Influence of Sewage Sludge on Ash Fusion during Combustion of

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Influence of Sewage Sludge on Ash Fusion during Combustion of Maize Straw Hongpeng Liu, Shiqiang Zhang, Shiyu Feng, Jiyong Liu, Guangrui Liu, Baizhong Sun, Deyong Che, and Qing Wang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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Influence of Sewage Sludge on Ash Fusion during Combustion of Maize Straw Hongpeng Liu[a]*, Shiqiang Zhang[a], Shiyu Feng[a], Jiyong Liu[a], Guangrui Liu[b], Baizhong Sun[a],Deyong Che[a],Qing Wang[a] [a]Engineering

Research Centre of Oil Shale Comprehensive Utilization, Ministry of Education, School of Energy and Power

Engineering, Northeast Electric Power University, Jilin 132012, Jilin Province, China. [b]Qingdao

City Investment Environmental Resources Corporation, Qingdao 266000, Shandong Province, China.

Corresponding e-mail: [email protected].

ABSTRACT A key issue associated with combustion of maize straw (MS) is slagging, in order to solve this problem, MS co-combustion with sewage sludge (SS) is a good choice. In this work, co-combustion experiments were conducted in a muffle furnace with SS percentages of 0, 10, 20 and 100% at the temperature of 700 °C, 800 °C, and 900 °C. The effect of SS on MS alkali metals release characteristics was investigated, the ash samples were analyzed by atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). The results indicate that the potassium in the MS can be captured by Si、Al、Ca、Mg and phosphorus containing inorganic substances in the SS to form a new high melting point compound such as KAlSi2O6, KAlSiO4, KCaFe(PO4)2. Slagging tendency was assessed based on the chemical components and ash fusion temperature (AFT). The evaluation results were highly consistent with the actual slagging conditions.

KEYWORDS Maize straw; Sewage sludge; Alkali metals; Slagging

1. INTRODUCTION As a green renewable energy source, biomass has a zero-greenhouse gas emission characteristic and can convert solar energy and carbon dioxide into useful chemical energy. The rational use of biomass energy can not only reduce the consumption of fossil fuels, but also effectively reduce environmental pollution. Therefore, the development of biomass energy is important for heat and power generation [1,2]. In the past few decades, woody biomass has mainly been used to produce electricity and heat. Due to the ever-increasing need of woody biomass in other fields (chemical products and liquid biomass fuels), the price of woody biomass has risen [3-5]. As a result, people pay more attention to the agricultural waste. The ash content of agricultural waste is usually much higher than that of the woody biomass, whereas the composition of ash is also more complex and varied [6]. Agricultural waste contains a large amount of alkali metal (potassium and sodium), as well as related inorganic elements including calcium, magnesium, chlorine and sulfur [2,6,7]. During the combustion process, most of the potassium in the fuels reacts with silicon to form a potassium silicates with a low melting point. These potassium-containing compounds with low melting points exist in molten state and lead to sintering and slagging at the bottom of furnace [6-8]. When using a fluidized bed as a combustion or gasification reactor, potassium may also react with the bed material to form low-melting eutectic compounds, which results in the agglomeration of particles, hinders fluidization, and even causes failure of the fluidization [9-11]. Some of the potassium-containing compounds evaporate in gas phase (such as, KOH, KCl, K2CO3, and K2SO4) and condense or deposit on the solid or liquid phase on a low-temperature heating surface, eventually destroying the heating surface [2,6,8,12]. Straw is the most common agricultural waste and has considerable potential for development in terms of combustion for heat and electric power [13]. During the combustion process, the chemical reaction mechanism and the theoretical knowledge of straw (mainly wheat, cotton and maize) ash have been extensively studied. For example, Radačovská et al. [14] studied ash content and ash melting characteristics of co-combustion of spruce and birch bark. Li et al. [15] used Factsage simulation software to study the melting characteristics and melting mechanism of mixed ash between peanut shell and coal. In recent years, a variety of chemical additives have been commonly used in industry to alleviate the problems of ash sintering and slagging. However, it might incur high investments and reducing economic viability of using these additives in industrial applications [16,17]. Therefore, it is necessary to find a new low-cost, environmentally-friendly, anti-slagging additive. The cocombustion of a suitable amount of sewage sludge (SS) and potassium-enriched biomass can alleviate the corrosion of heating surface, which is a good alternative to using chemical additives [18-22]. SS containing a large amount of silicon, aluminum, phosphorus, iron and calcium. It was found that SS can capture the alkali metal in the straw and react with it to form high melting point compounds, reducing the formation of low melting point potassium compounds [23]. Therefore, SS can effectively alleviate the problems of sintering and slagging of biomass ash. Wang et al. [24] studied the effect of SS as an additive on the melting point 1

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of biomass ash, and believed that the SS mainly affected the migration and transformation of potassium. Li et al. [25] studied the reaction mechanism of phosphorus in SS and potassium in wheat straw. The results showed that the reaction formed high melting point potassium aluminosilicate and alkali metal phosphate, which increased the potassium fixation rate of mixed ash. Skoglund et al. [26] conducted a co-firing experiment between biomass and municipal sludge. It was found that the alkali-chloride in biomass ash changed into alkali metal sulfate after adding SS, which could reduce the risk of alkali metal chloride-related corrosion and slagging. In general, SS can be used as an anti-slagging additive to the combustion of maize straw (MS), but scientific evidence for evaluating engineering application feasibility and conducting cost comparison analyses were necessary. The main objective of this work is to study the effect of SS on MS alkali metals release characteristics and slagging. Potassium retention rate and sodium retention rate of MS, SS, and their blends, as well as their slagging characteristics and ash characteristics were obtained. The results obtained in this experimental study can provide data and theoretical references for sewage sludge’s use as an anti-slagging additive.

