Energy & Fuels 2008, 22, 3471–3478
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Enhanced Conversion of Cellulosic Process Residue into Middle Caloric Fuel Gas with Ca Impregnation in Fuel Drying Guangwen Xu,*,†,‡ Takahiro Murakami,†,§ Toshiyuki Suda,† Yoshiaki Matsuzawa,† Hidehisa Tani,† and Yutaka Mito| Research Laboratory, IHI Corporation, Ltd., Nakahara-Cho 1, Isogo-Ku, Yokohama 235-8501, Japan, State Key Laboratory of Multi-Phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, National Institute of AdVanced Science and Technology (AIST), Onogawa 16-1, Tsukuba 305-8659, Japan, and Coal ConVersion Technology Department, Kobe Steel Ltd., Kobe, Hyogo 651-2271, Japan ReceiVed January 31, 2008. ReVised Manuscript ReceiVed May 15, 2008
Process residues, such as coffee and tea grounds, bagasse, vinegar lees, etc., represent a kind of concentrated biomass resources rich in cellulose. They contain water usually of about 60 wt % and are easy to rot to cause serious pollution to groundwater and air. Conversion of them into energy can not only control their induced pollution but also develop their CO2 neutralization function. The present paper concerns the production of middle caloric producer gas from such cellulose-rich process biomass residues. The involved technical process consisted of fuel drying and in turn gasification of the dried fuel in a dual fluidized bed system. Using coffee grounds containing 65 wt % water as a model high water content process residue, the paper found that with fuel drying calcium hydrate (or oxide) can be well-impregnated onto fuel to remarkably increase the gasification reactivity of the fuel and suppress tar evolution with producer gas. Thermal gravitational analysis clarified further that the increase in the gasification reactivity of the fuel was due to enhanced char gasification exclusively. In both scanning electron microscopy-energy dispersive X-ray (SEM-EDX) photographs and X-ray diffraction (XRD) spectrums, it was identified that the impregnated Ca species were present at micro sizes and dispersed uniformly on the matrix of fuel as well as char particles made from the fuel. This indicates essentially the necessary precondition for Ca-base material to catalyze biomass fuel gasification.
Introduction Efficient use of biomass is being highly demanded in the view of preventing a further increase of CO2 concentration in the atmosphere. There are various different types of biomass.1 While some are extremely scattering to cause difficulties in using them at large scales, the so-called “process residue” represents a kind of biomass resources that are concentrated already. The typical process residues are those generated in drink, seasoner, and food works, such as coffee grounds, tea grounds, vinegar lees, and bagasse. All these biomass residues have relatively high water contents. Even after dewatering in centrifuge, the usual fuel water content is still high as 60 wt %. This makes the process residues easy to rot and be actually a kind of potential pollution to water, ground, and air. The aforementioned high water content biomass residues are also rich in cellulose and can hardly be recycled as feeds of animals. They are thus usually disposed as wastes or simply combusted to generate heat. To fully take advantage of their CO2 neutralization function, we have started to work on converting them into middle caloric producer gas with heating values over 2000 kcal Nm-3. The produced gas can not only * To whom correspondence should be addressed. Telephone: +86-1062550075. Fax: +86-10-62550075. E-mail:
[email protected]. † IHI Corporation, Ltd. ‡ Chinese Academy of Sciences. § National Institute of Advanced Science and Technology (AIST). | Kobe Steel Ltd. (1) IEA Bioenergy. What is biomass? http://www.aboutbioenergy.info/ definition.html.
