Thermal Degradation of Alkaline Black Liquor from Straw

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Ind. Eng. Chem. Res. 2002, 41, 4714-4721

Thermal Degradation of Alkaline Black Liquor from Straw. Thermogravimetric Study Gloria Gea, Marıá B. Murillo, and Jesu ´ s Arauzo* Department of Chemical and Environmental Engineering, University of Zaragoza, Marıá de Luna, 3, 50015 Zaragoza, Spain

Both pyrolysis and gasification can be considered as alternatives to conventional boilers for recovering chemicals and energy from black liquor. The present work is focused on the study of pyrolysis of alkaline black liquor from straw, and its interest is enhanced by the fact that the material used has hardly been studied before. The influence has been studied of the final pyrolysis temperature (500-900 °C), the heating rate (5-30 °C/min), and the addition of a determined CO concentration in the N2 atmosphere (5-40% vol) on both the final solid conversion and the devolatilization rate. The results show that the thermal decomposition of the organic matter fraction of black liquor takes place at temperatures below 550 °C in N2 atmosphere. The weight loss observed at temperatures higher than 550 °C is mainly due to reduction reactions of alkaline compounds by the carbon. The final solid conversion and the devolatilization rate are also noticeably influenced by the addition of a certain CO flow rate in N2 atmosphere. Introduction Alkaline black liquor is a biomass waste resulting from digestion with NaOH of wood, straw or other fibrous plants in the papermaking industry. The main components of black liquor are organic and inorganic compounds deriving from the raw material used in the papermaking process and water. Black liquor can also be considered as a byproduct because approximately 35% of the total energy requirement in the pulp and paper industry comes from black liquor combustion.1 Nowadays, black liquor is concentrated and burned in recovery boilers to generate energy and to recuperate inorganic chemicals required in the papermaking process. However, recovery boilers present important problems from safety and environmental points of view. Therefore, new alternatives are being widely investigated in order to find more energy efficient, safer, and easier to control processes than the conventional ones. Both pyrolysis (or thermal devolatilization) and gasification are potential processes for using black liquor as a source to produce gaseous products for use as a fuel. Some very interesting works can be found on the reactivity of kraft black liquor from wood under burning and gasification conditions.2,3 Similarly, studies on the behavior of these black liquors under pyrolysis conditions have been reported.4 Black liquor gasification is widely viewed as the technology most likely to replace the recovery boiler.5 The main emphasis during recent years has been on the development of combined-cycle co-generation technology for pulp mills.6 If pressurized and coupled with gas turbines, gasification processes can provide more efficient utilization of the black liquor fuel value and produce more electrical power relative to steam.7 Black liquor gasification processes can be classified in two categories. One is low-temperature gasification, where the gasifier operates below the melting point of the inorganic compounds of the liquor (700-750 °C). The * Corresponding author. Telephone +34-976-761878. Fax +34-976-761879. E-mail: [email protected].

other category includes those processes that operate above the melting point, producing a molten product. Some studies are being conducted to check the possibility of using fluid bed processes for low-temperature gasification.7,8 Pyrolysis or thermal devolatilization is not only important as a thermochemical process itself, but also as the previous stage in gasification processes. Gasification would be more energy efficient if more of the carbon in black liquor were converted to combustible gases instead of char during pyrolysis.9 Thus, a good understanding of the behavior of black liquor during the pyrolysis or devolatilization process is required for the design and development of new alternative combustors and gasifiers. In the literature, some previous works about the pyrolysis of kraft black liquor from wood have determined the composition of the gas resulting from the process as a function of time and temperature.1,10 Thermogravimetric studies have also been carried out to evaluate the behavior of the organic components of kraft black liquor from wood during their devolatilization.4 More recently, rapid pyrolysis of black liquor from wood at high temperature and high pressures using pressurized reactors has been evaluated.6,11 However, limited information is available on the devolatilization of alkaline black liquor from straw. Sańchez has studied the devolatilization of this black liquor in different atmospheres (N2 and air) and heating rates (5-100 °C/ min) at temperatures below 600 °C.12 Pue´rtolas et al. have evaluated the influence of a previous oxidation stage at low temperature on the thermoplastic and swelling properties of straw black liquor.13 Excepting these works, there are relatively few data on alkaline black liquor from straw. Most of the available data deal with kraft black liquor from wood. It is known that different liquors can behave very differently in the same recovery boiler under the same operation conditions,14 and in consequence, each type of black liquor requires a specific study to evaluate its pyrolysis behavior and char reactivity in order to determine the optimal conditions for it to be burned or

