Key factors influencing the release and formation of HCN during pyrolysis of iron-containing coal

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 35, Issue 1, February 2007 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2007, 35(1), 5−9

RESEARCH PAPER

Key factors influencing the release and formation of HCN during pyrolysis of iron-containing coal XU Ming-yan, CUI Yin-ping, QIN Ling-li, CHANG Li-ping*, XIE Ke-chang Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and Shanxi Province, Taiyuan 030024, China

Abstract: Minerals inherently present in coal and iron-containing compounds externally added have an important influence on the formation and release of HCN—one of the main precursors of NOx during coal pyrolysis. Pyrolysis of raw coal and demineralized coal with different ranks and iron-containing compounds was studied in a fixed bed quartz reactor at temperature programmed in this article. The trend and influencing factors of HCN formation and release during coal pyrolysis were examined. The results showed that the effect of iron-containing compounds in coal on HCN release depended on the coal types, and the iron added by impregnation and precipitation methods played different roles on HCN formation for different coal types. Additionally, the effects of the particle size and the reaction time on HCN formation during pyrolysis of coal with iron were also studied. Key Words: coal; pyrolysis; mineral; HCN; iron

The low conversion and utilization efficiency of coal, as the main energy source for quite a long term for China, caused serious environment pollution. NOx is one of the main pollutants forming acid rain produced during coal combustion. Pyrolysis is the first and essential step in coal conversion processes and the nitrogen in coal will be released as volatile-N such as tar-N, NH3, HCN, and N2, and the remainder is retained in the residual solid as char-N during the primary coal pyrolysis process. Some of the precursors HCN and NH3 in gas and char-N in solid may then be converted to NOx through different reaction approach in the subsequent combustion process. To reduce and restrain the formation of NOx, it is important to study the release mechanism and rule of HCN[1−7], as one of the main precursors during coal pyrolysis. Researches have shown the effect of coal characteristics (coal types, geographical location, coal size, and so on) and experimental conditions (environmental gases, gas flow rate, heating rate, and so on) on the release and formation of HCN and NH3[1] as well as the role of iron-containing species and ashes[2]. The results illustrated that inherent minerals and iron-containing additives can greatly influence the transformation of nitrogen in coal. The results from Ohtsuka et al.[3,4] about the role of minerals and iron added in coal on the conversion of

coal-nitrogen to N2 during pyrolysis and gasification showed that the metallic iron particles formed from inherently present or externally added iron-containing species can catalyze the conversion of the coal-nitrogen to N2, and the temperature of N2 release can be decreased about 100 K. Guan et al.[5] has certified that iron added by the impregnation method can catalyze the conversion of HCN to N2. Zhao et al.[8−10] reported that iron, calcium, and sodium added into the char by impregnation and inherent minerals all promoted the reaction of NO-Char to N2. This article will particularly discuss the role of iron added by different methods on the formation of HCN during coal pyrolysis from 873 K to 1173 K to understand the transformation mechanism of coal nitrogen during these processes and to effectively suppress the formation and release of NOx.

1 1.1

Experimental Coal selection and preparation

Three coals from Shendong (SD), Pingshuo (PS), and Changcun (CC) coal fields in China were selected in this study. All samples were air-dried at room temperature, ground, and sieved to coal particles with size fraction of

Received: 2006-05-11; Revised: 2006-08-25 * Corresponding author. Tel: +86-351-6018080; Fax: +86-351-6018453; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (90410018, 20276046), the Shanxi Research Foundation to Returned Scholars (2003-25) and the Shanxi Province Natural Science Foundation (20041018). Copyright2007, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All right reserved.

