Activated carbons from carbon dioxide partial gasification of


Citation data is made available by participants in Crossref's Cited-by Linking service. For a more comprehensive list of citations to this article, us...
0 downloads 0 Views 1MB Size


Energy & Fuels 1993,7, 133-138

133

Activated Carbons from CO2 Partial Gasification of Eucalyptus Kraft Lignin J. Rodrlguez-Mirasol, T. Cordero, and J. J. Rodrlguez' Department of Chemical Engineering, University of M&laga,29071 M&laga,Spain Received March 13, 1992. Revised Manuscript Received September 25, 1992

The behavior of eucalyptus kraft lignin as activated carbon precursor is investigated. The development of porosity upon C02 partial gasification at 1073 and 1123 K of low-temperature precarbonized kraft lignin has been studied. Increase of micropore volume and broadening of micropore size distribution takes place as activation proceeds. At intermediate burn-off, corresponding to an overall yield of about 25 % , a substantial development of mesoporosity is already observed. Macroporosity becomes importantat higher activation degrees. The development of apparent surface area compares favorably, in an overall yield basis, with the reported for lignocellulosic materials upon physical activation. In the case of kraft lignin, the evolution of porosity shows a different pattern with a higher contribution of meso and macroporosity. The presence of S in the activated carbons is negligible, and the final ash content is easily reduced up to below 2% by simply washing with dilute H&O4 aqueoussolution. The reactivity curves for C02 gasification of low-temperature precarbonized kraft lignin have been obtained at differenttemperatureswithin the 1023-1173 K range. The reactivity values are indicative of significant catalytic effect which can be attributed to the presence of finely distributed Na, the major metallic componentof the inorganic impurities. This effect becomes more relevant at high conversion values where a higher Na/C atomic ratio is reached. No saturation is observed at least up to X 0.95, most probably as the result of the aforementionedfine distribution of Na. Apparent activation energy values in the range of 212-239 kJ/mol have been obtained, the lower values corresponding to higher conversions.

-

Introduction Lignin is one of the three basic componentsof wood and other lignocellulosics. Lignin percentage in wood ranges commonly between 20and 30% . A modified type of lignin, kraft lignin, represents a major waste of the cellulosepulp industry. Duringthe kraftprocess, chemical breakage and solubilization of polymeric lignin is achieved giving rise to a waste black liquor which is commonly evaporated and calcinated to recover energy and chemical reactant. Isolation of different forms of modified lignins from pulping black liquors and the development of further applications for them has gained substantial interest in the last decade.'-6 A potential way of lignin conversion is pyrolysis and gasification to obtain activated carbons. Although there are some examples in the technical patent literature6J on the use of different types of lignins as activated carbon precursors, there is still an important lack of comprehensive information on the evolution of the porous structure of this raw material upon activation. Li and Van Heiningen819have studied the kinetics of CO2 gasification of chars obtained from slow and fast pyrolysis (1) Sjortrom, E.Wood Chemistry: Fundamentals and Applicationa; Academic Prees: New York, 1981; Chapter 10. (2) Pearl. J. A. Tappi 1982, 65(2), 159-163. (3) Deechamp, A. Inf. Chim. 1982,223,187-188. (4) Garcla-Her". F.: Martin-JimBnez,F.: RodrIrmez. - J. J. Ina. _ Bulm. _ 1984,249,187-192. (5) G h e r . W. G.. Sarkanen. E. S.. E&. Linnin: Pro~ertiesand Materiale; h e r i c a n Chemical Society: Washington, DC,i989. (6) Suzuki, H. Jap. Pat. 73-50989; 1973. (7) Ivmchenko, A. V.; Simkin,Y. Y.; Voropaev, Y. M. USSR Pat. 1987, 1,328,288. (8) Li, J.; Van Heiningen, A. R. P. Can. J. Chem. Eng. 1989,67,693697. (9)Li, J.; Van Heiningen,A. R. P. Ind. Eng. Chem. Res. 1990, 29, 1776-1785.

of kraft black liquors. Although related in origin, there are some significantdifferences compared to our substrate which affect the reactivity. On the other hand, these authors do not report any information on the evolution of the porous structure of the solid since the main objective of their studies focuses with the kinetics of the gasification process. In this paper we study the physical activation (partial C02 gasification) of a substrate obtained from lowtemperature precarbonization of eucalyptus kraft lignin. The reactivity of this substrate and the development of porosity upon activation are analyzed.

Experimental Section The eucalyptus kraft lignin used as raw material was supplied by the Empresa Nacional de Celulosas as obtained in ita pilot plant from acid precipitation of kraft black liquors. The precipitate was centrifuged and dried in a fluidized bed. The kraft lignin used in all the experimentscame from the m e pilot plant run, and no significant differences were observed in the analytical characteristicsof different samples. A typical annlysis as received is given in Table I. To avoid the problems derived from the partial fusion and important swelling observed when heating washed low-ash lignin even at very low heating rates, we submitted our raw material as received (- 12 5% ash) to a previous treatment'o consisting basicallyin predevolatilizationat low temperature (623K)under Nz atmosphere. This temperature was reached at 10 K/min and maintained for 2 h. The resulting precarbonized substrate was washed with 1 % H&OI aqueous solution to lower ita final ash content. The lignin to precarbonized Substrate yield was 60.3% on a dry ash free (daf) basis. (10) Rodriguez, J. J., et al. in Pyrolysis and Gosification; Ferrem, G. L., et al., Me;Ehvier Applied Science: London, 1989; pp 436-438.

