Advanced Materials Research Vols. 622-623 (2013) pp 535-539 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.622-623.535
Catalytic deoxygenation derived from pyrolysis of oil palm shell Varin Han-u-domlarpyos1,2,a, Prapan Kuchonthara1,2,b, Napida Hinchiranan1,2,c 1
Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
2
Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, Bangkok 10330, Thailand a
[email protected],
[email protected],
[email protected]
Keywords: Pyrolysis, Bio-oil, Catalytic deoxygenation
Abstract. This work aimed to prepare the bio-oil with low oxygen content via two-step process involving pyrolysis and catalytic deoxygenation. The raw bio-oil was produced from the pyrolysis of oil palm shell in a screw pyrolyzer with a heating rate of 25°C/min. Then, 10 ml of obtained bio-oil with 54.5 % (w/w) of oxygen content was upgraded by catalytic deoxygenation carried out in a fixed-bed reactor containing 20 g of NiMoS/γ-Al2O3 catalyst under nitrogen atmosphere. When the temperature in the reactor reached to the target point, the bio-oil was dropped by using a syringe pump at a constant flow rate of 0.2 ml/min. The results indicated that this process was efficient to reduce the oxygen content in the bio-oil to 11.5% (w/w) when the reaction temperature was 500°C. Introduction Biomass is one of the renewable energy sources for the biofuel production. The biofuel has received more attentive to substitute for conventional fossil fuels. The advantage for the use of biofuel is the reduction of dependency on fossil fuels and the greenhouse gas emissions because the CO2 produced during fuels combustion can be re-absorbed by biomass via photosynthesis for closing the cycle of CO2 [1]. Pyrolysis, one of thermochemical conversion processes, to convert solid biomass in the absence of oxygen into three physical phases: liquid, solid and gases. The pyrolysis liquid product from biomass is normally called as “bio-oil”. However, it cannot be directly applied for the combustion engines without upgrading due to its high oxygenated compounds produced from the thermal degradation of cellulose, hemicelluloses and lignin in the structures of biomass. These oxygenated compounds such as phenol, ester, carboxylic acid, ether, ketone and aldehyde leads bio-oil having undesired properties such as poor heating value, corrosiveness, thermal instability and immiscibility with fossil fuels [2]. The direct substitution or blending of the bio-oil to petroleum fuel has also limited due to the difference in polarity. Consequently, many researches have studied the several processes to upgrade the bio-oil via thermal catalytic treatment to reduce the content of oxygenated compounds in the bio-oil. Conventional catalysts used for catalytic deoxygenation such as sulfided NiW, CoMo or NiMo supported on alumina. The mechanism to remove the oxygenated compounds in the bio-oil has been proposed as two possible pathways. The oxygenated compounds could be eliminated as water via hydrodeoxygenation and CO2 via decarboxylation [3,4]. In this work, oil palm shell were selected as the biomass feedstock for pyrolysis to produce the bio-oil containing high oxygen and water contents. The obtained bio-oil was then via deoxygenation catalyzed by using NiMoS/γ-Al2O3. The effects of studied parameters such as the height of bed, the reduction temperature of catalyst and the reaction temperature on the amount of oxygenated compounds in the final product were investigated. To avoid the mass transfer limitation for diffusion of bio-oil to the porous catalyst. The bio-oil was gradually dropped into the heated reactor to transform the bio-oil as vapor phase. This process also gave the advantage for non-requirement of seperation to remove solid catalyst from the product after upgrading.
