Synthesis gas (syngas) fermentation represents a promising biological method for converting industrial waste gases, particularly carbon monoxide (CO) and carbon dioxide (CO2) from industrial sources (e.g. steel production or municipal waste gasification), into high-value products such as biofuels, chemicals, and animal feed using acetogenic bacteria. This review identifies and addresses key challenges that hinder the large-scale adoption of this technology, including limitations in gas mass transfer, an incomplete understanding of microbial metabolic pathways, and suboptimal bioprocess conditions. Our findings emphasize the critical role of microbial strain selection and bioprocess optimization to enhance productivity and scalability, with a focus on utilizing diverse microbial consortia and efficient reactor systems. By examining recent advancements in microbial conditioning, operational parameters, and reactor design, this study provides actionable insights to improve syngas fermentation efficiency, suggesting pathways towards overcoming current technical barriers for its broader industrial application beyond the production of bulk chemicals.

Over the duration of the IEA Bioenergy triennium 2022 to 2024, a consortium of IEA Bioenergy Tasks – 32, 33, 34, 36, 37, 39, 40, 42, 44 and 45 – collaborated on an inter-task project called Synergies of green hydrogen and biobased value chains deployment. Hydrogen is a very crosscutting topic and the strategic inter-task project is a collaborative effort of the IEA Bioenergy TCP Tasks and also in collaboration with the Hydrogen TCP. The objective of the project was to identify and assess technologies for producing hydrogen from biomass as well as synergies in the deployment of green hydrogen and biobased value chains that can enhance the use of biobased value chains in the energy system. The descriptions of technologies and concepts - including 1) technology readiness and economic fundamentals and 2) climate effects and role in the energy system - are done through case studies. This serves to increase visibility and share state-of-the-art knowledge of promising applications. The report on hand looks into types of technologies for producing biohydrogen and their respective technology readiness level. It summarizes the findings of the work package 2 “Case studies on hydrogen from biomass” and provides a synthesized view on promising biomass technologies, major drivers and barriers for their deployment, and measures to overcome potential barriers. The considered case studies and their respective findings show that, there are various activities and projects in different parts of the world that look into the production of biomass-based hydrogen. The project developers mainly come from the fuel industry. Early stage concepts and their development is driven by research. All the presented production concepts are still under development and none of them has reached commercialization. The presented concepts are in the TRL level range of 4-7 and many of them show the “weakest link” in complete integrated operation for hydrogen production. For consistently evaluating the status of development a methodological framework has been developed addressing the TRL of key process components: feedstock handling, conversion processes, upgrading, and integrated operation. Each component is given a weight based on its relative importance in the overall technology resulting in an overall weighted average of technology development. 

With regards to the techno-economic performance of the different biohydrogen pathways first results show that biohydrogen production prices are competitive with production prices for power-based hydrogen. From a system perspective several of the biomass-based hydrogen production concepts also generate additional value-added commodities such as biochar, biocarbon, biomethane etc. This adds flexibility, resilience and likely also improved economic performance. Many of the concepts also generate a stream of CO2, what opens for opportunities to obtain negative CO2-emissions. Hence, biohydrogen can add to the portfolio of renewable hydrogen provision while at the same time allowing for additional energy and climate system services as carbon capture and storage. This makes it a relevant concept for further consideration within scenarios for reaching climate-neutrality.

This study evaluates the biological water-gas shift reaction of syngas into hydrogen (H2) using natural microbial consortia from wastewater treatment facilities. Various inoculum pretreatment techniques to inhibit methanogenesis and different operational conditions such as temperature, pH, and inoculum concentrations were tested to improve H2 production. An inoculum heat pretreatment of 85 °C for 30 min followed by a fermentation temperature of 65 °C and an initial pH value of 9, resulted in the maximum CO-to-H2 conversion (2.35 ± 0.00 mmol) in which Firmicutes dominated with a total inhibition of methanogens, such as with the chemical inhibition treatment. Additionally, volatile fatty acids (VFA) production was observed, being influenced by the pre-treatment. This study highlights the potential of natural consortia for efficient, cost-effective and scalable syngas fermentation processes, offering new insights into the bioconversion of H2 without costly chemical inhibitors. 

Bioflex - biobased energy carriers and their contribution to a flexible energy system

Bioflex aims at increasing the flexibility, redundancy, and robustness of the energy system by integrating biobased energy carriers (biohydrogen/biogas) with electrolytic hydrogen. By combining 2 energy carriers, the project intended to investigate possible synergy effects between the different production pathways. The combination of energy carriers is expected to generate 3 main effects: more efficient resource utilization, increased share of biobased energy and increased flexibility in local energy systems.

The project performed a lab and pilot study of a two-stage bioprocesses, investigated synergy effects and flexibility in the interaction between the bioprocesses and electrolysis, techno-economic analysis of the system and an actor and stakeholder analysis. The goals of the project were to 1) demonstrate the two-stage bioprocess in pilot scale (TRL 5), reduce the hydraulic retention time (HRT) for biogas production by 50% and optimize nutrient supply, and 2) formulate an implementation plan and recommendations to the stakeholders.

