Gasification of Alkaline Black Liquor from Wheat Straw. 2. Evolution of

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Kinetics of CO2 Gasification of Alkaline Black Liquor from Wheat Straw. 2. Evolution of CO2 Reactivity with the Solid Conversion and Influence of Temperature on the Gasification Rate Gloria Gea, Jose´ L. Sa´ nchez,* Marıá B. Murillo, and Jesu ´ s Arauzo Thermochemical Processes Group (GPT), Arago´ n Institute of Engineering Research (I3A), University of Zaragoza, Marıá de Luna 3, 50018, Zaragoza, Spain

Results obtained in recent studies have shown that black liquor (BL) gasification can be considered as an attractive recovery system. However, limited information has been published on the gasification kinetics of alkaline BL char (ABLC). Part 1 of this work (Gea, G.; Sańchez, J. L.; Murillo, M. B.; Arauzo, J. Ind. Eng. Chem. Res. 2004, 43, 3233) studied the CO2 gasification of ABLC, specifically the influence of [CO] and [CO2] on the gasification rate at different temperatures. The present work is focused on the variation of this reaction rate with solid conversion during the gasification process. The results obtained show that the gasification rate increases with the solid conversion up to a maximum value and then decreases. The effect of various factors such as pyrolysis operating conditions on this variation has also been analyzed. Furthermore, the dependence of the ABLC gasification rate on the temperature has been evaluated, and the apparent average activation energy over the range 170-234 kJ/mol has been determined for the temperature range 750-850 °C. Introduction The recent energy crisis suffered in some countries has shown how the lack of energy self-sufficiency can be a real problem. Fossil fuels can be assumed to be limited resources, and the search and development of renewable energy resources is therefore a current priority for many industries. Black liquor (BL), a residue in virgin pulp production, plays a key role in energy provision for paper mills. The results obtained in a recent study have shown that BL gasification can be considered an attractive powerhouse recovery cycle technology.1 However, important gaps remain in the knowledge of BL gasification fundamentals. As an example, more reliable data are needed on black liquor char (BLC) gasification kinetics as a function of temperature and of pressure, especially for chars produced at high heating rates. Up to now, interesting basic studies have been carried out2-5about the gasification kinetics of kraft black liquor char (KBLC). These works indicate that CO2 and steam KBLC gasification can be welldescribed by a Langmuir-Hinshelwood expression, which also describes both the uncatalyzed and catalyzed gasification of coal char. The temperature dependence of BL gasification has also been studied at a certain levels of [CO] and [CO2] for CO2 gasification and [H2] and [H2O] for steam gasification, obtaining an apparent activation energy similar to the values obtained for coal char gasification (about 200 kJ/mol).3,4 These studies showed that the gasification rate for KBLC produced at high heating rates increased with the solid conversion up to a maximum value reached at conversion close to 0.95, but the rate of KBLC produced at slow heating rates hardly varied with the solid conversion.3 Most of the works about gasification kinetics found in the literature deal with KBLC because the kraft process is one of the most common for the production of * Corresponding author. Phone: +34-976-761878. Fax: +34976-761879. E-mail: [email protected].

virgin pulp. However, some of the important pulp and paper-producing countries, such as China, are poor in wood resources, and most of their production comes from non-wood materials. This means that the study of the soda non-wood pulp production process could result interesting. This process has a major disadvantage as compared to the kraft wood process because the high content of silica in non-wood fibers can produce a loss of efficiency in the chemical and energy recovery cycle. However, new technologies based on saturation of the liquor with strong carbon dioxide appear to decrease the level of silica in the liquor, enhancing the efficiency in the recovery cycle.6 Fundamental studies of soda or alkaline BL from non-wood fibers are therefore becoming of greater interest and indeed necessary for the development of alternative gasification processes. Previous works about alkaline black liquor (ABL) were mainly focused on comparing KBL and ABL behavior with respect to combustion and swelling properties7,8 and on its pyrolysis behavior.9-12 However, limited information has been published on gasification kinetics of ABLC. Therefore, part 1 of this work studied the gasification rate of ABLC as a function of [CO] and [CO2] at different gasification temperatures.13 It was observed that the gasification process might be described by a Langmuir-Hinshelwood equation for a specific range of operating conditions. This range could be broader if the number of catalytic sites is considered as a function of [CO] and the final pyrolysis temperature rather than constant. Another important aspect of the gasification process that has not yet been evaluated for ABLC is the reaction rate variation with the solid conversion during the gasification process. Since ABLC gasification involves an alkali-catalyzed carbonaceous material process, the variation of effective catalyst concentration during gasification and its contribution to the variation in the gasification rate are of interest. The effect of certain factors such as the pyrolysis operating conditions on this variation has received very little attention. Further-

