Heat transfer fluids for concentrating solar power systems – A review

This paper reviews previous studies on iron and steel oxidation in oxygen or air at high temperatures. Oxidation of iron at temperatures above 700°C follows the parabolic law with the development of a three-layered hematite/magnetite/wustite scale structure. However, at temperatures below 700°C, inconsistent results have been reported, and the scale structures are less regular, significantly affected by sample-preparation methods. Oxidation of carbon steel is generally slower than iron oxidation. For very short-time oxidation, the scale structures are similar to those formed on iron, but for longer-time oxidation, because of the less adherent nature, the scale structures developed are typically much more complex. Continuous-cooling conditions, after very short-time oxidation, favor the retention of an adherent scale, suggesting that the method proposed by Kofstad for deriving the rate constant using continuous cooling or heating-oxidation data is more appropriate for steel oxidation. Oxygen availability has certain effects on iron and steel oxidation. Under continuous cooling conditions, the final scale structure is found to be a function of the starting temperature for cooling and the cooling rate. Different scale structures develop across the width of a hot-rolled strip because of the varied oxygen availability and cooling rates at different locations.

Review of the High-Temperature Oxidation of Iron and Carbon Steels in Air or Oxygen

Influence of temperature on the electrochemical and passivation behavior of 2507 super duplex stainless steel in simulated desulfurized flue gas condensates

Accurate knowledge of water splitting kinetics is essential for the design and optimization of high-temperature thermochemical cycles for solar-driven fuel production, but such crucial data are unavailable for virtually all redox materials of potential practical value. We describe an investigation of the redox activity and oxidation kinetics of cobalt ferrite, a promising material for this application that is representative of a broader class of metal-substituted ferrites. To enable repetitive cycling, ferrites must be supported on another oxide to avoid sintering and deactivation. Consequently, we synthesized a composite material using atomic layer deposition of cobalt and iron oxides on zirconia, a commonly used ferrite “support”, to create a well-controlled, uniformly distributed composition. Our results show that the support is not an innocent bystander and that dissolved iron within it reacts by a different mechanism than embedded iron oxide particles in the matrix. Samples were thermally reduced at 1450 °C under helium and oxidized with steam at realistic process temperatures ranging from 900 °C to 1400 °C. Experiments within a fluid-dynamically well-behaved stagnation-flow reactor, coupled with detailed numerical modelling of the transient H2 production rates, allow us to effectively deconvolve experimental artefacts from intrinsic material behaviour over the entire time domain of the oxidation reaction. We find that second-order reaction and diffusion-limited mechanisms occur simultaneously at different oxidation rates and involve iron in two separate phases: (1) reduced Fe dissolved in the ZrO2 support and (2) iron oxide located at the interface between embedded ferrite particles and the zirconia matrix. Surprisingly, we also identified a catalytic mechanism occurring at the highest temperatures by which steady-state production of H2 and O2 occurs. The results reported here, which include Arrhenius rate constants for both oxidation mechanisms, will enable high-fidelity computational simulation of this complex, but promising approach to renewable fuel production.

Kinetics and mechanism of solar-thermochemical H2 production by oxidation of a cobalt ferrite–zirconia composite

Bicrystalline nanowires of hematite (α-Fe2O3) have been successfully synthesized by the oxidation of pure iron. The product was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM in combination with focal series reconstruction, energy-dispersive X-ray spectroscopy, and electron energy-loss spectroscopy. The bicrystalline nanowires have diameters of 20−80 nm and lengths up to 20 μm. All of the investigated materials are found to be α-Fe2O3 with a rhombohedral crystal structure. Investigations indicate that most of the bicrystalline nanowires are nanotwins with ellipsoidal heads. The orientation relationship between the nanotwins can be described as (1110)M//(1110)T, [110]M//[110]T. An energy-filtered TEM investigation indicates that the ellipsoidal head is iron-rich. The growth mechanism of such unique nanostructures is considered to be a solid-phase growth via surface and internal diffusions of molecules from base to tip.