The results from the lab and pilot study show that biohydrogen and biogas can be efficiently produced in a two-stage bioprocess and that this leads to higher carbon utilization (from substrate), higher energy recovery and lower emissions (from digestate). Comparisons with existing biogas production with the same process water indicate that the two-stage bioprocess in total delivers equal amounts of biogas compared to the base case, but more energy is produced in total since biohydrogen is also produced. Calculations with linear scaling would give approx. 30% more energy already in the first bioreactor. That scaling up in reality would be linear is however unlikely where a number of other conditions come into play. Therefore, this process needs successively to find key parameters for scaling effects.

Calculations on integration with electrolytic hydrogens show that it is beneficial to use waste heat from electrolysis to heat the bioprocesses, since heat costs are otherwise a large cost item for the two-stage bioprocess.

The techno-economic calculations show that the major cost driver for the entire system is the electrolysis system. The payback period is also highly dependent on the selling price of hydrogen. If the selling price of hydrogen is high, the payback period will be shorter.

The results from the stakeholder mapping and scenario analysis show that: 1. Collaborations are desired for sharing the risk when investing in this type of new technology. In particular, clear collaborations are required when it comes to symbiosis solutions. 2. Clear prerequisites for policy and regulations are requested from many parts of the value chain. 3. Biohydrogen is a niche and its possible position in the market needs further investigation. 4. The two-step bioprocess combined with electrolysis can increase the supply security of renewable energy sources locally and thus increase preparedness. 5. The replicability of the concept with the two-step bioprocess is assessed as high, since there are many different ways it could be done depending on local conditions, but the replicability decreases with increased complexity in the symbiosis solution.

Bioflex pilot study

Today, there is an increasing demand for robust and flexible energy solutions to meet sustainable development goals. Renewable energy carriers, such as hydrogen and methane can play a key role in the sustainable transition. Although the simultaneous biological production of these components is promising, there is limited work on upscaling development. In the first part of this work, methane and hydrogen were produced from sugar-rich process water, in a two-stage process, by anaerobic microbes. Both pure strains and mixed culture were evaluated for hydrogen production at lab-scale. Mixed anaerobic culture was used after pretreatment for hydrogen production and the effluent was then used as substrate for methane production in pilot reactors of 10 L and 60 L in total volume. The pilot system was operated in continuous and semi-automated mode for 69 days, at 65oC (hydrogen) and 40oC (methane). The highest yields of hydrogen and methane obtained were 1.57 L/Lr/d and 0.91 L/Lr/d, respectively. Out of the 0.91 L/Lr/d production of methane, approximately 0.7 L/Lr/d were produced in the hydrogen reactor and 0.21 L/Lr/d in the methane reactor. anaerobic process. The process led to more efficient methane production and lower biogas emissions from the digestate compared to one-stage biogas production.

The second part of this work is a techno-economic analysis of the integrated two-stage bioprocess and electrolysis. The design of a large-scale integrated process is established, and mass and energy balances are calculated. A study-level cost analysis is performed based on the mass and energy balances. The results indicate that the electrolyser system and the hydrogen compression system are the major cost drivers of the integrated process. Electricity is the major contributor to the operating costs. The calculated pay-back period is highly dependent on the selling price of hydrogen.

A major source of CO2 emissions is the flaring of steel mill gas. This work demonstrated the enrichment of carboxydotrophic bacteria for converting steel mill gas into volatile fatty acids and H2, via gas fermentation. Several combinations of pure and mixed anaerobic cultures were used as inoculum in 0.5-L reactors, operated at 30 and 60 °C. The process was then scaled up in a 4-L membrane bioreactor, operated for 20 days, at 48 °C. The results showed that the enriched microbiomes can oxidize CO completely to produce H2/H+ which is subsequently used to fix the CO2. At 30 °C, a mixture of acetate, isobutyrate and propionate was obtained while H2 and acetate were the main products at 60 °C. The highest CO conversion and H2 production rate observed in the membrane bioreactor were 29 and 28 mL/LR/h, respectively. The taxonomic diversity of the bacterial community increased and the dominant species was Pseudomonas.

Raw syngas contains tar contaminants including toluene and naphthalene, which inhibit its conversion to methane. Cell encasement in a hydrophilic reverse membrane bioreactor (RMBR) could protect the cells from hydrophobic contaminants. This study aimed to investigate the inhibition of toluene and naphthalene and the effect of using RMBR. In this work, toluene and napthalene were added at concentrations of 0.5 - 1.0 and 0.1 - 0.2 g/L in batch operation. In continuous operation, concentration of 0 - 6.44 g/L for toluene and 0 - 1.28 g/L for napthalene were studied. The results showed that no inhibition was observed in batch operation for toluene and naphthalene at concentrations up to 1 and 0.2 g/L, respectively. In continuous operation of free cell bioreactors (FCBR), inhibition of toluene and naphthalene started at 2.05 g/L and 0.63 g/L, respectively. When they were present simultaneously, inhibition of toluene and naphthalene occurred at concentrations of 3.14 g/L and 0.63 g/L, respectively. In continuous RMBRs, no inhibition for toluene and less inhibition for naphthalene were observed, resulting in higher methane production from RMBR than that of FCBR. These results indicated that RMBR system gave a better protection effect against inhibitors compared to FCBR.