10.1021/ie020283z CCC: $22.00 © 2002 American Chemical Society Published on Web 08/24/2002

Ind. Eng. Chem. Res., Vol. 41, No. 19, 2002 4715 Table 1. Ultimate and Proximate Analyses (Dry Basis) for Two Types of Black Liquors (wt %) elements

alkaline black liquor from straw

kraft black liquor from wood

C H N S Cl K Na Si others

39.05 4.54 1.00 0.78 3.50 4.10 8.83 0.23 37.97

36.7 3.3 0.11 5.4 0.4 0.9 18.3 100 min) increases with the final pyrolysis temperature. Influence of the Heating Rate. Experiments with different heating rates (5, 15, and 30 °C/min) and the same final pyrolysis temperature (Tp ) 800 °C) were carried out to evaluate the effect of the heating rate on the final solid conversion and the devolatilization rate. Figure 7 shows the data of solid conversion vs time for the different β studied. Frederick et al. have observed higher final solid conversion at higher heating rates for black liquor from pulping of wood, due to a higher production of volatiles.24 However, the black liquor used in this work does not show the same trend, as can be seen in Figure 7. In fact, the trend is not clear, as the final solid conversion, As, obtained at 30 °C/min is higher than 5 °C/min but lower than 15 °C/min. These deviations might be attributed to differences in the initial composition of the dried black liquor solids as a consequence of a nonhomogeneous drying process, which could generate samples of solids with more organic, and less inorganic, matter

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Figure 7. Solid conversion vs time at different heating rate. Tp ) 800 °C.

Figure 8. Devolatilization rate vs temperature at different heating rate. Tp ) 800 °C.

than others, resulting in different final solid conversions.25 Therefore, the drying method could have noticeable effect on the pyrolysis process, both in the final solid conversion and in the devolatilization rate. With regard to the devolatilization rate, Figure 8 shows the values obtained vs temperature at the three heating rates studied. It can be appreciated that the devolatilization rate increases with the heating rate, both for the thermal decomposition of organic and inorganic matter. The maximum value of the devolatilization rate for the organic and the inorganic matter increases by 3.6 and 6.8 times respectively when the heating rate increases from 5 to 30 °C/min at 800 °C as the final pyrolysis temperature. Therefore, black liquor needs lower residence times in a reactor with high heating rates, such as fluidized bed reactors, to be transformed into gaseous products. The temperature for reaching the maximum devolatilization rate of the organic matter fraction (270-300 °C) seems to be fairly independent of the heating rate in the studied range (5-30 °C/min). Influence of the Addition of CO in the N2 Atmosphere. As has been shown previously, CO is the major gaseous product during the devolatilization of the inorganic matter of black liquor, which takes place at temperatures higher than 550 °C (eqs 4-6). These reduction reactions are partially inhibited by the addition of a certain CO concentration in the inert carrier gas, affecting the final solid conversion, the devolatilization rate, and probably the resulting char structure. Taking into account that there is always a CO presence

Figure 9. Solid conversion vs time in atmospheres containing different CO concentration. β ) 5 °C/min; Tp ) 700 °C.


inside any combustor or gasifier, it is important to evaluate its effects on the final solid conversion and the devolatilization rate (which determines the thermal degradation kinetic) and the resulting char structure (which noticeably influences further processes, such as gasification). In the present work, the effect of the addition of CO to the carrier gas on the final solid conversion and devolatilization rate is shown. As in previous works,3,26 CO is added to the N2 stream at 500 °C, just before the thermal degradation of the inorganic matter starts. Figures 9-11 show the solid conversion data obtained in experiments with different CO concentrations in the carrier gas at different final pyrolysis temperatures: 700, 800, and 850 °C, respectively. Table 3 presents the final solid conversion values obtained for the different runs. The addition of CO to N2 atmosphere results in a lower final solid conversion at the different final pyrolysis temperatures studied (Figures 9 and 10). CO can react to produce carbon and CO2 when the CO/CO2 ratio is high relative to its temperature-dependent equilibrium. Whitty observed some carbon deposits on the sample surface and sample holder during kraft black liquor pyrolysis experiments performed at elevated pressures in a thermobalance.27,28 It may be possible that carbon production explains the reduction on conversion with the CO addition to the atmosphere, but the experiments of the present work were performed at atmospheric pressure and carbon deposits were not

Ind. Eng. Chem. Res., Vol. 41, No. 19, 2002 4719


Figure 12. DSC plot of dried black liquor in N2 atmosphere. Tp ) 900 °C.

