Kinetics of CO2 Gasification of Alkaline Black Liquor from Wheat Straw

Moreover, new technologies are being developed to solve the problems caused by the silica present in alkaline black liquor (ABL) from straw in heat re...
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Ind. Eng. Chem. Res. 2004, 43, 3233-3241

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Kinetics of CO2 Gasification of Alkaline Black Liquor from Wheat Straw. Influence of CO and CO2 Concentrations on the Gasification Rate Gloria Gea, Jose´ L. Sa´ nchez, Marı´a B. Murillo, and Jesu ´ s Arauzo* Thermochemical Processes Group (GPT), Arago´ n Institute of Engineering Research (I3A), University of Zaragoza, Marı´a de Luna 3, 50018 Zaragoza, Spain

Interest in the use of nonwoody biomass as a raw material for pulp and paper production is growing worldwide. Moreover, new technologies are being developed to solve the problems caused by the silica present in alkaline black liquor (ABL) from straw in heat recovery systems. Because, among the various possible technologies, gasification is becoming attractive as a new alternative recovery system for ABL, in-depth studies of ABL gasification are therefore required. The present work is focused on the study of CO2 gasification of a char obtained from the ABL pyrolysis. Specifically, the influence of [CO] and [CO2] on the gasification rate at different gasification temperatures has been studied. The process may be described by a Langmuir-Hinshelwood equation for a specific range of operating conditions. The data obtained suggest that this operational range could be broader if the number of catalytic sites is considered as a function of [CO] and the final pyrolysis temperature rather than as constant. Introduction The finite nature of fossil fuels has led to increasing interest in using renewable biomass fuels as an energy source for power and heat production. Black liquor (BL), the main biomass-containing waste from chemical pulp and paper production, is also an important fuel supplying the major share of energy used in pulp mills.1 It consists of inorganic compounds from the chemicals used in the pulping process and organic compounds from the raw material used for the paper production. Recovery of the chemicals used, as well as the energy contained in the organic compounds, is important for the economics of the paper industry. Traditionally, the Tomlinson recovery boiler technology has been used for recovering chemicals and energy from BL. However, this technology has important drawbacks, and the many attempts to develop alternatives have not yet achieved commercial success at full industrial scale. Since their introduction, Tomlinson boilers have been subject to many improvements, but they continue to suffer from relatively low thermal efficiency and a low power output to total heat input ratio. For this reason, the pulp and paper industry is investing money and effort in developing new recovery systems. BL gasification promises to be a future alternative or a complement to conventional Tomlinson recovery boilers,2 because integrated gasification-combined-cycle power plants can produce electrical energy at about twice the efficiency of conventional steam-cycle power plants.3 Because the chemical kraft process is the most important process for virgin pulp production worldwide, the principal raw material is wood. However, about 10% of the total chemical pulp produced in the world is made using nonwoody material as the raw material, such as bagasse and, most commonly, straw. China, the third largest producer of pulp and paper in the world, is poor * To whom correspondence should be addressed. Tel.: +34976-761878. Fax: +34-976-761879. E-mail: [email protected].

in wood resources, and about 70% of its entire pulp output comes from straw. For nonwoody raw materials, the soda process is usually utilized for pulp production. Nowadays, interest in nonwood fibers for pulp production seems to be increasing. The chemical soda process could become a process that converts agricultural waste to a useful product, avoiding the felling of trees. Moreover, new technologies are under development for solving the specific problem of the alkaline process for nonwood fibers caused by silica. The high content of silica in nonwood fibers makes the recovery of chemicals and energy from BL more difficult because it provokes more deposits in the boiler and a higher viscosity of the liquor when evaporated. Both effects reduce the efficiency of the recovery cycle. However, new technology based on the saturation of the liquor with strong carbon dioxide and separation of the precipitate formed by an efficient filter press may decrease the level of silica in the liquor and could make the recovery boiler process for nonwood BL more profitable.4 Therefore, the study of gasification of alkaline BL (ABL) from straw is now of great interest, and fundamental work on this subject is required. Although there are a number of studies on kraft wood BL (KBL) gasification fundamentals, there are very little data available for ABL from straw. What data there are comes mainly from studies concerned with the pyrolysis of ABL5-8 and its swelling properties.9 However, there are hardly any data on gasification studies.10 Reliable data on the gasification kinetics of ABL and ABL char (ABLC) are therefore required to obtain sufficient information about char reactivity, with the objective of developing new combustors and gasifiers for this type of liquor. In the present work, a kinetic study of CO2 gasification of ABLC has been carried out to analyze the influence of CO and CO2 on the gasification rate. KBL char (KBLC) from wood- and alkali-catalyzed carbon gasification studies has served as a useful reference for gasification of ABLC. Some very interesting works can