2. EXPERIMENTAL 2.1. Samples The molding MS used in the experiments originated in Jilin province, China, and is a major crop in northeast China. MS is directly processed in the farmland, a small amount of black soil may be mixed in MS. The MS is first crushed and then compression to form molding MS. SS from Jilin sewage treatment plant was selected, as shown in Fig. 1. First, the molding MS and SS were naturally dried, and then, dried in an air-drying oven at 105 °C to a constant weight. Finally, they were pulverized to obtain particles with a size less than 200 μm. Ultimate and proximate analyses of raw materials are presented in Table 1. The raw samples were ashed in accordance with ASTM/E1755-01. The chemical composition of the MS and SS ashes were analyzed using X-ray fluorescence (XRF), (ZSX Primusll RIGAKU), and the results are listed in Table 2. In order to study the effect of SS as an additive on MS ash slagging, the blends of MS-SS with SMRs (sludge mass ratio in maize straw-sludge mixture) of 10% and 20% (by weight), and were represented using the M9S1 and M8S2, respectively. The amount of additive was selected based on two factors, which are as follows. (1) The amount of mixed additive should be sufficient to meet the expected reaction requirements, and can significantly reduce the issues related to ash melting and slagging. (2) The mixing ratio should be practical and feasible. When the additive is mixed, the increased ash content after combustion should not be too high. This is due to the reason that it becomes difficult for the combustion equipment to remove massive amounts of ash. Table 1

Ultimate and Proximate Analyses of Sample Element

Proximate Analysis (wt.%, ad)

Fuel

Ultimate Analysis (wt.%,ad)

Analysis

LHVad

(wt.%,ad)

(MJ/kg)

Moisture

Ash

Volatiles

Fixed Carbon

C

H

N

S

O

K

Na

MS

2.98

12.61

67.75

16.66

42.60

5.35

0.95

0.20

35.31

1.88

0.08

16.79

M9S1

2.86

17.83

64.12

15.19

40.01

5.23

1.11

0.24

32.72

1.86

0.13

15.84

M8S2

2.77

23.22

59.28

14.73

37.52

4.96

1.36

0.28

29.89

1.84

0.18

15.21

SS

2.34

63.19

31.14

3.33

18.65

3.01

3.13

0.62

9.06

1.65

0.56

8.03

ad= air dry. Table 2 Fuel

XRF Analysis Results of Different Sample Ash Chemical Composition (wt. %)

Na2O

MgO

Al2O3

SiO2

P2O5

SO3

Cl

K2O

CaO

TiO2

Fe2O3

MS

0.87

4.79

7.50

48.86

2.94

1.45

3.44

17.42

7.52

0.46

3.90

M9S1

0.97

4.19

11.23

49.58

4.34

1.48

2.09

12.38

6.18

0.60

6.12

M8S2

1.06

3.73

14.68

49.66

5.21

1.33

1.15

9.61

5.36

0.73

7.11

SS

1.14

2.73

19.55

50.00

7.30

1.24

0.02

3.10

3.84

0.88

9.56

2.2. Combustion process The combustion experiments were conducted in a muffle furnace. The door was kept semi-open to ensure that the sample was completely burnt in the air. The experiments were conducted at temperatures of 700 °C, 800 °C and 900 °C. When the furnace temperature reached the set value, four samples (MS, M9S1, M8S2 and SS) were sent to the muffle furnace. In order to ensure the 2


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burning of fuel, each experiment lasted 60 min. After this, ash was collected for subsequent analysis. 2.3. AAS (atomic absorption spectroscopy) The ash was digested according to the standard GB/T 1574-2007, and the alkali metal content in the raw material and the residual ash was determined using AAS (Analytik Jena, ContrAA700). It was difficult to analyze the alkali metal migration characteristics from the alkali metal content in the ash during the combustion process. The content of alkali metals and the ash yield for different samples were different from each other. Therefore, the retention rate of alkali metals (δ) was defined using Equation (1).

（%）

C2  YAsh  100 C1

(1)

where δ is the retention rate of alkali metals in ash (%), C1 and C2 represent the contents of alkali metals in the raw material and ash, respectively (%), and YAsh represents the ash yield (%). In order to investigate the extent of ability of MS with SS additive to fix the alkali metal, this article cites another variable called the alkali metal retention growth rate (β). When β increases, it shows that the ability of MS with SS additive to fix the alkali metal is significant. the alkali metal retention growth rate is calculated using Equation (2).

（%）

 MS  SS   MS  100  MS

(2)

where δMS+SS represents the alkali metals’ retention rate for MS-SS ash, the alkali metal in the MS and SS have been taken into account. and δMS represents the alkali metals’ retention rate for MS ash. 2.4. XRD (X-ray diffraction) Using XRD (RIGAKU, MiniFlex600) analyzer, the main crystal phase of the bottom ash was analysed. The experimental conditions were set to be as follows. Cu K-α radiation; sampling interval of 0.02°/step; pipe pressure of 40 kV; pipe flow 40 mA; scanning speed of 2°/min; scanning angle range of 10-70°. The results used MDI Jade 6.5 to analyze the composition of crystalline phase in the ash. 2.5. SEM-EDS (Scanning electron microscopy-energy dispersive spectroscopy) SEM (Zeiss Sigma 300) was used to observe the micro-morphology of ash. A representative area of the micro-topography was selected for X-ray EDS (Oxford Inca Energy 250x-max) to obtain the elemental composition and relative content of the area. 2.6. AFT (Ash fusion temperature experiments) In order to study the effect of SS on the fusion characteristics of MS ash, the ash fusion temperature (AFT) were measured using an ash fusion tester (SDAF2000e). The samples were completely ashed according to ASTM/E1755-01. According to the Chinese standard GB/T 3076-2014, deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT) were recorded based on the shape variations of the ash cone.

