Performance in a Fixed-Bed Reactor of Titania-Supported Nickel

Ind. Eng. Chem. Res. 2006, 45, 157-165

157

Performance in a Fixed-Bed Reactor of Titania-Supported Nickel Oxide as Oxygen Carriers for the Chemical-Looping Combustion of Methane in Multicycle Tests Beatriz M. Corbella,† Luis F. de Diego,‡ Francisco Garcıá-Labiano,‡ Juan Ada´ nez,‡ and Jose´ M. Palacios*,† Instituto de Cata´ lisis y Petroleoquı´mica (CSIC), Campus UAM-Cantoblanco, 28049 Madrid, Spain, and Department of Energy and EnVironment, Instituto de Carboquı´mica (CSIC), Miguel Luesma Casta´ n 4, 50015 Zaragoza, Spain

Chemical-looping combustion has been proposed as an alternative process for the complete elimination of CO2 emissions to the atmosphere in the combustion of carbonaceous products, such as natural gas. In this case, the combustion is a two-stage process. In the first stage, the structural oxygen contained in a reducible inorganic oxide is used for the combustion of the natural gas. In the second stage, the reduced oxygen carrier is regenerated with pure air to reinitiate a new combustion cycle. In this paper, nickel oxide supported on porous rutile is used as an oxygen carrier for the chemical-looping combustion of methane, as the main component of natural gas. The performance is assessed in 20-cycle tests in a fixed-bed reactor at 900 °C, using either dilute (20 vol % in N2) or pure methane for the reduction stage and pure air for the regeneration stage. The experimental results reveal that the reactions in the two involved processes are fast, as CO2, before breakthrough, is the only compound detected in the outlet gas of the reduction stage. However, in the reduction stage, the thermal decomposition of methane appears as a side reaction, already acting at the start of the test in clear competition for methane consumption with the main reaction of the chemical-looping combustion. In this case, carbon is mostly deposited as uniform coatings on Ni catalyst particles. Because this deposited carbon will evolve then as CO2 in the outlet gas of the next regeneration stage, its presence poses some limitations to the achievable maximum efficiency in CO2 capture in a chemical-looping process. Moreover, rutile does not behave as a completely inactive support, especially using pure methane. Conversely, through its partial reduction, it acts as an additional oxygen source for methane combustion that must be taken into account. A slight performance decay and significant porosity increase of the oxygen carriers with the number of cycles were observed in a 20-cycle test in a fixed-bed reactor, which should be assessed in further longterm tests in future work. 1. Introduction Because of pronounced greenhouse effects, CO2 emissions to the atmosphere, mainly coming from the combustion of carbonaceous materials including oil, natural gas, and coal, are causing significant concern in the developed countries in recent decades. Chemical-looping combustion (CLC) has been proposed recently as an alternative method for complete CO2 capture. In this case, the structural oxygen contained in a reducible inorganic oxide is used for the combustion of the carbonaceous material, instead of air as in conventional combustion processes.1-5 CLC is a two-stage process. (a) In the first stage, called the reduction stage, the carbonaceous material is burned using the lattice oxygen of an inorganic oxide. The outlet gas from the reactor is exclusively composed of steam and CO2. The pure CO2, obtained after separation of the steam by condensation, can either be used for multiple direct applications or be stored in suitable locations depending on local necessities and CO2 production. (b) In the second stage, called the regeneration stage, the carrier coming from a prior reducing stage, found in a reduced state, is regenerated with pure air, usually in a second reactor. The outlet gas from this regeneration stage is exclusively * To whom correspondence should be addressed. Tel.: +34-915854787. Fax: +34-91-5854760. E-mail: [email protected]. † Instituto de Cata´lisis y Petroleoquı´mica. ‡ Instituto de Carboquı´mica.

composed of N2 and, perhaps, some traces of unreacted oxygen. The operating temperatures in the two stages, determined by their respective reaction rates, should be high enough for thermal activation but low enough to prevent excessive thermal sintering. Usually, this balance is achieved at an operating temperature of around 900 °C, which is low compared to that used in a conventional combustion process. At this low operating temperature, the formation of thermal NOx in the regeneration stage becomes highly improbable, and consequently, this is another advantage derived from CLC utilization. Complete CO2 capture and low NOx emissions in the combustion of a carbonaceous material make CLC a highly attractive process. Unfortunately, many efforts have to be made before suitable oxygen carriers become available for industrial applications. In a single-cycle test, the involved chemical reactions in the respective reduction and regeneration stages should be fast to achieve high reactant conversions. Additionally, selectivity to CO2 in the reduction stage should be very high, preventing the occurrence of side reactions as possible, especially CO emissions and thermal decomposition promoting carbon deposition on the oxygen carrier, which can cause overall efficiency loss of the process in CO2 capture. In multicycle tests, the oxygen carrier is repeatedly subjected to severe chemical and thermal stresses, probably leading to progressive mechanical degradation and performance decay with increasing number of cycles. A prior study in thermobalance6 showed that nickel oxide exhibits high reactivities both in the reduction stage with

