Energy Fuels 2010, 24, 5139–5147 Published on Web 08/19/2010
: DOI:10.1021/ef100369g
Hydrogen Production from Steam Reforming of m-Cresol, a Model Compound Derived from Bio-oil: Green Process Evaluation Based on Liquid Condensate Recycling Ceng Wu and Ronghou Liu* Biomass Energy Engineering Research Centre, School of Agriculture and Biology, Shanghai JiaoTong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China Received March 26, 2010. Revised Manuscript Received July 15, 2010
Liquid pollutants and coke formation can cause many problems in steam reforming of bio-oil for hydrogen production. From an environmental and economic point of view, an operation of liquid condensate recycling aiming at eliminating secondary pollution as well as carbon deposition was applied in this work. Under the optimal reaction conditions, m-cresol (a heavy organic compound present in bio-oil) was steamreformed on a highly efficient commercial Ni-based catalyst for 6 h time-on-stream. Gas product distribution, liquid pollutant formation, and carbon deposition behavior were investigated, respectively. On the basis of one-time liquid condensate recycling, a green and efficient steam-reforming process can be achieved. Under different reaction conditions, the possibility of achieving this green process was evaluated. The results indicated that under a much higher temperature (900 °C), m-cresol becomes easier to steam reform but it is still impossible to achieve a green process just by a single steam reforming. Under a low temperature (800 °C), low steam/carbon ratio (2.5), or high weight hourly space velocity of bio-oil (1.0 h-1), it is difficult to eliminate the liquid pollutants completely by one-time liquid condensate recycling. However, for every single test, the operation of liquid condensate recycling can make a contribution to the increase of the hydrogen yield and the reduction of secondary pollution and coke formation. It provides an alternative route for steam reforming of bio-oil, especially for some heavy components in bio-oil.
pyrolysis stations easily, and in this process, the transportation costs can be reduced effectively. At present, many research institutes in the world have set up different devices to produce bio-oil. In addition, the main technologies of fast pyrolysis focus on fluidized-bed pyrolysis,5 vacuum pyrolysis,6 microwave pyrolysis,7 rotating-cone pyrolysis,8 and vortex pyrolysis.9 However, until now, the properties of bio-oil are still very bad. A high content of water, low calorific value, and high viscosity and corrosivity have limited the extensive use of bio-oil as a liquid fuel. At present, bio-oil is very difficult to use in the engine and can be only used as a boiler fuel.10 Nowadays, bio-oil can be produced from biomass easily through fast pyrolysis technology, and the yield can reach 80 wt % if reaction conditions are proper. With the development of biomass pyrolysis technologies, the use technologies for bio-oil have become a new challenge in the area of bioenergy conversion. Many researchers in the world have tried to use biooil production through different ways, such as bio-oil/diesel
1. Introduction In recent years, energy shortage and environmental problems have become big threats to the sustainable development of our society. With the depletion of fossil energy reserves in the world, renewable energy sources have attracted people’s great attention. As a kind of renewable energy source, biomass energy has the merits of being clean and abundant. In comparison to traditional energy sources, biomass energy will not run out by reasonable use in a foreseeable future. The emission of NOx and SOx can be reduced effectively, and a CO2-neutral energy supply can be achieved.1-4 Biomass can be converted to available energy products by many technological routes. Bioethanol production can be produced from biomass by acid hydrolysis and fermentation technologies. Through gasification, biomass can be converted to combustible gases (often used for power generation) or syngas [often used for liquid fuel synthesis, such as dimethyl ether (DME)]. Biomass pyrolysis for bio-oil production is another important technological route. Bio-oil has a much higher energy density than biomass. Bio-oil can be collected from scattered biomass
(5) Scott, D. S.; Piskorz, J. The flash pyrolysis of aspen-poplar wood. Can. J. Chem. Eng. 1982, 60, 666–674. (6) Pakdel, H.; Roy, C. Chemical characterization of wood oils obtained in a vacuum pyrolysis process development unit. Prepr. Pap.—Am. Chem. Soc., Div. Fuel Chem. 1987, 32 (2), 203–214. (7) Krieger-Bockett, B. Microwave pyrolysis of biomass. Res. Chem. Intermed. 1994, 20 (1), 39–49. (8) Wagenaar, B. M.; Prins, W.; Vanswaaij, W. P. M. Pyrolysis of biomass in the rotating cone reactor: Modeling and experimental justification. Chem. Eng. Sci. 1994, 49 (24), 5109–5126. (9) Diebold, J.; Scahill, J. Production of primary oils in a vortex reactor. Prepr. Pap.—Am. Chem. Soc., Div. Fuel Chem. 1987, 32 (2), 21–28. (10) Gust, S. Combustion experiences of flash pyrolysis fuel in intermediate size boiler. In Developments in Thermal Biomass Conversion; Bridgewater, A. V., Boocock, D. G. B., Eds.; Blackie Academic and Professional: London, U.K., 1997; pp 481-488.