Fig. 1 (a) MS raw material (b) SS drying raw material

3. RESULT AND DISCUSSION 3.1 Effect of SS on apparent morphology of MS ash Fig. 2 presents the characteristics of apparent morphology of different samples after the combustion at 700-900 °C for 60 min. At 700 °C, the MS ash was deep gray and had coarse bulks. Some of the black matter was similar to unburnt carbon in MS ash, 3


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these black matters were char. Low melting point compounds (amorphous potassium silicate) were formed during the oxidation of char [27]. The low melting compounds melted on the ash surface and the char was encapsulated, making it incapable of direct contact with air, which resulted in incomplete combustion of fuel. At 800 °C and 900 °C, the MS was fully burnt and the ash was metallic gray. The ash surface was hard and relatively smoother. Meanwhile, a lot of molten product adhered to the crucible and was difficult to be removed, suggesting that MS ash exhibited a serious melting problem. The SS ash was porous and powdery. Only at 900 °C, some of the SS ash adhered to the boat. The melting phenomenon did not occur. For the mixture of MS-SS, although the surface morphology changes of M9S1 is not as obvious as that of M8S2, it can be clearly observed that the combustion product became softer compared to MS, while the ash adhering to the crucible was greatly reduced in quantity. These results demonstrated that SS acted as an additive, which reacted with MS in a certain physical or chemical way, and effectively inhibited the melting and slagging of MS ash.

Fig. 2 Morphology of ash at different temperatures 3.2 Release characteristics of alkali metals 3.2.1 Effect of SS on the potassium retention rate As shown in Fig. 3(a), δK-MS reached the value of 54.69% at 700 °C, which indicated that MS exhibited a certain ability to fix K. As the temperature increased, δK-MS significantly decreased, indicating that the temperature promoted the release of potassium. Some previous studies have shown that, when the temperature was higher than 700 °C, KCl was released in the gaseous form [28]. At 900 °C, δK-MS in MS ash was 36.94%, which meant that in addition to KCl precipitation in gaseous form, KOH and K formed by the reaction of K2CO3 with water vapor also precipitated [29]. The remaining potassium was not released and was stored in the molten product in the ash. As the temperature increased, K was rarely released in the gaseous form in SS ash. Additionally, potassium was almost entirely stored in the ash. The contents of silicon, aluminum, phosphorus, and sulfur were high in SS, which had the ability to capture K in the SS ash, though the contents of potassium and chlorine were low. When different proportions of SS were added, the potassium retention rate in the mixed ash significantly increased, indicating that SS exhibited a trapping effect for alkali metals. Due to the reason that silicon, aluminum, iron and phosphorus are the main components of SS ash. In the combustion

process, aluminosilicates and phosphates can capture alkali metals present in MS. A large amount of sulfur in SS reacted with potassium of MS to form potassium sulfate, which was in the form of inorganic salt in the ash. 3.2.2. Effect of SS on the sodium retention rate In general, potassium and sodium are commonly referred to as alkali metals, and they are released in a similar way to each other. Therefore, only the release of potassium has been studied, the way sodium is released has rarely been studied in literature. Fig. 3(a) and Fig. 3(b) show that, δNa-MS in different samples was lower than that of δK-MS. Previous studies also have reported that Na release is higher than K [30-32]. There are several main reasons for this behavior. Firstly, K has more positive charge than Na, and the reducibility of K is greater than that of Na. This difference may affect the capture of Si、Al、Ca、Mg. Secondly, K is more likely to form silicate than Na, while a large amount of K will capture Si and reduce available silicate sites of Na, which may lead 4


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to a large release of Na. Thirdly, compared with K, Na has poor ability to bind with carbon-base. As the temperature increases, the bond between sodium and carbon-base can be easily broken. Therefore, the amount of sodium released is much more than that

of potassium. From Fig. 3(c) and Fig. 3(d), it can be clearly observed that, as the temperature increased, both βK and βNa increased, which meant that the ability to retain alkali metal increased after the addition of SS. Comparing the alkali metal retention growth rate, βNa was obviously higher than that of βK. This is mainly because that, during combustion, the retention characteristic of

potassium in MS are much better than those of sodium.

(a) Potassium retention ratio (b) Sodium retention ratio (c) Potassium retention growth rate (d) Sodium retention growth rate Fig. 3 Effect of SS on the ability to fix alkali metals

