Study on fusion characteristics of biomass ash

Bioresource Technology 101 (2010) 9373–9381

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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Study on fusion characteristics of biomass ash Yanqing Niu, Hongzhang Tan *, Xuebin Wang, Zhengning Liu, Haiyu Liu, Yang Liu, Tongmo Xu State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, 710049 Shaanxi, China

a r t i c l e

i n f o

Article history: Received 24 April 2010 Received in revised form 22 June 2010 Accepted 26 June 2010 Available online 22 July 2010 Keywords: Biomass Ash Fusion Temperature

a b s t r a c t The ash fusion characteristics (AFC) of Capsicum stalks ashes, cotton stalks ashes and wheat stalks ashes that all prepared by ashing at 400 °C, 600 °C and 815 °C are consistent after 860 °C, 990 °C and 840 °C, respectively in the ash fusion temperature test and TG. Initial deformation temperature (IDT) increases with decreased K2O and went up with increased MgO, CaO, Fe2O3 and Al2O3. Softening temperature (ST), hemispherical temperature (HT) and fluid temperature (FT) do not affected by the concentrations of each element and the ashing temperature obviously. Therefore, the IDT may be as an evaluation index of biomass AFC rather than the ST used as an evaluation index of coal AFC. XRD shows that no matter what the ashing temperature is, the biomass ashes contain same high-temperature molten material. Therefore, evaluation of the biomass AFC should not be simply on the proportion of elements except IDT, but the high-temperature molten material in biomass ash. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction With the depletion of fossil fuels and the increasing serious environmental problems, the utilization of biomass that as a rich ‘‘green” renewable energy source has attracted worldwide attention. The utilization of biomass will reach up to 50 million tons in China after ten years (Xiong et al., 2008). The European Union will exploit approximately 1.3 billion tons of biomass by 2010 (European Environment Agency, 2005), which is equivalent to 180 million tons of oil, and it will reach up to 210–250 million tons of oil equivalent in 2030 (European Commission, 2006). Currently, the exploration techniques of biomass energy mainly are combustion in boiler (Li et al., 2009; Liu et al., 2009); of course, there are some other techniques such as gasification, pyrolysis, liquefaction, etc. (Brown et al., 2000; Iliuta et al., 2010; Sheth and Babu, 2009; Sun et al., 2010; Wang et al., 2008a). But inside boiler, the residual inorganic materials forms slag and fly ash depositing on the tail heating surface, which deteriorate burning, retard heat transfer, cause high temperature corrosion and super-heater explosion (Aho and Silvennoinen, 2004; Knudsen et al., 2004; Szemmelveisz et al., 2009). The effective utilization of fossil fuel is directly affected by its owned AFC, namely IDT, ST, HT and FT. Considerable studies on the coal AFC has been carried out (Gupta et al., 1998; He et al., 2008; Neville, 2009; Song et al., 2009), the AFC are related to SiO2/Al2O, and the IDT is mainly affected by the concentration of K2O (Gupta et al., 1998). Wang et al. study the seaweed AFC and * Corresponding author. E-mail address: [email protected] (H. Tan). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.06.144

find that the melting point of seaweed ash is low, and neither China’s national standard (GB/T212-2001) nor the US ASTM standard are suitable for seaweed biomass (Wang et al., 2008b). With sawdust, wheat straw and rice straw prepared by ashing at 525 °C, except for the loss of 26% K2O in sawdust and 20% Cl loss in wheat straw, the loss of remaining elements could be ignored (Thy et al., 2006). The melting and slagging of biomass is dependent on the concentrations of K, Cl, S, Al and Si in ash (Arvelakis and Frandsen, 2007; Johansson et al., 2008; Kaufmann et al., 2000; Lillieblad et al., 2004; Szemmelveisz et al., 2009). To solve the melting and slagging problems to some extent, several authors propose using various mineral additives such as kaolin (Al2O32SiO2) which can significantly reduce super-heater deposits, corrosion and slagging (Davidsson et al., 2008, 2007; Jensen et al., 1997; Johansson et al., 2008). The slag quantities could decrease by half and one third with kaolin and calcite addition, respectively (Xiong et al., 2008). Adding high concentration of aluminosilicate to biomass can prevent deposition on heat transfer surface efficiently (Aho and Ferrer, 2005). Meanwhile, the efficiency is in positive correlation with the concentrations of aluminum and silicon in ash (Khan et al., 2008). In addition, there are some other methods that can enhance the operation of biomass-fired boiler such as co-combustion (Aho and Ferrer, 2005; Khan et al., 2008; Pettersson et al., 2008; Turn et al., 1997) and leaching (Aho and Ferrer, 2005; Turn et al., 1997). Although some studies have been carried out, the ashing temperature and the nature factors affecting boimass AFC are still uncertain or incompletely understood and further investigations are required. In this paper, capsicum stalk (CMS) ashes which has been single discussed in previous study (Niu et al., 2010), cotton stalks (CNS)

