Effect of NaOH treatment on combustion performance of Xilinhaote lignite

International Journal of Mining Science and Technology 24 (2014) 51–55

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Effect of NaOH treatment on combustion performance of Xilinhaote lignite Liu Xiangchun, Feng Li ⇑, Song Lingling, Wang Xinhua, Zhang Ying School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou 221116, China

a r t i c l e

i n f o

Article history: Received 28 May 2013 Received in revised form 30 June 2013 Accepted 25 July 2013 Available online 4 January 2014 Keywords: Lignite Alkali treatment Non-isothermal thermogravimetry Combustion performance

a b s t r a c t The combustion characteristics of NaOH treated and untreated Xilihaote lignite was investigated by thermogravimetric analysis. The relationship between physico-chemical properties, including the ash content, oxygen-containing functional groups, mean pore diameter and specific surface area and combustion performance, was also studied in this paper. Combustion kinetic parameters were calculated through Coasts Redfern Method. The results show that ignition of treated samples takes place at higher temperature compared to raw lignite, and peak temperature also occurs at higher temperature. The maximum combustion rate of the sample, which was treated by 0.01 mol/L NaOH lignite, was the biggest. Reaction orders of 0.6, 2.0, and 0.8 were found to be effective mechanism for definite three temperature regions. Average activation energies of these three temperature regions of XLHTR, XLHT0.01, XLHT0.50 and XLTH1.00 are 19.17, 23.87, 10.77, and 10.93 kJ/mol, respectively. Treatment of lignite with NaOH can reduce the reactivity of lignite at proper concentration. Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

1. Introduction Generally, lignite has strong thermal reactivity. There are a number of factors determining the combustion performance of lignite. Briefly, oxygen-containing functional groups, minerals, and the highly porous nature of lignite have the significant effect on its combustion performance [1–3]. Many early studies considered alkali treatment on coal. Some studies focused on demineralization and desulfuration of lignite by alkali treatment while others reported that alkali treatment could remove some oxygen-containing functional groups from lignite [4–8]. All of them focus on the effect of alkali treatment on removal of minerals or oxygen-containing functional groups. Few detailed investigations focus on the changes of some physico-chemical properties, including mean pore diameter, specific surface area, etc. Attempts to correlate variation in physico-chemical properties with alkali treatment are significant because these properties may play an important role in the combustion performance of lignite. The combustion characteristics of lignite were usually investigated by thermogravimetric analysis (TGA), which provides a rapid quantitative method to examine the overall combustion process and estimate the effective kinetic parameters during the overall decomposition reactions [9–11].

⇑ Corresponding author. Tel.: +86 13852488050. E-mail address: cumthgfl@163.com (L. Feng).

A few studies considered how the physico-chemical properties within alkali treatment process affect combustion performance. Here, this paper studies the combustion performance of untreated and treated Xilinhaote lignite by different NaOH concentrations. The effects of some physico-chemical properties, including the ash content, oxygen-containing functional groups, mean pore diameter and specific surface area on combustion performance, are also discussed. Non-isothermal thermogravimetry is used to investigate the combustion performance of all samples.

2. Experimental 2.1. Materials and samples preparation The lignite was sampled from Xilinhaote, Inner Mongolia, China. The lignite was ground to pass a 178 lm sieve for these experiments. The raw lignite was analyzed for ultimate and proximate analysis according to the Chinese Standard Method [12]. Table 1 shows the proximate and ultimate analyses of the lignite sample. Alkali treatment was carried out as described by Song et al [13]. Briefly, a series of 6.00 g portions of raw lignite was prepared by mixing with 100 mL of different concentrations (0.01, 0.50, and 1.00 mol/L) NaOH solution, placed in different beakers, which were called XLHT0.01, XLHT0.50 and XLHT1.00, respectively. Xilihaote raw lignite itself was called XLHTR. Each mixture was stirred at room temperature for 2.5 h. Then the sample was filtered and washed with distilled water until the filtrate was neutral. The

2095-2686/$ - see front matter Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2013.12.009

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X. Liu et al. / International Journal of Mining Science and Technology 24 (2014) 51–55

Table 1 Proximate and ultimate analyses of the lignite sample (%, by weight). Lignite

XLHTR *

Ultimate analysis*

Proximate analysis Mad

Vdaf

Ad

FCdaf

C

H

N

O (by difference)

S

22.23

40.12

14.59

59.88

68.14

6.06

1.32

23.75

0.73

Is daf.

residue was dried in vacuum at 100 °C for 6 h. The treated products were then stored for further analysis.

