Reforming of model gasification tar compounds

Transcription

1 Reforming of model gasification tar compounds Agata Łamacz 1*, Andrzej Krztoń 1, Andrea Musi 2, Patrick Da Costa 2, Gérald Djéga-Mariadassou 2 1 Centre of Polymer and Carbon Materials, Polish Academy of Sciences, Marii Curie-Sklodowskiej 34, Zabrze, Poland; agata.lamacz@cmpw-pan.edu.pl 2 Université Pierre et Marie Curie, Laboratoire de Réactivité de Surface, CNRS UMR 769, case 178, 4 place Jussieu, Paris, France Introduction Biomass gasification has attracted a lot of interest by producing a gas rich in and H 2 which is used for chemicals production, Fisher-Tropsch and methanol synthesis. It is also the most efficient, alternative method for energy production in the gas turbine. The impurities present in the gas consist of ash, volatile alkali metals and tar, which is a complex mixture of aromatics [1]. In fluid bed gasifiers tar content varies from 5 to 75 g/n m 3 [1, 2], what exceeds the maximum allowed for diesel engines and gas turbines. Tar can condense or polymerize into more complex structures in the pipes, filters or heat exchangers, causing process equipment problems (as choking and attrition), decreasing total efficiency and increasing the cost of the process. Because of the prohibition of direct gas stream utilisation, gas purification systems are needed. Tar removal technologies including cracking and mechanical separation using scrubbers or cyclones take place downstream the gasifier (hot gas cleaning). Tar treatment inside the gasifier may eliminate the need for downstream cleanup. For both tar removal technologies, catalytic steam reforming is very promising [1] however it s a endothermic process and the external source of heat is needed [3]. This process usually involves tar components oxidation on supported nickel-based catalyst, with the use of steam, at the temperatures above 65 C. Tar steam reforming leads to and H 2, enriching gas from biomass gasification in these components. The reaction pathway is described by reactions (1) and (2). Hydrocarbons react irreversibly to and H 2. A competing reaction leading to coke formation on the catalyst s surface can be prevented by selecting the type of the catalyst, steam to carbon ratio and reaction temperature [1]. C x H y + H 2 O +(x+y/2) H 2 (1) +H 2 O 2 + H 2 (2) Ceria-zirconia with the addition of metallic phase (Ni, Pt, Pd) was found to be very promising catalysts system in reforming reactions. The addition of ZrO 2 to ceria improves oxygen storage capacity (OSC), redox properties, thermal stability and catalytic activity [47]. High oxygen mobility and OSC makes ceria-zirconia very interesting in wide range of application involving catalytic reforming reactions. High temperature reaction between hydrocarbon and lattice oxygen can produce and H 2. The great benefit of Ce x Zr 1-x O 2 is its high resistance for carbon deposition [8]. Due to current requests in the field of energetic and the need of renewable energy sources application in society s everyday life our work is focused on finding active

2 Catalysis for Environment: Depollution, Renewable Energy and Clean Fuels Zakopane, 28 biomass tar decomposition catalysts. The goal of this work was to understand the chemistry involved in tar reforming process, using toluene as a model compound because it is the most reactive among all tar components and it represents a stable aromatic structure in tar formed in high-temperature processes. In this study, ceriazirconia supported Ni and Co catalysts were characterized and tested in toluene steam reforming reaction. Experimental Nickel and cobalt catalysts containing 1 and 1 wt % of active phase were obtained by incipient impregnation of commercial ceria-zirconia CZ 7/3 (RHODIA Electronics & Catalysts) with different quantities of Co(NO 3 ) 3 6H 2 O and Ni(NO 3 ) 2 2H 2 O water solutions. Ceria-zirconia support was previously calcined at 5 C for 5h. After impregnation each mixture was dried at room temperature overnight and deposited nitrates were decomposed at 11 C. Finally, catalysts were calcined at 7 C for 2 h. Obtained catalysts were characterized by S BET, X-ray diffraction (XRD), temperature programmed reduction (TPR) and temperature programmed desorption (TPD). Temperature programmed surface reactions (TPSR) were performed in quartz U-type reactor in the presence of 1 ppm of toluene and 1.7% of steam in Ar as a balance. The flow-rate of 25 ml/min, heating-rate of 3 C/min from 15 to 9 C and GHSV = 3 1/h were used during TPSR experiments. Catalytic tests in stationary conditions were performed in the same gas mixture and with the same flowrate as in TPSR. Outleting gas composition during isotherms in temperatures decreasing from 9 to 3 C was being detected until and 2 concentrations were stable. Results and discussion Specific surface areas of the support and catalysts revealed that with increasing metal loading surface area decreases. XRD patterns of the support and catalysts shown that crystallites did not appear for low loaded catalyst, what implies that Co and Ni species are highly dispersed on the support s surface due to metal-support interaction. XRD pattern of high loaded catalysts clearly show Co 3 O 4 and NiO crystallites presence. High metal loading takes effect in metal oxide particles agglomeration which is probably caused by metalmetal interaction [9, 1]. TPR-H 2 profiles for the support and Co/CZ catalysts are illustrated on figure 1A. The reduction peak at 55 C is assigned to bulk CeO 2 reduction (Ce 4+ Ce 3+ ). Shoulder at 35 C probably refers to CeO 2 surface reduction. The reduction peak at 35 C corresponds to CoO reduction. The shoulder at 24 C is probably due to reduction of Co 3 O 4 to CoO. The first peak at 3 C on Co(1)/CZ plot is assigned to the small amount of Co 3 O 4 reduction. The second one, at 4 C probably corresponds to CoO to Co reduction [11]. Peaks at 15 and 25 C and at 15 and 275 C (Fig. 1B) are assigned to adsorbed oxygen reduction, arised by formation of Ni-O-Ce solid solution. When Ni 2+ ions are incorporated into support s lattice, they replace Ce +4 ions. Within CeO 2 structure, unbalanced charge and lattice distortion occurs. In the consequence, very reactive 8