be a kind of high-grade gaseous fuels for combustion utilities, such as boilers and engines, but also be able to be upgraded into synthetic nature gas (SNG) to replace nature gas as a chemical. The conversion was implemented with fuel drying and in turn gasification of the dried fuel. A technology called slurry dewatering in kerosene, which was originally developed for dewatering of brown coal,2 was tested to dry and upgrade the process biomass residues.3,4 Not only the water content of the tested residue can be reduced to about 10 wt % but the residue itself can be also upgraded through reforming part of its containing fatty matters. This allows then the upgraded dry biomass residue to have certainly lower oxygen/carbon ratios and higher heating values. The dried biomass fuel containing water of about 10 wt % was gasified with the so-called dual fluidized bed gasification (DFBG) technology that allows fuel gasification to be isolated from char combustion generating endothermic heat.5,6 As a consequence, the producer gas with a higher heating value, such (2) Umar, D. F.; Daulay, B.; Usui, H.; Deguchi, T.; Sugita, S. Coal Prep. 2005, 25, 31–45. (3) Mito, Y.; Komatsu, N.; Hasegawa, I.; Mae, K. In 2005 International Conference on Coal Science and Technology (ICCS&T); Yamada, O., Ed.; IEA-Clean Coal Center: London, U.K., 2005; paper 2E01. (4) Shigehisa, T.; Mito, Y. Kobe Steel Eng. Rep. 2006, 56, 59–59 (in Japanese). (5) Pfeifer, C.; Rauch, R.; Hofbauer, H. Ind. Eng. Chem. Res. 2004, 43, 1634–1640. (6) Xu, G.; Murakami, T.; Suda, T.; Matsuzawa, Y.; Tani, H. Ind. Eng. Chem. Res. 2006, 45, 2281–2286.
10.1021/ef800073w CCC: $40.75 2008 American Chemical Society Published on Web 07/17/2008
3472 Energy & Fuels, Vol. 22, No. 5, 2008
Xu et al.
Figure 1. Process outline for fuel drying and Ca impregnation.
as over 2000 kcal Nm-3, is possible even if air is used to provide combustion-required oxidant. In our previous work,6–8 coffee grounds containing 65 wt % water were taken as a representative high water content biomass process residue. Results from a pilot DFBG facility with steam as the gasification reagent demonstrated that the heating value of the produced gas can be higher than 4500 kcal Nm-3, but the coffee grounds fuel is highly tarry. At gasification temperatures of about 1073 K and steam/fuel mass ratios of about 1.0 kg/kg, the tar content in the producer gas was as high as 40 g Nm-3.3 The fuel rich in cellulose exhibited as well-limited kinetic rate for gasification, causing the realized C conversions into gas to be rarely over 70% under the mentioned test conditions. Consequently, other advanced technical means are needed to promote gasification reaction and suppress tar evolution. The use of Ca-based catalyst was concerned. There are plenty of studies on using calcium-based additive to catalyze coal gasification.9–11 The literature clarified that high-degree dispersion of Ca species on fuel matrix is necessary to develop their activity.9,11 This has to be implemented with either the ion-exchange12,13 or impregnation method,13,14 incurring thus high additional cost. Because of this, up to now, there is yet no actual application of catalytic coal gasification after several tens of years that the activity of dispersed Ca species on coal gasification was observed in the 1960s. For biomass gasification, Ca-base additive has been extensively tested to decompose tars.15–17 The additive is usually physically mixed into the fuel or packed into an independent column through which the producer gas containing tars passes. For this kind of additive that is not dispersed on fuel, they have hardly activity to catalyze char gasification. On the other hand, it would be difficult and expensive to disperse a catalytic element to most cellulosic biomass, such as woody materials and agriculture residues. This may be why by far most of the reports regarding biomass catalytic pyrolysis and gasification using Cabased catalysts are confined to tests in micro reactors, such as in thermal gravitational analysis (TGA).18,19 (7) Xu, G.; Murakami, T.; Suda, T.; Matsuzawa, Y. Energy Fuels 2006, 20, 2695–2704. (8) Murakami, T.; Xu, G.; Suda, T.; Matsuzawa, Y.; Tani, H.; Fujimori, T. Fuel 2007, 86, 244–255. (9) Lang, R. J.; Neavel, R. C. Fuel 1982, 61, 620–626. (10) Ohtsuka, Y.; Asami, K. Catal. Today 1997, 39, 111–125. (11) Clemens, A. H.; Damiano, L. F.; Matheson, T. W. Fuel 1998, 77, 1017–1020. (12) Ohtsuka, Y.; Tomita, A. Fuel 1986, 65, 1653–1657. (13) Salinas-Martı´nez de Lecea, C.; Almela-Alarco´nh, M.; LinaresSolano, A. Fuel 1990, 69, 21–27. (14) Shibaoka, M.; Ohtsuka, Y.; Wornat, M. J.; Thomas, C. G.; Bennett, A. J. R. Fuel 1995, 74, 1648–1653. (15) Corella, J.; Aznar, M. P.; Gil, J.; Caballero, M. A. Energy Fuels 1999, 13, 1122–1127. (16) Sutton, D.; Kelleher, B.; Ross, J. R. H. Fuel Process. Technol. 2001, 73, 155–173. (17) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Biomass Bioenergy 2003, 24, 125–140. (18) Richards, G. N.; Zheng, G. J. Anal. Appl. Pyrolysis 1991, 21, 133– 146.