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Experimental Materials. The chemicals used for the synthesis of the NiMoS/γ-Al2O3 catalyst were activated γ-Al2O3 (KHD-12, Sumitomo Chemical Co., Ltd., Japan), [NH4]2MoS4 (Aldrich, 99.97%) and Ni(NO3)2⋅6H2O (QRec, 97%). The gases used for the reduction of catalyst were hydrogen (Praxair, 99.99%purity) and nitrogen (Praxair, 99.99%purity) used for the deoxygenation process. Preparation of bio-oil. The oil palm shell used for preparation of bio-oil was reduced and sieved as powder with a particle size of 1 mm. The obtained powder was dried at 105°C for 12 h before use. The results from proximate analysis exhibited that the dried oil palm shell powder had the contents of moisture, volatile matter, fixed carbon and ash as 7.4, 73, 17 and 2.6% (w/w). Then, it was fed into the hopper of a screw pyrolyzer with the screw speed of 20 rpm at 400°C. The feed rate of the oil palm shell powder was approximately 1 kg/h. The liquid fraction obtained from the pyrolyzer was collected by using a condensation process circulated by cooled water at 8°C. The yields of pyrolysis products could be shown in Table 1. Catalyst preparation and characterization. NiMoS/γ-Al2O3 was prepared by incipient wetness impregnation method [5]. The γ-Al2O3 was calcined in the hot air stream at 250°C for 3 h. Then, the calcined γ-Al2O3 was firstly impregnated by ammonium tetrathiomolybdate ([NH4]2MoS4) dissolved in de-ionized (DI) water (0.06 M). The mixture was stirred at 60°C for 1 h following drying in a rotary evaporator at 75°C for 30 min. The obtained product was further impregnated by nickel nitrite hexahydrate (Ni(NO3)2⋅6H2O) in the presence of DI water (0.02 M). The resulting product was also dried in the rotary evaporator at the same condition as described above and then calcined again at 250°C and 500°C for each 3 h under nitrogen atmosphere. The prepared NiMoS/γ-Al2O3 catalyst contained 1% (w/w) of metal with the atomic ratio of (Ni/(Ni+Mo)) 0.3. The reduction temperature of this catalyst was evaluated by Temperature-programmed reduction (TPR) under a nitrogen flow rate of 30 ml/min at 100°C for pre-treatment step following the reduction in the presence of hydrogen gas at a flow rate of 30 ml/min at 950°C (heating rate = 10°C/min). The amount of H2 uptake was monitored by using thermal conductivity detector (TCD). Catalytic deoxygenation. The catalytic deoxygenation of bio-oil was carried out in a fixed-bed stainless steel reactor (i.d. = 2.5 cm; ∅ = 60 cm). The packing bed in the reactor was consisted of NiMoS/γ-Al2O3 catalyst diluted by unimpregnated γ-Al2O3 to achieve the desired catalyst concentration and maintain the height of packing level. The catalyst in the reactor was reduced in the hydrogen stream at 20 ml/min and 400, 550 and 700°C for 4 h. After the reduction step, the reduced catalyst was kept in the nitrogen atmosphere at a flow rate of 40-80 ml/min until the temperature of system was cooled down to the target point (300-500°C). The 10 ml of bio-oil was then dropped into the reactor by using a syringe pump with a flow rate of 0.2 ml/min. The fed bio-oil was evaporated and passed through the packing bed zone. The upgraded vapor of bio-oil was trapped in glass tubes immersed in an ice bath to obtain the liquid product exhibiting two immiscible phases: oil phase and non-oil phase. The amounts of oxygenated compound and water in the bio-oil before and after catalytic deoxygenation were detected by CHN analyzer (Perkin Elmer, PE-2400) and Karl Fischer titration (Mettler Toledo-V20), respectively. Result and discussion TPR characterization of NiMoS/γγ-Al2O3 catalyst. The TPR profile of NiMoS/γ-Al2O3 catalyst (Fig.1) indicated that this catalyst was initially reduced from Ni1+ to Ni0 at 380-400°C and completely reduced at 680-700°C. The results indicated that this catalyst required high reduction temperature and hydrogen consumption possibly due to the interaction between Ni species and γ-Al2O3 [6,7]. Elemental analysis, water content and heating value of oil palm shell and bio-oil. Table 2 shows the elemental content, amount of water and heating value of oil palm shell and its pyrolysis liquid product. The results indicated that the oil palm shell had high oxygen content (47.7% (w/w)) with a heating value of 20.3 MJ/kg. After pyrolysis, the obtained liquid fraction or bio-oil had oxygen and water contents as 54.5% (w/w) and 25.3% (w/w), respectively with lower heating value as 18.2 MJ/kg. This bio-oil was used as raw material for further catalytic deoxygenation experiments.
