Influence of biomass addition on Jincheng coal ash fusion temperatures

Influence of biomass addition on Jincheng coal ash fusion temperatures

Fuel 160 (2015) 614–620 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Influence of biomass addition...

1MB Sizes 39 Downloads 125 Views

Fuel 160 (2015) 614–620

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Influence of biomass addition on Jincheng coal ash fusion temperatures Xueli Chen ⇑, Jianye Tang, Xiaojun Tian, Li Wang Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, P.O. Box 272, Shanghai 200237, PR China Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, P.O. Box 272, Shanghai 200237, PR China

h i g h l i g h t s  Ash fusion temperatures of Jincheng coal and biomass (cotton stalk and sargassum natans) blends were investigated.  The fusion temperatures of biomass and coal blending ash were lower than their individual ash.  Base to acid ratio (B/A) was related to the fusion temperatures.  The ash fusion mechanisms were discussed by minerals transformations.

a r t i c l e

i n f o

Article history: Received 22 April 2015 Received in revised form 4 August 2015 Accepted 6 August 2015 Available online 17 August 2015 Keywords: Coal Biomass Co-gasification Ash fusion temperature Mineral matters

a b s t r a c t The ash fusion temperatures (AFTs) of Jincheng (JC) coal, two kinds of biomass (cotton stalk (CS) and sargassum natans (SA)) and various coal–biomass mixtures were investigated in this study. Fluid temperatures (FT) were found 1531 °C, 1534 °C and 1428 °C for JC, CS and SA ash respectively, which were higher than the normal operating temperature of entrained bed gasifier. However, the FTs of JC and CS mixture ash were lower than that of either single JC or CS ash, which showed a minimum value of 1292 °C with 80% CS, and the FTs of JC and SA mixture ash showed the similar trend with a minimum value of 1258 °C when SA proportion was 40%. At high temperature, refractory minerals were presented in three respective ashes, which were mullite of JC ash, lime and periclase of CS ash, merwinite and akermanite of SA ash. And they could support the ash skeleton structure. Interaction between coal and biomass ash occurred during blending ash melting process, which formed co-eutectic and decreased the AFTs. The relationship between the ash fusion temperatures and the ash chemical composition was also discussed. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The world’s high dependence on fossil fuels has motivated research efforts to discover alternative sources to fulfill the energy demands. And biomass has been regarded as a potential renewable energy source [1]. Besides, effective use of bioresource suggests significant potential to mitigate the greenhouse emissions problem. Among all the thermal conversion technologies, gasification is an efficient and clean process in which feedstock undergoes partial oxidation reaction with an oxidized medium to produce synthesis gas [2,3]. However, the dispersed distribution and seasonal supply [4] of biomass restrict the further commercialization of biomass alone gasification technology. Although many of the techniques used for coal gasification have their analogues in biomass ⇑ Corresponding author at: Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, P.O. Box 272, Shanghai 200237, PR China. Tel.: +86 21 64250734. E-mail address: [email protected] (X. Chen). http://dx.doi.org/10.1016/j.fuel.2015.08.024 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

gasification, the differences both in the fuel properties and in the associated mineral matter, call in many cases for different solutions in detail [5]. Co-gasification of biomass with coal is considered to be an appropriate way to balance the transition from fossil fuel to renewable bioenergy [6,7]. Among the three gasification technologies, namely, fixed-bed, fluidized bed and entrained flow bed gasification, the last one is considered as one of the most promising technologies which is widely used on a commercial scale for its low pollution and high efficiency [8,9]. In the entrained flow gasifier, organic matters in fuels could be completely combusted, and the inorganic ash was melt to form slag. The gasifier operating temperature must be high enough so that the slag can be extracted at the bottom of the gasifier by a free-flowing liquid [5]. The ash fusion temperature is one of the crucial parameters for gasifier design. Generally, 1400 °C is considered as a breaking point with higher values of ash fusion temperatures requiring fluxing additives [10]. The reduction of ash fusion temperature can reduce the oxygen consumption and

