Influencing Mechanism of Additives on Ash Fusion Behaviors of Straw

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Influencing Mechanism of Additives on Ash Fusion Behaviors of Straw Fenghai Li,*,†,‡,§ Hongli Fan,† Mingxi Guo,† Qianqian Guo,† and Yitian Fang§ †

Department of Chemistry and Chemical Engineering, Heze University, Heze, Shandong 274015, China School of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan 454003, China § State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, China ‡

ABSTRACT: Ash fusion characteristics of straws [wheat straw (WS), corn stalk (CS), and rice stalk (RS)] and the effects of additives (kaolinite and dolomite) on their fusion behaviors under a reducing atmosphere were explored using an inductively coupled plasma atomic emission spectrometry, X-ray diffractometer (XRD), network theory, and Factsage calculation. The results shows that kaolinite and dolomite can promote the ash fusion temperatures (AFTs) of three straws, respectively. For three straw ashes, kaolinite makes their deformation temperatures (DTs) obviously increase, while dolomite results in obvious increases in their flow temperatures (FTs). The DT is closely related to the contents of low melting point (MP) minerals and amorphous matter, and the FT is mostly determined by the skeleton structure of high MP mineral. Dolomite addition makes three straw AFTs increase through the formations of high MP lime, magnesia, and merwinte, while SiO2 from kaolinite decomposition might lead to the generation of low MP mineral forsterite, kirschstenite, and amorphous matter, especially for WS and CS with low silicon content. High MP kaliophilite formation causes an AFT increase of the mixture of straw and kaolinite. The combination of XRD, network theory, and FactSage calculation provides a good method to explore the AFT variation mechanism of straw ashes from mineral evolution.

1. INTRODUCTION With increasing pressures of energy and the environment,1,2 increasing attention has been paid to biomass utilization worldwide because of its characteristics of being CO2 neutral and having a low cost and wide distribution and its potential provision of feed-stock for heat, power, transport, and chemical product.3−6 Among the conversion technologies of biomass (e.g., combustion, gasification, and biochemical conversion), fluidized-bed gasification or combustion is considered to be a promising way to utilize biomass due to its comparatively low operating temperature (generally at ∼900 °C), homogeneous temperature distribution, and wide adaptability to raw materials.7,8 However, compared with coal, biomass generally has high contents of alkali (potassium and sodium) and alkali earth, which makes its ash fusion temperature (AFT) low. This generally leads to the high occurrence of ash related problems (e.g., slagging and agglomeration) during biomass combustion or gasification.9−11 These issues might result in unstable operations and even shutdowns of the system. Thus, it is of importance to modify biomass ash fusion characteristics. During biomass conversion, formation of ash elements results from two ways: some are presented in biomass as salts, which chemically bond to the carbon structure (inherent ash); others are from mineral soil particles that have been caught during biomass growth or swiped during harvest and transport (foreign ash).12 In general, the inherent ash components are homogeneously distributed in biomass and contain much more alkali than the foreign ash. In the process of biomass combustion or gasification, with the decomposition of the organic structure (mainly composed of hemicelluloses, cellulose, lignin, protein, etc.), the alkali metals are released and © XXXX American Chemical Society

transferred in the form of either vapor species or solid particles.13 The alkali vapor species are mostly in the forms of M(g), MCl(g), (MCl)2(g), M2SO4(g), and MOH(g) (M represents K or Na).14 KCl and K2SO4 are the dominant alkalicontaining substances that influence biomass slagging formation.15 Sometimes K2Ca(SO4)2 and K3Na(SO4)2 also played important roles in slagging. These resulted in alkali-induced slagging. While alkali metals in solid particles are mostly in the form of M-silicates and M-aluminosilicates, some of the Msilicates generally have low AFT due to low temperature eutectics formation (e.g., K2O·2SiO2), and these low AFT silicates might attach to the surface of fly ash particles, which reduced the AFT of fly ashes and increased its viscosity, resulting in an increase in the formation of particle agglomeration.15 This was called silicate melt-induced slagging. In general, three methods are used to mitigate these issues: additives, coal blending, and leaching.11 The essence of these methods is to change ash composition through addition (additive, coal blending) or removal (leaching) from straw ashes.16−18 By comparison, leaching (water leaching or acid leaching) is an effective strategy to remove inherent alkalis, sulfur, and chlorine from straw (chlorine promotes the transfer of potassium in the gas phase). The alkali chlorides might either be deposited on cold surfaces (chlorine may be substituted by sulfur or by reaction with the protective oxide layer of the metal surface, causing corrosion) or take part in other further reactions.11,19,20 However, a lot of energy is consumed to dry Received: December 18, 2017 Revised: February 10, 2018 Published: February 13, 2018 A

DOI: 10.1021/acs.energyfuels.7b04012 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Characteristics of Three Biomass Samplesa proximate analysis (wt %)

a

ultimate analysis (wt %)

samples

Mad

Vad

Aad

FCad

WS CS RS

3.36 9.22 5.87

72.29 65.23 68.51

7.61 6.27 11.65

14.60 19.28 15.97

samples

Na2O

K2O

MgO

CaO

Fe2O3

SO3

WS CS RS

1.24 0.48 0.85

32.56 26.34 18.30

9.22 8.62 1.70

9.64 16.86 3.21

2.48 4.17 0.80

3.83 1.52 1.20

Cd 49.20 50.12 47.19 ash composition (wt %)

