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Reforming of tar contained in a raw fuel gas from biomass gasification using nickel-mayenite catalyst
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Reforming of tar contained in a raw fuel gas from biomass gasification using nickel-mayenite catalyst Andrea Di Carlo a,*, Domenico Borello b, Mario Sisinni b, Elisa Savuto c, Paolo Venturini b, Enrico Bocci d, Koji Kuramoto e a
University of L' Aquila, Via Campo di Pile, L'Aquila, Italy Sapienza-University of Rome, Via Eudossiana 18, Rome, Italy c Tuscia University, Via S. M. in Gradi 4, Viterbo, Italy d Marconi University, Via Plinio 24, Rome, Italy e Energy Technology Research Institute (ETRI), Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki, Japan b
article info
abstract
Article history:
Catalytic steam reforming of tar is a very efficient process to clean the gas produced by
Received 19 November 2014
biomass gasification. Ni catalysts remain an affordable solution for this problem, even if
Received in revised form
this catalytic material suffers degradation due to carbon deposition and sulphur poisoning.
16 May 2015
In this study, the properties of a new catalyst Ni/Mayenite (Mayenite as binder), prepared
Accepted 19 May 2015
by impregnation method and tested with a real gas obtained from a bench scale fluidized
Available online 11 June 2015
bed steam gasification of biomass, were investigated. Experiments were carried out in a microreactor fed by a slipstream coming from the bench scale gasifier to evaluate gas
Keywords:
cleaning and upgrading options. Preliminary tests were carried out at three different
Biomass gasification
temperatures, 700, 750 and 800 C. In all the tests, the catalysts showed high activities
Tar
reaching to a conversion rate of 90% in the case at highest temperature (800 C). The
Steam reforming catalyst
conversion efficiency remained stable around this value during a 3 h test. A decrease in the
Fluidized bed
performance was observed at 700 and 750 C, even if the conversion remained stable around a lower value. An increase of H2 (>50%) and a decrease of CH4 were observed at all the temperatures, due to the occurrence of steam reforming reaction. A long duration test (12 h) was carried out at 800 C and demonstrated that, at this temperature, the conversion was stable for a longer period. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The market of high efficiency devices for power generation, like high temperature fuel cells (SOFC, MCFC), is continuously
expanding. These devices are prone to be fed with wood-gas produced during biomass gasification, provided that gas was efficiently cleaned by tar and sulphur compounds produced during the gasification. Currently, one of the most efficient gasification technologies is based on the use of fluidized bed
* Corresponding author. Tel.: þ39 3381266995. E-mail address: [email protected] (A. Di Carlo). http://dx.doi.org/10.1016/j.ijhydene.2015.05.128 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature Abbreviation GHSV gas hourly space velocity (h1) XRD X-ray diffraction Symbols A Ctar dp Ea kapp rp t XC
Arrenhius pre-exponential factor (m3kg1h1) tar concentration in the gas (g/Nm3) particles diameter (mm) Arrenhius activation energy (kJ/mol) Arrhenius constant (m3kg1h1) particles density (kg/m3) catalyst space time (m3kg1h1) tar conversion
Subscript s l g in out T cat
solid liquid gas inlet outlet bed Temperature catalyst
reactors. Such reactors are commonly used in processes where catalysts must be continuously regenerated, also facilitating heat transfer, temperature uniformity, and higher catalyst effectiveness factors. In particular, referring to the double bed systems, the fluidization state allows a steady circulation of the bed material among different reactors. In this way it is possible to exploit the bed material as thermal carrier to enhance heat exchange among different reactors, for example from a combustion reactor (exothermic) to gasification/reforming reactor (endothermic) [1,2]. In this gasifier concept, the fuel is fed into the gasification zone, and gasified with steam. The bed material, together with some charcoal, circulates in the combustion zone. This zone is fluidized with air and the charcoal is burned, heating the bed material at a temperature that is higher than the entrance one. The hot bed material from the combustor is circulated to the gasifier supplying the thermal power needed for the gasification reactions. With this concept the two reaction chambers (combustion with air and steam gasification) are physically separated and it is possible to get a high-quality gas, with a reduced N2 content even if air (and not pure oxygen) is used for the combustion. Biomass steam gasification produces a fuel gas rich in hydrogen and carbon monoxide, with a significant content of methane and carbon dioxide. However, also organic impurities (tar) are present. As defined in Ref. [3] the organics compounds, produced under thermal or partial-oxidation regimes (gasification) of any organic material, are called “tars”. Generally, it is assumed that tars are aromatic hydrocarbons. They are undesirable and noxious by-products, known for their toxic and carcinogenic properties. The tar concentration in gas produced in a fluidized bed gasifier ranges between 5 and 100 g/Nm3. Moreover, corrosive and pollutant characteristics of tar compounds prohibit direct utilization of the
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produced gas. Therefore, tar reduction strategies are actively pursued. Phuphuakrat et al. [4] demonstrated that the reduction of the gravimetric tar mass was 78% in the case of the thermal cracking, whereas it was in the range of 77e92% in the case of the steam reforming. Gao et al. [5] investigated the production of hydrogen-rich gas in an updraft biomass gasifier. A ceramic reformer was used and temperature, equivalence ratio and steam to biomass ratio were varied to assess their influence on wood-gas properties. Catalytic steam reforming seems to be the best way to eliminate tar compounds, converting these into syngas, and thus recovering their energy content whilst reducing pollution. Thus, the clean fuel gas can be delivered at temperatures as high as those required to exploit it in high efficiency power generation devices like fuel cells (SOFC or MCFC [6e13]). There are different reports on steam reforming of representative tar compounds, as well as toluene [14e23], or naphthalene [24]. However further investigations are necessary for a successful application of the biomass-derived gas [22] because of the high degradation of the catalyst due to carbon deposition or sintering. This aspect is specifically addressed in this paper. Zhang et al. [25] investigated the efficiency of potassium catalysts in steam gasification of biomass in a fixed bed reactor. Tests were carried out at different temperatures and with variable potassium compounds load. Optimal conditions (hydrogen yield of 73% vol.) were obtained when working at 700 C and with a 20% K loading. Corella et al. [26] and Aznar et al. [27] evaluated different Ni catalyst for steam reforming. They showed that, in a flue gas from a fluidized bed biomass gasifier, the commercial catalysts for steam reforming of naphtha or heavier (than natural gas) hydrocarbons are more active for tar elimination than the ones for steam reforming of light hydrocarbons or natural gas. A guard bed of dolomite was inserted upstream the Ni catalyst bed in order to lower tar and thus to avoid Ni catalyst deactivation. The tested commercial catalysts for steam reforming of naphtha have a similar activity. In the studies of Corella et al. and Aznar et al., the NiO content in the catalyst varied from 15% to 25% in weight. Consequently, GHSV varied between 2700 and 16,000 Nm3(wet)/m3 h depending on the catalyst used. No deactivation was observed with tar concentration lower than 2 g/Nm3. This low concentration was obtained when using the dolomite guard bed before the Ni catalyst. In most cases, commercial Ni catalysts have been tested for steam reforming, demonstrating that the catalyst deactivation due to coke deposition can be one of the most serious problems. Steam reforming of hydrocarbons (both methane and tar) is therefore relevant to enhance the applicability of biomass gasification processes and to produce a gas that is sufficiently pure to be considered suitable for renewable hydrogen production. To investigate the syngas clean-up in a laboratoryscale updraft biomass gasifier, Wang et al. [28] produced Nibased catalysts by mechanically mixing NiO and char particles at various ratios. The Ni/wood-char catalysts removed more than 97% of tars in syngas at 800 C (for 8 h) reforming temperature, 15% NiO loading, and 0.3 s gas residence time. Analysis of syngas composition indicated that concentrations of H2 and CO in syngas significantly increased. A slight
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deactivation of the catalyst, followed by a successive stabilization, was noticed in the early stage of tar/syngas reforming. Mayenite (Ca12Al14O33) was previously developed and used by Fujita et al. [29,30] as an alternate catalyst. It proved to have high performance on hydrocarbon catalytic combustion because of the oxidation ability of the “free” oxide anions O 2 and O2 2 that are generated on the surface of the solid catalyst. Li et al. [31,32] used a Ni/Ca12Al14O33 catalyst to support steam reforming of toluene, selected as reference tar compound, in a fixed bed reactor. They noticed a high resistance to coke formation and to H2S poisoning. The excellent characteristics of the catalyst was attributed to the special structure of mayenite and to its high amount of ‘‘free oxygen’’, which was related to the presence of hydroxide, peroxide and superoxide radicals in the cages. The superoxide radicals in the cages are transferred to nickel site to gasify the carbon deposit that is present on nickel, thus producing CO. In the present work, the new catalyst Ni/Ca12Al14O33 was prepared following an original synthesis route. This new catalyst was tested at different operating temperatures (700e750e800 C) using a raw gas directly obtained by biomass steam gasification. The catalyst efficiency in steam reforming process was evaluated in terms of conversion rate of the most abundant heavy hydrocarbons obtained by steam gasification of biomass (benzene, toluene, styrene, xylene, napthalene, phenol, anthracene, phenantrene, pyrene) and in terms of generated permanent gas.