3.3 Effect of SS on crystal phase composition of MS As shown in Fig. 4(a), SiO2 exhibited the highest intensity of diffraction in MS ash. This is mainly due to the following two aspects: on the one hand, SiO2 from soil or dusts that are from external sources, due to for example contamination of straw during storage and transportation. On the other hand, SiO2 formed during combustion of straw, which already exits in the microstructure of straw as part of nutrient. At 700 °C, MS ash also contained mineral phase microcline (KAlSi3O8). According to some relevant previous reports, KAlSi3O8 did not have thermal stability at high temperatures, and reacted with the ash that had already melted, thereby forming more Si-rich, low-melting amorphous phase material, which also increased the MS ash slagging [33]. At 700 °C, the diffraction peak of KCl was relatively strong. With the increase in temperature, the number and intensity of diffraction peaks of KCl were significantly reduced. When the temperature was around 800 °C, chlorine was completely released, and no KCl was generated. This is consistent with the decrease in potassium retention rate in MS ash with the increase in temperature (see Section 3.2.1). As the temperature increased, potassium constantly combined with silicon and silica to form K2SiO3 crystals. As shown in Fig. 4(b), the main crystal phase in the SS ash was very different from MS ash. The main crystalline phases in SS ash were aluminum silicate (Al2SiO5), iron oxide (Fe2O3), and calcium phosphate (Ca7Mg2P6O24 and Ca2P2O7). The appearance of Fe2O3 explained why the SS ash color was reddish. As shown in Fig. 4(c) and Fig. 4(d), no characteristic peak of KCl was found in M9S1 and M8S2 ash samples. With the addition of SS, the formation of KAlSi3O8 having poor thermal stability was reduced, and the high-melting aluminosilicate materials (such as KAlSi2O6 and KAlSiO4) were formed. This is due to the reason that, with the addition of SS, the modification in skeleton of 5


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Al2O3 was obvious. During the ash melting process, Al2O3 can be used as a kind of modified intermediate oxide that entered the SiO2 network structure and played a strengthening role, thus significantly increasing the ash melting characteristic temperature. KAlSi2O6 and KAlSiO4 existed in the K2O-Al2O3-SiO2 ternary system. In the mixed ash, the alkali metal existed in the form of KCaFe(PO4)2 except for the potassium aluminosilicate, which was related to rich phosphorus content in the SS. In the process of biomass combustion, there was an affinity between K and P. and when they encountered calcium oxide and iron oxide, the potassium phosphate salt formed by further reaction was present in the K2O-P2O5-CaO-Fe2O3 multicomponent system. In addition, CaAl2Si2O8 is a high-melting compound, which is also present in the co-fired ash. Comparing M9S1 and M8S2, the characteristic peaks of ash were basically coincident to each other, and differed only slightly in intensity of individual peaks. This shows that the addition of different blending amounts of SS exhibited a weak effect on the crystal phase of ash. These results show that, during the MS and SS co-firing process, the potassium in MS was captured by the Si、Al、Ca、Mg and phosphorus-containing inorganic substances in SS, thus resulting in high-melting aluminosilicate and phosphate compounds, which can reduce the ash sintering and melting phenomena.

1-SiO2 2-KCl

3-K2SO4

8-Ca2P2O7 9-Al2SiO5

4-KAlSi3O8 5-K2SiO3

10-KAlSi2O6

11-KAlSiO4

6-Ca7Mg2P6O24

12-CaAl2Si2O8

7-Fe2O3

13KCaFe(PO4)2

Fig. 4 XRD pattern of ash mixed with MS and SS under different conditions (a: MS; b: SS; c: M9S1; d: M8S2)

3.4. Analysis of micro-morphology of ash mixed with MS and SS Fig. 5 shows the micro-morphology of ash burnt at 700 - 900 °C. At 700 °C, the MS ash did not show significant melting. However, the MS ash began to adhere and large area sintering occurred. The EDS analysis (Spots 1 and 2 in Table 3) found that there were quite a few K and Si along with small amounts of Al, Ca and Fe, indicating that the ash was mainly composed of different forms of potassium silicate. The two main elements of K and Cl were detected from the surface of white particles (Spot 3 in Table 3), which may be KCl crystal. The result was consistent with that of XRD analysis. At 800 °C, the MS ash began to melt and the particles agglomerated to form large spherical and sticky melts. From Fig. 5, a representative region was selected for EDS analysis (Spot 5 in Table 3), in which the K and Si elements were the major components (K+Si＞90%). Meanwhile, the sum of K and Si elements was also greater than 50% (Spots 4 and 6 in Table 3). The results demonstrate the presence of amorphous potassium silicate, which has lower melting point, and can easily lead to sintering and slagging of biomass ash. At 900 °C, the MS ash 6


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underwent a severe melting phenomenon, during which, different ash particles melted together, and the formed surface was smooth with a metallic luster. Additionally, a series of pores appeared on the surface of product, which were caused by the release of gases from the thermal decomposition of some carbonates during the melting of MS combustion. Fig. 6 shows the micro-morphology of SS ash that burnt at 700 - 900 °C. At 700 °C and 800 °C, although granular ash was present, the surface was still dense and rough, and did not exhibit sintering. The EDS analysis (Spots 1 and 2 in Table 4) detected abundant amounts of Al, Si and P. In Spot 3, the content of Si was 86.9%, which may be SiO2 in the soil. At 900 °C, the surface of SS ash particles remained dense and rough, however the ash had a small area of slight sintering. The analysis showed that there were few alkali metals in SS, and the Si、Al、P、Fe were abundant, especially the aluminum (Spots 7 - 10 in Table 4) and iron (Spots 9 and 10 in Table 4). Therefore, a large amount of Si、Al、Fe and phosphorous present in SS ash may help alleviate MS ash slagging. Fig. 7 shows the microscopic appearance of ash at 700 - 900 °C for M9S1 and M8S2. With the increase in temperature, SS significantly decreased the tendency of MS ash to sinter. In order to further observe the effect of SS, Fig. 8 shows the ash of MS and M9S1 magnified 5000 times at 800 °C. It can be seen that, after the addition of SS, the M9S1 ash structure became loose, and the MS ash melting trend was alleviated. With the increase of SS, the melting mitigation effect of M8S2 is only slightly better than that of M9S1. Therefore, it was deemed appropriate to select 10% of SS to be added in the blend. At 900 °C, although a partial agglomeration phenomenon occurred, the entire surface was still rough and loose. The EDS analysis was performed on representative regions selected from the ash. The results are presented in Table 5. It is evident that the content of Al at all points increased significantly. This is mainly due to the addition of SS, the formation of Al2O3-SiO2-K2O ternary system, and the formation of a high-melting potassium aluminum silicate. In Spot 3, the molar ratio of K, Si, and Al was approximately 1:2:1, which was close to the molar ratio of elements in KAlSi2O6. The result was consistent with that of XRD analysis. Due to the addition of SS, the content of P increased significantly at Spots 1, 2, 4 and 8. The change in the content of P implies the formation of new phosphate compounds, which may also be one of the reasons for morphological changes in MS ash. Combined with the results of XRD analysis, the EDS analysis verified that SS can react with potassium in MS to generate a higher-melting K-Ca-Fe-P compound (Spot 2 in Table 5). However, in the process of biomass combustion, accompanying a series of complex reactions, the phosphate formed initially can be mixed with silicate [34]. The formed silicate-phosphate mixture had a lower melting temperature than its individual precursors. A small part of the molten ash was also observed, showing a smooth continuous surface (Spots 4 and 8 in Table 5). It can be seen that the formation of silicate-phosphate may increase the tendency of melting in MS ash.