Hd

Od

Nd

Sd

6.85 4.50 10.65

37.87 44.04 40.28

0.63 1.13 0.92

0.79 0.21 0.96

Al2O3

SiO2

Cl2O

P2O5

1.92 5.97 2.37

30.14 26.58 62.51

7.87 8.49 6.03

1.12 0.97 1.03

M, moisture; A, ash; V, volatile; FC, fixed carbon; ad, air-dry basis; d, dry basis. 1.92%; CS: 5.97%; RS: 2.37%), and the silica content in RS is higher than those of the other two (RS: 62.51%), while alkali content is comparatively high (especially for K2O, WS: 32.56%; CS: 26.34%; RS: 18.30%). Kaolinite and dolomite were the products of a chemical factory affiliated with Tianjin University. 2.2. Preparation of Ash Samples. The straw laboratory ashes were prepared on the basis of ASTM E1755-01 standards. The dried raw materials (∼10 g) were put into the muffle furnace (SX2-8-16ASP, Kewei Co., Beijing, China) and heated through a temperatureprogrammed route. First, the temperature was heated to 250 °C at 10 °C/min and maintained for 30 min, and then, it increased to 575 °C within 30 min and was maintained for 3 h.26 The ash samples of straw ashes and their mixture at different temperatures were prepared as follows. The additives (kaolinite and dolomite) were crushed to less than 0.120 mm, put into three straw ashes at a certain mass ratio (0, 5%, 10%, 15%, and 20%), and mixed until reaching uniformity, respectively. First, the ash samples (∼1.0 g) were placed into a ceramic crucible, and then, the crucible was transferred into the low-temperature zone of a fixed bed tube furnace (Figure 1). Then, the mixture gases of 50% carbon dioxide and 50%

these leaching straws. Blending coal is favorable because it allows one to overcome the seasonal shortage of biomass, enhance coal conversion efficiency (high alkali and alkalineearth metal in biomass act as the catalyst of coal conversion21), and increase the AFT of mixed biomass. However, the differences in their densities and reactivity make it impossible to mix evenly and react simultaneously, and two-segment reaction might occur during their coconversion.22,23 The additive in the process of biomass conversion has the advantages of enhancing the mixture AFT by altering the ash composition with refractory elements (e.g., Si and Al), converting low melting point (MP) mineral into high MP compounds through chemical reaction, and reducing the problematic specie (i.e., KCl and K2SO4) concentration by absorption.24 Lots of investigations have been performed on the modification in the AFT of biomass by additives; however, due to the variations in biomass ash composition (e.g., agricultural residues are low in Ca yet high in Si and K; woody biomass is low in Si and K yet high in Ca; animal residues are high in P and Ca) and the complexities in mineral interaction, the modification behavior and mechanism of the additives on the biomass AFT have not been clarified. Moreover, straw (rice, wheat, and corn account for 70−75%) is plentiful in China, making up for approximately 50−60% of the total biomass from the perspective of energy amount.25 The aim of the paper was to explore the influences of two typical additives, kaolinite (Al 2 O 3 ·2SiO 2 ·2H 2 O) and dolomite (MgCO3·CaCO3), on straw AFT (wheat straw [WS], corn stalk [CS], and rice stalk [RS]) and their regulating mechanisms. It might provide the basic data for the regulation in straw ash characteristics and the research and development of straw conversion technology.

Figure 1. Schematic diagram of a fixed bed tube furnace. Legend: 1 hydrogen gas cylinder; 2 carbon dioxide cylinder; 3 gas valve; 4 mass flow meter; 5 moving sample injector; 6 sealing element; 7 stainless steel tube; 8 electric heater; 9 temperature controller; 10 thermoelectric couple.

2. EXPERIMENTAL SECTION 2.1. Sample Characterization. Three air-dried typical straws, i.e., WS, CS, and RS, were selected from the countryside in Heze city (southwest of Shandong province, China). The samples were crushed to 0.180−0.250 mm, respectively. The samples were put into a vacuum drying oven (FZG-32W, Ruiao Co. Ltd., Nanjing, China) and dried at 105 °C for 12 h. The results of proximate analyses (conducted on a SDLA 718 proximate analyzer (SUNDY Co. Ltd., Changsha, China)) and ultimate analyses (performed on a PE 2400 analyzer (PerkinElmer, USA)) are shown in Table 1. The ash compositions determined by an inductively coupled plasma atomic emission spectrometry (ICP-AES, icap 6300, Thermo Fisher Scientific, USA) are also presented in Table 1. As can obviously be seen, the volatile contents in three straw samples are high (WS: 72.30%; CS: 66.43%; RS: 68.51%), and the ash content in RS is higher than those of the other two. The contents of alumina in three straws are low (WS:

hydrogen were introduced to simulate a reducing atmosphere during the process of biomass gasification,21,26,27 and the furnace was heated at 5 °C/min until it reached the preset temperature. After that, the ceramic crucible was inserted into the constant-temperature zone of the furnace, and the temperature was maintained for 15 min. Finally, the samples were taken out and put into ice water rapidly to avoid the crystal segregation and phase transformation of ash samples. The quenched samples were dried at 105 °C for 36 h in the vacuum drying oven and then stored in a drying cabinet.27 The samples were milled to less than 0.074 mm before measurement. 2.3. Fusion Temperature Test. The AFTs of ash samples (straw ashes or the mixed ashes) are measured on a KDHR-8 intelligent AFT B


