Thermal Degradation of Alkaline Black Liquor from Wheat Straw. 2

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Ind. Eng. Chem. Res. 2003, 42, 5782-5790

Thermal Degradation of Alkaline Black Liquor from Wheat Straw. 2. Fixed-Bed Reactor Studies G. Gea, M. B. Murillo, J. L. Sa´ nchez, and J. Arauzo* Centro Polite´ cnico Superior, Chemical and Environmental Engineering Department, University of Zaragoza, Marıá de Luna 3, 50018 Zaragoza, Spain

The usual problems experienced with common recovery boilers are aggravated in processes involving alkaline black liquor derived from straw. The consequent need to develop alternative processes for the use of black liquor for energy purposes, such as pyrolysis or gasification, requires a good understanding of the thermochemical behavior of this substance. There is, however, little information available. Part 1 of this work (Gea, G.; Murillo, M. B.; Arauzo, J. Ind. Eng. Chem. Res. 2002, 41, 4714), therefore, studied the thermal degradation of alkaline black liquor from straw in a thermogravimetric system. The present work also focuses on its pyrolysis. In particular, the influence has been analyzed of the final pyrolysis temperature (250-900 °C) and the heating rate (5-30 °C/min) on the product yields, gas composition, and specific surface area of the resulting char in a fixed-bed reactor. The results obtained show that both the energy recovery and the specific surface area of the char increase with a rise in the final pyrolysis temperature and the heating rate. Introduction Black liquor is an industrial lignocellulosic waste generated in pulp and paper processes, as well as a biomass fuel with high ash content. Approximately 2 × 108 tonnes of black liquor solids, with a fuel value of 2.4 × 1015 J, are generated annually worldwide.2 Some 90% of black liquor from straw production currently takes place in China and India. For more than 60 years, black liquor has been burned in recovery boilers to make use of its energetic value and recover the cooking chemicals required in pulp and paper production. However, although it is an important industrial fuel, there are fundamental aspects of the pyrolysis, gasification, and combustion of black liquor that are not well understood. Recovery boilers used to burn black liquor, such as the Tomlinson type, suffer from several weaknesses, including relatively low energy efficiency and various safety and operational problems. This situation has resulted in the search and development of alternative processes, such as pyrolysis and gasification, that are more energy efficient, as well as safer and easier to control. The problems of most recovery boilers are aggravated with the use of alkaline black liquor deriving from straw rather than wood because it contains more silicon and potassium due to the composition of the straw.1 The higher silicon content provokes more deposits in the boilers, decreasing their efficiency, while the higher level of potassium reduces the melting point of the black liquor, producing serious corrosion problems. Furthermore, alkaline black liquor from straw has a higher viscosity than kraft black liquor from wood,1,3 resulting in lower concentration yields in the evaporators before the liquor enters the boiler. Consequently, the use of an auxiliary fuel is necessary to maintain the combus* To whom correspondence should be addressed. Tel.: +34976-761878. Fax: +34-976-761879. E-mail: qtarauzo@ posta.unizar.es.

tion conditions in the boiler. Black liquor gasification seems to be the technology most likely to replace the recovery boiler.4 Since the 1950s, several pilot-scale trials have been carried out in order to test whether the gasification process could operate more efficiently and safely than conventional Tomlinson boiler systems. For example, Rockwell International evaluated this possibility in 1978.5 Today, there are two main processes under continuing development and expected to be commercialized: Chemrec6,7 and MTCI processes. Both concentrate on the development of combined-cycle cogeneration technology for pulp mills with the objective of producing more electrical power than in that existing steam cycles with conventional recovery boilers.8 Parallel with these developments, a great deal of bench-scale work has been done to study the pyrolysis and gasification of kraft black liquor from wood. For example, thermogravimetric studies of CO2 gasification processes have been conducted in order to determine the influence of the reaction atmosphere composition on black liquor char gasification, modeled using Langmuir-Hinshelwood-type kinetics.9,10 The impact of pressure on the black liquor gasification rate was subsequently evaluated, revealing that, at constant gas composition and temperature, the gasification rate of black liquor char with CO2 diminished as the pressure was increased.11 Similarly, steam gasification of black liquor was studied and, as far as kinetic equations and activation energy are concerned, results similar to those of CO2 gasification were obtained.12 Van Heiningen et al. also evaluated the effect of the heating rate on black liquor char during the pyrolysis stage, finding that the gasification rate increased as the heating rate was raised.13 Because pyrolysis is the initial stage in gasification processes, the gasification energy efficiency depends on the conversion of the black liquor carbon to combustible gases during pyrolysis.14 Thus, a good understanding of this previous stage is required for the design of new gasifiers. Several works studying the pyrolysis of kraft

