The Sedimentary Carbon-Sulfur-Iron Interplay – A Lesson From East Anglian Salt Marsh Sediments

We explore the dynamics of the subsurface sulfur, iron and carbon cycles in salt marsh sediments from East Anglia, United Kingdom. We report measurements of pore fluid and sediment geochemistry, coupled with results from laboratory sediment incubation experiments, and develop a conceptual model to describe the influence of bioturbation on subsurface redox cycling. In the studied sediments the subsurface environment falls into two broadly defined geochemical patterns – iron-rich sediments or sulfide-rich sediments. Within each sediment type nearly identical pore fluid and solid phase geochemistry (in terms of concentrations of iron, sulfate, sulfide, dissolved inorganic carbon (DIC), and the sulfur and oxygen isotope compositions of sulfate) are observed in sediments that are hundreds of kilometers apart. Strictly iron-rich and strictly sulfide-rich sediments, despite their substantive geochemical differences, are observed within spatial distances of less than five meters. We suggest that this bistable system results from a series of feedback reactions that determine ultimately whether sediments will be sulfide-rich or iron-rich. We suggest that an oxidative cycle in the iron-rich sediment, driven by bioirrigation, allows rapid oxidation of organic matter, and that this irrigation impacts the sediment below the immediate physical depth of bioturbation. This oxidative cycle yields iron-rich sediments with low total organic carbon, dominated by microbial iron reduction and no methane production. In the absence of bioirrigation, sediments in the salt marsh become sulfide-rich with high methane concentrations. Our results suggest that the impact of bioirrigation not only drives recycling of sedimentary material but plays a key role in sedimentary interactions among iron, sulfur and carbon

Temperature optimization for improving polymer electrolyte membrane-water electrolysis system efficiency

Most of the hydrogen produced today is made using fossil fuels, making a significant contribution to global CO$_2$ emissions. Although polymer electrolyte membrane water-electrolyzers can produce green hydrogen by means of excess electricity generated from renewable energy sources, their operation is still not economical. According to industry experts, the necessary cost reductions can be achieved by 2030 if system efficiency can be improved. The commonly stated idea is to improve efficiency by increasing the stack temperature, which requires the development of more resistant materials. This study investigates not only the efficiency of an electrolysis cell, but of the entire electrolysis process, including gas compression of hydrogen. The results indicate that an optimal stack temperature exists for every operating point. It is shown that the optimal temperature depends solely on the electrode pressure and cell voltage and can be analytically calculated. In addition, the temperature optimization leads to significantly reduced hydrogen permeation at low current densities. In combination with the pressure optimization, the challenging safety issues of pressurized electrolysis can be eliminated for the entire load range and, at the same time, the efficiency of the overall system be maximized

Effect of Power Quality on the Design of PEM Water Electrolysis Systems

Water electrolyzer technologies may play a key role in the decarbonization of the fossil-fueled world economy. Electrolytic hydrogen production could bridge emission-free power generation and various energy end-use sectors to drive the energy system towards a net zero-emission level. In order to reduce the economic cost of the required energy transition, both the overall system efficiency in converting electrical energy into the chemical energy carried by hydrogen, and the material used to build electrolytic cell stacks, should be optimal. The effect of power quality on the specific energy consumption of proton exchange membrane (PEM) water electrolyzers is investigated with a semi-empirical cell model. An experimentally-defined polarization curve is applied to analyze cell-specific energy consumption as a function of time in the case of sinusoidal current ripples and ripples excited by an industrial 12-pulse thyristor bridge. The results show that the effective electrolyzer cell area should be up to five times as high as an ideal DC power supply when powered by the 12-pulse thyristor rectifier supply to match the specific energy consumption between the two power supply configurations. Therefore, the improvement of power quality is crucial for industrial PEM water electrolyzer systems. The presented approach is applicable to simulate the effect of power quality for different proton exchange membrane electolyzers

PEM-Electrolysis: On the Tradeoff between Pressure and Performance

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Establishing a Fed-Batch Process for Protease Expression with Bacillus licheniformis in Polymer-Based Controlled-Release Microtiter Plates

Introducing fed‐batch mode in early stages of development projects is crucial for establishing comparable conditions to industrial fed‐batch fermentation processes. Therefore, cost efficient and easy to use small‐scale fed‐batch systems that can be integrated into existing laboratory equipment and workflows are required. Recently, a novel polymer‐based controlled‐release fed‐batch microtiter plate is described. In this work, the polymer‐based controlled‐release fed‐batch microtiter plate is used to investigate fed‐batch cultivations of a protease producing Bacillus licheniformis culture. Therefore, the oxygen transfer rate (OTR) is online‐monitored within each well of the polymer‐based controlled‐release fed‐batch microtiter plate using a µRAMOS device. Cultivations in five individual polymer‐based controlled‐release fed‐batch microtiter plates of two production lots show good reproducibility with a mean coefficient of variation of 9.2%. Decreasing initial biomass concentrations prolongs batch phase while simultaneously postponing the fed‐batch phase. The initial liquid filling volume affects the volumetric release rate, which is directly translated in different OTR levels of the fed‐batch phase. An increasing initial osmotic pressure within the mineral medium decreases both glucose release and protease yield. With the volumetric glucose release rate as scale‐up criterion, microtiter plate‐ and shake flask‐based fed‐batch cultivations are highly comparable. On basis of the small‐scale fed‐batch cultivations, a mechanistic model is established and validated. Model‐based simulations coincide well with the experimentally acquired data