*To whom correspondence should be addressed. Telephone: 0086-2134205744. E-mail:
[email protected]. (1) Turner, J. A. A realizable renewable energy future. Science 1999, 285, 687–689. (2) Okkerse, C.; Bekkum, H. From fossil to green. Green Chem. 1999, 1, 107–114. (3) Marquevich, M.; Sonnemann, G. W.; Castells, F.; Montane, D. Lifecycle inventory analysis of hydrogen production by the steamreforming process: Comparison between vegetable oils and fossil fuels as feedstock. Green Chem. 2002, 4, 414–423. (4) Sharma, R. K.; Bakhshi, N. N. Catalytic upgrading of biomassderived oils to transportation fuels and chemicals. Can. J. Chem. Eng. 1991, 69, 1071–1081. r 2010 American Chemical Society
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achieve an efficient steam reforming. The literature25 has proven that it is much more difficult to steam reform heavy bio-oil compounds than light fractions. Some serious problems will occur in this process when the reaction is carried out in a long-time duration. First of all, it is usually difficult to convert these heavy compounds to gas production completely even under a high temperature and high steam/carbon ratio. Some unreacted material will cause a wasting of resources. At the same time, because of the secondary reactions, some toxic liquid byproduct formed can cause secondary pollution. Second, the carbon deposition will cause catalyst deactivation and even block the reactor tube. In recent years, the research emphasis for bio-oil steam reforming focuses on catalyst preparation, selectivity, and modification,26-42 in which the primary target is to improve the catalyst activity as well as the resistance to carbon deposition. The materials chosen in the
emulsion for an engine fuel, bio-oil upgrading via catalytic cracking,13 hydrotreatment,14 and esterification,15 bio-oil separation for valuable chemicals,16-18 bio-oil gasification for syngas,19 and bio-oil catalytic steam reforming for hydrogen production.20-24 Among the technologies mentioned above, bio-oil steam reforming for hydrogen production will be a promising route. In comparison to biomass, for the liquid state of bio-oil, the material feeding can be operated conveniently using a liquid feed pump. For the high energy density of bio-oil, a small-scale fixed- or fluidized-bed reactor can be qualified for a certain quantity of hydrogen production requirement. The literature20 has proven that bio-oil or its aqueous fraction can be efficiently reformed to generate hydrogen by a thermocatalytic process using commercial, Ni-based catalysts. The hydrogen yield is as high as 85% of the stoichiometric value. The literature23 has proven that production of 1 kmol/s hydrogen from bio-oil (model compounds) steam reforming required almost the same amount of energy as with natural gas reforming. Generally, the components in bio-oil are numerous and complex. For some light fractions in bio-oil (such as acetic acid, etc.), it is usually easy to steam reform even under a relatively lower temperature and lower steam/carbon ratio. However, for crude bio-oil or some heavy components present in bio-oil (such as phenol, xylene, glucose, etc.), a high temperature and sufficient water feeding are necessary to