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black liquor from wood can be found in the literature. The composition of the gas resulting from this process as a function of time and temperature was first analyzed in systems allowing pyrolysis at low heating rates15 and subsequently in laminar entrained flow reactors allowing pyrolysis at very high heating rates.16,17 The kinetics of black liquor pyrolysis has also been studied in thermogravimetric systems by several authors.18-20 More recently, the effect of high pressures and high temperatures on the rapid pyrolysis of black liquor from wood has been measured.21,22 These works have provided theoretical support for the development of larger scale plants with the objective of commercializing gasification processes. Although there are numerous studies involving kraft black liquor from wood, data for black liquor deriving the alkaline pulping of straw are scarce. It is known that different liquors can behave very differently in the same gasifier under the same operating conditions.23 Thus, each type of black liquor requires a specific study in order to evaluate its pyrolysis and gasification behavior and determine the most appropriate conditions for gasification in a pilot-scale plant. What data there are about the pyrolysis of alkaline black liquor from straw come mainly from experiments performed in thermogravimetric systems. Sańchez24 has studied its devolatilization in different atmospheres (N2 and air) and with different heating rates (5-100 °C/min) at temperatures below 600 °C. Gea et al.1 have evaluated the influence of the temperature (500-900 °C), heating rate (5-30 °C), and concentration of CO in inert atmospheres (0-40% by volume) on the final solid conversion and the devolatilization rate. In the latter study, the experiments were performed in a thermogravimetric system under operating conditions (Tp and β) similar to those described in this paper. However, because of the small quantity of black liquor (5 mg) used in the thermogravimetric experiments, only the solid conversion and the devolatilization rate were analyzed. In the present work, the product yields, gas composition, and characteristics of the resulting char have been studied. Both works are therefore complementary. Limited information has been published on the effect of the temperature and heating rate on the product distribution and composition from the pyrolysis of this type of black liquor. These data are needed to quantify the potential energy power of black liquor, as well as to evaluate the optimum conditions for its pyrolysis and gasification. The objective of the current work is to determine the effect of the final pyrolysis temperature (250-900 °C) and heating rate (5-30 °C/min) on the product yields and gas composition obtained during the pyrolysis of alkaline black liquor from straw. The pyrolysis experiments were carried out in a fixed-bed reactor. Additionally, measurements of the specific surface area of the resulting black liquor char were performed in order to evaluate its evolution under different pyrolysis conditions. Experimental Section Material. The black liquor used in this work comes from the soda pulping of straw. It contains organic components, mainly degraded alkali lignin, aliphatic carboxylic acids, and a minor fraction of extractives, and a large amount of inorganic components derived from the chemicals used during the digestion of the straw.