10.1021/ie034338o CCC: $27.50 © 2004 American Chemical Society Published on Web 05/22/2004

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be found on CO2 gasification of KBLC.11-15 These works have reported that the CO2 gasification rate of KBLC can be well described by the kinetic expressions used for the CO2 gasification rate of carbon. It has been shown in a number of studies that the following Langmuir-Hinshelwood expression describes both uncatalyzed and catalyzed CO2 gasification of carbon:16-18

-rWg )

k′[CO2] 1 + K′CO2[CO2] + K′CO[CO]

(1)

where k′ (mol-1 m3 min-1) is the reaction constant and K′CO2 (m3 mol-1) and K′CO (m3 mol-1) are the equilibrium adsorption constants of CO2 and CO, respectively. Other authors have proposed another Langmuir-Hinshelwood expression to describe catalyzed CO2 gasification of carbon:19,20

-rWg )

k′′[CO2] [CO2] + K′′CO[CO]

(2)

where k′′ is the gasification reaction constant (min-1) and K′′CO (dimensionless) is the equilibrium adsorption constant of CO. Because KBLC contains alkaline metals in its structure, Li and Van Heiningen suggested that KBLC gasification by CO2 can better be described by the eq 2 expression,12 whereas other authors have preferred the eq 1 expression.14,15 Both Langmuir-Hinshelwood expressions have been derived from the same mechanism proposed by Sams and Shadman as follows:21

[-COM](s) + CO2(g) S [-CO2M](s) + CO(g) (3) [-CO2M](s) + C(s) f [-COM](s) + CO(g)

(4)

where [-COM] and [-CO2M] represent alkali surface compounds in the carbonaceous matrix. The fast rate of the oxidation reaction (3) suggests that the rate-limiting step of the gasification process is reaction (4). In the present work, the gasification rate of ABLC obtained after slow pyrolysis is studied as a function of CO and CO2 concentrations at three different gasification temperatures (750, 800, and 850 °C). The data are analyzed to check if the proposed mechanism and the corresponding Langmuir-Hinshelwood equation types can well describe the ABLC CO2 gasification process. Experimental Section Material. The ABL used in this study derives from the soda pulping of wheat straw, and it was produced by an integrated paper mill, which uses recovered paper and cereal straw as raw materials. In the straw pulping process, straw is cooked at about 90 °C with a caustic soda solution. The ultimate and proximate analyses 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. The ABL used as the raw material was 90% water in its original state but was dried to 100% solid for use in the thermogravimetric (TG) runs. The drying method