(25) Wu., C.; Sui, M.; Yan, Y. A comparison of steam reforming of two model bio-oil fractions. Chem. Eng. Technol. 2008, 31 (12), 1748– 1753. (26) Garcia, L.; French, R.; Czernik, S.; Chornet, E. Catalytic steam reforming of bio-oils for the production of hydrogen: Effects of catalyst composition. Appl. Catal., A 2000, 201, 225–239. (27) Rioche, C.; Kulkarni, S.; Meunier, F. C.; Breen, J. P.; Burch, R. Steam reforming of model compounds and fast pyrolysis bio-oil on supported nobel metal catalysts. Appl. Catal., B 2005, 61, 130–139. (28) Basagiannis, A. C.; Verykios, X. E. Reforming reactions of acetic acid on nickel catalysts over a wide temperature range. Appl. Catal., A 2006, 308, 182–193. (29) Basagiannis, A. C.; Verykios, X. E. Steam reforming of the aqueous fraction of bio-oil over structured Ru/MgO/Al2O3 catalysts. Catal. Today 2007, 127, 256–264. (30) Takanabe, K.; Aika, K.; Seshan, K.; Lefferts, L. Sustainable hydrogen from bio-oil-steam reforming of acetic acid as a model oxygenate. J. Catal. 2004, 227, 101–108. (31) Takanabe, K.; Aika, K.; Inazu, K.; Baba, T.; Seshan, K.; Lefferts, L. Steam reforming of acetic acid as a biomass derived oxygenate: Bifunctional pathway for hydrogen formation over Pt/ ZrO2 catalysts. J. Catal. 2006, 243, 263–269. (32) Vagia, E. C.; Lemonidou, A. A. Hydrogen production via steam reforming of bio-oil components over calcium aluminate supported nickel and noble metal catalysts. Appl. Catal., A 2008, 351 (1), 111–121. (33) Hu, X.; Lu, G. Investigation of steam reforming of acetic acid to hydrogen over Ni-Co metal catalyst. J. Mol. Catal. A: Chem. 2007, 261, 43–48. (34) Hu, X.; Lu, G. Investigation of the steam reforming of a series of model compounds derived from bio-oil for hydrogen production. Appl. Catal., B 2009, 88, 376–385. (35) Iojoiu, E.; Domine, M.; Davidian, T.; Guilhaume, N.; Mirodatos, C. Hydrogen production by sequential cracking of biomass-derived pyrolysis oil over noble metal catalysts supported on ceria-zirconia. Appl. Catal., A 2007, 323, 147–161. (36) Wang, Z.; Pan, Y.; Dong, T.; Zhu, X.; Kan, T.; Yuan, L.; Torimoto, Y.; Sadakata, M.; Li, Q. Production of hydrogen from catalytic steam reforming of bio-oil using C12A7-O--based catalysts. Appl. Catal., A 2007, 320, 24–34. (37) Hou, T.; Yuan, L.; Ye, T.; Gong, L.; Tu, J.; Yamamoto, M.; Torimoto, Y.; Li, Q. Hydrogen production by low-temperature reforming of organic compounds in bio-oil over a CNT-promoting Ni catalyst. Int. J. Hydrogen Energy 2009, 34, 9095–9107. (38) Medrano, J. A.; Oliva, M.; Ruiz, J.; Garcı´ a, L.; Arauzo, J. Catalytic steam reforming of acetic acid in a fluidized bed reactor with oxygen addition. Int. J. Hydrogen Energy 2008, 33, 4387–4396. (39) Medrano, J. A.; Oliva, M.; Ruiz, J.; Garcı´ a, L.; Arauzo, J. Catalytic steam reforming of model compounds of biomass pyrolysis liquids in fluidized bed reactor with modified Ni/Al catalysts. J. Anal. Appl. Pyrolysis 2009, 85, 214–225. (40) Bimbela, F.; Oliva, M.; Ruiz, J.; Garcı´ a, L.; Arauzo, J. Hydrogen production by catalytic steam reforming of acetic acid, a model compound of biomass pyrolysis liquids. J. Anal. Appl. Pyrolysis 2007, 79, 112–120. (41) Bimbela, F.; Oliva, M.; Ruiz, J.; Garcı´ a, L.; Arauzo, J. Catalytic steam reforming of model compounds of biomass pyrolysis liquids in fixed bed: Acetol and n-butanol. J. Anal. Appl. Pyrolysis 2009, 85, 204– 213. (42) Yan, C.; Hu, E.; Cai, C. Hydrogen production from bio-oil aqueous fraction with in situ carbon dioxide capture. Int. J. Hydrogen Energy 2009, 35, 2612–2616.