Table 1. Ultimate and Proximate Analyses (Dry Basis) for the Black Liquor under Study (wt %)

element

alkaline black liquor from straw

C H N S Cl

39.05 4.54 1.00 0.78 3.50

component


ashes volatiles

20.63 65.98

element


K Na Si others

4.10 8.83 0.23 37.97

component


fixed carbon

13.39

The ultimate and proximate analyses for this alkaline black liquor are listed in Table 1. The elemental analysis was obtained in a CHNS Carlo Erba elemental analyzer (model EA1108). The proximate analysis was carried out following the standard norms, ISO 1171 for ashes and ISO 5623 for volatiles. The metal analysis was performed by atomic absorption after previous alkaline fusion in accordance with ASTM D368296. For the present work, the black liquor, 90% water in its original state, was initially concentrated to 45% on a heating plate under continuous stirring and subsequently completely dried in an oven at 105 °C. The drying process needs to be gradual and carefully controlled in order to achieve a homogeneous distribution of the inorganic compounds inside the solid matrix.10 After drying, the black liquor solids were ground and sieved between 75 and 53 µm, with the particles used for the experimental procedure being those retained in the 53 µm particle size sieve. Experimental Procedure. The dried and ground black liquor was pyrolyzed in a fixed-bed reactor. Figure 1 illustrates the laboratory-scale plant used for performing the experiments. Samples of approximately 4 g were placed in a cylindrical stainless steel basket (35 mm inner diameter and 23 mm length) of 40 µm mesh hung inside a reactor consisting of a stainless steel tube (43.5 mm inner diameter and 150 mm length) placed inside a furnace. Several thermocouples were used: five were placed in the core of the sample at different lengths and radii and the other was on the sample. This last thermocouple was the one connected to the temperature controller. The highest difference between the measured internal temperature and the controller temperature (approximately 50 °C) took place at the highest heating rate (30 °C/min) and at a temperature below 400 °C. Once the reactor was closed and purged with a N2 stream, the sample was heated in the upward flowing N2 (100 cm3/min NTP) up to the preset final pyrolysis temperature (Tp, °C) at the selected heating rate (β, °C/ min). The final temperature was maintained for 90 min. The residence time of the gas in the hot environment downstream of the sample was approximately 0.2 min. The gas was then cooled. Two ice traps and a cotton filter were installed downstream of the reactor exit to eliminate the tar from the exit gas stream. A continuous CO/CO2 IR analyzer was positioned after the gas cleaning system in order to monitor the composition of the exit gas. Samples were collected at constant time intervals and analyzed by chromatography in order to determine the gas yield and composition. After the experiment, the reactor was cooled and the resulting

5784 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003

Figure 1. Fixed-bed laboratory-scale reactor.

char sample weighed in a balance. Tar production was also determined by weighing the tar collection device. The char was ground and sieved in a 75 µm particle size sieve, taking those passed through the sieve and the specific surface area (Sg) obtained by the BrunauerEmmett-Teller (BET) adsorption method, using a Micromeritics Pulse Chemisorb 2700 apparatus with N2 at 77 K as the adsorbent gas. Results and Discussion The pyrolysis experiments produced the results described below, allowing an evaluation of the influence of two important variables, the final pyrolysis temperature and the heating rate, on the product yields (% char, % tar, and % gas), gas composition, and specific surface area of the resulting char. Influence of the Final Pyrolysis Temperature (Tp). The pyrolysis of the black liquor under study was performed at 14 different final temperatures in the range 250-900 °C (at 50 °C intervals) at a heating rate of 5 °C/min. For each experiment, gas samples were collected every 5 min until the selected final temperature was reached and subsequently every 10 min until the end of the experiment. Influence on the Gas Composition. The gases analyzed in the samples were H2, CO, CO2, CH4, C2’s (acetylene, ethylene, and ethane), and C3H8. A thermocouple placed just upon the sample recorded the temperature when the gas samples were collected. The temperature axes in the graphs of this paper refer to that temperature. Figure 2 shows the evolution with the temperature of each of the gases analyzed for the experiment performed with a final pyrolysis temperature of 900 °C. For the sake of clarity, only this experiment is shown. The evolution for the other runs (with different final pyrolysis temperatures) is similar. In the figure, the composition is expressed as a percentage volume of each compound in the total product gas (N2 included). As can