10.1021/ie050756c CCC: $33.50 © 2006 American Chemical Society Published on Web 11/17/2005

158

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006

methane and in the regeneration stage with air. Consequently, nickel oxide is a potential oxygen carrier suitable for use in the CLC of methane that should be assessed in further reactor tests. However, the appearance of deposited carbon from the thermal decomposition of methane might be a drawback in the use of this type of oxygen carrier. Further studies showed that enhanced reactivities and/or simultaneous suppression of carbon deposition could be achieved by using nickel oxide doped with cobalt oxide7 and including small steam concentrations in the composition of the feed gas in the reducing stage.8,9 Nickel oxide-based oxygen carriers exhibiting high reactivities and high mechanical strengths can be obtained by impregnation of a porous support.10 To decrease the derived effects of severe chemical and thermal stresses, it is very important to check support passivity along the two stages involved in a CLC process. Moreover, the maximum amount of oxygen available for CLC could be higher if this support were highly macroporous. Support inactivity will probably enhance the carrier ability to withstand a high number of successive cycles without substantial mechanical degradation. A recent study11 using nickel oxide-based carriers prepared by wet impregnation on porous rutile showed that increasing nickel oxide loadings can be achieved through successive impregnations with a saturated aqueous solution of nickel nitrate. The mechanical strength of the prepared fresh carriers was very high, and the performance in five-cycle tests of CLC with pure methane in a fixed-bed reactor was excellent regardless of the nickel oxide loading, despite the progressive decrease in pore volume observed with increasing load. However, the concentration of carbon deposited in the reactor bed increased almost linearly with time, and the partial reduction of rutile, as an additional oxygen source for methane combustion not taken into account, was also observed. The appearance of deposited carbon implies some limitations to the maximum carrier efficiency to be achieved in CO2 capture, and the partial reduction of the support enhanced the appearance of low-level CO emissions in the reduction stage that need to be assessed in long-term tests. In this paper, the performance of a sample 7NT with the highest available nickel content is studied in 20-cycle CLC tests in a fixed-bed reactor using for the reduction stage of either dilute methane (20 vol % in N2), to increase the quality of data acquisition and analysis resolution, or pure methane, more useful from a practical standpoint. Their respective CO2 breakthrough curves, the evolution of deposited carbon, and the partial reduction of rutile as the support in the reduction stage are studied through an analysis of the evolved gases by GC and solid characterization using different techniques including powder X-ray diffraction and scanning and transmission electron microscopies. 2. Experimental Section 2.1. Preparation of Oxygen Carriers. The studied fresh oxygen carrier was prepared as 0.2-0.4-mm-diameter particles by incipient wet impregnation with a saturated aqueous solution of nickel nitrate as the active phase precursor on titania as the porous support. A detailed description of the method of preparation can be found in ref 11. This sample is denoted 7NT, in which N ) NiO as the active phase and T ) TiO2 as the support. To achieve convenient thermal carrier stability at the operating conditions appropriate for the CLC of methane, all studied samples were calcined at 900 °C for 6 h before testing. 2.2. Characterization Techniques. Because the studied samples are macroporous in nature, the textural properties were determined by porosimetry of Hg intrusion in a Micromeritics

9310 apparatus filling pores down to 3 nm in size. For identification, quantification, and structural characterization of crystalline phases, powder X-ray diffraction (XRD) patterns were acquired on a Seifert 3000 diffractometer using Ni-filtered Cu KR radiation. A database including powder patterns of most known crystalline inorganic compounds was used for the identification of the crystalline chemical species present in the samples. Semiquantitative analysis of the crystalline phases found was subsequently obtained by obtaining complete structural data from ICSD database and further pattern processing using Rietveld methods with the program Powder Cell v. 2.3. The evolution of the concentration of the carbonaceous material deposited on the samples in the reduction stage and the morphology of the oxygen carriers were studied by scanning electron microscope (SEM; ISI DS-130) coupled to an ultrathin window (PGT Prism) detector for energy-dispersive X-ray (EDX) analysis. Thermoprogrammed reduction (TPR) profiles were obtained by passing a gas containing 10 vol % of H2 in Ar through 50 mg of sample at a flow rate of 51 cm3/min and heating at a rate of 10 °C/min to 950 °C. H2 evolution in the outlet gas along a thermal scan was continuously recorded using a thermal conductivity detector (TCD). High-resolution micrographs of the reduced titania used as the support and the carbon deposited during the reduction stage in a 20-cycle CLC test were obtained by transmission electron microscope (TEM; JEOL JEM-3010) operating at accelerating voltage of 300 kV, using a hot field-emission cathode as the electron source, with spherical and chromatic aberration constants of the objective lens of Cs ) 0.5 mm and Cc ) 1.1 mm, respectively, to achieve a point resolution of 0.2 nm. 2.3. Performance Tests. The performance of the prepared nickel oxide carrier 7NT was studied in 20-cycle tests of methane CLC, in a 1.6 cm i.d. × 34 cm height upflow fixedbed quartz reactor at an operating temperature of 900 °C and atmospheric pressure, using 30-40 g of sample and a total gas flow rate of 80 cm3/min of either dilute CH4 (20 vol % in N2) or pure CH4 for the reduction stage and pure air for the regeneration stage. The compositions of the inlet and outlet gases from the reactor were obtained by gas chromatography (GC) using a Chrompack column and a thermal conductivity detector (TCD). Steam was not detected in the outlet gas of the reduction stage because it was condensed out before reaching the detector to prevent filament damage. Because the amount of active phase present in the reactor bed along a multicycle reductionregeneration test is variable, because of the repeated sample extractions for solid characterization, the breakthrough curves were plotted as a function of the dimensionless time ratio t/t0. t0 in the reduction stage is the theoretical time necessary to achieve complete reduction of the active phase contained in the reactor bed, assuming complete conversion of CH4 in the inlet gas and taking into account the stoichiometry of the main reduction reaction, the molar flow rate of methane used as the reactant in the feed gas, and the initial number of moles of active phase contained in the reactor bed. 3. Results and Discussion 3.1. Characterization of the Fresh Sample 7NT. The crystalline phases found in sample 7NT and their respective weight concentrations and domain sizes as determined by powder XRD and subsequent treatment of patterns by Rietveld methods are reported in Table 1. In sample 7NT calcined at 500 °C immediately after the seventh impregnation, rutile is found as the major phase, and its domain size is high (175 nm). Rutile associated with the support comes from the transformation