(11) Chiaramonti, D.; Bonini, M.; Fratini, E.; Tondi, G.; Gartner, K.; Bridgwater, A. V.; Grimm, H. P.; Soldaini, I.; Webster, A.; Baglioni, P. Development of emulsions from biomass pyrolysis liquid and diesel and their use in engines—Part 1: Emulsion production. Biomass Bioenergy 2003, 25, 85–99. (12) Chiaramonti, D.; Bonini, M.; Fratini, E.; Tondi, G.; Gartner, K.; Bridgwater, A. V.; Grimm, H. P.; Soldaini, I.; Webster, A.; Baglioni, P. Development of emulsions from biomass pyrolysis liquid and diesel and their use in engines—Part 2: Tests in diesel engines. Biomass Bioenergy 2003, 25, 101–111. (13) Sharma, R. K.; Bakhshi, N. N. Conversion of non-phenolic fraction of biomass-derived pyrolysis oil to hydrocarbon fuels over HZSM-5 using a dual reaction system. Bioresour. Technol. 1993, 45, 195–203. (14) Fisk, C. A.; Morgan, T.; Ji, Y.; Crocker, M.; Crofcheck, C.; Lewis, S. A. Bio-oil upgrading over platinum catalysts using in situ generated hydrogen. Appl. Catal., A 2009, 358 (2), 150–156. (15) Miao, S.; Shanks, B. H. Esterification of biomass pyrolysis model acids over sulfonic acid-functionalized mesoporous silicas. Appl. Catal., A 2009, 359 (1-2), 113–120. (16) Wang, S.; Gu, Y.; Liu, Q.; Yao, Y.; Guo, Z.; Luo, Z.; Cen, K. Separation of bio-oil by molecular distillation. Fuel Process. Technol. 2009, 90 (5), 738–745. (17) Murwanashyaka, J. N.; Pakdel, H.; Roy, C. Separation of syringol from birth wood-derived vacuum pyrolysis oil. Sep. Purif. Technol. 2001, 24, 155–165. (18) Chen, C. A.; Pakdel, H.; Roy, C. Separation of phenols from eucalyptus wood tar. Biomass Bioenergy 1997, 13 (1-2), 25–37. (19) Panigrahi, S.; Dalai, A. K.; Chaudhari, S. T.; Bakhshi, N. N. Synthesis gas production from steam gasification of biomass-derived oil. Energy Fuels 2003, 17, 637–642. (20) Wang, D.; Czernik, S.; Chornet, E. Production of hydrogen from biomass by catalytic steam reforming of fast pyrolysis oils. Energy Fuels 1998, 12, 19–24. (21) Wang, D.; Montane, D.; Chornet, E. Catalytic steam reforming of biomass-derived oxygenates: Acetic acid and hydroxyacetaldehyde. Appl. Catal., A 1996, 143, 245–270. (22) Davidian, T.; Guihaume, N.; Iojoiu, E.; Provendier, H.; Mirodatos, C. Hydrogen production from crude pyrolysis oil by a sequential catalytic process. Appl. Catal., B 2007, 73, 116–127. (23) Vagia, E. C.; Lemonidou, A. A. Thermodynamic analysis of hydrogen production via steam reforming of selected components of aqueous bio-oil fraction. Int. J. Hydrogen Energy 2007, 32 (2), 1133– 1142. (24) Wu, C.; Huang, Q.; Sui, M.; Yan, Y.; Wang, F. Hydrogen production via catalytic steam reforming of fast pyrolysis bio-oil in a two-stage fixed bed reactor system. Fuel Process. Technol. 2008, 89 (12), 1306–1316.
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Figure 1. Schematic diagram of the experimental apparatus.
tests are usually some light compounds (such as acetic acid or the aqueous phase of bio-oil). It is still a big challenge to steam reform crude bio-oil or its heavy compounds. Phenols are very common heavy components present in crude bio-oil. In our previous research work,24 their proportion in fast pyrolysis bio-oil can reach as high as 13% with sawdust as pyrolysis material. In this work, to solve the problems mentioned above, the heavy bio-oil model compound m-cresol was chosen as feeding material. The experiment was carried out on a highly efficient commercial Ni-based catalyst. An operation of liquid condensate recycling aiming at secondary pollution elimination and carbon deposition reduction was applied to evaluate the possibility to achieve green and efficient steam-reforming processes. At the same time, our recent research paper43 has pointed out the carbon deposition problem as well as secondary pollution problem in steam reforming of m-cresol. Therefore, this paper can be also regarded as an extended study of our previous research work.