be seen, the two compounds that first appeared in the product gas were CO and CO2, which could occur even before 250 °C. CH4 and H2 then appeared at around 300 °C, followed by the other hydrocarbons at around 350 °C. From 450 to 600 °C, H2 is the most significant gas product. The CO production began to be significant again at 600 °C, with both CO and H2 being the two main compounds of the product gas from 600 to 900 °C. These results can be explained considering that the descarboxylation of the black liquor takes place during the earlier stage of pyrolysis, yielding high concentrations of CO and CO2, whereas H2 production increases with the temperature as a result of the breaking of the aromatic rings, which requires more energy.15 Influence on the Gas Production Rate. The production rate (expressed in g/min) for each gas produced during the experiment performed at Tp ) 900 °C in the fixed-bed reactor is plotted versus temperature in Figure 3. The dashed line represents the total number of grams of gas produced during the experiment. These data can be compared with the devolatilization rate

Figure 2. Product gas composition versus temperature. Tp ) 900 °C; β ) 5 °C/min.


Figure 4. Pyrolysis product yields versus final pyrolysis temperature (Tp).

Figure 3. Gas production rate versus temperature. Tp ) 900 °C; β ) 5 °C/min. The gas production data can be compared with the thermogravimetric data (dXs/dt) obtained in a previous work.1

versus temperature obtained in the experiment with Tp ) 900 °C and β ) 5 °C/min during the prior thermogravimetric study cited previously;1 the data of the devolatilization rate obtained in that study are also represented in Figure 3 for comparison. In the thermogravimetric study, the weight loss observed between 250 and 550 °C was explained by the thermal degradation of the organic matter fraction of the black liquor. This explanation is supported by the data obtained in this work because, as can be appreciated in Figure 3, the production of CO2, the main product of the thermal devolatilization of organic matter, was observed in the same temperature range. In addition, the second most important weight loss detected during the thermogravimetric experiment at temperatures higher than 600 °C is coincident with the CO production observed in Figure 3 for the work presented here. In the previous work, the increase of conversion at temperatures higher than 600 °C was considered to be the consequence of reduction reactions of alkaline compounds, mainly coming from the inorganic matter fraction of the black liquor, with the carbon present in the carbonaceous matrix. According to the mechanism proposed for these reactions,1 CO is the main product of the reduction, which is in agreement with the results presented in this paper. Influence on the Product Distribution. Global mass balances were made for each of the pyrolysis experiments performed in the fixed-bed reactor. Figure 4 displays the product yields as a function of the final pyrolysis temperature. It can be seen that the tar yield was the lowest for all of the final pyrolysis temperatures

studied, and this remained constant at around 14-16% from 400 °C. The char yield decreased with the temperature, whereas the gas yield increased. For a final pyrolysis temperature of approximately 800 °C, the gas production, expressed as a percentage of the sample initial weight, was higher than the char yield. These results are similar to the data reported for kraft black liquor from wood by other authors.2,15,25 The mass balance closure for all of the runs ranged from 98 to 115%. One of the most interesting calculations that can be made from the results relating to the product yields is the energy recovery in the form of gas. The energy recovery is here defined as the ratio of the heating value of the generated gas and the gross heat combustion of the black liquor. The low heating value of the gas produced was calculated by multiplying the mass of each gas generated with its individual low heating value. The following equation was used for the calculation:

energy recovery )

∑i LHVgasimgasi LHVliquormliquor

(1)

The lower heating value of the black liquor under study, determined at the Institute of Paper Science and Technology of Atlanta, is 3.683 kcal/g. Figure 5 shows both the percentage of energy recovery and the gas yield obtained for the different Tp values studied. It can be seen that both increased with the final pyrolysis temperature. Over the temperature range studied, the energy recovery varied from 3% at 250 °C to 45% at 900 °C. Bhattacharya reported an energy recovery of 28% at 720 °C for kraft black liquor from wood, similar to the value of roughly 33% obtained for the black liquor used in this study15 at approximately the same temperature. Influence on the Resulting Char. The char residue remaining at the end of devolatilization in the thermogravimetric experiment was not sufficient to perform an elemental analysis in order to check whether the weight loss detected between 750 and 900 °C was due to the evaporation of the alkaline metals, in accordance with the mechanism proposed in the previous work.1 However, the char remaining after devolatilization in the fixed-bed reactor was sufficient, and Table 2 shows the elemental analysis of the char obtained at different