615

X. Chen et al. / Fuel 160 (2015) 614–620

avail the entrained flow gasifier operating [11]. Therefore, knowledge on fusion characteristic of coal and biomass blending ash is required to support the development of commercial entrained flow co-gasification of coal and biomass. Several researchers have studied the ash fusion temperatures or slag characteristics of different rank coals [12,13] and various biomasses [14,15]. However, only a few investigations [16,17] are closely related to the coal and biomass blending ash. It has been reported that the addition of hazelnut shell to the Turkish Elbistan lignite reduced the ash sintering temperatures from 1320 °C to 919 °C and 730 °C for the blends of 5 and 10 wt.%, respectively. However, the addition of rice husk showed limited effect on the sintering temperature as well the deformation temperature of the lignite [16]. The noticeable effect of hazelnut shell is due to the interaction of potassium from biomass with silicon compounds found in mineral matter of lignite. Fang and Jia [17] studied the ash fusion characteristics of corn straw and bituminous coal blends. The four characteristic temperatures decreased when the content of coal reduced from 100% to 50%, and increased when the content of corn straw increased from 50% to 100%. They explained why the ‘‘V” shape curve of the ash melting temperature was due to the different porous structures of different ash. It is well known that the ash melting process of biomass and coal blends is complex because of the various kinds of mineral compositions in biomass and coal ash. And mineral compositions have important influence on the ash melting characteristics. For one thing, melting point of the mineral directly affects the ash fusion temperature. For another, mineral could react with each other to form new phase, thus the ash fusion temperature would be affected. In this study, fusion temperatures of Jincheng coal and biomass (cotton stalk and sargassum natans) blending ash were measured. The relationship between the different constituents in different ash and the fluid temperature of ash were discussed. The underlying ash fusion mechanisms were analyzed by mineralogy. The aims of this work were to study the melting characteristics coal and biomass blending ash. 2. Material and methods 2.1. Materials Jincheng coal (JC), which has abundant reserves in China, is selected as a coal sample in this study. Two kinds of representative biomass, cotton stalk (CS) and sargassum natans (SA), are chosen as biomass samples. CS is taken from Hengshui in Heibei province of China, which is a major crop residue in China. SA has great potential as a feedstock for commercialization because of its short life cycle and no-land breeding [18]. The proximate and ultimate analysis of the samples is shown in Table 1.

2.2. Ash preparation and ash fusion temperature determination Coal was ground in a ball milling and biomass was shredded in a pulverizer. The particle samples were sieved and the final particle size less than 180 lm were used. The ash samples of biomass and coal were prepared according to the standards ASTM E1755-01. About 1 g sample was first placed in a ceramic crucible, which was in turn placed in a muffle furnace. Initially heat the samples to 250 °C at a rate of 10 °C/min and hold for 30 min, then increasing the temperature to 575 °C within 30 min and kept for 3 h. This ashing procedure is used to produce ash samples under controlled temperatures and to avoid the possible volatilization of inorganic elements because of flame burning [19]. Based on the ash yields at this condition, biomass ash and coal ash were mixed manually with various biomass mass ratios (0, 20, 40, 60, 80 and 100 wt.%) on the dry basis. The ashes with different biomass ratio were ground to particle size below 75 lm for ash fusion temperature tests and further analysis. The fusion temperatures of the ash were determined using a 5EAFIII auto-analyzer (Changsha, China) according to Chinese Standard GB/T 219-2008. The ash was made to a triangular ash cone with a height of 20 mm and a bottom of 7 mm equilateral triangle. Then the cone was heated at a rate of 15 °C/min before 900 °C and at 5 °C/min after it under reducing atmospheres. Four characteristic temperatures, deformation temperature (DT), soft temperature (ST), hemisphere temperature (HT) and fluid temperature (FT) could be obtained according to the shape changes of the ash cone. The characteristic temperatures reported in this study are the average value of two duplicate experiments. 2.3. Ash chemical compositions and mineral analysis The chemical compositions of various ashes were determined by ADVANT’X IntelliPower 3600 X-ray fluorescence (XRF) produced by Thermo Electron Company. The XRF operating conditions are 60 kV and 40 mA. Different crystal mineral displays different absorbed or reflected X-rays pattern [11]. To explain ash fusion mechanism, XRD (X-ray diffraction) analysis was used to determine the mineral composition. The ash prepared at 575 °C was gradually heated under reducing atmosphere from 1000 °C to 1400 °C via 100 °C intervals, and then was quenched in liquid nitrogen [20] to avoid the crystal phase changes. The quenched sample was crushed to a particle size of less than 75 lm. The mineral composition and type of the ash samples at various temperatures were measured using a PANalytical X’Pert PRO powder diffractometer, which was operated with 40 kV and 40 mA. 3. Results and discussion 3.1. The influence of biomass on ash fusion temperatures of coal

