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Production of Biofuels
Published in K.A. Subramanian, Biofueled Reciprocating Internal Combustion Engines, 2017
Photofermentation is a process in which photosynthetic bacteria convert organic materials and biomass into hydrogen and carbon dioxide by using solar energy. The process takes place under anaerobic conditions. The optimal temperature is about 30°C–35°C and a neutral pH of 7.0. Nonsulfur purple bacteria perform this process using simple organic acids. In the absence of nitrogen and solar energy, organic acids or biomass are converted into hydrogen as described in Equation 2.13 (Das et al., 2008).
Biohydrogen Production by Photobiological Processes
Published in Debabrata Das, Jhansi L. Varanasi, Fundamentals of Biofuel Production Processes, 2019
Debabrata Das, Jhansi L. Varanasi
Certain photoheterotrophic bacteria have the capability to produce biohydrogen under anoxic conditions by a process known as photofermentation. These bacteria comprise of light-harvesting photosystems (bacterial photosystems) similar to photosynthetic organisms. However, they are incapable of water oxidation and thus in turn oxygen evolution due to the lack of a PSII reaction center (Levin 2004). Two classes of photoheterotrophic bacteria have been observed that possess photofermentative capabilities: green bacteria and purple bacteria (McKinlay and Harwood 2010). They evolve hydrogen with the help of nitrogenase enzyme under nitrogen-limiting conditions by using light energy and organic compounds as substrates (Manish and Banerjee 2008). The most widely studied organisms among the photofermentative bacteria are the purple non-sulfur (PNS) bacteria such as Rhodobacter sp., Rhodopseudomonas sp., and Rhodospirillum sp. (Basak and Das 2007). Compared to biophotolysis, photofermentation has several advantages which include (1) higher yields, (2) no oxygen evolution that facilitates the activity of key H2 producing enzymes, (3) use of broad spectrum of light, and (4) capability for using exogenous organic substrates thereby aiding in bioremediation. Compared to biophotolysis, PNS bacteria require less free energy (~8.5 KJ mol−1 H2 for lactate) to produce hydrogen from organic substrates (Basak and Das 2007). Moreover, several PNS bacteria are reported to produce hydrogen using inorganic substrates such as S2O32−, H2S, and Fe2+ (McKinlay and Harwood 2010). Another major advantage of photofermentation is the ability to drive thermodynamically unfavorable reactions by using solar energy (i.e., it can potentially convert an entire substrate to hydrogen) (Azwar et al. 2014). Since these bacteria can metabolize short chain organic acids such as acetate, propionate, and butyrate, the photofermentation process can be integrated with the dark fermentation process to obtain higher substrate conversion efficiency and overall hydrogen yields (Guwy et al. 2011). In spite of several advantages, the production volume of hydrogen is still too low. One of the possible reasons is the presence of uptake hydrogenases (which consume hydrogen during nitrogen fixation) that lower the maximum possible biohydrogen yields. By eliminating the uptake hydrogenases from the PNS bacteria using genetic tools, the hydrogen yields can be significantly enhanced. Moreover, as in case of biophotolysis, several attempts have been made to improve the light conversion efficiencies of photosynthetic bacteria to further improve the biohydrogen yields (Table 7.3).
Green hydrogen production by Rhodobacter sphaeroides
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2023
Dahbia Akroum-Amrouche, Hamza Akroum, Hakim Lounici
Photofermentation is a process that takes advantage of solar energy for the growth of photo-heterotropic anoxygenic microorganisms. However, unlike photosynthesis, which uses water as an electron donor, photofermentation instead uses organic molecules to provide the electrons that are required for growth. In addition, a very large variety of substrates can be used as electron donors by photosynthetic bacteria (Das and Veziroglu 2001) such as organic acids (purple non-sulfur bacteria “PNSB”) or reduced sulfates (purple or green sulfur bacteria) that will be converted in H2 and CO2/oxidized sulfate compounds anaerobically and under anoxic conditions. R. sphaeroides, R. capsulatus, Thiocapsa roseopersicina, Chloroflexus aurantiacus, and Chloroflexus aggregans are cited as examples.
Waste into energy conversion technologies and conversion of food wastes into the potential products: a review
Published in International Journal of Ambient Energy, 2021
Jeya Jeevahan, A. Anderson, V. Sriram, R. B. Durairaj, G. Britto Joseph, G. Mageshwaran
Hydrogen has now become one of the promising alternative fuels to fossil fuels, due to its many advantages. It has high energy content (approximately 122 kJ/g), which is about 2.75 times higher than that of the hydrocarbon fuels. On combustion, it produces water, which makes it a clean and environment-friendly fuel. It can be used in fuel cells for direct conversion into electricity (Han and Shin 2004). Hydrogen is conventionally produced by reforming methods such as steam reforming of methane and other hydrocarbons, partial oxidation of fossil fuels and autothermal reforming, desulphurisation, pyrolysis, plasma reforming, aqueous phase reforming and ammonia reforming. Other hydrogen generation technologies that are capable of producing hydrogen from the renewable resources such as biomass and water are getting attention in order to reduce the dependence of fossil sources. There are also non-reforming methods, such as gasification and biological process, for hydrogen production (Holladay et al. 2009). Biohydrogen can be produced using four processes: (a) direct photolysisof water using algae/cyanobacteria, (b) photofermentation using photosynthetic bacteria, (c) dark fermentation using anaerobic bacteria and (d) hybrid systems. Among all these processes, dark fermentation is considered as the most feasible technology as no external energy or light source is required. This makes dark-fermentation process low cost method (Kothari et al. 2012) (McKinlay and Harwood 2010). The process conditions of each fermentation processes are summarised in Table 5.