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

alkaline black liquor from straw

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

component

alkaline black liquor from straw

ashes volatiles fixed carbon

20.63 65.98 13.39

more, the temperature influence on the gasification rate for ABLC has not been studied, and the apparent activation energy for this material is not available. In the present work, the variation of the gasification rate for an ABLC with the solid conversion at different gasification temperatures (750-850 °C) and several concentrations of [CO] and [CO2] have been evaluated. The effect of certain factors such as the final pyrolysis temperature on this variation has also been analyzed. Moreover, the dependence of the ABLC gasification rate on the temperature has also been studied and the apparent activation energy determined over the temperature range of 750-850 °C. The potential effect of the pyrolysis operating conditions on this apparent activation energy has been analyzed. Finally, a brief comparison has been made between ABLC, KBLC, and alkali impregnated coal char gasification rates in order to evaluate ABLC reactivity with respect to that of other similar carbonaceous materials. Experimental System Materials. The ABL used in this work was supplied by an integrated paper mill, which uses recovered paper and cereal straw as raw materials. The BL came from the soda pulping of wheat straw. The material was dried to 100% solid. Afterward, it was ground and sieved to less than 53 µm in particle size for the runs performed in this work. Table 1 shows the ultimate and approximate analyses of the material. Both the drying method and the standard methods used for the analysis have been explained in detail elsewhere.10 Experimental Procedure. The thermogravimetric runs in this study were performed in a Cahn TG-131, already described in a previous work.10 Two different experimental procedures were used; both explained in detail in the first part of this work.13 However, a brief reminder may be helpful since constant reference is made to these procedures in this second part. For the runs performed according to procedure 1, after adding 5 mg of ABL to the pan inside the reactor, the thermobalance was purged with a nitrogen flow, and then the temperature was raised from room temperature to the selected gasification temperature (750, 800, and 850 °C) with a heating rate of 5 °C/min in a nitrogen atmosphere. Once the gasification temperature was reached and the weight was steady, the N2 flow was changed to a mix of N2 and CO2 (tg ) 0). Under

experimental procedure 2, after introducing about 5 mg of ABL in the thermobalance reactor and purging it with nitrogen, the temperature was raised from room temperature to 900 °C with the same heating rate and maintained for 1 h. Afterward, the temperature was reduced to the selected gasification temperature (the same as in procedure 1). A mixture of N2 and CO2 then replaced the N2 flow, and the gasification process started. A selected CO flow was added at 500 °C and maintained up to the end of the experiment for all the runs performed in this work under both procedures. The operating conditions for the runs were as follows. The total flow rate was 100 cm3 NTP min-1, inert gas included. The influence of the gasification temperature was evaluated over the range of 750-850 °C. At each temperature, the concentration of CO added ([CO]) was varied from 5 to 30 vol %, and the concentration of CO2 ([CO2]) was varied from 10 to 95 vol %. Since the experiments in this study were also used for obtaining a kinetic equation in part 1 of the work,13 two groups of runs can be differentiated. In the first group of runs, the [CO] and [CO2] were simultaneously varied (the [CO] from 5 to 29% and the [CO2] from 95 to 71 vol %), and both experimental procedures were used. In the second group of runs, the [CO] was maintained constant at 4% and the [CO2] varied from 10 to 40 vol % in some experiments while in others the [CO2] was maintained at 20% and the [CO] varied from 4 to 30 vol %. In this second group of runs only procedure 2 was used. Results and Discussion The typical temperature and weight loss curves for the runs performed under both experimental procedures were described in detail in the first part of the work.13 Since one of the objectives of this work is to evaluate the evolution of the CO2 gasification rate of ABLC with the solid conversion, both variables need to be defined. Equations 1 and 2 indicate the expressions for the gasification rate and the solid conversion, respectively:

-rWg )

dWg -1 Wg - Wg∞ dt

(1)

where -rWg is the instantaneous gasification rate per unit mass, Wg is the sample weight remaining at time tg, and Wg∞ is the weight remaining at the end of the gasification process;

Xg )