Table 1 Proximate and ultimate analyses of used samples. Samples

CS

SA

Proximate analysis (dry basis, wt.%) Volatile 12.34 Fixed carbon 64.25 Ash 23.41

JC

83.25 13.67 3.08

55.87 9.21 34.92

Ultimate analyses (dry ash free basis, wt.%) C 89.16 H 3.68 N 1.75 S 0.82 O 4.59

45.97 3.95 1.71 0.30 48.07

34.93 5.11 2.52 1.99 55.45

The ash fusion temperatures of JC, CS and SA were showed in Table 2. It can be seen that the ash fusion temperature of JC is close to CS. However, the ash fusion temperature of SA is lower than that of JC and CS. The maximum temperature differences of ST and FT of JC, CS and SA are 161 °C, 158 °C and 78 °C. FT is generally Table 2 Ash fusion temperature of coal and biomass. Samples

DT (°C)

ST (°C)

HT (°C)

FT (°C)

JC CS SA

1370 1376 1289

1450 1515 1350

1483 1523 1398

1531 1534 1428

616

X. Chen et al. / Fuel 160 (2015) 614–620

considered to be the most important temperature parameter for entrained flow gasification technology. When FT was above 1400 °C, flux agent must be used [10]. The FTs are 1531 °C, 1534 °C and 1428 °C for JC, CS and SA ash respectively. Thus those samples cannot be directly used as entrained flow gasifier feedstock. The influence of CS and SA on ash fusion temperatures of JC coal is presented in Fig. 1. The ash fusion temperatures are plotted as histogram against the mass ratio of biomass in blending fuels. From Fig. 1(a), JC and CS blending ash fusion temperatures decrease with the CS proportion increasing from 0 to 80 wt.%. The FT values of blend ash are 1481 °C, 1365 °C, 1300 °C, 1292 °C corresponds to 20%, 40%, 60%, 80% of the CS proportion, which are lower than that of either single JC or single CS ash. Compared to the FT of JC, the max decline value of FT can reach 239 °C when the CS proportion is 80%. As shown in Fig. 1(b), the SA and JC blending ash fusion temperatures decrease initially and then increase with increasing the ratio of SA, and the FT of each blending ash sample is below 1400 °C. The FT decreases to the minimum value of 1258 °C when SA ratio is 40%. And the decline value can reach 273 °C and 175 °C compared with the FT of single JC and SA ash. Obvious interactions between different ashes take place in the blending ash melting process. Thus both CS and SA can be used as natural fluxing agents by co-gasification with JC coal. 3.2. Ash chemical composition analysis The ash chemical composition could influence the transition of minerals during the ash fusion process. Table 3 shows the ash chemical composition of JC coal and two kinds of biomass. It can

(a) CS

(b) SA Fig. 1. Influence of CS and SA on JC coal ash fusion temperatures.

be seen that SiO2 and Al2O3 are the major oxides in JC coal, and their contents are exceeding 85% in total composition. However, CS ash mainly consists of K2O, CaO, MgO, SO3, Na2O and P2O5. The SA ash mainly contains K2O, CaO, Na2O and Cl. Those distinct differences of ash composition in different fuels are determined by the formation environment of coal and biomass. The main ash compositions of blending ash are shown in Fig. 2. It is clear that SiO2 and Al2O3 contents are reduced with CS and SA ratio increasing. With the increasing of CS ratio in JC and CS blending ash, the contents of K2O, CaO, SO3 are increasing. Similarly, in the JC and SA blending ash, the contents of K2O, CaO, Na2O and Cl are increasing with the rising of SA proportion. Based on the network theory of tetrahedral silicates, ash compositions can be divided into basic oxides and acidic oxides. Basic oxides include K2O, CaO, Na2O, MgO and Fe2O3 and acidic oxides include SiO2, Al2O3 and TiO2. The ratio of basic oxides to acidic oxides (B/A) is usually used to evaluate slagging characteristics [16,21], which can be calculated by Eq. (1).