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

ABL from straw

element

ABL from straw

C H N S Cl

39.05 4.54 1.00 0.78 3.50

K Na Si others

4.10 8.83 0.23 37.97

component

ABL from straw

component

ABL from straw

ashes volatiles

20.63 65.98

fixed carbon

13.39

has been explained in detail elsewhere.5 After drying, the solids were ground and sieved to less than 53 µm in particle size in order to minimize internal diffusion problems in the experiments. Experimental Procedure. A Cahn TG-131 thermobalance was used to perform the CO2 gasification kinetic runs. The schematic picture of the TG system was shown in a previous work.5 A flat pan of high-purity alumina (99.8%) was used to eliminate chemical interaction with the sample. In the two experimental procedures used in this study, BL solids were introduced into the thermobalance, a pyrolysis step was carried out to produce char, and subsequently CO2 char gasification was performed. The experimental procedure was as follows. Once about 5 mg of ABL was placed on the pan, the thermobalance was purged with 100 cm3 NTP min-1 of N2 for 30 min at room temperature. After this prestep, the runs can be classified into two experimental procedures, both of which had been previously used in order to study the KBLC gasification rate.13,14 For the runs performed according to experimental procedure 1, the furnace temperature was raised from room temperature to the selected gasification temperature (750, 800, and 850 °C) in a N2 atmosphere at a heating rate of 5 °C min-1. Once the sample achieved a constant weight at the gasification temperature, the N2 flow was changed to a mix of N2 and CO2 (tg ) 0). For the runs performed according to experimental procedure 2, after purging of the system, the temperature was raised from room temperature to 900 °C with a heating rate of 5 °C min-1 in a N2 atmosphere. After being maintained at 900 °C for 1 h in order to allow the sample weight to stabilize, the temperature was reduced to the selected gasification temperature (750, 800, and 850 °C, same as that in experimental procedure 1), and once the sample weight was steady, the N2 flow was replaced by a mix of N2 and CO2 and the gasification process started. At about 500 °C, a selected CO flow rate (0-35 cm3 NTP min-1) was also added to the N2 atmosphere and maintained up to the end of the experiment in both experimental procedures. The run was considered finished when the sample weight remained constant for 2 h. For the experiments performed according to experimental procedure 1, the final pyrolysis temperature (Tp) is a variable of the gasification process that affects the properties of the BL char (BLC), whereas Tp is constant for the runs performed according to experimental procedure 2. The influence of CO and CO2 on ABLC gasification has been studied following both experimental procedures at a temperature range of 750-850 °C. At each temperature studied, [CO] was varied from 0 to 35% by volume and the [CO2] from 10 to 100% by volume. The flow rate in all of the experiments was 100 cm3 NTP min-1, inert gas included.

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Figure 1. Temperature and weight loss curves for a typical gasification run performed according to experimental procedure 1.

Figure 3. Effect of the total flow rate on the gasification rate.

Figure 2. Temperature and weight loss curves for a typical gasification run performed according to experimental procedure 2.

Although originally alkali metals are present as organic and inorganic compounds in BL, it has been shown that essentially all organic metal compounds are converted into alkali-metal carbonates during pyrolysis below 675 °C.11 Because CO2 gasification of ABLC is studied at temperatures above 700 °C in this work, ABL has been pyrolyzed at temperatures above 675 °C in all of the runs. Therefore, the alkali metals in ABLC are in the form of surface compounds ([-CO2M], [-COM], and [-CM]) in this study. The addition of CO at 500 °C shifts the equilibrium of reactions (5) and (6) to the left and, as a consequence, reduces the evaporation of the alkaline metals (eq 7), which are catalytic species for the gasification process.5,11,12 The gasification process begins with the addition of CO2 (tg ) 0). The weight loss during the process is due to the reaction given in eq 9.

C(s) + CO2(g) S 2CO(g) Results The typical temperature and weight loss curves for runs performed according to both experimental procedures are shown in Figures 1 and 2, respectively. The weight loss below 500 °C is due to the thermal degradation of the organic matter of the BL.5 The weight loss above 600 °C is caused mainly by the release of CO.6,7 The formation of these gases might be due to the reduction in the inert atmosphere of some alkali-surface compounds, represented in this paper by [-COM] and [-CO2M], with the carbon present in the carbonaceous matrix according to the following reactions:22,23

[-CO2M](s) + C(s) S [-COM](s) + CO(g)

(5)

[-COM](s) + C(s) S [-CM](s) + CO(g)

(6)

[-CM](s) S [-C](s) + M(g)

(7)

where M represents the alkali metals, Na or K. The alkali compounds [-COM] and [-CO2M] are mainly formed by the decomposition of alkali carbonates present in BL according to the reaction