Experimental techniques
CaðCH3 COOÞ2 ðsÞ/CaCO3 ðsÞ þ CH3 COCH3 ðgÞ
(2)
CaCO3 ðsÞ/CaOðsÞ þ CO2 ðgÞ
(3)
The resultant components were calcined for 3 h at 950 C to obtain Ca12Al14O33.
Synthesis of Ni/Ca12Al14O33 For the synthesis of Ni/Ca12Al14O33, the mayenite powder previously prepared was milled using a mortar. Then it was sieved in between 150 and 600 mm. At this point, the catalyst (Ni) was added to the Ca12Al14O33 powder by impregnation under magnetic stirrer at 60 C, with adequate amount of nickel(II) nitrate hexa-hydrate (Ni(NO3)2 6H2O) salt dissolved in distilled water. The amount of Ni(NO3)2 6H2O was chosen to produce a catalyst with 5% in weight of Ni. The resulting slurry was dried overnight at 120 C (around 12 h) and then calcined for 3 h at 850 C. The catalyst obtained at this stage was milled using a mortar and sieved to select the granulometry in between 200 and 600 mm, suitable for the experiments at laboratory scale of steam reforming. It is worth noting that at this stage the powder is NiO/Ca12Al14O33 and it needs to be activated under hydrogen flux to obtain the desired Ni/ Ca12Al14O33.
Test bench
Chemicals aspects Calcium acetate Ca(CH3COO)2 is used as the calcium oxide precursor. Ca(CH3COO)2, is obtained from calcium carbonate CaCO3 (CarloErba, technical grade) and acetic acid CH3COOH (Sigma Aldrich, >99.7%). Aluminium oxide Al2O3 (Sigma Aldrich e ACS reagent, 98%) and nickel(II) nitrate hexahydrate Ni(NO3)2 6H2O (Sigma Aldrich purum p.a., crystallized, 97.0% KT) were used to obtain mayenite and nickel oxide catalyst.
Synthesis of Ca12Al14O33 The procedure for the synthesis of Ca12Al14O33 started with the synthesis of the precursor, Ca(CH3COO)2, and pulverised Al2O3. Al2O3 was firstly grinded in a Planetary Ball Mill apparatus (Pulverisette-7, Fritsch) for 4 h at speed of 300 rpm. The Ca(CH3COO)2 was obtained by adding calcium carbonate (corresponding to a molar ratio of CH3COOH/CaCO3 ¼ 9) to a solution of distilled water and acetic acid (1:1 ratio), under magnetic stirring at 90 C. During the production of Ca(CH3COO)2 carbon dioxide is released as shown in (1). CaCO3ðsÞ þ 2CH3 COOHðlÞ /CaðCH3 COOÞ2ðsÞ þ H2 OðlÞ þ CO2ðlÞ
resulting mix of calcium acetate Ca(CH3COO)2 and Al(OH)3 was milled using a mortar and calcined at 800 C for 3 h under air atmosphere in order to decompose it to pure CaO (reaction (2) and (3)).
(1)
After the completion of reaction (1) no release of CO2 was observed. During the production of Ca(CH3COO)2 the pulverised Al2O3 was added in a stoichiometric ratio. The slurry obtained was thus dried overnight (around 12 h) at 120 C. The
Fig. 1, shows a schematic representation of the bench scale facility used during the work. The relevant components are: a solid fuel feeding system, a fluidized bed gasifier, a microreactor for catalyst tests, a gas cooling system, and metering and analysing systems for the off-gases. The fluidized bed gasifier consists of an austenitic stainless steel cylindrical vessel of internal diameter 80 mm fitted with stainless steel porous distributor plate, designed to allow a good gas distribution at all temperatures. In particular, a sintered stainless steel plate is used. The pressure drops through this plate are higher than 40% of those through the fluidized bed yet at ambient temperature. The entire reactor is located in a cylindrical electric furnace provided with temperature and heating rate control systems. Temperature within the reactor is measured by means of two thermocouples, one immersed in the bed and the other located under the distributor. The bed inventory consists of olivine sand. The fluidized gas is a mix of steam and nitrogen. The steam flowrate is adjusted to obtain the desired steam to biomass ratio, while nitrogen is added to guarantee a superficial velocity equal to two times of the minimum fluidization velocity. Water for the steam generation is fed to an electrically heated boiler by means of a peristaltic pump at a constant flow rate. The biomass feeding system is designed to deliver the biomass inside the bubbling bed. During the start-up phase, the entire raw gas generated by biomass gasification feeds a torch where it is completely burned. When gasification
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Fig. 1 e Experimental rig for gasification/catalytic steam reforming tests.
process reaches the working conditions (specified in Section 3.1), a vacuum pump is switched on to feed the stainless steal microreactor (2 cm ID) for a catalysis test with a slipstream of raw gas. A heated ceramic filter assures that no fine particles reach the microreactor. The microreactor (filled with catalyst) is located in a cylindrical electric furnace provided with a temperature and a heating rate control systems. The temperature of the catalyst bed is measured by the thermocouple inside the bed. For each run, the permanent gas yield is measured by means of a volumetric gasmeter, after separation of the condensate (water and organic phases) in a cold bath of isopropanol (at 10 C). The flow rate of the slipstream is controlled by a needle valve downstream the microreactor. Tar is measured after each test by means of Agilent GC-MS. Gas products are analysed by Varian micro-GC. Tests are carried out at three different temperatures (700, 750, 800 C) by using fresh catalyst (15 ml). After any tests catalyst is reduced in H2 flow of 100 Nml/min at the operative temperature.