The reforming products are first condensed through a condenser, and the liquid condensate can be collected in a liquid receiver. Then, the gas products are dried through a drying tower and collected in a gas bag. Through a wet gas flow meter, the accumulative volume of gas products can be recorded. Gas chromatography (GC) is used for the determination of gas product composition. When a single bio-oil steam-reforming test is completed, a liquid condensate recycling can still be applied immediately based on the initial temperature and reacted catalyst. 2.2. Materials, Analytical Methods, and Data Analysis. Generally, the composition of bio-oil depends upon the types of biomass and the pyrolysis conditions, with its main components belonging to the following groups: acids, aldehydes, alcohols, ketones, phenols, sugars, and furans.44,45 For the complex composition and unstable nature of bio-oil, coke formation and catalyst deactivation are nearly unavoidable when the reaction proceeds in a long-time duration. Especially for some heavy components in bio-oil, carbon deposition can be formed even in a short-time duration.25,34 In this work, pure m-cresol (a heavy model compound derived from bio-oil) was chosen as the feeding material, so that the results obtained can be valuable and forceful to evaluate the steam reforming of crude bio-oil itself. Pure m-cresol (CP, purity >98%) was provided by Sinopharm Chemical Reagent Co., Ltd., China. For the merits of low cost and high selectivity for hydrogen, a highly efficient commercial Ni-based catalyst Ni/MgO (YWC series, provided by Wuxi Qiangya Co. Ltd.) was used in the experiment. The catalyst composition is MgO > 80%, Al2O3 > 6%, Ni > 6%, and rareearth element < 0.5%. The specific surface area is 1-3 m2/g.
2. Experimental Section 2.1. Flow Sheet and Operation. In this work, the experiment was carried out on the basis of a fixed-bed reactor system. The schematic diagram of the experimental apparatus is shown in Figure 1. The feeding rate of material and water is controlled by two syringe pumps. A quartz supporter is fixed in the reactor, and the catalyst grains can be loaded in the inner middle part of the reactor tube. A sealed stainless-steel drive pipe stretching into the reactor tube is fixed in the bottom flange. The thermocouple can be inserted into the dirve pipe to measure the temperature of the catalyst bed. An electric furnace connected with a temperature controller works to keep the reactor at a constant reaction temperature.
(44) Scott, D. S.; Piskorz, J.; Radlein, D. Liquid products from the continuous flash pyrolysis of biomass. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 581–588. (45) Iordanidis, A. A.; Kechagiapoulos, P. N.; Voutetakis, S. S.; Lemonidou, A. A.; Vasalos, I. A. Autothermal sorption-enhanced steam reforming of bio-oil/biogas mixture and energy generation by fuel cells: Concept analysis and process simulation. Int. J. Hydrogen Energy 2006, 31, 1058–1065.
(43) Wu, C.; Liu, R. Carbon deposition behavior in steam reforming of bio-oil model compound for hydrogen production. Int. J. Hydrogen Energy 2010, 35, 7386–7398.
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Table 1. Analytical Methods Used in the Experiment method
model
function
gas chromatography (GC) thermal gravimetry (TG) gas chromatography-mass spectrometry (GC-MS) scanning electron microscopy (SEM)
Agilent-6890 PerkinElmer Instrument, TGA/DTA PerkinElmer Instrument, Clare-500 JEOL, JSM-6360LV
gas product composition carbon deposition content of the coked catalyst organic compounds in liquid condensate carbon deposition state on the catalyst surface
Table 2. Reaction Conditions in Steam Reforming of m-Cresol T (°C)
water feeding rate (g/h)
m-cresol feeding rate (g/h)
S/C
catalyst content (g)
WBHSV (h-1)
reaction time (h)
850
29.2
5.0
5:1
10.0
0.5
6
Figure 2. (a) Gas product mole fraction and (b) gas product yield in 6 h time-on-stream.
