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Production of Calcium Carbide


Communications
Biomass
DOI: 10.1002/anie.201004169

Production of Calcium Carbide from Fine Biochars**
Guodong Li, Qingya Liu, Zhenyu Liu,* Z. Conrad Zhang,* Chengyue Li, and Weize Wu
Carbon is the most abundant source of energy and chemicals on the earth. Biomass produced from photon-activated conversion of atmospheric CO2, and biomass fossils such as coal and petroleum are all carbon-rich sources. In around only one century of heavy industrial use of petroleum, this hydrocarbon source has already depleted to a point of a widespread concern over its scarcity in the decades to follow.[1] Biocarbon, also known as biochar, can be readily produced from a vast sustainable supply of lignocellulosic biomass through pyrolysis.[2] It is often in fine form and characterized by low mechanical strength and high activity in comparison to coal-derived chars. The ability to use biochar for the production of chemicals with high energy efficiency will largely alleviate our dependence on shrinking petroleum feedstock. Herein, we show reaction of fine biochars with fine CaO for the production of CaC2, an important starting material for production of many commodity chemicals. The process offers the potential to redirect the carbon conversion pathway. CaC2 is produced by the reaction 3 C + CaO + E1! CaC2 + CO, where E1 is the energy required for the process, about 445.6 kJ mol?1 at above 2000 8C.[3] CaC2 can be readily converted into acetylene by treatment with water: CaC2 + 2H2O!C2H2 + Ca(OH)2. Acetylene is an oxygen-free platform chemical for production of chemicals, for example, polyvinylchloride (PVC), vinyl acetate, and 1,4-butanediol. In this carbon conversion process, the main products CaC2 and then C2H2 are readily separated from other components. The current CaC2 production technology dates back to 1892 and has not changed much since then.[4, 5] It uses an electric arc furnace, which is limited only to small-scale
[*] G. Li, Q. Liu, Prof. Dr. Z. Liu, C. Li, W. Wu State Key Laboratory of Chemical Resource Engineering Beijing University of Chemical Technology Beijing, 100029 (PR China) Fax: (+ 86) 10-6442-1073 E-mail: liuzy@mail.buct.edu.cn Dr. Z. C. Zhang KiOR Inc., 13001 Bay Park Road, Pasadena, TX 77507 (USA) Fax: (+ 1) 713-299-4165 E-mail: conrad.zhang@kior.com zczhang@yahoo.com [**] We thank the Nature Science Foundation of China (20821004 and 20976011) for financial support. Z.L. conceived the research and developed the interpretation of the results. Z.C.Z. proposed and designed some of the experiments and structure of the paper. G.L. performed all the experiments and Aspen Plus simulation. Q.L. designed some of the experiments and contributed to the structure of the paper. C.L. proposed interpretation of the solid-solid reaction. W.W. contributed to calculation of thermal efficiency. All the authors discussed the results and commented on the manuscript. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201004169. Figure 1. TG/DTG curves and MS signal for release of CO from the reaction of CaO and a pine wood char of size 0.022 mm at a C/CaO molar ratio of 3.6.

operations, typically less than 40 kt CaC2 per year. This process requires granular char and CaO of 5–30 mm in size and with sufficient mechanical strength, such as coal char, to allow unrestricted release of byproduct CO. Because of the low reaction rate resulting from the low surface area and poor contact between the large feed particles, high temperatures (about 2200 8C) and long reaction times (1–2 h) are usually required.[6] These constraints inevitably result in high energy consumption (4000 kW h tCaC2?1), high production cost, and high CO2 emissions in electricity generation.[7] Autothermal heating by combustion of chars has been studied as an alternative process for CaC2 preparation.[8–10] However, the use of large feed particles, as with the electric arc process, requires long production times and high reaction temperatures to obtain sufficient yields; therefore, the process is not economically attractive for commercial development. We report a considerably reduced temperature (by 500 8C), shortened reaction time, and increased thermal efficiency in converting fine biochar into CaC2. More importantly an autothermal reactor similar to the modern high throughput gasification reactors could be used. Figure 1

