Materials challenges in hydrogen-fuelled gas turbines

With the increased pressure to decarbonise the power generation sector several gas turbine manufacturers are working towards increasing the hydrogen-firing capabilities of their engines towards 100%. In this review, we discuss the potential materials challenges of gas turbines fuelled with hydrogen, provide an updated overview of the most promising alloys and coatings for this application, and highlight topics requiring further research and development. Particular focus is given to the high-temperature oxidation of gas turbine materials exposed to hydrogen and steam at elevated temperatures and to the corrosion challenges of parts fabricated by additive manufacturing. Other degradation mechanisms such as hot corrosion, the dual atmosphere effect and hydrogen diffusion in the base alloys are also discussed.acceptedVersio

Effect of coating density on oxidation resistance and Cr vaporization from solid oxide fuel cell interconnects

Manganese cobalt spinel oxides are promising materials for protective coatings for solid oxide fuel cell (SOFC) interconnects. To achieve high density such coatings are often sintered in a two-step procedure, involving heat treatment first in reducing and then in oxidizing atmospheres. Sintering the coating inside the SOFC stack during heating would reduce production costs, but may result in a lower coating density. The importance of coating density is here assessed by characterization of the oxidation kinetics and Cr evaporation of Crofer 22 APU with MnCo1.7Fe0.3O4 spinel coatings of different density. The coating density is shown to have minor influence on the long-term oxidation behavior in air at 800 °C, evaluated over 5000 h. Sintering the spinel coating in air at 900 °C, equivalent to an in-situ heat treatment, leads to an 88% reduction of the Cr evaporation rate of Crofer 22 APU in air-3% H2O at 800 °C. The air sintered spinel coating is initially highly porous, however, densifies with time in interaction with the alloy. A two-step reduction and re-oxidation heat treatment results in a denser coating, which reduces Cr evaporation by 97%

Experimental review of the performances of protective coatings for interconnects in solid oxide fuel cells

Ferritic stainless steel interconnects are used in solid oxide fuel cells; however, coatings are required to improve their performance. Although several types of coatings have been proposed, they have been scarcely investigated under similar conditions. This study compares the characteristics of uncoated Crofer 22 APU and eight different coatings on Crofer 22 APU for up to 3000\ua0h at 800\ua0\ub0C. The coatings were deposited at various research laboratories around the world, and the experiments were performed at Chalmers University of Technology, Sweden. Cross-sections of the samples were analysed using scanning electron microscopy and energy-dispersive x-ray spectroscopy. The (Co,Mn)-based coated steels showed more than 50-fold lower chromium evaporation and at least 3 times thinner Cr2O3 scale thickness compared to uncoated steel. The coated steel samples showed lower area-specific resistance (ASR) values than the uncoated steel after 3000\ua0h of exposure, irrespective of the coating thickness, composition and deposition method

Metallic Interconnects for Solid Oxide Fuel Cells: High Temperature Corrosion and Protective Spinel Coatings

Metallic Interconnects for Solid Oxide Fuel Cells: Increasing the share of renewable energy is important in the effort to limit global warming. The challenge with increased dependence on renewable energy sources such as sun and wind power is handling the fluctuations in energy production with changes in the weather. A promising solution to this challenge is using hydrogen as an energy carrier. Solid oxide fuel cells (SOFC) offer an environmentally friendly way of efficiently converting hydrogen to electrical energy. These cells can also be operated in the reverse mode to produce hydrogen by electrolysis of water. However, high costs and a limited lifetime inhibit the commercialization of SOFC. These issues are in part related to degradation of the ferritic stainless steel interconnect material. There are two main challenges with the use of ferritic stainless steel as the interconnect material: i) the increase in electrical resistance due to growth of modestly conductive oxide scales; and ii) the vaporization of Cr(VI) species leading to poisoning of the SOFC cathode. This thesis investigates the use of protective spinel oxide coatings as a way of mitigating these issues. The principal aim of the thesis is to contribute to a better understanding of the performance of spinel oxide coatings and their interaction with ferritic stainless steel during operation. It was shown that coatings based on MnCo2O4 are effective for reducing the oxidation rate of ferritic stainless steel and maintaining a low electrical resistance. In the first part of this thesis, iron and copper substitutions in MnCo2O4 were explored as a way of improving coating performance. Iron substitutions proved beneficial for lowering the thermal expansion coefficient, resulting in a better match with the interconnect and other SOFC materials. Copper substituted materials suffered from poor stability, but provided acceptable protection when applied as a coating. In the second part of this thesis the possibility to reduce coating costs by simplifying the heat treatment procedure was investigated. To ensure high density while avoiding excessive damage of the interconnect alloy, spinel coatings have typically been sintered in a two-step reduction and re-oxidation procedure. It was shown that coatings sintered in a single step (in air) can be nearly as effective in reducing chromium evaporation, despite being significantly more porous

