Ind. Eng. Chem. Res. 1998, 37, 2611-2617
2611
Reductive Regeneration of Sulfated CuO/Al2O3 Catalyst-Sorbents in Hydrogen, Methane, and Steam C. Macken and B. K. Hodnett* Department of Chemical and Environmental Sciences, University of Limerick, Limerick, Ireland
A study of the reductive regeneration of sulfated 4.3% CuO/Al2O3 catalyst-sorbents suitable for the simultaneous removal of SO2 and NOX from flue gases was carried out with various reductants. H2 demonstrated a propensity to form CuS (ca. 20% CuS at 400 °C) and subsequent formation of H2S above 550 °C. Surface aluminum sulfate species are slowly reduced with H2 to the sulfide at temperatures in excess of 450 °C. Subsequently, H2S forms at higher temperatures, while bulk aluminum sulfate species were found to reduce in H2 directly to Al2O3 with the formation of H2S above 550 °C. Although a much weaker reducing agent, regeneration with CH4 leads to significantly less CuS formation and no H2S production. The formation of CuS, which was not reduced by CH4, occurred via the readsorption of product SO2 gases on the catalyst-sorbent. Interestingly, no CuS was formed when water vapor was added to the CH4 reductant gases and more SO2 evolved in these conditions. Introduction The CuO/Al2O3 catalyst-sorbent system has received much attention for the simultaneous removal of SO2 and NOX from flue gases because (i) it readily adsorbs SO2 in the presence of O2 to form CuSO4, at typical flue gas temperatures (Lowell et al., 1971); (ii) it is relatively easy to regenerate under reducing conditions, liberating SO2 (McCrea et al., 1970); (iii) copper compounds (Cu, CuO, or CuSO4) are active for the selective reduction of NOX gases to N2, with NH3 as a reducing agent, thus giving the potential for simultaneous removal of SO2 and NOX (Strakey et al., 1979); and (iv) it has high stability over an extended reaction (i.e., SO2 and NOX removal) followed by regeneration in cyclic operation (Centi et al., 1995). This research focuses on the continued development of a CuO/Al2O3 catalyst-sorbent suitable for the simultaneous removal of SO2 and NOX (by NH3 injection) from flue gases, specifically to treat flue gases from power generation plants burning low to medium sulfur fuel oils. The first step in the process involves the catalytic oxidation of SO2 to chemisorbed SO3 species by the CuO/ Al2O3 catalyst-sorbent, forming copper sulfate in a radial-type mobile bed reactor at the temperature of the flue gas (ca. 350 °C). CuO
SO2 + 1/2O2 98 SO3
(1)
SO3 + CuO f CuSO4
(2)
The copper sulfate, together with the unreacted oxide, catalyzes the reduction of NOX to N2, with NH3 as the reducing agent. CuO-CuSO4
4NO + 4NH3 + O2 98 4N2 + 6H2O
(3)
CuO-CuSO4
2NO2 + 4NH3 + O2 98 3N2 + 6H2O (4) * Corresponding author. Tel: 353-61-202246. Fax: 353-61202568. E-mail:
[email protected].
The sulfated CuO/Al2O3 catalyst-sorbent can be transferred to a regeneration unit at 400-500 °C, where a reducing agent (CH4, H2 etc.) regenerates the catalystsorbent with the evolution of a concentrated stream of SO2 (>30%). It is possible to convert the SO2 stream to sulfuric acid or sulfur.
CuSO4 + 2H2 f Cu0 + SO2 + 2H2O
(5)
CuSO4 + 2CO f Cu0 + SO2 + 2CO2
(6)
CuSO4 + 1/2CH4 f Cu0 + SO2 + 1/2CO2 + H2O (7) The regenerated Cu/Al2O3 catalyst-sorbent is then oxidized to CuO/Al2O3 before commencing a new reaction-regeneration cycle.