0887-0624/93/2507-0133$04.00/0Q 1993 American Chemical Society

Rodrfguez-Mirasol et al.

134 Energy & Fueb, Vol. 7, No. 1, 1993

Table I. Analytical Characteristics of the Eucalyptus Kraft Lignin (Ai Reoeived) and the Low-Temperature Precarbcnized Substrate (As Prepared for Advation) eucalyptus precarbonized kraft lignin substrate proximate analysis (9% db) 35.2 64.2 fiied carbon volatile matter 52.4 33.7 12.4 2.1 ash content ultimate analysis (9% daf) 64.4 72.3 C 5.0 3.2 H

S 0 (by difference) apparent surface area (m2/g) BET (N2 77 K)

DR (Con 273 K)

1.2 29.4

0.3 24.2 23 336

micropore volume (cm3/g) N2 77

K (as)

C02 273 K (DR)

0.01 0.13

Table I summarizes the analyticaland structural characteristics of the final precarbonized substrate used in the reactivity studies and for activated carbons preparation. The substantial differences observed between the No and COzvaluee of apparent surface area and micropore volume are indicative of a narrow microporosity. The adsor9tion values obtained with N2 at 77 K most probably do not correspond to true equilibrium values since the aforementioned narrow microporosity gives to important diffusionalproblems. In fact, a very slow adsorption was observed at low relative pressures. The reactivity experiments were carried out in a CI Electronics thermogravimetricsystem consistingbasically of a 200mm length and 20 mm i.d. silica-glass tube fitted to an electrobalance and heated by a cylindrical electric resistance furnace with temperature controller and heating rate programmer. The temperature signal for the controller was measured by means of a chromelalumel thermocouple placed inside the reaction tube closer than 5 mm from the sample. The sample weight was continuously recorded in a microcomputer data acquisition system. In each run, the reaction tube was previously evacuated with N2 (99.998%) at room temperature and then heated under continuous flow of NOat a 50 K/min heating rate up to 50 K below the reaction temperature and at 10 K/min up to the final reaction temperature. The fed gaa waa switched to high purity (99.998%) oxygen-free C02 at 100 mL(STP)/min, and the gasification experiment was carried out isothermally. Approximately 10 mg of precarbonized substrate with a particle size of 45-53 pm (-270 + 325 mesh) was always used. The reactivity values, r, were obtained from r = - ( l / ~ dw/dt ) (1) where w represents the daf solid weight remaining at time t . All the conversion values are referred to the initial sample weight (daf) measured when the reaction temperature has been reached and the fed gas switched to CO2. The activated carbonswere obtained in a conventionalstainless steel cylindrical reactor (160mm length and 80 mm i.d.) heated by electric resistance at controlled temperature and heating rate. The temperature was measured with a chromel-alumel thermocouple inside the reactor and close to the sample holder. This consisted of a flat pan where samples of the precarbonized substrate, approximately 40 g, were placed. The reactor was evacuated with Ns at room temperature and heated at 50 K/min under continuous flow of 150 mL(STP)/min of high-purity oxygen-free C02 up to 50 K below the activation temperature and at 10K/min up to that temperature. Activation experimenta were performed a t 1073 and 1123 K using holding times ranging from 4 to 40 h in order to cover a wide range of burn-off or yield values and consequently of activation degrees. The abovedescribed experimental system is conceptWy similar to that used for previous investigators dealing with the preparation of activated carbons on a lab scale to study the evolution of the

nn

0.0

0.2

0.4

0.6

0.8

1.0

X

Figure 1. Reactivity versus conversion for COZ gasification of the low-temperature precarbonized kraft lignin. porous structure of a given precursor upon activation.ll-l3 The gasification rates also fall within a comparable range.lS-l6 To characterize the porous structure of the activated carbons we obtained the N2 adsorption-deeorption isotherms at 77 K and the CO2 adsorption isotherms at 273 K, both using a Carlo Erba Sorptomatic 1800instrumentwhich was operated manually. Equilibrium times longer than 2 h were used when needed as was the case at low relative pressures for low-activated carbons.The samples were outgassed at 548 K far 12 h to a residual pressure Torr. Mercury poroeimetry measurements were also accomplished using a Carlo Erba Poroeimeter 4OOO which allows to cover the pore range above 3.7 nm diameter. Scanning electron micrographs were obtained using a JEOL JSM-840instrument.

Results and Discussion Reactivity Study. Figure 1 shows the reactivity curves for COZgasification of the low-temperatureprecarbonized kraft lignin at different temperatures ranging from 1023 to 1173 I