Figure 5. Pyrolysis gas yield and energy recovery versus final pyrolysis temperature (Tp). Table 2. Elemental Analysis of Char Resulting from Pyrolysis of Black Liquor at Different Final Pyrolysis Temperatures Tp (°C)

%C

%H

%N

%S

% Na

%K

Nachar/ NaBL

Kchar/ KBL

250 300 350 400 450 550 600 650 700 750 850 900

33.38 33.35 33.36 34.50 35.08 31.94 34.03 32.49 29.97 25.96 36.61 35.72

3.84 3.37 2.73 1.95 1.37 0.78 0.86 0.81 0.61 0.65 0.93 0.74

0.91 0.85 0.72 0.76 0.75 0.76 0.8 0.86 0.76 0.77 2.95 2.98

0.56 0.59 0.61 0.74 0.80 1.07 1.13 1.06 1.25 1.12 1.67 1.60

12.22 14.05 16.75 16.86 16.79 17.33 16.66 17.14 18.02 18.40 13.84 15.21

5.90 6.21 7.49 7.45 7.24 7.21 7.19 8.04 8.37 9.40 7.42 8.43

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.6 0.6

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.7 0.7

final pyrolysis temperatures. This analysis was also obtained in a CHNS Carlo Erba elemental analyzer (model EA1108), and the metal analysis was performed by atomic absorption. The relation between the sodium content in the resulting char and that in the matrix of black liquor is also shown in the table (Nachar/NaBL). The parameter Kchar/KBL expresses the same relation for potassium. Both parameters indicate that the amount of sodium and potassium in the char obtained after devolatilization at 750 °C was similar to the initial amount in the black liquor but that it decreased by 40% in the case of sodium and 30% in the case of potassium after devolatilization at 850 °C. Therefore, the increase in weight loss observed in the thermogravimetric experiment between 750 and 900 °C could be largely due to the evaporation of alkaline metals. Regarding the influence of the final pyrolysis temperature on the specific surface area of the resulting char, Figure 6 shows the variation of the surface area, determined by the application of the BET adsorption method, versus Tp for a heating rate of 5 °C/min. As can be seen, the values of Sg were lower than 10 m2/g for temperatures lower than 600 °C. These values of Sg correspond to low specific surface area solids, indicating that the thermal degradation of the organic matter fraction of the black liquor could result in a macroporous char up to values of Tp ) 600 °C. However, at about 650 °C, a higher surface area char (Sg ≈ 28 m2/g) was measured, and from 700 to 800 °C, the Sg of chars increased noticeably (Sg ≈ 100 m2/g). Finally, as can be observed in Figure 6, the measured Sg values went on increasing with Tp and reached their highest value (Sg

Figure 6. Specific surface area (Sg) of the black liquor char obtained at different final pyrolysis temperatures.