Wg0 - Wg Wg0 - Wg∞

(2)

where Wg0 is the solid weight remaining when the gasification step starts. Evolution of ABLC Reactivity with the Solid Conversion. Figures 1 and 2, respectively, show an example of the gasification rate curves for the CO2 gasification of the ABLC at different temperatures for both experimental procedures. The shape of the reactivity curves is similar for all the runs carried out in this work, independently of the CO and CO2 concentrations and the experimental procedure used. As can be seen, the gasification rate initially increases with the conversion up to a certain conversion value and then decreases. Previous works3,5 about CO2 gasification of KBLC also reported that the CO2 gasification rate of BLC showed

Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6585

Figure 1. Gasification rate vs solid conversion (Xg) for experiments performed at different temperatures according to experimental procedure 1: [CO] ) 17%, [CO2] ) 83 vol %.

Figure 2. Gasification rate vs solid conversion (Xg) for experiments performed at different temperatures according to experimental procedure 2: [CO] ) 5%, [CO2] ) 95 vol %.

a maximum with regard to the solid conversion. This behavior had previously been observed in CO2 catalyzed gasification of coal chars.14,15 The factors that mainly contribute to the variation of the rate with the solid conversion are the catalyst loss, the change in the catalyst/carbon ratio, the change in catalyst distribution, and the change in the surface area.14 It has frequently been reported that the gasification rate of coal chars increases with the catalyst/carbon ratio up to a certain value of this ratio, where the carbonaceous matrix is saturated with catalytic sites, after which the rate levels out.14,16,17 During coal char CO2 gasification, the variation of the rate with the solid conversion depends on

the catalyst/carbon ratio before the catalyst saturation level is reached and then on the surface area. Before saturation, the rate increases with the solid conversion during gasification if the catalyst loss rate is slower than the carbon loss rate, otherwise it decreases.14 For ABLC gasification, the maximum observed in the variation of the rate with the conversion could also indicate catalyst saturation of the carbonaceous matrix. Taking into account that the sample weight is steady before starting the gasification process, the catalyst loss rate is likely to be slower than the carbon gasification rate, which could explain why the rate increases with the solid conversion at the beginning of the gasification process up to a certain conversion value for all the experiments performed in this study. Once the catalyst saturation level is reached, the surface area may determine the gasification rate evolution. It has been reported for coal chars that the surface area increases with the solid conversion up to a certain value and then decreases.14,17 For ABLC gasification, the decrease of the rate after the maximum is reached could also be a consequence of the decrease of the surface area with the conversion once the saturation level has been reached. The maximum gasification rate is reached for ABLC gasification at different solid conversion values (Xgmax) depending on the operating conditions. The effect of the gasification temperature (Tg), the final pyrolysis temperature (Tp), the [CO], and the [CO2] on this solid conversion value has also been evaluated in this work. Effect of Tg on Xgmax. As can be observed in Figure 1, the maximum gasification rate is reached at higher conversion values when the gasification temperature increases for the runs performed under experimental procedure 1. However, the Tg has less influence on Xgmax for the experiments performed according to procedure 2 (see Figure 2). For the sake of clarity, Table 2 shows the values of the solid conversion at which the maximum gasification rate is reached for some of the experiments carried out according to both experimental procedures. A comparison of the variation coefficient obtained in both cases shows that the Tg has a greater influence on the solid conversion when procedure 1 is used. This difference in effect of the Tg is probably a consequence of the different final pyrolysis temperatures used in both procedures. The Tp is variable for procedure 1 experiments and constant under procedure 2 (900 °C). Since all the procedure 2 experiments have undergone the same pyrolysis treatment, the ABLC, which is subsequently gasified in the various runs, must have very similar properties. Therefore, the surface area, the amount of catalyst and the catalyst/carbon ratio of the

Table 2. Solid Conversion Values at Which the Maximum Gasification Rate Is Reached for Runs Performed According to both Experimental Procedures at Different Tg [CO] ) 5%; [CO2] ) 95% (vol)

[CO] ) 17%; [CO2] ) 83% (vol)

Tg (°C) Xgmax average Xgmax SD coeff of variation, %

750 0.083

800 0.37 0.46 0.43 93

Experimental Procedure 1 850 750 800 0.93 0.18 0.32 0.38 0.23 61

Tg (°C) Xg average Xg SD coeff of variation, %

750 0.83

800 0.97 0.92 0.076 8

Experimental Procedure 2 850 750 800 0.95 0.81 0.93 0.88 0.061 7

[CO] ) 29%; [CO2] ) 71% (vol) 850 0.63

750 0.26

800 0.22 0.45 0.37 82

850 0.88

850 0.89

750 0.82

800 0.96 0.90 0.070 8

850 0.91

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Figure 3. Gasification rate vs solid conversion at Tg ) 750 °C for both experimental procedures.