The catalyst was crushed, and the grains in particle size range of 0.9-1.1 mm were sieved out for the test. The literature28,37-41,45 indicates that Ni-based catalysts are the primary choice in steam reforming of bio-oil or its model compounds. It has been proven that the catalyst used in this work has good performance in steam reforming of methane and can keep good stability under very high reaction temperatures. The reforming products can be collected in three phases, which are gas products (desirable products), solid carbon (carbon deposition), and liquid condensate, respectively. The analytical methods for reforming products can be illustrated in Table 1. For gas products, the hydrogen yield can be defined as the ratio of the hydrogen content obtained to the hydrogen content in stoichiometric potential. H2 content obtained H2 yield ð%Þ ¼ H2 content in stoichiometric potential 100
The carbon deposition content can elucidate the carbon deposition degree of severity. It can be defined as eq 5. carbon deposition content ðg=gcat Þ ¼
3.1. Problems in the Single m-Cresol Steam-Reforming Process. In this section, a single steam-reforming process was carried out. The optimal reaction conditions of high temperature (T), sufficient steam/carbon mole ratio (S/C), and low weight hourly space velocity of bio-oil (WBHSV) were applied to increase the efficiency of steam reforming. The detailed reaction conditions can be illustrated in Table 2. To elucidate the necessity of liquid condensate recycling, two problems in the single steam-reforming process will be revealed. 3.1.1. Catalyst Deactivation Caused by Carbon Deposition. Coke formation is a tough problem in steam reforming of bio-oil. Carbon deposition can block the catalyst active center and cause catalyst deactivation. In this section, the steam reforming of m-cresol was carried out in a long-time duration (6 h) and the variation of gas product distribution with time-on-stream was investigated. The results including gas product mole fraction and the yield (H2 yield is based on the H2 content in stoichiometric potential; C-containing product yield is based on the carbon in feed) are shown in Figure 2. During the entire time-on-stream, the H2 mole fraction shows a little downtrend from 67.8 to 65.8%. CO shows an uptrend, and CO2 shows an opposite trend. Although the composition of CH4 is maintained at a very
ð1Þ
The carbon-containing product yield is based on the carbon in feed. It can be defined as eq 3. C-containing product yield ð%Þ carbon content in product 100 carbon content in feed
ð3Þ
Carbon convertion to gases can be determined as the ratio of the carbon content in gases to the carbon content in feed. carbon content in gases carbon conversion ð%Þ ¼ carbon content in feed 100
ð5Þ
Results and Discussion
Hydrogen in stoichiometric potential is based on the ideal reaction shown as eq 2. ð2Þ C7 H8 O þ 13H2 O f 7CO2 þ 17H2
¼
weight of carbon deposition weight of catalyst
ð4Þ 5142
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Figure 3. (a) External appearance and (b) SEM image of the reacted catalyst.
Figure 4. Total ion current of the liquid condensate.
low value in 6 h time-on-stream, it shows an uptrend from 0.1 to 1.7%. The result of the gas product yield shows a similar variation tendency to that for the mole fraction. The gaseous carbon conversion drops from 97.5 to 95.5%, and the H2 yield shows a more obvious drop, from 84.5 to 75.7%. The results indicate that, as the reaction proceeds, the catalyst activity decreases gradually and the reaction becomes inclined to produce CO, CH4, and some byproduct in solid or liquid phase. When the reaction was completed and the temperature dropped to ambient value, the deactivated catalyst sample was collected and characterized. Its external appearance picture and SEM image are shown in Figure 3. It is indicated that the reacted catalyst is coked deeply from its black color. The SEM detection shows that the carbon nanofiber has blocked the entire catalyst surface. A heavy carbon coating has formed, and the catalyst surface structure can no longer be discerned. An operation aiming at catalyst regeneration must be applied to eliminate the severe carbon deposition. 3.1.2. Secondary Pollution Caused by Liquid Condensate. Generally, the carbon element in crude bio-oil is difficult to be converted to the gaseous phase completely just by a single steam-reforming process. Usually, there will be a carbon loss in the form of solid carbon (carbon deposition) and liquid
carbon (carbon in liquid condensate). In this section, the liquid condensate collected in steam reforming of m-cresol was determined by the GC-MS method, and the result is shown in Figure 4. The result shows that unreacted m-cresol is the main organic compound in liquid condensate. Some liquid byproducts, such as benzene, toluene, phenol, o-cresol, and naphthalene, were also detected. From the compounds above, it can be deduced that some secondary reactions, such as dehydroxylation, demethylation, isomerization, chain scission, and polymerization, occurred in the m-cresol steamreforming process. The organic compounds in liquid condensate will make two problems. On one hand, it will lead to a wasting of resources and a low hydrogen yield. On the other hand, the byproduct formed in the reaction and the unreacted material itself are toxic. They will become a big threat to the environment. Therefore, to achieve a green steam reforming, the liquid condensate must be recycled to eliminate these toxic compounds completely. 3.2. Steam Reforming of m-Cresol Integrated with Liquid Condensate Recycling. In view of problems caused by carbon deposition and secondary pollution, a liquid condensate recycling was applied in this section. For the high water proportion (S/C > 145) in liquid condensate, the recycling 5143
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Figure 5. (a) Overall gas product mole fraction of single m-cresol steam reforming, (b) liquid condensate steam reforming, and (c) m-cresol steam reforming integrated with liquid condensate recycling.