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ashes and wheat stalks (WTS) ashes prepared by ashing at 400 °C, 600 °C and 815 °C are studied by ash fusion temperatures test, TG, XRF and XRD. It aims at revealing the effect of ashing temperature on biomass AFC and the nature factors which influence the biomass AFC.

Table 2 Results of ashing test. T (°C)

Ash (w/%) CMS

CNS

WTS

400 600 815

7.4560 6.3372 5.9536

7.6718 7.2592 4.9797

7.6150 7.0082 6.0100

2. Methods 2.1. Experimental apparatus

2.2. Materials

Fusion temperature test on biomass ash is conducted in a sintering instrument (SJY, Xiangtan Instrument Co., Ltd., Hunan). Using a figuring machine, cylinder samples (u6  8 mm) are made from the collected ash. The heating process is observed and photographed using a high definition video camera with 1.3 million pixels. It is based on the changes in shape detected during the heating of the ash cylinder from room temperature to 1600 °C with a heating rate of 10 °C min1. The atmosphere in furnace is air, and storage interval is 1 °C. The schematic diagram of the sintering instrument is plotted in Fig. 1. Sample fixed in the sample slot is placed in the centre of the furnace, and thermocouple is also fixed in the sample slot. Heating process and the capture of photographs are all controlled by a computer program. A thermal balance (NETZSCH-490PC, Germany) is applied in the TG test of ashes. The sample weights approximately 6 mg and the temperature range is from room temperature to 1300 °C at a constant heating rate of 10 °C/min. The flow rate of N2 and O2 is 80 ml/ min and 20 ml/min, respectively. Repeated experiments are carried out twice. XRF (S4-Pioneer, Bruker Co., Germany) is used for elemental determinations. And the main crystalline compounds in ash samples are identified by XRD using a D/max2400X powder diffractometer (Japan) with the characteristic Cu Ka radiation. Operating conditions are 40 kV and 100 A. Peak identification is performed through comparison with standards coming from JADE5 software package.

CMS, CNS and WTS used in experiment are common in northern China, the former two are from the second power plant of Baoji, Shaanxi, and the last one from the biomass-fired power plant of Bachu, Xinjiang. Corresponding ultimate and proximate analysis are listed in Table 1. The fuels dry at 105 °C for 12 h and then are milled to a maximum 2 mm particle size are ashed at 600 °C (ASTM/E870-82, US) and 815 °C (GB/T212-2001, China) for 2.5 h in muffle furnace, respectively. For comparison, ashes are also prepared by ashing at 400 °C. 3. Results and discussion It is imaginable that the composition including Cl, K, Na and S are great different among three temperature’s ashes. CMS ashes show light gray, nave white, brown colour at 400 °C, 600 °C, 815 °C, respectively. WTS ashes present blue black, black and offwhite, respectively. CNS ashes experience the change from dark gray to light yellow. Moreover, fusion in ashing test is not observed. Ashing results weighed by AEL-200 electronic scale (Shimadzu, Japan) with an accuracy of 1/10,000 are summarized in Table 2. It can be seen from Table 2 that with increasing ashing temperature, ashing rate drops. The weight loss of CMS is higher than CNS and WTS at 400–600 °C, while the weigh loss of CNS is highest at

Fig. 1. Schematic diagram of sintering instrument.

