Review on Bio-hydrogen Production Methods

Abstract

Hydrogen is a promising replacement for fossil fuels as a long-term energy source. It is a clean, recyclable, high efficient nature and environmentally friendly fuel. Hydrogen is now produced mostly using water electrolysis and natural gas steam reformation. However, biological hydrogen production has substantial advantages over thermochemical and electrochemical. Hydrogen can be produced biologically by bio-photolysis (direct and indirect), photo fermentation, dark fermentation. The methods for producing biological hydrogen were studied in this study.

Introduction

I. INTRODUCTION

In recent years, the provision of high energy demands, as well as serious environmental challenges like greenhouse gas emissions and global warming, need the use of clean, renewable, and long-term energy sources [1]. Among various alternative clean fuels, hydrogen is a promising green fuel that is accepted as an environmentally safe, renewable, and high energy yield (122 kJ g-1) [2]. Hydrogen is one of the foremost abundant elements within the universe in its ionic form. It is an odorless, colorless, tasteless, and non-poisonous gas. When hydrogen is employed as a fuel, it generates no pollutants but produces water [3]. It also contents the highest energy per unit weight of any known fuel [4]. Hydrogen has the calorific value (higher heating value) of 141 MJ kg-1, which is the highest of all the known commercial fuels. But the lower heating value (120 MJ kg-1) is considered, 1 kg of hydrogen is equivalent to about 2.75 kg of gasoline [5]. Hydrogen is expected to have about 11% of the total renewable energy share of 36% by 2025 and up to 34% of the total renewable energy share of 69% by 2050 [6]. Hydrogen can be generated by thermochemical, electrochemical, and biological processes [2]. Both thermochemical and electrochemical processes are non-renewable in nature and depend on fossil fuels [7]. These methods not only consume energy but pollute the environment, so the depletion of fossil fuels has led the researcher to find renewable and non-polluting energy resources [8].

Biological hydrogen production is a promising alternative approach for the production of fuel from low cost, renewable, environmentally friendly resources, and non-polluting in nature [9]. Globally, the demand for hydrogen is increasing so the use of organic wastes as a substrate it is a renewable way for the production [10]. As cost of the substrate is a major factor in the economics of bio-hydrogen production, it is necessary to use less expensive and abundant feedstocks to keep the process affordable. Organic waste from industry and agriculture is not only used to generate green energy, but it also aids in bioremediation [11]. Lignocellulosic biomass, which makes up a major portion of municipal, industrial, and agricultural waste, is both renewable and inexpensive, making it ideal for biofuel generation [12]. The utilization of organic waste as a substrate to generate hydrogen provides a simultaneous solution of waste treatment along with clean energy generation [13-14]. Currently, biological hydrogen can be produced through different metabolic pathways, including direct

water bio-photolysis by green algae, indirect water bio-photolysis by cyanobacteria, photo-fermentation by sulfur-free purple photosynthetic bacteria, water gas exchange reaction to synthesize hydrogen by microalgae, and heterotrophic anaerobic bacteria by dark fermentation [15-16].

At present, there are numerous studies on several aspects of biological hydrogen production. Tami et al. [17] reviewed bio-hydrogen production by dark fermentative bacteria using starch-containing waste as a substrate. Vinod et al. [18] published a review on bio-hydrogen production from waste materials. Prakash et al. [19] reviewed bio-hydrogen production through dark fermentation.

However, a review article on bio-hydrogen production methods is limited in the literature. Based on these facts, the objective of this paper is to present an overview of biological hydrogen production methods by using organic waste biomass.

II. BIOLOGICAL HYDROGEN PRODUCTION METHODS

Bio-hydrogen production methods can be classified as

A. Direct bio-photolysis

This method is analogous to the processes found in plants and algal photosynthesis. In this process, solar energy is directly converted to H2 via photosynthetic reactions.

2H2O + light energy → 2H2 + O2

1. Organisms: Cyanobacteria and green algae are two types of organisms that aid in the generation of hydrogen through direct bio-photolysis [20]. Algae use photosynthesis to split water molecules into hydrogen ions and oxygen. The generated H+ is converted into hydrogen gas by the hydrogenase enzyme. Chlamydomonas reinhardtii is one of the best-known hydrogen-producing algae [15].
2. Advantage: The advantage of this method is that the primary feed is water, which is cheap and almost universally available (Table 1).
3. Disadvantages: Three characteristics that restrict hydrogen production by direct photolysis using green algae: (i) Solar conversion efficiency of photosynthetic system; (ii) H2 synthesis processes (i.e. the necessity to separate the processes of H2O oxidation and H2 synthesis); and (iii) bioreactor design and cost [21].

The simultaneous synthesis of hydrogen and molecular oxygen in direct bio-photolysis is a disadvantage that has resulted in the inhibition of hydrogen-related processes ranging from gene expression to hydrogenase catalytic activity. While constant purging of an inert gas (argon) to remove the produced oxygen could be a viable option, it is not cost-effective for long-term large-scale hydrogen generation. Several methods for improving H2 generation by green algae are currently under investigation.

B. Indirect Bio-photolysis

In indirect bio-photolysis, involves the separation of H2 and O2 evolution reactions into different stages, which are then linked via CO2 fixation and evolution. Cyanobacteria have unique characteristics in that they use CO2 in the air as a carbon source and solar energy. CO2 is first taken by the cells to form cellular components, which are then used to produce hydrogen. The reactions below describe the general mechanism of hydrogen generation in cyanobacteria:

12H2O + 6CO2+ light energy → C6H12O6 + 6O2

C6H12O6 + 12H2O + light energy → 12H2 + 6CO2

Key enzymes (nitrogenase and hydrogenase) in cyanobacteria carry out metabolic functions to generate hydrogen [22].

1. Advantage: One of the benefits of indirect bio-photolysis is that it requires minimal nutrients. However, adding trace metals like Ni2+ to the culture medium can help nitrogen-fixing cyanobacteria like Nostoc muscorum and Anabaena cylindrical grow faster and fix more CO2 [23]. Anabaena species and strains have been studied extensively due to their greater rates of H2 generation [21].

C. Fermentation

Fermentation is the process of generating energy by utilizing an endogenous electron acceptor derived from the oxidation of organic waste products by a variety of microorganisms. The fermentation process can be either aerobic or anaerobic in nature [24]. Organic waste materials can be fermented with microbes under anaerobic conditions to yield H2 and various organic alcohols or acids as by-products. Bio-fermentation can be classified into two categories, (a) Photo fermentation and (b) Dark fermentation [25].

1. Photo Fermentation: Photo-fermentation produces hydrogen by decomposing organic acids with the help of light-dependent, sulphur, and non-sulphur purple bacteria. Purple sulphur bacteria refers to a group of bacteria that can perform photosynthesis. Photobacteria are purple non-sulphur bacteria, which produces H2 mainly due to the presence of nitrogenase under nitrogen-deficient conditions which uses light energy and reduced compounds [4]. Photo fermentative bacteria produce H2 and CO2 by oxidizing organic acids such as acetic acid, propionic acid, butyric acid, lactic acid, and malic acid. As a result, the two-stage dark fermentation method is frequently followed by photo fermentation to obtain a larger H2 yield [26]. The generation of ATP by photophosphorylation provides the energy required for microbial development [27].

C6H12O6+6H2O→6CO2+12H2

2CH3COOH + 4H2O→8H2+ 4CO2

.a. Effect of Substrate: Carbon source is one of the most crucial aspects of producing sustainable biofuels. The benefits of such a sustainable process with high efficiency, cost-effective, and minimal pollution are highly dependent on selecting an appropriate feedstock [28].

b. Effect of Illumination: The most significant factor in the photo-fermentation process is light; specifically, the intensity, sources, and distribution of light. Because the generation of ATP in photosynthetic organisms is a light-dependent process, making efficient use of light is critical. In the nitrogen fixation process and subsequent hydrogen synthesis, a large amount of ATP is required to encourage electron transit [29].

c. Effect of Trace Metals and Minerals: Nitrogenase is an important enzyme in the metabolism of PNS bacteria. It is a binary enzyme and made up of two proteins: (i) a molybdenum and iron-containing protein (MoFe protein or dinitrogenase) and (ii) an iron-containing protein (Fe protein or dinitrogenase reductase). In the presence of both proteins, the nitrogenase complex is active [20]. The electron transfer chain (ETC) uses Fe ions as electron carriers, such as Fd and cytochromes. As a result, the availability of an adequate amount of molybdenum and iron in the medium is critical for photo-fermentation hydrogen production, particularly when wastewater is employed as a feedstock [30].

