Laura Fonseca, Process Engineer, Aqua Enviro/SUEZ
Hydrogen production and use have been identified as potentially significant contributors to Net Zero energy strategies around the world. Hydrogen is considered a clean fuel as it leaves only water after combustion, it can be used in the production of electricity, fuel cells and as a raw material in chemical industries (IEA – International Energy Agency, 2019).
The source of produced hydrogen is extremely important to ensure the sustainability of the value chain, and several routes are available for producing biohydrogen. These include steam reformation of biomethane produced by anaerobic digestion of organic waste, the fermentation of organic materials by bacteria under light (photo-fermentation) or dark (dark fermentation) conditions, and the breakdown of acetic acid via microbial electrolysis cells. Extensive information about EU and US research projects on hydrogen from sustainable sources is made available by the European Technology and Innovation Platform, ETIP Bioenergy (https://www.etipbioenergy.eu/).
Dark fermentation is a pre-commercial technology which resembles the well-established anaerobic digestion process. While in a conventional AD reactor the two major microbial consortia, the acidogenic and methanogenic microorganisms, are kept together in balanced environment, in dark fermentation different operational conditions are applied to decrease methanogenic activity to avoid consuming the hydrogen that has been produced. Techniques directed to inhibit methanogenic microorganisms include heat treatment of the inoculum to select for certain bacteria, reducing the hydraulic retention time (HRT), using a separate reactor or reducing pH.
“In dark fermentation different operational conditions are applied to decrease methanogenic activity and avoid consuming the hydrogen that has been produced”
Essentially, the dark fermentation process converts organic matter in two stages: Hydrolysis, where molecules are broken down, and Acidogenesis, where the hydrolysis products are converted into carbon dioxide, hydrogen, ammonia, alcohols and organic acids (Bastidas-Oyanedel et al., 2015). These are also the first two steps in anaerobic digestion, which is why various strategies need to be implemented to allow hydrogen to be extracted prior to its biological conversion to methane.
Figure 1 Integrated hydrogen and methane production in a two-stage system (Tapia-Venegas et al., 2015)
Compared to the other biological processes for hydrogen production, the main advantages of dark fermentation is the simplicity of the reactors, the capability to take place without requiring light or strict anaerobic conditions, the low energy input and the potential of using waste as feedstock. However, the low hydrogen yield and productivity rates are its main challenges. Randolph & Studer (2017) estimated a cost of hydrogen of $8.56/kg H2 from dark fermentation, although any credits from by-products such as electricity and chemical compounds could help reduce costs to as low as $3/kg H2.
Hydrogen produced in dark fermentation can be blended with biogas to form biohythane, a blend of 10% H2, 30% CO2 and 60% CH4. This is considered an enhanced combustible as it has a higher energy density compared to biogas (23.8 MJ/m3 versus 22.6 MJ/m3 for biogas) and can also be considered as biosyngas, finding use as a feedstock or precursor for a wide range of possible fine chemicals and/or liquid fuels (Cavinato et al., 2012).
The necessity to improve system efficiency has driven an increase of research on this topic during the last decade, mostly focused on operational conditions. Assays like the Bio-Hydrogen Potential (BHP) batch test can be very helpful for guiding the conditions for dark fermentation. The test can help to evaluate inoculum suitability, get a sense of the possible process performance with options (for instance, different organic loads, feedstock types, pre-treatment processes, etc.), check possible inhibitions and determine achievable specific hydrogen and methane productions (Cavinato et al., 2012). Moreover some studies have monitored the concentration of Clostridium perfringens sp as the most dominant hydrogen producing culture to evaluate the suitability of different inoculum and pre-treatments (Pecorini, Baldi and Iannelli, 2019).
“Assays like the Bio-Hydrogen Potential (BHP) batch test can be very helpful for guiding the conditions for dark fermentation”
Reactor configuration is also an important factor in improving process efficiency. On one hand, a low HRT (hydraulic retention time) of between 6 to 12 hours is preferred to control VFA and methane production. On the other hand, the control of SRT (solids’ retention time) achieved in reactors such as membrane bioreactors, fixed film or microbial granulation systems is important to prevent biomass washout. A comprehensive review and comparison of different reactors was completed by Carolin Christopher et al. in 2021.
Since hydrogen corresponds to only 4% of the total substrate mass consumed, dark fermentation can produce a number of other compounds of interest – such as acetic acid, which is one of the main products in the liquid phase and is widely used in the synthesis of molecules such as vinyl acetate, itself used for production of paper bags, adhesives, paints, and textiles among others. Bastidas-Oyanedel et al., 2015 explored the role of dark fermentation as a core bioprocess in a biorefinery concept as shown in the figure below.
Figure 2 Biorefinery concept taken from Bastidas-Oyanedel et al., 2015
“Dark fermentation could play a central role in the future circular economy of bioresources” – Bastidas-Oyanedel et al., 2015
It is a technology able to produce a non-polluting fuel such as H2, flexible enough to be integrated in the current infrastructure of natural gas and anaerobic digestion facilities, whilst retaining the potential to sit at the heart of a biorefinery acting as a renewable source of raw materials for the chemical industry.
Our experience and laboratory facilities can be instrumental in providing support on Net Zero strategies and biohydrogen production, from initial biohydrogen potential (BHP) testing, larger lab-scale feasibility studies and even technical and economic evaluations. Furthermore, our expertise in metagenomics (the genetics of populations) can provide an essential understanding of the types and abundance of microbes present in biological reactors, giving valuable data that can inform operational approaches and process optimisation.
Aqua Enviro is a specialist environmental consultancy, conference organiser and training provider. Aqua Enviro has its own laboratory, developing solutions for even the most complex issues. Water, wastewater, bio-resources and organic waste are just some of the sectors that our services cover. Aqua Enviro has been a proud member of the SUEZ group of companies since 2016.
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