Bicrystalline Hematite Nanowires

The oxide-scale structure developed on commercial hot-rolled steel strip at the mid-coil position was examined. The initial oxide scale after rolling and cooling on the run-out table had a three-layer (hematite, magnetite, and wustite) structure; the thickness was found to be a function of the finishing temperature. From this initial structure, various final scale structures developed after coiling, depending on the coiling temperature, oxygen availability, and cooling rate. For relatively low coiling temperatures (e.g., at 520°C), the final scale structure comprised an inner magnetite/iron mixture layer, an outer magnetite layer, and, at regions away from the center, a very thin outermost hematite layer. For higher coiling temperatures (e.g., in the range of 610 to 720°C), a two-layer hematite/magnetite structure was observed at the edge regions, whereas at the center regions, these two layers were absent and the entire scale layer comprised a mixture of the wustite-transformation products, i.e., a mixture of proeutectoid magnetite, magnetite+iron eutectoid, and a certain amount of retained wustite. At regions between the edges and the center, the oxide structures were similar to those developed at low coiling temperatures (<570°C), i.e., an inner layer comprising a mixture of the wustite-transformation products, an intermediate magnetite layer and at regions near the edges, an outermost hematite layer. In addition, two distinct structures were observed on strips with a coiling temperature of 720°C. One structure comprised a very thick hematite layer (3–5 μm) formed near the edges (within 10–20 mm from the edges), while the other structure comprised a substantial amount of retained wustite formed at the center regions. The formation mechanisms of various oxide scale structures are discussed.

Oxide-Scale Structures Formed on Commercial Hot-Rolled Steel Strip and Their Formation Mechanisms

In this study, the oxidation kinetics of low-carbon, low-silicon steel in flowing air at 850–1,180 °C within 30 or 60 s were examined. The parabolic kinetics were established from the very early stage at 850° and 1,000 °C, whereas the oxidation kinetics at 1,100–1,180 °C appeared to obey a linear law initially and a more-parabolic one at a later stage. When the oxidation kinetics followed the linear law, “rough”-scale with an undulating, saw-teeth like microstructure developed, whereas when the parabolic law was followed, smooth scale developed. It appeared that a critical scale thickness existed, at which the scale-growth mechanism changed from linear to parabolic. This thickness was less than 7 μm at 850 °C, about 10 μm at 1,000 °C, about 50 μm at 1,100 °C and in the range of 60–80 μm at 1,180 °C under the conditions examined. Blister formation at 900 °C prevented clear observation of the linear-to-parabolic transition.

Short-time Oxidation Behavior of Low-carbon, Low-silicon Steel in Air at 850–1,180 °C––I: Oxidation Kinetics

The oxidation and copper enrichment behaviors of several copper-containing mild steels under isothermal and step-isothermal conditions at 980–1220°C in ambient air, and the effects of nickel additions, are examined. The oxidation kinetics for all steels does not obey the parabolic law because of the formation of blisters in the scale. At 980°C, decreased parabolic oxidation kinetics is observed, whereas at 1120 and 1220°C, the oxidation kinetics exhibits an undulating pattern. High nickel content and high oxidation temperature are two prerequisites for the occlusion mechanism to operate during steel oxidation. For the low nickel steel, a planar scale-steel interface develops at all temperatures, and a copper phase is always seen to form and spread along the scale-steel interface. For the high nickel steels, a planar scale-steel interface develops at 980 and 1120°C, but at 1220°C, the scale-steel interface becomes rugged and the copper-rich phase is occluded into the scale. Introduction of a 980°C and/or 1220°C oxidation step significantly affects the copper enrichment behaviors of all steels normally exhibited at 1120°C.

Isothermal and Step Isothermal Oxidation of Copper-Containing Steels in Air at 980–1220°C

The effect of water vapour additions in the range of 2.5–24.8 % on the oxidation behaviour of a low carbon and low silicon steel in 1 %O2–N2 at 1073 K (800 °C) was examined. It was found that the characteristic of steel oxidation was completely changed by addition of water vapour in the atmosphere. First, the kinetics was changed from non-parabolic to parabolic. Second, the scale formed in 1 %O2–N2 for 30 min or longer was easy to spall upon cooling whereas the scales formed in the water-vapour containing atmospheres did not spall easily. Third, additions of 2.5–10 % of water vapour in 1 %O2–N2 resulted in the formation of numerous depressed locations in the scale, but the scale-steel adherence in the areas surrounding the depressed locations was very much strengthened. Finally, additions of 17.2 and 24.8 % of water vapour in the atmosphere prevented both scale spallation upon cooling and formation of depressed areas in the scale. The mechanisms of forming various scale structures and the roles of water vapour additions under different conditions were discussed.

R. Y. Chen

Papers

Review of the High-Temperature Oxidation of Iron and Carbon Steels in Air or Oxygen

Oxide-Scale Structures Formed on Commercial Hot-Rolled Steel Strip and Their Formation Mechanisms

Short-time Oxidation Behavior of Low-carbon, Low-silicon Steel in Air at 850–1,180 °C––I: Oxidation Kinetics

Isothermal and Step Isothermal Oxidation of Copper-Containing Steels in Air at 980–1220°C

Effects of the Presence of Water Vapour on the Oxidation Behaviour of Low Carbon–Low Silicon Steel in 1 %O2–N2 at 1073 K