КИНЕТИКА И КАТАЛИЗ, 2013, том 54, № 3, с. 352-357
UDC 541.128:547.211:546.211.1
THE MAIN FACTORS CONTROLLING GENERATION OF SYNTHETIC NATURAL GAS BY METHANATION OF SYNTHESIS GAS IN THE PRESENCE OF SULFUR-RESISTANT Mo-BASED CATALYSTS
© 2013 Zhenhua Li1, Haiyang Wang1, Erdong Wang1, Jing Lv1, Yuguang Shang1, Guozhong Ding1,
Baowei Wang1, Xinbin Ma1, *, Shaodong Qin2, Qi Su2
1Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology,
Tianjin University, Tianjin, 300072 China 2NationalInstitute of Clean-And-Low-Carbon Energy, Beijing, 102209 China *E-mail: xbma@tju.edu.cn Received 15.02.2012
The Co—Mo—Al and Co—Mo—Ce—Al catalysts were prepared and tested for their activity in the methanation of synthesis gas in the presence of hydrogen sulfide. The results showed that the Co—Mo—Ce—Al series was superior to the Co—Mo—Al system in terms of CO conversion. The former system was used to examine the main factors controlling the methanation behavior. Among these are: H2S concentration in the reaction mixture, reaction temperature and pressure, concentrations of CO2, CH4, and H2O, H2/CO ratio, and gaseous hourly space velocity. The methanation activity increased with increasing temperature, pressure and H2/CO ratio. The reason why adding CO2 or H2O decreases the methanation activity is discussed.
DOI: 10.7868/S0453881113030118
Coal continues to be a strategic primary energy source in a number of countries and its global reserves and resources are abundant. Therefore, it becomes necessary to develop clean-coal technologies to convert coal into more convenient energy carriers of high quality [1, 2]. At the same time, biomass is an inexhaustible and renewable energy source. It can help in developing a secure energy supply and reducing the green house gas emission. Accordingly, substitute natural gas (SNG) production from coal or biomass is an ideal alternative for energy security. The advantages of SNG production and application are high conversion efficiency, an extensive available gas distribution infrastructure such as pipelines and the well-established and efficient end — use technologies, e.g., compressed natural gas cars, heating facilities, power stations [3]. Coal and solid biomass have to be converted to SNG by thermo-chemical processes via gasification and subsequent methanation to reach an overall chemical efficiency [4]. Wet biomass such as crops, sewage sludge and manure produce SNG via a hydrothermal gasification process [5]. Under the hydrothermal environment, the wet biomass is directly converted in the presence of a catalyst to methane, carbon dioxide and water [6, 7].
The production of SNG via a thermo-chemical process requires several conversion steps. The first step is the gasification of the solid carbon source (coal [8], refinery residues [9—12] and biomass/wastes in combination with coal [13, 14]) with steam and/or oxygen to make the producer gas, a gas mixture containing
mainly H2, CO, CO2, H2O, CH4, and some impurities such as sulphur and chlorine species [15]. The composition of the produced gas is influenced to a large extent by the gasification technology, and the outlet gases from coal gasifiers have usually H2/CO ratios between 0.3 and 2 [16—18]. Methanation of carbon monoxide with hydrogen is the key step, which needs the presence of a catalyst. Ni-based catalyst is highly active for methanation reaction, but it is extremely sensitive to sulfur. Conceivably, the syngas fed to methanation should be desulfurized to have sulfur compounds in concentrations less than 0.1 ppm. Before methanation, a H2/CO ratio of 3 : 1 in the syngas needs to be achieved by water gas shift reaction (WGSR). Due to this reason, there has been considerable activity on the subject of sulphur-resistant molybdenum catalysts for methanation. The catalytic properties of these catalysts were reported and their resistance to sulfur poisoning recognized [19—21].
Here we report the catalytic performance of supported MoO3 catalysts prepared via the incipient impregnation method. The catalysts were characterized by N2 adsorption—desorption and X-ray photoelec-tron spectroscopy (XPS). The results showed that the 5%CoO—15%MoO3/25%CeO2—Al2O3 catalyst was more active for the methanation of syngas than the 5%CoO—15%MoO3/Al2O3 catalyst. The effect of reaction parameters on methanation activity of the former catalyst was thoroughly investigated.
Table 1. Effect of support on activity in methanation
Catalyst T, °C X CO, % Xh2, % ^CH4, % V, % ^ %
Co-Mo-Al 440 27.1 29.9 38.1 49.2 13.7
500 36.4 43.4 44.7 49.0 7.8
556 42.1 52.9 50.6 46.5 4.5
607 45.2 56.9 52.4 44.1 3.2
Co-Mo-Ce-Al 446 32.5 31.9 35.2 48.3 15.0
512 48.2 53.1 43.1 47.0 8.6
563 51.1 59.0 48.3 45.1 6.0
611 51.0 60.7 51.3 44.5 4.6
EXPERIMENTAL
Catalyst Preparation
A commercial y-Al2O3 ("Yixing", China) was used as a support. The 25%CeO2—Al2O3 composite support was prepared by coprecipitation from aqueous solutions of cerium and aluminum nitrates. An ammonia solution was added to a continuously stirred mixed solution of Ce(NO3)2 and Al(NO3)3 at 40°C. After standing for 8 h at 40°C, the preparation was filtered and thoroughly washed with deionized water until the final pH is approximately 7.0. The precipitate was dried overnight at 120°C and heated in air for 4 h at 5°C/min to 600°C.
