Revising the dark fermentative H2 research and development scenario – An overview of the recent advances and emerging technological approaches
Introduction
The growing concerns about the anthropogenic CO2 emissions and depletion of natural resources have resulted in an enormous search for sustainable energy resources [[1], [2], [3]]. Therefore, a wide variety of clean technologies are being investigated [4,5]. Hydrogen (H2) is seen as one of the most appealing energy resources as a result of its qualities such as high energy content (120 kJ g−1), its production using various techniques (e.g. steam reforming, gasification, water electrolysis and fermentation), its carbon-sequestration abilities during downstream processing, and its diverse industrial applications [[6], [7], [8], [9], [10]].
Hydrogen is expected to play a pivotal role in decarbonising the energy and transport sector [11]. The potential of a H2-driven economy is already being recognized in several countries. For example, a total of 8000 fuel cell vehicles are now registered with the International Energy Agency, of which 4500 vehicles are coming from the United States and another 2500 vehicles from Japan [12]. It is estimated that more than 10 000 H2 fuel cell-powered forklifts are already in use in several warehouses in the United States [13]. Furthermore, 192 fuel cell vehicles are currently running under demonstration projects in Europe and it is anticipated that around 350 000 vehicles will be sold to the general public by 2020 [14]. Technology roadmaps for H2 and fuel cells have been established already in countries like Japan, China and the United States, to accelerate the industrialization of H2 related technologies [15]. According to recent reports, the global H2 market was estimated at 129 billion US dollars in 2017 and is expected to increase to 183 billion US dollars by 2023 [16].
Currently, steam reforming and gasification of fossil-derived fuels are still considered the primary sources of H2 worldwide [11]. However, these processes undermine the purpose of using H2 as a clean technology, as more CO2 is emitted during the processing of fossil fuels [17,18]. It is therefore vital for a H2 driven-economy to make environmental and economic sense [19]. Hydrogen from waste biomass represents an economical and environmentally-friendly approach because this process uses diverse feedstocks [[20], [21], [22], [23]]. Amongst the H2 bioprocesses, DF is considered as the most promising clean technology of the 21st century because it can valorise diverse feedstocks including waste materials under mild fermentation conditions [[24], [25], [26]]. However, the establishment of a large-scale DF process has not yet been realized due to the incomplete conversion of feedstocks that results in low yields [27]. Therefore, this calls for the implementation of robust technologies which will fast-track the development of this process.
In recent years, there has been a surge in biofuel development initiatives in Sub-Saharan African countries with the aim of boosting economic growth, energy security, and rural development within the region [28]. The development of clean energy is fuelled by several factors such as high availability of non-arable land, abundance in biomass resources, and warm climate [28]. Biofuels such as bioethanol and biodiesel are being explored in many Sub-Saharan African nations, and it is hoped that these initiatives will lead to their commercialization [4]. DF is also receiving significant attention due to its socio-economic benefits, and the fact that this process can be incorporated into a biorefinery concept.
As the body of knowledge is constantly expanding in DF, it is imperative to update the scientific community with novel and emerging technologies that can fast-track the advancement of this technology. This article examines the novel technologies that are gaining prominence in DF based on recent scientific publications. Biogenic H2 enhancement methods such as cell immobilization, nanotechnology, mathematical optimization tools, and biogas upgrading, are reviewed in this article. A section that highlights the potential of biofuels in Sub-Saharan Africa including South Africa is included. Finally, conclusions and suggestions for further research in DF, particularly from organic wastes, are provided.
Section snippets
An overview of the process barriers facing the DF process
Despite the efforts that have been undertaken over the past decade, DF is still hindered by low yields which delay its industrialization (Fig. 1). The substrates are partially converted into H2 and remain in the medium in the form of volatile fatty acids (acetic acid, butyric acid, propionic acid, etc.) and alcohols (butanol, ethanol, propanol, etc.) [[29], [30], [31]]. These by-products shift the reactions from acidogenesis to solventogenesis, resulting in low H2 yields [[32], [33], [34], [35]
Cell immobilization
Studies are now using cell immobilization to improve the H2 yields and also minimize the accumulation of inhibitors during DF [59,60]. This approach favours the DF systems by: (i) maintaining high cell concentrations; (ii) stabilizing the fermentation pH; (iii) resisting the effect of H2-consumers; (iv) enabling an easier downstream process; and (v) allowing the reusability of cells [61,62]. Immobilization methods involves adsorption, entrapment, encapsulation, cross-linking, and covalent
The potential of bioenergy in Sub-Saharan Africa
Several Sub-Saharan African countries are listed amongst the world's fastest-growing economies [411,412]. These nations need a thriving energy sector to cater for this rapid industrialization and population growth [411]. Biofuels will serve as a catalyst to strengthening the region's energy sector, infrastructural development programmes, rural development, and economic growth. In recent years, efforts have been made to fast-track the development of biofuel-related technologies in this region.
The way forward: a proposed roadmap for surpassing the current process barriers
In spite of the enormous amount of research that has been conducted over the past years, major hurdles still need to be overcome to realize the potential of DF process as an alternative fuel. The proposed optimal technologies are mostly conducted under laboratory-scale conditions and have not been evaluated at large-scale. Secondly, scientists are currently investigating the individualistic effects of these technologies on DF pathways but are yet to examine the synergistic interactions of these
Conclusions and recommendations for future studies
This article provides a critical review of recent technological methods that are used to enhance H2 yields in DF process. Herein, novel biogenic H2 optimization methods such as cell immobilization, nanotechnology, empirical optimization tools, and biogas upgrading from renewable H2, are suggested to be the most promising methods that can be used to overcome the technical barriers facing DF process. However, most of the studies discussed in this article are still at the infancy stage and are far
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