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Ind. Eng. Chem. Res. 2010, 49, 10144–10148
Regeneration of Sulfur Deactivated Ni-Based Biomass Syngas Cleaning Catalysts Liyu Li,* Christopher Howard, David L. King, Mark Gerber, Robert Dagle, and Don Stevens Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99354
Nickel-based catalysts have been widely tested for reforming undesired tar and methane from hot biomassderived syngas. However, nickel catalysts readily deactivate through the adsorption of sulfur compounds in the syngas. We report a new regeneration process that can effectively regenerate sulfur-poisoned Ni reforming catalysts. This process consists of four sequential treatments: (1) controlled oxidation at 750 °C in 1% O2, (2) decomposition at 900 °C in inert gas, (3) reduction at 900 °C in 2% H2, and (4) reaction at 900 °C under reforming condition. This 4-step regeneration process might have advantages over the conventional steam regeneration process. Introduction Because of their successful application for hydrogen production via steam reforming of hydrocarbons and methane, nickel-based catalysts have been widely tested for decomposing tar and reforming excess methane in hot biomass syngas cleanup processes.1,2 However, the sulfur gases in the biomass syngas greatly decrease the reforming activity of Ni catalysts because of the strong chemisorption of sulfur on the Ni surface.1-3 Because the Ni surface chemisorption of sulfur is reversible, the sulfurdeactivated Ni catalysts can be regenerated in a reducing environment at high temperatures.4 The major disadvantage of this regeneration process is its slow sulfur removal rate, which declines exponentially with time. This process also requires expenditure of a large volume of sulfur-free reducing gas. In industrial hydrogen production practice, under desulfurization unit upset conditions, sulfur-poisoned steam reforming catalysts are regenerated by sequential treatments of steam, steam-air mixture, and steam-hydrogen mixture (H2O/H2 molar ratio of 100) at about 650 °C.5-7 The steaming treatment removes sulfur in the form of SO2 and H2S and oxidizes Ni to NiO via the following reactions Ni - S + H2O ) NiO + H2S
(1)
H2S + 2H2O ) SO2 + 3H2
(2)
Ni + H2O ) NiO + H2
(3)
Carbon formation is always observed on sulfur-poisoned Ni catalysts. The introduction of a small amount of air with steam can completely remove aged carbon deposits as CO2 2Ni - C + 3O2 ) 2NiO + 2CO2
(4)
Some NiSO4 always forms during the steam and steam/air treatments, which requires further treatment with steam/ hydrogen mixtures at molar ratio of H2O/H2 higher than 100. A high ratio of H2O/H2 can prevent reduction of NiO to metallic Ni, which can readsorb H2S on its surface. Under this condition, NiSO4 decomposes to NiO and sulfur is removed as H2S: NiSO4 + 4H2 ) NiO + H2S + 3H2O
(5)
After sulfur removal, the catalysts are further reduced in H2 and then put back to steam reforming service. Normally this * Corresponding author. Tel: (509) 375-2572. Fax: (509) 375-2186. E-mail:
[email protected].
process can effectively remove the sulfur adsorbed on the surface of Ni catalysts and restores their reforming performance. One disadvantage with this regeneration process is its relatively long duration, which can easily require 2-3 days. Aguinaga and Montes reported a multicycle oxidationreduction process for regeneration of a Ni-SiO2 catalyst poisoned by thiophene.8 Three cycles of reduction (in pure H2) and oxidation (5% O2 in Ar) treatment at 500 °C were reported to be able to remove 86% sulfur from the catalyst and recover 60% of its hydrogenation activity. Treatment at temperatures higher than 500 °C was not studied because of the expected severe sintering effect for the Ni-SiO2 hydrogenation catalyst. Here we report on a new regeneration process, which can remove sulfur from the Ni-based reforming catalyst and restore its catalytic activity in a more effective and time-efficient manner than the conventional treatment. Experimental Section Steam reforming of CH4 in a tar-free biomass syngas simulant (18.4% H2, 11.4% CO2, 12.7% CO, 6.2% CH4, 2.9% N2, and 