Baseline tests The operating conditions in the gasifier were kept constant for all runs and are reported in Table 2. All tests were performed after the fluidized bed reached the steady state conditions. Hazelnut shells were used for the gasification tests. The biomass elemental analysis is reported in Table 3. The proximate analysis (moisture, ash, volatile matter and fixed carbon) was carried out by means of a TGA Mettler Toledo, the elemental analysis (CHNO) was carried out by means of Leco 2000 CHN analyzer, and the LHV by means of a Parr 6200 isoperibol oxygen bomb calorimeter. Table 4 report on the reference gas composition. In Fig. 3, tar concentration obtained in the tests without catalyst is shown. As shown in Fig. 3, tars obtained by steam gasification at 850 C are mainly tertiary tars. The most abundant components are benzene, toluene, and napthalene, reaching concentrations of 18.2, 3.0, 4.2 g/Nm3 respectively, while the concentration of phenantrene, anthracene, xylene, styrene
Results XRD analysis of the reduced catalyst was carried out to detect the presence of Mayenite and Ni in the powder. The Philips X'Pert Pro with Cu radiation was used for such analysis. The results are shown in Fig. 2. Both Ca12Al14O33 and Ni can be identified from XRD patterns, the remaining peaks was identified to be residual CaO and Al2O3 in small quantities. X-ray fluorescence analysis (performed on a SPECTRO X-LAB 2000 instrument) of the reduced catalyst showed that metallic Ni was around 4.1% in weight. This value is very close to the foreseen 5% (see Section 2.3). Specific surface area was obtained by the B.E.T method with a Micromeritics instrument (ASAP 2000). The physical properties of the as-synthesized catalyst are presented in Table 1.
Fig. 2 e X-ray diffraction pattern of reduced Ni/mayenite.
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Table 1 e Physical properties of the catalyst used in this work.
Table 4 e Reference composition of the gas obtained by steam gasification of biomass at 850 C (dry, N2 free).
Ni/Ca12Al14O33
Compound 2
Surface area (cm /g) Pore volume (cm3/g) Average pore diameter (nm)
9.15 0.06 20.5
Table 2 e Operating conditions in the gasifier during tests. dp ¼ 351 mm, rp ¼ 2640 kg/m3 dp ¼ 10002000 mm rp ¼ 1026 kg/m3 850 C 347 g/h 0.7
Bed Inventory, olivine Biomass, almond shells Bed temperature Biomass feed rate Steam to biomass ratio
3
and phenol are 0.1, 0.04, 0.17, 0.36, 0.17 g/Nm respectively. Total tar concentration (including benzene) is 26 g/Nm3.
Catalytic tests at different temperatures Tests were carried out at three different temperatures 700, 750 and 800 C by using fresh catalyst (15 mle5 cm Bed Height). Pressure drop through the bed has an order of magnitude of few mbars. Test at 850 C (gasification process) was not carried out as the thermal losses occurring in a real plant operation usually reduces the working temperature to 800 C maximum. The gas flowrate through the catalyst bed was chosen to obtain a GHSV ranging between 3000 and 4000 h1. Each test (at each temperature) was performed for 3 h. At the end of a test, the catalyst was regenerated with air at 800 C to burn solid carbon possibly deposited. This leads to an oxidation of catalyst (forming NiO). Then, before a successive test, the catalyst is reduced with H2 before starting a new test. During catalyst regeneration and reduction, air was blown in the fluidized bed gasifier to burn the residual char collected in the bed. To measure the activity of a catalyst, the tar conversion in given conditions is evaluated. The results are expressed in terms of conversion of total heavy hydrocarbon and in term of individual tar classes defined as:
Vol. fraction (dry, N2 free)
H2 CO CH4 CO2
0.48 0.21 0.09 0.22
2-rings: napthalene 3-rings: phenantrene, anthracene 4-rings: pyrene Once the average mass flow rate of each tar class was measured before and after the catalytic reactor, conversion for each class is calculated as follow: XC ¼
in out m_ tar m_ tar in m_ tar
(4)
The results of total heavy hydrocarbon conversion at different temperatures are shown in Fig. 4. Fig. 5 shows the conversion results per each tar class. As expected, temperature plays a very crucial role in decomposing tars. For any tar class, conversion rate of total hydrocarbons increases when the temperature is raised from 700 to 800 C. At 700 C the catalyst shows a moderate activity in terms of hydrocarbon reduction. Total hydrocarbon conversion at this temperature is close to 0.61 while the conversion increases to 0.84 at 800 C. In particular benzene and 2rings (napthalene) show low conversion rates at 700 C.
benzene 1-ring: toluene (most abundant), xylene, styrene phenol
Table 3 e Physical and chemical properties of biomass. Type Status Moisture (wt %) Ash (wt %) Volatile matter (wt %) Fixed carbon (wt %) Hydrogen (wt %) Oxygen (wt %) LHV (kJ kg1)
Hazelnut shells Raw
Dry
Dry-ash-free (daf)
7.9 1.16 72.45 46.65 5.55 38.74 17 228
1.26 78.66 50.65 6.03 42.06 18 727
79.67 51.3 6.1 42.6 18 966
Fig. 3 e Reference TAR concentration in the gas obtained from steam gasification of biomass at 850 C (dry, N2 free), a) benzene, toluene, napthalene b) phenantrene, anthracene, xylene, styrene, phenol.
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Table 5 e Comparison of the results obtained by this work and that of Corella et al. [33]. Conversion Benzene 1-ring 2-ring Phenol 3-4-Rings
Fig. 4 e Conversion of heavy hydrocarbons at 700, 750 and 800 C.
Moving from 700 C to 800 C benzene conversion varies between 0.59 and 0.83 while 2-rings (napthalene) ranges from 0.52 to 0.86. 1-ring hydrocarbons (mainly toluene) instead, shows a high conversion (compared with the total conversion) even at 700 C. Conversion of this class varies between 0.81 and 0.88 at 700 and 800 C respectively. Phenol, 3-rings hydrocarbon and 4-rings hydrocarbons show the same trend with temperature and their conversion is also high at 700 C. It must be noticed that the amount of these compounds is almost negligible even in the reference test (see Fig. 3b) thus their conversion does not influence significantly the total conversion rate. Table 5 shows the comparison of the results of this work (at 800 C) compared with those of Corella et al. [33] at 840 C. During tests it was not possible to estimate the amount of residual water contained in the gas feeding the micro-reactor and thus the residence time to make a proper comparison with wet syngas. In any case, it can be estimated that the residence time was between 0.03 and 0.04 kgcat h/[m3(Tcat,wet)] thus in the table the values obtained in this work and those by Ref. [33] in the range mentioned above, are reported. As shown in Table 5 the catalyst studied in this work shows an highest activity when compared with the commercial one
Fig. 5 e Heavy hydrocarbon conversions at 700, 750 and 800 C divided per classes: benzene, 1-ring, phenol, 2rings, 3-rings and 4 rings.