M2CO3(s) + C(s) S [-COM](s) + [-CO2M](s)

(8)

(9)

As has already been pointed out, this work evaluates the influence of CO and CO2 concentrations on the CO2 gasification rate of ABLC. The instantaneous gasification rate is defined according to eq 10, where Wg is the

-rWg ) -

1 dWg Wg dt

(10)

sample weight remaining at time tg. Figure 3 shows the ABLC gasification rate versus solid conversion for two runs performed at the same gasification temperature (800 °C) and CO and CO2 percentages but at different total gas flow rates. As can be seen in this figure, there are no appreciable differences between the two experiments. Similar results were obtained at all of the gasification temperatures studied in this work. It can therefore be concluded that the experiments were performed without heat- and mass-transfer limitations. In this paper, the solid conversion (Xg) is defined as

Xg )

Wg0 - Wg Wg0

(11)

where Wg0 is the solid weight remaining at time tg ) 0. Gasification Kinetics. From the mechanism proposed for alkali-catalyzed carbon gasification21 and

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Figure 4. Inverse of the gasification rate as a function of [CO]/[CO2] at different gasification temperatures. Runs performed according to experimental procedure 1.

assuming that reactions (3) and (6) approach equilibrium and eq 4 is irreversible and the rate-limiting step, the following Langmuir-Hinshelwood equation is obtained:

-rWg )

k4LKCOKCO2[CO2] 1 + [CO]

KCO [C]

+ [CO2]

KCOKCO2

(12)

[C]

where k4 is the reaction constant (g2 mol-2 min-1), KCO (m3 g-1) and KCO2 (dimensionless) are the equilibrium constants of eqs 6 and 3, respectively, [C] is the carbon concentration in the carbonaceous matrix (mol g-1), [CO] and [CO2] are the CO and CO2 concentrations in the reaction atmosphere (mol m-3), and L is the total number of active sites (mol g-1), as can be seen in eq 13.

L ) [-CO2M] + [-COM] + [-CM]

(13)

A comparison of eqs 1 and 12 shows that both equations can be considered to be similar by assuming [C] and L as constants. However, such an assumption would not be acceptable in all of the operation conditions studied in this work, as will be explained in the following sections. Taking into account that the complete reduction of the alkali-metal compounds by reactions (6) and (7) in an oxidizing atmosphere is not likely because reaction (3) is much faster than reactions (6) and (7), the value of [-CM] could be ignored. In that case, the LangmuirHinshelwood equation (12) can be simplified and eq 14 is obtained as the expression that describes the mechanism proposed for the gasification process:

-rWg )

k4[C]L[CO2] 1 [CO2] + [CO] KCO2

(14)

Once again, if L and [C] are considered as constants, eqs 2 and 14 are similar. The objective of this work is to study whether eq 12 or eq 14 can be used to represent the results obtained for the CO2 gasification rate of ABLC. Validation of Equation 14 as the Kinetic Expression of CO2 Gasification of ABLC. First, the validity of the simplified equation was tested, so the LangmuirHinshelwood expression of eq 14 was rewritten as

-

[CO] 1 1 1 + ) rWg k4[C]L k4[C]LKCO [CO2] 2

(15)

Accordingly, if both [C] and L remain constant during the reaction, the inverse of the ABLC gasification rate should only depend on the [CO]/[CO2] ratio rather than on [CO] and [CO2] independently. To test whether eq 14 can be used to represent the data obtained for the ABLC gasification rate, several sets of experiments were performed whereby only the [CO]/[CO2] ratio was varied. This variation was performed in this part of the work by changing both [CO] and [CO2] simultaneously. The results obtained for both experimental procedures used in this work are explained below. Experimental Procedure 1. The inverse of the gasification rate calculated at different conversions (Xg) for runs performed following experimental procedure 1 is plotted versus the ratio [CO]/[CO2] in Figure 4. There is one plot for every gasification temperature studied. As can be observed, there is not a linear relationship