Fig. 5 Micro-morphology of MS at different temperatures (magnification of ×500) Table 3 EDS spot analysis results referring to Figure 6 Maize Straw Ash

Element (wt.%)

1

2

3

4

5

6

7

8

9

10

Na

1.66

2.06

0.00

7.52

0.63

16.53

4.19

12.16

11.28

2.75

Mg

0.17

1.06

1.07

4.31

1.46

0.87

2.94

3.96

8.52

5.34

Al

6.14

3.72

0.68

4.35

2.23

17.84

3.03

8.87

13.73

11.73

Si

35.50

25.01

6.46

32.36

57.23

22.95

46.93

36.80

17.24

29.22

P

0.82

5.31

1.36

2.20

0.00

1.14

0.00

12.91

4.63

2.61

S

0.42

1.71

0.19

0.00

0.00

3.11

0.00

0.00

0.00

0.00

7


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Cl

3.02

3.22

34.54

0.00

0.57

0.76

0.00

0.00

0.00

0.00

K

36.75

37.11

52.4

24.29

33.15

27.96

37.28

14.79

31.06

23.03

Ca

7.05

15.14

2.36

16.69

2.71

3.37

3.33

8.77

6.85

24.44

Fe

8.47

4.68

0.94

8.28

2.02

5.47

2.30

1.74

6.69

0.88

Ti

0.00

0.98

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Fig. 6 Micromorphology of SS at different temperatures (magnification of ×500) Table 4

EDS spot analysis results referring to Figure 7 Sewage Sludge Ash

Element (wt.%)

1

2

3

4

5

6

7

8

9

10

Na

1.41

1.25

0.00

1.31

1.52

0.96

2.40

19.73

2.16

1.96

Mg

6.28

5.68

1.13

5.62

5.41

3.43

4.05

0.00

5.32

4.98

Al

38.80

27.14

6.14

32.80

33.84

29.10

32.20

36.24

29.89

33.99

Si

17.52

24.21

86.9

14.91

13.41

18.37

16.56

30.96

14.33

13.25

P

13.81

16.85

1.40

15.52

16.30

20.64

14.54

1.44

13.69

18.39

S

2.14

0.00

0.80

0.00

0.00

1.14

0.00

1.20

0.00

0.00

Cl

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

K

5.23

4.02

0.97

6.74

6.20

4.46

4.27

7.56

5.58

5.74

Ca

6.91

8.52

1.50

8.09

14.35

9.27

12.37

1.30

8.90

7.74

Fe

7.90

12.33

1.16

13.70

8.00

12.63

11.59

1.57

14.66

12.89

Ti

0.00

0.00

0.00

1.31

0.97

0.00

2.02

0.00

5.47

1.06

8


Page 9 of 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Fig. 7 Micro-morphology of M9S1 and M8S2 at different temperatures (magnification of ×500)

Fig. 8 Micro-morphology of MS and M9S1 at 800 °C (magnification of ×5000) Table 5. EDS spot analysis results referring to Figure 8 M9S1 ash

Element

M8S2 ash

(wt.%)

1

2

3

4

5

6

7

8

9

10

Na

0.00

1.27

1.45

2.78

2.49

8.79

1.20

1.04

0.00

1.26

Mg

2.99

5.83

4.62

4.81

2.56

4.14

2.17

3.03

7.64

8.55

Al

19.66

19.89

22.98

16.8

27.51

29.18

28.73

29.34

23.05

14.46

Si

12.05

14.55

39.07

12.44

13.65

24.96

25.17

12.74

22.28

13.06

P

10.40

14.73

0.00

22.12

4.72

1.87

0.00

15.98

10.00

8.08

S

0.00

0.00

0.00

0.00

2.72

0.00

0.00

0.00

0.00

0.00

Cl

2.74

2.79

0.00

0.00

0.00

0.57

0.00

0.65

0.00

0.00

K

18.19

19.70

17.72

23.22

31.77

17.08

36.51

22.62

10.15

15.73

Ca

12.64

11.02

7.01

15.05

3.16

7.00

3.18

5.39

20.97

26.42

Fe

18.85

9.75

7.15

2.78

10.58

6.41

3.04

9.21

5.91

12.44

Ti

2.48

0.47

0.00

0.00

0.84

0.00

0.00

0.00

0.00

0.00

3.5. Evaluation of slagging tendency The high concentration of K and Cl may cause problems such as slagging, agglomeration and contamination of the combustion equipment when the straw is burned in the furnace. These problems not only reduce the heat exchange efficiency, but also destroy the superheater, causing the boiler to be shut down frequently. Slagging is obviously affected by the quality of the fuel, and slagging characteristics are usually evaluated based on the melting point and chemical composition of the ash. At present, there is no uniform standard for the biomass slagging discriminant, and the coal slag discriminant index is used to discriminate the biomass slagging tendency, which has low credibility. Many scholars have revised the discriminant limit based on the coal slagging tendency criterion, and obtained the method for judging the tendency of biomass slagging. The discriminant method derived from literature [35]. The limits are shown in Tables 6 and 8. 3.5.1. Evaluation of slagging tendency on the basis of AFTs 9