Table 1 Proximate and ultimate analysis of samples. Sample

CMS CNS WTS

Proximate analysis (w/%)

Ultimate analysis (w/%)

Mad

Aad

Vad

FCad

Cad

Had

Oad

Nad

St,ad

Clad

4.44 7.22 3.88

5.17 5.50 6.01

71.79 67.65 72.10

18.6 19.63 18.01

44.04 43.99 43.92

3.94 3.40 4.47

41.19 45.42 40.98

0.91 0.75 0.44

0.31 0.32 0.30

0.218 0.630 0.486

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Fig. 2. Ash fusion temperature of CMS, CNS and WTS.

600–815 °C. With increasing temperature, organic matter burns out, and the low melting point substances evaporate, while the high melting point substances are generated. Different ashing rates

in different ashing temperatures lead to different compositions in ashes, which again affect the AFC. Therefore, it is essential to establish an appropriate standard to distinguish the biomass AFC.

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3.1. Ash fusion temperature test

100

TG/ %

CMS400 CMS600 CMS815

80

1

0 70 -1 60 0

200

400

600

800

1000

1200

T/

a. CMS 100

3

90

2 CNS400

1

CNS600

70

CNS815

0

60 50

DTG/ %/min

TG/ %

80

-1

40 0

200

400

600

800

1000

1200

1400

T/

b. CNS 3

100

90

TG/ %

2

80

WTS400 1

WTS600 WTS815

70

DTG/ %/min

3.2. TG test The combustion characteristics of the three kinds of biomass ashes are conducted by TG. Results are shown in Fig. 3. It can be seen from Fig. 3 that the CMS, CNS and WTS ashes prepared by ashing at 400 °C, 600 °C and 815 °C experience three, two and only one weight loss peaks, respectively. CMS ashes prepared by ashing at 400 °C experience three weight loss peaks at 684 °C (corresponding to DTG peak), 824 °C and 1170 °C, respectively. Ash prepared at 600 °C also has two weight loss peaks at 824 °C and 1170 °C. In addition, ash prepared at 815 °C only has a small weight loss peak at 1170 °C. The three weight loss peaks for CNS ashes are at 698 °C, 943 °C and 1376 °C, and 655 °C, 917 °C and

2

90

DTG/ %/min

In order to study the biomass AFC, the CMS, CNS and WTS prepared by ashing at 400 °C, 600 °C and 815 °C are heated with the SJY Image-Melt Point experimental apparatus. Results are shown in Fig. 2. As seen from Fig. 2, all biomass ashes experience initial deformation, softening, hemispherical, and fluid with an obvious narrowing. As can be seen from Fig. 2a, with increasing ashing temperature from 400–815 °C, IDT of CMS increases 43 °C, while ST, HT and FT drops 19 °C, 14 °C and 8 °C, respectively. But compared with 1100 °C, these changes can be ignored. It seems to be that when the ashes are heated up to above 1100 °C, compositions in three kind of ashes and the AFC are basically the same. There may be the same high melting point substances providing supporting effect of skeleton structure. As seen from Fig. 2b, the IDT, ST, HT and ST of CNS are higher than that of CMS. Meanwhile, compared with the CNS ashing at 400 °C and 600 °C, the IDT of 815 °C is higher above 200 °C. Nevertheless, the difference of ST, HT and ST can be ignored. When the ashes are heated up to above 1100 °C, compositions in three kinds of ashes may be the same and the AFC are the same. As seen from Fig. 2c, compared with CMS, the IDT, ST, HT and FT are relatively lower. Meanwhile, compared with the CNS ashing at 400 °C and 600 °C, the IDT and ST of 815 °C are also higher above 200 °C. However, the change of HT and FT related to more than1150 °C are little. When the ashes are heated up to above 1150 °C, compositions and AFC may be the same. Overall, no matter what the ashing temperature is, the compositions in CMS, CNS and WTS ashes may be the same after 1100 °C?1100 °C and 1150 °C, respectively. It could be inferable that the ashes contain the same high-temperature molten material that provides a supporting effect of skeleton structure in biomass ashes. It seems that the fusion characteristics of biomass ash depend on the high-temperature molten material, rather than simply on the proportion of elements in ashes. In order to confirm this inference, three kinds of ashes are analyzed by XRF and TG quantitatively and by XRD qualitatively.