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Figure 5. Inverse of the gasification rate as a function of the [CO]/[CO2] ratio at different gasification temperatures. Runs performed according to experimental procedure 2.

between the inverse of the gasification rate and the [CO]/[CO2] ratio at any temperature for the complete range of [CO]/[CO2] tested. Therefore, CO2 gasification of ABLC cannot be described by eq 14 for the whole operating range studied in this work. However, it can be seen in Figure 4 that there are some specific operational conditions under which the relationship between the inverse of the gasification rate and the [CO]/[CO2] ratio can be considered to be linear. For example, the linear regression coefficients of the data obtained at 750 °C and values of the [CO]/[CO2] ratio lower than 0.6 are approximately 0.98 for all of the solid conversions. The data obtained at 800 °C show linear relationships only for [CO]/[CO2] ratios lower than 0.4. The results obtained at 850 °C do not show linear relationships in any range of the ratio. Thus, the validity of the kinetic expression of eq 14 could be confirmed for values of the [CO]/[CO2] ratio lower than 0.6 at temperatures lower than 800 °C. This result is in agreement with the observations reported for KBLC at gasification temperatures lower than 775 °C by Li and van Heiningen.12,13 Experimental Procedure 2. Similarly, the inverse of the gasification rate calculated at different conversions (Xg) for runs performed following experimental procedure 2 is plotted versus the [CO]/[CO2] ratio in Figure 5. As can be seen, the relationships between both variables could be considered linear at 750 and 800 °C but not at 850 °C. However, the correlation data obtained at 750 °C according to eq 14 give negative values for the kinetic constant k4, which does not make any chemical sense. The constants k4L[C] and KCO2 were determined at 800 °C from the correlation equation, and their values are listed in Table 2. This table also includes kinetic data from the literature for KBLC.12 Although the values are not directly comparable because

Table 2. Values of the Kinetic Constants Calculated from the Correlation of the Data According to Equation 14 for CO2 Gasification of ABLCa type of BLC

Xg

T (°C)

KCO2

k4L[C] (min-1)

ABLC KBLC12

0.25 0.25

800 725

0.11 0.24

0.13 0.24

a Experiments were carried out following experimental procedure 2.

they are calculated at different temperatures, it can be observed that the kinetic constants for ABLC and KBLC have similar orders of magnitude. Apart from the results obtained at 800 °C, the data shown above indicated that the relationship between the inverse of the gasification rate and the [CO]/[CO2] ratio is nonlinear. Thus, the CO2 gasification rate of ABLC cannot be well described by the Langmuir-Hinshelwood expression of eq 14 for the whole operational range studied in this work. This conclusion is confirmed by the results shown in Figure 6. The variation of the gasification rate with time for runs performed under the same [CO]/[CO2] ratio but different values of [CO] and [CO2] is plotted in Figure 6 at different gasification temperatures. It can be seen that the gasification rate of ABLC seems not to depend on the [CO]/[CO2] ratio but on the values of [CO] and [CO2] independently. Validation of Equation 12 as the Kinetic Expression of CO2 Gasification of ABLC. Once it was checked that the simplified equation (14) did not describe ABLC gasification kinetics, eq 12 was tested. With this aim, two sets of experiments were performed in which respectively only [CO] and [CO2] were varied. Because, as has been shown above, the effect of CO and CO2 on gasification kinetics seems to be similar for both experimental procedures used in this work, only the experiments performed according to procedure 2 have

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Figure 6. Variation of the gasification rate with time under the same [CO]/[CO2] ratio at different gasification temperatures.

Figure 7. Inverse of the gasification rate as a function of the inverse of [CO2] at different gasification temperatures and CO concentrations.

been analyzed in this part of the study. To test eq 12, [CO] was maintained constant and the inverse of the gasification rate was plotted versus the inverse of [CO2] in the first set of experiments, while in the second set, [CO2] was constant and the inverse of the gasification rate was plotted versus [CO]. The validation of eq 12 for describing the gasification rate of ABLC would only be confirmed if both relationships were linear.