2. Dark Fermentation: Dark fermentation is one of the most well-known bio-hydrogen generation systems. H2 can be produced by anaerobic bacteria grown in the dark on carbohydrate-rich substrates [31]. Enterobacter, Bacillus, and Clostridium species are found to generate hydrogen [21]. Carbohydrates, specifically glucose, are the preferred carbon sources for fermentation processes that produce acetic and butyric acids as well as hydrogen gas [32]. The theoretical output of hydrogen can be determined based on the microorganisms metabolic pathway and the initial sugar concentration in the fermentation medium. Hexose to acetic acid, hexose to butyric acid, and acetate to ethanol are three thermodynamically favorable dark fermentation metabolic routes for the conversion of organic substrates to bio-hydrogen [33-34]. Depending on the microorganisms and substrate, a mixed gas containing H2 and CO2 is produced, along with additional trace gases such as CH4, CO, and H2S [35-38]. Bacteria can convert glucose to pyruvic acid via glycolytic pathways by simultaneously generating ATP from ADP and NADH. With the help of pyruvate ferredoxin- oxidoreductase and hydrogenase, pyruvic acid is further converted to CO2 and H2. The level of bio-hydrogen generation can be determined by the conversion of pyruvate to acetyl-CoA and then to acetate, butyrate, and ethanol. The availability of hydrogen from glucose is determined by the ratio of butyrate to acetate. H2 can be produced from acetic acid and butyric acid in Eq. (a) and Eq. (b) [39-40]. H2 production from glucose is also represented in Eq. (c) and Eq. (d).

C6H12O6+ 2H2O→4H2+ 2CH3COOH + 2CO2                                                           (a)

7C6H12O6+ 6H2O→24H2 + 6CH3CH2CH2COOH + 18CO2                                      (b)

C6H12O6+ 4H2O→2CH3COO?+ 2HCO3-+ 4H+ + 4H2                                                 (c)  

C6H12O6+12H2O→6HCO3- + 12H2 + 6H+                                                                      (d)

a. Organisms: Some of the bacterial communities which generate H2 include anaerobes, such as Clostridia, methanogenic bacteria, and archaea, facultative anaerobes, such as Enterobacter, Escherichia coli, and Citrobacter, and some aerobes, such as Alcaligenes and Bacillus.  Some other bacterial species that aid in hydrogen production belong to the group of Bacillaceae, Gram-positive cocci, such as Micrococcaceae and Peptococcaceae.

III. FACTORS INFLUENCING DARK FERMENTATION

Factors that influence the dark fermentation are temperature, pH, hydraulic retention time, volatile fatty acids, and partial pressure of H2 and inorganic content.

A. Effect of temperature

The process temperature has a direct impact on bacterial growth and metabolic activity, as well as the rate of hydrogen production. Bacteria that produce hydrogen by dark fermentation include those that are mesophilic (25–40°C), thermophilic (40–65°C), extreme thermophilic (65–80°C), and hyper-thermophilic (>80°C). The temperature range of 35–55°C is ideal for dark fermentation processes. When bacterial species are exposed to high temperatures, they produce more bio-hydrogen than when they are exposed to low temperatures [41].

B. Effect of pH

The pH level has the potential effect to improve biohydrogen generation. Microorganisms enzymatic activity during the bioconversion process works best at a certain pH range. According to most experts, the pH level of 5.5 is ideal for biohydrogen generation [42-44].

C. Effect of partial pressure

The aspect that influences hydrogen generation is the partial pressure of hydrogen inside the bioreactor. When the partial pressure inside the bioreactor drops, the amount of hydrogen transferred from the liquid to the gas phase increases [45-46]. The activity of hydrogenase is influenced by the reversible oxidation-reduction of ferredoxin. As a result of the higher hydrogen content in the liquid phase, ferredoxin oxidation becomes unfavorable, lowering ferredoxin [47], and clearing the way for bio-hydrogen synthesis.

D. Effect of Hydraulic retention time

The hydraulic retention time (HRT) is another crucial factor in biohydrogen synthesis by dark fermentation. Short HRTs are used to clean up the methanogens in a typical continuous stirred-tank reactor (CSTR) system by choosing acid-producing bacteria [48]. Kim et al. suggest that [49], in a CSTR system, a short HRT of fewer than 3 days could enhance hydrogen synthesis. For hydrogen generation, simultaneous effects of pH and HRT are occasionally reported. In general, a short HRT can result in a low pH level during anaerobic activity [50]. The dilution rate, on the other hand, has an impact on the hydrolysis of organic wastes [51].

Table1. Advantages and disadvantages of different hydrogen production processes

Conclusion

For the future of the zero-carbon economy, hydrogen production through biological processes plays a significant role as it can utilize sustainable energy sources, and it will also contribute to the world’s economy long-term viability by ensuring a steady supply of energy and reducing future greenhouse gases. This review study provides an overview of biological hydrogen production methods by using organic waste biomass. Among the various methods, In the photo fermentation process use of waste resources such as wastewater which is a suitable carbon source for PNS bacteria. This bacteria allows for long-term hydrogen synthesis from low-cost substrates and abundant solar energy. This will minimize the issues concerning with fossil fuel and ultimately benefits the environment from pollution.

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Copyright

Copyright © 2022 Syed A. R. Ahmad, Mritunjai Singh, Archana Tiwari. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.