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 159 Table 1. Composition and Crystalline Domain Sizes of the Fresh Sample 7NT Determined by Powder XRD rutile sample 7NT 500 °C 7NT 900 °C

NiO

NiTiO3

concentration size concentration size concentration size (wt %) (nm) (wt %) (nm) (wt %) (nm) 66 56

175 140

34 22

64 70

22

200

of anatase used as the raw material. Prior to impregnation, the titania extrudates were calcined at 1000 °C, and rutile was formed as the most stable phase of TiO2 at this high temperature. Ni species appear as a single phase of NiO, revealing that complete decomposition of the nitrate phase as the precursor of NiO was achieved at 500 °C. However, the absence of any sort of mixed oxides implies there was no substantial interaction between NiO and rutile at this temperature. The NiO concentration was 34 wt %, and consequently, the oxygen concentration in the carrier available for methane combustion in the reduction stage was 7.3 wt %. Because the complete reduction of the reactor bed is presumably reached in a short time, better control of the operating variables and better resolution in the gas analysis are obtained using dilute methane (20 vol % CH4 in N2) in the reduction stage of a 20-cycle test of the CLC of methane. The domain size of NiO is small (64 nm), indicating that it is mostly found occluded in support pores. After the seventh impregnation, sample 7NT was subsequently calcined at 900 °C for 1 h before testing, to achieve the desired thermal stability of the carrier at the operating conditions used in the CLC process. The presence of rutile in the sample calcined at this higher temperature was again evidenced, and a part of the active phase was also found to be NiO. However, the presence of a mixed oxide, NiTiO3 (22 wt %), was also evidenced in this sample, indicating that, at this higher calcination temperature, there was substantial interaction between the active phase and the support. The domain size of NiTiO3 is large (200 nm), close to that of the rutile domain size, suggesting that this mixed oxide is found as a more or less uniform-thickness coating on rutile also evidenced in other study.11 Before impregnation, the fresh rutile cylindrical extrudates showed a pore volume, Vp, of 0.29 cm3/g; a specific surface area, S, of 2 m2/g; and a pore size, dp, of 1.3 µm, indicating the macroporous nature of the support. Additionally, a crush strength of 30 N/mm reveals that this titania support imparts a high mechanical strength to the oxygen carrier. After impregnation, however, the fresh sample 7NT with a high nickel oxide content

Figure 1. Evolution of the CO2 breakthrough curves with the number of cycles in the reduction stage of a 20-cycle test of the CLC of dilute methane in a fixed-bed reactor.

had a pore volume, Vp, of 0.19 cm3/g with porosity reduced to about 65%. This suggests that the incorporation of the nickel phase, as NiO and/or NiTiO3, implies significant pore plugging of the support. 3.2. Carrier Performance in a 20-Cycle Fixed-Bed Reactor Test of the CLC of Dilute Methane. The chemical reactions, including free energies, ∆G; reaction enthalpies, ∆H; and equilibrium methane conversions, XCH4, for nickel oxide-based catalyst supported on rutile as oxygen carriers for the CLC of methane are the following:11

Reduction Stage 4NiTiO3 + 3/2CH4 f 4Ni + 2Ti2O3 + 3/2CO2 + 3H2O (1) ∆G ) -21 kJ/mol, ∆H) 573 kJ/mol, XCH4 ) 0.9272 4NiO + CH4 f 4Ni + CO2 + 2H2O

(2)

∆G) -259 kJ/mol, ∆H ) 134 kJ/mol, XCH4 ) 0.9999 CH4 f C + 2H2

(3)

∆G) -38 kJ/mol, ∆H ) 88 kJ/mol, XCH4 ) 0.9633 Regeneration Stage Ni + 1/2Ti2O3 + 3/4O2 f NiTiO3