XU Ming-yan et al. / Journal of Fuel Chemistry and Technology, 2007, 35(1): 5−9

0.250 − 0.420 mm, 0.125 − 0.149 mm, and 0.074 − 0.083 mm. Demineralized coal (DM-coal) were obtained from raw coal in 18% HCl solution at 333 K for 12 h, followed by filtration to separate the sample from the solution. The resulting sample was washed with ion-free water repeatedly to remove the remaining Cl− ions and was finally dried at 353 K in a vacuum oven. Iron addition was done at room temperature using the FeCl3·6H2O (A.R.). The impregnated iron were loaded by immersing the demineralized coal in a saturated aqueous solution of FeCl3 with the iron loading content as 0.3 wt%[2] (samples denote as DSD + Fe1, DPS + Fe1, and DCC + Fe1),

and the precipitated iron was obtained by first immersing the demineralized coal in a FeCl3 aqueous solution for 2 h, followed by adding sufficient Ca(OH)2 powder to the mixture of DM-coal and FeCl3 aqueous solution (samples denoted as DSD + Fe2, DPS + Fe2, and DCC + Fe2). After the solutions were held for 12 h, the samples were separated from the solutions by filtration, and then the coal samples with additives were dried in a vacuum oven at 353 K. The proximate and ultimate analyses of the coals, and mineral matter composition of the coals used in experiment are provided in Table 1 and Table 2. Table 3 provides the analysis of the demineralized coals.

Table 1 Proximate and ultimate analyses of the coals used in experiments Proximate analysis w / %

Coal type

Ultimate analysis wdaf / %

Ad

Mad

Vdaf

C

H

N

S

O*

SD

9.80

4.50

33.72

79.53

4.16

0.91

0.48

14.39

PS

2.23

17.93

37.19

80.41

5.20

1.38

1.06

11.95

CC

0.23

12.13

13.33

87.75

4.02

1.39

6.62

0.22

Note: ad is air-dried basis; d is dry basis; daf is dry and ash-free basis; *: by difference

Table 2 Ash analyses of the coals used in experiments Ash composition w / %

Coal type

SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

Other

SD

23.28

9.66

15.98

27.48

3.23

0.08

2.30

17.99

PS

48.31

44.66

2.25

1.33

0.13

0.17

0.06

3.09

CC

61.83

19.95

3.79

9.23

1.01

2.89

0.47

0.82

Table 3 Ultimate analysis of the demineralized coals Coal type

H

N

S

Odiff

78.95

4.90

1.08

0.64

14.43

DPS

79.09

5.46

1.57

0.99

12.89

DCC

89.18

5.05

1.65

0.49

3.63

DSD

1.2

Ultimate Analysis wdaf / % C

Prolysis experiment and product analysis

Pyrolysis experiments were carried out in a stream of Ar to avoid the influence of N2. The reactor was a quartz tube with a quartz frit in the middle described elsewhere[1,2]. About 0.5 g of the sample was heated at 15 K/min to the prearranged temperature under Ar with a flow rate of 360 mL/min and was then held for 40 min. Pyrolysis products were separated into char, tar, and gas. Char remained on the reactor, while tar was captured by a trap and gaseous product from the outlet of the reactor passed through the tar trap to an absorption bottle. HCN was absorbed by 0.1 mol/L NaOH solution. The solution samples collected were quantified using DX-500 ion chromatography equipped with an ED40 electrochemical detector. Char-N was characterized with element analysis. The yields of HCN in the figures are the

accumulated yields, which are expressed as the percentage of HCN released during coal pyrolysis on the total nitrogen contained in the feed coal sample. The yield of HCN was calculated according to the formula: Y= N in HCN / N in coal, and the C/N atom ratio was calculated according to the formula: C/N = (Cchar / 12) / (Nchar / 14).

2

Results and discussion

2.1 The formation of HCN during pyrolysis of different types of coal Yields of HCN in pure Ar at 1073 K held for 40 min from three raw coals, DM-coals with and without iron are illustrated in Fig. 1, and the C/N mole ratios of the residual chars after pyrolysis are provided in Table 4. It is seen that the yield of HCN changed in the sequence of SD > PS > CC during pyrolysis for all samples from Fig. 1. This change trend was not identical with the carbon content in coal shown in Tables 1 and 3, indicating that the carbon content is one of the key factors influencing the yield of HCN during pyrolysis of raw coal and demineralized coal. It is known that the formation of HCN originates mainly from the direct thermal