Figure 7 shows the inverse of the gasification rate versus the inverse of [CO2] for different solid conversions at three gasification temperatures (750, 800, and 850 °C). It can be seen that linear relationships are obtained for all of the conditions tested. The values of the constant k4L[C] calculated from the correlation of the data according to the inverse of eq 12 are listed in Table 3 as a function of Tg and Xg. By comparison of these

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Figure 8. Inverse of the gasification rate as a function of [CO] at different gasification temperatures and CO2 concentrations. Table 3. Values of the Constant k4L[C] Calculated from the Correlation of the Kinetic Data According to Equation 12 for CO2 Gasification of ABLCa T (°C)

[CO] (% by volume)

Xg

k4L[C] (min-1)

750

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 10 10 10 10 10

0.05 0.10 0.15 0.20 0.25 0.05 0.10 0.15 0.20 0.25 0.05 0.10 0.15 0.20 0.25 0.05 0.10 0.15 0.20 0.25

3.1 × 10-2 3.6 × 10-2 4.6 × 10-2 5.9 × 10-2 7.1 × 10-2 7.0 × 10-2 1.2 × 10-1 1.5 × 10-1 1.5 × 10-1 1.5 × 10-1 1.1 × 10-1 1.3 × 10-1 1.4 × 10-1 1.4 × 10-1 1.6 × 10-1 7.7 × 10-2 1.8 × 10-1 2.0 × 10-1 1.8 × 10-1 1.7 × 10-1

800

850

800

a Experiments were carried out following experimental procedure 2.

data, it can be calculated that the value of the constant increases 2.5 times when the temperature goes up from 750 to 800 °C. However, there is not such a significant difference between the data obtained at 800 and 850 °C. A comparison of the data obtained at 800 °C and different [CO] shows that the value of the constant increases by 23% when [CO] goes up from 4 to 10% by volume. Finally, it can be seen that the constant values seem to increase with Xg, at least up to values of Xg lower than 15%. Figure 8 shows the inverse of the gasification rate versus [CO] for different solid conversions at three gasification temperatures (750, 800, and 850 °C). As can be seen, there is not a linear relationship between the inverse of the gasification rate and [CO] over the whole

[CO] range studied. The relationship is linear only up to a [CO] of 20% by volume (9 mol m-3 NTP), for the gasification temperature range studied. Therefore, it can be concluded that ABLC gasification by CO2 can be well described by eq 12 for values of [CO] lower than 20% by volume in the temperature range 750-850 °C. Discussion BLC gasification can be considered as an alkali-metalcatalyzed gasification of a carbonaceous material. Therefore, the BLC gasification rate depends on the number of catalytic sites. Previous works12 about CO2 gasification of KBLC reported that the effect of CO and CO2 on the gasification rate can be described by a simplified Langmuir-Hinshelwood kinetic equation (eq 2). However, from the data obtained in this study, it seems that ABLC gasification can be better described by a nonsimplified Langmuir-Hinshelwood equation (eq 12). Equations 1 and 2, which are published in the literature,16-20 would be respectively similar to eqs 12 and 14 whether L and [C] were constant values. Because, in this work, the validity of eqs 12 and 14 has been studied assuming constant values for L and [C], it can be assumed that the validity of eqs 1 and 2 has also been studied for describing ABLC CO2 gasification. The important assumption for the derivation of the simplified equation (14) is that [-CM] is neglected. The value of [-CM] at the commencement of the gasification step depends on the pyrolysis conditions. A high [CO] in the inert atmosphere and a low Tp could favor a lower [-CM] at the beginning of the gasification step. A high [CO2] could also result in lower values of [-CM] during the gasification process. Therefore, the assumption of neglecting [-CM ] would be valid only in some specific operating conditions. This reason could explain why the simplified Langmuir-Hinshelwood equation (eq 14) seems not to describe well ABLC gasification in all of the operating conditions used in this study.