(4)

∆G ) -293 kJ/mol, ∆H ) -444 kJ/mol Ni + 1/2O2 f NiO

(5)

∆G ) -134 kJ/mol, ∆H ) -234 kJ/mol Ti2O3 + 1/2O2 f 2TiO2(rutile)

(6)

∆G ) -259 kJ/mol, ∆H ) -381 kJ/mol C + O2 f CO2

(7)

∆G ) -394 kJ/mol, ∆H ) -394 kJ/mol Reactions 1 and 2 are the main reactions involved in the reduction stage. Reaction 3 is the thermal decomposition of methane occurring as a side reaction in clear competition for methane consumption with reactions 1 and 2. In a fixed-bed reactor, reaction 3 might play an important role, especially at the end of the reduction stage when almost all of the reactor bed lies completely converted and all oxygenated nickel species are absent. In the reduction stage, at the operating conditions used, rutile reduction is thermodynamically not possible, and consequently, it has not been included. In the regeneration stage, reactions 4 and 5 are the main involved reactions. Reaction 6 has been included because it was shown in a prior study11 that, at the operating conditions used, rutile can be partially reduced into phases not found in available structural databases. Chemical analysis and helium density measurements revealed that the stoichiometry of this reduced rutile was close to Ti2O3. Reaction 7 shows the conversion of the carbon deposited in the prior reduction stage into CO2, appearing as a component of the regeneration gas. The respective breakthrough curves of CO2, as the main reaction product of the reduction stage, in a 20-cycle test of CLC with dilute methane are shown in Figure 1. At first, the molar fraction xCO2 in the outlet gas is high, close to 1, indicating that methane combustion is highly selective. However, the molar fraction rapidly falls to xCO2 ) 0.30 at t/t0 ratios still low when

160


Figure 2. Evolution of the cumulative pore volume, Vp, of regenerated samples with the number of cycles in a 20-cycle test of the CLC of dilute methane.

the reactor bed, presumably, is still far from being completely converted. Afterward, the slope of the curve remains apparently low and almost constant, with breakthrough occurring at t/t0 ratios much higher than 1. From a chemical standpoint, these features of the breakthrough curves suggest the presence of two reducible chemical species in the bed reactor. According to the above XRD study of the fresh samples, these species might be NiO and NiTiO3 (Table 1). NiO is easy reduced by methane (reaction 2), and its presence in the fresh and regenerated samples could explain the shape of the breakthrough curves at low t/t0 ratios (high slope). NiTiO3, however, is not as easily reduced by methane (reaction 1), and its presence could explain the shape of the breakthrough curves at the highest t/t0 ratios (low slope). The NiO concentration, about 25 wt %, can be directly estimated from the respective t/t0 ratio at breakthrough in Figure 1. This value agrees well with the data in Table 1 for the NiO concentration in the sample calcined at 900 °C. It could be argued that, although both samples were prepared at the same temperature of 900 °C, the fresh sample in Table 1 was prepared by calcination of NiO impregnated on porous rutile, whereas regenerated samples were produced in the regeneration stage of a CLC process through the oxidation of a reduced sample, mostly composed of metallic Ni, in the prior reduction stage. Even assuming that the final compounds, NiO and NiTiO3, could be the same, their relative concentrations might be quite different. To obtain more conclusive results, additional studies are necessary. Another plausible explanation for the observed shape of the CO2 breakthrough curves, shown in Figure 1, could be associated with possible changes in reactivity, as a consequence of substantial changes in the textural properties in the oxygen carrier. Additionally, the apparent slow but progressive performance decay exhibited by carrier 7NT with the number of cycles, as shown in Figure 1, is also worth mentioning. Consequently, the evolution of the textural, chemical, and structural properties of carrier 7NT with the number of cycles must be pursued. 3.3. Evolution of the Textural, Chemical, and Structural Properties of the Oxygen Carriers in a 20-Cycle Test of the CLC of Dilute Methane. The evolution of the cumulative pore volume, Vp, determined by porosimetry of Hg intrusion is shown in Figure 2. Surprisingly, the porosity of regenerated samples increases substantially with the number of cycles, especially from cycle 10 to cycle 20. At first, one should expect a porosity decrease with the number of cycles accounting for possible accumulative thermal sintering. This porosity increase, however, is also evidenced through SEM micrographs taken from

Figure 3. Morphological appearance of regenerated samples in a 20-cycle test of CLC with dilute methane: (a) 5th cycle, (b) 20th cycle.

