Energy Fuels 2010, 24, 5686–5695 Published on Web 09/21/2010
: DOI:10.1021/ef1009605
Deoxygenation of Bio-oil during Pyrolysis of Biomass in the Presence of CaO in a Fluidized-Bed Reactor Yuyu Lin, Chu Zhang, Mingchuan Zhang,* and Jian Zhang School of Mechanical Engineering, Shanghai Jiaotong University, Shanghai 200240, China Received February 3, 2010. Revised Manuscript Received August 30, 2010
The direct deoxygenation effect of CaO on bio-oil during biomass pyrolysis in a fluidized-bed reactor was studied. Bio-oils were produced from white pine in the presence and absence of CaO at 520 °C and a carrier gas flow rate of 50 L/min (standard temperature and pressure). The results showed that the oxygen content of the organic components in the bio-oils was 39, 39, 39, 36, 32, and 31 wt % for white pine alone and white pine accompanied with CaO at CaO/biomass mass ratios of 1, 2, 3, 4 and 5, respectively. At a CaO mass ratio of 5, the relative reduction of oxygen content in the bio-oil reached 21%. Detailed gas chromatography-mass spectrometry analysis showed that the relative abundances of high oxygen content laevoglucose, formic acid, and acetic acid were highly reduced by CaO, indicating direct fixation of “the active quasi-CO2 intermediates” produced during biomass pyrolysis. Furthermore, the relative abundances of furfural, furfuryl alcohol, etc., mainly derived from dehydration reactions, all increased, showing that CaO addition could also catalyze dehydration reactions. X-ray diffraction and Fourier transform infrared analyses of the solid residues prepared by a thermal balance confirmed the direct fixation of the active quasi-CO2 intermediates and showed that some organic calcium salts appeared at 350 °C, which would decompose below 400 °C to form easily regenerated CaCO3. This further confirmed the feasibility of CaO recycling for in situ deoxygenation of bio-oil. have been several reports on using CaO to remove CO2 from the syn-gas produced by coal or biomass gasification.10-13 Meanwhile, Khan found that a relatively high-quality liquid fuel with low sulfur, low oxygen and low viscosity could be produced from coal pyrolysis in the presence of CaO.14 Yeboah et al. found that calcined dolomite (MgO þ CaO) decreased tar yield with some oxygen removal and a modest increase in the H/C ratio during fluidized-bed (FB) pyrolysis of coal.15 Zhu et al. reported that the H/C ratio in tar increased and the (O þ N þ S)/C ratio decreased in coal pyrolysis in the presence of CaO.16 These observations indicate that, in addition to absorbing CO2 in the gas phase, CaO can also fix CO2like substances directly in the liquid product of coal pyrolysis. If it was also the case for biomass pyrolysis, the latter, viz. direct fixation of CO2-like substances in the liquid, would be a superior route for deoxygenation in the biomass-to-liquid (BTL) conversion process. On the basis of these considerations, a tentative scheme for deoxygenation in bio-oil production by fast biomass pyrolysis with CaO recycle was proposed by the authors.17 Preliminary
1. Introduction Increasing concerns over energy security and global climate change have recently led to an explosion of interest in biomass energy use.1,2 In comparison to other usages of biomass, pyrolysis to produce bio-oil is now considered to be an advanced and promising methodology. However, raw bio-oil usually has very poor fuel properties, such as high oxygen content, high instability, low calorific value, high viscosity, and high corrosiveness.3-5 Refining and subsequent purification of the raw oil increase the production cost and greatly reduce the efficiency of energy use.6-8 In work on the effect of alkali and alkaline earth metals on the product distribution from biomass pyrolysis, Liao et al.9 discovered that Ca2þ could strongly catalyze the dehydration reaction in the pyrolysis process. When 2.5 wt % of Ca2þ was impregnated in cellulose fibers, the water content of the liquid product increased by more than 10 wt % and the quality of the bio-oil was improved. Consequently, modifications to in situ pyrolysis may be a way to improve the quality of bio-oil. There
(10) Corella, J.; Toledo, J. M.; Molina, G. Ind. Eng. Chem. Res. 2006, 45 (18), 6137–6146. (11) Mondal, K.; Piotrowski, K.; Dasgupta, D.; Hippo, E.; Wiltowski, T. Ind. Eng. Chem. Res. 2005, 44 (15), 5508–5517. (12) Kuramoto, K.; Ohtomo, K.; Suzuki, K.; Fujimoto, S.; Shibano, S.; Matsuoka, K.; Suzuki, Y.; Hatano, H.; Yamada, O.; Shi-Ying, L.; Harada, M.; Morishita, K.; Takarada, T. Ind. Eng. Chem. Res. 2004, 43 (25), 7989–7995. (13) Acharya, B.; Dutta, A.; Basu, P. Energy Fuels 2009, 23 (10), 5077–5083. (14) Khan, M. R. Pet. Sci. Technol. 1987, 5 (2), 185–231. (15) Yeboah, Y. D.; Longwell, J. P.; Howard, J. B.; Peters, W. A. Ind. Eng. Chem. Process Des. Dev. 1980, 19 (4), 646–653. (16) Zhu, T.; Zhang, S.; Huang, J.; Wang, Y. Fuel Process. Technol. 2000, 64 (1-3), 271–284. (17) Lin, Y.; Tao, R; Zhang, C.; Zhang, J.; Zhang, M. Chin. J. Power Eng. 2009, 29 (5), 492–496 (in Chinese).
*To whom correspondence should be addressed. E-mail: mczhang@ sjtu.edu.cn. (1) Mohan, D.; Pittman, C. U., Jr.; Steele, P. H. Energy Fuels 2006, 20 (3), 848–889. (2) Oasmaa, A.; Solantausta, Y.; Arpiainen, V.; Kuoppala, E.; Sipil€a, K. Energy Fuels 2010, 24 (2), 1380–1388. (3) Czernik, S.; Bridgwater, A. V. Energy Fuels 2004, 18 (2), 590–598. (4) Lu, Q.; Li, W. Z.; Zhu, X. F. Energy Convers. Manage. 2009, 50 (5), 1376–1383. (5) Jiang., X.; Ellis, N. Energy Fuels 2010, 24 (2), 1358–1364. (6) Bridgwater, A. V.; Cottam, M. L. Energy Fuels 1992, 6 (2), 113– 120. (7) Demirbas, M. F. Appl. Energy 2009, 86 (1), S151–S161. (8) Naik, S. N.; Goud, V. V.; Rout, P. K.; Dalai, A. K. Renewable Sustainable Energy Rev. 2010, 14 (2), 578–597. (9) Liao, Y.; Wang, S.; Luo, Z.; Cen, K. Chem. Ind. For. Prod. 2005, 25 (2), 25–30 (in Chinese). r 2010 American Chemical Society
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pubs.acs.org/EF
Energy Fuels 2010, 24, 5686–5695
: DOI:10.1021/ef1009605
Lin et al.
studies carried out by the authors on the pyrolysis of biomass with CaO in a thermal balance and in a drop-tube pyrolyzer17,18 indicated that CaO could extract and fix some active quasi-CO2 intermediates directly by semi-liquid to solid contact rather than via the very slow gas-solid reaction. In this study, a small FB reactor and the related oil-production system were constructed and a series of experiments were carried out to explore the deoxygenation effect and the mechanism of CaO addition. Besides, X-ray diffraction (XRD) and Fourier transform infrared (FTIR) analyses of the solid residues from biomass pyrolysis with CaO in a thermal balance were carried out to obtain more evidence regarding the mechanism.
composed of a feeding system, an electrically heated FB reactor, and a filter-cooling system. The feeding system consisted of a screw feeder driven by a speed-controllable direct-current (DC) motor and a feeding pipe with a water-cooling jacket to avoid pyrogenation and caking within the pipe during biomass feeding. The FB reactor was heated by a 3 3 kW tubular electrical furnace. SCR temperature controllers and fast-response armored thermocouples were used to measure and control the temperatures of each part of the reaction system. The FB reaction pipe was made of 65 5 mm thermostable stainless steel with a total height of 1600 mm, of which 600 mm was below the air distribution plate. The lower part was filled with thermostable stainless-steel screens of 200 mesh, working as the preheater of the reactor. The air distribution plate was covered by a stainless-steel screen of 800 mesh (18 μm). An overflow pipe was installed to keep the dense phase within the FB and maintain continuous operation of the whole system. A differential manometer was used to monitor the fluidization condition in the reactor. The cooling system was made up of two parts. The first part was a shell-and-tube-type heat exchanger cooled by water. The second part was a copper bottle surrounded by a dryice-acetone mixture, with the temperature of about -40 °C. A scrubbing bottle, filled with pure alcohol, was used at the end of the system. The scrubbing bottle was checked after each run to make sure that it remained colorless and transparent, indicating that the cooling system could entirely cool and collect the condensable products. In addition, a vacuum pump was set at the end of the system to adjust the pressure in the reactor. A nitrogen bottle was used to provide fluidizing gas, and quartz sand with a particle size of 0.2-0.35 mm was used as the bed material. After the temperature and pressure in the reactor and the flow rate of fluidizing gas attained their preset values, the reactants (biomass and CaO) were fed into the reactor using the screw feeder. The biomass mixed quickly with the hot sand, and pyrolysis began. During reaction, the pyrolysis products passed through the filter, removing solid impurities. Finally, the pyrolysis product entered the two cooling devices, where the condensable gas was converted to biocrude. Non-condensable gas was collected in a gas bag. A WRT-2P microthermobalance18 was used to prepare the solid residues from biomass pyrolysis with CaO.
2. Experimental Section 2.1. Materials. A white pine powder of 50-100 μm was used as the biomass feed material. CaO used was of commercial grade (>95% purity), with particle sizes less than 50 μm for the FB experiments, and of analytical grade (>98% purity), with the same particle size for the preparation of the solid residue. The white pine powder was dried for 10 h at 105 °C and then mechanically mixed with CaO before feeding. Six mixtures with different mass ratios, R, of CaO/white pine were used as feeds: R = 0 (white pine alone), 1, 2, 3, 4, and 5. The proximate and ultimate analyses of the white pine are given in Table 1. 2.2. Experimental Equipment. Figure 1 shows the FB experimental apparatus and bio-oil system. The apparatus was mainly Table 1. Proximate and Ultimate Analyses of White Pine
proximate analysis (wt %, air-dried basis)
ultimate analysis (wt %, dry ash-free basis)
item
value
moisture ash volatile matter fixed carbon C H N S O
2.4 0.3 83.0 14.3 47.3 6.1 0.1