3

0 60 -1 50 0

200

400

600

800

1000

1200

1400

T/

c. WTS Fig. 3. TG/DTG curves of CMS, CNS and WTS.

Table 3 Results of XRF analyses. Sample

K2O

Na2O

MgO

CaO

Fe2O3

Al2O3

SiO2

TiO2

Cl@O

CMS400 CMS600 CMS815 CNS400 CNS600 CNS815 WTS400 WTS600 WTS815

27.18 23.22 18.85 47.27 43.84 30.52 36.13 35.49 29.99

1.40 1.39 1.43 5.75 9.88 5.16 0.37 0.53 0.78

9.37 11.40 12.56 7.57 7.60 12.05 3.90 4.57 7.01

19.94 21.47 22.90 20.85 20.32 27.83 14.24 15.44 19.27

3.55 3.69 4.10 3.16 3.07 4.25 1.04 1.05 1.35

7.43 7.89 8.47 2.35 2.22 3.53 1.95 2.18 3.63

29.64 29.42 31.19 8.76 8.45 16.49 39.60 37.73 37.70

0.42 0.41 0.46 0.18 0.19 0.17 0.14 0.13 0.23

1.08 1.12 0.03 4.11 4.43 0.00 2.62 2.88 0.04

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1214 °C for WTS. It can be inferred that no matter what the ashing temperature is, when the heating temperature is higher than a certain value, the compositions in the ashes are nearly the same or they contain the same high-temperature molten material. After 860 °C, 990 °C and 940 °C, the AFC are nearly the same as the re-

sults of the above ash fusion temperature test that the AFC are the same and do not depend on the ashing temperature. It is presumed that the AFC depend on its own high-temperature molten material, rather than simply on the proportion of elements in ashes prepared at different temperatures.

K2O IDT/50 / ST/50 / HT/50 / FT/50 /

50

40

Na2O IDT/50 / ST/50 / HT/50 / FT/50 /

30

20 Value

Value

30

20

10

10

0

0

CMS400 CMS600 CMS815 CNS400 CNS600 CNS815 WTS400 WTS600 WTS815

CMS400 CMS600 CMS815 CNS400 CNS600 CNS815 WTS400 WTS600 WTS815

Sample

Sample

b. Na 2O vs. AFC

a. K2O vs. AFC MgO IDT/50 / ST/50 / HT/50 / FT/50 /

30

CaO IDT/50 / ST/50 / HT/50 / FT/50 /

30

20 Value

Value

20

10

10

0

0 CMS400 CMS600 CMS815 CNS400 CNS600 CNS815 WTS400 WTS600 WTS815

CMS400 CMS600 CMS815 CNS400 CNS600 CNS815 WTS400 WTS600 WTS815

Sample

Sample

c. MgO vs. AFC

d. CaO vs. AFC Fe2O3 IDT/50 / ST/50 / HT/50 / FT/50 /

30

Al2O3 IDT/50 / ST/50 / HT/50 / FT/50 /

30

20

Value

Value

20

10

10

0

0 CMS400 CMS600 CMS815 CNS400 CNS600 CNS815 WTS400 WTS600 WTS815

CMS400 CMS600 CMS815 CNS400 CNS600 CNS815 WTS400 WTS600 WTS815

Sample

Sample

e. Fe2O3 vs. AFC

f. Al2O3 vs. AFC

Fig. 4. Concentration of different elements vs. AFC.