regenerated samples in cycles 5 and 20, shown in Figure 3a,b. Consequently, exclusively on the basis of the observed changes in the textural properties of the oxygen carriers, one should expect an improvement in the carrier performance with the number of cycles in a 20-cycle test in a fixed-bed reactor. In fact, these predictions were not observed, and the opposite tendency is rather apparent in Figure 1. It can be concluded that the apparent performance decay of sample 7NT with the number of cycles in a 20-cycle reactor test must be associated exclusively with evolving chemical effects. H2 TPR profiles of fresh and regenerated sample 7NT at different cycles are shown in Figure 4. All H2 TPR profiles show two reduction peaks, one at the low-temperature range of 300-500 °C and another at the higher-temperature range of 600-800 °C. The first can be clearly assigned to the reduction of NiO because it is the only reducible species found in the fresh sample (Table 1). The second should be assigned to a less reducible species of NiTiO3, as revealed by XRD, not found in the fresh sample. The most remarkable differences shown among the respective profiles with the number of cycles are as follows: (a) The intensity of the reduction peak at low temperature (NiO) decreases progressively, whereas the reduction peak at higher temperature (NiTiO3) becomes increasingly intense. (b) The low-temperature reduction peak becomes increasingly complex, and both reduction peaks are shifted progressively toward higher temperatures. These features suggest that the relative concentration of the less-reducible species in the sample, NiTiO3, increases with the number of cycles with respect to the more easily reduced species, NiO, as a consequence of the increasing interaction of NiO with rutile through the formation of this mixed oxide. This increasing interaction

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 161

Figure 4. H2 TPR profiles of fresh sample 7NT calcined at 900 °C and regenerated in different cycles in a 20-cycle test of the CLC of dilute methane.

also explains the progressive shifting toward higher temperatures of the respective reduction peaks. Consequently, the observed slight carrier performance decay in a 20-cycle test of CLC with dilute methane, evidenced by the respective breakthrough curves in the reduction stage shown in Figure 1, must be associated with a progressive increase in the NiTiO3 concentration with the number of cycles. The evolution of the chemical and structural properties of the carrier 7NT in the reduction stage of a 20-cycle CLC test with dilute methane was also studied by powder XRD, and the respective patterns are shown in Figure 5a,b. Figure 5a shows patterns of samples taken from the bottom of the reactor bed, i.e., the most reduced layers in an upflow fixed-bed reactor, at the end of the reduction stage (t/t0 > 2). All of the corresponding XRD patterns reveal the presence of metallic Ni as the only NI-containing phase. This indicates that all oxygenated nickel species, NiO and NiTiO3, initially present in the regenerated carrier were reduced to metallic Ni. Consequently, the two oxygenated Ni species contributed as oxygen donors in the reducing stage of the CLC of dilute methane. However, the breakthrough curves shown in Figure 1 indicate that the reactivity of NiTiO3 with dilute methane was much lower than that of NiO. Additionally, rutile, initially present as the only phase associated with the support in the regenerated carrier, has vanished in the reduced carrier. In its place, new reflections are apparent in the pattern, corresponding to a phase denoted X because these new reflections could not be assigned to any known compound listed in the available databases of powder X-ray patterns. A single reflection, denoted G in the patterns, can be assigned to graphitic carbon with basal plane spacing d(002) ) 0.337 nm. Carbon deposition from the chemical decomposition of methane is thermodynamically favored at the operating conditions used (reaction 3). Additionally, carbon can be deposited especially in layers located at the bottom of the reactor bed because metallic Ni is present there almost always during the reduction stage. It is well known that metallic Ni is a good catalyst, promoting methane decomposition at operating temperatures similar to that used in this work.12-14 No significant differences among powder XRD patterns corresponding to samples at 1, 10, or 20 cycles were found, as shown in Figure 5a, indicating that no differential structural changes occur with the number of cycles in the reduced samples.

Figure 5. Powder XRD patterns of samples extracted from the reactor bed after the reduction stage in a 20-cycle CLC test with dilute methane: (a) samples from the bottom of the bed at different cycles, (b) samples from the top of the bed at different t/t0 ratios of the same cycle.

Powder XRD patterns obtained from samples extracted from the top bed at different t/t0 ratios of the same cycle in the reduction stage of a 20-cycle test of CLC with dilute methane are shown in Figure 5b. At t/t0 ) 0.2, the top bed in an upflow fixed-bed reactor is almost completely composed of unconverted carrier, and consequently, Ni oxidized species are clearly identified. In fact, only NiTiO3 is detected, whereas NiO is absent. At t/t0 ) 1.2, NiTiO3 is still detected in the top layers of the reactor bed, indicating that its reactivity in the reduction stage is low, as compared with that of NiO, and its presence is extended to relatively very high t/t0 ratios. At the much higher value of t/t0 ) 2.0, NiTiO3 becomes undetectable, although deposited carbon and/or reduced rutile, denoted as phase X, are not detected. This indicates that the presence of difficult-toreduce species with low reactivity as NiTiO3 is always beneficial in preventing carbon deposition. Its presence also prevents the partial rutile reduction, otherwise probably resulting in detrimental CO emissions. 3.4. Reduced Rutile Phase and the Evolution of Carbon Deposition in the Reduction Stage of the CLC of Dilute Methane. The structural changes undergone by rutile in the reduction stage of a CLC process with dilute methane were vstudied by transmission electron microscopy (TEM). The reduction process of rutile using hydrogen as the reducing agent has been widely documented in the literature.15 It starts with the formation of randomly distributed oxygen vacancies in the rutile lattice while the number of point defects is still small. The lattice strain increases with the number of oxygen vacancies, which eventually coagulate to form ordered structures. The first