B=A ¼

K2 O þ CaO þ Na2 O þ MgO þ Fe2 O3 SiO2 þ Al2 O3 þ TiO2

ð1Þ

The relationships between B/A and the FT of different ash are shown in Fig. 3(a). It is observed that FT decreased at the beginning and then increased with the increasing of B/A value. The minimum value of FT is 1258 °C when B/A is 0.9. The high temperature ashes are silicate melts with SiO2-containing material. Generally, the SiO2 network is stable when silicon atoms are organized in a tetrahedral structure through oxygen bonds. If alkaline oxides, such as K2O, CaO and MgO are introduced, a loosening of the network structure will be the result. According to Fig. 2(a), with the increasing of CS ratio in JC and CS blending ash, the contents of alkali metals and alkaline earth metal are increasing. The increasing of CS ratio can enhance the decomposition of the organized tetrahedral structure. Thus the JC and CS blending ash fusion temperatures decreased with the CS proportion increasing from 0 to 80 wt.%. However, with B/A value continued to increase, more and more alkaline oxides were introduced. The redundant alkaline oxides exist independent in the silicate melts, which could form refractory materials and increase the ash fusion temperature. As shown in Fig. 3(a), the B/A value increased with the increasing of biomass ratio. And the influence of SA addition on B/A value is more obvious than the addition of CS with the same biomass ratio. The redundant alkaline oxides exist independently with high SA ratio can increase the ash fusion temperature. Thus there is a different behavior of the ash fusion temperature between a mixture of CS– JC and a mixture of SA–JC. It has been found that intensive slagging was observed when the B/A index is in the 0.75–2 range [21]. It can be concluded that the B/A value of the ash should be in the range of 0.2–3 to ensure the FT below 1400 °C and match for entrained flow gasifier. Except for the basic oxides and acidic oxides, Cl and SO3 are rich in common biomass ash. The relationship between Cl + SO3 and the FT is shown in Fig. 3(b). The results demonstrate that except for CS ash, the trend of FT changes with increasing Cl + SO3 is the same as that of FT changes with increasing B/A. For the blending ash, the B/A index of coal and biomass blending ash could also be predicted using individual B/A index of coal ash and biomass ash, and the blending ratio of biomass ash in the mixture. The calculated and the measured B/A value are listed in Table 4. It can be seen that the calculated B/A value of the blending ash is very closed to the measured value, which is foreseeable because the ash is only mixed and ground without other process. The slight deviation is due to the inaccuracy of XRF measurement. Therefore, it could be concluded that the lowered ash fusion temperature can be also predictable using the calculated B/A index.

617

X. Chen et al. / Fuel 160 (2015) 614–620 Table 3 Chemical composition of coal and biomass ash. Samples

JC CS SA

Ash chemical composition (wt.%) SiO2

K2O

Al2O3

CaO

MgO

SO3

Na2O

P2O5

TiO2

Cl

Fe2O3

Others

53.89 1.63 8.56

1.51 31.67 16.35

32.92 0.28 2.11

2.81 22.51 11.28

0.94 13.10 5.62

1.12 12.13 4.07

1.27 8.41 21.65

0.32 7.26 0.39

1.28 0.05 0.10

0.00 1.48 28.16

3.75 1.15 1.08

0.19 0.33 0.63

(a) FT vs. B/A

(a) JC and CS blending ash

(b) FT vs. Cl+SO3 Fig. 3. Relationship between ash compositions and FT.

(b) JC and SA blending ash Fig. 2. Main ash chemical composition in blending ash with different biomass ratio. (The oxides below 10% in each various biomass ratio are not shown in the figure).