3 Zakopane, 28 Catalysis for Environment: Depollution, Renewable Energy and Clean Fuels oxygen is generated and its reduction occurs at low temperatures. Peaks at 35 C for both Ni(1)/CZ and Ni(1)/CZ may be attributed to NiO reduction [12]. Hydrogen consumption [a.u.] (A) (b) (c) (a) Hydrogen consumption [a.u.] (B) (b) (c) (a) Fig. 1. TPR profiles of (A) CZ (a), Co(1)/CZ (b), Co(1)/CZ (c) and (B) CZ (a), Ni(1)/CZ (b), Ni(1)/CZ (c) Ceria-zirconia s TPSR profile (Fig. 2) reveals toluene s consumption up to 45 C and bimodal toluene s oxidation. The 2 formation with maximum at 5 C corresponds to slight toluene s oxidation. Reforming reaction starts at about 65 C. Figure 3A shows Co(1)/CZ TPSR. There s toluene consumption observed up to 55 C. From 31 to 45 C part of the toluene is oxidized to 2, whereas from 55 C it starts to be completely converted to and 2. While concentration increases, toluene conversion to 2 begins decrease from 68 C. Analogically to low Co loaded catalyst, for Co(1)/CZ constant toluene consumption to 55 C was observed. While concentration increases, 2 starts to decrease from 75 C (Fig. 3B). Higher toluene to conversion on Co(1)/CZ and more important 2 participation on Co(1)/CZ were noticed , 2 concentration [ppm] Fig. 2. TPSR of CZ 9

4 Catalysis for Environment: Depollution, Renewable Energy and Clean Fuels Zakopane, (A) , 2 concentration [ppm] (B) , 2 concentration [ppm] Fig. 3. TPSR of Co(1)/CZ (A) and Co(1)/CZ (B) (A) , 2 concentration [ppm (B) , 2 concentration [ppm] Fig. 4. TPSR of Ni(1)/CZ (A) and Ni(1)/CZ (B) In case of Ni catalysts (Fig. 4) conversion of toluene to 2 up to 5 and 45 C (for low and high loaded catalysts respectively) was also observed, nevertheless its complete oxidation started at 475 C. It was noticed, that concentration was increasing while 2 was decreasing rapidly. Analogically to Co catalysts, more was produced on low loaded Ni/CZ. Generally, toluene s oxidation to 2 may occur by the oxygen stored on catalyst s surface or by the oxygen produced in water decomposition reaction, taking place on ceria-zirconia surface (reaction 3). may be produced in steam reforming reaction (reaction 4) or by partial toluene oxidation (reaction 6). H 2 O H 2 + ½ O 2 (3) C 7 H H 2 O H 2 (4) C 7 H ½ O H 2 (5) Activity measurements performed in isotherms showed, that at 532 C toluene s conversion is close to 1% (Fig. 5). Toluene oxidation toward during steady state experiments is presented on figure 5. GC-MS analysis carried out at temperature range between 9 and 7 C revealed presence of 2 and H 2 O only. Below 7 C, toluene and benzene were also detected. 1

5 Zakopane, 28 Catalysis for Environment: Depollution, Renewable Energy and Clean Fuels 1 8 C 7 H 8 conversion [%] 6 4 selectivity toward Fig. 5. Toluene conversion and selectivity toward in steady state experiments on Co(1)/CZ Conclusions XRD analysis indicated the presence of Co 3 O 4 and NiO crystallites only for high loaded catalysts due to strong metal-metal interaction occurring. TEM observations for Co(1)/CZ revealed that Co is not dispersed well, forming clusters and the catalyst lattice is heterogeneous. In case of Ni(1)/CZ metallic phase is well dispersed and the catalysts is homogeneous in his structure. TPR experiments showed the reduction of Co 3 O 4 and CoO for both, low and high loaded catalyst. In case of Ni/CZ, NiO reduction occurred. Toluene-TPD experiments proved hydrocarbon desorption at low temperatures. Bimodal toluene oxidation occurred for Co(1)/CZ. Combination of toluene desorption with 2 formation on all samples indicates two kinds of hydrocarbon adsorption on catalyst s surface weak and strong. Toluenes desorption and 2 production is shifted toward higher temperatures for the experiments performed with H 2 O, what can be explained by its blocking on catalysts surface by water particles. TPSR experiments revealed good results toward production, which was more significant on low loaded catalysts. Nevertheless, total toluene s oxidation occurred at lower temperatures for high loaded catalysts. In case of Co/CZ bimodal hydrocarbon s oxidation was observed, while on Ni/CZ oxidation to occurred at lower temperatures. TPSR results for Co(1)/CZ were confirmed by steady state experiments, which proved almost total toluene s oxidation and 5% selectivity toward production at 535 C. Obtained catalysts active in toluene steam reforming reaction starting at 5 C for Ni/CZ and seem to be very promising in biomass tar decomposition. Acknowledgment: Authors acknowledge financial support of the Ministry of Science and Higher Education (project PBZ-MEN-2/2/26) and International Group of Research (GDRI) Catalysis for Environment: Depollution, Renewable Energy and Clean Fuels. 11

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