Continuous Hydrogen Production from Syngas via the Biological Water-Gas Shift Reaction in a Trickle Bed Reactor: Effect of Nutrient Management, Inhibitors, and Hydraulic Retention Time

Hydrogen is a clean fuel that can power everything from factories to cars, but making it today usually requires burning fossil fuels, which defeats the purpose of using it to fight climate change. Scientists are looking for ways to produce this energy carrier from waste and renewable sources instead. One promising method involves taking a gas mixture called syngas, which is rich in carbon monoxide, and using tiny living organisms to transform it. These microbes act like biological factories, eating the carbon monoxide and releasing hydrogen gas as a byproduct. This process, known as the biological water-gas shift reaction, offers a way to turn industrial waste gases into clean energy without the need for extreme heat or expensive chemicals. However, keeping these microbial factories running smoothly in a continuous flow is difficult, as the waste products they create can eventually poison the very organisms doing the work.

Researchers at the University of Borås and RISE Research Institutes of Sweden set out to solve this problem by building a specialized reactor designed to keep the microbes happy and productive for the long haul. They used a natural community of microbes harvested from food waste, rather than a single, carefully bred species, hoping that this diverse group would be tougher and more adaptable. The team packed a vertical column with pieces of polyethylene sponge, creating a surface for the microbes to cling to, and then trickled nutrient-rich liquid down over them while pushing the syngas up from the bottom. This setup allowed the gas and liquid to meet closely, giving the microbes the best chance to eat the carbon monoxide and spit out hydrogen. The goal was to see if they could keep this process running continuously for months without the system shutting down due to the buildup of toxic byproducts.

The experiment began with a startup phase where the team added concentrated nutrients to the system every week to jumpstart the microbial activity. Initially, the microbes worked well, converting nearly all the carbon monoxide into hydrogen. However, as the weeks passed, the system began to struggle. The researchers discovered that the repeated addition of concentrated nutrients caused certain trace metals, like copper, to build up to levels thousands of times higher than intended. At the same time, the microbes started producing large amounts of volatile fatty acids, a type of organic acid that can be harmful in high concentrations. As these acids accumulated, the conversion of carbon monoxide dropped significantly, and the production of hydrogen became unstable. The team realized that while the microbes needed food, too much of a good thing was throwing off the chemical balance of their environment.

To understand exactly what was killing the process, the researchers moved to a second phase where they tested the effects of specific inhibitors. They introduced synthetic versions of the fatty acids and ethanol, a type of alcohol, into the reactor to see which one was the real culprit. The results were clear: the fatty acids were far more damaging than the ethanol. When the team added high levels of these acids, the carbon monoxide conversion plummeted, and the microbes could not recover quickly even after the acids were removed. The ethanol had a much weaker effect, causing only a temporary dip in performance. This confirmed that the buildup of fatty acids was the primary reason the system was failing, and that simply replacing the liquid occasionally was not enough to keep the reactor healthy.

In the final phase, the team changed their strategy to focus on controlling how long the liquid stayed in the reactor, a factor known as hydraulic retention time. Instead of just adding nutrients periodically, they set the system to continuously wash out the liquid, ensuring that harmful byproducts were removed before they could reach toxic levels. They also replaced the old microbes and the sponge packing with fresh materials to start with a clean slate. This approach worked remarkably well. By controlling the flow of liquid, the team maintained a stable environment where the fatty acids never built up to dangerous levels. The reactor achieved a perfect conversion rate, turning one hundred percent of the incoming carbon monoxide into hydrogen for twelve consecutive days. At a specific flow rate of twelve milliliters per minute, the system produced hydrogen at a rate of 3.2 liters per liter of reactor volume every day, with a yield of 0.66 moles of hydrogen for every mole of carbon monoxide consumed.

The researchers also pushed the system to its limits to find out how much gas it could handle. They gradually increased the flow of syngas, watching closely to see when the microbes would fail to keep up. The reactor held steady and maintained full conversion up to a flow rate of 26 milliliters per minute. Once they pushed it to 28 milliliters per minute, the system could no longer convert all the carbon monoxide, and the excess gas began to exit the reactor unprocessed. This identified a clear operational boundary for this specific setup. Throughout the entire experiment, no methane was detected, confirming that the process was strictly producing hydrogen and not drifting into other types of gas production.

Looking at the microscopic world inside the reactor, the researchers found that the community of microbes changed dramatically depending on how the reactor was run. In the early phases, the system was dominated by one group of bacteria, but as the conditions shifted to the stable, flow-controlled environment of the final phase, a different group took over. The new dominant bacteria were species of Stenotrophomonas and Pseudomonas, which had been rare in the original mixture. While the team could not pinpoint exactly which specific microbes were doing the heavy lifting of converting the gas, the shift in the community suggested that the reactor conditions themselves were selecting for the most efficient workers. The study concludes that managing the liquid flow to prevent the buildup of toxic byproducts is the key to making this technology work continuously. While the system is not yet a commercial reality, this proof-of-concept shows that using waste-derived microbes in a simple, non-sterile reactor is a viable path toward producing clean hydrogen from industrial waste gases.