This work
Corella et al. [33]
84% 93% 87% 97% 90%
30e40% 60e70% 60e65% 85e90% 75e80%
used in Ref. [33]. This is particularly true for Benzene, 2-rings and 1-ring compounds where conversions range between 84 and 93% while in Ref. [33] ranged between 30 and 40 to 60e70%. Fig. 6, shows the gas composition obtained from the catalytic tests. H2 increased from 0.48 (reference test) to values always higher than 0.54 in all the catalytic tests. Furthermore, the presence of catalyst improves steam-reforming of methane, even if reactivity was low despite the adequate temperatures. As a matter of fact, the methane molar fraction is reduced to 0.05 while it was 0.09 in the reference test The low reactivity could be due to the low Ni loading on catalyst (<5%). Higher Ni loading could lead to sintering of the metallic Ni at the process temperature, increasing carbon deposition problems. No appreciable variation can be observed in the composition with temperature varying between 700 and 800 C. The variations are in the range 0.54e0.55 for H2 and 0.06e0.05 for CH4. Similar results were observed for CO and CO2 varying between 0.19 and 0.20 and between 0.21 and 0.22, respectively. As shown in Fig. 6 the H2/CO ratio has a slight decrease with temperature, which can be ascribed to Water Gas Shift reaction that is favoured at lower temperature even if this reaction is a minority because the H2/CO ratio is always very close to 3.
Long-time test To verify the stability of the Ni/mayenite catalyst on time, a durability test of 12 h was performed at 800 C, with GHSV of 3000e4000 h1 and 15 ml of fresh catalyst. The isopropanol in the cold bath was changed every 60e70 min to collect tar and to evaluate changes in conversion rate.
Fig. 6 e Gas composition at 700, 750 and 800 C.
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were carried out at 800 C to verify that conversion was stable at this temperature for a longer period. No catalyst deactivation was observed during the 12 h tests. Total heavy hydrocarbon conversion was stable around 0.9 during the entire experiment. Results for H2 and CH4 were similar to those obtained during previous tests thus demonstrating that Ni/ mayenite catalyst is active and stable also for steam reforming of methane. Finally a Temperature Programmed Oxidation was performed to verify the presence of deposited carbon after the 12 h tests. No CO and CO2 were measured. This result confirmed the excellent capability of this new catalyst to resist at carbon deposition even at real gasification condition. Fig. 7 e Total Heavy HydroCarbon (HHC) conversion, H2 and CH4 molar fraction in the gas during 12 h tests at 800 C.
Fig. 7 shows the total hydrocarbon conversion obtained during the test and the average molar fraction of H2 and CH4 measured. No catalyst deactivation was observed during the 12 h test. Total heavy hydrocarbon conversion remained constant at a value around 0.9. Ni/mayenite catalyst is active and stable also for steam reforming of methane. Then, H2 and CH4 exhibited results similar to the ones obtained during previous tests. This means that no carbon deposit was forming. At the end of the long time test, TPO (Temperature Programmed Oxidation) was performed, feeding microreactor with 590 ml/min of air. Temperature was varied from 25 to 850 C with an heating rate of 10 C/min. No CO and CO2 were measured in this test, thus demonstrating that not appreciable carbon was deposited on the catalyst. This result confirms the excellent capacity of mayenite to resist at carbon deposition in real gasification condition.
Summary and conclusions In this work several research objectives were pursued to assess the performance of a new catalyst for biomass gasification with reference to: a) tar conversion at different temperature; b) steam reforming of bio-syngas aiming at increase the H2 content; c) durability. All these aspects will be summarized in the following presenting the relevant conclusions to be drawn. A new catalyst Ni/mayenite (5% of Ni) for biomass tar steam reforming was developed. Previous works [31,32] showed the high capability of this catalyst in the steam reforming of toluene. Here, the catalyst was tested in a microreactor fed by a slipstream from the bench scale gasifier to assess the efficiency in conversion of tar when using a real raw gas obtained from biomass gasification with steam at 850 C. The experiments were carried out at three microreactor bed temperatures i.e. 700, 750 and 800 C. As expected, temperature played a crucial role in decomposing tars. Conversion of total hydrocarbons and of each tar class increases when the temperature is raised from 700 to 800 C. At 700 C the catalyst showed a moderate activity and a total hydrocarbon conversion of 0.61. The higher total hydrocarbon conversion was observed at 800 C with a value of 0.84. The catalyst also showed a moderate activity for the steam reforming of methane. Finally a long time tests (12 h)
Acknowledgement The authors gratefully acknowledge Prof. Stefano Natali at Dipartimento Ingegneria Chimica Materiali Ambiente of Sapienza University of Rome, and Ing. Ilaria Aloisi of Department of Industrial and Information Engineering and Economics at University of L'Aquila for their availability in carrying out XRD analysis and X-ray fluorescence analysis.
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[13] Di Carlo A, Borello D, Bocci E. Process simulation of a hybrid SOFC/mGT and enriched air/steam fluidized bed gasifier power plant. Int J Hydrogen Energy 2013;38:5857e74. [14] Swierczynski D, Courson C, Kiennemann A. Study of steam reforming of toluene used as model compound of tar produced by biomass gasification. Chem Eng Process Process Intensif 2008;47:508e13. ski D, Libs S, Courson C, Kiennemann A. Steam [15] Swierczy n reforming of tar from a biomass gasification process over Ni/ olivine catalyst using toluene as a model compound. Appl Catal B Environ 2007;74:211e22. [16] Jess A. Mechanisms and kinetics of thermal reactions of aromatic hydrocarbons from pyrolysis of solid fuels. Fuel 1996;75:1441e8. [17] Taralas G, Kontominas MG, Kakatsios X. Modeling the thermal destruction of toluene (C7H8) as tar-related species for fuel gas cleanup. Energy Fuels 2003;17:329e37. [18] Abu El-Rub Z, Bramer EA, Brem G. Experimental comparison of biomass chars with other catalysts for tar reduction. Fuel 2008;87:2243e52. [19] Polychronopoulou K, Bakandritsos A, Tzitzios V, Fierro JLG, Efstathiou AM. Absorption-enhanced reforming of phenol by steam over supported Fe catalysts. J Catal 2006;241:132e48. [20] Devi L, Ptasinski KJ, Janssen FJJG. Pretreated olivine as tar removal catalyst for biomass gasifiers: investigation using naphthalene as model biomass tar. Fuel Process Technol 2005;86:707e30. [21] Dou B, Gao J, Sha X, Baek SW. Catalytic cracking of tar component from high-temperature fuel gas. Appl Therm Eng 2003;23:2229e39. [22] Li C, Suzuki K. Tar property, analysis, reforming mechanism and model for biomass gasificationdan overview. Renew Sustain Energy Rev 2009;13:594e604. [23] Srinakruang J, Sato K, Vitidsant T, Fujimoto K. A highly efficient catalyst for tar gasification with steam. Catal Commun 2005;6:437e40.
