Freeze Granulation Research Articles

Freeze Granulation for the Processing of Silicon Nitride Ceramics

Freeze Granulation for the Processing of Silicon Nitride Ceramics

Multicycle Reduction and Oxidation of Different Types of Iron Oxide ParticlesApplication to Chemical-Looping Combustion

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 that transfers oxygen from the combustion air to the fuel. The use of iron oxide as an oxygen carrier has been investigated. Particles composed of 40−80 wt % Fe2O3, together with Al2O3, ZrO2, TiO2, or MgAl2O4, have been prepared by freeze granulation. Particles have been sintered at different temperatures in the range of 950−1400 °C, and particles 0.125−0.180 mm in diameter have been obtained by sieving. The reactivity of the oxygen-carrier particles has been evaluated in a laboratory fluidized bed of quartz, where the alternating atmosphere that an oxygen carrier encounters in a CLC system is simulated by exposing the sample to alternating reducing (50% CH4, 50% H2O) and oxidizing (5% O2) conditions at a temperature of 950 °C. The oxides are characterized prior to, and following, reactivity testing, with respect to crushing s...

Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion

For combustion with CO 2 capture, chemical-looping combustion (CLC) with inherent separation of CO 2 is a promising technology. Two interconnected fluidized beds are used as reactors. In the fuel reactor, a gaseous fuel is oxidized by an oxygen carrier, e.g. metal oxide particles, producing carbon dioxide and water. The reduced oxygen carrier is then transported to the air reactor, where it is oxidized with air back to its original form before it is returned to the fuel reactor. The feasibility of using oxygen carrier based on oxides of iron, nickel, copper and manganese was investigated. Oxygen carrier particles were produced by freeze granulation. They were sintered at 1300 °C for 4 h and sieved to a size range of 125–180 μm. The reactivity of the oxygen carriers was evaluated in a laboratory fluidized bed reactor, simulating a CLC system by exposing the sample to alternating reducing and oxidizing conditions at 950 °C for all carriers except copper, which was tested at 850 °C. Oxygen carriers based on nickel, copper and iron showed high reactivity, enough to be feasible for a suggested CLC system. However, copper oxide particles agglomerated and may not be suitable as an oxygen carrier. Samples of the iron oxide with aluminium oxide showed signs of agglomeration. Nickel oxide showed the highest reduction rate, but displayed limited strength. The reactivity indicates a needed bed mass in the fuel reactor of about 80–330 kg/MW th and a needed recirculation flow of oxygen carrier of 4–8 kg/s, MW th.

Controlling hydrolysis and dispersion of AlN powders in aqueous media

Aqueous suspensions of aluminum nitride (AlN) powders have been prepared in the presence of different surface-active agents, namely, H 3PO 4 and an anionic surfactant, to avoid the hydrolysis of AlN powders and to enhance dispersion. The most determinant parameters to the hydrolysis process (ΔpH and time of contact) and the stabilization of AlN particles in water (surface crystallinity, surface chemical modification, and surface ionic charge) were seen to be strongly dependent on the acidic agent. The H 3PO 4 treatment was effective against hydrolysis of AlN due to the formation of a phosphate-based protection layer on the particles' surface, and, although it keeps the pH of the suspension below 4, it does not guarantee a good dispersion. The individual adsorption of the anionic surfactant at the surface of AlN particles suspensions did not completely suppress the hydrolysis but it did enhance the degree of dispersion. A proper combination of the two types of surface-active agents enabled the preparation of AlN aqueous suspensions of relatively low viscosity and high AlN concentration, which can be a good starting point for aqueous-based colloidal shaping techniques or for freeze granulation or spray drying to obtain suitable granulate powder characteristics for dry-pressing technologies. An adsorption mechanism of the surface-active agents onto the particles' surface is proposed and supported by NMR and FT-IR analyses.

Homogeneous Distribution of Sintering Additives in Liquid‐Phase Sintered Silicon Carbide

The addition of sintering additives to silicon carbide particles by electrostatic adsorption of colloidal A12O3 and Y2O3 sols has been studied as a way to achieve an optimum homogeneity in the microstructure. The adsorption behavior of the sol particles was examined by electrophoretic measurements and X‐ray fluorescence analysis. Both A12O3 and Y2O3 sols could simultaneously be adsorbed on the SiC particle surfaces. Viscosity measurements showed that the colloidal sol particles had a stabilizing effect on the slip, and hence slips with relatively high solid loadings could be prepared without adding extra dispersing agent. Liquid‐phase‐sintered silicon carbide materials (LPS‐SiC) with 2 wt% A12O3 and 1 wt% Y2O3 were prepared by freeze granulation/ pressing and sintering at 1880deg;C for 4 h. The homogeneity of the green compacts was quantified using a spot analysis technique in an electron probe microanalyzer. It was clearly shown that the addition of sols gave a more homogeneous microstructure than the reference sample with Y2O3 and A12O3 added as powders. The addition of sintering additives as sols also enhanced the sintering behavior.