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Energy & Fuels determination meter (Keda Ltd. Co., Hebi, China) under reducing atmosphere (CO2/H2, 1:1, volume ratio) according to a standard test method (ASTM D1857). The four characteristic temperatures, i.e., deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT), were recorded on the basis of the specific shapes of ash cones. 2.4. Analytical Measurement. 2.4.1. Ash Composition Measurement. The ash compositions were conducted on the icap 6300 ICPAES with high-frequency power (1150 W) and 0.65 L/min gas velocity, and the results are presented in the forms of oxides. Under these conditions, the ICP-AES can detect concentrations from per billion to percent levels with high precision (relative standard deviation, RSD, < 0.50%). 2.4.2. X-ray Powder Diffractometer Measurements. A D/max-rB X-ray powder diffractometer (XRD, Rigaku Co., Tokyo, Japan) with CuKα radiation (40 kV, 100 mA) and software MDI JADE 6.5 were used to analyze the mineral compositions of ash samples. The samples were scanned at 5° 2θ/min with 0.01° step size in 10−70° 2θ. A semiquantitative normalized reference intensity ratio (RIR) method was used to determine the mineral content; the ratio is the diffraction intensity of each pure phase divided by that of Al2O3 standard material.28 The precision of the method changes with the diffraction intensity of each phase; it is estimated as ±25% for weakly diffracting phases and ±10% for strongly diffracting phases, and the amorphous matter content is the difference between the total crystalline-phase content and the bulk chemical composition of ash samples. 2.5. Thermodynamic Equilibrium Calculations. The multiphase equilibria and phase transitions of ash samples in a reducing atmosphere (H2/CO2, 1:1, volume ratio) at 0.10 MPa were predicted by the FactSage (version 7.1) software package. The oxides SiO2, Al2O3, K2O, CaO, Na2O, MgO, and Fe2O3 were selected as input data in the equilibrium module. Phase formation data for these oxides and their combinations were chosen from the database of FToxid and FACTPS. The calculations were carried out from 900 to 1500 °C with an interval of 50 °C. The liquid-phase contents in the mixed ashes with increasing temperatures were also calculated by the FactSage software.

straws (WS or CS), and the difference in the AFT variation trend in mixed RS for two additives at the same mass ratio is small, which might be related to its high silicon content (SiO2: 62.51%). However, for low silicon content WS and CS, the increasing trend is different for different additives at the same mass ratio. As kaolinite content increases, DT increases more than that of the other three temperatures (for CS, the FT of mixed ashes with kaolinite mass ratio at 5% or 10% is a little lower than that of CS original ash), and for dolomite addition, FT increases more rapidly than that of the other three temperatures. 3.2. Network Theory Analyses. Ashes under high temperature can be considered to be complex structure silicate melts with SiO2-containing materials.31,32 The relationship between silicon and oxygen atom is described by the bond of bridging (BO, Si−O−Si) and nonbridging oxygen bonds (NBO, Si−O).33 The AFT is correlated to the network stability, which is determined by the ratio of network formers and network modifiers. In ash compositions, the acidic oxides (e.g., SiO2 and Al2O3) with high ionic potential are easy to combine with oxygen to generate silicate and alumino-silicate stable networks, which makes an increase in AFT, while network modifiers (e.g., Na2O, K2O, CaO, and MgO) can convert BO to NBO through the transformation of the stable network from tectosilicates, ino-silicates, cyclo-silicates, and soro-silicates to nesosilicates,26 resulting in a decrease in the AFT. The NBO/BO ratio in the network of blends varies with composition, which results in the changes in their AFTs.34 The NBO/BO ratio was calculated according to the equation

3. RESULTS AND DISCUSSION 3.1. AFT Variation with Additive Content. 3.1.1. Ash Fusion Characteristics of Three Origin Straw. The AFTs of three origin straws are presented in Table 2, which decrease in

