Effect of char on the melting characteristics of coal ash

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY

Volume 35, Issue 2, April 2007 Online English edition of the Chinese language journal

Cite this article as: J Fuel Chem Technol, 2007, 35(2), 136−140

Effect of char on the melting characteristics of CHEN Dong-xia1, TANG Li-hua1,*, ZHOU Ya-ming2, WANG Wen-mei2, WU YonZHU Zi-bin1

1 Research Institute of Chemical Technology ECUST, Shanghai 200237, China 2 Shanghai Coking & Chemical Corporation, Shanghai 200241, China

Abstract: The effect of char on the melting characteristics of coal ash was studied in the argon atmosphedeterminator connected with a camera and a computer. The results show that the coal char in ash has remarkabmelting points as well as the ash melting behavior. The effects are related to the coal rank and the char contenashes investigated, the deforming temperatures (DT), the softening temperatures (ST), and the flowing temsamples with the addition of coal char are higher than those of corresponding initial ashes. The ash melting paddition of coal char. When the content of coal char is 20% and when the temperature reaches or exceeds the ash melting point, an infusible skeleton can be formed because of the sticking interaction between char and ashformed, the softening temperature and flowing temperature of the sample are difficult or impossible to occur. Key Words: coal ash; melting characteristics; coal char

Ash melting characteristics are an important index to guide the usage of coal in power plants and the gasification process. Ash melting characteristics determine the aggregation properties of coal ash in combustion, which have close relation to the smoke temperature of the furnace outlet and the method of residual slag deposition[1]. In the gasification process, the type of the gasifier chosen and the suitable operation temperature are determined by the ash melting points. Many factors affect the melting behavior of coal ash and great efforts have been put on these[2−7]. However, there is little research carried out on the effect of carbon residue on the coal ash melting behavior. Considering the fact that the reaction of coal combustion or gasification can not be performed completely in the practical operation, it is necessary to study the effect of carbon residue on the ash melting characteristics. In the present work, the influence of coal char on ash melting behavior was analyzed and discussed. 1 Experimental 1.1 Raw material

Shenfu, Datong, Dongsheng, and Houbulian coals were

used as the raw materials and their properties are listed in

Table 1. The ash samples were Chinese standard GB212−91, ancompositions are presented in Table

Table 1 Properties of coProximate analysis wd / % Coal

V A FC

Shenfu 33.16 6.91 59.93

Datong 26.33 8.73 64.94

Dongsheng 33.56 8.82 57.62

Houbulian 27.43 19.15 53.42

Table 2 Ash composition o

Ash coCoal ash sample

Al2O3 SiO2 CaO

Shenfu 28.03 34.47 13.6

Datong 21.59 37.57 17.6

Dongsheng 22.85 53.93 7.5

Houbulian 28.19 32.76 15.7

1.2 Preparation of coal char

Coal samples with the diameter

placed in a carbonization furnace.replace the air in the furnace and

Received: 2006-10-21; Revised: 2007-01-27 * Corresponding author. E-mail: [email protected] Copyright2007, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All right reserved.

RESEARCH PAPER

coal ash g-qiang1,

re using an ash fusion le influence on the ash t addition. For the coal

peratures (FT) of all oints increase with the corresponding original . When the skeleton is

prepared based on the d their corresponding 2.

al samples Ultimate analysis wd / %

C H N+O S

79.20 5.92 14.58 0.30

82.79 4.65 11.45 1.11

75.07 4.54 20.08 0.31

81.00 3.98 14.69 0.33

f coal samples ntent w / %

Fe2O3 MgO TiO2

9 19.02 2.88 0.71

8 16.96 2.32 0.50

9 8.10 2.69 0.17

7 16.52 3.69 0.25

of 200 − 280 µm were Nitrogen was used to the flow rate was 20

CHEN Dong-xia et al. / Journal of Fuel Chemistry and Technology, 2007, 35(2): 136−140

mL/min. Samples were heated to 900 °C at a heating rate of 5 °C/min, and were kept for 2 h at this temperature. After they cooled down to the room temperature, coal chars were crushed again to obtain particles with the diameter of 200 − 280 µm. Table 3 shows properties of coal char samples.

Table 3 Properties of coal char samples Proximate analysis wd / % Ultimate analysis wd / % Coal char

sample V A FC C H N+O S

Shenfu 4.83 8.77 86.40 91.32 3.39 5.16 0.13

Datong 4.23 11.06 84.71 92.94 3.15 4.47 0.14

Dongsheng 5.56 11.72 82.72 89.07 3.25 7.57 0.11

Houbulian 3.43 21.15 75.42 93.32 2.58 3.94 0.16

1.3 Testing of ash melting characteristics

Fig. 1 shows the ash fusion determinator which is

composed of a tube furnace, a temperature controller, and a computer with CCD which is used to monitor the change of ash cones in situ with increasing temperature. Ash cone shapes at different temperatures were imaged and stored in the computer.

Fig. 1 Testing apparatus of ash fusibility

1: temperature control instrument; 2: thermocouple; 3: high temperature

furnace; 4: ash cone; 5 refractory bracket; 6: lead rail; 7: CCD; 8: computer

Ash melting characteristics, deforming temperature (DT),

softening temperature (ST), and flowing temperature (FT), were determined by the standard ash fusion temperature test (pyrometric cone method, Chinese standard GB219−74). Ash

fusion temperatures of four original coal ash samples were detected under the weak reduction atmosphere, which are shown in Table 4.