48.4% H2O) at 750 °C was used as a model reaction to monitor activity. A commercial Ca-promoted 20 wt % Ni on Al2O3 reforming catalyst (G90-B from United Catalyst Inc.) was used throughout this work. About 0.5 g of 60-100 mesh catalyst particles was loaded into a 1/4 in. i.d. stainless steel fixed bed reactor, which was heated in a clam-shell furnace. Before the CH4 reforming test, the catalyst was reduced in 200 sccm 10% H2 in Ar (∼24 000 h-1 gas hourly space velocity, GHSV) at 500 °C for 4 h. The reforming activity of the reduced catalyst was measured with 300 sccm sulfur-free syngas (36 000 h-1 GHSV) at 750 °C for 12-16 h. After that, 50 ppmv H2S (source gas, 1000 ppmv H2S in He) was introduced into the biomass syngas to deactivate the Ni catalyst. This sulfur treatment normally lasted 4 h. The catalyst was then regenerated following various procedures. After regeneration, the CH4 reforming activity was measured again in 300 sccm sulfur-free syngas at 750 °C. All the experiments were carried out under ambient pressure. The flow rates of biomass syngas, 1000 ppm H2S in He, and regeneration gases (air, Ar, N2, H2) were metered using MKS mass flow controllers. Steam was generated using a small cartridge vaporizer, and steam flow was controlled by metering water to the vaporizer using a HPLC pump. Downstream of the catalyst bed, water was removed with a condenser followed by a 50-tube Nafion membrane dryer (Perma Pure LLC, Toms
10.1021/ie101032x 2010 American Chemical Society Published on Web 09/14/2010
Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010
Figure 1. Sulfur effect on the CH4 steam reforming activity of a commercial 20 wt % Ni-Al2O3 catalyst.
River, NJ, USA). The off-gas composition was monitored continuously during the reaction using a micro gas chromatograph (micro-GC, Agilent 3000A). The dry off-gas flow rate was calculated using N2 concentration and N2 flow rate as the internal standard. The sulfur concentration in the off-gas was monitored using a sulfur chemiluminescence detector (SCD) installed on an Agilent 6890 GC. This GC-SCD system has a sulfur detection limit of 10 ppbv. The sulfur-free biomass syngas used in this work contains about 20 ppbv sulfur. The CH4 conversion was calculated as CH4 conversion % ) (CH4 concentration in dry off-gas)(dry off-gas flow rate) 1100 (CH4concentration in dry feed gas)(dry feed gas flow rate)
(
)
Results and Discussion Figure 1 shows that at 50 ppm, H2S can dramatically decrease the CH4 steam reforming activity of the Ni-based catalyst G90-B operating at 750 °C. After H2S was removed from the syngas, the catalyst’s reforming activity was only partially recovered. At 750 °C, under the employed test conditions, the equilibrium H2S concentration with bulk Ni3S2 is about 750 ppm (3Ni + 2H2S T Ni3S2 + 2H2 Kp ) 6.8 × 104), much higher than the 50 ppm H2S concentration in the feed. Since formation of bulk Ni3S2 is thermodynamically not favorable, most sulfur must be chemisorbed on the active Ni surface. About 0.06 wt % sulfur was absorbed by this catalyst during the 4 h sulfur exposure treatment, which was less than 6% of the total sulfur fed in. To regenerate this sulfur-poisoned reforming catalyst, we evaluated several different regeneration methods, including the currently practiced sequential steam-steam/air-steam/hydrogen treatment, high-temperature (900 °C) sulfur-free reaction treatment, high-temperature (900 °C) steaming, controlled oxidation in 1% O2 gas at 750 °C, and an oxidation-decomposition treatment. The regeneration conditions are given in Table 1. The CH4 reforming performance at 750 °C after these different regeneration treatments are shown in Figure 2. Although no sulfur was added to the feed gas in these tests, a certain amount of sulfur, previously absorbed on the catalyst and not effectively removed during the regeneration treatment, is released into the gas stream, as shown also in Figure 2. It is clear that none of these regeneration methods were able to completely remove sulfur from the deactivated catalyst, notably the conventional sequential steam, steam/air, and steam/hydrogen treatment. Note that in this work, a