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Energy & Fuels 2009, 23, 1748–1753
Hydrogen-Rich Gas Production from Pyrolysis of Biomass in an Autogenerated Steam Atmosphere Hu Guoxin,* Huang Hao, and Li Yanhong School of Mechanical and Power Engineering, Shanghai Jiaotong UniVersity, 200240 Shanghai, China ReceiVed NoVember 12, 2008. ReVised Manuscript ReceiVed January 1, 2009
Hydrogen-rich gas production from pyrolysis of biomass in an autogenerated steam atmosphere was proposed. The scheme aims to utilize steam autogenerated from biomass moisture as a reactant to react with the intermediate products of pyrolysis to produce additional hydrogen. In the present Article, the effect of reactor temperature, moisture content, heating rate, and sweeping gas flow rate on the process was investigated experimentally. Measurement of the atomic structures of chars produced from pyrolysis of wet/predried biomass was carried out with XRD. The results show that heating rate is a key role in the process. Under fast-heating conditions, drying and pyrolysis occurred in a relatively shorter time, which enhances the interactions between the autogenerated steam and the intermediate products and hence produces more hydrogen. Under slow-heating conditions, however, the autogenerated steam from moisture will be partially purged away from the reaction zone, leading to a weakened effect on the subsequent reactions. The use of sweeping gas is unfavorable to hydrogen production due to the reduced residence time of both the autogenerated steam and the volatile. Moisture content has a great effect on hydrogen production. The H2 yield and content increases with the moisture content. Under the conditions of fast-heating rate and without the use of sweeping gas, the pyrolysis of BW (wet biomass with a moisture content of 47.4%, wet basis) exhibits higher H2 yield of 495 mL/g, H2 content of 38.1 vol %, and carbon conversion efficiency of 87.3% than those (267 mL/g, 26.9 vol %, and 68.2%) from the pyrolysis of BTD (the predried biomass with a moisture content of 7.9%, wet basis). The comparison of atomic structure of chars from BW and BTD further confirms that directly pyrolyzing wet biomass without predried treatment favors the pyrolysis to a deeper extent.
1. Introduction Both the growing awareness of the decreasing availability of fossil fuels and the increasing pressure on the environment from production and combustion of fossil fuels have nowadays led to a deeper interest in sustainable heat and power generation using biomass.1,2 The energy from biomass can be transformed into a source of higher value products for the chemical industry3,4 by using a thermochemical method, such as pyrolysis, liquefaction, and gasification. In the past two decades, hydrogen production from biomass pyrolysis and/or gasification has become the subject of extensive research in the field of biomass utilization.5 Previous studies6-8 reported that the introduction of steam during biomass gasification resulted in a remarkable enhancement of hydrogen yield due to the reforming of tar and higher * To whom correspondence should be addressed. Telephone/fax: +86 21 34206569. E-mail:
[email protected]. (1) Johansson, T. B.; Kelly, H.; Reddy, A. K. N.; Williams, R. H. Renewable Energy: Sources for Fuel and Electricity; Island Press: CA, 1993; ISBN 1-55963-138-4. (2) Jong, W. D.; Pirone, A.; Wo´jtowicz, M. A. Fuel 2003, 82, 1139– 1147. (3) Judit, A.; Eleni, A.; Angelos, L.; Michael, S.; Merete, H. N.; Aud, B. Microporous Mesoporous Mater. 2006, 96, 93–101. (4) Bridgewater, A. V. Appl. Catal., A 1994, 116, 5–47. (5) Ni, M.; Dennis, Y. C. L.; Michael, K. H. L.; Sumathy, K. Fuel Process. Technol. 2006, 87, 461–472. (6) Turn, S.; Kinoshita, C.; Zhang, Z.; Ishimura, D.; Zhou, J. Int. J. Hydrogen Energy 1998, 8, 641–648. (7) Lv, P. M.; Chang, J.; Xiong, Z. H.; Huang, H. T.; Wu, C. Z.; Chen, Y.; Zhu, J. X. Energy Fuels 2003, 17, 677–682. (8) Chaudhari, S. T.; Dalai, A. K.; Bakhshi, N. N. Energy Fuels 2003, 17, 1062–1067.