Table 4 Ash fusion temperatures of coal ashes

Temperature t / °C Ash samples

DT ST FT

Shenfu 1155 1215 1235

Datong 1038 1195 1250

Dongsheng 1100 1218 1240

Houbulian 1100 1146 1180

Melting characteristics of ash samples with the addition of

coal char were analyzed under an argon stream to eliminate the possible influence of the atmosphere in accordance with the above method. 2 Results and discussion 2.1 Melting points of ashes with char addition

Melting points of Shenfu, Datong, Dongsheng, and

Houbulian ashes with coal char addition were shown in Fig. 2. It can be seen that the influences of coal char on the melting characteristics are almost the same for the four coals. The ash melting points of ashes with coal char addition are higher than those of corresponding initial coal ashes. However, the influence degrees of coal char are different for four coals. The melting points of Shenfu ash samples are greatly affected by coal char; while the coal char has relatively small influence on the melting points of Datong ash samples; Dongsheng and Houbulian ash samples show the similar influence degree of the coal char. When the addition of coal char content is 20%, DT still shows the increased trend for four coals, while ST and FT do not occur even at the temperature as high as 1350 °C.

0 5 10 15 201100

1150

1200

1250

1300

1350 DT ST FT

Tem

pera

ture

(o C)

Coal char content (%)

0 5 10 15 201100

1150

1200

1250

1300

1350

Tem

pera

ture

(o C)


DT ST FT

(a) Shenfu (b) Datong


0 5 10 15 201100

1150

1200

1250

1300

1350

Tem

pera

ture

(o C)


DT ST FT

0 5 10 15 20

1100

1150

1200

1250

1300

1350


Tem

pera

ture

(o C)

DT ST FT

(c) Dongsheng (d) Houbulian

Fig. 2 Effect of coal char content on ash melting points

2.2 Melting behavior of ashes with char addition

During the test, the height changes of coal ash cone were

recorded and shown in the ash melting curve. The ash melting curve can reflect the ash melting behavior comprehensively and directly in the whole temperature range[8]. The results for four coals are shown in Fig. 3. It shows that the effects of coal char on ash melting behavior

are basically the same for four coals. The temperatures of the ash cone height initiating change of the samples with coal char addition are higher than those of initial ashes, and they also increase with the increase of coal char content. When the addition of coal char is 20%, it can be seen that the heights of the ash cone samples only gradually decrease to 30 − 40% of the initial samples, and they will not change any more even at the temperature as high as 1350 °C.

1050 1100 1150 1200 1250 1300 13500

20

40

60

80

100Shenfu

Prop

ortio

n of

hei

ght (

%)

Temperature (oC)

0% 5% 10% 15% 20%

1100 1150 1200 1250 1300 13500

20

40

60

80

100Datong

Pro

porti

on o

f hei

ght (

%)

Temperature (oC)

0% 5% 10% 15% 20%

1200 1250 1300 13500

20

40

60

80

100Dongsheng

Prop

ortio

n of

hei

ght (

%)

Temperature(oC)

0% 5% 10% 15% 20%

1050 1100 1150 1200 1250 1300 1350

0

20

40

60

80

100Houbulian

Pro

porti

on o

f hei

ght(%

)

Temperature (oC)

0% 5% 10% 15% 20%

Fig. 3 Effect of different coal char content on ash melting characteristics


2.3 Shapes of cone after cooled down

Shapes of the ash cone samples were cooled down after

experimental tests as shown in Fig. 4. The surfaces of the samples without coal char addition are smooth, while granular coal char can be seen on the surface of the samples with the addition of coal char. The shapes of ash cones with 20% coal char addition show almost similar image for four

coals, and the changes of the heights of cones may be decided by the coal char content. The coal char can stick together with the coal ash when the temperature reaches or exceeds the original ash melting point to form an infusible skeleton. It is the skeleton that keeps the shapes of the ash cone, and ST and FT are difficult or impossible to occur even at the temperature as high as 1350 °C.

Content of coal char addition

samples 0% 10% 20%

Shenfu

Datong

Dongsheng

Houbulian

Fig. 4 Shapes of samples with or without coal char addition 3 Conclusions

Coal char in ash has remarkable influence on ash melting points and ash melting behavior. Ash melting points increase with the content of coal char addition. When the content is 20%, an infusible skeleton can be formed, and then the height of ash cone does not change with the rise of temperature.

References

[1] Wang Q Q, Zeng P J. Present situation of the study of coal

ash-fusibility and its main points. Coal Conversion, 1997, 20(2): 33−36.

[2] Qiu J R, Li F, Zheng Y, Zheng C G, Zhou H C. The influences of mineral behavior on blended coal ash fusion characteristics.


Fuel, 1999, 78(8): 963−969. [3] Gupta S K, Gupta R P, Bryant G W, Wall T H. The effect of

potassium on the fusibility of coal ashes with high silica and alumina levels. Fuel, 1998, 77(11): 1195−1201.

[4] Li F, Qiu J R, Zheng Y, Zheng C G. Study on behavior of minerals in blended coal ash during heating process. Journal of Fuel Chemistry and Technology, 1997, 25(5): 400−403.

[5] Yao X Y. Relationship of coal ash fusibility to chemical composition. Journal of Fuel Chemistry and Technology, 1965, 6(2): 151−161.

[6] Chen W M, Jiang N. Relation between the coal ash composition and fusibility. Clean Coal Technology, 1996, 2(2): 34−37.

[7] Li Y G. Discussion on the fusing characteristics of coal ash and the safe operation of the boiler. Hubei Electric Power, 2004, 28(1): 40−41.

[8] Sun F M, Zhao X, Xie Y G, Xu H F. Study on coal ash-fusibility of effluent and fine coal-water slurry. Journal of Engineering for Thermal Energy and Power, 2004, 19(5): 471−473.

Documents

Effect of char on the melting characteristics of coal ash