Figure 13. Devolatilization rate vs time in atmospheres containing different CO concentration. β ) 5 °C/min; Tp ) 700 °C.


Table 3. Final Solid Conversion, As, Obtained at Different Final Pyrolysis Temperatures and Several CO Concentrations under Inert Atmosphere solid conversion Tp (°C) 700 800 850

[CO] ) [CO] ) [CO] ) [CO] ) [CO] ) [CO] ) 0% vol 5% vol 10% vol 17% vol 30% vol 40% vol 0.58 0.71 0.71

0.46 0.67 0.64

0.50

0.38 0.51

detected. Another assumption to explain the decrease in the final solid conversion could be the partial inhibition of the reduction of alkaline compounds (reactions 4 and 6) by CO, which would provoke a lower weight loss during the thermal degradation of the inorganic matter. However, control of the reduction of the alkaline compounds by adding CO to the atmosphere becomes more difficult at higher temperatures due to the endothermic character of these reactions. Figure 12 shows the DSC curve for 20 mg of the dried black liquor under study in a N2 atmosphere with a heating rate of 20 °C/ min. The analysis was performed in a standard apparatus (SETARAM TG-DTA 92) in the Carboquı´mica Institute (CSIC, Zaragoza, Spain). This Figure serves to illustrate that the devolatilization of the organic matter fraction (T < 550 °C) is mainly exothermic while the decomposition of the inorganic matter fraction (T > 600 °C) is an endothermic process. The data listed in Table 3 shows that, in the presence of CO, the final solid conversion obtained at 800 °C is


25% lower than that at 850 °C within the same CO concentration. This fact indicates that achieving a certain control of the alkaline compound decomposition requires a higher level of CO at higher final pyrolysis temperatures. For example, As decreases by 20% when the CO concentration is increased from 0 to 5% vol at 700 °C. However, it only decreases by 5.6% at 800 °C. When the CO concentration is increased from 0 to 40% vol, As decreases by 46.5% at 800 °C and only 39% at 850 °C. Figures 13-15 show the devolatilization rate of the black liquor vs time for the experiments with different

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Table 4. Maximum Values of Devolatilization Rate for the Inorganic Matter Fraction Obtained at Different CO Concentrations rate [CO] ) [CO] ) [CO] ) [CO] ) [CO] ) [CO] ) Tp (°C) 0% vol 5% vol 10% vol 17% vol 30% vol 40% vol 700 800 850

0.0038 0.0079 0.0095

0.0022 0.0044 0.0067

0.0036

0.0011 0.0027

CO concentrations at final pyrolysis temperatures of 700, 800, and 850 °C, respectively. In general, the devolatilization rate of the inorganic matter fraction (T > 550 °C) decreases with the CO concentration for all the temperatures studied. Table 4 shows the maximum values of the devolatilization rate for the inorganic matter fraction obtained in the presence of different CO concentrations. By comparison of these data, it can be calculated that the maximum decreases by 50% when the CO concentration goes up from 0 to 5% vol at 700 and 800 °C. However, by comparison of the runs carried out at 800 and 850 °C, it can be appreciated that the maximum devolatilization rate decreases by 84% at 800 °C and only 68% at 850 °C when the CO increases from 0 to 40% vol. This observation confirms that a higher CO concentration is required at temperatures higher than 800 °C in order to inhibit the alkaline compound devolatilization to some extent. Conclusions The thermogravimetric study performed in the present work provides quantitative information about the thermal devolatilization process of black liquor from alkaline pulping of straw, required in order to determine the thermal devolatilization kinetics. The main conclusions obtained from the evaluation of the influence of the final pyrolysis temperature, the heating rate and CO addition to the N2 atmosphere on the final solid conversion and devolatilization rate can be summarized as follows: • The thermal decomposition of the organic matter fraction of black liquor takes place at temperatures below 550 °C in N2 atmosphere. The weight loss which black liquor suffers at temperatures above 550 °C in N2 atmosphere is due to the devolatilization of alkaline (Na and K) compounds, which mainly come from the inorganic matter fraction of the black liquor. This devolatilization process consists of reduction reactions of the alkaline compounds with the carbon remaining in the char. The final solid conversion reached for dried black liquor in N2 atmosphere increases by 56% when the final pyrolysis temperature goes up from 550 to 600 °C due to the commencement of devolatilization of the black liquor inorganic fraction. • The maximum devolatilization rate obtained is higher for organic than inorganic matter for all the final temperatures studied. The devolatilization rate of the inorganic matter fraction increases with the final pyrolysis temperature. The devolatilization rate of both organic and inorganic matter increases with the heating rate. • The addition of CO to the N2 atmosphere results in a lower final solid conversion. The reduction of the alkaline compounds with carbon is partially inhibited by CO. Therefore, the amounts of both carbon and alkaline metals in the resulting char after the total devolatilization will be higher for devolatilization in