ABLC before the gasification step are likely to be similar for all these runs. This fact would explain why the catalyst saturation level is reached at a similar conversion value in the procedure 2 experiments. However, the procedure 1 experiments with their different gasification temperatures have withstood different final pyrolysis temperatures; therefore, the ABLC shows differences that could affect the Xgmax value. According to the results obtained and taking into account that the main difference between both procedures is the Tp, it would appear that Tp that has a greater influence on Xgmax rather than Tg. Influence of Tp on Xgmax. Since the difference between both procedures is the Tp, a comparison be-

tween experiments performed at the same Tg allows us to study the effect of Tp on the gasification rate. Figures 3-5 show a comparison between both procedures at different Tg and different [CO] and [CO2]. As can be observed in these figures, the solid conversion at which the maximum rate is reached is higher for the experiments that follow procedure 2. However, this difference decreases when the Tg increase. The Xgmax value under both procedures is similar for the experiments carried out at Tg ) 850 °C. Therefore, since the procedure 2 runs have a higher Tp (900 °C), the results obtained seem to indicate that the higher Tp, the higher the Xgmax value. As has previously been reported, during pyrolysis of ABL a reduction of some alkali-surface compounds


Figure 6. rWg vs Xg as function of [CO]: Tg ) 800 °C, [CO2] ) 20 vol %.

Figure 7. rWg vs Xg as function of [CO2]: Tg ) 800 °C, [CO] ) 4 vol %.

(which will be the catalytic sites in the gasification step) with the carbon present in the carbonaceous matrix takes place according to the following reactions:18,19

The tendency observed for the other Tg values studied was similar. The maximum gasification rate was obtained at similar Xg values for different [CO]. However, the increase of rWg with Xg is slower when the [CO] increases. The gasification rate is slower in the presence of CO so that the carbon loss is slower at higher [CO] during the gasification step. Consequently, the catalyst/ carbon ratio will increase more slowly at higher [CO], and the slope of rWg versus Xg will be lower at higher [CO] before reaching Xgmax (see Figure 6). Influence of [CO2] on Xgmax. Figure 7 shows the variation of the gasification rate versus Xg at Tg ) 800 °C and [CO] ) 4 vol % for different [CO2] values. As can be seen, the [CO2] hardly influences the value of Xgmax. However, the higher the [CO2], the faster is the increase of rWg with Xg. These [CO2] effects were also observed at other Tg and [CO] values. The gasification rate increases with an increase in [CO2]. The catalyst/ carbon ratio therefore increases faster at higher [CO2] values during the gasification step, and as a consequence, the slope of the rWg versus Xg plot is steeper before reaching Xgmax. Variation of ABLC Gasification Rate with the Temperature, Tg. The temperature dependence of the ABLC gasification rate was studied at different [CO] and [CO2] values for both procedures. As an example, Arrhenius plots for the gasification rate at different conversion values over the temperature range 750-850 °C for 2 runs performed at [CO] ) 5% and [CO2] ) 95% following both procedures are shown in Figure 8. The plots for other [CO] and [CO2] values are similar. The linear coefficient regressions obtained are between 0.92 and 0.99 for all the runs carried out. The apparent activation energies (E) obtained for the different experiments are listed in Table 3. As can be seen, the E values are in general higher for the procedure 1 experiments. The activation energy should remain constant with the conversion, if the reaction mechanism does not change. However, an observation of the results shows that the variation coefficients of E with Xg for the procedure 1 experiments are too high. These coefficients are smaller for procedure 2. The high values in the case of procedure 1 seem to indicate a certain variation of E with Xg. It has been observed in the previous section that the Xgmax values vary significantly with the Tg for the procedure 1 experiments. Before reaching Xgmax, the gasification rate evolution can depend on the number of catalyst sites and then on the surface area. Therefore, the

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

(3)

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

(4)