Table 4 The calculated and measured B/A value of coal and biomass blending ash. Biomass ratio

0%

20%

40%

60%

80%

100%

3.3. Minerals analysis

CS

Calculated Measured

0.12 0.12

0.16 0.15

0.21 0.19

0.25 0.29

0.59 0.57

39.15 39.15

The main mineral transitions in JC coal ash is shown in Fig. 4. The low temperature ash is mainly composed of quartz (SiO2) and muscovite [KAl2AlSi3O10(OH)2]. And there also identified some low melt point minerals with weak diffraction peak, such as calcite (CaCO3), anhydrite (CaSO4), dolomite [CaMg(CO3)2] and hematite (Fe2O3). The ash at 1000 °C contained mullite (3Al2O32SiO2), quartz (SiO2) and anorthite (CaAl2Si2O8). In the temperature interval of 575–1000 °C, calcite, anhydrite, dolomite and muscovite could decompose to their corresponding oxides [22–24]. Simultaneously mullite (3Al2O32SiO2) and anorthite (CaAl2Si2O8) are formed by the reaction of the decomposition matters. With temperature continue increasing, the diffraction peak intensity of mullite increased and quartz decreased. Though the melting point of anorthite is 1553 °C [25], the XRD peak of anorthite in the JC ash increased to the maximum value at 1200 °C and disappears at

SA

Calculated Measured

0.12 0.12

0.41 0.36

0.68 0.71

1.32 1.29

2.45 2.37

5.20 5.20

1300 °C. This phenomenon is also been reported by Wu et al. [26], which is due to the partial melting of the phase assemblage. Generally, quartz and mullite are main minerals of JC coal ash at high temperature. Especially when temperature rises to 1400 °C, mullite is the dominant and single crystalline phase in JC ash. The melting point of quartz and mullite are 1710 °C and 1850 °C respectively. Their skeleton structures can act as supporting role against the shape changes of the ash cone. Fig. 5 shows the main mineral transitions of CS ash during heating process. The CS ash at 575 °C is mainly composed of arcanite (K2SO4), periclase (MgO) and sylvite (KCl), calcite (CaCO3). Du

618

X. Chen et al. / Fuel 160 (2015) 614–620

Fig. 4. XRD patterns of JC ash at different temperatures. A: Anorthite(CaAl2Si2O8); Al: Anhydrite(CaSO4); C: Calcite(CaCO3); D: Dolomite[CaMg(CO3)2]; H: Hematite (Fe2O3); M: Mullite(3Al2O32SiO2); Mu: Muscovite[KAl2Si3O10(OH)2]; Q: Quartz (SiO2).

Fig. 5. XRD patterns of CS ash at different temperatures. A: Arcanite(K2SO4); C: Calcite(CaCO3); H: Huntite[Mg3Ca(CO3)4]; L: Lime(CaO); P: Periclase(MgO); S: Sylvite(KCl).

et al. [22] has found the intense volatilization of sylvite occurred in biomass ash from 750 °C to 950 °C. In this study, diffraction signals associated with sylvite was no longer detected in the CS ash at 1000 °C. The calcite diffraction peak also disappeared along with the formation of lime (CaO) at temperature above 1000 °C. Meanwhile, new phase huntite is formed. At the typical entrained flow gasifier operation temperature (1400 °C), arcanite, periclase, huntite and lime are the main crystalline phase in CS ash. Those phases can provide a supporting effect of the skeleton structure and go against the deformation of the ash cone. Fig. 6 shows the XRD diffractograms of 60% JC and 40% CS blending ash at different temperatures. The diffraction peak of quartz decreases with the increase of temperature, because it reacts with other oxides. At 1000 °C, albite (NaAlSi3O8) and akermanite (Ca2MgSi2O7) are observed. This is different with both individual JC coal and CS ash at the same temperature. From 1000 °C to 1300 °C, the transformations of feldspar minerals are the main reactions in the JC and CS blending ash. Co-existing of feldspar minerals and quartz will produce a low-melting eutectic mixture

Fig. 6. XRD patterns of JC and CS blending ash at different temperatures. A: Albite(NaAlSi3O8); Ad: Andesine[Na0.499Ca0.491(Al1.488Si2.506O8)]; Ak: Akermanite (Ca2MgSi2O7); Al: Anhydrite(CaSO4); An: Anorthite(CaAl2Si2O8); As: Anorthite sodian [(Na0.45Ca0.55)Al1.55Si2.45O8]; C: Calcite(CaCO3); D: Dolomite[CaMg(CO3)2]; H: Hematite(Fe2O3); M: Mullite(3Al2O32SiO2); Mu: Muscovite[KAl2Si3O10(OH)2]; Q: Quartz (SiO2); Ri: Ringwoodite(Mg2SiO4); S: Sanidine(KAlSi3O8).