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Page 10 of 13

Among the slagging characteristics of biomass, the relationship between ash melting point and slagging is the most direct. The melting properties of biomass are key indicators of thermochemical treatment. At the macro level, the ash melting characteristics can be used to determine the dangerous level of ash deposition and slagging during biomass combustion. Therefore, it is important to study the relationship between the characteristic temperature of the biomass ash and the slagging. The ST of biomass ash is commonly used to determine the slagging tendency. The limits based on the biomass ash softening temperature are shown in Table 6. Table 6. Limits for slagging evaluation based on ST ST

Slagging degree

＞1180

Slight

1010～1180

Medium

＜1010

Severe

Table 7 shows the characteristic temperatures of ash from the co-combustion of MS and SS and the evaluation results based on ST. DT, ST, HT and FT represent the deformation temperature, softening temperature, hemispherical temperature and flow temperature of the ash, respectively. Table 7. AFTs of ashes and slagging evaluation based on ST Sample

DT (℃)

ST (℃)

HT (℃)

FT (℃)

Slagging evaluation

Actual slagging

basing on ST

degree

MS

1108

1146

1201

1239

Medium

Medium

M9S1

1124

1193

1244

1298

Slight

Slight

M8S2

1135

1214

1260

1315

Slight

Slight

SS

1150

1237

1278

1339

Slight

Slight

It can be seen from Table 7 that all characteristic temperatures of the SS ash are higher than that of the MS ash, and the MS displayed a medium slagging tendency. This is mainly due to the following two aspects: on the one hand, SS and MS contain a large amount of high melting point SiO2 and Al2O3 (melting point: 1730 ° C and 2054 °C respectively). Besides, the SiO2 and Al2O3 content in SS ash is higher than MS ash. On the other hand, the MS ash also contains an amount of fusible compounds such as K2SO4 and KAlSi3O8. These compounds have a relatively low melting point and fluxing ability increases, so MS ash has a medium slagging tendency. In contrast, SS ash contains an amount of high melting point compounds such as Ca2P2O7, Al2SiO5, and Ca7Mg2P6O24. These high melting point calcium phosphate salts and aluminum silicates have a relatively high melting point and increase the melting point of ash. With the addition of SS, the characteristic temperature of ash is improved to certain extent, this is because, the components in the MS react with the components in the SS to form the new substances having a high melting point, such as KAlSi2O6, KAlSiO4, CaAl2Si2O8, KCaFe(PO4)2. Thereby the ash melting point is increased to some extent. The discrimination result based on ST is completely consistent with the actual slagging situation.

3.5.2. Evaluation of slagging tendency on the basis of the chemical compositions of the ash The difference of ash chemical composition is the root cause of the difference in biomass ash melting temperature. The most commonly used indicators for determining the degree of biomass slagging are the ratio of alkali to acid, the ratio of silicon to aluminum, and the ratio of iron to calcium. The alkali acid ratio refers to the ratio of the sum of the alkaline components (oxides of iron, calcium, magnesium, potassium, etc.) to the sum of the acidic components (oxides of silicon, aluminum, titanium) in the biomass ash. The ratio of silicon to aluminum refers to the ratio of the oxide of silicon to the oxide of aluminum in the biomass ash; the ratio of iron to calcium refers to the ratio of the oxide of iron to the oxide of calcium in the biomass ash. Table 8 shows the limits for slagging evaluation based on the chemical compositions. Table 8. Limits for slagging evaluation based on the chemical compositions Discriminant index

Discriminant boundary

Base-to-acid ratio （B/A）

R b/a =

Fe 2 O3 +CaO+MgO+K 2 O+Na 2 O Al2 O3 +SiO 2 +TiO 2

10


Prediction of slagging degree ＜1（Low slagging inclination） 1~2（Medium）＞2（High）

Page 11 of 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels Iron-to-calcium ratio

＞2（Low slagging inclination） Fe2O3/CaO

（Fe/Ca）

0.3~2（Medium）＜0.3（High）

Silicon-to-aluminum ratio

＜7（Low slagging inclination） SiO2/Al2O3

（Si/Al）

7~20（Medium）＞20（High）

Evaluation results of slagging tendency based on B/A, Fe/Ca, and Si/Al are shown in Table 9. It can be seen that the results of these three evaluation methods are basically the same. The evaluation results of the B/A and Si/Al are highly consistent with the actual slagging conditions, but there is a deviation in the evaluation results of MS, the deviation is not very large. The value of the indicator is close to the range of the actual slagging level. The evaluation results of Fe/Ca is different from the actual slagging conditions in M9S1 and M9S2, and the value of the index differs greatly from the range of the actual slagging degree. It can be seen from the data in the table that the B/A value decreases with the addition of SS. According to the B/A limit, the MS slagging tendency becomes smaller; this is because the addition of SS increases the content of acid oxides in MS, such as Al2O3 and SiO4. Acidic oxides and alkali metals in MS produce high melting point aluminosilicates, which is consistent with previous analyses. Similarly, with the addition of SS, the index values of Fe/Ca and Si/Al develop towards slagging resistance. Table 9. Slagging evaluation based on chemical compositions Sample

