Mixed oxides, prepared by calcination at 950 °C for 6 h of powder mixtures of manganese and copper oxides, have been studied as regenerable sorbents for hot coal gas desulfurization. For the stabilization of copper in the oxidation states 2+ or 1+ under strong reducing conditions of coal gas, different concentrations of component oxides have been used enhancing the formation of different mixed oxides. Copper was not stabilized by manganese oxides, but its presence in the fresh sorbent was completely necessary because it increases the sorbent reactivity and keeps the H 2 S concentration in the outlet gas from a reactor below 50 ppmv. Thermogravimetric experiments and performance tests in a fixed bed reactor have allowed the optimization of the operating conditions for a sorbent MC (1:0.1) showing a good performance in multicycle tests without apparent decay.
The performance of a ZnO-doped manganese oxide MZ(1:0.1) as a regenerable sorbent for hot
coal gas desulfurization in 70-cycles tests in a fixed bed reactor has been studied. Sulfidation
has been carried out at 700 °C using a simulated RKW coal gas, while regeneration of the sulfided
sorbent has been conducted at 800 °C using pure air. The performance was very good as deduced
from low pre-breakthrough H2S concentration in the outlet gas, high sorbent efficiency, and long
durability. Despite that, a slight efficiency decay was observed that will make the sorbent become
inactive after a certain number of successive cycles in use. Simple methods of preparation, similar
to those used in the manufacture of the original fresh sorbents, are shown to be effective in
recovering the activity of this sorbent. Additionally, because manganese sulfate plays an important
role in the long durability exhibited by manganese based sorbents, the evolution of this compound
in the reactor bed in a typical regeneration test has been studied by FTIR spectroscopy. It is
formed during the first stages of regeneration but it decomposes then, although not completely,
in the last stages of the process, acting as a por-forming additive, like graphite, in every cycle.
New technologies for power generation demand the development of regenerable sorbents for hot coal gas desulfurization down to very low levels of H 2 S. Unfortunately, long-term tests in different reactors show that usual regenerable sorbents exhibit such efficiency decay and degradation of mechanical properties that their use is impractical in the current state of the art. In a previous study, some long-term tests in a fixed bed reactor have shown that Zn-or Cu-doped manganese oxide sorbents exhibit good performance. The highlighting features in the behavior of these sorbents are a substantial change in sulfidation reactivity as they are doped with zinc or copper oxides and a pronounced efficiency increase during the first 1-5 sulfidation-regeneration cycles. To understand this apparent behavior, a detailed characterization study of fresh, sulfided, and regenerated sorbents has been carried out using X-ray diffraction and transmission electron microscopy techniques. This study shows that the structure and degree of dispersion of the active phases in the fresh and regenerated sorbents and those of sulfides in the sulfided sorbents are highly dependent on the chemical nature of the dopant used. Zn enhances the formation of mixed oxide or sulfide species, while Cu increases mostly the degree of dispersion. These structural and textural changes can be associated with the high sulfidation reactivity exhibited by the studied manganese-based sorbents.
In a previous study it was shown that the overall reactivity in thermobalance tests and the efficiency at breakthrough in 2-cycle tests in a fixed bed reactor of a sorbent used for hot coal gas desulfurization increased substantially by the addition of a 5 wt % concentration of graphite during the preparation of sorbent ZT(0.8:1). In this paper, the study of the graphite effect as a pore forming additive is extended to other types of sorbents whose performance is investigated through the sulfidation breakthrough curves obtained in 20-cycle tests in a fixed bed reactor. Experiments show that the efficiency of the studied sorbents, ZFC, ZFT, and ZT, a zinc oxide, and a zinc ferrite doped with TiO 2 or CuO, respectively, are dramatically increased by the addition of a 5 wt % graphite concentration as compared with the efficiency shown by the same sorbents prepared without graphite. The graphite effect is also evidenced by the achievement of a substantial delay of spalling appearance, a detrimental process consisting in the appearance and development of a crack network in the extrudate, decreasing the mechanical strength of the sorbent progressively down to excessively low values, after enough number of successive sulfidation-regeneration cycles. A first study trying to recover the reactivity of a sorbent, that became inactive after 20-cycles operation in a fixed bed reactor, by graphite addition is also included.
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