Fig. 7. XRD patterns of SA ash at different temperatures. C: Calcite(CaCO3); H: Halite(NaCl); S: Sylvite(KCl); P: Periclase(MgO); Q: Quartz(SiO2); L: Larnite (Ca2SiO4); M: Merwinite[Ca3Mg(SiO4)2]; Ak: Akermanite(Ca2MgSi2O7).

[27]. At 1400 °C, all of the minerals have melted. Enhanced baselines are observed in the XRD patterns of the blending ash, which indicates that significant amounts of amorphous glassy materials are detected. In SA ash made at 575 °C, halite (NaCl), sylvite (KCl) and quartz (SiO2) crystalline phase are both identified as the major crystalline compounds. Also, calcite (CaCO3) and periclase (MgO) are present. Calcite can be decomposed as previous ash samples. Halite and sylvite can be volatilized or melt at relatively low temperature. As shown in Fig. 7, their diffracted intensity are gradually weakened and disappeared at 1100 °C. Larnite (Ca2SiO4) is formed at 1100 °C. The XRD peaks of larnite and periclase (MgO) are strongest at 1200 °C. When the temperature over 1200 °C, larnite can be reaction with SiO2 and MgO to form merwinite [Ca3Mg(SiO4)2] and akermanite (Ca2MgSi2O7), which could be illustrated by the diffraction peaks disappearing of larnite, quartz and periclase at

X. Chen et al. / Fuel 160 (2015) 614–620

619

the ash is heated to 1400 °C, those high temperature ash melts with low mobility cause sluggish flow and depressed dissolution of the residual refractory constituents. Besides, arcanite, periclase, and lime are the main minerals for CS ash at 1400 °C, and merwinite and akermanite co-exsit for SA ash, which leads to the CS and SA also have high ash fusion temperatures. However, in the blending ash, the alkali metal as a corrector of network is easy to crystallize with SiO2, bridged oxygen bond between silicon and oxygen would convert to non-bridged oxygen bond, which could result in original tetrahedral structure loose and decomposition and form some eutectic [11]. The formation of initial and active low-temperature eutectic melts with high mobility that can quickly dissolve the residual refractory minerals [25]. Thus the coal and biomass blending ash fusion temperatures were lower than that of both coal and biomass ash. 4. Conclusions

Fig. 8. XRD patterns of JC and SA blending ash at different temperatures. Ak: Akermanite(Ca2MgSi2O7); H: Halite(NaCl); S: Sylvite(KCl); A: Anhydrite(CaSO4); Q: Quartz(SiO2); M: Muscovite[KAl2Si3O10(OH)2]; N: Nepheline[KNa3(AlSiO4)4]; Ha: Hauyne[K1.6Ca2.4Na4.32(Al6Si6O24)(SO4)1.52]; Hauyne [Na6Ca2Al6Si6O24(SO4)2]; D: Diopside(CaMgSi2O6).

1300 °C. The generated merwinite and akermanite have a melt point of 1575 °C and 1554 °C individually. Those two refractory minerals could stable co-exist, which are also found by Thy et al. [28] and Lindstrom et al. [29] in high temperature biomass slag. For JC and SA blending ash with 40% SA ratio, ash presented in Fig. 8, some new phase, like nepheline (KNa3(AlSiO4)4), akermanite (Ca2MgSi2O7) and hauyne (K1.6Ca2.4Na4.32(Al6Si6O24)(SO4)1.52) are formed at 1000 °C. Minerals contained potassium and sodium were not detected in SA ash when temperature was above 1000 °C because of the volatilization of halite and sylvite. The JC ash is rich in silicon and aluminum, which can retard the volatilization of alkali metal and trap it into slag [28]. XRD result shows that diopside (CaMgSi2O6) formed at 1100 °C. Diopside can exist in crystalline form when temperature was below 1263 °C [28] and its diffraction peaks vanished at 1300 °C in this study. Hauyne and akermanite co-exist at 1300 °C and eutectic at 1400 °C. The XRD peak of JC and SA blend ash at 1400 °C is similar to that of JC and CS blending ash at the same temperature, which only reflects some amorphous phase. 3.4. The flux mechanism analysis Ash melting process is accompanied by the reaction and fusion of minerals. When the ash is heated, the open pore network transforms to micropore leading to the decrease of porosity level [30]. Thus shrinkage and deformation of the ash cone took place. Some liquid phases produced firstly when the ash is heated to a certain temperature. Those liquid phases can flow and permeate into the micropore. The ash cone structure collapses and becomes to hemispheric. Eventually it tiles to the cone salver when temperature is high enough. In the ash melting process, ash fusion temperatures are influenced by the liquid melts content and the flowability of the liquid melts [31,32]. The more liquid melts produce when ash cone is heated, the easier deformation for the ash cone. In addition, the liquid melts with good flowability can increase the reaction rate of minerals [32]. JC ash is mainly consisted of SiO2 and Al2O3. The fusion mechanism of ash with high content of silicon and aluminum is prolonged softening and subsequent melting of solid phases and formation of less active and high temperature melts with low mobility [25]. Large amounts of solid phase mullite (3Al2O32SiO2) existed when