B/A

Fe/Ca

Si/Al

B/A-evaluation

Fe/Ca- evaluation

Si/Al- evaluation

Actual slagging degree

MS

0.61

0.52

6.51

Low

Medium

Low

Medium

M9S1

0.49

0.99

4.41

Low

Medium

Low

Low

M8S2

0.41

1.33

3.38

Low

Medium

Low

Low

SS

0.29

2.49

2.56

Low

Low

Low

Low

Comparison of Table 8 and Table 9 shows that there is a bias in the reliability of the evaluated results when the slagging tendency evaluation is based on B/A, Fe/Ca, Si/Al. The results can be explained as follows: B/A, Fe/Ca, Si/Al evaluation calculation formula is mainly composed of various ash components. When the slagging tendency is evaluated based on B/A, Fe/Ca, Si/Al, only the percentage of each component in the ash is considered, and the matching ratio and possible eutectic reaction among these components are neglected, therefore, a big mistake may occur. In contrast, AFT visually displays the deformation, softening and melting information of the ash in a high temperature environment, thereby providing a relatively accurate prediction of slagging behavior [36].

4. CONCLUSIONS In this paper, the SS is selected as anti-slag additive and conducts co-combustion experiments with MS. The alkali metals release characteristics, ash slagging characteristics were studied. The noteworthy findings are summarized as follows: (1) By the apparent morphology analysis of ash, MS has undergone severe melting under 800 ° С and 900 ° С. When an appropriate amount of SS is added, the ash particles adhering to the magnetic boat can be effectively reduced, thereby suppressing fusion and slagging of MS. (2) Through the analysis of AAS, the MS itself has a certain ability to fix the alkali metal (K, Na), and the potassium fixation rate and the sodium fixation rate decrease as the temperature increases. The addition of SS significantly improved the ability of mixed ash to fix the alkali metal, indicating that SS has a trapping effect on the alkali metal in the MS. (3) Combined with XRD and SEM-EDX analysis, it was found that MS ash contained more low-melting potassium compounds such as KCl, K2SO4 and KAlSi3O8. With the increase of the amount of SS added, the potassium in the MS was captured by the Si、Al、Ca、Mg and the phosphorus-containing inorganic substance in the SS to form new high melting point compounds such as KAlSi2O6, KAlSiO4, KCaFe(PO4)2. 5. ACKNOWLEDGEMENTS This work was supported by the China Scholarship Council (CSC); the National Natural Science Foundation of China (Grant No. 51676032); the National Key Research and Development Program of China (Grant No.2018YFB0905104). 6. REFERENCES 1. Roy, M. M.; Corscadden, K. W., An experimental study of combustion and emissions of biomass briquettes in a domestic wood 11


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stove. Applied Energy 2012, 99, 206-212. 2. Niu, Y. Q.; Tan, H. Z.; Hui, S. E., Ash-related issues during biomass combustion: Alkali-induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, and related countermeasures. Progress in Energy and Combustion Science 2016, 52, 1-61. 3. Dassanayake, G. D. M.; Kumar, A., Techno-economic assessment of triticale straw for power generation. Applied Energy 2012, 98, 236-245. 4. Tosin, O. S.; Athanasios, J. K.; Alison, P.; Ewan, M.; Leon, W., Sean, T., Faecal-wood biomass co-combustion and ash composition analysis. Fuel 2017, 203, 781-791. 5. Borello, D.; Venturini, P.; Rispoli, F.; Rafael, S. G. Z., Prediction of multiphase combustion and ash deposition within a biomass furnace. Applied Energy 2013, 101, 413-422. 6. Boström, D.; Skoglund, N.; Grimm, A.; Boman, C.; Öhman, M.; Broström, M.; Backman, R., Ash transformation chemistry during combustion of biomass. Energy & Fuels 2011, 26, (1), 85-93. 7. Wang, L.; Becidan, M.; Skreiberg, Ø., Sintering behavior of agricultural residues ashes and effects of additives. Energy & Fuels 2012, 26, (9), 5917-5929. 8. Knudsen, J. N.; And, P. A. J.; Damjohansen, K., Transformation and release to the gas phase of Cl, K and S during combustion of annual biomass. Energy & Fuels 2004, 18, (5), 1385-1399. 9. Grimm, A.; Skoglund, N.; Boström, D.; Öhman, M., Bed agglomeration characteristics in fluidized quartz bed combustion of phosphorus-rich biomass fuels. Energy & Fuels 2011, 25, (3), 937-947. 10. Khan, A. A.; De Jong, W.; Jansens, P. J., Spliethoff, H., Biomass combustion in fluidized bed boilers: Potential problems and remedies. Fuel Processing Technology 2009, 90, (1), 21-50. 11. Brus, E.; Öhman, M.; Nordin, A., Mechanisms of bed agglomeration during fluidized-bed combustion of biomass fuels. Energy & Fuels 2005, 19, (3), 825-832. 12. Johansen, J. M.; Jakobsen, J. G.; Frandsen, F. J.; Glarborg, P., Release of K, Cl and S during pyrolysis and combustion of highchlorine biomass. Energy & Fuels 2011, 25, (11), 4961-4971. 13. He, J. X.; Liu, Y.; Lin, B.Q., Should China support the development of biomass power generation? Energy 2018, 163, (15), 416425. 14. RadaČovská, L.; HolubČík, M.; Nosek, R.; Jandačka, J., Influence of bark content on ash melting temperature. Procedia Engineering 2017, 192, 759-764. 15. Li, F. H.; Ma, M. J.; Fang, Y. T.; Ma, X. W., Fusion characteristics of blended ash from Changzhi coal and biomass. Journal of Fuel Chemistry & Technology 2018, 46, (2), 129-137. 16. Wang, L.; Hustad, J. E.; Skreiberg, Ø.; Skjevrak. G.; Grønli, M., A critical review on additives to reduce ash related operation problems in biomass combustion applications. Energy Procedia 2012, 20, 20-29. 17. Wang, L.; Skjevrak, G.; Hustad, J. E.; Grønli, M.; Skreiberg, Ø., Effects of Additives on Barley Straw and Husk Ashes Sintering Characteristics. Energy Procedia 2012, 20, 30-39. 18. Aho, M.; Yrjas, P.; Taipale, R.; Hupa, M.; Silvennoinen, J., Reduction of superheater corrosion by co-firing risky biomass with sewage sludge. Fuel 2010, 89, (9), 2376-2386. 19. Amand, L.; Leckner, B.; Eskilsson, D.; Tullin, C., Deposits on heat transfer tubes during co-combustion of biofuels and sewage sludge. Fuel 2006, 85, (10-11), 1313-1322. 20. Elled, A. L.; Davidsson, K. O.; Åmand, L. E., Sewage sludge as a deposit inhibitor when co-fired with high potassium fuels. Biomass and Bioenergy 2010, 34, (11), 1546-1554. 21. Wang, L.; Skjevrak, G.; Hustad, J. E.; Grønli, M. G., Effects of sewage sludge and marble sludge addition on slag characteristics during wood waste pellets combustion. Energy & Fuels 2011, 25, (12), 5775-5785. 22. Elled, A. L.; Åmand, L. E.; Leckner, B.; Andersson, B. Å., Influence of phosphorus on sulphur capture during co-firing of sewage sludge with wood or bark in a fluidised bed. Fuel 2006, 85, (12-13), 1671-1678. 23. Werther, J.; Ogada, T., Sewage sludge combustion. Progress in energy and combustion science 1999, 25, (1), 55-116. 24. Wang, L.; Skreiberg, Ø.; Becidan, M.; Li, H. L., Investigation of rye straw ash sintering characteristics and the effect of additives. Applied Energy 2016, 162, 1195-1204. 12