3. Results and discussion 3.1. Effect of alkali treatment on acidic functional groups

2.2. Characterization Mean pore diameter was obtained by mercury intrusion porosimetry (MIP) using an AUTOPORE IV 9500 mercury porosimeter (Micromeritics). A set of MIP experiments were run in series on each vacuum dried, 1.20 g sub-sample, which had been crushed to form a chunk of approximately 1 cm diameter and then dried. The specific surface area was measured by N2 adsorption using an Autosorb-1 (Quantachrome). Experiments were carried out on vacuum dried, 0.50 g sub-samples, which were further dried under vacuum at 120 °C for 5 h to ensure complete drying and removal of adsorbed gases. Specific surface areas were calculated using the BET model. The number of carboxyl groups as well as the total acidic values was measured by an improved barium exchange technique [14]. TGA tests were carried out on NETZSCH STA 409 Pc simultaneous analyzer. Sample of about 15 mg was placed in an Al2O3 ceramic pan. The samples were heated from room temperature to 825 °C under artificial air (20/80 in O2/N2) flow rate of 100 mL/min at heating rate 10 K/min.

2.3. Calculation of kinetic parameters Coats Redfern Method is one of the most widely used methods for the determination of thermal kinetics of lignite [15].

  dx E ¼ A exp  ð1  X Þn dt RT

ð1Þ

When n = 1, Eq. (1) is simplified as Eq. (2).

ln

lnð1  XÞ T2

¼ ln

AR E  qE RT

ð2Þ

Table 2 presents the concentrations of carboxyl groups and total acidic groups of all samples. As can be seen, an increase in NaOH concentration results in a decrease in the total numbers of carboxyl groups and acidic functional groups. When the concentration of NaOH treated lignite is very low, the reaction of NaOH with acidic functional groups is limited. Compared to XLHTR, carboxyl groups and the total acidic values are reduced significantly for XLHT0.50. This phenomenon shows that large amount of acidic functional groups is removed from the lignite. Changes between XLHT0.500 and XLHT1.000 are small, and this suggests that no more amount of oxygen-functional groups could remove from lignite above the concentration of 0.500 mol/L. In other words, significant increase in alkali concentration is associated with a small change in the dissolution of acidic functional groups. 3.2. Effect of alkali treatment on the ash content Table 3 shows the effect of NaOH concentration on the ash contents of raw and treated samples. As can be seen from Table 3, the ash contents of NaOH treated samples are always increased relative to XLHTR. This phenomenon shows that the percentage of removed organic matter exceeds the removed minerals. Up to the NaOH concentration of 0.500 mol/L, the increase in NaOH concentration is associated with the increase in the ash content, which suggests that the changes are mainly removal of the organic matter. Increased NaOH concentrations are associated with a decrease in the ash content above the alkali concentration of 0.500 mol/L, and this suggests that the reaction between NaOH and minerals dominates in this concentration region. Thus, dissolution of certain amount of organic matter could lead to corresponding gain in the ash content, and a decrease in the ash content is seen since more and more amounts of the minerals are removed from the lignite.

When n – 1, Eq. (1) is simplified as Eq. (3) 3.3. Effect of NaOH treatment on combustion characteristics

ln

1  ð1  X Þ1n ð1  nÞT 2

AR E ¼ ln  qE RT

ð3Þ

where X is the conversion degree of sample; A the pre-exponential factor; E the activation energy (kJ/mol); T the absolute temperature, K; R the gas constant (kJ/(mol); and q the heating rate (K/min).