where compositions were expressed by molar fractions of oxides.35,36 The ash compositions of the CS and kaolonite mixture based on Table 1 and kaolinite composition (kaolinite is calculated by silica and alumina) are shown in Table 3. The variations in NBO/BO ratio with increasing kaolinite content based on Table 3 are shown in Table 4. For three mixed straw ashes, the BO content increases with an increase in kaolinite. This might explain the AFT increase for three straw ashes with kaolinite addition. Moreover, the NBO/BO ratio of WS mixed ashes decreases more obviously with increasing kaolinite (WS: 0.524 − 0.312 = 0.192) than that of CS mixed ashes (CS: 0.249 − 0.187 = 0.062), which reveals the AFT variation difference for them with kaolinite content increase. The ratio of NBO/BO of WS is higher than that of CS, which might result in the CS with a higher AFT. However, the NBO/BO ratio cannot explain the RS with higher AFT than that of WS and the AFT variation of three straws with dolomite addition; it might be related to the ionic potentials of calcium ion (Ca2+) and magnesium ion (Mg2+), because the species and amounts of mineral matter also have important influences on ash fusion characteristics.35 The ionic potential of Ca2+ (20 nm−1) or Mg2+ (30 nm−1) is higher than that of potassium ion (K+, 7.5 nm−1); it tends to combine with oxygen, resulting in a decrease in Si− O combination force.22,37 As the elements of calcium and magnesium increase, Ca2+ or Mg2+ gradually replace K+ in the semimolten alumino-silicate and cause the higher MP formations of monticellite (MP: 1390 °C) and merwinite (MP: 1550 °C). 3.3. Mineral Variation Analyses. 3.3.1. Mineral Evolution as Straw Ashes Are Heated. The AFT mainly depends on the

NBO/BO = (CaO + MgO + K 2O + FeO − (Al 2O3 + Fe2O3))/(SiO2 + 2(Al 2O3 + Fe2O3))

Table 2. Ash Fusion Characteristics of Three Origin Straw Ashes WS CS RS

DT (°C)a

ST (°C)b

HT (°C)c

FT (°C)d

962 1065 1120

1055 1105 1170

1092 1124 1250

1123 1164 1310

a

Deformation temperature. bSoftening temperature. cHemispherical temperature. dFlow temperature.

the order RS > CS > WS (e.g., DT is a more important parameter than the three others to asses the ash-related problems during biomass conversion29). The effects of potassium oxide (K2O) content on the AFT are affected by silicon, aluminum, and calcium content, and for biomass ashes with high silica/alumina ratio, the AFT decreases with increasing K2O content.30 The fact that K2O content in three straw ashes decreases in the order WS > CS > RS might explain the AFT variation in three straws. 3.1.2. AFT Variation through Additives. The modifications in AFT for three straws by additive addition at different mass ratios are given in Figure 2. The AFTs of three mixed straws all increase with increasing additive mass ratio. For high AFT RS, the AFT increase is slower than that of the other two low AFT C


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Figure 2. AFT variation with increasing additive mass ratio.

samples at different temperatures (575, 700, 800, 900, 1000, and 1100 °C) were prepared in the fixed bed furnace. The XRD patterns of CS ash samples with increasing temperature are shown in Figure 3, and its mineral compositions at different temperatures determined by RIR are presented in Table 4. At 575 °C, the minerals in the CS ashes were made up of sylvite, potassium carbonate, quartz, and goothite (α-FeO(OH)). As the temperature increased, some

generation of mineral species and content during ash being heated, which can be determined by XRD.32 For a given mineral, the crystal content is approximately proportional to its peak height ratio in XRD patterns.38 To explore the AFT variation mechanism with an increase in additive content, it is necessary to investigate the mineral evolution in biomass ash with increasing temperature. Take CS for example, the ash D


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Energy & Fuels Table 3. Ash Composition of Straw and Kaolinite Mixtures ash composition (wt %) samples

Na2O

K2O

MgO

CaO

Fe2O3

SO3

Al2O3

SiO2

Cl2O

P2O5

100% WS + 0% kaolinite 95% WS + 5% kaolinite 90% WS + 10% kaolinite 85% WS + 15% kaolinite 80% WS + 20% kaolinite 100% CS + 0% kaolinite 95% CS + 5% kaolinite 90% CS + 10% kaolinite 85% CS + 15% kaolinite 80% CS + 20% kaolinite 100% RS + 0% kaolinite 95% RS + 5% kaolinite 90% RS + 10% kaolinite 85% RS + 15% kaolinite

1.24 1.19 1.13 1.08 1.02 0.41 0.39 0.37 0.36 0.34 0.85 0.37 0.34 0.31

32.56 31.15 29.72 28.27 26.80 26.34 25.20 24.04 22.87 21.68 18.30 24.11 21.95 19.85

9.22 8.82 8.42 8.01 7.59 8.69 8.31 7.93 7.54 7.15 1.70 7.95 7.24 6.54

9.64 9.22 8.80 8.37 7.93 16.86 16.13 15.39 14.64 13.88 3.21 15.43 14.05 12.71

2.48 2.37 2.26 2.15 2.04 4.17 3.99 3.81 3.62 3.43 0.80 3.82 3.49 3.14

3.83 3.66 3.50 3.33 3.15 1.52 1.45 1.39 1.32 1.25 1.20 1.39 1.27 1.15

1.93 3.84 5.77 7.73 9.72 5.99 7.72 9.48 11.26 13.06 2.37 9.40 12.71 15.90

30.15 31.19 32.24 33.3 34.38 26.56 27.75 28.96 30.19 31.43 62.51 28.87 31.05 33.27

7.87 7.53 7.18 6.83 6.48 8.49 8.12 7.75 7.37 6.99 6.03 7.77 7.08 6.40

1.12 1.07 1.02 0.97 0.92 0.97 0.93 0.89 0.84 0.80 1.03 0.89 0.82 0.73

magnesium (MgO: 8.62%). The occurring reaction could be deduced and listed as follows.41,42

