Controlling dissolved hydrogen for the optimization of hydrogenotrophic processes

There are only a few investigations regarding phase separation in combination with thin sludge recirculation in anaerobic digestion applications. To measure dissolved gases in biogas media, the technical possibilities of a robust, long-term stable and at the same time cost-effective measuring technology have so far been lacking.
In this project, plant operation shall be made more flexible with regard to substrates, energy generation and the production of value-added by-products. This can be achieved by means of phase separation in combination with thin-slurry recirculation within the configuration of predominant stirred tank reactors.
Laboratory tests on phase separation with thin sludge recirculation were carried out at in a specially designed two-stage laboratory fermenter system for biogas production. A phase separation of the biogas process was achieved, in which the hydrolytic and acidogenic phases take place in the first fermenter and the acetogenesis and methanogenesis in the second fermenter. The laboratory system was examined under dynamic operating conditions (substrate use, recirculation volume, power input, etc.) with regard to its robustness and performance. A measuring system for the detection of dissolved hydrogen developed by the project partner KSI Meinsberg was extensively tested and used in both fermenters. The injection of hydrogen gas into the methane stage could be monitored and optimized beyond the typical operation of the fermenters.

n the first phase, the acid-forming microorganisms produced up to 10 g/L of short-chain carboxylic acids when using maize silage and stable straw as substrate (50/50 m/m%). In the second stage, the carboxylic acids in the effluent of the first fermenter were then completely converted into biogas (300 mL/gCOD) with a methane content of up to 65%. Thanks to phase separation and thin-slurry recirculation, the fermenter system has proven to be very robust and efficient and made methane production more flexible.
The first ever applied membrane-free dissolved hydrogen sensor based on metal-oxide technology from KSI was successfully tested and detected the dissolved hydrogen from the effluent of the first fermenter, as well as the externally added hydrogen gas in the liquid phase of the methane stage reliably. The sensors showed good long-term stability. Overall, it was possible to develop a system that, despite the dynamic mode of operation, has the process stability and controllability that is needed for industrial operation for power-to-X applications.