Table 2 Concentration of carboxyl groups and total acidic groups of all samples (mmol/g) (dry basis). Sample

Carboxyl group

Total acidity

XLHTR XLHT0.01 XLHT0.50 XLHT1.00

14.50 14.18 7.63 6.89

17.24 16.82 10.56 9.85

Burning characteristics obtained from a TGA analyzer may be a good guide to compare the combustibility of the lignite. Although operating conditions such as heating rate and oxygen concentration affect the combustion characteristics, under the same condition the technique is a valuable tool for comparing combustion characteristics between raw lignite and alkali treated samples [9]. According to the approximate starting and end points of DTG curves, it can be observed that DTG curves of samples XLHTR and XLHR0.01 are separated into two regions (as shown in Table 3 Effect of alkali treatment on the ash contents (dry basis) of lignite and NaOH-treated lignite (%, by weight). Sample

XLHTR

XLHT 0.01

XLHT 0.50

XLHT1.00

Ash content

12.68

13.95

18.51

14.29

X. Liu et al. / International Journal of Mining Science and Technology 24 (2014) 51–55

0

C

100

Derivative weight (%/min)

DTG

90

-2

Weight percent (%)

80

st 1 region

70

2

60

nd

-4

region

B -6

50

XLHTR

40

-8

30

MCR

A TG

20 25

225

T

i

425 Temperature

TP

-10 825

625

2

Fig. 1. TG and DTG curves of the raw lignite (Ti as ignition temperature, TP2 the peak temperature of the 2nd region).

0 100

DTG

-2

60

-4

rd

st

nd

1 region

2

3

TG

region

region

40

-6

20 -8

XLHT0.50

Derivative weight (%/min)

Weight percent (%)

80

0 -10 25

225

425 Temperature (

)

625 TP

TP

2

3

825

Fig. 2. TG and DTG curves of sample XLHT0.50 (TP2 and TP3 are the peak temperature of the second and third regions, respectively).

100

0 DTG

60 st 1 region

-2

nd

2

rd

region

40

3

-4 region -6

TG

XLHT0.01 XLHT1.00

20

-8

Derivative weight (%/min)

Weight percent (%)

80 st nd 1 region 2 region

53

ignition and combustion of the volatile matters. The 3rd region with sufficiently high temperature corresponds to ignition and burning of the char remaining after the volatiles are removed. Ignition of coal can be described as a process of achieving a continuing reaction between coal and an oxidizer. It influences flame stability and extinction. The definition of ignition temperature (Ti) refers to study investigated by Li et al. [16]. Briefly, as shown in Fig. 1, firstly through the DTG peak point A, a vertical line is made upward to meet the TG curve at point B; secondly a tangent line to TG curve is made at point B, which meets the extended TG initial level line at point C; thirdly another vertical line is made downwards through point C, which meets the cross axle at Ti defined as ignition temperature. The peak temperatures represent (TP2 and TP3) the point at which the rate of weight loss is at maximum due to rapid volatilization accompanied by the formation of char, and the maximum weight loss rate at the peak temperature is called the maximum combustion rate (MCR). Ti, TP2 and TP3, MCR and some physical properties of all samples are given in Table 4. Ti, TP2, TP3, and MCR of the raw lignite and the treated samples vary in large ranges, indicating that the combustion characteristics of treated samples are very different with the raw lignite. Berkowitz et al. reported that at sufficiently high temperature only if sufficient oxygen can reach the particle surface, ignition and burning of the residual solid particle itself [17]. The mean pore diameters of samples XLHT0.05 and XHLT1.00 are greater than that of the other two, thus oxygen could reach the particle surface and support combustion, and the 3rd region appears. The peak temperature, which is mainly used to measure the combustibility of a coal, is inversely proportional to the reactivity of lignite. The lower the maximum peak temperature is, the more reactive a coal may be considered [18]. During the 2nd region, the peak temperature increases with the alkali concentration, which suggests that the reactivity of the sample reduces within the process of alkali treatment. Ti is determined by early release of volatiles and the speed that heat being released by volatiles combustion. It is clear that the large amount of volatiles and the energy released could reduce the Ti [9]. MCB depends on the accessibility of the oxidant to the active sites of lignite and the specific surface area for oxidation [19]. From Sections 3.1 and 3.2, we know that, take XLHT 0.01 as a case in point, the amount of organic matter removed is very smaller, and the specific surface area is the largest. These facts lead to the increase in Ti with the highest MCR. Hence, it could observe a maximum peak with increased curvature during the 2nd region of volatiles combustion. As for XLHT0.50 and XLHT1.00, the removal of more amount of organic matter from the lignite leads to the amount of volatile matters for combustion obviously reduced. This affects the combustion characteristics of XLHT0.05 and XLHT1.00, so they have the higher Ti and the lower MCR Ti of XLHT1.00 is reduced from 502 to 478 °C compared to XLHT0.5, since the content of the dissolved minerals increases the decomposition of minerals retained in lignite and it leads to further decrease in Ti. 3.4. Effect of NaOH treatment on combustion kinetics