relatively short treatment duration (850 °C) after oxidation. Figure 3a gives the sulfur removal profile during controlled oxidation in 1% O2 at 750 °C. When a low flow rate of the oxidizing gas mixture (105 mL/min, 12 000 h-1 GHSV) was introduced, about 35% of the sulfur absorbed on the Ni catalyst was removed as SO2 (reaction 6). However, when 1% O2 gas mixture was introduced at a higher flow rate (210 mL/min, 24,000 h-1 GHSV), almost no sulfur was removed (see the initial 3/4 h’s performance in Figure 3b). This unexpected result implies that with the higher flow of O2, all the sulfur was directly oxidized to NiSO4, and reaction 7 dominates reaction 6. Clearly, effective sulfur removal requires careful control of a number of variables, including flow rate. Ni - S + 3/2O2 ) NiO + SO2
(6)
Ni - S + 2O2 ) NiSO4
(7)
To regenerate metallic hydrogenation catalysts, Katzer and Windawi described an oxidation process using gases with oxygen concentration of about 1-10 ppm at ∼400 °C.9,10 Very long treatment times (up to 600 h) were required to completely regenerate the deactivated metal catalysts since an extremely low oxygen partial pressure was used. Regeneration using gas with higher oxygen concentration (>10 ppm) at 400 °C was reported to be unsuccessful. Hughes patented a similar process for sulfur decontamination of conduits and vessels communicating with hydrocarbon conversion catalytic reactors.11 Gases with oxygen concentration of 12 h) was required. Conclusion A new regeneration method has been developed that can effectively and efficiently remove adsorbed sulfur from Ni-based steam reforming catalysts. This regeneration method includes four steps: (1) oxidation at 750 °C in low flow 1% O2 to generate SO2 and NiSO4; (2) decomposition of the NiSO4 to NiO and SO2 at 900 °C in inert gases; (3) reduction at 900 °C in 2% H2 to slowly generate Ni from NiO and release additional H2S; (4) reaction at 900 °C with clean biomass syngas containing steam to further remove traces of residual adsorbed sulfur. This novel regeneration is faster than the conventional regeneration process. After regeneration, the reforming performance of the deactivated catalyst was fully recovered. Although it seems possible to rapidly regenerate sulfurpoisoned Ni-based tar cleaning catalysts, under normal operation conditions, it is still better to remove all the sulfur molecules
(1) Torres, W.; Pansare, S. S.; Goodwin, J. G., Jr. Hot Gas Removal of Tars, Ammonia, and Hydrogen Sulfide from Biomass Gasification Gas. Catal. ReV. 2007, 49 (4), 407. (2) Yung, M. M.; Jablonski, W. S.; Magrini-Bair, K. A. Review of Catalytic Conditioning of Biomass-Derived Syngas. Enery Fuels 2009, 23, 1874. (3) Ma, L.; Verelst, H.; Baron, G. V. Integrated High Temperature Gas Cleaning: Tar Removal in Biomass Gasification with a Catalytic Filter. Catal. Today 2005, 105, 729. (4) Rostrup-Nielsen, J. R. Some Principles Relating to the Regeneration of Sulfur-Poisoned Nickel Catalyst. J. Catal. 1971, 21, 171. (5) Rostrup-Nielsen, J. R. Catalytic Steam Reforming; Anderson, J.R., Boudart, M., Eds.; Catalysis, Science and Technology; Springer: Berlin, 1984; Vol. 5, p 1. (6) Houken, J. How to Regenerate Sulfur-poisoned Steam Reforming Catalyst. AIChE Symp. 1985, 26, 19. (7) Lombard, C.; Le Doze, S.; Marencak, E.; Marquaire, P.-M.; Le Noc, D.; Bertrand, G.; Lapicque, F. In situ Regeneration of the Ni-Based Catalytic Reformer of a 5kW PEMFC System. Int. J. Hydrogen Energy 2006, 31, 437. (8) Aguinaga, A.; Montes, M. Regeneration of a Nickel/Silica Catalyst Poisoned by Thiophene. Appl. Catal., A 1992, 90, 131. (9) Katzer, J. R.; Windawi, H. Process for the Regeneration of Metallic Catalysts. U.S. Patent 4 260 518, 1981. (10) Windawi, H.; Katzer, J. R. AES Study of Oxidation of Surface and Bulk Sulfides of Ni. J. Vac. Sci. Technol. 1979, 16 (2), 497. (11) Hughes, T. R. Sulphur Decontamination of Conduits and Vessels Communicating with Hydrocarbon ConVersion Catalyst Reactor during in situ Catalyst Regeneration. U.S. Patent 4 610 972, 1986.
ReceiVed for reView May 5, 2010 ReVised manuscript receiVed August 5, 2010 Accepted September 1, 2010 IE101032X