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3.3. XRF analysis Results of quantitative analysis by XRF are listed in Table 3. It can be seen that with increasing ashing temperature, the relative content of K2O drops, while MgO, CaO, Fe2O3 and Al2O3 rise, and Cl is nearly zero at 815 °C with a light rise at 600 °C. In addition, by the equilibrium calculation of Cl based on Tables 1–3, the Cl in CMS, CNS and WTS decreases by 28.52%, 6.11% and 21.03% at 400 °C, respectively. Except that, the loss of K, Na, Mg, Ca, Fe, Al, Si and Ti could be ignored (Arvelakis and Frandsen, 2007). All elements are translated into oxides (Cl converted to equivalent oxygen), and then normalized. Previous study argues that the potassium began to volatilize at 750 °C (Thy et al., 2006). However, seen from Table 3, there is varying degree of potassium loss at 400–600 °C. Fig. 4 illustrates the relation between the concentrations of K2O, Na2O, MgO, CaO, Fe2O3, Al2O3, SiO2, TiO2, Cl and the AFC. It can be seen that the IDT is negatively correlated with the concentration of K2O and positively correlated with the concentrations of MgO, CaO, Fe2O3 and Al2O3, and the relations between IDT and Na2O, SiO2, Cl are uncertainly. Maybe that the significant decrease of K2O in CNS and WTS ashes and the significant increase of MgO, CaO, Fe2O3 and Al2O3 at 600–815 °C result the sharp rise of IDT seen in Fig. 2. Meanwhile,

compared with CMS ashes, the higher concentration of K2O and lower MgO, CaO, Fe2O3 and Al2O3 in CNS and WTS ashes lead to the relatively lower IDT. In addition, the ST, HT and FT are not affected by the concentrations of each element. Therefore, the biomass AFC depend on its own high-temperature molten material, rather than simply on the proportion of elements in ashes prepared at different temperatures except IDT. Meanwhile, considering that high concentrations of Si and Al (according to Eq. (1)) could trap alkali halide (KCl and NaCl), which promotes the deposits formation on medium temperature heating surface in boilers, via the formation of aluminosilicate and hydrogen chloride that can directly react with CaO to form calcium chloride (according to Eq. (2)) and thus prevent the re-formation of alkali halide and enhance the operation of biomass-fired boiler (Yanqing et al., 2010).

Al2 O3  2SiO2 þ 2MCl þ H2 O ! M2 O  Al2 O3  2SiO2 þ 2HCI; where, M is K, Na and so on.

CaOðsÞ þ 2HClðgÞ ! CaCl2ðsÞ þ H2 OðgÞ

40

ð2Þ

By comprehensive consideration of the low ashing temperature with high concentrations of K2O and Cl as well as low concentrations of Al2O3 and CaO that aggravate the deposits formation in

SiO2 IDT/50 / ST/50 / HT/50 / FT/50 /

50

TiO2 IDT/50 / ST/50 / HT/50 / FT/50 /

30

20 Value 10 10

0

0

CMS400 CMS600 CMS815 CNS400 CNS600 CNS815 WTS400 WTS600 WTS815

CMS400 CMS600 CMS815 CNS400 CNS600 CNS815 WTS400 WTS600 WTS815

Sample

Sample

h. TiO2 vs. AFC

g. SiO2 vs. AFC

Cl IDT/50 / ST/50 / HT/50 / FT/50 /

30

20 Value

Value

30

20

ð1Þ

10

0 CMS400CMS600CMS815CNS400CNS600CNS815WTS400WTS600WTS815

Sample

j. Cl vs. AFC Fig. 4 (continued)

Y. Niu et al. / Bioresource Technology 101 (2010) 9373–9381

biomass-fired boilers, the ashing temperature of biomass therefore should be low and the IDT may be as an evaluation index of biomass AFC rather than the ST used as an evaluation index of coal AFC. 3.4. XRD analysis To reveal the nature factors used to determine the biomass AFC, the CMS, CNS and WTS ashes prepared by ashing at 400 °C, 600 °C

9379

and 815 °C are analyzed by XRD. Results of XRD analysis are illustrated in Fig. 5. It can be seen from Fig. 5a that the main compositions of the CMS ashes are periclase, quartz, potassium iron oxide and arcanite. In addition, calcite, fairchildite and sylvite exist in the ash prepared at 400 °C. However, calcite and fairchildite disappear in 600 °C ash (Eq. (3)), in which the calcium silicate presents by Eq. (4) and Eq. (5), and sylvite disappears in 815 °C accompanied by the appearance of zeolite(Eq. (6)). The AFC of CMS are therefore mainly

Fig. 5. Results of XRD analyses of biomass ash.