Table 4. NBO/BO Ratio Variation of Three Mixed Straw Ashes with Increasing Kaolinite Content kaolinite mass ratio (%)

NBO/BO ratio

lime(CaO) + magnesia(MgO) + quartz (SiO2 )

ash types

0

5

10

15

20

WS CS RS

0.524 1.246 0.249

0.449 1.090 0.230

0.392 0.983 0.215

0.348 0.888 0.2000

0.312 0.810 0.187

→ monticellite(CaMgSiO4 )

(1)

kylvite(KCl) + quartz (SiO2 ) + alumina(Al 2O3) + vapor(H 2O) → kaliophilite(KAlSiO4 ) + hydrogen chloride(HCl)

(2)

goethite(α‐FeO(OH)) → wustite(FeO)

(3)

wustite(FeO) + lime(CaO) + silicon(SiO2 ) → kirschsteinite(CaFeSiO4 )

(4)

magnesia(MgO) + quartz (SiO2 ) → forsterite(Mg 2SiO4 ) (5)

monticellite(CaMgSiO4 ) + forsterite(Mg 2SiO4 ) → merwinite(Ca3MgSi2O8) + magnesia(MgO)

(6)

magnesia(MgO) + wustite(FeO) → magnesiumferrousoxide((MgO)0.77 (FeO)0.23 ) Figure 3. XRD patterns of CS ashes at different temperatures. 1 sylvite (KCl); 2 potassium carbonate (K2CO3); 3 quartz (SiO2); 4 goothite (α-FeO(OH)); 5 monticellite (CaMgSiO4); 6 kaliopilite (KAlSiO4); 7 forsterite (Mg2SiO4); 8 kirschsteinite (CaFeSiO4); 9 merwinite (Ca3MgSi2O8); 10 magnesium ferrous oxide ((MgO)0.77(FeO)0.23)).

(7)

3.3.2. Variations in Mineral Evolution with Increasing Additive Content. The interaction of mineral with increasing temperature results in the variation in its species and content; the mineral composition in ash samples at a certain temperature can be used to assess its fusion characteristics.22 Thus, the mineral compositions of CS mixed ash at 1000 °C were conducted by XRD to explore the CS AFT variation difference by the additions of kaolinite and dolomite. Figure 4 shows the ash XRD patterns of CS and their mixtures, and Table 5 indicates the mineral composition based on the RIR method. As can be seen from Figure 4 and Table 5, the mineral content increased and the amorphous matter content decreased with an increase in additive content (kaolinite or dolomite). The content of amorphous matter decreased more obviously with kaolinite addition (20.48 − 8.28 = 12.20) than with dolomite addition (20.48 − 14.29 = 6.19); this could be explained

sylvite changed into gaseous phase, which resulted in an decrease in sylvite content.39 The monticellite (CaMgSiO4), kaliophilite (KAlSiO4), forsterite (Mg2SiO4), and kirschsteinite (CaFeSiO4) were generated at 800 °C, and then, some mineral transformed into amorphous matter, which was indicated by the distorted baseline in XRD pattern from 10° to 30° 2θ (900 °C).40 The formation of merwinite (Ca3MgSi2O8) resulted from the interaction of monticellite and forsterite. At 1000 °C, the magnesium ferrous oxide ((MgO)0.77(FeO)0.23) was formed because of its higher contents of iron (Fe2O3: 4.17%) and E


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Table 5. Mineralogical Composition of CS Ashes and Its Mixtures at 1000 °C mineral skylite monticellite lime merwinite magnesium ferrous oxide kirschsteinite kaliophilite amorphous mattera mineral skylite monticellite lime merwinite magnesium ferrous oxide kirschsteinite magnesia amorphous mattera a

CS (wt/%)

CS + 5% kaolinite (wt/%)




18.38 5.37 6.28 32.17 11.21

15.27 12.74 3.71 22.19 8.37

11.85 20.01

5.76 23.98

28.31

15.57 4.25

11.28

9.42

6.11 20.48

7.04 12.37 18.31

8.64 24.36 15.32

9.27 34.47 13.24

11.75 42.24 8.28

CS (wt/%)

CS + 5% domolite (wt/%)



CS + 20% dolomite (wt/%)

18.38 5.37 6.28 32.17 11.21

15.37 4.39 7.87 37.24 10.24

7.87 12.09 44.24 9.78

14.67 51.47 5.37

16.27 57.46

6.11

5.79

20.48

19.10

4.12 3.24 18.23

3.22 9.17 16.10

11.98 14.29

Includes both amorphous phase and any carbon (char) components.