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The data obtained in the gasification runs performed in this work fit better the nonsimplified kinetic equation (eq 12), as long as [CO] is lower than 20% by volume. This kinetic expression includes the total number of catalytic sites (L) in the equation; however, this term has been assumed to be constant, an assumption which might not be acceptable for the whole operating range studied in this work. The number of catalytic sites in BLC depends on the catalyst load or the M/C atomic ratio in BL, the internal surface area of BL, and the pyrolysis conditions before initiation of the gasification step.12 Both the catalyst load and the internal surface area of BL could be considered constant because all of the ABLC samples used in the TG experiments came from the same raw material, which was dried in a single batch. The drying process was carefully carried out in order to obtain a homogeneous alkali-metal distribution in the solid matrix. However, the pyrolysis conditions (final pyrolysis temperature, Tp, and [CO] added at 500 °C in the inert atmosphere) were changed for the different runs performed. As has been reported in previous works,5,6 [CO] added to the inert atmosphere at 500 °C influences the number of catalytic sites presented in the ABLC and Tp influences both the number of catalytic sites of ABLC and the internal surface area. Therefore, L could be variable in the different experiments carried out and not constant, as has been assumed, which would explain why the gasification data obtained only followed a Langmuir-Hinshelwood expression of eq 12 within a specific operating range. Previous works5,6 also show that the addition of CO to the N2 atmosphere could partially inhibit the reduction of alkali compounds by the reactions (5)-(7), preventing the loss of alkali metals (catalytic sites) by evaporation. However, [CO] required to control the loss of alkali metals strongly depends on the final pyrolysis temperature. Because of the endothermic character of the reduction reactions, a higher [CO] is required at higher final pyrolysis temperatures (especially at temperatures higher than 800 °C) in order to achieve a certain degree of inhibition of the alkali reduction reactions. Therefore, CO seems to have a double effect on the ABLC gasification rate: (i) CO, the main gasification product, decreases the gasification rate. This effect is expressed in the Langmuir-Hinshelwood kinetic equation type used in this work (eq 12). (ii) On the other hand, CO also inhibits the alkali reduction reactions, which causes the loss of catalytic sites during the previous pyrolysis step. Thus, CO could increase the total number of active sites (L) available for the gasification step, which, in turn, increases the gasification rate. This effect is not expressed in eq 12, which assumes L as constant rather than as a function of [CO]. The observation made in this work about the validation range of the Langmuir-Hinshelwood equation type ([CO] < 20%) could be explained as a result of this double effect of [CO] on the gasification rate. The runs studied to validate eq 12 were performed under experimental procedure 2. Therefore, the final pyrolysis temperature was 900 °C for all of the experiments. However, [CO] added at 500 °C in the pyrolysis step was one of the variables of the process. Because 900 °C is a high final pyrolysis temperature, [CO] required to control the