162


Figure 6. HREM micrograph of reduced rutile phase X in sample 7NT extracted from the reactor bed in the reducing stage of the CLC of dilute methane.

step in lattice relaxation is the generation of crystallographic shear (CS) planes. These CS planes substantially reduce the number of oxygen vacancies visible in TEM images as planar defects. In mode diffraction, the presence of such planes is observed by the appearance of directional streaking along specific crystallographic directions in their respective selective area electron diffraction (SAED) patterns. In image mode, highresolution electron microscopy (HREM) images show at this stage the progressive formation of corrugated structures of variable wavelength of a few tens of nanometers. At more advanced stages of the reduction process, the number of oxygen vacancies becomes high, and lattice relaxation gives way to the formation of ordered phases, usually denoted as Magnelli phases of general formula TinO2n-1 16 that are often described in the crystallographic triclinic system. An HREM micrograph of reduced rutile (phase X) in a sample extracted from the reactor bed at the end of the reducing stage of a CLC process with dilute methane is shown in Figure 6. It reveals the presence of many crystallographic defects, including boundary grains, corrugations, and stacking faults, together with other apparently well-crystallized areas. Fast Fourier transforms (FFTs) taken from these small well-crystallized areas reveal interplanar spaces, d, of 0.249, 0.258, and 0.252 nm that are also observed in their respective powder XRD patterns but cannot be assigned to rutile. Chemical analysis and helium density measurements11 of different reduced samples, denoted phase X, reveal an uncertain and surely variable stoichiometry. Direct evidence for the presence of deposited carbon in the reduced samples, and the study of its evolution in the reactor bed during the reducing stage of a CLC with dilute methane was obtained by SEM-EDX analysis of samples extracted from the reactor bed at different t/t0 ratios of the same cycle (Figure 7a) or at different cycles but at fixed t/t0 (Figure 7b) in a 20cycle test. SEM-EDX with a Si/Li detector enclosed in a highvacuum chamber fitted with ultrathin windows allowed direct identification of carbon in the reduced samples. Its evolution through the respective atomic ratio C/Ni was obtained from quantitative analysis of EDX spectra taken of large areas of sample pellets at very low SEM magnification. The amount of

Figure 7. Evolution of carbon deposited in the bottom of the reactor bed during the reduction stage of the CLC of dilute methane: (a) evolution along a single cycle, (b) evolution with the number of cycles in 20-cycle test at high t/t0.

deposited carbon on the bottom layers of the reactor bed is displayed in Figure 7a,b. Figure 7a shows that carbon is already present in these layers at the start of the reduction stage. The C/Ni atomic ratio increases almost linearly with time, indicating that the catalytic decomposition of methane acts as a side reaction for methane consumption in clear competition with the main CLC reactions (reactions 1 and 2). The appearance of deposited carbon imposes some limitations on the achievable maximum carrier efficiency for CO2 capture using nickel-based materials as oxygen carriers for the CLC of methane. Additionally, the amount of deposited carbon in a 20-cycle CLC test with dilute methane seems to be independent of the number of cycles, as shown in Figure 7b. Some studies of the thermal decomposition of methane carried out at lower temperatures, in the range of 600-700 °C, than that used in this study using small Ni particles a few nanometers in diameter as catalyst show that carbon is usually deposited as thin wires a few micrometers long emerging from catalyst particles of approximately the same size. These carbon forms are usually designated in the literature as filamentous17 carbon, but because of their nanometric dimensions and graphene orientation, often parallel to the wire axis, they are also known as nanowires, nanofibers, or even nanotubes,18-23 although in this last case the presence of tubes is not always assessed. At the operating conditions used in this study, however, the deposited carbon in the reduction stage of the CLC of dilute methane does not alter the morphological appearance of the oxygen carriers, and the presence of extended carbon forms

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 163

Figure 9. CO2 breakthrough curves in the reduction stage of a 20-cycle test of CLC with pure CH4.

Figure 8. HREM images of carbon deposited on Ni particles in the reducing stage of a CLC test with dilute methane.