made the FT increase again, because the FT was mostly determined by the skeleton structure of the high MP mineral. As domolite mass ratio increased, the decomposition of dolomite led to a high amount of MP lime and a magnesia (MP: 2852 °C) increase. Equations 5 and 6 or monticellite (CaMgSiO4) + lime (CaO) + quartz (SiO2) → merwinite (Ca3MgSi2O8) led to an increase in merwinite content and a decrease in montecllite content. 3.4. FactSage Software Calculation. 3.4.1. Ash Sample Equilibrium Calculations. To further explore mineral transformation and analyze the effect of the mechanism of additive on the ash fusion characteristics of straw, the FactSage7.1 software was used to calculate the ideal equilibrium mineral compositions of two mixed ashes (85% CS + 15% kaolinite and 85% CS + 15% dolomite) under reducing atmosphere (H2/ CO2, 1:1 volume ratio). As the temperature increases, the formations of low MP chloride compounds (e.g., KCl) in the straw ashes might lead to an increase in particle size, which decrease its sintering temperature. As the temperature increased further, the potassium is mostly released in the form of KCl at 700−800 °C,38,43 and sulfur was released above 900 °C.13 Thus, the contents of SO3 and Cl2O and its corresponding K2O content in the form of KCl were subtracted from its raw ash composition before they were normalized and put into the FactSage calculation. The variations in ideal equilibrium mineral compositions of ash sample with temperature increases are shown in Figure 5. As can be seen from Figures 4 and 5, most mineral compositions from the software calculation are consistent with XRD experimental results; the small difference results from the calculation based on ideal chemical equilibrium state.25 Forsterite was not formed in the CS ash (Figure 5a) through the FactSage software simulation calculation; this might result from the low content of magnesia (8.62%) in CS ashes. In the temperature range, the 85% CS +

Figure 4. XRD patterns of CS and its mixture ashes at 1000 °C. (a) CS+kaolinite; (b) CS+dolomite. 1 sylite (KCl); 2 monticellite (CaMgSiO 4); 3 lime (CaO); 4 merwinite (Ca3MgSi2O8); 5 magnesium ferrous oxide ((MgO)0.77(FeO)0.23); 6 kirschsteinite (CaFeSiO4); 7 kaliophilite (KAlSiO4); 8 magnesia (MgO).

because DT of CS mixed ashes decreased more quickly with the addition of kaolinite than with the addition of dolomite. With an increase in kaolinite mass ratio, the contents of lime, skylite, merwinite, and magnesium ferrous oxide decrease and the contents of kaliophilite, monticellite, and kirschstenite increase. This might be explained as follows: The decomposition of kaolinite resulted in an increase in quartz and alumina content. The skylite transferred into kaliophilite due to the eq 2). Due to the influence of chemical equilibrium, the magnesium ferrous oxide might be decomposed into the magnesia and wustite, the reaction (merwinite (Ca3MgSi2O3) + quartz (SiO2) + magnesia (MgO) → monticellite (CaMgSiO4)), and eq 4 led to merwinite and lime content decreases and an increase in the contents of monticellite and kirschteinite. High MP lime (MP: 2580 °C) and merwinite content decreased with increasing kaolinite causing the FT to decrease, and an increase in high MP kaliophilite (MP: 1800 °C) content F


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3.4.2. Variation Analyses of Liquid-Phase Content. FactSage was used to predicate the proportions of solid and liquid phases of ash samples under reducing atmosphere with increasing temperature.44,45 The proportions of liquid phase in ash samples at different temperatures are shown in Figure 6. As

Figure 6. Liquid-phase content variation in CS ash mixture with increasing temperature.

can be seen from Figure 6b, the liquid-phase proportion decreases with an increase in dolomite content at the same temperature, which is consistent with the CS AFT variation trend of blended dolomite. For kaolinite addition (Figure 6a), at low temperature, liquid-phase content decreased more obviously than that of dolomite at the same mass ratio; this might be explained because its DT increases more quickly than when dolomite is added. At high temperature, the liquid-phase content was higher than that of dolomite addition at the same mass ratio, which could explain the fact that its ash FT is lower than that of CS mixed ashes with dolomite addition. The contents of liquid phase of CS mixed with kaolinite ashes were higher than that of CS ashes at higher temperature, which result in the lower FT.

Figure 5. Phase assemblage temperature curves for ash samples.

15% kaolinite ashes changed into liquid entirely at 1200 °C, while the ash samples of 85% CS + 15% dolomite could not be changed into liquid at 1500 °C; this might explain why the FT of CS mixed dolomite was higher than that of the mixed ashes of CS and kaolinite. G