The 5%CoO—15%MoO3/Al2O3 and 5%CoO-15%MoO3/25%CeO2—Al2O3 samples were prepared by impregnating a y-Al2O3 support or a composite support with aqueous solutions of ammonium hepta-molybdate and cobalt nitrate through the incipient-wetness impregnation method. The impregnated samples were dried at 30°C for 24 h in an open-air atmosphere, then dried at 120°C for 6 h and heated at 3°C/min for 4 h to 600°C. Depending on the nature of the support the samples obtained were designated as the Co-Mo-Al and Co-Mo-Ce-Al catalysts.
Catalyst Characterization
The BET specific surface area was determined from the N2 adsorption isotherm at -196°C using the commercial Micromeritics TRISTAR 3000 unit (USA) with samples evacuated under vacuum at 300°C for 4 h.
X-ray photoelectron spectroscopy analysis was performed using a PHI-1600 ESCA XPS spectrometer (USA) with Mg^a X-ray radiation as an incident beam. The C1s binding energy at 284.6 eV was used as a reference to calibrate the binding energies.
Catalytic Activity Measurements
Catalytic activity measurements were carried out in a continuous flow fixed-bed reactor. The stainless re-
actor (i.d. of 12 mm, length of 700 mm) was heated with an enclosed electric furnace. The temperature was controlled using a K-type thermocouple placed in the middle of the furnace and reaction temperature was monitored using a K-type thermocouple placed in the middle of the 3 mL catalyst bed. The catalysts were sulfurized by 3% H2S/H2 mixture gas at 400°C for 5 h before activity tests. The reaction was studied at 560°C, 5000 h-1, CO : H2 : N2 = 2 : 2 : 1, 0.2% H2S and 3 MPa. The feed gas mixture was preheated to a certain temperature before entering the reactor and pressure was controlled by a back pressure regulator. Exit gases were analyzed using an Agilent 7890A gas chromatograph (USA) to monitor the concentration of CO, CO2, CH4, H2, N2, and C2 hydrocarbons. Conversion (X) and selectivity (S) data were rounded off to three decimal places except for the specific cases. All the catalytic data presented in Tables were obtained by averaging the stable results registered after the catalyst was exposed to the reaction mixture for 4-5 h.
RESULTS AND DISCUSSION
Effect of Support
The effect of the support on the catalytic activity for methanation is shown in Table 1. It can be seen that the Co-Mo-Ce-Al catalyst exhibited an enhanced methanation activity compared with the Co-Mo-Al sample. This meant that adding CeO2 to Al2O3 modifies the property of the support to increase the activity. In order to understand the effect of CeO2, XPS analysis was carried out for the catalyst after conducting reaction tests. High-resolution Mo3d and S2s spectra are shown in Fig. 1.
The two catalysts had almost the same XPS spectra of Mo3d, and the binding energies of Mo3d were nearly identical which indicates that MoS2 is the dominant form of molybdenum. A small amount of molybdenum occurs as a Mo+6 species. According to the reference data of S2p, the sulfur predominantly exists as MoS2 in both catalysts as shown by a strong peak at about 162 eV (Fig. 1). However, a weaker peak at 169 eV
238 234 Co—Mo—Ce—Al
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170 166 162 Binding energy, eV
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Fig. 1. The spectra of Mo3d and S2s for the Co-Mo-Al and Co-Mo-Ce-Al catalysts after reaction.
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Fig. 2. The change of the catalysts activity with the time on stream.
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due to SO4 appeared in the Co—Mo—Al sample rather than in the Co—Mo—Ce—Al catalyst, and this signal can be assigned to a species containing CeO2 that is formed on the surface of the Al2O3 support.
Although the Co—Mo—Al catalyst had a larger surface area, it had only a modest activity for methanation process. The existence of SO2 _ can inhibit the activity of molybdenum sulfide [22]. Introducing CeO2
to Al2O3 suppresses the formation of SO4 species and therefore increases the activity in methanation.
Figure 2 shows the change of catalyst activity with time on stream. In the lifetime test, the selectivity for CH4 remains stable while the conversion of CO slightly decreases. It can be argued that stability of the Co—Mo— Ce—Al catalyst is higher than that of the Co—Mo—Al sample. Xavier [23] reported that doping cerium oxide into Al2O3 imparts a promoting effect to the methanation process due to the electronic interaction. The reduction of cerium oxide in the course of the reaction might be a reason why the CeO2—Al2O3 composite support is superior to the Al2O3 support f
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