0 25

225

425 Temperature ( )

625

-10 825

Fig. 3. TG and DTG curves of samples XLHT0.01 and XLHT1.00.

Figs. 1 and 3), while three regions are determined for samples XLHT0.50 and XLHT1.00 (see Figs. 2 and 3). The first region on DTG curve is attributed to the evaporation of moisture and low boiling point organic matter in the sample and the 2nd region to

TG curve of the sample is used to calculate the kinetic parameters through the method of Coasts-Redfern, which is one of the most widely used methods and considered as a good approach for the determination of thermal kinetics [20]. According to Eq. (1), when n is correct, plot of ln[g(a)/T2] against 1/T should give straight line with high correlation coefficient (R2) of linear regression analysis. Fig. 4 displays the best value of n for definite temperature regions. The temperature region has been divided into three regions, as defined in Table 5. Combustion kinetics of temperature region (T from 25 to 120 °C) where the water is mainly removed is not

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X. Liu et al. / International Journal of Mining Science and Technology 24 (2014) 51–55

Table 4 Effect of alkali treatment on the combustion characteristics and some physical properties of lignite.

XLHTR XLHT0.01 XLHT0.50 XLHT1.00

353 370 669 702

-13.6

3rd peak temperature, TP3 (°C)

Max. combustion rate, MCR (%/min)

Ignition temperature, Ti (°C)

Specific surface area (m2/g)

Mean pore diameter (nm)

755 752

8.64 10.01 3.59 2.81

279 322 502 478

2.76 3.27 1.19 0.69

33.4 42.3 96.9 78.7

-11.5

Temperature region 1 -12.0

2

ln(-ln((1-x)/T ))

y= -14.94+206.57x

(3) (4) y= -15.14+267.36x

y= -16.23+535.96x

0.0020

(1)

(1) XLHTR (2) XLHT0.01 (3) XLHT0.50 (4) XLHT1.00 n=2

-13.2

(2) y= -15.29+1596.30x

-13.6

-13.5 -14.0

Temperature region 3

y= -13.32-536.76x

(3)

y= -13.42-524.24x

(4)

(1) y= -14.87+1557.57x

(1) XLHTR (2) XLHT0.01 (3) XLHT0.50 (4) XLHT1.00 n=0.8

-14.0 (3) y= -10.88-3123.13x

(2)

0.0022 T (1/K) -1

y= -5.96-4523.67x

-13.0

-14.5

-14.8

-15.2 0.0018

-12.5 2

(1) XLHTR (2) XLHT0.01 (3) XLHT0.50 (4) XLHT1.00 n=0.6

-14.0

-14.4

(2) y= -2.88-6520.44x

(1)

ln(-ln((1-x)/T ))

y= -15.72+795.81x

-12.8

Temperature region 2

2

2nd peak temperature, TP2 (°C)

ln(-ln((1-x)/T ))

Sample

-15.0

0.0024

0.0026

-15.5 0.0011

0.0013

0.0015-1 0.0017 T (1/K)

0.0019

0.0021

-14.4 0.0009

0.0010

0.0011

(4) y=-10.94-3152.89x 0.0012 0.0013

0.0014

-1

T (1/K)

Fig. 4. Plots of nth order for the three temperature regions of NaOH all samples by Coasts-Redfern method.

