition.
These classes are Ni/Fe, Fe and metal-free where the Fe hydrogenase has a unique active centre giving the enzyme a 100-fold higher activity:
2e-+ 2H++ 4ATP = Hnitrogenase 2+ 4ADP + 4Pi
(5.10) These hydrogenases and nitrogenase are responsible for hydrogen production by a number of microorganisms.
Anaerobic metabolism (dark fermentation)
Anaerobic breakdown of organic material can yield hydrogen in a number of cases.
Anaerobic digestion of sewage sludge by a consortium of microorganisms can produce small amounts of hydrogen in addition to the major product biogas (section ‘Anaerobic digestion’, Chapter 5, this volume). Anaerobic digestion or fermentation of organic compounds by Clostridium sp. and some microalgae was also found to produce hydrogen under specific conditions. One of the best-known examples is the production of acetone and butanol by Clostridium acetobutylicum growing anaerobically on glucose (molasses). This process was used from 1915 until the 1950s to produce acetone and butanol for the munitions and chemical industries. The biological process has now been replaced by the production of acetone and butanol from petrochemicals. However, other products are formed along with acetone and butanol and include ethanol, butyrate, acetic acid, carbon dioxide and hydrogen. Depending on the culture conditions, and the strain used, the amount of the various products formed can vary including the amount of hydrogen produced. The pathway involved in the production of acetone and butanol is shown in Fig. 5.11. It has been estimated that 2 mol of hydrogen are formed per mole of glucose consumed (Ni et al., 2006).
Green algae such as C. reinhardtii respond to anaerobic conditions or nutrient reduction (sulfur) by producing hydrogen. The induction of anaerobic conditions switches the organism’s metabolism to fermentative which produces a number of harmful end products such as ethanol and organic acids. Under these conditions hydrogenase activity is inhibited and hydrogen acts as an electron sink, avoiding some of the problems of aerobic conditions. The low sulfur condition causes the downregulation of the photosystem II where the lack of sulfur-containing amino acids blocks the repair cycle for photosystem II.
Photosynthetic hydrogen production (direct biophotolysis)
In photosynthesis, solar energy is used by photosystem II to split water and release oxygen, electrons and protons. Photosystem I uses solar energy to produce the reducing power required to fix carbon dioxide, and the electrons are passed along the electron transfer system, eventually generating ATP. In the direct use of solar energy to convert water into hydrogen the electrons are transferred along the electron transfer chai until the penultimate step catalysed by ferredoxin, where the electrons are transferred to a hydrogenase, converting protons into hydrogen (Fig. 5.12). These reactions are carried out by green microalgae and blue-green Cyanobacteria sp.
However, hydrogenases are inhibited by oxygen so that the concentration of oxygen needs to be kept below 0.1%. In the case of C. reinhardtii oxygen is removed
H2hydrogenase« 2H++ 2e
by respiration but because substrate is consumed the efficiency is low. In some cyano- bacteria such as Anabena cylindrica photosynthesis is split between two types of cells.
Photosystem II functions in the vegetative cells whereas the heterocysts contain the carbon-fixing portion and a hydrogenase enzyme which is protected from the inhibi- tory effects of oxygen by a thick cell wall.
Ferredoxin Hydrogen
Butyryl-phosphate
Butyric acid
Butyryl-CoA
Butyaldehyde
Butanol Crotonyl-CoA 3-Hydroxybutyryl-CoA
Acetoacetyl-CoA Acetyl-phosphate
Acetate CO2
Glucose
Pyruvate
Acetyl-CoA Acetaldehyde
Ethanol
Acetoacetate
Acetone
Fig. 5.11. The pathway involved in the production of acetone and butanol by Clostridium acetobutylicum.
PS 1 & PS 11 2e– Ferredoxin 2e– Hydrogenase
H2O + H+ 2H+
H2
O2
Fig. 5.12. The direct production of hydrogen using light carried out by green algae and Cyanobacteria.
Indirect hydrogen production
In this case, growing in the light photosynthesis is used for growth and to store car- bohydrates. When the organism is switched to aerobic dark conditions the stored carbohydrates or cell material is metabolized in the same way as in Fig. 5.12, yielding hydrogen. This type of two-stage process has been observed in Cyanobacteria sp.
Photo-fermentation
Some photoheterotrophic bacteria (purple non-sulfur bacteria) such as Rhodobacter sp. and Rhodospirillum sp. convert organic acids in the light into carbon dioxide and hydrogen (Fig. 5.13). The key enzyme in these organisms is nitrogenase which requires ATP to produce hydrogen. The nitrogenase is inhibited by oxygen, ammonia and high nitrogen to carbon ratios so that oxygen-free conditions are required.
Carbon monoxide metabolism (water-shift reaction)
Some photoheterotrophic bacteria, for example Rhodospirillum rubrum, can metab- olize carbon monoxide in the dark in a reaction similar to the water-shift reaction:
CO+ H2O= CO2+ H2 (5.11)