inert atmospheres containing CO. The CO concentration required to control the inorganic matter devolatilization strongly depends on the final pyrolysis temperature. Because of the endothermic character of such reduction reactions, a higher CO concentration is required at higher final pyrolysis temperatures (especially at temperatures higher than 800 °C) in order to reach a certain degree of inhibition of these reactions. • The devolatilization rate of the inorganic matter fraction decreases when the CO concentration increases. Acknowledgment The authors express their gratitude to the CICYT (Project AMB 95-0575) for providing frame support for this work, and to MEC for research grants awarded to G.G. Nomenclature [CO]: concentration of CO in N2 atmosphere, % vol As: final solid conversion, solid conversion at the end of the experiment r: devolatilization rate, min-1 T: temperature at a specific point in time, °C t: time, min Tp: final pyrolysis temperature, °C W∞: black liquor solid weight at the end of the experiment, mg W: black liquor solid weight at a specific point in time, mg W0: initial black liquor solid weight, mg Xs: black liquor solid conversion β: heating rate, °C/min

Literature Cited (1) Bhattacharya, P. K.; Shrinath, A. S.; Kunzru, D. Pyrolysis of Black Liquors. J. Chem. Tech. Biotechnol. 1985, 35, 223. (2) Noopila, T.; Aleń, R.; Hupa, M. Combustion Properties of Laboratory-Made Black Liquors. J. Pulp Paper Sci. 1991, 17 (4), J105. (3) Frederick, W. J.; Wag, K.; Hupa, M. Rate and Mechanism of Black Liquor Char Gasification with CO2 at Elevated Pressures. Ind. Eng. Chem. Res. 1993, 32, 1747. (4) Aleń, R.; Rytko¨nen, S.; McKeough, P. Thermogravimetric Behaviour of Black Liquors and their Organic Constituents. J. Anal. Appl. Pyrolysis 1995, 31, 1. (5) Larson. E. D.; Raymond, D. R. Commercializing Black Liquor and Biomass Gasifier/Gas Turbine Technology. Tappi J. 1997, 80 (12), 50. (6) Heginuz, E.; Sjo¨stro¨m, K.; Sorvari, V.; Backman, R. Pyrolysis of Black Liquor in a Pressurized Free Fall Reactor and in a Pressurized Grid Heater. Presented at the International Chemical Recovery Conference, Tampa, FL, June 1998; no. 1129. (7) Verrill, C. L.; Dickinson, J. A. Development and Evaluation of a Low-Temperature Gasification Process for Chemical Recovery from Kraft Black Liquor. Presented at the International Chemical Recovery Conference, Tampa, FL, June 1998; no. 1067. (8) Gea. G.; Murillo, M. B.; Sańchez, J. L.; Bilbao, R.; Arauzo, J. Straw Black Liquor Gasification Studies at the University of Zaragoza. Presented at the International Chemical Recovery Conference, Whistler, Canada, June 2001, no. 389. (9) Koepke, S. A.; Zhu, J. Y. Pyrolysis of Black Liquor in a High -Intensity Acoustic Field. Presented at the International Chemical Recovery Conference, Tampa, FL, June 1998; no. 971. (10) Sricharoenchaikul, V.; Phimolmas, V.; Frederick, W. J.; Grace, T. M. Pyrolysis of Kraft Black Liquor: Formation and Thermal Conversion of Volatile Products and Char. J. Pulp Paper Sci. 1998, 24 (2), J43. (11) Whitty, K.; Wilsson, A. Experience from a High Temperature, Pressurised Black Liquor Gasification Pilot Plant. Presented at the International Chemical Recovery Conference, Whistler, Canada, June 2001, no. 281.