Page 13 of 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

25. Li, L. N.; Ren, Q. Q.; Li, S. Y.; Lu, Q. G., Effect of phosphorus on the behavior of potassium during the co-combustion of wheat straw with municipal sewage sludge. Energy & Fuels 2013, 27,(10), 5923-5930. 26. Skoglund, N.; Grimm, A.; Öhman, M.; Boström, D., Effects on ash chemistry when co-firing municipal sewage sludge and wheat straw in a fluidized bed: Influence on the ash chemistry by fuel mixing. Energy & Fuels 2013, 27, (10), 5725-5732. 27. Krishnarao, R.V.; Subrahmanyam, J.; Jagadish Kumar, T., Studies on the formation of black particles in rice husk silica ash. Journal of the European Ceramic Society 2001, 21, 99-104. 28. Jensen, P. A.; Frandsen, F.; Dam-Johansen, K.; Sander, B., Experimental investigation of the transformation and release to gas phase of potassium and chlorine during straw pyrolysis. Energy & Fuels 2000, 14, (6), 1280-1285. 29. Lith, S. C. V.; Jensen, P. A.; Frandsen, F. J.; Glarborg, P., Release to the gas phase of inorganic elements during wood combustion. Part 2: Influence of fuel composition. Energy & Fuels 2008, 22, (3), 1598-1609. 30. Fagerström, J.; Steinvall, E.; Boström, D.; Boman, C., Alkali transformation during single pellet combustion of soft wood and wheat straw. Fuel Processing Technology 2016, 143, 204-212. 31. Van Lith, S. C.; Alonso-Ramírez, V.; Jensen, P. A.; Frandsen, F. J.; Glarborg, P., Release to the gas phase of inorganic elements during wood combustion. Part 1: development and evaluation of quantification methods. Energy & Fuels 2006, 20, (3), 964-978 . 32. Wornat, M. J.; Hurt, R. H.; Yang, N. Y.; Headley, T. J., Structural and compositional transformations of biomass chars during combustion. Combustion and Flame 1995, 100, (1-2), 131-143. 33. Lindström, E.; Öhman, M.; Backman, R.; Dan, B., Influence of sand contamination on slag formation during combustion of wood derived fuels. Energy & Fuels 2008, 22, (4), 2216-2220. 34. Lindberg, D.; Backman, R.; Chartrand, P., Towards a comprehensive thermodynamic database for ash-forming elements in biomass and waste combustion-Current situation and future developments. Fuel Processing Technology 2013, 105, (1), 129-141. 35. Ruan, R. B.; Long, B.; Liu, Z. Q.; Xu, A. X., Study on discriminant index of biomass ash slagging. Journal of Engineering for Thermal Energy and Power, 2013, 28, (06), 650-654+666. (in chinese) 36. Zhao, Z. H.; Wang, R. K.; Wu, J. H.; Yin, Q. Q,; Wang, C, B., Bottom ash characteristics and pollutant emission during the cocombustion of pulverized coal with high mass-percentage sewage sludge. Energy, 2019, 170, 809-818.

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Influence of Sewage Sludge on Ash Fusion during Combustion of

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