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Energy & Fuels

(10) Ding, L.; Gong, Y.; Wang, Y.; Wang, F.; Yu, G. Characterisation of the morphological changes and interactions in char, slag and ash during CO2 gasification of rice straw and lignite. Appl. Energy 2017, 195, 713−724. (11) Niu, Y.; Tan, H.; Hui, S. Ash-related issues during biomass combustion: Alkali-induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, and related countermeasures. Prog. Energy Combust. Sci. 2016, 52, 1−61. (12) Sahu, S. G.; Chakraborty, N.; Sarkar, P. Coal−biomass cocombustion: an overview. Renewable Sustainable Energy Rev. 2014, 39, 575−586. (13) Niu, Y.; Du, W.; Tan, H.; Xu, W.; Liu, Y.; Xiong, Y.; Hui, S. Further study on biomass ash characteristics at elevated ashing temperatures: The evolution of K, Cl, S and the ash fusion characteristics. Bioresour. Technol. 2013, 129, 642−645. (14) Wei, X. L.; Schnell, U.; Hein, K. R. G. Behaviour of gaseous chlorine and alkali metals during biomass thermal utilisation. Fuel 2005, 84, 841−848. (15) Garba, M. U.; Ingham, D. B.; Ma, L.; Porter, R. T. J.; Pourkashnian, M.; Tan, H.; Williams, A. Prediction of potassium chloride sulfation and its effect on deposition in biomass-fired boilers. Energy Fuels 2012, 26, 6501−6508. (16) Li, H.; Zhang, Z.; Ji, A.; Zhao, R.; Yang, P. Behavior of slagging and corrosion of biomass ash. J. Environ. Eng. Technol. 2017, 7, 107− 113. (17) Davidsson, K. O.; Korsgren, J. G.; Pettersson, J. B. C.; Jaglid, U. The effects of fuel washing techniques on alkali release from biomass. Fuel 2002, 81, 137−142. (18) Arvelakis, S.; Vourliotis, P.; Kakaras, E.; Koukios, E. G. Effect of leaching on the ash behavior of wheat straw and olive residue during fluidized bed combustion. Biomass Bioenergy 2001, 20, 459−470. (19) Tonn, B.; Thumm, U.; Lewandowski, I.; Claupein, W. Leaching of biomass from semi-natural grasslands−Effects on chemical composition and ash high temperature behaviour. Biomass Bioenergy 2012, 36, 390−403. (20) Liaw, S. B.; Wu, H. Leaching characteristics of organic and inorganic matter from biomass by water: differences between batch and semi-continuous operations. Ind. Eng. Chem. Res. 2013, 52, 4280− 4289. (21) Li, F.; Fang, Y. Modification of ash fusion behavior of lignite by the addition of different biomasses. Energy Fuels 2015, 29, 2979−2986. (22) Li, F.; Xu, M.; Wang, T.; Fang, Y.; Ma, M. An investigation on the fusibility characteristics of low-rank coals and biomass mixtures. Fuel 2015, 158, 884−890. (23) Rizkiana, J.; Guan, G.; Widayatno, W. B.; Hao, X.; Li, X.; Huang, W.; Abudula, A. Promoting effect of various biomass ashes on the steam gasification of low-rank coal. Appl. Energy 2014, 133, 282−288. (24) Wang, L.; Becidan, M.; Skreiberg, O. Sintering behavior of agricultural residues ashes and effects of additives. Energy Fuels 2012, 26, 5917−5929. (25) Jiang, D.; Zhuang, D.; Fu, J.; Huang, Y.; Wen, K. Bioenergy potential from crop residues in China: availability and distribution. Renewable Sustainable Energy Rev. 2012, 16, 1377−1382. (26) Ma, X.; Li, F.; Ma, M.; Fang, Y. Investigation on blended ash fusibility characteristics of biomass and coal with high silica−alumina. Energy Fuels 2017, 31, 7941−7951. (27) Li, F.; Li, Z.; Huang, J.; Fang, Y. Understanding mineral behaviors during anthracite fluidized-bed gasification based on slag characteristics. Appl. Energy 2014, 131, 279−287. (28) Li, F.; Ma, X.; Guo, Q.; Fan, H.; Xu, M.; Liu, Q.; Fang, Y. Investigation on the ash adhesion and deposition behaviors of lowrank coal. Fuel Process. Technol. 2016, 152, 124−131. (29) Fang, X.; Jia, L. Experimental study on ash fusion characteristics of biomass. Bioresour. Technol. 2012, 104, 769−774. (30) Wang, Y.; Li, H.; Wang, D.; Dong, C.; Liu, Q.; Li, W. The effect of K2O in coal and biomass ash on ash fusibility. Chem. Ind. Eng. Prog. 2016, 35, 2759−2765.

4. CONCLUSION Two additives (kaolinite and dolomite) can promote the AFT of three straws (WS, CS, and RS). For three straws, kaolinite addition made the DTs obviously increase, while dolomite addition resulted in a clear increase in FT. DT was mostly determined by the contents of low MP minerals and amorphous matter, and FT was closely related to the skeleton structure of high MP minerals. The addition of dolomite made straw AFT increase through the formation of high MP lime, magnesia, and merwinte, while SiO2 from kaolinite decomposition was transferred into the low MP mineral forsterite and kirschstenite or amorphous matter, especially for low silicon WS and CS. The high MP kaliophilite formation resulted in an increase in the AFTs of the straw mixture with kaolinite. The combination of XRD experiments, network theory, and FactSage calculation simulation can be used to explore the ash fusion variation mechanism of straw ashes from the perspective of mineral evolution.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: qfl[email protected]. ORCID

Fenghai Li: 0000-0003-1690-3346 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA07050103) and the Natural Science Foundation of Shandong Province, China (ZR2014BM014, ZR201702180212).