XU Ming-yan et al. / Journal of Fuel Chemistry and Technology, 2007, 35(1): 5−9

cracking of thermally less stable N-containing heteroaromatic ring systems with the release of volatiles from the coal heated[1]. It is reasonable that CC coal with the highest carbon content and the lowest volatile content has the lowest HCN yield. However, PS coal with the highest volatile has lower HCN formation than SD coal, implying that the carbon content is not the only key factor influencing the formation of HCN. The results of the HCN yields from pyrolysis of the raw coal, the demineralized coal, and the iron-containing coal are approximately the same for CC coal, but these vary greatly for PS and SD coals. From the above discussion, it is seen that CC coal with the highest carbon content may have the stable nitrogen-containing group, which is difficult to form HCN. The inorganic mineral composition in CC coal has slight effect on HCN formation during CC coal pyrolysis. The stability of the nitrogen-containing group is dominant among the factors controlling the release of HCN for CC coal. For the PS and SD coals with medium carbon content and coal-N stability, inherent mineral and iron added play a significant role in HCN formation. 7 R-coal DM-coal DM-coal+Fe1 DM-coal+Fe2

6

Y / % coal-N

5 4 3 2 1 0

CC

PS

SD

Coal types

Fig. 1 Release of HCN from different types of raw coals and demineralized coals with and without iron additive during pyrolysis at 1073 K

From Fig. 1, it is obviously seen that all the demineralized coals have higher HCN yields than the raw coals, which indicates that minerals present in the raw coals have an ability to vary the distribution of coal nitrogen between gaseous and solid phases or promote HCN transformation to other nitrogen-containing species. The demineralization by hydrochloric acid washing mainly removed the Fe, Ca, Na, or K-containing minerals and some Al and Si-containing minerals in this study. It can be deduced that one or more minerals among Fe, Ca, Na, or K-containing minerals can restrain the release of HCN or promote the transformation of HCN. Yields of HCN of iron-containing coals mainly depend on the coal properties and the additive methods. The yields

of HCN increased for SD and CC iron-containing demineralized coals from the impregnation method, but it decreased for PS demineralized coal. The PS and SD iron-containing demineralized coals from the precipitation method restrained the release of HCN; however, CC coal promoted the release of HCN. In fact, coal nitrogen can transform not only to HCN, NH3, and N2 but also to tar-N and char-N during pyrolysis. Moreover, some gaseous species can transform to other nitrogen-containing species by secondary reaction. Thus, the role of iron added in the formation of HCN can be attributed to the transformation of coal-N to the gaseous nitrogen species and their secondary reaction. Table 4 Mole ratio of C/N from residual char after pyrolysis at 1073 K for raw coals and demineralized coals with and without iron additive Sample

DSD

C/N

83.41

DSD+Fe1 DSD+Fe2 83.92

85.06

DPS+Fe1 DPS+Fe2

SD 86.10

Sample

DPS

C/N

52.29

52.75

55.01

PS

Sample

DCC

CC+Fe1

CC+Fe2

CC

C/N

58.93

62.21

64.95

62.46

53.25

According to the data in Table 4, it is found that the char from the pyrolysis of demineralized coal has the lowest C/N atom ratio, which indicates that the least nitrogen is transformed in char after pyrolysis of demineralized coal. The inherent minerals and the iron added all show an effect to decrease the nitrogen content remaining in the char, i.e. all these promote the transformation of nitrogen from solid phase to gaseous phase. However, the role of iron in nitrogen transformation during coal pyrolysis is closely related to the coal types and the method of iron addition. Usually the dispersion and state of iron added can greatly control its effect. Wu et al.[11] reported that iron added by precipitation to the low rank coal can be reduced to highly dispersed metallic iron particles, because low rank coal has high hydroxyl and carboxyl contents. The iron added by impregnation exists mostly in the pore and only a few are highly dispersed as ion-exchangeable state through hydrolysis[12]. The dispersion of iron in coal and the interaction with coal substrate are the two dominant factors controlling the role of iron added in the release and formation of HCN. 2.2 The formation of HCN during coal pyrolysis under different temperatures Fig. 2 shows the yields of HCN during pyrolysis of SD raw coal, demineralized coal with and without iron in pure Ar at 873 K, 973 K, 1073 K, and 1173 K held for 40 min,