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[24] Sato K, Fujimoto K. Development of new nickel based catalyst for tar reforming with superior resistance to sulfur poisoning and coking in biomass gasification. Catal Commun 2007;8:1697e701. [25] Zhang Y, Gong X, Zhang B, Liu W, Xu M. Potassium catalytic hydrogen production in sorption enhanced gasification of biomass with steam. Int J Hydrogen Energy 2014;39:4234e43. [26] Corella J, Orio A, Toledo J-M. Biomass gasification with air in a fluidized bed: exhaustive tar elimination with commercial steam reforming catalysts. Energy Fuels 1999;13:702e9. [27] Aznar MP, Caballero MA, Gil J, Martı´n JA, Corella J. Commercial steam reforming catalysts to improve biomass gasification with steamoxygen mixtures. 2. Catalytic tar removal. Ind Eng Chem Res 1998;37:2668e80. [28] Wang D, Yuan W, Ji W. Char and char-supported nickel catalysts for secondary syngas cleanup and conditioning. Appl Energy 2011;88:1656e63. [29] Fujita S, Nakano H, Suzuki K, Mori T, Masuda H. Oxidative destruction of hydrocarbons on Ca12Al14-xSixO33þ0.5x (0 x 4) with radical oxygen occluded in nanopores. Catal Lett 2006;106:139e43. [30] Fujita S, Suzuki K, Ohkawa M, Mori T, Iida Y, Miwa Y, et al. Oxidative destruction of hydrocarbons on a new zeolite-like crystal of Ca12Al10Si4O35 including O 2 and O22 radicals. Chem Mater 2003;15:255e63. [31] Li C, Hirabayashi D, Suzuki K. A crucial role of O 2 and O22 on mayenite structure for biomass tar steam reforming over Ni/ Ca12Al14O33. Appl Catal B Environ 2009;88:351e60. [32] Li C, Hirabayashi D, Suzuki K. Development of new nickel based catalyst for biomass tar steam reforming producing H2-rich syngas. Fuel Process Technol 2009;90:790e6. [33] Corella J, Caballero MA, Aznar M-P, Brage C. Two advanced models for the kinetics of the variation of the tar composition in its catalytic elimination in biomass gasification. Ind Eng Chem Res 2003;42:3001e11.
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Reforming of tar contained in a raw fuel gas from biomass gasification using nickel-mayenite catalyst Andrea Di Carlo a,*, Domenico Borello b, Mario Sisinni b, Elisa Savuto c, Paolo Venturini b, Enrico Bocci d, Koji Kuramoto e a
University of L' Aquila, Via Campo di Pile, L'Aquila, Italy Sapienza-University of Rome, Via Eudossiana 18, Rome, Italy c Tuscia University, Via S. M. in Gradi 4, Viterbo, Italy d Marconi University, Via Plinio 24, Rome, Italy e Energy Technology Research Institute (ETRI), Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki, Japan b
article info
abstract
Article history:
Catalytic steam reforming of tar is a very efficient process to clean the gas produced by
Received 19 November 2014
biomass gasification. Ni catalysts remain an affordable solution for this problem, even if
Received in revised form
this catalytic material suffers degradation due to carbon deposition and sulphur poisoning.
16 May 2015
In this study, the properties of a new catalyst Ni/Mayenite (Mayenite as binder), prepared
Accepted 19 May 2015
by impregnation method and tested with a real gas obtained from a bench scale fluidized
Available online 11 June 2015
bed steam gasification of biomass, were investigated. Experiments were carried out in a microreactor fed by a slipstream coming from the bench scale gasifier to evaluate gas
Keywords:
cleaning and upgrading options. Preliminary tests were carried out at three different
Biomass gasification
temperatures, 700, 750 and 800 C. In all the tests, the catalysts showed high activities
Tar
reaching to a conversion rate of 90% in the case at highest temperature (800 C). The
Steam reforming catalyst
conversion efficiency remained stable around this value during a 3 h test. A decrease in the
Fluidized bed
performance was observed at 700 and 750 C, even if the conversion remained stable around a lower value. An increase of H2 (>50%) and a decrease of CH4 were observed at all the temperatures, due to the occurrence of steam reforming reaction. A long duration test (12 h) was carried out at 800 C and demonstrated that, at this temperature, the conversion was stable for a longer period. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The market of high efficiency devices for power generation, like high temperature fuel cells (SOFC, MCFC), is continuously
expanding. These devices are prone to be fed with wood-gas produced during biomass gasification, provided that gas was efficiently cleaned by tar and sulphur compounds produced during the gasification. Currently, one of the most efficient gasification technologies is based on the use of fluidized bed
* Corresponding author. Tel.: þ39 3381266995. E-mail address: [email protected] (A. Di Carlo). http://dx.doi.org/10.1016/j.ijhydene.2015.05.128 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature Abbreviation GHSV gas hourly space velocity (h1) XRD X-ray diffraction Symbols A Ctar dp Ea kapp rp t XC
Arrenhius pre-exponential factor (m3kg1h1) tar concentration in the gas (g/Nm3) particles diameter (mm) Arrenhius activation energy (kJ/mol) Arrhenius constant (m3kg1h1) particles density (kg/m3) catalyst space time (m3kg1h1) tar conversion
Subscript s l g in out T cat
solid liquid gas inlet outlet bed Temperature catalyst
reactors. Such reactors are commonly used in processes where catalysts must be continuously regenerated, also facilitating heat transfer, temperature uniformity, and higher catalyst effectiveness factors. In particular, referring to the double bed systems, the fluidization state allows a steady circulation of the bed material among different reactors. In this way it is possible to exploit the bed material as thermal carrier to enhance heat exchange among different reactors, for example from a combustion reactor (exothermic) to gasification/reforming reactor (endothermic) [1,2]. In this gasifier concept, the fuel is fed into the gasification zone, and gasified with steam. The bed material, together with some charcoal, circulates in the combustion zone. This zone is fluidized with air and the charcoal is burned, heating the bed material at a temperature that is higher than the entrance one. The hot bed material from the combustor is circulated to the gasifier supplying the thermal power needed for the gasification reactions. With this concept the two reaction chambers (combustion with air and steam gasification) are physically separated and it is possible to get a high-quality gas, with a reduced N2 content even if air (and not pure oxygen) is used for the combustion. Biomass steam gasification produces a fuel gas rich in hydrogen and carbon monoxide, with a significant content of methane and carbon dioxide. However, also organic impurities (tar) are present. As defined in Ref. [3] the organics compounds, produced under thermal or partial-oxidation regimes (gasification) of any organic material, are called “tars”. Generally, it is assumed that tars are aromatic hydrocarbons. They are undesirable and noxious by-products, known for their toxic and carcinogenic properties. The tar concentration in gas produced in a fluidized bed gasifier ranges between 5 and 100 g/Nm3. Moreover, corrosive and pollutant characteristics of tar compounds prohibit direct utilization of the