Figure 6. (a) Overall H2 yield and (b) C-containing product yield of single m-cresol steam reforming and m-cresol steam reforming integrated with liquid condensate recycling.
can favor the gasification of carbon deposition (see eq 6), which has formed in the initial m-cresol steam-reforming process. Also, it will have a great potential to eliminate the liquid pollutants through this secondary steam-reforming process. ð6Þ C þ 2H2 O f 2H2 þ CO2
to the final overall composition is tiny. Figure 6 shows the overall gas product yield for both single steam reforming and steam reforming integrated with liquid condensate recycling. The results indicate that liquid condensate recycling indeed made a contribution to the increase of the gas product yield. The H2 yield increases from 77.4 to 81.1%; the CO yield increases from 34.8 to 35.3%; and the CO2 yield increases from 58.8 to 62.0%. 3.2.2. Carbon Distribution and Carbon Deposition Behavior. In view of catalyst regeneration, carbon distribution and carbon deposition behavior after liquid condensate recycling were investigated in this section. Figure 7 shows the overall carbon distribution of single steam reforming and the steam reforming integrated with liquid condensate recycling. The results indicate that a successful elimination for solid and liquid carbon can be obtained through the operation of liquid condensate recycling. The liquid carbon can be eliminated completely, and a green steam-reforming process can be achieved. The solid carbon can also be reduced effectively, and 94.6% of the initial carbon deposition was converted to gas production. Figure 8 shows the carbon deposition behavior on reacted catalyst after liquid condensate recycling. From the external
3.2.1. Distribution of Gas Products. To evaluate the possibility of achieving green steam reforming under the same reaction conditions, for the operation of liquid condensate recycling, the reaction temperature kept the same value and the feeding of liquid condensate was maintained at the same liquid hourly space velocity (LHSV) as that for initial steam reforming. The results including the overall gas product mole fraction and overall yield are shown as Figures 5 and 6 (“overall” means that the values are based on all gas products collected in the reaction). From Figure 5, liquid condensate recycling obtained a better result in gas product composition. With the drops of CO and CH4, the H2 mole fraction shows an obvious increase from 66.2 to 70.2%. However, because the organic compound content in liquid condensate is very low, its contribution 5144
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Figure 7. (a) Overall carbon distribution of single m-cresol steam reforming and (b) m-cresol steam reforming integrated with liquid condensate recycling.
Figure 8. (a) External appearance and (b) SEM image of the reacted catalyst after liquid condensate recycling. Table 3. Evaluation of Green Steam-Reforming Processes under Different Reaction Conditions T (°C) run number
900
1 2 3 4 5 6 7 8 9 10 11 12
850
800
5
WBHSV (h-1)
S/C
2.5
1.0
0.5
single
integrated
Cs (%)
Cl (%)
Cs (%)
Cl (%)
evaluation
2.89 1.62 3.07 1.82 3.66 2.41 4.72 2.76 5.23 4.97 5.80 5.09
1.90 0.57 2.94 1.16 4.02 1.48 4.55 1.90 19.20 9.03 20.74 9.80
0.88 0.09 0.95 0.09 1.06 0.13 1.34 0.28 3.34 2.22 3.75 2.75
0.05 0.00 0.18 0.00 0.52 0.00 0.59 0.09 3.55 2.19 3.82 2.73
O O O
reforming temperature points (800, 850, or 900 °C) and higher or lower S/C (5 or 2.5) and WBHSV (1.0 or 0.5 h-1) were applied in the experiment. A total of 12 different runs were carried out based on the reaction conditions chosen above. The distribution of solid carbon (Cs) and liquid carbon (Cl) as well as evaluation results are illustrated in Table 3. For the comparability among these tests, the total feeding content of m-cresol for each run is based on a constant value of 30.0 g. The feeding rate of liquid condensate recycling for each run is maintained at the same LHSV as that for initial steam reforming. The same applies to the reaction temperature. The results shown in Table 3 indicate that under a much higher temperature of 900 °C, two green processes, runs 2 and 4, can be achieved. As a green process, run 4 proves the fact that the high temperature can compensate for its low S/C. However, under a higher WBHSV of 1.0 h-1, it is still difficult to eliminate the liquid organic pollutants completely. Under a temperature of 850 °C, only one green process, which corresponds with the former reaction conditions