XU Ming-yan et al. / Journal of Fuel Chemistry and Technology, 2007, 35(1): 5−9

iron-containing coal than the demineralized coal in the heating period. The results show the catalytic effect of inherent minerals and iron added on promoting the release of HCN in the heating period.

respectively. 6

Y / % coal-N

5 4 3

SD DSD DSD+Fe1 DSD+Fe2

2 1 800

900

1000

1100

1200

Temperature / K

Fig. 2 Release of HCN from pyrolysis of SD raw coal and demineralized coal with and without iron additive at different temperatures

There is a characteristic temperature at which the yield of HCN reaches maximum for all coal samples in Fig. 2. The yield of HCN increases with increasing temperature at lower temperature and then decreases at higher temperature. These results must be attributed to the difference of the competition capacity between the formation of HCN from coal/char solid and the secondary reaction of HCN at different temperatures. In other words, the formation of HCN is dominant at lower temperature, while the secondary reaction of HCN becomes obvious at higher temperature. When compared with demineralized coal, it is found that the inherent minerals and the iron added all promote the transformation reaction of HCN at higher temperature. It is also shown that iron added by impregnation has strong capacity to promote HCN formation and release, while that added by precipitation greatly improves the HCN secondary reaction. Asami et al.[13] reported that the diffusion in coal is important for the role of the iron added. Ohtsuka et al.[3,4,11] found that iron added by precipitation can promote the transformation of nitrogen in char to NH3 and N2 and can reduce the formation of HCN, and also induced that the highly dispersed iron metallic particles can catalyze HCN transformation to NH3 and N2. 2.3

The distribution of HCN in the period of heating and

holding Yields of HCN in heating and holding periods during pyrolysis of SD raw coal and demineralized coal with and without iron additive in pure Ar at 1073 K held for 40 min are shown in Fig. 3. It is obvious that most HCN was formed in the heating period and some HCN was from the holding period. There are higher HCN yields for the raw coal and the

Fig. 3 Percentage of HCN in heating and holding periods from pyrolysis of SD coal samples

The results in Fig. 2 indicate that HCN yields of the raw coal and the precipitated coal are lower than the demineralized coal, while it is higher for the impregnated coal at all temperatures. The dominative effect of inherent minerals and precipitated iron in coal is to promote the secondary reaction of HCN, while the impregnated iron in coal mainly accelerates the release of HCN from coal/char solid. The data in Table 5 illustrate that C/N atom ratios in chars increase in the sequence of DSD, DSD+Fe1, DSD+Fe2, SD, i.e., the untransformed nitrogen remain in the char after pyrolysis decrease in the sequence of DSD, DSD+Fe1, DSD+Fe2, SD. It is reasonable to induce that the inherent minerals, the precipitated iron, and the impregnated iron facilitate coal nitrogen transformation from solid coal\char particles to gaseous phase. 2.4 The formation of HCN during pyrolysis of coal with different particle sizes Yields of HCN during pyrolysis of SD raw coal, demineralized coal with and without iron additive with different particle sizes in pure Ar at 1073 K held for 40 min are provided in Fig. 4. The yields of HCN from raw coal, demineralized coal, and iron-containing coal change with the coal particle size in the same trend. The HCN yield from coal with particle size of 0.125 − 0.149 mm is higher than that of the other two. Chang et al.[1] reported that with decreasing coal particle size the release path of volatiles from inside the particle to the surface is shorter, leading to less secondary volatile-char

XU Ming-yan et al. / Journal of Fuel Chemistry and Technology, 2007, 35(1): 5−9

reaction to form HCN and a decrease in the yield of HCN. However, with the decrease in the coal particle size, the less chance for HCN to react with inherent minerals or iron-containing additions in the char results in less HCN to be transformed to other nitrogen-containing species and an increase in the yield of HCN. Thus, the influence of the coal particle size on the HCN formation and release is complex. For the iron-containing coal, the dispersion of iron on the coal surface may be changed with particle size to affect the catalytic activity of iron. By comparing the yields of HCN for raw coal, demineralized coal, and iron-containing coal at the same particle size, it is found that the inherent minerals and iron added play the same role in HCN formation.