Table 1. Proximate and Ultimate Analyses of Pine Sawdust ultimate analysis
wt %
proximate analysis
wt %
C H O N
49.25 5.37 45.1 0.28
fixed carbon volatiles moisture ash
10.8 80.2 7.9 1.1
hydrocarbon, the steam reforming of methane, as well as the water gas shift reaction. Using steam as an oxidizer, the following balanced chemical equation can be written: CH1.31O0.69 + 1.31H2O f 1.97H2+ CO2
(1)
with the biomass chemical formula obtained from the elemental analysis, Table 1. According to eq 1, a minimum of S/B (steam/ biomass) of 0.97 is calculated for the theoretical maximum yield of H2. Taking specific heat of water and incomplete utilization of steam into account, the energy required for steam generation counts for a big proportion in whole energy consumption of the conventional biomass steam gasification (CBSG). Now, the research is mostly focused on varying operating conditions,6,7 adopting different types of reactor,3,7 adding various catalysts,9 etc. Unfortunately, little attention is paid to reducing the running cost through improving the current steam supply mode,10 as even it consumes quite a large amount of energy of the whole process. Normally, fresh biomass contains a large amount of moisture, which even exceeds 50 wt % (wet basis). In CBSG, such moisture is removed through a predry process before gasifica(9) Demirbas¸, A. Energy ConVers. Manage. 2002, 43, 897–909. (10) Domı´nguez, A.; Mene´ndez, J. A.; Pis, J. J. J. Anal. Appl. Pyrolysis 2006, 77, 127–132.
10.1021/ef800988r CCC: $40.75 2009 American Chemical Society Published on Web 02/09/2009
Hydrogen-Rich Gas Production from Pyrolysis of Biomass
Energy & Fuels, Vol. 23, 2009 1749
Figure 1. Schematic diagram of the experimental setup: (1) nitrogen, (2) gas flow meter, (3) valve, (4) thermocouples, (5) electric tube furnace, (6) quartz tube reactor, (7) quartz boat, (8) furnace, (9) quartz tube, (10) stainless steel reactor, (11) data acquisition unit, (12) computer, (13) quartz serpentine cooler, (14) glass serpentine cooler, (15) cotton filter, (16) gas sample bag, (17) sealed vessel, (18) wet gas meter.
tion, for instance, heating wet biomass with hot air or sun drying it naturally. Next, the predried biomass was gasified with steam, which is usually produced from an auxiliary steam generator or boiler. Both the predrying process and the specific steam generation involve high levels of energy consumption and make CBSG more complicated. Furthermore, in view of energy consumption, the drying process of wet biomass before it is fed into reactor and the subsequent steam generation are also illogical and cause lots of energy waste, leading to an increase of the running cost considerably. For all of these considerations, a scheme of hydrogen-rich gas production from pyrolysis of biomass in an autogenerated steam atmosphere is proposed. In this process, wet biomass with high moisture content is directly used as feedstock of pyrolysis. The drying process of wet biomass will generate a steam-rich atmosphere where steam gasification of the biomass takes place as a result. The scheme aims to utilize steam autogenerated from biomass moisture as a reactant to react with the intermediate products of pyrolysis to produce additional hydrogen. In the present Article, to obtain basic information, the effect of several parameters such as reactor temperature, moisture content, heating rate, and sweeping gas flow on the process was investigated experimentally. For further investigation, the atomic structure of char obtained from pyrolysis of wet biomass and predried biomass was studied with X-ray diffraction (XRD). 2. Experimental Section 2.1. Raw Materials and Preparation of Samples. Wet pine sawdust (particle size:
