Catalytic effects of NaOH and Na2CO3 additives on alkali lignin pyrolysis and gasification

Applied Energy 95 (2012) 22–30

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Catalytic effects of NaOH and Na2CO3 additives on alkali lignin pyrolysis and gasification Da-liang Guo a, Shu-bin Wu a,⇑, Bei Liu a, Xiu-li Yin b, Qing Yang b a b

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, PR China The Renewable Energy and Gas Hydrate Key Laboratory of CAS, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China

a r t i c l e

i n f o

Article history: Received 23 October 2011 Received in revised form 12 January 2012 Accepted 15 January 2012 Available online 17 March 2012 Keywords: Lignin Black liquor Pyrolysis and gasification NaOH and Na2CO3 TG-FTIR

a b s t r a c t The interest in utilization of alkali lignin in black liquor by pyrolysis and gasification is increasing due to the wish to produce bio-fuel and bio-chemical. Besides alkali lignin, the sodium salts are also the basic component of black liquor; they exist in two main forms: as phenolic sodium (ACONa) and carboxylate sodium (ACOONa) groups forming a part of alkali lignin or as dissolved salts (NaOH and Na2CO3). In this paper, the influences of these dissolved salts on the pyrolysis and gasification characteristics of alkali lignin were discussed. Five lignin samples, including pure acid precipitated lignin, 10% and 60% NaOHloaded lignin, 10% and 60% Na2CO3-loaded lignin, and black liquor solids were selected as the testing samples. Following experimental research on the evolution patterns of volatile products were carried out on a thermogravimetric analyzer coupled with Fourier transform infrared spectrometry. The experimental data indicated that the pyrolysis and gasification reaction of alkali lignin could be catalyzed by NaOH and Na2CO3. In the pyrolysis stage, the maximum mass loss rate decreased with increasing amount of NaOH and Na2CO3 additives, while in the gasification stage it increased. In the gasification stage, the temperature of the maximum mass loss rate shifted to lower value with increasing amount of NaOH and Na2CO3 additives, but did not significantly change in the pyrolysis stage. FTIR analysis showed that the influences of NaOH and Na2CO3 additives on the pyrolysis and gasification products mainly varied in amounts but not in species. FTIR results also suggested that the release time of the volatile was affected by increasing NaOH and Na2CO3 additives amount. Moreover, NaOH and Na2CO3 markedly improved the evolution of CO in the gasification stage. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction In recent years more and more biomass-containing wastes such as agricultural and industrial wastes have begun to supplement future energy needs in place of fossil fuel [1–9]. One of the potential conversion methods for such wastes is via pyrolysis and gasification processes [1]. Black liquor (BL), a major waste from chemical pulp and paper production, contains, on a dry basis, about 40% of inorganic compounds and 60% of organic compounds [1,3]. The organic compounds are composed mainly of degraded lignin (alkali lignin), and the inorganic compounds are mostly recyclable pulping chemicals (alkali salts) [4,5]. In a conventional mill powerhouse, black liquor is fired in a recovery boiler to produce combustion heat for electrical generation, and more importantly to recover pulping chemicals [6,7]. In the recent 20 years, the pyrolysis and gasification of black liquor has attracted great interest and been extensively studied [1–7,10–16]. Furthermore, it is considered as an alternative

⇑ Corresponding author. Tel./fax: +86 02022236808. E-mail address: [email protected] (S.-b. Wu). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2012.01.042

technology to replace conventional recovery boiler and convert this waste into more valuable fuel products [7,10]. In the soda pulping process, sodium hydroxide is used as the cooking chemical to dissolve lignin. Chemical action and solubilization make Na+ connect with phenolic hydroxyl groups (AOH) and carboxyl acid groups (ACOOH) of lignin by chemical bonds, forming phenolic sodium (ACONa) and carboxylate sodium (ACOONa) groups (PCSG) [17–19]. So, the sodium salts in black liquor exist in two main forms: as organic bound Na salts (PCSG) and as soluble salts (NaOH and Na2CO3). While the former form of sodium salts can be considered as a part of the alkali lignin, the latter form of sodium salts exist as ionic species associated with the water. Upon drying, the soluble salts would tend to be more concentrated in the abundant pore structures of alkali lignin, weakly associated with the internal pore surface [4,20]. Thus, the sodium salts species exist in black liquor in different chemical and physical forms. During the past decade, a number of studies have illustrated the strong effects of different factors such as the reaction temperature [11,12], pressure [13,14], and pretreatment [15] on the technical performance. According to the previous works [17,21], the high

D.-l. Guo et al. / Applied Energy 95 (2012) 22–30

23

Nomenclature T 0i;i¼1;2

temperature ranges of un-catalyzed pyrolysis and gasification Ti,i = 1,2,3,4 temperature ranges of catalyzed pyrolysis and gasification T 0i max;i¼1;2 temperature at maximum mass loss rate in un-catalyzed pyrolysis and gasification Timaxv,i = 1,2,3,4 temperature at maximum mass loss rate in catalyzed pyrolysis and gasification

content of sodium salts in black liquor has a significant effect on the pyrolysis and gasification reaction of alkali lignin. Unfortunately, the influence mechanisms of both the dissolved salts and PCSG on the pyrolysis and gasification characteristic of alkali lignin are not clearly understood. In our previous research [19], we have investigated the effect of organic bound Na groups (PCSG) on pyrolysis and gasification behaviors of alkali lignin. In particular, we have found that the pyrolysis and gasification characteristic of alkali lignin depended on PCSG. Similarly, Raveendran et al. [22] proposed the alkali and alkaline earth metallic species have a strong influence on both the pyrolysis characteristics and the product distribution. Teng et al. [23] suggested that de-ashing increases the volatile yield, initial decomposition temperature and rate of pyrolysis. Jensen et al. [24] concluded that the existence of inorganic salts influences the property of volatiles, decreases the yield of tar and promotes the formation of char. Based on these previous studies, we believe that the dissolved salts in black liquor are some of the important parameters to evaluate the pyrolysis and gasification characteristic of alkali lignin. Furthermore, fundamental pyrolysis and CO2-gasification characteristic of alkali lignin with and without sodium salts catalysis have so far not been studied. The objective of the present study was to investigate if NaOH and Na2CO3 as soluble Na salts would have catalytic effects on the pyrolysis and gasification reaction of alkali lignin. In addition, basic data on the pyrolysis and CO2-gasification of alkali lignin and black liquor solid (BLS) were obtained. A thermogravimetric analyzer coupled with Fourier transform infrared spectrometry (TG-FTIR) was used in this study. TGA was adopted to keep a detailed recording of the mass loss data, and FTIR was used to predict the product evolution pattern and yield. 2. Materials and methods 2.1. Materials preparation The black liquor sample, with solids content of 73.2%, was obtained from a soda–anthraquinone (AQ) wheat straw pulp mill in Shandong Province, China. Samples were filtered through a 200mesh screen to remove any suspended matter. Pure acid precipitated lignin (APL) sample, without NaOH and Na2CO3, was separated from the black liquor by acid precipitation with 1 N H2SO4 to pH 2.5. The acid precipitated solids were filtered from the solution, washed thoroughly with distilled water, and dried by freeze-drying. Subsequently, the dried solids were dissolved in 1,4-dioxane, filtered to remove any inorganic impurities, and reclaimed by rotary evaporation of solvent under vacuum and dried. Finally, the absolutely dry APL was used for this research without further purification. According to previous researches [25–27], the acid precipitation process of alkali lignin is relevant to precipitation pH. As the pH value decreases, the organic bound Na on lignin structure will be gradually replaced by hydrogen ions; as the pH value increases, the hydrogen on lignin structure will be also replaced by sodium

Dm0i;i¼1;2 mass loss in un-catalyzed pyrolysis and gasification Dmi,i = 1,2,3,4 mass loss in catalyzed pyrolysis and gasification ðdm=dt 0i;i¼1;2 Þ=ðm0  mf Þ reactivity in un-catalyzed pyrolysis and gasification ðdm=dt 0i;i¼1;2;3;4 Þ=ðm0  mf Þ reactivity in catalyzed pyrolysis and gasification

ions. Based on this principle, 10% NaOH- and 10% Na2CO3-loaded APL samples were prepared by impregnating method [19,28,29]. A known amount of APL solid was first added into two beakers. 0.1 mol/L NaOH and Na2CO3 solution were then added into the beakers, respectively, until the APL samples just dissolved; the loading amount of NaOH and Na2CO3 was 10% at this time. In order to avoid loading excessive NaOH and Na2CO3, the process of addition needed to be as gradual and slow as possible. 60% NaOH- and 60% Na2CO3loaded APL samples were also prepared by the same method; the only difference is that, the adding amount of 0.1 mol/L NaOH and Na2CO3 solution were 60%. Finally, the lignin solution was dried in an oven at 105 °C, and then the NaOH- and Na2CO3-loaded APL sample were obtained. Black liquor solids (BLSs) were prepared by totally drying black liquor in an air dry oven at 105 °C. Afterward, it was ground and sieved to less than 200 lm in particle size for the runs performed in this work. 2.2. Experimental methods The elemental analysis of the samples was performed at an elemental analyzer (Perkin-II CNHS/O2400 Elmer) and an inductively coupled plasma atomic emission spectrometer (Vario-I elementar Germany). The proximate analysis was carried out using a thermo-gravimetric analyzer [1]. Gross calorific values of the samples were determined by an automatic bomb calorimeter. The results of ultimate and proximate analysis of the samples are shown in Table 1. From Table 1, the marked difference of constituents between APL and other samples were the contents of ash and Na. Fundamental tests were carried out on a Jupiter Thermo Gravimetric Analyzer STA 449 F3, coupled with a thermo electron corporation Fourier Transformation Infrared Spectrometer TENSOR 27 by using a pipe. The experiments were done on TGA at a heating rate of 20 °C/min within the temperature range from 50 to 1000 °C. High purity nitrogen was used as carrier gas with a flow rate of 20 mL/min, and carbon dioxide was used as gasifying agent with a flow rate of 40 mL/min throughout the pyrolysis and gasification process. The sample weight was required to be less than 10 mg. The volatile released from pyrolysis and gasification would be swept into a Fourier Transform Infrared Spectrometer gas cell quickly by pure nitrogen. Moreover, the FTIR gas cell and the pipe were already preheated to 150 °C before each experiment. The spectrum scope was located in the range 4000–667 cm1 and the resolution factor was set at 4 cm1. 3. Results and discussion 3.1. Determine the time ranges of pyrolysis and gasification reaction Identification of pyrolysis and gasification reactions was mainly based on the DTG curve and the first order derivative of DTG curve

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D.-l. Guo et al. / Applied Energy 95 (2012) 22–30

Table 1 Utimate and proximate analysis of the materials. Materials

APL BLS 10% 10% 60% 60% a

Proximate analysis (dry basis) (wt%)

NaOH-loaded APL Na2CO3-loaded APL NaOH-loaded APL Na2CO3-loaded APL

Ultimate analysis (wt%)

Ash

Volatiles

C

H

O

5.95 42.47 21.93 21.80 40.25 40.37

78.83 50.14 72.48 72.65 52.45 52.46

56.01 32.46 40.47 40.58 34.58 34.63

8.54 6.41 4.31 4.32 3.21 3.30

35.34 60.99 55.15 55.03 62.17 62.03

T'1

QHHV (MJ/kg)

– 8.11  104 0.72  104 0.72  104 6.12  104 6.12  104

22.87 11.32 18.77 18.83 12.35 12.46

N 0.08 0.14 0.07 0.07 0.04 0.04

DTG (%/s)

pyrolysis and gasification. As shown in Fig. 1b, the appreciable difference between catalyzed and un-catalyzed pyrolysis was the number of the pyrolysis stage. There were three DTG peaks during BLS pyrolysis process, however, only one DTG peaks can be found during APL pyrolysis. It indicated that BLS pyrolysis underwent three different weight losses, whereas APL pyrolysis underwent only one weight loss. The pyrolysis and gasification reaction regions of APL, BLS and other samples are shown in Table 2. From Table 2, adding 10% NaOH and Na2CO3 to pure APL, the start temperature of gasification decreased from 818 °C to 642 and 643 °C, respectively, especially this value decreased to 617 and 616 °C as adding 60% NaOH and Na2CO3 to APL. This phenomenon may be indicated that the start temperature of gasification reaction was decided by the adding quantities of NaOH and Na2CO3.

T'2

-0.05 -0.10

D(DTG)

0.0004 0.0000 -0.0004 0

500

1000

1500

2000

2500

3000

3500

4000

3.2. TG and DTG analysis

Time (s) Fig. 1a. Determine the time-limit of each stage during APL pyrolysis and gasification.

T1

0.00

DTG (%/s)

Na (ppm)

Oxygen content was determined by difference.

0.00

T2

T4

T3

-0.03 -0.06 0.0002

D (DTG)

a

0.0000

-0.0002 0

500

1000

1500

2000

2500

3000

3500

4000

Time (s) Fig. 1b. Determine the time-limit of each stage during BLS pyrolysis and gasification.

(D(DTG)). As shown in Fig. 1a, as soon as the APL pyrolysis reaction occurred, the DTG curve varied from zero to y direction along y-axis, reached the maximum, and then moved from the maximum value to zero. In mathematics, the D(DTG) value contained three zero values in above process. The first and the third zero values of D(DTG) corresponded to the start and end of the APL pyrolysis reaction. The second zero value of D(DTG) due to the maximum value of DTG. As shown in Fig. 1a, the temperature range of APL pyrolysis was marked as T 01 , and the temperature range of APL gasification was marked as T 02 . For NaOH and Na2CO3 catalytic reactions, the same method was adopted to distinguish between the pyrolysis stage and the gasification stage. Fig. 1b presented the time-limit of each stage during BLS

The TG and DTG curves of APL pyrolysis and CO2-gasification with NaOH and Na2CO3 additives are shown in Figs. 2 and 3. For comparison, the TG and DTG curves of pure APL and BLS are also presented in Figs. 2 and 3. As shown in Figs. 2 and 3, pure APL pyrolysis and CO2-gasification was characterized by two weight loss stages between 112 and 1000 °C (Table 2). The first stage between 112 and 688 °C was responsible for the pyrolysis of APL, accounting for 54.79% weight loss with a maximum mass loss rate at 382 °C (Table 3). The second stage from 818 to 1000 °C was the main gasification stage, accounting for a weight loss of 30.32% with a maximum rate of thermal degradation at 1000 °C. However, BLS, 10% NaOH and 10% Na2CO3 catalyzed pyrolysis and gasification exhibited four distinct weight loss stages: the first weight loss process was due to the release of water with the maximum mass loss rate at 135, 165 and 157 °C, respectively (Table 3). The second and third stages could be ascribed to the catalyzed pyrolysis processes. The fourth stage occurred at higher temperatures (900–1000 °C) was the main CO2 gasification stage. For 60% NaOH-loaded and 60% Na2CO3-loaded APL, the pyrolysis and gasification process was characterized by three weight loss stages. From Table 2, it could be found that the end temperatures of this two samples pyrolysis (616 and 615 °C) were lower than the end temperatures of 10% NaOH and 10% Na2CO3 catalyzed pyrolysis (641 and 642 °C). The main reason was quite likely the higher ash content of 60% NaOH-loaded and 60% Na2CO3-loaded APL. In this paper, in order to eliminate the effect of additives content on the weight fraction of volatile matter in raw sample, dm/dt was replaced by 1/(m0  mf) (dm/dt) to compare the reactivity among different samples. For clear description, a summary of pyrolysis and CO2 gasification parameters are given in Table 3. Table 3 shows that the absolute value of ðdm=dt01 Þ=ðm0  mf Þ was higher than the absolute value of (dm/dti,i = 1,2,3)/(m0  mf), however, the absolute value of ðdm=dt 02 Þ=ðm0  mf Þ was lower than the absolute value of (dm/dt4)/(m0  mf). It meant that pure APL had the highest pyrolysis reactivity but had the lowest gasification reactivity. Meanwhile, according to the value of T 02 max was higher than

25

D.-l. Guo et al. / Applied Energy 95 (2012) 22–30 Table 2 The reaction regions of pyrolysis and gasification. Experiments

Pyrolysis T 01

APL 10% 10% 60% 60% BLS

Gasification

(°C)

112–688 – – – – –

NaOH-loaded APL Na2CO3-loaded APL NaOH-loaded APL Na2CO3-loaded APL

T1 (°C)

T2 (°C)

T3 (°C)

T 02 (°C)

T4 (°C)

– 103–214 94–201 92–211 96–211 94–191

– 215–435 202–460 212–616 212–615 192–414

– 435–641 461–642 – – 415–612

818–1000 – – – – –

– 642–1000 643–1000 617–1000 616–1000 613–1000

Heating Curve

100

1000

800

60

APL BLS

600

)

10% NaOH + APL 60% NaOH + APL 60% Na2CO3 +APL

40

400

20

200

0

Temperature (

Mass (%)

10% Na2CO3 +APL

80

0 0

1000

2000

3000

4000

5000

gasification reactivity increased with increasing the amounts of NaOH and Na2CO3. The absence of (dm/dt3)/(m0  mf) during 60% NaOH and 60% Na2CO3 additives pyrolysis presumably due to the increased amount of additives; the reduced T4max was mainly attributed to the catalysis of NaOH and Na2CO3 [19,30]. For BLS, containing quantity of NaOH and Na2CO3 were between 10% and 60%, the absolute value of (dm/dt4)/(m0  mf) and T4max also accorded with above tendency. In summary, we proposed that, by increasing the total NaOH and Na2CO3 additives in gasification reactions, the temperature of the maximum mass loss rate may be decreased due to an increase in the number of the catalytic surface groups [18]. Also, increased NaOH and Na2CO3 additives amount in the system may result in enhanced gasification reactivity of alkali lignin.

6000

Time (s)

3.3. Analysis the composition of the volatile products

Fig. 2. Influence of NaOH and Na2CO3 additives on TG curves of lignin samples and BLS.

the value of T4max, It can be speculated that the presence of NaOH and Na2CO3 moved the max mass loss rate to lower temperature side in the gasification stage. As shown in Table 3, the loading quantities of NaOH and Na2CO3 also had a significant effect on (dm/dt4)/ (m0  mf) and T4max. An important observation of this study was that the absolute value of (dm/dt4)/(m0  mf) and T4max both decreased with increasing amount of NaOH and Na2CO3 additives (Table 3). In the case of pure APL gasification, the absolute value of ðdm=dt 01 Þ=ðm0  mf Þ was 2.74E4; when 10% NaOH- and 10% Na2CO3- loaded APL gasification, the absolute values of the former was 8.37E4, the latter was 1.03E3; while in the case of that with 60% NaOH and 60% Na2CO3 additives, these values increased to 1.51E3 and 1.42E3, respectively. It indicated that the catalyzed

The evolved volatiles from TGA were real-timely swept into the gas cell. Then the information of absorbance at various wave numbers and times could be obtained by the 3D infrared spectrum. When a time was fixed, the absorbance information at various wave numbers could be obtained to analyze the composition of the gas. When a wave number was fixed, the absorbance information at various times could be obtained to analyze a given composition as a function of time. According to the concentration of a volatile component was in proportion to its absorbance [31], the variation of absorption intensity in the whole process can reflect the tendency of product yields of the gas species. Typical 3D infrared spectra from pyrolysis and gasification of 60% Na2CO3-loaded APL is presented in Fig. 4. As shown in Fig. 4, it could be observed that water, methane, hydrocarbons, alcohols, phenols, aldehydes, and ketones (C@O) were released out as vola-

(a)

(b) 0.00

DTG (%/s)

DTG (%/s)

0.00

-0.05

-0.05

60% Na2CO3 + APL

10% Na2CO3 + APL -0.10

-0.10

10% NaOH + APL APL BLS 0

1000

2000

3000

Time (s)

4000

5000

6000

60% NaOH + APL APL BLS 0

1000

2000

3000

Time (s)

Fig. 3. Influence of NaOH and Na2CO3 additives on TG curves of lignin samples and BLS.

4000

5000

6000

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D.-l. Guo et al. / Applied Energy 95 (2012) 22–30

Table 3 Characteristic parameters of pyrolysis and gasification. Un-catalytic

APL

Catalytic

10% NaOH loaded APL

10% Na2CO3 loaded APL

60% NaOH loaded APL

60% Na2CO3 loaded APL

BLS

ðdm=dt01 Þ=ðm0  mf Þ

1.28E3

(dm/dt1)/(m0  mf) (dm/dt2)/(m0  mf) (dm/dt3)/(m0  mf)

1.10E4 8.60E4 1.28E3

8.36E5 9.63E4 9.25E4

2.58E4 8.65E4 –

2.39E4 7.35E4 –

2.58E4 8.70E4 5.48E4

ðdm=dt02 Þ=ðm0  mf Þ

2.78E4

(dm/dt4)/(m0  mf)

8.37E4

1.03E3

1.51E3

1.42E3

1.08E3

T 01 max

382 °C

T1max T2max T3max

165 °C 350 °C 492 °C

157 °C 341 °C 522 °C

115 °C 365 °C –

115 °C 354 °C –

135 °C 308 °C 464 °C

(°C)

T 02 max (°C)

1000 °C

T4max

935 °C

909 °C

797 °C

828 °C

908 °C

Dm01 (%)

54.79

Dm1 (%) Dm2 (%) Dm3 (%)

2.48 24.00 26.16

2.77 27.05 12.10

3.43 31.00 –

4.32 32.21 –

3.45 21.73 9.82

Dm02 (%)

24.16

Dm4 (%)

32.95

38.78

24.27

25.74

28.66

Table 4 The main products of pyrolysis and gasification by 3D infrared spectra.

Fig. 4. Typical 3D infrared spectra from 60% Na2CO3-loaded APL pyrolysis and gasification.

tiles between 500 s and 2000 s [31–38]. The release of these products mainly occurred at low temperatures, corresponding to the main pyrolysis temperature zone in the TG curve (Fig. 2). However, after 2000 s, carbon monoxide was the main gas product evolving from BLS, NaOH and Na2CO3 catalyzed gasification. The typical functional groups and the IR signal with the possible compounds are listed in Table 4. 3.4. Influence of NaOH and Na2CO3 additives on the species of volatile products Based on the maximum mass loss rate of DTG curves, three levels of temperature were selected to compare the IR spectra of pure APL pyrolysis with those of NaOH and Na2CO3 catalyzed pyrolysis and gasification (Fig. 5). For example, the spectra of volatiles released at 135, 308, and 908 °C for BLS are shown in Fig. 5(a1), (a2), and (a3), respectively. As shown in Fig. 5(a1)–(f1), a conclusion could be drawn that the weight loss in the initial pyrolysis stage was mainly caused by the release of H2O. For pure APL pyrolysis, the generation of H2O was not conspicuous (Fig. 5(a1)), while this was obvious for BLS, NaOH and Na2CO3 catalyzed pyrolysis (Fig. 5(b1)–(f1)). From Table 3, the weight losses of BLS, 10% NaOH- and Na2CO3-loaded APL, 60% NaOH- and Na2CO3-loaded APL in the initial pyrolysis were 3.45%, 2.48%, 2.77%, 3.43%, 4.32%, respectively. According to previous researches [32,39], the generated H2O at low temperature is released out by the cracked of aliphatic hydroxyl groups in the lateral chains. This elucidated that the presence of NaOH and

Wavenumber (cm1)

Functional group

Vibration

Evolution Product

References

3559–3964 1275–1775 2058–2131 2150–2212 3024 2850–3200 2775–3115 1300–1400 1495–1525

OAH HAOAH CAO CAO AOCH3 CAH CAH OAH

Stretching Bending Stretching Stretching

H2O

[23,24]

CO

[25,26]

CH4

[24,29,37]

Hydrocarbons Phenols

[14,24,29] [24,28,29]

960–1131 3559–3964 1145–1211

CAO OAH CAC

Stretching Stretching Stretching

Alcohols

[24,37]

Aldehydes and ketones

[14,29]

1700–1740

C@O

Stretching

Stretching Stretching Stretching Skeleton

Na2CO3 stimulated the aliphatic hydroxyl groups to convert to H2O. With temperature increasing, pyrolysis reaction proceeded into the main pyrolysis stage, and then more gaseous products were released out. As shown in Fig. 5(a2)–(f2), the absorption band of H2O was still present. Besides H2O, the evolution of alcohols, phenols, aldehydes, ketones, and hydrocarbons were also responsible for the weight loss in this stage. The absorption bands of these volatiles for pure APL, BLS, and NaOH- and Na2CO3-loaded APL appeared to be at the same wave numbers, while some diversities of the absorbance existed. Therefore, it was reasonable to postulate that the catalytic effects of NaOH and Na2CO3 additives on the pyrolysis products mainly varied in amounts but not in species. Meanwhile, as shown in Tables 2 and 3, the temperature range (T2) and the weight loss (Dm2) in the main pyrolysis stage increased with increasing the NaOH and Na2CO3 amount. Specifically, it could be seen that the characteristic absorbance intensity of alcohols at 1040–1180 cm1 apparently increased with 10% NaOH and 10% Na2CO3 catalyzed pyrolysis. This observation further demonstrated the above analysis that the released H2O was indeed affected by NaOH and Na2CO3 additives during the initial pyrolysis stage. Moreover, the characteristic absorbance intensity of aldehydes and ketones first increased and then decreased with the increasing Na2CO3 additive amount, while this was decreased with the increasing NaOH additive amount. In contrast, the characteristic absorbance intensity of phenols decreased with NaOH catalyzed pyrolysis, while it was increased with and Na2CO3 catalyzed pyrolysis. In summary, NaOH-loaded and Na2CO3-loaded APL have

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D.-l. Guo et al. / Applied Energy 95 (2012) 22–30

0.02

(a1)

0.02 (b1) 0.01

(a) - APL

0.00 0.04 0.02

3000

2500 2000

1500

1000

(a2)

0.00 0.10

(b) - BLS

0.00 3500

3500

3000

2500 2000

1500

Absorbance

Absorbance

0.01

1000

0.04

0.05

3000

2500 2000

1500

1000

3000

2500 2000

1500

1000

3000

2500 2000

1500

1000

0.02 0.00 0.10

(a3)

3500

(b2)

3500

b3)

0.05

0.00

0.00 3500

0.02 (c1)

3000

2500 2000

1500

1000

3500

0.02 (d1)

(c) - 10% Na2CO3 loaded APL

(d) - 60 %Na2CO3 loaded APL

0.01

0.01

3500

3000

2500 2000

1500

1000

(c2) 0.02 0.00 0.10

3500

3000

2500 2000

1500

Absorbance

Absorbance

0.00 0.00 0.04

1000

(e1)

0.01

3000

2500 2000

1500

1500

1000

3000

2500 2000

1500

1000

3000

2500 2000

1500

1000

0.00

1000

3500

(d3)

3500

0.02

(e) - 10% NaOH loaded APL

(f1)

(f) - 10% NaOH loaded APL

0.01

0.00

0.00 3500

3000

2500 2000

1500

1000

(e2)

0.02 0.00 3500

3000

2500 2000

1500

1000

(e3)

0.05

Absorbance

Absorbance

2500 2000

0.00 3500

0.10

3000

0.05

0.00

0.04

3500

(d2)

0.02

0.10

(c3)

0.05

0.02

0.04

0.04

3500

3000

2500 2000

1500

1000

3000

2500 2000

1500

1000

3000

2500 2000

1500

1000

(f2)

0.02 0.00 0.10

3500

(f3)

0.05

0.00

0.00 3500

3000

2500 2000

1500

1000

Wavenumbers (cm-1)

3500

Wavenumbers (cm-1)

Fig. 5. FTIR spectra of volatile products at the maximum weight loss rate from lignin samples and BLS.

the same change trend on the quantity of alcohols, but different change trend on the quantity of phenols, aldehydes, and ketones. Fig. 5(a3)–(f3) compared the IR spectra at the maximum mass loss in the gasification stage. It could be seen that CO was the main gas released in this stage. Unlike pure APL gasification, which only limited CO was detected, the absorbance intensity of CO was largely enhanced in BLS, NaOH and Na2CO3 catalyzed gasification. The results were also deduced from the regular weight loss in the gasification. As shown in Table 3, the value of Dm02 (%) was 24.16%, for 10% NaOH- and Na2CO3-loaded APL, BLS, 60% NaOHand Na2CO3-loaded APL, the values of Dm4 were 32.95%, 38.78%, 28.66%, 24.27%, 25.74%%, respectively. It appeared clearly that NaOH and Na2CO3 increased the weight loss in the gasification stage. To sum up the above arguments, NaOH and Na2CO3 favored the generation of CO in the gasification stage. On the contrary, Fig. 5(a2)–(f2) showed that all the samples appeared very weak absorption intensity at 2260–1990 cm1, this meant that the pres-

ence of NaOH and Na2CO3 had an insignificant catalytic effect on the formation of CO in the pyrolysis stage. 3.5. Influence of NaOH and Na2CO3 additives on the evolution patterns of volatile products during the pyrolysis stage Fig. 6 presented the evolution profiles of the volatile products, which were constructed by selected the characteristic absorption bands and followed the intensity with time. The sample mass of APL for each run had been accurately controlled at 10 mg, thus the effect of NaOH and Na2CO3 additives on the evolution of the volatile products could be compared by the absorption intensity of the IR spectra [40,41]. According to the literatures [37,42], alcohols are mainly released with cracking of the aromatic methoxyl groups and cinnamyl alcohol-type propanoid side chains. For pure APL pyrolysis, alcohols release started at 1197 s (397 °C), reached a max at 1361 s (454 °C)

28

D.-l. Guo et al. / Applied Energy 95 (2012) 22–30

0.025

0.015

(a) Alcohols

(b) Phenols

Absorbance

Absorbance

0.020

0.015

0.010

0.010

0.005

0.005

0.000

0.000 1000

1500

1000

2000 0.08

(c) Hydrocarbons

1500

2000

(d) Aldehydes & Ketones

0.03

Absorbance

Absorbance

0.06 0.02

0.01

0.04

0.02 0.00 0.00 1000

1500

2000

2500

Time (s)

1000

1500

2000

Time (s)

Fig. 6. Volatile evolution profiles during pyrolysis stage (h: pure APL; }: BLS; : with 10% NaOH; : with 60% NaOH; : with 10% Na2CO3; : with 60% Na2CO3).

and then decreased dramatically till 1512 s (504 °C) followed by a slow continuous decreased (Fig. 6a). However, in the case of BLS, NaOH and Na2CO3 catalyzed pyrolysis, the evolution patterns of alcohols were apparently different. Alcohols release begun at 1525 s (508 °C), reached a max at 1701 s (567 °C) and finished at 1840 s (613 °C). These results implied that the addition of NaOH and Na2CO3 pushed back the time of alcohols release. As a result of the cleavage of the ether bonds between the lignin building units, followed by cracking and reforming of the alkyl side chains of these units [37], phenols were released from APL pyrolysis over a time range between 1121 s (374 °C) and 1487 s (496 °C) (Fig. 6b). However, in the presence of 10% NaOH and 10% Na2CO3, the generation time of phenols was postponed for about 370 s (123 °C); in the case of 60% NaOH and 60% Na2CO3, the delayed time was approximate 619 s (206 °C) (Fig. 6b). Therefore, we may be concluded that the release time of phenols was delayed with increasing NaOH and Na2CO3 additive amount. A possible reason could be that phenols generated by the secondary reaction between sodium phenolate and the volatile need to higher reaction temperature. With regard to hydrocarbons, it is produced mainly from the cracking of the methoxy group (AOCH3A) and the methylene group (ACH2A) [32,37]. From Fig. 6c, for pure APL pyrolysis, hydrocarbons were released in a single peak between 1134 s (378 °C) and 1676 s (559 °C). In the presence of NaOH or Na2CO3, the formation of hydrocarbons showed a double peak, the first peak between 1537 s (512 °C) and 1846 s (615 °C), was from the decomposition of methyl functional groups and the second higher temperature peak from 2000 s (667 °C) to 2420 s (807 °C), might be attributed to the secondary pyrolysis of the volatile. As shown in Fig. 6d, the absorbance intensity of aldehydes and ketones were

both reduced during NaOH and Na2CO3 catalyzed pyrolysis. In addition, the increasing amount of NaOH and Na2CO3 additives seemed to have no further effect on the absorbance intensity of aldehydes and ketones. It is well known that aldehydes and ketones are probably attributable to the CbACc cleavage in the alkyl side chains with ACH2OH or ACOOH groups in –c position [37,43]. It might be reasonable to speculate that the aliphatic ACH2ONa and ACOONa groups were difficultly removed by alkyl CAC fragmentation of phenyl-propane side chains, yielding aldehydes and ketones. 3.6. Influence of NaOH and Na2CO3 additives on the evolution patterns of volatile products during the gasification stage Unlike NaOH and Na2CO3 catalyzed pyrolysis reaction, The CO evolution in gasification stage was obviously affected by the additives. As shown in Figs. 5b and 7, a small quantity of CO was generated from pure APL, BLS, NaOH- and Na2CO3-loaded APL pyrolysis. The reason could be that NaOH and Na2CO3 additives had little effect on the cracking of ether bridges which is the main source of CO at the lower temperature range below 667 °C (2000 s) [37,43]. However, in the gasification stage, the absorbance intensity of CO was totally increased under all NaOH and Na2CO3 catalyzed gasification conditions. From Fig. 7, for 60% NaOH-loaded APL gasification, the evolution range of CO was from 2199 s (733 °C) to 2755 s (918 °C); when 10% NaOH was added to APL, CO was released from 2367 s (789 °C) to 2934 s (978 °C). Similarly, for 60% Na2CO3-loaded APL, CO formation was from 2251 s (750 °C) to 3240 s (1000 °C); when 10% Na2CO3-loaded APL gasification, CO generation was from 2338 s (779 °C) to 3070 s (1000 °C). For BLS, in the presence of NaOH

D.-l. Guo et al. / Applied Energy 95 (2012) 22–30

was relatively concentrated and intensively increased from 754 to 920 °C. Several investigated volatile species, including water, carbon monoxide, methane, hydrocarbons, alcohols, phenols, aldehydes, and ketones, were identified from 3D FTIR spectra. The FTIR analysis revealed that the effect of NaOH and Na2CO3 additives on the pyrolysis products mainly varied in amounts but not in species. Specifically, the weight loss in the initial pyrolysis stage was mainly caused by the release of H2O. The evolution of alcohols, phenols, aldehydes, ketones, and hydrocarbons were responsible for the mass loss in the main pyrolysis stage, and CO was the main gas released in the gasification stage. Moreover, The addition of NaOH and Na2CO3 pushed back the releasing time of alcohols and phenols.

Absorbance

0.08

0.06

0.04

0.02

0.00 1500

29

2000

2500

3000

3500

Acknowledgements

Time (s) Fig. 7. The evolution profiles of CO from the gasification stage (h: pure APL; }: BLS; : with 10% NaOH; : with 60% NaOH; : with 10% Na2CO3; : with 60% Na2CO3).

and Na2CO3, The generation of CO was concentrated and increased from 2262 s (754 °C) to 2760 s (920 °C). These observations were consistent with the mechanisms proposed by Sams and Shadman [44] for the gasification of organic carbon catalytic with Na2CO3 by carbon dioxide (Eqs. (1)–(5)).

Na2 CO3 ðsÞ þ CðsÞ ! ACOONa þ ACONa

ð1Þ

ACOONa þ CðsÞ ! ACONa þ COðgÞ

ð2Þ

ACONa þ CðsÞ ! ACNa þ COðgÞ

ð3Þ

ACNa þ CO2 ðgÞ ! ACONa þ COðgÞ

ð4Þ

ACONa þ CO2 ðgÞ ! ACOONa þ COðgÞ

ð5Þ

Eqs. (2) and (3) are conceived as the reduction of the catalytic sites by the organic carbon. Eqs. (4) and (5) are identified as the oxidation of the NaAC catalytic sites by CO2. The gasification reactions for BLS by Na2CO3 were best described by the combination of Eqs. (2) and (5) as follows:

ACOONa þ CðsÞ ! ACONa þ COðgÞ

ð2Þ

ACONa þ CO2 ðgÞ ! ACOONa þ COðgÞ

ð5Þ

CðsÞ þ CO2 ðgÞ ! COðsÞ

ð6Þ

According to Connolly [18] who pointed out that the reaction rate of ACONa with CO2 is far faster than that of ACOONa with organic carbon, so the Eq. (2) will be the rate determining step of the gasification of BLS. For APL, without the presence of NaOH or Na2CO3, The gasification reactions should only be represented as Eq. (6), which is highly endothermic and will not proceed until significantly high temperatures are reached [44]. Therefore, the presence of NaOH and Na2CO3 has a significant effect on CO formation between 754 and 920 °C during alkali lignin gasification. 4. Conclusions The effects of NaOH and Na2CO3 additives on the thermogravimetric characteristics and the evolutions patterns of volatile products during alkali lignin pyrolysis and gasification were studied by using TG-FTIR analysis. In the pyrolysis stage, the maximum mass loss rate decreased with increasing amounts of NaOH and Na2CO3 additives. In the gasification stage, NaOH and Na2CO3 moved the max mass loss rate to lower temperature side. The CO evolution in gasification stage was obviously affected by NaOH and Na2CO3. For pure APL gasification, a small quantity of CO was generated; for NaOH and Na2CO3 catalyzed gasification, the evolution of CO

This work was supported by the National Natural Science Foundation of China (NSFC, No. 21176095 and No. 51176195), the National High Technology Research and Development Program of China (863 Program, 2012AA101806) and the Major Research Projects of Guangdong Province, China (No. 2011A090200006).

References [1] Sricharoenchaikul V. Assessment of black liquor gasification in supercritical water. Bioresour Technol 2009;100:638–43. [2] Cardoso M, Oliveira KD, Costa G, Passos ML. Chemical process simulation for minimizing energy consumption in pulp mills. Appl Energy 2009;86:45–51. [3] Naqvi M, Yan J, Dahlquist E. Black liquor gasification integrated in pulp and paper mills: a critical review. Bioresour Technol 2010;101:8001–15. [4] Sánchez J, Gea G, Gonzalo A, Bilbao R, Arauzo J. Kinetic study of the thermal degradation of alkaline straw black liquor in nitrogen atmosphere. Chem Eng J 2004;104:1–6. [5] Pettersson K, Harvey S. CO2 emission balances for different black liquor gasification biorefinery concepts for production of electricity or secondgeneration liquid biofuels. Energy 2010;35:1101–6. [6] Eriksson H, Harvey S. Black liquor gasification–consequences for both industry and society. Energy 2004;29:581–612. [7] Naqvi M, Yan J, Fröling M. Bio-refinery system of DME or CH4 production from black liquor gasification in pulp mills. Bioresour Technol 2010;101:937–44. [8] Lou R, Wu SB. Products properties from fast pyrolysis of enzymatic/mild acidolysis lignin. Appl Energy 2011;88:316–22. [9] He J, Zhang W. Techno-economic evaluation of thermo-chemical biomass-toethanol. Appl Energy 2011;88:1224–32. [10] Andersson E, Harvey S. Comparison of pulp-mill-integrated hydrogen production from gasified black liquor with stand-alone production from gasified biomass. Energy 2007;32:399–405. [11] Demirbas A. Pyrolysis and steam gasification processes of black liquor. Energy Convers Manage 2002;43:877–84. [12] Carlsson P, Wiinikka H, Marklund M, Grönberg C, Pettersson E, Lidman M, et al. Experimental investigation of an industrial scale black liquor gasifier. 1. The effect of reactor operation parameters on product gas composition. Fuel 2010;89:4025–34. [13] Whitty K, Kullberg M, Sorvari V, Backman R, Hupa M. Influence of pressure on pyrolysis of black liquor: 2. Char yields and component release. Bioresour Technol 2008;99:671–9. [14] Whitty K, Backman R, Hupa M. Influence of pressure on pyrolysis of black liquor: 1. Swelling. Bioresour Technol 2008;99:663–70. [15] Puértolas R, Gea G, Murillo MB, Arauzo J. Pyrolysis of black liquors from alkaline pulping of straw. Influence of a preoxidation stage on the char characteristics. J Anal Appl Pyrol 2001;58–59:955–66. [16] Zhan X, Zhou ZJ, Wang F. Catalytic effect of black liquor on the gasification reactivity of petroleum coke. Appl Energy 2010;87:1710–5. [17] Kumar V. Pyrolysis and gasification of lignin and effect of alkali addition. PhD thesis, Georgia Institute of Technology, US; 2009. [18] Connolly TS. CO2 pyrolysis and gasification of kraft black liquor char. PhD thesis, The University of Maine; 2006. [19] Guo DL, Wu SB, Lou R, Yin XL, Yang Q. Effect of organic bound Na groups on pyrolysis and CO2-gasification of alkali lignin. BioResources 2011;6:4145–57. [20] Quyn DM, Wu H, Bhattacharya SP, Li CZ. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part II. Effects of chemical form and valence. Fuel 2002;81:151–8. [21] Yang H, Yan R, Chen H, Lee DH, Zheng C. Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel 2007;86:1781–8. [22] Raveendran K, Ganesh A, Khilar KC. Pyrolysis characteristics of biomass and biomass components. Fuel 1996;75:987–98.

30

D.-l. Guo et al. / Applied Energy 95 (2012) 22–30

[23] Teng H, Wei YC. Thermogravimetric studies on the kinetics of rice hull pyrolysis and the influence of water treatment. Ind Eng Chem Res 1998;37:3806–11. [24] Jensen A, Johansen K, Wójtowicz MA, Serio MA. TG-FTIR study of the influence of potassium chloride on wheat straw pyrolysis. Energy Fuels 1998;12:929–38. [25] Sun RC, Tomkinson J, Bolton J. Effects of precipitation pH on the physicochemical properties of the lignins isolated from the black liquor of oil palm empty fruit bunch fibre pulping. Polym Degrad Stabil 1999;63: 195–200. [26] Luo CD, Chen ZT, Chen. Preliminary studies on the mechanism of lignin precipitation during acidification of kraft wheat straw black liquor. Trans Chin Pulp Pap 1990;5:33–42. [27] Lawther JM, Sun RC, Banks W. Rapid isolation and structural characterization of alkali-soluble lignins during alkaline treatment and atmospheric refining of wheat straw. Ind Crop Prod 1996;5:97–105. [28] Quyn DM, Wu H, Li C-Z. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part I. Volatilisation of Na and Cl from a set of NaClloaded samples. Fuel 2002;81:143–9. [29] Ohtsuka Y, Zhiheng W, Furimsky E. Effect of alkali and alkaline earth metals on nitrogen release during temperature programmed pyrolysis of coal. Fuel 1997;76:1361–7. [30] Lv D, Xu M, Liu X, Zhan Z, Li Z, Yao H. Effect of cellulose, lignin, alkali and alkaline earth metallic species on biomass pyrolysis and gasification. Fuel Process Technol 2010;91:903–9. [31] Tao L, Zhao GB, Qian J, Qin Yk. TG-FTIR characterization of pyrolysis of waste mixtures of paint and tar slag. J Hazard Mater 2010;175:754–61.

[32] Liu Q, Wang S, Zheng Y, Luo Z, Cen K. Mechanism study of wood lignin pyrolysis by using TG-FTIR analysis. J Anal Appl Pyrol 2008;82:170–7. [33] Bassilakis R, Carangelo RM, Wójtowicz MA. TG-FTIR analysis of biomass pyrolysis. Fuel 2001;80:1765–86. [34] Zhu HM, Yan JH, Jiang XG, Lai YE, Cen KF. Study on pyrolysis of typical medical waste materials by using TG-FTIR analysis. J Hazard Mater 2008;153:670–6. [35] Lee SB, Fasina O. TG-FTIR analysis of switchgrass pyrolysis. J Anal Appl Pyrol 2009;86:39–43. [36] Jiang X, Li C, Wang T, Liu B, Chi Y, Yan J. TG-FTIR study of pyrolysis products evolving from dyestuff production waste. J Anal Appl Pyrol 2009;84:103–7. [37] Lv GJ, Wu SB, Lou R, Yang Q. Analytical pyrolysis characteristics of enzymatic/ mild acidolysis lignin. Cellul Chem Technol 2010;44:335–42. [38] Caballero JA, Font R, Marcilla A. Study of the primary pyrolysis of Kraft lignin at high heating rates: yields and kinetics. J Anal Appl Pyrol 1996;36:159–78. [39] Madarasz J, Varga PP, Pokol G. Evolved gas analyses (TG/DTA-MS and TG-FTIR) on dehydration and pyrolysis of magnesium nitrate hexahydrate in air and nitrogen. J Anal Appl Pyrol 2007;79:475–8. [40] Nowakowski DJ, Jones JM, Brydson R, Ross AB. Potassium catalysis in the pyrolysis behaviour of short rotation willow coppice. Fuel 2007;86:2389–402. [41] Han L, Wang Q, Ma Q, Yu C, Luo Z, Cen K. Influence of CaO additives on wheatstraw pyrolysis as determined by TG-FTIR analysis. J Anal Appl Pyrol 2010;88:199–206. [42] Caballero J, Conesa J, Font R, Marcilla A. Pyrolysis kinetics of almond shells and olive stones considering their organic fractions. J Anal Appl Pyrol 1997;42:159–75. [43] Wang S, Wang K, Liu Q, Gu Y, Luo Z, Cen K, et al. Comparison of the pyrolysis behavior of lignins from different tree species. Biotechnol Adv 2009;27:562–7. [44] Sams D, Shadman F. Mechanism of potassium-catalyzed carbon/CO2 reaction. AICHE J 1986;32:1132–7.