Ind. Eng. Chem. Res., Vol. 41, No. 19, 2002 4721 (12) Sańchez, J. L. Thermochemical Processing at low temperature of Black Liquors from Alkaline Pulping of Straw. Ph.D. Dissertation, University of Zaragoza, Zaragoza, Spain, 1999. (13) Pue´rtolas, R.; Gea, G.; Murillo, M. B.; Arauzo, J. Pyrolysis of Black Liquors from Alkaline Pulping of Straw. Influence of a Preoxidation stage on the Char Characteristics. J. Anal. Appl. Pyrolysis 2001, 58-59, 955. (14) Whitty, K.; Backman, R.; Forsseń, M.; Hupa, M.; Rainio, J.; Sorvari, V. Liquor-to-Liquor Differences in Combustion and Gasification Processes: Pyrolysis Behaviour and Char Reactivity. J. Pulp Paper Sci. 1997, 23 (3), J119. (15) Frederick, W. J.; Noopila, T.; Hupa, M. Swelling of Spent Pulping Liquor Droplets During Combustion. J. Pulp Paper Sci. 1991, 17 (5), J164. (16) Frederick, W. J. Black Liquors Properties. In Kraft Recovery Boilers; Adams, T. N., Ed.; Tappi Press: Atlanta, GA, 1997. (17) Gea, G.; Murillo, M. B.; Arauzo, J. Swelling Behaviour of Black Liquors from Alkaline Pulping of Straw. Presented at the Second Olle Lindstro¨m Symposium on Renewable Energy-Bioenergy, Stockholm, Sweden, June 1999; no. 161. (18) Aleń, R.; Siistonen, H. Relationships Between the Chemical Composition and the Combustion Properties of Black Liquor. Presented at the International Chemical Recovery Conference, Tampa, FL, June 1998; no. 185. (19) Millera, A. Influence of Physical and Chemical Characteristics of Lignocellulosic Materials on their Thermochemical Use. Ph.D. Dissertation, University of Zaragoza, Zaragoza, Spain, 1988. (20) Wa¨g, K. Characterization and Modelling of Black Liquors Char Combustion Processes. Ph.D. Dissertation, Oregon State University, Corvallis, OR, 1996. (21) Grace, T. M.; Frederick, J. W.; Iisa, K.; Wag, K. New Black Liquor Drop Burning Model. Presented at the International Chemical Recovery Conference, Tampa, FL, June 1998; no. 257.

(22) Gea, G.; Pue´rtolas, R.; Murillo, M. B.; Arauzo, J. Pyrolysis and Gasification of Black Liquors from Alkaline Pulping of Straw in a Fixed Bed Reactor. In Progress in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackwell Science: Oxford, U.K., 1999. (23) Gea, G. Thermochemical Behaviour of Alkaline Black Liquor from Straw. Ph.D. Dissertation, University of Zaragoza, Zaragoza, Spain, 2000. (24) Frederick, W. J.; Hupa, M. The effects of Temperature and Gas Composition on Swelling of Black Liquor Droplets during Devolatilization. J. Pulp Paper Sci. 1994, 20 (10), J274. (25) Li, J.; Van Heinigen, A. R. P. Kinetics of CO2 Gasification of Fast Pyrolysis Black Liquor Char. Ind. Eng. Chem. Res. 1990, 29, 1776. (26) Li, J.; Van Heinigen, A. R. P. Reaction Kinetics of Gasification of Black Liquor Char. The Can. J. Chem. Eng. 1989, 67. (27) Whitty, K.; Backman, R.; Hupa, M. Influence of Char Formation Condition on Pressurised Black Liquor Gasification Rates. Carbon 1998, 36, 1683. (28) Whitty, K.; Backman, R.; Hupa, M. Influence of Pressure on Swelling, Char Morphology and Char Yields During Black Liquors Pyrolysis. Presented at the International Chemical Recovery Conference, Tampa, FL, June 1998; no. 733.

Received for review April 15, 2002 Revised manuscript received July 11, 2002 Accepted July 17, 2002 IE020283Z

Thermal Degradation of Alkaline Black Liquor from Straw

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