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REFERENCES

(1) Shen, J.; Igathinathane, C.; Yu, M.; Pothula, A. K. Biomass pyrolysis and combustion integral and differential reaction heats with temperatures using thermogravimetric analysis/differential scanning calorimetry. Bioresour. Technol. 2015, 185, 89−98. (2) Emami-Taba, L.; Irfan, M. F.; Daud, W. M. A.; Chakrabarti, M. H. Fuel blending effects on the co-gasification of coal and biomass − a review. Biomass Bioenergy 2013, 57, 249−263. (3) Wang, Q.; Han, K.; Gao, J.; Wang, J.; Lu, C. Investigation of maize straw char briquette ash fusion characteristics and the influence of phosphorus additives. Energy Fuels 2017, 31, 2822−2830. (4) Capablo, J. Formation of alkali salt deposits in biomass combustion. Fuel Process. Technol. 2016, 153, 58−73. (5) Cao, J.; Xiao, X.; Zhang, S.; Zhao, X.; Sato, K.; Ogawa, Y.; Wei, X.; Takarada, T. Preparation and characterization of bio-oils from internally circulating fluidized-bed pyrolyses of municipal, livestock, and wood waste. Bioresour. Technol. 2011, 102, 2009−2015. (6) Garcia-Maraver, A.; Mata-Sanchez, J.; Carpio, M.; Perez-Jimenez, J. A. Critical review of predictive coefficients for biomass ash deposition tendency. J. Energy Inst. 2017, 90, 214−228. (7) Zhou, C.; Rosén, C.; Engvall, K. Biomass oxygen/steam gasification in a pressurized bubbling fluidized bed: Agglomeration behavior. Appl. Energy 2016, 172, 230−250. (8) Saidur, R.; Abdelaziz, E. A.; Demirbas, A.; Hossain, M. S.; Mekhilef, S. A Review on biomass as a fuel for boiler. Renewable Sustainable Energy Rev. 2011, 15, 2262−2289. (9) Konsomboon, S.; Pipatmanomai, S.; Madhiyanon, T.; Tia, S. Effect of kaolin addition on ash characteristics of palm empty fruit bunch (EFB) upon combustion. Appl. Energy 2011, 88, 298−305. H


Article

Energy & Fuels (31) Ren, X.; Zhang, W.; Ouyang, S. Effect of multiple foreign ion doping on meta-stable foreign ion doping on meta-stable structure of alite. J. Chin. Ceram. Soc. 2012, 40, 664−670. (32) Li, F.; Ma, X.; Xu, M.; Fang, Y. Regulation of ash-fusion behaviors for high ash-fusion-temperature coal by coal blending. Fuel Process. Technol. 2017, 166, 131−139. (33) Xu, J.; Yu, G.; Liu, X.; Zhao, F.; Chen, X.; Wang, F. Investigation on the high-temperature flow behavior of biomass and coal blended ash. Bioresour. Technol. 2014, 166, 494−499. (34) Vargas, S.; Frandsen, F. J.; Dam-Johansen, K. Rheological properties of high temperature melts of coal ashes and other silicates. Prog. Energy Combust. Sci. 2001, 27, 237−429. (35) Senior, C. L.; Srinivasachar, S. 1995. Viscosity of ash particles in combustion systems for prediction of particle sticking. Energy Fuels 1995, 9, 277−283. (36) Van Dyk, J. C.; Melzer, S.; Sobiecki, A. Mineral matter transformation during Sasol-lurgi fixed bed dry bottom gasificationUtilization of HT-XRD and FactSage modelling. Miner. Eng. 2006, 19, 1126−1135. (37) Bai, J.; Li, W.; Li, B. Q. Characterization of low-temperature coal ash behaviors at high temperatures under reducing atmosphere. Fuel 2008, 87, 583−591. (38) Pang, C.; Hewakandamby, B.; Wu, T.; Lester, E. An automated ash fusion test for characterization of the behaviour of ashes from biomass and coal at elevated temperatures. Fuel 2013, 103, 454−466. (39) Niu, Y.; Tan, H.; Ma, L.; Pourkashanian, M.; Liu, Z.; Liu, Y.; Wang, X.; Liu, H.; Xu, T. Slagging characteristics on the superheaters of a 12MW biomass-fired boiler. Energy Fuels 2010, 24, 5222−5227. (40) Li, F.; Fang, Y. Ash fusion characteristics of a high aluminum coal and its modification. Energy Fuels 2016, 30, 2925−2931. (41) Borello, D.; Venturini, P.; Rispoli, F.; Rafael, S. G. Z. Prediction of multiphase combustion and ash deposition within a biomass furnace. Appl. Energy 2013, 101, 413−422. (42) Li, F.; Li, M.; Zhao, H.; Fang, Y. Experimental investigation of ash deposition behaviour modification of straws by lignite addition. Appl. Therm. Eng. 2017, 125, 134−144. (43) Vassilev, S. V.; Baxter, D.; Andersen, L. K.; Vassileva, C. G. An overview of the composition and application of biomass ash. Part 1. Phase mineral and chemical composition and classification. Fuel 2013, 105, 40−76. (44) Kong, L.; Bai, J.; Li, W.; Wen, X.; Li, X.; Bai, Z.; Guo, Z.; Li, H. The internal and external factor on coal ash slag viscosityat high temperatures, Part 3: Effect of CaO on the pattern of viscosity temperature curves of slag. Fuel 2016, 179, 10−16. (45) Li, F.; Fan, H.; Fang, Y. Investigation on the regulation mechanism of ash fusion characteristics in coal blending. Energy Fuels 2017, 31, 379−386.

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Influencing Mechanism of Additives on Ash Fusion Behaviors of Straw

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