≈ 300 m2/g) at 900 °C. As has been pointed out before, the reduction of alkaline compounds, mainly coming from the inorganic matter of the black liquor, seems to begin at 600-650 °C. Therefore, the increase in the surface area measured at 650 °C is probably due to the thermal decomposition of the inorganic compounds such as sodium and potassium carbonates. This decomposition provides small molecular gases, CO (bond length 1.12 Å) and CO2 (bond length 1.16 Å), which could create a microporous structure as they leave the carbonaceous matrix. By comparison of Figures 7 and 8, the change in the porous structure of the char can be seen when Tp increases from 600 to 700 °C. Figure 7 shows the complete adsorption-desorption isotherms and the pore-size distribution of the char prepared at Tp ) 500 °C, which are typical of a macroporous solid (average diameter pore ) 520 Å). However, Figure 8, which shows the isotherm and the pore-size distribution for the char obtained at Tp ) 750 °C, is completely different and represents a microporous solid with an average diameter pore of 32 Å. Other authors have reported similar values of Sg, between 5 and 300 m2/g, for chars obtained from the pyrolysis of kraft black liquor from wood in the same temperature range (300-1000 °C).13,26 Influence of the Heating Rate (β). Four experiments with different heating rates (β ) 5, 10, 15, and 30 °C/min) and the same final pyrolysis temperature (Tp ) 800 °C) were performed in order to evaluate the effect of the heating rate on the product gas composition, product distribution, and specific surface area of the resulting char. Influence on the Gas Composition. In regards to the product gas composition, Figure 9 shows the percentage of each compound (H2, CO, CO2, and hydrocarbons) in the exit product gas versus temperature for the different β values studied. As can be appreciated in the figure, the temperature for reaching the maximum production of each gas slightly increased with the heating rate. Except for the percentage of H2, which clearly increased in line with the heating rate, the rest of the gases only increased from β ) 5 to 10 °C/min, remaining fairly constant for higher heating rate values. H2 production mainly could come from the breaking of the aromatic rings, which requires more energy than the other reactions that take place for the decarboxylation of the organic matter.15 The reactions that require more energy seem to be improved by higher heating rates. However, more data about the reaction mecha-


Figure 7. Adsorption-desorption isotherm and volume pore distribution plots of black liquor char obtained at Tp ) 500 °C.

Figure 8. Adsorption-desorption isotherm and volume pore distribution plots of black liquor char obtained at Tp ) 750 °C.

nism are required to explain why H2 is more affected than the other gases when the heating rate is increased. Influence on the Product Distribution. The effect of the heating rate on the product distribution was noticeable neither in the range studied nor in the experimental system used in this work. The data are presented in Figure 10. Heating rates much higher than 30 °C/min seem to be required to notice the effect of this parameter on the product yields, in cases where the reactor used can present problems of temperature profiles in the sample. However, because the production of H2, the compound with the highest heating value, increased with the heating rate, the energy recovery showed a 50% increase from β ) 5 to 15 °C/min, as can be seen in Figure 11.

Influence on the Char Properties. Figure 12 summarizes the results obtained for the effect of the heating rate on the specific surface area of the resulting char. The data show that the surface area increased with the heating rate. The Sg value increased approximately 70% when the heating rate went up from 5 to 30 °C/min. Similar works on coals have shown an important increase in the surface area of chars with the heating rate.27 This result suggests that it is appropriate to use reactors with high heating rates to transform black liquor into gaseous products. Conclusions Because limited information is available about the product distribution and yield of gases from the pyroly-


Figure 9. Evolution of product gas composition versus temperature for the different heating rates studied. Tp ) 800 °C.

Figure 10. Variation of gas production rate versus temperature for the different heating rates studied. Tp ) 800 °C.

sis of alkaline black liquor from straw, the work presented here examines the effect of two important process variables, temperature and heating rate, on three parameters: the gas composition, the product yields, and the specific surface area of the resulting char. These data are required to quantify the potential energy power of the black liquor as well as to evaluate the optimum conditions for its pyrolysis or gasification. The pyrolysis has been performed in a fixed-bed reactor, and the results obtained have been compared with those obtained in a previous study carried out in a thermogravimetric system under the same operating conditions.1 Both works are complementary: the ther-

Figure 11. Pyrolysis product yields versus heating rate (β). Tp ) 800 °C.

mogravimetric experiments provided information about the conversion and the devolatilization rate, whereas the experiments performed in the fixed-bed reactor have provided data about the composition of the products and their distribution. By comparison of the weight loss observed in the thermogravimetric system from the pyrolysis of the black liquor between 250 and 500 °C with the gas production (H2, CO, CO2, and small hydrocarbons) obtained in this work for the same temperature range, it can be concluded that the thermal degradation of the organic matter fraction of the black liquor mostly takes


Basque Country (Campus of San Sebastian) for his assistance with the surface area measurements. Nomenclature

Figure 12. Pyrolysis gas yield and energy recovery versus heating rate (β). Tp ) 800 °C.

