Yngve Larring

Ammonia is emerging as a promising carbon-free fuel that can be converted to energy using combustion engines and fuel cells or converted to hydrogen in a cracker. The structural alloys in these applications are exposed to harsh conditions, notably high temperature and presence of NH ₃ and H ₂ O, under which their behavior remains insufficiently understood. In this work, the nitridation behavior of seven alloys (625, 600, 800HT, AISI 316L, AISI 310S, AISI 444 and EF101) was systematically studied as a function of temperature (495–670 °C), NH ₃ concentration (6–85%) and H ₂ O content (0–2%). Mass gain measurements combined with microstructural characterization were used to quantify nitridation and identify degradation mechanisms. Nitridation resistance increased with decreasing Fe-content and increasing Ni-content of the alloy, with alloy 625 showing the highest resistance across the investigated conditions. Increasing temperatures and the presence of 2% H ₂ O accelerated nitridation in alloys 600, 800HT and 316L, but suppressed it in alloy 444. The improved nitridation resistance of alloy 444 is attributed to the formation of a protective oxide scale, highlighting the critical interplay between oxidation and nitridation in environments containing both NH ₃ and H ₂ O.

Direct fuelling of solid oxide fuel cells with ammonia is attractive but can lead to reduced stack durability due to nitridation of the metallic interconnect material. This works investigates the stability of the interconnect material Crofer 22 APU in ammonia-containing atmosphere and evaluates the effects of pre-oxidation and the application of nickel or yttrium coatings to mitigate nitridation. Exposures were conducted in dry and wet NH3-H2-N2 mixtures at 650 °C and 750 °C and 20 bar for up to 800 h. Bare Crofer 22 APU suffered from nitridation under all exposure conditions with more severe degradation observed at 650 °C compared to 750 °C and in dry conditions. At 750 °C, pre-oxidation and Y coating both provided excellent resistance against nitridation in wet conditions, while these treatments had no effect in dry conditions. A Ni coating provided best protection in dry conditions and at 650 °C although it did not completely supress nitridation.

A numeric scale of acidity and basicity, developed by D.W. Smith, allows for quantitative comparison between different oxides and has been useful in explaining various oxide behaviors. In this study, we aim to predict oxide acidity on the Smith scale using a machine learning approach. Previous attempts using a linear fit based on electronegativity showed a clear trend but lacked precision due to the simplicity of the model and the multi-valent nature of metal oxides. We propose a multi-parameter model incorporating four features: electronegativity, metal valence (raised to the power of 1/3), ionic radius, and dipole polarizability. A simple linear neural network outperformed a more complex 1D convolutional neural network, demonstrating superior accuracy and interpretability. Our model significantly improved predictive performance as compared to the single-parameter model. These results were validated through predictions of testing data and unknown oxides acidity were predicted in this work.

The understanding and development of stable redox materials based on cheap and abundant elements, forming Ca-Mn-Ti-Fe-O-based perovskites, have been in focus for applications in renewable technologies such as chemical looping combustion and thermal energy storage. The present research focuses on developing stable materials to be utilized up to 1050 degrees C in a CLC process and has shown that the structure stability and oxygen transfer capacity can be achieved by tuning the content of different elements on B-sites of the perovskites. Various experiments, such as redox cycling under various fuels, temperatures, and pO(2), were carried out to evaluate the oxygen transfer capacity, reaction rates under various fuels, etc. The redox stability at high temperatures was evaluated by redox cycles at 1050 degrees C followed by post SEM analyses on surface and depth profiling. The three developed materials can avoid phase change during redox due to the moderate oxygen transfer capacity of up to 5.6 wt % O-2 for CaMn0.5Ti0.375Fe0.125O3-delta at 1050 degrees C, which is important for having stable particles. Cation diffusion was also investigated during redox cycling in the development of stable redox materials, and only a minor diffusion of Mn to the grain boundaries is seen in the least stable material. The findings show that perovskites with high stability can be obtained with more Ti on B-sites, termed as CaMn0.375Ti0.5Fe0.125O3-delta. The developed stable oxides, to some extent, have a reduced activity compared to the less stable composition with less Ti and more Mn, termed as CaMn0.5Ti0.375Fe0.125O3-delta, which possesses a higher oxygen release to inert ca. 1.1 wt % O-2 compared to more stable CaMn0.375Ti0.5Fe0.125O3-delta that can release up to 0.8 wt % O-2. Two of the materials have faster kinetics than ilmenite by a factor of 2 in H-2.

Upscaling of the laboratory oxygen carriers while keeping comparable performance as laboratory powders remains a challenge in the field of chemical looping combustion (CLC). In this work, two perovskite compositions of CaMn_0.375Ti_0.5Fe_0.125O₃ (termed CMTF8341) and CaMn_0.5Ti_0.375Fe_0.125O₃ (CMTF8431) have been fabricated by means of spray granulation. The fabricated granules were evaluated by redox cycles under CLC conditions and showed comparable performance with laboratory-prepared powders, i.e. high oxygen transfer capacity and fast redox kinetics. Under chemical looping oxygen uncoupling (CLOU) conditions, it has been found that steam leads to a slower kinetics and lower oxygen transfer capacity for these two compositions. When it comes to sulphur effect on the oxygen carriers, redox cycles with deep reduction showed no influence with H₂S since the CaSO₄ formed in air decompose but it affected the materials under mild CLOU due to calcium sulphate formed under such condition. Nonetheless, redox performance can be recovered by deep reduction with an enhanced oxygen capacity. These two oxygen carriers showed no degradation as compared to other tested minerals in the field of CLC. Microstructure analyses from SEM showed high degree of structure integrity after redox cycles at temperatures up to 1050 °C for the CMTF8341. Good mechanical strength was evidenced from cold attrition test, and the lifetime of the two granules was also predicted based on a standard reference.

Double-perovskite Ca₂AlMnO_5+δ exhibits promising oxygen uptake and release capacity at intermediate temperatures (400–700 °C), which makes it an interesting candidate for in situ oxygen production in an integrated gasification combined cycle (IGCC) process. Experiments were conducted at 10 bars by alternating gas feeds of air and various sweep gases to a packed bed filled with 300 g of granular oxygen carrier materials. These realistic operating conditions demonstrated that 15–20% oxygen can be introduced to the sweep gas, which is sufficient for autothermal gasification of solid fuels in IGCC. Argon performed slightly better than CO₂ as a sweep gas, presumably because of some CO₂ absorption or the higher O₂ partial pressure of CO₂ that inhibited O₂ release. Further O₂ concentration increases can be expected from increasing the temperature under reduction by feeding a fuel gas to combust with the released O₂, but experiments with H₂ did not produce the desired effect because the combustion reaction was too slow at the optimal reactor temperature (~ 600 °C). In general, the reduction stage was more prone to kinetic limitations, as illustrated by a significant decrease in O₂ concentration when the sweep flowrate was increased. A longer oxidation stage to fully charge the oxygen carrier also increased O₂ concentrations in the sweep, but this requires a process integration such as IGCC where the large quantity of warm depleted air can be effectively utilized. Furthermore, the enthalpy of oxidation of Ca₂AlMnO_5+δ was obtained from density functional theory modeling, equilibrium conditions in thermogravimetric analysis, packed bed experiments and directly from differential scanning calorimetry. The enthalpy of oxidation obtained by these techniques range from − 166 to − 196 kJ mol⁻¹ O₂.