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dependent on the high-temperature molten material built up by periclase, quartz, potassium iron oxide, arcanite, zeolite and calcium silicate, which provide a supporting effect of skeleton structure in the CMS biomass ashes. Seen from Fig. 5b, the main compositions of the CNS ashes are arcanite, periclase, quartz, calcium silicate and potassium iron oxide. Additionally, calcite, dolomite and sylvite exist in the ash prepared at 400 °C. Nevertheless, calcite and dolomite disappear in 600 °C ashes (Eq. (3)), in which the calcium silicate presents (Eq. (7) and Eq. (8)), and sylvite disappears in 815 °C ashes. Arcanite, quartz and potassium iron oxide are all present in CNS ashes prepared by ashing at 400 °C, 600 °C and 815 °C, as well as calcium silicate and periclase exist in 600 °C and 815 °C ashes. The AFC of CNS are therefore mainly dependent on the high-temperature molten material built up by arcanite, periclase, quartz, calcium silicate and potassium iron oxide, which provides a supporting effect of skeleton structure in the CNS biomass ashes. As seen from Fig. 5c, the main compositions of the WTS ashes are arcanite and quartz. In addition, both calcite (Eq. (3)) and sylvite disappear in 600 °C and 815 °C ashes, respectively exist in the ash prepared at 400 °C. The AFC of WTS mainly depend on the high-temperature molten material built up by arcanite and quartz, which provides a supporting effect of skeleton structure in the CNS biomass ashes. Comprehensive consideration, the main reactions in different ashing temperatures can be summarized as follows?

MCO3 ! MO þ CO2 "

ð3Þ

SiO2 þ CaO þ 5C ! CaSi2 þ 5CO "

ð4Þ

or : 2SiO2 þ CaCO3 þ 5C ! CaSi2 þ CO2 " þ5CO "

ð5Þ

2KCl þ Al2 O3  2SiO2 þ H2 O ! 2KAlSiO4 þ 2HCl "

ð6Þ

SiO2 þ 2CaO ! Ca2 SiO4 or : SiO2 þ 2CaCO3 ! Ca2 SiO4 þ 2CO2 "

ð7Þ ð8Þ

where, M represents Ca or the mixture of Ca, Mg and K. Seen from the results of XRD, the AFC of CMS, CNS and WTS ashes prepared by ashing at 400 °C, 600 °C and 815 °C depend on its own high-temperature molten material. The nature factor used to determine the biomass AFC is its own high-temperature molten material providing a supporting effect of skeleton structure in ashes rather than simply on the proportion of elements in ashes prepared at different temperatures. In this paper, after 860 °C, 990 °C and 940 °C, the AFC of CMS, CNS and WTS ashes prepared by ashing at 400 °C, 600 °C and 815 °C are the same and do not depend on the ashing temperature. It is presumed that the AFC are dependent on its own high-temperature molten material, rather than simply on the proportion of elements in ashes prepared at different temperatures. Unfortunately, the high-temperature molten material is not presented in this paper duo to the highest ashing temperature of 815 °C, which is significantly lower that the critical temperature (860 °C, 990 °C and 940 °C). Notwithstanding its limitation, this study can clearly indicate that the AFC are not related with the ashing temperatures and the proportions of the elements in ashes except the IDT that depends on the concentrations of K2O, MgO, CaO, Fe2O3 and Al2O3, meanwhile, the IDT may be as an evaluation index of biomass AFC rather than the ST used as an evaluation index of coal AFC. Moreover, this study does suggest that the ashing temperature of biomass should be low and the study on biomass AFC should be focused on the biomass owned high-temperature molten material formed at high temperature. Meanwhile, it shows the predominant intermediate precursor of the high-temperature molten material in the ashing process of CMS, CNS and WTS, which provide a preliminary study for the further study on the high-temperature molten material.

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