Cu + 1/2O2 f CuO
(8)
The regeneration of the sulfated CuO/Al2O3 catalystsorbent can be carried out by reductive treatment with H2, CH4, or C3H8 (McCrea et al., 1970; Dautzenberg et al., 1971; Yeh et al., 1985). H2 is a very strong reducing agent and is capable of reducing the sulfated CuO/Al2O3 catalyst-sorbent at the temperature of the flue gas desulfurization step. However, when H2 is employed as the reductant, there is a strong propensity to form copper sulfide (ca. 33%) by a competitive parallel reaction, thus lowering the amount of SO2 sorbed by the CuO/Al2O3 catalyst-sorbent in the following oxidativesulfation step (Dautzenberg et al., 1971). The formation of Cu2S was predicted by Dautzenberg et al. (1971), Princiotta et al. (1979), and more recently by Kiel et al. (1992) while McCrea et al. (1970) and Centi et al. (1990) proposed that CuS forms during reduction of the sulfated catalyst-sorbent with H2. In addition, H2S formation is also a possibility at temperatures above 500 °C, from the reduction of copper sulfide (Cu2S or CuS) or aluminum sulfate species (McCrea et al., 1970; Centi et al., 1990, 1994). The regeneration of sulfated CuO/Al2O3 with CH4 is slower and requires higher temperatures (ca. 150 °C),
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2612 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
compared to the use of H2 as a reducing agent (McCrea et al., 1970; Centi et al., 1995). McCrea et al. (1970) found that temperatures in excess of 400 °C were required for efficient regeneration of the sulfated CuO/ Al2O3 catalyst-sorbent with CH4. These authors also found that the regeneration rate was relatively independent of methane partial pressure but strongly dependent on temperature. When CH4 is employed as the reducing agent, SO2 is the only sulfur-containing species found in the gas stream (Centi et al., 1995). To date, there is no literature evidence to suggest the formation of copper sulfide during the reduction of the sulfated CuO/Al2O3 catalyst-sorbent with CH4. Work described in this paper focuses on the regeneration of the sulfated CuO/Al2O3 catalyst-sorbents. The reducing power of different reductants (H2, CH4, C3H8, and NH3) was evaluated by comparison of their temperature-programmed reduction (TPR) profiles in the reduction of sulfated 4.3% CuO/Al2O3 catalyst-sorbents. The regeneration of the sulfated 4.3% CuO/Al2O3 was tested isothermally with (i) 5% H2 or (ii) 5% CH4 at temperature intervals between 300 and 600 °C, with the reduction rates, gaseous product distribution, and conversion of CuSO4 (with SO2 evolution) evaluated. Finally, the effect of the addition of the reaction products (H2O and SO2) on the regeneration reaction with CH4 was investigated. Experimental Section The 4.3 wt % CuO/Al2O3 sample (coded 4.3CuO/Al2O3 hereafter) was prepared by the incipient wetness impregnation technique according to the following procedure. A sufficient amount of RP535 Al2O3 beads (mean diameter, 2 mm; surface area, 130 m2‚g-1) supplied by Rhoˆne Poulenc was calcined at 500 °C for 4 h. The copper sulfate precursor (140 g dissolved in 1000 mL of purified water) was added dropwise (2 h) to 1 kg of dry Al2O3 (pore volume, 1.2 cm3‚g-1). The preparation mixture was stirred constantly, while under a slight vacuum during the addition of the precursor. The sample was initially dried under vacuum at 90 °C for 4 h, followed by a further drying stage at 110 °C for 15 h in an electric oven. Finally, the catalyst-sorbent was calcined in a muffle furnace at 250 °C for 8 h and subsequently at 500 °C for 3 h (the heating rate in each step was 1 °C‚min-1). Temperature-programmed reductions were performed on sulfated CuO/Al2O3 and oxidized CuO/Al2O3 catalystsorbents with various reductants. The TPRs of sulfated CuO/Al2O3 samples were monitored by following the product gases such as SO2 and H2S (with 5% H2 in He as the reductant) and SO2 and CO2 (with 5% CH4 in He as the reductant) with a mass-selective detector. The selection of appropriate experimental conditions and their effect on a TPR profile has received a lot of attention in the literature (Monti and Baiker, 1983; Bosch et al., 1984). To avoid distortions in the TPR profiles, Malet and Caballero (1988) defined the parameter P:
P ) βS0/FC0
(9)
where β ) heating rate (K‚min-1), S0 ) initial amount of reducible species (µmol), F ) flow rate (cm3‚min-1), and C0 ) initial H2 concentration (µmol‚cm-3).
This parameter should be kept as low as possible (i.e.,