The ash fusion temperatures of coal and biomass (cotton stalk and sargassum natans) blends were investigated in this study. The high value fluid temperature (FT) of three individual fuel ash could be reduced effectively by the mixing of coal and biomass ash. The FT of different ash was related to B/A but not related to Cl and SO3 contents, which had a relative low value when the B/A is in the range of 0.2–3. At high temperature, the main minerals for coal ash, CS ash and SA ash were mullite and quartz, lime and periclase, merwinite and merwinite respectively, which provided skeleton structures to hinder the deformation of ash cone. While the mixing of coal and biomass ash could form low melting eutectic minerals in the melting process, such as leucite and nepheline, decreasing the ash fusion temperatures. Thus the JC coal with high AFT could be effective utilized by co-gasification with CS or SA. Acknowledgements This study was supported by Major State Basic Research Development Program of China (2011CB200906), Program for New Century Excellent Talents in University (NCET-12-0854) by the Ministry of Education of China and the National Natural Science Foundation of China (21306050). References [1] Wan KD, Wang ZH, He Y, Xia J, Zhou ZJ, Zhou JH, et al. Experimental and modeling study of pyrolysis of coal, biomass and blended coal–biomass particles. Fuel 2015;139:356–64. [2] Nemanova V, Abedini A, Liliedahl T, Engvall K. Co-gasification of petroleum coke and biomass. Fuel 2014;117:870–5. [3] Qin K, Lin WG, Jensen PA, Jensen AD. High-temperature entrained flow gasification of biomass. Fuel 2012;93:589–600. [4] Li SD, Chen XL, Liu AB, Wang L, Yu GS. Study on co-pyrolysis characteristics of rice straw and Shenfu bituminous coal blends in a fixed bed reactor. Bioresour Technol 2014;155:252–7. [5] Higman C, Tam S. Advances in coal gasification, hydrogenation, and gas treating for the production of chemicals and fuels. Chem Rev 2014;114:1673–708. [6] Jeong HJ, Park SS, Hwang J. Co-gasification of coal–biomass blended char with CO2 at temperatures of 900–1100 °C. Fuel 2014;116:465–70. [7] Zhang GJ, Reinmoller M, Klinger M, Meyer B. Ash melting behavior and slag infiltration into alumina refractory simulating co-gasification of coal and biomass. Fuel 2015;139:457–65. [8] Gong X, Lu WX, Guo XL, Dai ZH, Liang QF, Liu HF, et al. Pilot-scale comparison investigation of different entrained-flow gasification technologies and prediction on industrial-scale gasification performance. Fuel 2014;129:37–44. [9] Qin K, Jensen PA, Lin WG, Jensen AD. Biomass gasification behavior in an entrained flow reactor: gas product distribution and soot formation. Energy Fuels 2012;26:5992–6002. [10] Collot AG. Matching gasification technologies to coal properties. Int J Coal Geol 2006;65:191–212. [11] Li WD, Li M, Li WF, Liu HF. Study on the ash fusion temperatures of coal and sewage sludge mixtures. Fuel 2010;89:1566–72. [12] Song WJ, Tang LH, Zhu XD, Wu YQ, Zhu ZB, Koyama S. Effect of coal ash composition on ash fusion temperatures. Energy Fuels 2009;24:182–9.