Figure 2. Dewatering characteristics of coffee grounds in the cases with and without inclusion of Ca impregnation.
In the conversion process with fuel drying in advance, the impregnation of Ca species can be implemented in fuel drying to avoid complicated additional operation and high extra cost. A preliminary investigation on this method and its realized catalytic effect was reported in Xu et al.20 Spurred by the prospective results obtained, the present paper is devoted to further detailing the impregnation method and to demonstrating how the impregnated Ca species catalyze gasification reactions. Furthermore, the partition and chemical status of the Ca species after fuel gasification are also clarified. Calcium Impregnation Figure 1 outlines the adopted fuel dewatering process in kerosene and how to integrate Ca impregnation into the fuel drying. According to Mito et al.,3 the dewatering process runs usually at 0.2-0.3 MPa and 423-453 K, and the fuel-oil slurry is made with a kerosene/fuel mass ratio of about 1.5. High energy efficiency of the process is ensured via a thermal pump that reuses the vaporized fuel water to heat the fuel-oil slurry. Testing on coffee grounds containing 65 wt % water clarified that the residual oil in the dried fuel was low as 0.2 wt % to allow 99.5% kerosene to be recycled. As highlighted in the dashed-line box of Figure 1, the Ca impregnation was implemented by simply dispersing either CaO or Ca(OH)2 into kerosene before making the kerosene-fuel slurry. In this work, chemical Ca(OH)2 with 95% purity was used and the CaO-equivalent dosing ratio was 4-5 wt % of dry fuel. Figure 2 compares the remaining water content in coffee grounds varying with fuel-oil slurry temperature in the dewatering tank for the cases with and without Ca impregnation. Very little difference can be identified between the two cases, indicating essentially that the drying process was subject to a very similar performance, irrespective of the dose of Ca(OH)2 into kerosene. This verifies that no additional cost could be incurred in the Ca impregnation according to the devised method. (19) Risnes, H.; Fjellerup, J.; Henriksen, U.; Moilanen, A.; Norby, P.; Papadakis, K.; Posselt, D.; Sørensen, L. H. Fuel 2003, 82, 641–651. (20) Xu, G.; Murakami, T.; Suda, T.; Matsuzawa, Y.; Tani, H.; Mito, Y.; Ashizawa, M. AIChE J. 2006, 52, 3555–3561.
Enhanced ConVersion of Cellulosic Process Residue
Figure 3. CODMn values of the condensed water from a dewatering unit at a gradually raised dewatering temperature for the cases with and without inclusion of Ca impregnation. Table 1. Properties of Dried Coffee Grounds and Other Materials Employed proximate (wt %) moisture VM FC ash ultimate (wt % db) C H N S O HHV (MJ/kg db) sizes
no Ca
Ca impregnated
10.5 71.8 16.7 1.0
14.8 64.9 15.8 4.5
52.97 6.51 2.80 0.05 36.62 21 980
50.17 6.07 2.78 0.08 35.78 20 457
