Chemical-looping combustion (CLC) is a combustion technology with inherent separation of the greenhouse gas CO2. The technique involves the use of a metal oxide as an oxygen carrier, which transfers oxygen from the combustion air to the fuel. Two reactors are used in the process:  (i) a fuel reactor where the metal oxide is reduced by reaction with the fuel, and (ii) an air reactor where the reduced metal oxide from the fuel reactor is oxidized with air. The possibility of using oxides of Cu, Co, Mn, and Ni as oxygen carriers was investigated. Particles were prepared by deposition of the metal oxides on γ-Al2O3 particles by so-called dry impregnation. The reactivity of the oxygen carrier particles was evaluated in a thermogravimetric analyzer (TGA), where the alternating atmosphere which an oxygen carrier encounters in a CLC system was simulated by exposing the sample to alternating reducing (10% CH4, 5% CO2, 10% H2O) and oxidizing (10% O2) conditions at temperatures between 750 and 950 °C. The particles of Ni and Cu showed high reactivity at all temperatures and cycles, with reduction rates of up to 100%/min for CuO and 45%/min for NiO and oxidation rates of up to 25%/min for the oxidation of both reduced metals. Oxides of Mn and Co showed a limited extent of reaction, which was explained by the chemical reaction of the metal oxide with the alumina, with the formation of highly irreversible phases which do not react with methane and oxygen. Thus, Mn and Co are not suitable as oxygen carriers for CLC when supported on Al2O3. From the reactivity data of the nickel and copper oxygen carriers, it was estimated that 460−620 kg/MW oxygen carrier would be needed in the reactors. Similarly, from the oxygen transfer capacity of the particles, the solids circulation rate between the fuel and air reactor would need to be between 1 and 8 kg MW-1 s-1.

A Rapidly Solidified Powder (RSP) metallurgical FeCrAl alloy, Kanthal APMT, was exposed in dry and humid O 2 for 72 h at 900-1,100°C. The formed oxide scales were characterized using gravimetry in combination with advanced analysis techniques (SEM, EDX, TEM, XRD, AES and SIMS). The oxide scales were at all exposures composed of two-layered a-Al 2 O 3 scales exhibiting a top layer of equiaxed grains and a bottom layer containing elongated grains. A Cr-rich zone, originating in the native oxide present before exposure, separated these two layers. The top a-Al 2 O 3 layer is suggested to have formed by transformation of outwardly grown metastable alumina, while the inward-grown bottom a-Al 2 O 3 layer had incorporated small Zr-, Hf-and Ti-rich oxide particles present in the alloy matrix. The scale also contained larger Y-rich oxide particles. Furthermore, in the temperature range studied, the presence of water vapour accelerated alloy oxidation somewhat and affected scale morphology.

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The effect of sulphur dioxide on the oxidation of alloy 304L in O 2

Pure iron has been exposed to pure O 2 and O 2 with 100 ppm SO 2 at 525°C for 1 and 24 h. The samples were investigated by FIB, SEM, TEM, EDX and EBSD. The oxide scales formed on iron at 525°C in O 2 and in O 2 + 100 ppm SO 2 are dense and adherent and consist of three layers. The outermost layer consists of hematite. Beneath it there is a duplex-magnetite scale. The two magnetite layers are separated by a straight interface. It is concluded that the inner-magnetite layer grows inward while the outer magnetite layer grows outwards. In the presence of SO 2 the inner-magnetite layer is much thinner, iron sulphate forms at the oxide surface and discrete iron sulphide grains nucleate at the metal/oxide interface. The amount of sulphide at the metal/oxide interface increases with exposure time. The oxidation of iron in oxygen at 525°C is inhibited by 100 ppm SO 2 . The inhibitive effect of SO 2 is attributed to iron sulphate that blocks active sites on the hematite surface, slowing down the formation of oxygen ions. This explains the strong inhibition of the inward growth of magnetite by SO 2 . There is also a marked effect on the morphology of the outer oxide, producing hematite whisker growth and a less porous surface in the presence of SO 2 .