The influence of the preparation method of NiAl 2 O 4 binders on their thermal stability was studied. For this purpose, the reactivity of two different NiAl 2 O 4 binders with CO as fuel was studied in a fixed bed reactor device. Successive oxidation-reduction cycles were performed-2/19-on the two binders to study their reactivity with the fuel and their structural modifications as cycles proceed. Results reveal that binders are not inert in reducing atmosphere; they both react with the fuel to produce CO 2. The total reduction capacity (TRC) of the first binder (B1, synthetized by pyrolytic pulverization) increases during the first cycles and levels off after 20 cycles. However, the TRC of the second binder (B2, synthetized by calcination of a mixture of Ni(OH) and γ-Al 2 O 3), increases progressively and reaches a maximum after 80 cycles. The growing amount of available oxygen in the binders leads both binders to structural modifications. X-Ray Diffraction studies performed on fresh and aged binders presented a shift of the peaks related to NiAl 2 O 4. Moreover, quantitative X-Ray Diffraction studies and Temperature Programmed Reduction measurements were performed in order to quantify the NiO present in each binder before and after oxidation-reduction cycles. These experiments revealed the presence of NiO in fresh binders due to the preparation method, and an increase of this amount after oxidation-reduction cycles. Therefore, NiAl 2 O 4 in the binder is progressively decomposed producing NiO and Al 2 O 3. Finally, the decomposition of the binder NiAl 2 O 4 as cycles proceed was also observed in studies performed on the oxygen carrier NiO/NiAl 2 O 4. This work showed that the binder reacts with the fuel and therefore it can contribute to the modification of the oxygen carrier reactivity.
The aim of this work was to desensitize keto‐RDX, respectively 2‐oxo‐1,3,5‐trinitro‐1,3,5‐triazacyclohexane (K6). For this purpose, two different methods were employed. First, nano‐K6 was produced by means of the Spray Flash Evaporation process. Particles with a median size of 74 nm were obtained. Sensitivity to friction and electrostatic discharge were reduced by downscaling particle size of K6. Second, due to their molecular analogy, the mixing of K6 and RDX was studied. For that reason, a physical nanometric mixture of K6 and RDX was produced by the same technique. In the latter case, an inter‐particular synergy between both compounds was noticed but without forming a cocrystal. The median particle size of the mixture is about 82 nm, and its sensitivity is between the ones of raw nano‐materials concerning friction and electrostatic discharge. Moreover, the mixture is less sensitive to impact (3.03 J) than nano‐K6 (<1.56 J) and nano‐RDX (threshold is 2.0 J).
Chemical Looping Combustion is a promising technology for clean power generation with integrated CO 2 capture. In this process the oxygen required for combustion is provided by a metal oxide. This work deals with the development of an experimental procedure to study performances of an oxygen carrier during oxidation/reduction cycles and the influence of the oxidation step on its behaviour. Tests were performed in a laboratory fixed bed reactor, with NiO/NiAl 2 O 4, a promising oxygen carrier, and CO as fuel. Two different protocols of oxidation were studied. Results reveal that the oxidation step conditions can change the performances of the oxygen carrier. A significant decrease in total reduction capacity was observed using the regeneration step at high temperature due to structural changes in particles. SEM analysis reveals that particle surface contains different crystallites according to this procedure. With the second procedure (oxidation in temperature ramp), nickel is partially agglomerated.
Abstract:This work was devoted to study experimentally and numerically the oxygen carrier (NiO/NiAl 2 O 4 ) performances for Chemical-Looping Combustion applications. Various kinetic models including Shrinking Core, Nucleation Growth and Modified Volumetric models were investigated in a one-dimensional approach to simulate the reactive mass transfer in a fixed bed reactor. The preliminary numerical results indicated that these models are unable to fit well the fuel breakthrough curves. Therefore, the oxygen carrier was characterized after several operations using Scanning Electronic Microscopy (SEM) coupled with equipped with an energy dispersive X-ray spectrometer (EDX). These analyses showed a layer rich in nickel on particle surface. Below this layer, to a depth of about 10 µm, the material was low in nickel, being the consequence of nickel migration. From these observations, two reactive sites were proposed relative to the layer rich in nickel (particle surface) and the bulk material, respectively. Then, a numerical model, taking into account of both reactive sites, was able to fit well fuel breakthrough curves for all the studied operating conditions. The extracted kinetic parameters showed that the fuel oxidation was fully controlled by the reaction and the effect of temperature was not significant in the tested operating conditions range.
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