Table 5 Combustion kinetic parameters calculated from DTG date of treated and untreated samples. Reaction order

Temp. region 1–3 (°C)

A (min1)

E (kJ/mol)

R2

(a) XLHTR n = 0.6 n=2 n = 0.8

120–265 266–440 441–830

5.34  1010 1.75  107 6.97  1010

6.62 37.61 13.27

0.996 0.951 0.986

Average (b) XLHT0.01 n = 0.6 n=2 n = 0.8 Average

19.17 120–227 228–450 451–830

5.99  1010 1.16  106 4.47  1010

4.46 54.21 12.95 23.87

0.993 0.918 0.975

(c) XLHT0.50 n = 0.6 n=2 n = 0.8 Average

120–221 222–608 609–830

6.36  109 3.27  109 1.66  109

1.72 4.46 25.97 10.77

0.945 0.954 0.969

(d) XLHT1.00 n = 0.6 n=2 n = 0.8 Average

120–218 219–608 609–830

1.01  1010 3.53  109 1.78  109

2.22 4.36 26.21 10.93

0.958 0.951 0.930

Note: E is the activation energy; A the pre-exponential factor for Arrhenius statement; and R2 the correlation coefficient.

calculated. The kinetic parameters activation energy (E) and pre-exponential factor for Arrhenius statement (A) of the three separate temperature regions are calculated from the slopes and the intercepts of the straight lines. All of them are shown in Table 5. As noted in Table 5, average activation energy of the samples varies from 10.77 to 23.87 kJ/mol, which is attributed to the competition between organic matter and mean pore diameter. In temperature region 1, due to the loss of low boiling point organic matter in the sample, E decreases with the increase of NaOH concentration. This is because more and more amount of organic matter is removed from the lignite with the increase of NaOH concentration. In temperature region 2, E of samples XLHT0.50 and XLHT1.00 reduces as compared to raw lignite. This could be

attributed to the bigger mean pore diameter than that of raw lignite. As a result, heat capacity and thermal conductivity improve with a lower E. It is interesting to see that E of sample XLHT0.01 is the highest in the temperature region. In temperature region 3, which is ignition and burning of the char, E of XLHT0.50 and XLHT1.00 is obvious higher compared to that of the other two because the combustion of volatiles requires relatively low activation energy in comparison with char combustion [1]. And results of Section 3.2 shows that more amount of organic part removed from XLHT0.50 and XLHT1.00 results in this phenomenon. In Tables 4 and 5, relatively higher Ti and average activation energy, and slightly higher MCR of XLHT0.01, indicates that NaOH treatment may reduce the reactivity of lignite at proper concentration.

X. Liu et al. / International Journal of Mining Science and Technology 24 (2014) 51–55

4. Conclusions (1) More amount of the loss in organic matter during alkali treatment process increases ignition, peak temperature and average activation energy of XLHT0.50 and XLHT1.00, and reduces their maximum combustion rate. (2) Increased amount of the dissolved minerals results in that the ignition of XLHT1.00 is lower than that of XLHT0.50. More amount of removing organic matter and smaller specific surface area results in a lower maximum combustion rate of XLHT1.00 than that of XLHT0.50. (3) Ignition, peak temperature, maximum combustion rate and average activation energy of XLHT0.01 are 322 °C, 370 °C, 10.01 %/min and 23.87 kJ/mol, respectively. Compared with XLHTR, combustion performance of XLHT0.01 improves, indicating that alkali treatment can reduce the reactivity of lignite at proper concentration.

Acknowledgments The authors are grateful to the National Basic Research Program of China (No. 2012CB214901), the National Natural Science Foundation of China (No. 51274197) and the Fundamental Research Funds for the Central Universities (No. 2010LKHX07) for the financial support. References [1] Küçükbayrak S, Haykırı-Açma H, Ersoy-Meriçboyu A, Yaman S. Effect of lignite properties on reactivity of lignite. Energy Convers Manage 2001;42(5):613–26. [2] Bai ZQ, Li W, Yu CW, Bai J. Low temperature pyrolysis of lignite in the presence of syngas and combustion characteristics of derived char. J Chin Univ Min Technol 2011;40(5):726–32.

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