place in that temperature range. This conclusion is supported by the fact that H2, CO, and CO2 are the main product gases of the pyrolysis of organic matter. However, the weight loss observed in the thermogram at temperatures above 600 °C coincides with the production of CO, the main gas product of the devolatilization of alkaline (Na and K) compounds, which mainly come from the inorganic components of the black liquor. Finally, the loss of Na and K at temperatures above 750 °C has been detected by means of elemental analysis of the resulting char at different final pyrolysis temperatures. The final pyrolysis temperature has an influence on the product distribution. Although the tar yield remains approximately constant at around 14%, the char yield decreases with the temperature whereas the gas yield increases. This increase supposes higher energy recovery when the black liquor is pyrolyzed at higher temperatures. Over the temperature range studied, the energy recovery varied from 3% at 250 °C to 45% at 900 °C. Furthermore, although the heating rate does not seem noticeably to influence the product yields in the β range studied, it does influence the energy recovery, which increases 50% when β goes up from 5 to 15 °C/ min. The reason for this is that H2 production also increases with the heating rate. In regards to the specific surface area of the resulting char, both variables studied, Tp and β, have an influence. The surface area values increase by more than 90% when Tp goes up from 600 to 700 °C. The char obtained at temperatures below 600 °C seems to be a macroporous solid with Sg lower than 10 m2/g, while the char obtained above 650 °C appears to be a microporous solid with Sg higher than 100 m2/g. Thus, the devolatilization of the organic matter of the black liquor could result in a macroporus solid, whereas the devolatilization of its alkaline compounds could generate a microporous solid. The specific surface area of the resulting char also increases with the heating rate (e.g., approximately 60% when the heating rate goes up from 15 to 30 °C/min). Acknowledgment The authors express their gratitude to the University of Zaragoza (286-68) for providing frame support for this work and to Prof. Dr. Mario Montes from the Department of Applied Chemistry of the University of the

LHVgasi ) low heating value of gas (kcal/g) LHVliquor ) low heating value of dried alkaline black liquor under study (kcal/g) mgasi ) mass of gas generated during pyrolysis (g) mliquor ) mass of dried alkaline black liquor under study (g) P0 ) pressure for adsorption isotherm determination (bar) PN2 ) saturation vapor pressure of N2 at 77 K (bar) Sg ) specific surface area of resulting char (m2/g) Tp ) final pyrolysis temperature (°C) Vads ) volume of N2 adsorbed (cm3 NPT/g) β ) heating rate (°C/min)

