46
Energy & Fuels 2003, 17, 46-53
Swelling Behavior of Black Liquor from Soda Pulping of Wheat Straw G. Gea, M. B. Murillo, and J. Arauzo*,† Chemical and Environmental Engineering Department, University of Zaragoza, Marı´a de Luna, 3, 50015 Zaragoza, Spain
W. J. Frederick*,‡ Department of Forest Products and Chemical Engineering, Chalmers University of Technology, SE-412, 96 Gothenburg, Sweden Received February 25, 2002
The primary aim of this research work was to characterize the swelling properties of black liquor and their dependence on the temperature and gas composition of the combustion or gasification environment. The measurements were made using an industrial black liquor from soda pulping of wheat straw. The swelling measurements were carried out in a single droplet reactor. This paper presents data that show the influence of the reactor temperature and the gas composition on the swelling during devolatilization, and compares the swelling characteristics of a straw black liquor with similar published data on black liquors from kraft pulping of wood. Experiments were carried out in inert, oxiding, and gasifying environments, at temperatures from 500 to 900 °C. One interesting result was that the straw liquor swelled much more than most kraft or soda liquors from pulping of wood. Another result was that droplets subjected to hot gas atmospheres containing O2, CO2 and/or H2O(v) swelled less than those subjected to hot N2 atmospheres. The data obtained suggest that the straw soda black liquor would burn more rapidly than most kraft liquors from pulping of wood because of its high external surface area when swollen. This highly swelling liquor caused problems with fluidization when straw liquors were injected in a laboratory-scale fluidized bed gasifier.
Introduction Black liquor is a byproduct in the production of pulp from wood and nonwood materials. It contains in addition to residues of inorganic cooking chemicals, various degradation products of feedstock materials dissolved during delignification.1 Thus, the organic solids are composed mainly of degraded lignin and polysacharides (hemicelluloses and cellulose) degradation products (aliphatic carboxylic acids), together with a minor fraction of extractives. Black liquor is an important fuel in pulp and paper manufacturing countries and currently, black liquor is burned in a Tomlinsontype recovery boilers to generate energy from the organics and to recover the cooking chemicals. Although this technology has been used almost exclusively in alkaline pulp mills for nearly 70 years, it has several weaknesses: relatively low energy efficiency and some safety and environmental problems. New alternative processes, based on gasification technology, are currently being developed. These processes could be more energy efficient, cheaper, safer, and easier to control than conventional recovery boilers. One of the most promising alternatives is gasification. A good understanding of the burning and gasifying behavior * To whom correspondence should be addressed. † E-mail:
[email protected]. Fax: 976-761879. ‡ E-mail:
[email protected]. (1) Ale´n, R. Bioresour. Technol. 1994, 49, 99-103.
of black liquor is important for developing these new processes. Black liquor, like coal or wood, burns in three stages:2 drying, devolatilization, and char combustion. One of the most important properties of black liquor is its tendency to swell when heated. The swelling characteristics during the three stages are very different. When the droplet enters the furnace, it begins almost immediately to dry and devolatilize simultaneously. The droplet swells considerably to a maximum volume that is reached at the end of devolatilization. In the char combustion stage, the char reacts with oxidizing gases (O2, CO2, and water vapor), and the diameter of droplet decreases until only an inorganic smelt remains. The degree of black liquor swelling can be an important variable for the efficient combustion of black liquor in a recovery furnace.3 Swelling plays an important role in rate of combustion of black liquors. Several authors have reported that high swelling liquors burn faster than low swelling liquors.3,4 Black liquor burning is controlled mainly by the rate of (2) Hupa, M.; Solin, P.; Hyo¨ty, P. J. Pulp Pap. Sci. 1987, 13 (2), J67-72. (3) Miller, P. T.; Clay, D. T.; lensky, W. I. W. In Proceedings from the 1996 Tappi Engineering Conference, Tappi Press: Atlanta, GA, 1996; pp 225-234. (4) Whitty, K.; Backman, R.; Hupa, M. In Proceedings from the 1998 International Chemical Recovery Conference, Tappi Press: Atlanta, GA, 1996; pp 733-745.