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produced gas. Therefore, tar reduction strategies are actively pursued. Phuphuakrat et al. [4] demonstrated that the reduction of the gravimetric tar mass was 78% in the case of the thermal cracking, whereas it was in the range of 77e92% in the case of the steam reforming. Gao et al. [5] investigated the production of hydrogen-rich gas in an updraft biomass gasifier. A ceramic reformer was used and temperature, equivalence ratio and steam to biomass ratio were varied to assess their influence on wood-gas properties. Catalytic steam reforming seems to be the best way to eliminate tar compounds, converting these into syngas, and thus recovering their energy content whilst reducing pollution. Thus, the clean fuel gas can be delivered at temperatures as high as those required to exploit it in high efficiency power generation devices like fuel cells (SOFC or MCFC [6e13]). There are different reports on steam reforming of representative tar compounds, as well as toluene [14e23], or naphthalene [24]. However further investigations are necessary for a successful application of the biomass-derived gas [22] because of the high degradation of the catalyst due to carbon deposition or sintering. This aspect is specifically addressed in this paper. Zhang et al. [25] investigated the efficiency of potassium catalysts in steam gasification of biomass in a fixed bed reactor. Tests were carried out at different temperatures and with variable potassium compounds load. Optimal conditions (hydrogen yield of 73% vol.) were obtained when working at 700 C and with a 20% K loading. Corella et al. [26] and Aznar et al. [27] evaluated different Ni catalyst for steam reforming. They showed that, in a flue gas from a fluidized bed biomass gasifier, the commercial catalysts for steam reforming of naphtha or heavier (than natural gas) hydrocarbons are more active for tar elimination than the ones for steam reforming of light hydrocarbons or natural gas. A guard bed of dolomite was inserted upstream the Ni catalyst bed in order to lower tar and thus to avoid Ni catalyst deactivation. The tested commercial catalysts for steam reforming of naphtha have a similar activity. In the studies of Corella et al. and Aznar et al., the NiO content in the catalyst varied from 15% to 25% in weight. Consequently, GHSV varied between 2700 and 16,000 Nm3(wet)/m3 h depending on the catalyst used. No deactivation was observed with tar concentration lower than 2 g/Nm3. This low concentration was obtained when using the dolomite guard bed before the Ni catalyst. In most cases, commercial Ni catalysts have been tested for steam reforming, demonstrating that the catalyst deactivation due to coke deposition can be one of the most serious problems. Steam reforming of hydrocarbons (both methane and tar) is therefore relevant to enhance the applicability of biomass gasification processes and to produce a gas that is sufficiently pure to be considered suitable for renewable hydrogen production. To investigate the syngas clean-up in a laboratoryscale updraft biomass gasifier, Wang et al. [28] produced Nibased catalysts by mechanically mixing NiO and char particles at various ratios. The Ni/wood-char catalysts removed more than 97% of tars in syngas at 800 C (for 8 h) reforming temperature, 15% NiO loading, and 0.3 s gas residence time. Analysis of syngas composition indicated that concentrations of H2 and CO in syngas significantly increased. A slight
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deactivation of the catalyst, followed by a successive stabilization, was noticed in the early stage of tar/syngas reforming. Mayenite (Ca12Al14O33) was previously developed and used by Fujita et al. [29,30] as an alternate catalyst. It proved to have high performance on hydrocarbon catalytic combustion because of the oxidation ability of the “free” oxide anions O 2 and O2 2 that are generated on the surface of the solid catalyst. Li et al. [31,32] used a Ni/Ca12Al14O33 catalyst to support steam reforming of toluene, selected as reference tar compound, in a fixed bed reactor. They noticed a high resistance to coke formation and to H2S poisoning. The excellent characteristics of the catalyst was attributed to the special structure of mayenite and to its high amount of ‘‘free oxygen’’, which was related to the presence of hydroxide, peroxide and superoxide radicals in the cages. The superoxide radicals in the cages are transferred to nickel site to gasify the carbon deposit that is present on nickel, thus producing CO. In the present work, the new catalyst Ni/Ca12Al14O33 was prepared following an original synthesis route. This new catalyst was tested at different operating temperatures (700e750e800 C) using a raw gas directly obtained by biomass steam gasification. The catalyst efficiency in steam reforming process was evaluated in terms of conversion rate of the most abundant heavy hydrocarbons obtained by steam gasification of biomass (benzene, toluene, styrene, xylene, napthalene, phenol, anthracene, phenantrene, pyrene) and in terms of generated permanent gas.
Experimental techniques
CaðCH3 COOÞ2 ðsÞ/CaCO3 ðsÞ þ CH3 COCH3 ðgÞ
(2)
CaCO3 ðsÞ/CaOðsÞ þ CO2 ðgÞ
(3)
The resultant components were calcined for 3 h at 950 C to obtain Ca12Al14O33.
Synthesis of Ni/Ca12Al14O33 For the synthesis of Ni/Ca12Al14O33, the mayenite powder previously prepared was milled using a mortar. Then it was sieved in between 150 and 600 mm. At this point, the catalyst (Ni) was added to the Ca12Al14O33 powder by impregnation under magnetic stirrer at 60 C, with adequate amount of nickel(II) nitrate hexa-hydrate (Ni(NO3)2 6H2O) salt dissolved in distilled water. The amount of Ni(NO3)2 6H2O was chosen to produce a catalyst with 5% in weight of Ni. The resulting slurry was dried overnight at 120 C (around 12 h) and then calcined for 3 h at 850 C. The catalyst obtained at this stage was milled using a mortar and sieved to select the granulometry in between 200 and 600 mm, suitable for the experiments at laboratory scale of steam reforming. It is worth noting that at this stage the powder is NiO/Ca12Al14O33 and it needs to be activated under hydrogen flux to obtain the desired Ni/ Ca12Al14O33.
Test bench
Chemicals aspects Calcium acetate Ca(CH3COO)2 is used as the calcium oxide precursor. Ca(CH3COO)2, is obtained from calcium carbonate CaCO3 (CarloErba, technical grade) and acetic acid CH3COOH (Sigma Aldrich, >99.7%). Aluminium oxide Al2O3 (Sigma Aldrich e ACS reagent, 98%) and nickel(II) nitrate hexahydrate Ni(NO3)2 6H2O (Sigma Aldrich purum p.a., crystallized, 97.0% KT) were used to obtain mayenite and nickel oxide catalyst.
Synthesis of Ca12Al14O33 The procedure for the synthesis of Ca12Al14O33 started with the synthesis of the precursor, Ca(CH3COO)2, and pulverised Al2O3. Al2O3 was firstly grinded in a Planetary Ball Mill apparatus (Pulverisette-7, Fritsch) for 4 h at speed of 300 rpm. The Ca(CH3COO)2 was obtained by adding calcium carbonate (corresponding to a molar ratio of CH3COOH/CaCO3 ¼ 9) to a solution of distilled water and acetic acid (1:1 ratio), under magnetic stirring at 90 C. During the production of Ca(CH3COO)2 carbon dioxide is released as shown in (1). CaCO3ðsÞ þ 2CH3 COOHðlÞ /CaðCH3 COOÞ2ðsÞ þ H2 OðlÞ þ CO2ðlÞ
resulting mix of calcium acetate Ca(CH3COO)2 and Al(OH)3 was milled using a mortar and calcined at 800 C for 3 h under air atmosphere in order to decompose it to pure CaO (reaction (2) and (3)).