alkali reduction reactions would be relatively high. For this value of Tp, a [CO] value higher than 20% by volume would seem to be required in order to detect an increase in L and, as a consequence, a deviation from the Langmuir-Hinshelwood expression, which considered L as constant. For [CO] values lower than 20% by volume, the loss of alkali metals could occur to the same degree independently of the [CO] used. Therefore, the total number of active sites (L) could be considered to be constant for [CO] values lower than 20% by volume. In this case, the data obtained in this study validate eq 12 as the Langmuir-Hinshelwood expression, which describes the ABLC CO2 gasification. However, L could increase when [CO] goes up from 20 to 25% by volume, because a CO concentration of 20% by volume could be enough to control better the alkali reduction reactions at a final pyrolysis temperature of 900 °C. In this case, the value of L would not be constant and a deviation from the kinetic equation expressed by eq 12 would be appreciated. To avoid such a deviation, eq 12 must be corrected. New studies are required to obtain the correlation between L and [CO]. The total number of active sites also depends on the final pyrolysis temperature. Tp is constant for the runs performed according to experimental procedure 2 but variable for experimental procedure 1. Therefore, it would be useful to obtain the expression of L as a function of [CO] and Tp. The results obtained in this work show that the application of eq 12 assuming L as constant would strongly depend on the operating temperature and type of gasification agent used to carry out the process. If air were used, [CO] obtained in the gas product would probably be lower than 20% by volume because nitrogen from the air dilutes the product gases from the gasification. [CO] of 6.5% by volume has been reported in a dry gas base for the gas obtained in a real KBL gasification plant based on the Chemrec technology using air as the gasification agent at 950 °C.24 Similar values could be expected for ABL gasification. If steam were used as the gasification media, the excess of steam in the reactor would displace the water gas shift reaction to the production of CO2 and H2, consuming part of the CO present, so [CO] would be expected to be even lower than 6.5% by volume. This is the case with the MTCI technology for BL gasification. Tests carried out with the same ABL was used in this work using MTCI technology7 showed [CO] of 2% by volume at 650 °C. The highest [CO] in a gasification process would be expected using oxygen as the gasifying agent, as there would be no nitrogen to dilute the gas produced. However, we have found no data for BL using this process. In the case of [CO] being higher than 20% by volume, a L variable would have to be taken into account. Conclusions ABLC gasification can be considered as an alkalimetal-catalyzed gasification of a carbonaceous material. Therefore, the ABLC gasification rate depends on the number of catalytic sites. The influence of CO and CO2 on the ABLC gasification rate can be described by a Langmuir-Hinshelwood-type kinetics for the temperature range of 750-850 °C, as long as [CO] added in the N2 atmosphere at 500 °C is lower than 20% by volume. This restriction is due to the nature of the LangmuirHinshelwood equation used, which assumes that the

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total number of active sites (L) is constant. However, L may depend on pyrolysis conditions, especially on Tp and [CO]. The addition of CO to the N2 atmosphere could partially inhibit the reduction of alkali compounds present in the carbonaceous matrix, preventing the loss of catalytic sites by evaporation, which could even increase the gasification rate. However, because of the endothermic character of the reduction reactions, a higher CO concentration is required at higher final pyrolysis temperatures (especially at temperatures higher than 800 °C) in order to achieve a significant degree of inhibition. Therefore, the Langmuir-Hinshelwood equation would better represent the ABLC gasification rate if L were included as a function of [CO] rather than as a constant. If the total number of active sites (L) is assumed to be constant, only the inhibition effect of CO on the gasification rate is considered in the kinetic equation, regardless of the [CO] used. Further studies are therefore required to determine the relationship between the total number of active sites with [CO] and the final pyrolysis temperature in order to be included in the Langmuir-Hinshelwood kinetic equation, which appears to describe ABLC gasification with CO2. Acknowledgment The authors express their gratitude to the University of Zaragoza (286-68) for providing frame support for this work. Nomenclature [CO] ) CO concentration added at 500 °C and maintained up to the end of the gasification experiment, % by volume or mol m-3 NTP [CO2] ) CO2 concentration, % by volume or mol m-3 NTP [C] ) carbon concentration in the carbonaceous matrix, mol g-1 k′ ) reaction constant, mol-1 m3 min-1 K′CO2 ) equilibrium adsorption constants of CO2 in the carbonaceous matrix, m3 mol-1 K′CO ) equilibrium adsorption constants of CO in the carbonaceous matrix, m3 mol-1 K′′CO ) equilibrium adsorption constants of CO in the carbonaceous matrix k4 ) reaction constant of eq 4, g2 mol-2 min-1 KCO ) equilibrium constants of eq 3, m3 g-1 KCO2 ) equilibrium constants of eq 6 L ) total number of active sites in the alkaline black liquor char, mol g-1 -rWg ) instantaneous gasification rate of alkaline black liquor char, min-1 tg ) time from the introduction of CO2 into the system, min Tp ) final pyrolysis temperature, °C Wg ) sample weight remaining at a specific point in time during the gasification step, mg Wg0 ) initial sample weight during the gasification step, when CO2 is introduced into the TG system, mg Xg ) alkaline black liquor char conversion during the gasification step

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Received for review December 24, 2003 Revised manuscript received April 12, 2004 Accepted April 16, 2004 IE034338O