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

(5)

where M represents the alkali metals, Na or K. The loss of catalyst by evaporation (reaction 5) is an endothermic reaction that is favored at high temperatures. Therefore, reaction 5 takes place more extensively at a high Tp. At higher Tp, this catalyst loss is likely to be higher than the carbon loss as a consequence of the previous reduction reactions during the pyrolysis step. These reactions can provoke a decrease in the catalyst/ carbon ratio at the beginning of the gasification following which there is an increase in the Xgmax value because the catalyst saturation level will be reached later. Before reaching the maximum gasification rate, the increase of rWg with Xg is faster for the procedure 1 experiments. The final pyrolysis temperature is lower for procedure 1 than for procedure 2 with the same gasification temperature. The catalyst/carbon ratio will therefore be higher at the beginning of the gasification step for the runs under procedure 1. As a consequence, the gasification rate is likely to be faster for procedure 1 than for 2 for the same Tg. Thus, the carbon amount will also decrease faster and the catalyst/carbon ratio will increase faster under procedure 1 until saturation level is reached. This could explain why the slopes of the plot of rWg versus Xg are higher for procedure 1 than for procedure 2 before reaching Xgmax, as can be observed in Figures 3-5. However, after Xgmax, the decrease is slower for the procedure 1 runs. This trend is less noticeable when the Tg increases. At Tg ) 850 °C, the slopes for experiments performed under both procedures are quite similar, because here the Tp for both experimental procedures are closer. Therefore, the increase of the rWg with the solid conversion is higher, and the decrease is lower for lower Tp. Influence of [CO] on Xgmax. To study the effect of [CO] on the variation of rWg with the solid conversion, experiments with different [CO] and the same [CO2] were performed following procedure 2. Figure 6 shows, as an example, the variation of rWg versus Xg as a function of [CO] at Tg ) 800 °C and [CO2] ) 20 vol %.

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Figure 8. Arrhenius plot of ABLC gasification rate. Table 3. Apparent Activation Energies and Pre-exponential Factors for ABLC Gasification Runs pre-exponencial factors (min-1)

apparent activation energy (kJ/mol)

[CO] ) 5%, [CO2] ) 95%

Xg

procedure 1

procedure 2

procedure 1

procedure 2

0.05 0.25 0.50 0.75

143.7 236.5 340.8 332.7 324.1 85.4 37

179.9 192.6 158.3 147.2 169.5 20.5 12

2.2e5 1.4e9 4.7e15 1.9e15 1.7e1 2.2e15 135

3.8e7 3.7e8 1.4e7 7.4e6 1.1e8 1.78e8 164

average SD coeff of variation (%)

reaction mechanism controlling step could change with the solid conversion value if the conversion range studied includes Xgmax. Since the conversion range used in order to evaluate the apparent activation energy was 0.05-0.75, Xgmax is included in this range for the procedure 1 experiments. This could explain the variation of the E values with the solid conversion. However, Xgmax is not included in the conversion range studied for the procedure 2 experiments; therefore, the variation of E with the solid conversion is lower in this case. This fact could also explain the higher E values for the procedure 1 experiments for conversions higher than Xgmax, in that case, the gasification rate could more depend on surface area than on catalyst sites provoking an increase in the apparent activation energy. From the results obtained it could be said that the apparent activation energy for ABLC gasification is 234 ( 85 kJ/mol for the procedure 1 experiments and 170 ( 21 kJ/mol for procedure 2 as long as the [CO] is lower than 20% in volume. If the [CO] is higher than 20%, the E value increases noticeably. An average E value of 365 ( 30 kJ/mol is obtained for experiments performed according to procedure 2 over the temperature range of 750-850 °C at [CO] ) 30% and [CO2] ) 20%. This is in accordance with the results obtained in part 1 of this work, which shows that CO2 gasification of ABLC can be well-described by a Langmuir-Hinshelwood equation as long as the [CO] is lower than 20%. The values of apparent activation energy for KBLC gasification obtained from the literature are similar to the values obtained in this study. Li and Van Heiningen2 obtained an average E value of 187 kJ/mol for KBLC gasified according to procedure 1 over the temperature range of 700-775 °C at [CO] ) 10% and [CO2] ) 20 vol %.2 These authors obtained an average E value of 205 kJ/mol for procedure 2 type experiments over the temperature range of 700-800 °C at [CO] ) 5% and [CO2] ) 20 vol %.3 The E values obtained for ABLC

Table 4. CO2 Gasification Rates for ABLC, KBLC, and Impregnated Coal Char ABLC (750 °C) CO ) 4%, CO2 ) 20% (vol) KBLC (725 °C) CO ) 5%, CO2 ) 20% (vol)

impregnated coal char (Xg ) 0.25, 700 °C)

Xg

-rWg (min-1)

0.25 0.50 0.75 0.25 0.50 0.75

0.17 0.34 0.60 0.13 0.19 0.22

% of metal

-rWg (min-1)

5% Na2CO3 10% Na2CO3 20% Na2CO3

4.4E-4 2.0E-3 2.0E-3

gasification were also close to the E values obtained for CO2 gasification of both catalyzed and uncatalyzed impregnated coals.20 The apparent activation energy is similar for impregnated carbon, KBLC, and ABLC CO2 gasification, suggesting that the mechanism is similar for all three processes. The gasification rates obtained for ABLC in this study have been compared to rates from the literature for KBLC3 at 725 °C and Na2CO3-impregnated carbon21 at 700 °C and 20% CO2 and 5% CO. It can be observed in Table 4 that ABLC shows higher gasification rates than the impregnated carbon (up to 300 times higher) but similar to KBLC. A general comparison between BLC and impregnated alkali coal chars shows two main differences: a major dispersion of catalyst on BLC and a much higher surface area for impregnated coal chars.3 The higher gasification rates of BLC than of impregnated coal chars could be explained by the dispersion of the catalyst in the carbonaceous matrix in the former case. The catalyst is chemically bound with the organic material in the precursor of BLC, but is impregnated on the internal carbon surface in the coal chars3. This fact explains the major catalyst distribution for BLC. The sodium and


than for impregnated coal chars and similar to the rate for KBLC. The fine dispersion of the catalyst in the carbonaceous matrix of BL could explain its higher gasification rate despite its lower surface area. Acknowledgment The authors express their gratitude to the Fullbright Commission (99017) for providing frame support for this work. Nomenclature

Figure 9. (a, top left) EDS potassium mapping of ABLC surface. (b, top right) background. (c, bottom left) EDS silica mapping of ABLC surface. (d, bottom right) EDS sodium mapping of ABLC surface.

potassium distribution in ABLC was determined by the electronic dispersion scanning (EDS) technique and can be seen in Figure 9. However, the BET surface area is much higher for impregnated coal chars: 1000 m2/g for the coal chars,3 160 m2/g for KBLC,3 and only 20 m2/g for ABLC.11 These data seem to indicate that the effect of the major dispersion of alkali metals is more important for the gasification kinetics than the effect of a higher surface area. Conclusions The results obtained in this work confirm that ABLC gasification can be considered as an alkali-metal catalyzed gasification of a carbonaceous material. As occurs in the case of CO2 gasification of alkali impregnated coal chars, the CO2 gasification rate of ABLC increases with the solid conversion up to a maximum value and subsequently decreases. The maximum gasification rate is attained at different solid conversions depending on the operating conditions. The final pyrolysis temperature is the variable that has the greatest influence on the solid conversion. When the Tp increases, the solid conversion at which the maximum rate is reached is higher. More severe pyrolysis conditions result in fewer catalyst sites available for gasification; thus, the saturation level is attained at higher solid conversion. In addition, the variation of the gasification rate with the solid conversion before reaching the point of catalyst saturation of the surface is higher at lower Tp. [CO2], [CO], and the gasification temperature hardly influence the value of the solid conversion at which the maximum rate is attained. However, the variation of the rate with the solid conversion is higher when [CO2] and Tg increase and [CO] decreases. The average apparent activation energy obtained for ABLC gasification is over the range of 170-234 kJ/mol. The apparent activation energy values obtained from the literature for KBLC and alkali impregnated coal chars are similar to that obtained for ABLC gasification. However, the gasification rate is much higher for ABLC

[CO] ) CO concentration added at 500°C and maintained up to the end of the gasification experiment, % vol [CO2] ) CO2 concentration, % vol -rWg ) instantaneous gasification rate of alkaline black liquor char per unit mass, min-1 tg ) time from the introduction of CO2 into the system, min Tg ) gasification temperature, °C 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, dimensionless Xgmax ) alkaline black liquor char conversion at which the maximum gasification rate is reached, dimensionless

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Received for review December 21, 2004 Revised manuscript received April 1, 2005 Accepted June 7, 2005 IE048772H

Gasification of Alkaline Black Liquor from Wheat Straw. 2. Evolution of

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