Synthesis, characterization, and kinetics of mixed copper-aluminum and iron-aluminum oxides for high-temperature desulfurization

Abstract

Hot-gas desulfurization is an important step for optimizing the process economics of new schemes for power generation from coal. Mixed oxides such as CuO•Al[2]O[3] and Fe[2]O[3]•Al[2]O[3] are attractive as high-temperature, regenerable, desulfurization sorbents because they exhibit higher performance than CuO and Fe[2]O[3]. Mixed copper-aluminum and iron-aluminum oxides were prepared in porous form by the citrate process under various calcination conditions for subsequent reduction and sulfidation studies. The oxide samples were characterized by several techniques to determine chemical structure and texture. For the mixed copper-aluminum oxides, atomic absorption spectroscopy (AAS) provided the fractions of copper, soluble and insoluble, in hot nitric acid which closely corresponded to CuO and CuAl[2]O[4], respectively; x-ray diffraction (XRD) provided complementary information about the content of the pure and compound oxides; and a combination of x-ray line broadening, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) provided an estimate of the size of crystallites or phase domains. For the mixed iron-aluminum oxides, XRD identified crystalline phases, SEM revealed the changing surface texture with iron composition, and BET surface area measurements indicated the content of free alumina. Temperature-programmed reduction (TPR) of mixed oxides was more complex than TPR of the pure, reducible oxides. The compound oxide, CuAl[2]O[4], and part of CuO closely associated with Al[2]O[3] were reduced much more slowly than bulk CuO. Similarly, the compound oxide, FeAl[2]O[4], a solid solution between Fe[3]O[4] and FeAl[2]O[4], and Fe[3]O[4] in close association with alumina were reduced much more slowly than bulk Fe[3]O[4]. While oxides of +1 oxidation state, Cu[2]O and CuAlO[2], were identified as reduction intermediates for TPR of CuAl[2]O[4], no oxides of +1 oxidation state were identified for reduction of iron-aluminum oxides. Mixed copper-aluminum oxides were studied more extensively than mixed iron-aluminum oxides. The interaction between CuO and Al[2]O[3] seen in TPR studies was further examined by XRD, diffuse reflectance spectroscopy, and laser Raman spectroscopy. Pronounced sintering of CuAl[2]O[4] was observed to commence at temperatures in excess of 700°C, and the dispersion of copper on reduction of CuAl[2]O[4] was poorer than that obtained by reduction of mixed oxide, CuO and Al[2]O[3]. In studies using a thermogravimetric analyzer, sulfidation of reduced sorbents produced high-temperature digenite (Cu[9+x]S[5]) in the case of copper aluminum samples, and high-temperature pyrrhotite (Fe[1]-[x]S) in the case of iron-aluminum samples as the major crystalline products. Both CuAl[2]O[4] and FeAl[2]O[4] were found to be resistant to sulfidation as compared to the pure oxides, CuO and Fe[2]O[3], and to mixed oxides, CuO-Al[2]O[3] and Fe[2]O[3]-Al[2]O[3]. Formation of copper sulfate during air regeneration of sulfided Cu-Al-O samples was increased in the presence of free alumina