(1)
After the completion of reaction (1) no release of CO2 was observed. During the production of Ca(CH3COO)2 the pulverised Al2O3 was added in a stoichiometric ratio. The slurry obtained was thus dried overnight (around 12 h) at 120 C. The
Fig. 1, shows a schematic representation of the bench scale facility used during the work. The relevant components are: a solid fuel feeding system, a fluidized bed gasifier, a microreactor for catalyst tests, a gas cooling system, and metering and analysing systems for the off-gases. The fluidized bed gasifier consists of an austenitic stainless steel cylindrical vessel of internal diameter 80 mm fitted with stainless steel porous distributor plate, designed to allow a good gas distribution at all temperatures. In particular, a sintered stainless steel plate is used. The pressure drops through this plate are higher than 40% of those through the fluidized bed yet at ambient temperature. The entire reactor is located in a cylindrical electric furnace provided with temperature and heating rate control systems. Temperature within the reactor is measured by means of two thermocouples, one immersed in the bed and the other located under the distributor. The bed inventory consists of olivine sand. The fluidized gas is a mix of steam and nitrogen. The steam flowrate is adjusted to obtain the desired steam to biomass ratio, while nitrogen is added to guarantee a superficial velocity equal to two times of the minimum fluidization velocity. Water for the steam generation is fed to an electrically heated boiler by means of a peristaltic pump at a constant flow rate. The biomass feeding system is designed to deliver the biomass inside the bubbling bed. During the start-up phase, the entire raw gas generated by biomass gasification feeds a torch where it is completely burned. When gasification
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Fig. 1 e Experimental rig for gasification/catalytic steam reforming tests.
process reaches the working conditions (specified in Section 3.1), a vacuum pump is switched on to feed the stainless steal microreactor (2 cm ID) for a catalysis test with a slipstream of raw gas. A heated ceramic filter assures that no fine particles reach the microreactor. The microreactor (filled with catalyst) is located in a cylindrical electric furnace provided with a temperature and a heating rate control systems. The temperature of the catalyst bed is measured by the thermocouple inside the bed. For each run, the permanent gas yield is measured by means of a volumetric gasmeter, after separation of the condensate (water and organic phases) in a cold bath of isopropanol (at 10 C). The flow rate of the slipstream is controlled by a needle valve downstream the microreactor. Tar is measured after each test by means of Agilent GC-MS. Gas products are analysed by Varian micro-GC. Tests are carried out at three different temperatures (700, 750, 800 C) by using fresh catalyst (15 ml). After any tests catalyst is reduced in H2 flow of 100 Nml/min at the operative temperature.
Baseline tests The operating conditions in the gasifier were kept constant for all runs and are reported in Table 2. All tests were performed after the fluidized bed reached the steady state conditions. Hazelnut shells were used for the gasification tests. The biomass elemental analysis is reported in Table 3. The proximate analysis (moisture, ash, volatile matter and fixed carbon) was carried out by means of a TGA Mettler Toledo, the elemental analysis (CHNO) was carried out by means of Leco 2000 CHN analyzer, and the LHV by means of a Parr 6200 isoperibol oxygen bomb calorimeter. Table 4 report on the reference gas composition. In Fig. 3, tar concentration obtained in the tests without catalyst is shown. As shown in Fig. 3, tars obtained by steam gasification at 850 C are mainly tertiary tars. The most abundant components are benzene, toluene, and napthalene, reaching concentrations of 18.2, 3.0, 4.2 g/Nm3 respectively, while the concentration of phenantrene, anthracene, xylene, styrene
Results XRD analysis of the reduced catalyst was carried out to detect the presence of Mayenite and Ni in the powder. The Philips X'Pert Pro with Cu radiation was used for such analysis. The results are shown in Fig. 2. Both Ca12Al14O33 and Ni can be identified from XRD patterns, the remaining peaks was identified to be residual CaO and Al2O3 in small quantities. X-ray fluorescence analysis (performed on a SPECTRO X-LAB 2000 instrument) of the reduced catalyst showed that metallic Ni was around 4.1% in weight. This value is very close to the foreseen 5% (see Section 2.3). Specific surface area was obtained by the B.E.T method with a Micromeritics instrument (ASAP 2000). The physical properties of the as-synthesized catalyst are presented in Table 1.
Fig. 2 e X-ray diffraction pattern of reduced Ni/mayenite.
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Table 1 e Physical properties of the catalyst used in this work.
Table 4 e Reference composition of the gas obtained by steam gasification of biomass at 850 C (dry, N2 free).
Ni/Ca12Al14O33
Compound 2
Surface area (cm /g) Pore volume (cm3/g) Average pore diameter (nm)
9.15 0.06 20.5
Table 2 e Operating conditions in the gasifier during tests. dp ¼ 351 mm, rp ¼ 2640 kg/m3 dp ¼ 10002000 mm rp ¼ 1026 kg/m3 850 C 347 g/h 0.7
Bed Inventory, olivine Biomass, almond shells Bed temperature Biomass feed rate Steam to biomass ratio
3
and phenol are 0.1, 0.04, 0.17, 0.36, 0.17 g/Nm respectively. Total tar concentration (including benzene) is 26 g/Nm3.
Catalytic tests at different temperatures Tests were carried out at three different temperatures 700, 750 and 800 C by using fresh catalyst (15 mle5 cm Bed Height). Pressure drop through the bed has an order of magnitude of few mbars. Test at 850 C (gasification process) was not carried out as the thermal losses occurring in a real plant operation usually reduces the working temperature to 800 C maximum. The gas flowrate through the catalyst bed was chosen to obtain a GHSV ranging between 3000 and 4000 h1. Each test (at each temperature) was performed for 3 h. At the end of a test, the catalyst was regenerated with air at 800 C to burn solid carbon possibly deposited. This leads to an oxidation of catalyst (forming NiO). Then, before a successive test, the catalyst is reduced with H2 before starting a new test. During catalyst regeneration and reduction, air was blown in the fluidized bed gasifier to burn the residual char collected in the bed. To measure the activity of a catalyst, the tar conversion in given conditions is evaluated. The results are expressed in terms of conversion of total heavy hydrocarbon and in term of individual tar classes defined as:
Vol. fraction (dry, N2 free)
H2 CO CH4 CO2
0.48 0.21 0.09 0.22
2-rings: napthalene 3-rings: phenantrene, anthracene 4-rings: pyrene Once the average mass flow rate of each tar class was measured before and after the catalytic reactor, conversion for each class is calculated as follow: XC ¼
in out m_ tar m_ tar in m_ tar
(4)
The results of total heavy hydrocarbon conversion at different temperatures are shown in Fig. 4. Fig. 5 shows the conversion results per each tar class. As expected, temperature plays a very crucial role in decomposing tars. For any tar class, conversion rate of total hydrocarbons increases when the temperature is raised from 700 to 800 C. At 700 C the catalyst shows a moderate activity in terms of hydrocarbon reduction. Total hydrocarbon conversion at this temperature is close to 0.61 while the conversion increases to 0.84 at 800 C. In particular benzene and 2rings (napthalene) show low conversion rates at 700 C.
benzene 1-ring: toluene (most abundant), xylene, styrene phenol
Table 3 e Physical and chemical properties of biomass. Type Status Moisture (wt %) Ash (wt %) Volatile matter (wt %) Fixed carbon (wt %) Hydrogen (wt %) Oxygen (wt %) LHV (kJ kg1)
Hazelnut shells Raw
Dry
Dry-ash-free (daf)
7.9 1.16 72.45 46.65 5.55 38.74 17 228
1.26 78.66 50.65 6.03 42.06 18 727
79.67 51.3 6.1 42.6 18 966
Fig. 3 e Reference TAR concentration in the gas obtained from steam gasification of biomass at 850 C (dry, N2 free), a) benzene, toluene, napthalene b) phenantrene, anthracene, xylene, styrene, phenol.