10.1021/ef0200469 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/21/2002
Swelling Behavior of Black Liquor
transport of oxidizing gases to the external surface of the char particle that remains after devolatilization. A reduction in the swelling decreases the specific external surface area available for the reaction of the char particle. This reduces the rate of transport of the oxidizing gases to the char. The swelling characteristics of liquor droplets and their rate of weight loss also determine their trajectories in a recovery boiler.5 These trajectories in turn have influence on the capacity of the boiler. Combustion stability is sometimes related to time-varying swelling behavior. Therefore, knowledge of swelling black liquors behavior is important in designing and optimizing the performance of combustors and gasifiers. Traditionally, two parameters have been used to quantify the degree of swelling of black liquors. Some authors used a swelling factor, defined as the ratio of the diameter of swollen particle to its initial diameter,2,6 and others used the maximum swollen volume (SVmax), defined as the ratio of the maximum volume of the particle to the initial mass of the dried droplet.3,4,7,8 Other researchers have investigated the swelling characteristics of black liquor. Miller et al.3 have studied the swelling of kraft black liquor during pyrolysis and attempted to determine the factors responsible of the swelling extension. Hupa et al.2 have reported the combustion behavior of kraft black liquor including the swelling properties. Frederick et al.6 have studied how droplet size, dry solids content, reactor temperature, and gas composition influence the kraft black liquor swelling. Whitty et al.7 have studied 15 different black liquors to investigate how differences in liquor type affected the swelling. It has been demonstrated that the original composition of black liquor influences on the swelling properties.1 However, although several studies of the swelling behavior of kraft black liquors from pulping of wood can be found in the literature there are only very few data on black liquors from pulping of straw. In this work, the purpose was to determine the influence of the reactor temperature and gas composition on the swelling of a straw soda black liquor, and to compare the results with similar data obtained from the literature on kraft black liquor from pulping of wood. Experimental Section The apparatus used to measure the swelling of the soda straw black liquor droplets was based on a single droplet reactor developed by Hupa et al.2 This reactor consisted of a constant temperature chamber in which single droplets were suspended, and through which a preheated gas mixture of preset composition flowed. Experiments were carried out in inert, oxiding, and gasifying environments at temperatures from 500 to 900 °C. The swelling droplet evolution was recorded on video. The experimental apparatus and procedure were essentially similar to those used by Verrill et al.9 (5) Frederick, W. J.; Hupa, M. In Proceedings from the 1990 European Conference on Industrial Furnaces and Boilers, Vilanova, Algare, Portugal. (6) Frederick W.; Noopila, T.; Hupa, M. J. Pulp Pap. Sci. 1991, 17 (5), 164-170. (7) Whitty, K.; Backman, R.; Forsse´n, M.; Hupa, M.; Rainio, J.; Sorvari, V. J. Pulp Pap. Sci. 1997, 23 (3). (8) Frederick, W. J.; Hupa, M. J. Pulp Pap. Sci. 1994, 20 (10), 274280. (9) Verrill, C. L.: Grace, T. M.; Nichols, K. M.J. Pulp Pap. Sci. 1994, 20 (12), 354-360.
Energy & Fuels, Vol. 17, No. 1, 2003 47 Table 1. Elemental Composition for the Straw Soda Black Liquor elements
amount (% of DS)
C H N S Na K Si Cl others
39.05 4.54 1.00 0.78 8.83 4.10 0.23 3.50 37.97
Table 2. Proximate Analysis for the Soda Straw Black Liquor parameter
amount (% of DS)
volatiles fixed carbon ash
66.0 13.4 20.6
Table 3. The Main Organic Constituents of Straw Soda Black Liquora
a
component
amount (% DS)
lignin aliphatic carboxylic acids hemicelluloses structural units glucose xylose mannose arabinose extractives
25.2 39.3 14.9 2.7 6.6 2.5 3.1