shows thermogravimetric/differential thermogravimetric (TG/DTG) data and mass spectra for released CO for the reaction of pine wood char and CaO at a nominal particle size of 0.022 mm with a C/CaO molar ratio of 3.6. The total mass loss at temperatures higher than 1510 8C corresponds to a CaC2 yield of 99 % based on CaO. XRD analysis (Figure S1 in the Supporting Information) confirms CaC2 as the main product. Gas chromatograph analysis of the C2H2 released from CaC2 hydrolysis shows a CaC2 yield of 97 %.
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Figure 2 shows initial and DTG peak temperatures of the reaction using chars of different particle sizes derived from pine wood and bamboo. The initial temperature, defined in Figure 1 as the temperature when DTG signal reaches the reactants and CaC2 except for the release of CO. The high CaC2 yield indicates constant mass transfer between the chars and CaO at the conditions used. Since the melting points of char and CaO are much higher (3625 8C for graphite and 2580 8C for CaO), the mass transfer may be attributed to nanocrystal migration. The temperatures in Figure 2 are all higher than the Tammann temperature of CaO, one half of its melting point in Kelvin (1154 8C), and above which nanocrystal migration becomes significant.[11] This hypothesis is supported by the experiments shown in Figure 4. Figure 4 a,b

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Figure 2. The initial and DTG peak temperatures of CaC2 formation through CaO and biochars (from pine wood and bamboo) of different particle sizes (dp).

?0.5 mg min?1 (corresponding to a CaO conversion rate of about 1.2 % min?1), decreases from about 1700 8C for a size of 0.675 mm to 1510 8C for a size of 0.022 mm; correspondingly, the peak temperature decreases from about 1850 to 1710 8C. Figure 3 shows mass-loss rates (DTG) at 1650 8C for reactions of CaO with various biochars as well as other carbon materials. Clearly the biochars have higher d002 values (from XRD) and are more active than the coal chars, petroleum coke, and graphite. The results indicate that CaC2 can be produced at temperatures much lower than that required by the current electric arc process. The process involves little vaporization of

Figure 4. Photos of fine reactants of size 0.022 mm and the products obtained after heating to 2000 8C at 20 8C min?1. Top: at a C/CaO molar ratio of 3.5, a) before and b) after the reaction; bottom: at a C/ CaO ratio of 1.8 c) before and d) after the reaction.

Figure 3. Conversion rates of CaO (R) at 1650 8C for reaction of CaO with chars. The d002 values are the interplanar distances from XRD analysis of the chars treated at 1650 8C. ~: biochars (from apricot shell, willow wood, bamboo, corncob, and pine wood, in the order of increasing d002); *: coal chars (from anthracite, bituminous coal, and lignite, in the order of increasing d002); &: petroleum coke; ^: graphite.
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shows the appearance of a sample before and after the reaction when a C/CaO mixture with a molar ratio of 3.5 was heated to 2000 8C at a rate of 20 8C min?1. The carbidization between 1510 and 1900 8C yielded a final mixture containing 86.5 % CaC2 and 13.5 % char (Figure S2 in the Supporting Information). The product (Figure 4 b) is loose with dense particles. When the C/CaO molar ratio (1.8) was below the reaction stoichiometry (Figure 4 c,d), the carbidization temperature was unchanged but additional mass loss after the full conversion of the char at 1828 8C was observed (Figure S3 in the Supporting Information). Figure 4 d indicates the formation of eutectics consisting of CaC2 and CaO. When a larger particle size (e.g. 5 mm in diameter) is used (Figure 5), the initial temperature is shifted to 2020 8C and the DTG peak temperature to 2090 8C (Figure S4 in the Supporting Information). The core (char) and shell (CaC2) structure in the products indicate migration of CaO into the char during the process. Contacting the core–shell structure (Figure 5 b) with H2O transformed the CaC2 shell into loose Ca(OH)2 that peels off from the core, yielding a CaO conversion of 77.8 % to CaC2 and a mass loss of 4.8 % in addition to the release of CO. The visual spillage on the mouth of the crucible indicates formation of foaming slurry during the experiment and explains the additional weight loss at temperatures higher than 2128 8C (Figure S4 in the Supporting Information). The formation of a eutectic mixture by CaO and CaC2 under the www.angewandte.org

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Figure 5. Appearance of the reactants and the product. a) Two CaO and two char granules of size 5 mm at a C/CaO molar ratio of 3.15 in a TG crucible; b) after heating at a rate of 20 8C min?1 to 2150 8C in Ar; c) cut product particles (top: black char core and gray CaC2 shell; bottom: black char core and white Ca(OH)2 shell, upon contact with moisture). Figure 7. Conversion rate of CaO (R) at different particle sizes (dp) at a pine wood char/CaO molar ratio of 3.6.

experimental conditions agrees with published results, which record the lowest eutectic temperature of 1650 8C for a CaO/ CaC2 molar ratio of 0.62.[12] The experimental observations suggest that CaC2 production at temperatures between 1154 8C (the Tammann temperature of CaO) and 2580 8C (the melting point of CaO) is controlled initially by CaO nanocrystal migration. Eutectic mixtures of different compositions may be formed at the interface of CaO and CaC2 when temperature is higher than 1650 8C. The eutectic phase accelerates CaO migration towards the char. This mechanism is schematically shown in Figure 6.