Development of Asymmeteric Membranes for Oxygen Separation by Tape Casting and Dip Coating

Ceramic membranes made from mixed ionic and electronic conductive oxide materials have received much attention over the last decade due to their ability to separate oxygen from air at 100 % selectivity. The flux through these mem- branes may be optimized by reducing their thickness. A porous support of the same composition is applied to ensure sufficient mechanical stability. The pro- cessing of these so-called asymmetric membranes is addressed in this work; for the technology to become attractive from a commercial point of view, a reliable and cost-effective processing procedure needs to be established.Phase pure La0.2Sr0.8Fe0.8Ta0.2O3−δ (LSFTa) and La0.2Sr0.8Fe0.8Al0.2O3−δ (LSFAl) powders were synthesized by solid state reaction. The powders were used to prepare porous supports by the means of aqueous based tape casting and hot-press lamination. The supports were pre-sintered at various temperatures and dip coated with an ethanol-based suspension containing sub-micrometer sized spray pyrolysis powder. Different parameters believed to affect dense layer for- mation by dip coating are discussed and related to the experimental observations. It was found that an important criteria for success is to have a similar shrinkage property in the functional and porous layer of the membrane. The most promis- ing asymmetric membrane was obtained for the LSFTa composition where dip coating two times and sintering at 1230◦C resulted in a 6?7 μm thick membrane layer and a support with 38 % open porosity.The fracture strength of LSFAl supports with ∼ 64 % porosity was also charac- terized in this work. Testing 11 specimens with the ball-on-ring method resulted in a characteristic strength of 10.7±0.5 MPa and a Weibull modulus of 5.9±1.8

Development of Asymmeteric Membranes for Oxygen Separation by Tape Casting and Dip Coating

Ceramic membranes made from mixed ionic and electronic conductive oxide materials have received much attention over the last decade due to their ability to separate oxygen from air at 100 % selectivity. The flux through these mem- branes may be optimized by reducing their thickness. A porous support of the same composition is applied to ensure sufficient mechanical stability. The pro- cessing of these so-called asymmetric membranes is addressed in this work; for the technology to become attractive from a commercial point of view, a reliable and cost-effective processing procedure needs to be established.Phase pure La0.2Sr0.8Fe0.8Ta0.2O3−δ (LSFTa) and La0.2Sr0.8Fe0.8Al0.2O3−δ (LSFAl) powders were synthesized by solid state reaction. The powders were used to prepare porous supports by the means of aqueous based tape casting and hot-press lamination. The supports were pre-sintered at various temperatures and dip coated with an ethanol-based suspension containing sub-micrometer sized spray pyrolysis powder. Different parameters believed to affect dense layer for- mation by dip coating are discussed and related to the experimental observations. It was found that an important criteria for success is to have a similar shrinkage property in the functional and porous layer of the membrane. The most promis- ing asymmetric membrane was obtained for the LSFTa composition where dip coating two times and sintering at 1230◦C resulted in a 6?7 μm thick membrane layer and a support with 38 % open porosity.The fracture strength of LSFAl supports with ∼ 64 % porosity was also charac- terized in this work. Testing 11 specimens with the ball-on-ring method resulted in a characteristic strength of 10.7±0.5 MPa and a Weibull modulus of 5.9±1.8