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Table 5 e Comparison of the results obtained by this work and that of Corella et al. [33]. Conversion Benzene 1-ring 2-ring Phenol 3-4-Rings
Fig. 4 e Conversion of heavy hydrocarbons at 700, 750 and 800 C.
Moving from 700 C to 800 C benzene conversion varies between 0.59 and 0.83 while 2-rings (napthalene) ranges from 0.52 to 0.86. 1-ring hydrocarbons (mainly toluene) instead, shows a high conversion (compared with the total conversion) even at 700 C. Conversion of this class varies between 0.81 and 0.88 at 700 and 800 C respectively. Phenol, 3-rings hydrocarbon and 4-rings hydrocarbons show the same trend with temperature and their conversion is also high at 700 C. It must be noticed that the amount of these compounds is almost negligible even in the reference test (see Fig. 3b) thus their conversion does not influence significantly the total conversion rate. Table 5 shows the comparison of the results of this work (at 800 C) compared with those of Corella et al. [33] at 840 C. During tests it was not possible to estimate the amount of residual water contained in the gas feeding the micro-reactor and thus the residence time to make a proper comparison with wet syngas. In any case, it can be estimated that the residence time was between 0.03 and 0.04 kgcat h/[m3(Tcat,wet)] thus in the table the values obtained in this work and those by Ref. [33] in the range mentioned above, are reported. As shown in Table 5 the catalyst studied in this work shows an highest activity when compared with the commercial one
Fig. 5 e Heavy hydrocarbon conversions at 700, 750 and 800 C divided per classes: benzene, 1-ring, phenol, 2rings, 3-rings and 4 rings.
This work
Corella et al. [33]
84% 93% 87% 97% 90%
30e40% 60e70% 60e65% 85e90% 75e80%
used in Ref. [33]. This is particularly true for Benzene, 2-rings and 1-ring compounds where conversions range between 84 and 93% while in Ref. [33] ranged between 30 and 40 to 60e70%. Fig. 6, shows the gas composition obtained from the catalytic tests. H2 increased from 0.48 (reference test) to values always higher than 0.54 in all the catalytic tests. Furthermore, the presence of catalyst improves steam-reforming of methane, even if reactivity was low despite the adequate temperatures. As a matter of fact, the methane molar fraction is reduced to 0.05 while it was 0.09 in the reference test The low reactivity could be due to the low Ni loading on catalyst (<5%). Higher Ni loading could lead to sintering of the metallic Ni at the process temperature, increasing carbon deposition problems. No appreciable variation can be observed in the composition with temperature varying between 700 and 800 C. The variations are in the range 0.54e0.55 for H2 and 0.06e0.05 for CH4. Similar results were observed for CO and CO2 varying between 0.19 and 0.20 and between 0.21 and 0.22, respectively. As shown in Fig. 6 the H2/CO ratio has a slight decrease with temperature, which can be ascribed to Water Gas Shift reaction that is favoured at lower temperature even if this reaction is a minority because the H2/CO ratio is always very close to 3.
Long-time test To verify the stability of the Ni/mayenite catalyst on time, a durability test of 12 h was performed at 800 C, with GHSV of 3000e4000 h1 and 15 ml of fresh catalyst. The isopropanol in the cold bath was changed every 60e70 min to collect tar and to evaluate changes in conversion rate.
Fig. 6 e Gas composition at 700, 750 and 800 C.
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were carried out at 800 C to verify that conversion was stable at this temperature for a longer period. No catalyst deactivation was observed during the 12 h tests. Total heavy hydrocarbon conversion was stable around 0.9 during the entire experiment. Results for H2 and CH4 were similar to those obtained during previous tests thus demonstrating that Ni/ mayenite catalyst is active and stable also for steam reforming of methane. Finally a Temperature Programmed Oxidation was performed to verify the presence of deposited carbon after the 12 h tests. No CO and CO2 were measured. This result confirmed the excellent capability of this new catalyst to resist at carbon deposition even at real gasification condition. Fig. 7 e Total Heavy HydroCarbon (HHC) conversion, H2 and CH4 molar fraction in the gas during 12 h tests at 800 C.
Fig. 7 shows the total hydrocarbon conversion obtained during the test and the average molar fraction of H2 and CH4 measured. No catalyst deactivation was observed during the 12 h test. Total heavy hydrocarbon conversion remained constant at a value around 0.9. Ni/mayenite catalyst is active and stable also for steam reforming of methane. Then, H2 and CH4 exhibited results similar to the ones obtained during previous tests. This means that no carbon deposit was forming. At the end of the long time test, TPO (Temperature Programmed Oxidation) was performed, feeding microreactor with 590 ml/min of air. Temperature was varied from 25 to 850 C with an heating rate of 10 C/min. No CO and CO2 were measured in this test, thus demonstrating that not appreciable carbon was deposited on the catalyst. This result confirms the excellent capacity of mayenite to resist at carbon deposition in real gasification condition.
Summary and conclusions In this work several research objectives were pursued to assess the performance of a new catalyst for biomass gasification with reference to: a) tar conversion at different temperature; b) steam reforming of bio-syngas aiming at increase the H2 content; c) durability. All these aspects will be summarized in the following presenting the relevant conclusions to be drawn. A new catalyst Ni/mayenite (5% of Ni) for biomass tar steam reforming was developed. Previous works [31,32] showed the high capability of this catalyst in the steam reforming of toluene. Here, the catalyst was tested in a microreactor fed by a slipstream from the bench scale gasifier to assess the efficiency in conversion of tar when using a real raw gas obtained from biomass gasification with steam at 850 C. The experiments were carried out at three microreactor bed temperatures i.e. 700, 750 and 800 C. As expected, temperature played a crucial role in decomposing tars. Conversion of total hydrocarbons and of each tar class increases when the temperature is raised from 700 to 800 C. At 700 C the catalyst showed a moderate activity and a total hydrocarbon conversion of 0.61. The higher total hydrocarbon conversion was observed at 800 C with a value of 0.84. The catalyst also showed a moderate activity for the steam reforming of methane. Finally a long time tests (12 h)
Acknowledgement The authors gratefully acknowledge Prof. Stefano Natali at Dipartimento Ingegneria Chimica Materiali Ambiente of Sapienza University of Rome, and Ing. Ilaria Aloisi of Department of Industrial and Information Engineering and Economics at University of L'Aquila for their availability in carrying out XRD analysis and X-ray fluorescence analysis.
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