620

X. Chen et al. / Fuel 160 (2015) 614–620

[13] Guo QH, Zhou ZJ, Wang FC, Yu GS. Slag properties of blending coal in an industrial OMB coal water slurry entrained-flow gasifier. Energy Convers Manage 2014;86:683–8. [14] Niu YQ, Tan HZ, Wang XB, Liu ZN, Liu HY, Liu Y, et al. Study on fusion characteristics of biomass ash. Bioresour Technol 2010;101:9373–81. [15] Leiser S, Cieplik MK, Smit R. Slagging behavior of straw and corn stover and the fate of potassium under entrained-flow gasification conditions. Energy Fuels 2013;27:318–26. [16] Haykiri-Acma H, Yaman S, Kucukbayrak S. Effect of biomass on temperatures of sintering and initial deformation of lignite ash. Fuel 2010;89:3063–8. [17] Fang X, Jia L. Experimental study on ash fusion characteristics of biomass. Bioresour Technol 2012;104:769–74. [18] Wang S, Jiang XM, Han XX, Wang H. Fusion characteristic study on seaweed biomass ash. Energy Fuels 2008;22:2229–35. [19] Thy P, Grundvig S, Jenkins BM, Shiraki R, Lesher CE. Analytical controlled losses of potassium from straw ashes. Energy Fuels 2005;19:2571–5. [20] Wang H, Qiu P, Shi X, Zhang J, Chen Y, Wu S. Effect of key minerals on the ash melting behavior in a reducing atmosphere. Energy Fuels 2011;25:3446–55. [21] Kupka T, Mancini M, Irmer M, Weber R. Investigation of ash deposit formation during co-firing of coal with sewage sludge, saw-dust and refuse derived fuel. Fuel 2008;87:2824–37. [22] Du SL, Yang HP, Qian KZ, Wang XH, Chen HP. Fusion and transformation properties of the inorganic components in biomass ash. Fuel 2014;117:1281–7. [23] Vassileva CG, Vassilev SV. Behaviour of inorganic matter during heating of bulgarian coals: 1. Lignites. Fuel Process Technol 2005;86:1297–333.

[24] Vassileva CG, Vassilev SV. Behaviour of inorganic matter during heating of Bulgarian coals: 2. Subbituminous and bituminous coals. Fuel Process Technol 2006;87:1095–116. [25] Vassilev SV, Baxter D, Andersen LK, Vassileva CG. An overview of the composition and application of biomass ash. Part 1. Phase-mineral and chemical composition and classification. Fuel 2013;105:40–76. [26] Wu XJ, Zhang ZX, Chen YS, Zhou T, Fan JJ, Piao GL, et al. Main mineral melting behavior and mineral reaction mechanism at molecular level of blended coal ash under gasification condition. Fuel Process Technol 2010;91:1591–600. [27] Reifenstein AP, Kahraman H, Coin C, Calos NJ, Miller G, Uwins P. Behaviour of selected minerals in an improved ash fusion test: quartz, potassium feldspar, sodium feldspar, kaolinite, illite, calcite, dolomite, siderite, pyrite and apatite. Fuel 1999;78:1449–61. [28] Thy P, Jenkins BM, Lesher CE, Grundvig S. Compositional constraints on slag formation and potassium volatilization from rice straw blended wood fuel. Fuel Process Technol 2006;87:383–408. [29] Lindstrom E, Larsson SH, Bostrom D, Ohman M. Slagging characteristics during combustion of woody biomass pellets made from a range of different forestry assortments. Energy Fuels 2010;24:3456–61. [30] Gupta SK, Wall TF, Creelman RA, Gupta RP. Ash fusion temperatures and the transformations of coal ash particles to slag. Fuel Process Technol 1998;56:33–43. [31] Huggins FE, Kosmack DA, Huffman GP. Correlation between ash fusion temperatures and ternary equilibrium phase diagrams. Fuel 1981;60:577–84. [32] Ninomiya Y, Sato A. Ash melting behavior under coal gasification conditions. Energy Convers Manage 1997;38:1405–12.