appearance shown in Figure 8a, the carbon coatings on most of the catalyst grains have been eliminated in comparison to the result shown in Figure 3a. The SEM image of the reacted catalyst dovetails nicely with its external appearance. The initial carbon nanofiber has been eliminated effectively, and the catalyst surface structure can be discerned clearly. 3.3. Green Process Evaluation under Different Reaction Conditions. From the results obtained above, under the optimal reaction conditions, a green steam-reforming process can be achieved on the basis of one-time liquid condensate recycling. Whether or not this green process can be achieved just by a single steam reforming under a much higher temperature point (such as 900 °C) and whether or not the green process can be also achieved on the basis of onetime liquid condensate recycling under a relatively lower temperature, lower S/C, or higher WBHSV are important questions related to energy consumption and economic viability in the bio-oil steam-reforming process. In this section, the possibility to achieve this green process will be evaluated under different reaction conditions. Three typical 5145
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: DOI:10.1021/ef100369g
Wu and Liu
Figure 9. Gas product mole fraction of (a and b) run 2 and (c and d) run 4.
Figure 10. Gas product yield of runs 2 and 4: (a) hydrogen yield, (b) C-containing product yield of run 2, and (c) C-containing product yield of run 4.
shown in Table 2, can be obtained. A low S/C has become an inhibitive factor when the temperature drops to this value. Under a temperature of 800 °C, the carbon deposition in single m-cresol steam reforming becomes much more severe and the liquid organic compounds become much more difficult to steam reform. The liquid carbon nearly occupies 20% of the total carbon feeding. Under this temperature, it is impossible to achieve green steam reforming just by one-
time liquid condensate recycling. It is important to mention that a high temperature, high S/C, and low WBHSV can positively favor the carbon element shift from solid and liquid phases to the gaseous phase. Although most runs in Table 3 are difficult to achieve green steam reforming, the recycling of liquid condensate has made a great contribution to the reductions of organic pollutants and carbon deposition. 5146
Energy Fuels 2010, 24, 5139–5147
: DOI:10.1021/ef100369g
Wu and Liu
in hydrogen yield and carbon balance. In the literature, m-cresol can be converted completely to gases in all of the experiments, even for a S/C ratio as low as 3. In addition, carbon was not deposited substantially onto the surface of the catalyst. In this research, we obtained different results, including secondary liquid pollution and severe carbon deposition. It is always true that the results can be different because of a different reactor or different catalyst. However, the most important thing is that the basic idea can be raised by this paper to solve the problems existing in steam reforming of some model compounds or biocrude itself. 4. Conclusions Some heavy organic compounds in bio-oil are usually difficult to steam reform because of the extensive liquid pollutant formation and severe carbon deposition. The problems are nearly unavoidable when the reaction is carried out in a long-time duration. In this paper, an operation of liquid condensate recycling aiming at solving the two problems was applied. Under the optimal reaction conditions, m-cresol (a heavy bio-oil model compound present in bio-oil) was steamreformed on a highly efficient commercial Ni-based catalyst for 6 h time-on-stream. The results of single m-cresol steam reforming showed that unreacted material, liquid byproduct, and carbon deposition indeed made trouble to achieve a green and sustainable steam-reforming process for hydrogen production. With the operation of one-time liquid condensate recycling, the liquid pollutants can be eliminated completely and the carbon deposition can be reduced effectively. Naturally, the hydrogen yield as well as gaseous carbon conversion gave a further increase. A green and efficient steam-reforming process can be achieved. Finally, the possibility of achieving this green process was evaluated under different reaction conditions. The results indicated that m-cresol is a component very difficult to steam reform. Even under a much higher temperature (900 °C), the liquid pollutants cannot be eliminated completely just by a single steam-reforming process. Liquid condensate recycling is necessary to eliminate the carbon residual in liquid and solid phases. Every single green process obtained a high gaseous carbon conversion (>99%) and maintained a very low catalyst carbon content (