Literature Cited (1) Gea, G.; Murillo, M. B.; Arauzo, J. Thermal Degradation of Alkaline Black Liquor from Straw. Thermogravimetric Study. Ind. Eng. Chem. Res. 2002, 41, 4714. (2) Schricharoenchaikul, 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 Pap. Sci. 1998, 24 (2), J43. (3) Gea, G.; Murillo, M. B.; Arauzo, J.; Frederick, W. J. Swelling Behaviour of Black Liquors from Soda Pulping of Wheat Straw. Energy Fuels 2003, 17, 46. (4) Larson, E. D.; Raymond, D. R. Commercializing Black Liquor and Biomass Gasifier/Gas Turbine Technology. Tappi J. 1997, 80 (12), 50. (5) Kohl, A. L. Black Liquor Gasification. Can. J. Chem. Eng. 1986, 64, 299. (6) Brown, C. A.; Hunter, W. D. Operating Experience at North America’s First Commercial Black Liquor Gasification Plant. International Chemical Recovery Conference, Tampa, FL, June 1998; TAPPI Press: Atlanta, GA, p 655. (7) Stigsson, L. Chemrec Black Liquor Gasification. International Chemical Recovery Conference, Tampa, FL, June 1998; TAPPI Press: Atlanta, GA, p 663. (8) Kreutz, T. G.; Consonni, S.; Larson, E. D. Performance and Preliminary Economics of Black Liquor Gasification Combined Cycles for a Range of Kraft Pulp Mill Sizes. International Chemical Recovery Conference, Tampa, FL, June 1998; TAPPI Press: Atlanta, GA, p 675. (9) Li, J.; Van Heiningen, A. R. P. Reaction Kinetics of Gasification Of Black Liquor Char. Can. J. Chem. Eng. 1989, 67, 693. (10) Li, J.; Van Heningen, A. R. P. Kinetics of CO2 Gasification of Fast Pyrolysis Black Liquor Char. Ind. Eng. Chem. Res. 1990, 29, 1776. (11) Frederick, W. J.; Wa˚g, K.; Hupa, M. Rate and Mechanism of Black Liquor Char Gasification with CO2 at Elevated Pressures. Ind. Eng. Chem. Res. 1993, 32, 1747. (12) Li, J.; Van Heiningen, A. R. P. Kinetics of Black Liquor Char by Steam. Ind. Eng. Chem. Res. 1991, 30, 1594. (13) Van Heiningen, A. R. P.; Aleń, R.; Arpianen, V. T. Effect of Liquor Type and Pyrolysis Rate on the Steam Gasification Reactivities of Black Liquors. International Chemical Recovery Conference, Seattle, WA, 1992; TAPPI Press: Atlanta, GA, p 641. (14) Koepke, S. A.; Zhu, J. Y. Pyrolysis of Black Liquor in a High -Intensity Acoustic Field. International Chemical Recovery Conference, Tampa, FL, June 1998; TAPPI Press: Atlanta, GA, p 971. (15) Bhattacharya, P. K.; Shrinath, A. S.; Kunzru, D. Pyrolysis of Black Liquors. J. Chem. Technol. Biotechnol. 1985, 35, 223. (16) 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 Pap. Sci. 1998, 24 (2), J43. (17) Frederick, W. J.; Iisa, K.; Phimolmas, V. The Effect of Temperature and Residence Time on the Distribution of Carbon, Sulfur, and Nitrogen Between Gaseous and Condensed Phase Products from Low-Temperature Pyrolysis of Kraft Black Liquor. Fundam. Adv. Pulp Pap. Ind. 1999, 95 (322), 119.

5790 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 (18) Li, J.; Van Heiningen, A. R. P. Sodium Emission during Pyrolysis and Gasification of Black Liquor Char. Tappi J. 1990, 213. (19) Backman, R.; Frederick, W. J.; Hupa, M. Basic Studies on Black Liquor Pyrolysis and Char Gasification. Bioresour. Technol. 1993, 46, 153. (20) Aleń, R.; Rytko¨nen, S.; McKeough, P. Thermogravimetric Behaviour of Black Liquors and Their Organic Constituents. J. Anal. Appl. Pyrolysis 1995, 31. (21) 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. International Chemical Recovery Conference, Tampa, FL, June 1998; TAPPI Press: Atlanta, GA, p 1129. (22) Whitty, K.; Nilsson, A. Experience from a High Temperature, Pressurised Black Liquor Gasification Pilot Plant. International Chemical Recovery Conference, Whistler, Canada, June 2001; PAPTAC: Montreal, Canada, p 281. (23) 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 Pap. Sci. 1997, 23 (3), J119. (24) 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. (25) Frederick, W. J.; Hupa, M.; Uusikartano, T. Volatiles and Char Carbon Yields During Black Liquor Pyrolysis. Bioresour. Technol. 1994, 48, 59. (26) Wa˚g, K. Characterization and Modelling of Black Liquors Char Combustion Processes. Ph.D. Dissertation, Oregon State University, Corvallis, OR, 1996. (27) Maloney, D. J.; Jenkins, R. G. Influence of Coal Preoxidation and the Relation between Char Structure and Gasification Potential. Fuel 1985, 64, 1415.

Received for review February 6, 2003 Revised manuscript received June 30, 2003 Accepted August 20, 2003 IE030116E

Thermal Degradation of Alkaline Black Liquor from Wheat Straw. 2

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