could not be evidenced by SEM. Carbon is conclusively not deposited as fibers, and the deposited carbon could only be revealed by TEM at higher magnification. As an example, an HREM micrograph of a polyhedral Ni particle is shown in Figure 8. Point EDX analysis taken from a micrometric area reveals the presence of Ni in the particle core and C in the uniform coating. The lattice fringes, visible in the central dark area of the micrograph, correspond to interplanar spaces d ) 0.202 nm that can be assigned to (111) planes of metallic Ni. Additionally, the separation of the lattice fringes located around the Ni particle is d ) 0.350 nm, which can be assigned to the basal plane spacing in graphitic carbon. Consequently, the HREM micrograph shows a Ni crystal orientated along the [111] axis zone, with the deposited carbon growing epitaxially on exposed Ni(111) faces. Some studies of the thermal decomposition of methane have shown that this sort of carbon growth is promoted by high carbon deposition rates and/or Ni crystals exposing low-index faces of low surface energy [octahedral (111) and/or cubic (001) planes].24,25 3.5. Performance of Carrier 7NT in a 20-Cycle CLC with Pure Methane. Using dilute methane for the reduction stage of a CLC process has been demonstrated useful for achieving good control of the reaction conditions, better resolution in the analysis of the inlet and outlet gases, and some convenience in sample extraction from the reactor bed for characterization studies; however, it makes no sense in practical terms. Moreover, even from a basic point of view, some features revealed at low kinetics with dilute methane might not be evidenced at higher kinetics with pure methane. Consequently, a 20-cycle test of the CLC of pure methane was also carried out, and the respective CO2 breakthrough curves in the reducing stage are shown in Figure 9. Before breakthrough, the molar fraction, xCO2, in the outlet gas is close to 1 because CO2 is the only detected compound. This indicates there is a high selectivity to the main reaction of the reducing stage of CLC at these operating conditions. However, breakthrough appears shifted toward t/t0 ratios higher than 1, revealing that there are additional oxygen sources not taken into account in the calculation of t0. Pure methane is a stronger reducing agent than dilute methane, and consequently, rutile can be more

Figure 10. Evolution of carbon deposited in the reactor bed along the reduction stage of CLC with pure methane.

extensively be reduced. The reduction with pure methane, independently of the number of cycles, was always fast, and the second part of the breakthrough curves, associated with the presence of NiTiO3 with low reactivity in the case of using dilute methane (Figure 1), was not found in the breakthrough curves with pure methane. Similarly, the evolution of carbon deposited along a single cycle in the reduction stage of the 20-cycle CLC of pure methane is shown in Figure 10. These results can be compared to similar results obtained with dilute methane shown in Figure 7a. Before breakthrough (t/t0 < 1.5), the apparent carbon deposition rates derived from the catalytic decomposition of methane are similar for both pure or dilute methane. These results suggest that the kinetics of the main reaction of the reduction stage of CLC is more affected by the partial pressure of methane than is that for methane decomposition. SEM images recorded from sample 7NT regenerated in the first cycle (Figure 11a) and 20th cycle (Figure 11b) clearly reveal that the porosity of the carrier increases with the number of cycles in a much more pronounced way than was found for dilute methane (Figure 3). Eventually, this porosity increase might promote the development of some cracks that can substantially reduce the mechanical strength of the carrier, although this feature should be assessed in longer-term tests of many hundreds of cycles.

164


Acknowledgment This research was carried out with financial support from the European Coal and Steel Community (Project 7220-PR125) and CICYT Project CTQ2004-025565/PPQ. Literature Cited

Figure 11. SEM micrographs of regenerated samples in a 20-cycle test of CLC with pure methane: (a) 1st cycle, (b) 20th cycle.

Conclusions From the study of nickel oxide supported on porous rutile prepared by the application of seven successive impregnations, as an oxygen carrier for a 20-cycle test of the CLC of dilute or pure methane for the reducing stage and pure air for the regeneration stage one can conclude that, in the reduction stage before breakthrough, the carrier reduction is fast with pure methane, as CO2 is the only compound detected in the outlet gas from the reactor. However, the thermal decomposition of methane appears soon as a side reaction in clear competition for methane consumption, especially at high t/t0 ratios. Additionally, rutile acts as an additional oxygen source that must be taken into account, especially in CLC tests of pure methane. Carbon is deposited as uniform coatings on Ni particles, and its concentration in the reactor bed increases almost linearly with time. This feature imposes some limitations on the maximum achievable efficiency of the CLC of methane using these types of nickel-based oxygen carriers in CO2 capture. In a 20-cycle test in a fixed-bed reactor, a slight decay in carrier performance with the number of cycles was observed, together with an increase in carrier porosity that might result in the development of cracks. These features might impose additional restrictions on the use of this type of oxygen carrier, but such issues should be assessed in longer-term tests of many hundreds or even thousands of cycles.