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Catalytic deoxygenation of bio-oil. To evaluate the appropriate height of packing bed and prevent the accumulation of volatiles above the bed zone, the unimpregnated γ-Al2O3 was loaded into the fixed bed reactor as 0, 5 and 10 cm. The catalytic deoxygenation was carried out at 400°C under nitrogen atmosphere. The results shown in Table 3 indicated the fixed-bed reactor without the loading of packing bed could decrease the amount of oxygenated compounds from 54.5 to 31.9% (w/w) due to the effect of thermal cracking [8]. In the presence of unimpregnated γ-Al2O3, the increase in the amount of γ-Al2O3 increased the height of packing bed resulting to the reduction of liquid product content with increase in the solid and gaseous products due to the longer contact time between the volatile derived from bio-oil and the surface of γ-Al2O3 exhibiting the acid property to promote the cracking reaction [9]. However, the use of high loading of γ-Al2O3 could enhance the elimination of oxygen compounds in the bio-oil to 28.3% (w/w) and 14.4% (w/w) for 5 and 10 cm of the bed height, respectively since the γ-Al2O3 was the acid catalyst which could accelerate hydrogenation and dehydration for catalytic deoxygenation [10]. The appropriate height of packing bed selected for further experiments was 5 cm to obtain the sufficient content of oil fraction with low amount of oxygenated compounds. For the effect of reduction temperature, the fixed-bed reactor contained catalyst/γ-Al2O3 at a ratio of 2.85 (w/w) with 5 cm of the height of packing bed. The catalyst/γ-Al2O3 mixture was reduced under hydrogen atmosphere at 400-700°C following the catalytic deoxygenation in the presence of nitrogen stream at 60 ml/min at 400°C. The results exhibited in Table 4 showed that the no reduction of this catalyst could decrease the oxygen content in the bio-oil to 24.6% (w/w) since Ni+ could convert acids in the bio-oil as ketones and released CO2 [11]. However, the reduction of catalyst before using indicated the higher performance to reduce the oxygen content to 12.4% (w/w) when the reduction temperature was 550°C. This could be explained that the fully reduction of catalyst could convert Ni+ to Ni0 which was more active for hydrogenation [12]. The use of high reduction temperature as 700°C promoted the sintering and metal loss of the catalyst resulting to lower reaction efficiency [7].
H2 comsumption (a.u.)
Table 1. Yields of pyrolysis products obtained from oil palm shell Product Yield (%w/w) Liquid 37.1 Gas 20.7 Char 42.2
100
300
500
700
Temperature (oC)
Fig.1 TPR profile of 1 wt% of NiMoS/γ-Al2O3 catalyst
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To study the effect of the reaction temperature, the catalyst was reduced at 550°C and operated under nitrogen flow rate of 60 ml/min. From the results in Table 4, the increase in the reaction temperature from 400–500°C slightly enhanced the activity of the catalyst to deoxygenate the bio-oil in form water, CO2 and/or CO [13]. However, the higher reaction temperature increased the gas product with decrease the deoxygenated bio-oil and solid yields via thermal cracking of the volatiles and decomposition of the solid particles in the reactor [14]. Table 2.
Elemental analysis, water content and heating value of oil palm shell and bio-oil Content (% (w/w)) Oil palm shell Bio-oil Carbon 46.0 37.5 Hydrogen 6.02 8.04 Nitrogen 0.28 0.0 Oxygen 47.7 54.5 Water content 7.4 25.3 Heating value (MJ/kg) 20.3 18.2 Table 3. Height of bed (cm) 0 5 10
Effect of the height of packing bed on product yields and oxygen content Yield (% (w/w)) Oxygen content of Liquid oil phase (% (w/w)) Solid Gas Oil fraction Non-oil fraction 11.3 65.6 0.0 23.1 31.9 10.4 49.6 21.6 18.4 28.3 7.99 42.4 28.3 21.3 14.4
Table 4.Effect of the reduction and reaction temperature on product yields and oxygen content in bio-oil Yield (% (w/w)) The reduction The reaction Oxygen content of Liquid temperature temperature oil phase Solid Gas Oil Non-oil (°C) (°C) (% (w/w)) fraction fraction No reduced 400 10.3 45.0 23.0 21.7 24.6 400 400 10.2 46.8 22.5 20.5 20.9 550 400 9.98 48.8 21.8 19.5 12.4 700 400 9.44 46.9 21.8 21.9 20.5 550 500 5.78 42.0 19.6 32.7 11.5 Summary This article studied the catalytic deoxygenation of bio-oil derived from oil palm shell pyrolysis in fixed-bed reactor and operated at low pressure in the presence of nitrogen atmosphere. The vapor phase of the bio-oil generated in the fixed-bed reactor could improve the mass transfer limitation of the bio-oil molecules into NiMoS/γ-Al2O3 catalyst. The optimum condition for reducing the oxygenated compounds was 550°C of reducing temperature and 400°C of reaction temperature to obtain bio-oil with 12.4% (w/w) of oxygen content and 9.98% of liquid yield. Acknowledgements The authors are thankful to the Center of Excellence on Petrochemical and Materials Technology, “CU.GRADUATE SCHOOL THESIS GRANTD”, Graduate School and Department of Chemical Technology, Faculty of Science, Chulalongkorn University for the financial support.
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