References [1] Chang L P. Study on the formation and release of nitrogen-containing compounds during coal pyrolysis and gasification.

Taiyuan,

China:

Taiyuan

University

of

Technology, 2004. [2] Zhao Y H. Effect of minerals on transformation of nitrogen during coal pyrolysis/gasification. Taiyuan, China: Taiyuan University of Technology, 2003. [3] Tsubouchi N, Ohtsuka Y. Nitrogen release during high temperature pyrolysis of coals and catalytic role of calcium in N2 formation. Fuel, 2002, 81(18): 2335−2342. [4] Ohtsuka Y, Watanabe T, Asami K, Mori H. Char-nitrogen

8 7

Y / % coal-N

6

functionality and interactions between the nitrogen and iron in 0.250-0.420mm 0.125-0.149mm 0.074-0.083mm

the iron-catalyzed conversion process of coal nitrogen to N2. Energy Fuels, 1998, 12(6): 1356−1362. [5] Guan R G, Li W, Chen H K, Li B Q. The release of nitrogen

5

species during pyrolysis of model chars loaded with different

4

additives. Fuel Process Technol, 2004, 85(8−10): 1025−1037.

3

[6] Zhao W, Chang L P, Feng Z H, Xie K C. Formation of nitrogenous species during coal pyrolysis. Journal of Fuel

2

Chemistry and Technology, 2002, 30(5): 408−412.

1 0

[7] Cao X Y, Niu Z G, Ying L Q, Wang Z H, Zhou J H, Liu J Z, SD

DSD

DSD+Fe1

DSD+Fe2

Coal types

Fig. 4 Release of HCN from the pyrolysis of SD coal samples with different particle sizes

3

Conclusions

Cen K F. Releasing of fuel-nitrogen during blind coal pyrolysis. Journal of Fuel Chemistry and Technology, 2003, 31(6): 538−542. [8] Zhao Z B, Li W, Li B Q. Reduction of NO over coal chars loaded with Na-Fe catalysis. Environmental Chemistry, 2002, 21(1): 19−25. [9] Zhao Z B, Li W, Li B Q. Catalytic reduction of NO by chars loaded with Ca and Fe. Chinese Journal of Environmental

The research on the release and formation of HCN during pyrolysis of minerals and iron-containing coal in a fixed-bed reactor show that the yield of HCN is closely related to the coal rank. The higher the carbon content in coal, the lower is the HCN formation. The release trend of HCN for iron-containing coal is the same as for raw coal, which is formed by the decomposition of unstable nitrogen-containing group in coal. The HCN percentage from the demineralized coal in the heating period is less than that from the other coal samples. The role of iron added in the release of HCN during pyrolysis greatly depends on the coal types. Iron added by precipitation to the low rank coal greatly influences the formation of HCN, whereas there is less effect for the high rank coal. There is no obvious relation between the coal particle size and the HCN formation during pyrolysis.

Science, 2001, 22(5): 17−20. [10] Zhao Z B, Guan R G, Li B Q. Influence of mineral matter in coal on NO-char reaction in presence of CO and O2. Journal of Fuel Chemistry and Technology, 2001, 29(3): 232−237. [11] Wu Z, Sugimoto Y, Kawashima H. Effect of demineralization and catalyst addition on N2 formation during coal pyrolysis and on char gasification. Fuel, 2003, 82(15−17): 2057−2064. [12] Abotsi G M K, Bota K B, Saha G. Interfacial phenomena in coal impregnation with catalysts. Energy Fuels, 1992, 6(6): 779−782. [13] Asami K, Sears P, Furimsky E, Ohtsuka Y. Gasification of brown coal and char with carbon dioxide in the presence of finely dispersed iron catalysts. Fuel Process Technol, 1996, 47(2): 139−151.