(1) Anheden, M.; Svedberg, G. Exergy analysis of chemical-looping combustion systems. Energy ConVers. Manage. 1998, 39, 1967-1980. (2) Yu, J.; Corripio, A. B.; Harrison, D. P.; Copeland, R. J. Analysis of the sorbent energy transfer system (SETS) for power generation and CO2 capture. AdV. EnViron. Res. 2003, 7, 335-345. (3) Ishida, M.; Jin, H. Chemical-looping combustion power generation plant system. U.S. Patent 5,447,024, 1995. (4) Ishida, M.; Jin, H. A new advanced power-generation system using chemical-looping combustion. Energy 1994, 19, 415-422. (5) Lyngfelt, A.; Leckner, B.; Mattison, T. A fluidised-bed combustion process with inherent CO2 separation; application of chemical-looping combustion. Chem. Eng. Sci. 2001, 56, 3191-3113. (6) Ishida, M.; Jin, H.; Okamoto, T. A fundamental study of a new kind of medium material for chemical-looping combustion. Energy Fuels 1996, 10, 958-963. (7) Jin, H.; Okamoto, T.; Ishida, M. Development of a novel chemicallooping combustion: Synthesis of a looping material with a double metal oxide of CoO-NiO. Energy Fuels 1998, 12, 1272-1277. (8) Ishida, M.; Jin, H.; Okamoto, T. Kinetic behavior of solid particle in chemical-looping combustion: Suppressing carbon deposition in reduction. Energy Fuels 1998, 12, 223-229. (9) Jin, H.; Okamoto, T.; Ishida, M. Development of a novel chemicallooping combustion: Synthesis of a solid looping material of NiO/NiAl2O4. Ind. Eng. Chem. Res. 1999, 38, 126-132. (10) Adańez, J.; de Diego, L. F.; Garcıá-Labiano, F.; Gayań, P.; Abad, A.; Palacios, J. M. Selection of oxygen carriers for chemical-looping combustion. Energy Fuels 2004, 18, 371-377. (11) Corbella, B. M.; de Diego, L. F.; Garcıá-Labiano, F.; Adańez, J.; Palacios, J. M. Characterization study and 5-cycle tests in a fixed bed reactor of titania-supported nickel oxide as oxygen carrier for chemical looping combustion of methane. EnViron. Sci. Technol. 2005, 39, 5796-5803. (12) Kuvshinov, G. G.; Mogilnykh, Y. I.; Kuvshinov, D. G. Kinetics of carbon formation from CH4-H2 mixtures over a nickel containing catalyst. Catal. Today 1998, 42, 357-360. (13) Choudary, T. V.; Sivadinarayana, C.; Chusuei, C. C.; Klinghoffer, A.; Goodman, D. W. Hydrogen production via catalytic decomposition of methane. J. Catal. 2001, 199, 9-18. (14) Piao, L.; Li, Y.; Chen, J.; Chang, L.; Lin, J. Y. S. Methane decomposition to carbon nanotubes and hydrogen on an alumina supported nickel aerogel catalyst. Catal. Today 2002, 74, 145-155. (15) Sorensen, O. T. Nonstoichiometric Oxides; Academic Press: New York, 1981. (16) Le Page, Y.; Strobel, P. Structural chemistry of Magnelli phases TinO2n-1 (4 < n < 9). Valence ordering of titanium in Ti6O11 at 130 K. J. Solid State Chem. 1983, 47, 6-15. (17) Fenelonov, V. B.; Derevyankin, A. Y.; Okkel, L. G.; Avdeeva, L. B.; Zaikovskii, V. I.; Moroz, E. M.; Salanov, A. N.; Rudina, N. A.; Likholobov, V. A.; Shaikhutdinov, S. K. Structure and texture of filamentous carbons produced by methane decomposition on Ni and Ni-Cu catalysts. Carbon 1997, 35, 1129-1140. (18) Ren, W. C.; Cheng, H. M. Herringbone-type carbon nanofibers with a small diameter and large hollow core synthesized by the catalytic decomposition of methane. Carbon 2002, 41, 1645-1687. (19) Reshetenko, T. V.; Avdeeva, L. B.; Ismagilov, Z. R.; Chuvilin, A. L. Catalytic filamentous carbon as supports for nickel catalysts. Carbon 2004, 42, 143-148. (20) Kuvshinov, G. G.; Mogilnykh, Y. I.; Kuvshinov, D. G.; Zaikovskii, V. I.; Avdeeva, L. B. Peculiarities of filamentous carbon formation in methane decomposition on Ni-containing catalysts. Carbon 1998, 36, 8797. (21) Wang, P.; Tanabe, E.; Ito, K.; Jia, J.; Morioka, H.; Shishido, T.; Takehira, K. Filamentous carbon prepared by catalytic pyrolisis of CH4 on Ni/SiO2. Appl. Catal. A: Gen. 2002, 231, 35-44. (22) Otsuka, K.; Ogihara, H.; Takenaka, S. Decomposition of methane over Ni catalysts supported on carbon fibers formed from different hydrocarbons. Carbon 2003, 41, 223-233.

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 165 (23) Wang, S. G.; Zhang, Q.; Yoon, S. F.; Ahn, J. Synthesis and characterization of MWNTs with narrow diameter over nickel catalyst by MPCVD. Scripta Mater. 2003, 48, 409-412. (24) Wang, X.; Jia, Y.; Yao, Q.; Wang, F.; Ma, J.; Hu, X. The calculation of the surface energy of high-index surfaces in metals at zero temperature. Surf. Sci. 2004, 551, 179-188. (25) Hoeft, J. T.; Polcik, M.; Sayago, D. I.; Kittel, M.; Terbog, R.; Toomes, R. L.; Robinson, J.; Woodruff, D. P.; Pascal, M.; Nisbet, G.;

Lamont, C. L. A. Local adsorption sites and bond length changes in Ni(100)/HCO and Ni(100)/CO. Surf. Sci. 2003, 540, 442-456.

ReceiVed for reView June 24, 2005 ReVised manuscript receiVed October 3, 2005 Accepted October 18, 2005 IE050756C

Performance in a Fixed-Bed Reactor of Titania-Supported Nickel

Recommend Documents