Figure 6. Reaction process of CaO and C for the production of CaC2.

Figure 8. Carbon yields to CaC2 (C % CaC2), CO (C % CO), and CO2 (C % CO2), and thermal efficiency (h) of autothermal and electric arc processes determined by Aspen Plus simulation. The autothermal process includes preheating by the high-temperature side-product CO. *: C % CaC2, &: C % CO, !: C % CO2, ~: h.

Clearly CaC2 production is controlled by mass transfer of CaO toward the char phase. A smaller particle size results in a greater contact surface area and a reduced mass transfer barrier for the reaction; thus, the reaction rate is enhanced even at lower temperature (Figure 7). From the data in Figure 7, a reaction time of 4.6 min for 95 % conversion of CaO at 1750 8C was determined when the particle size was 0.022 mm (assuming a first-order reaction to CaO; see the Supporting Information). The dramatic decrease in temperature and reaction time, compared to 1–2 h at about 2200 8C for the electric arc process, point to a tremendous potential for energy saving. This result also implies that CaC2 production can be carried out in autothermal mode, since the temperature becomes comparable to many autothermal processes such as pulverized coal gasification (1400– 2000 8C) and the BASF process of oxidative dehydrogenation of methane for acetylene production (1540 8C).[6, 13, 14] Computer simulation using Aspen Plus (Figure 8) confirms that autothermal CaC2 production is achievable by oxidizing a fraction of char to CO to meet the energy requirement of the reaction, and the process is of higher

thermal efficiency than the electric arc process especially when the reactants are preheated by the high-temperature CO generated from the CaC2 production (see the Supporting Information). In summary, fine biochars as feed for CaC2 production make the autothermal process feasible at a temperature about 500 8C lower than that of the current electric arc process. The reaction time is also reduced from 1–2 h to less than 5 min. The reaction mechanism may involve eutectic mixtures at the CaO/char interphase. Coupling this process with biomass pyrolysis forms a new route for biomass conversion, which is extendable to coal-based processes.

Experimental Section
A thermoanalyzer from Setaram (Setsys Evolution 24) and a mass spectrometer from Balzers (Omnistar 200) were used. CaO had a purity of 98.3 %. The carbon sources include five biochars (derived from apricot shell, willow wood, bamboo, pine wood, and corncob), three coal chars (derived from anthracite, bituminous coal, and lignite), petroleum coke, and graphite. The preparation procedure and conditions of all the chars include pyrolysis in N2 at 900 8C for 3 h, acid washing with a HF + HCl solution at 60 8C for 24 h, and drying at
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120 8C for 5 h. Proximate and ultimate compositions determined for all the carbon sources are shown in Table S1 in the Supporting Information. Blank TG-MS experiments (with empty crucible) were made to account for the effect of buoyancy caused by the temperature rise. Received: July 8, 2010 Published online: September 10, 2010 [4] J. Lacson, U. Lochner, G. Toki, Chemical Economics Handbook, SRI International, Menlo Park, CA, 2001, pp. 1 – 48. [5] J. T. Morehead, G. De Chalmot, J. Am. Chem. Soc. 1896, 18, 311 – 331. [6] Acetylene 93S14. Chem. Systems 1996. [7] M. H. El-Naas, R. J. Munz, F. Ajersch, Plasma Chem. Plasma Process. 1998, 18, 409 – 427. [8] M. Fujishige, H. Yokogawa, S. Ujiie, M. Tokiya, Japan, 61– 178412, 1986. [9] S. C. Yue, China, 1843907A, 2005. [10] J. J. Mu, R. A. Hard, Ind. Eng. Chem. Res. 1987, 26, 2063 – 2069. [11] C. N. Satterfield, Heterogeneous catalysis in practice, McGrawHill Book Company, USA, 1980, pp. 86 – 94. [12] M. B. Müller, Scand. J. Metall. 1990, 19, 210 – 217. [13] A. G. Collot, Int. J. Coal Geol. 2006, 65, 191 – 212. [14] L. Zheng, E. Furinsky, Energy Convers. Manage. 2005, 46, 1767 – 1779.

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Keywords: biomass · calcium · carbides · nanostructures · sustainable chemistry

[1] A. P. C. Faaij, Energy Policy 2006, 34, 322 – 342. [2] J. Lehmann, Nature 2007, 447, 143 – 144. [3] C. S. Kim, R. F. Baddour, J. B. Howard, H. P. Meissner, Ind. Eng. Chem. Process Des. Dev. 1979, 18, 323 – 328.

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