Ash Fusion Characteristics and Transformation Behaviors during

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

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Ash Fusion Characteristics and Transformation Behaviors during Bamboo Combustion in Comparison with Straw and Poplar Youjian Zhu,†,‡ Junhao Hu,† Wei Yang,† Wennan Zhang,§ Kuo Zeng,† Haiping Yang,*,† Shenglei Du,† and Hanping Chen† †

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, Hubei Province China School of Energy and Power Engineering, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, China § Department of Chemical Engineering, Mid Sweden University, Holmgatan 10, 85170 Sundsvall, Sweden ‡

ABSTRACT: In this work, the bamboo ash fusion and sintering characteristics were studied to evaluate its potential application in combustion for the production of heat and power. Poplar and wheat straw were used in the experimental test as the reference fuels for comparison. Standard ash fusion tests and ash sintering tests were carried out at elevated temperatures. The results indicate that bamboo has a low ash melting temperature of 862 °C, much lower than that of poplar. In spite of the high K content in bamboo ash, no severe melting and sintering was observed under the temperature lower than 1000 °C. The ashes after the tests were analyzed using SEM/EDX, XRF, and XRD techniques to illustrate the ash transformation behavior. Standard ash fusion tests indicated that the melting temperatures of bamboo, wheat straw, and poplar ashes are 862 °C, 770 °C, and 1088 °C, respectively. No severe sintering can be observed for poplar due to the large existence of refractory compounds. Ash sintering occurred when the temperature is higher than 800 °C, for wheat straw, due to the formation of the low melting temperature Krich silicate. Additionally, bamboo ash has a relatively high P content compared to that of wheat straw, which facilitates the formation of high melting temperature compounds of K−Ca/Mg phosphates. Moreover, the ash content in bamboo is low. As a conclusion, bamboo is a good quality biofuel which can be fired in biomass combustion plants without severe sintering at a temperature lower than 1000 °C.

1. INTRODUCTION Bamboo is a group of perennially large woody grass which belongs to the subfamily of bambusoideae under the family of Gramineae1 and is widely distributed in Asia, Africa, and Latin America. It includes approximately 1500 species within 87 genera worldwide.1 China is the largest bamboo cultivation country and has over 300 species within 44 genera, accounting for 3% of the global forest area.2,3 Currently bamboo is widely used in paper, textile, reinforcing fibers, food, and construction industry.4 Its utilization as an energy resource has received increasing attention considering the characteristics of high biomass productivity. Research on bamboo utilization in the energy industry has mainly concentrated on the fuel properties and combustion characteristics. As given by Liu et al.,5 the calorific value of bamboo is 18−19 MJ/kg, which is higher than that of wheat straw, corn straw, and rice straw (15−17 MJ/kg) . Chandrashekar6 and Engler et al.7 indicated that bamboo has similar fuel properties to woody biomass and is a good fuel for combustion. In addition to the fairly high calorific value, other properties such as low ash content and high combustion rate make it a potential fuel for the production of heat and power.5 Liu et al.8 found that the addition of bamboo in the rice straw fuel during the densification process can significantly improve the properties of rice straw pellets. Pellets from pure rice straw cannot meet the requirement of commercial standards due to the high ash content and low gross calorific value. However, these issues can be solved by adding 60 wt % bamboo in the densification process.8 © XXXX American Chemical Society

Co-firing torrefied/carbonized bamboo with coal is also an attractive application. After torrefaction/carbonization, the calorific value of bamboo can increase to 20−27 MJ/kg, comparable to that of brown coal. The release of volatile matter and molecular modification during the torrefaction/carbonization process9 obviously increased the ignition and burnout temperature of bamboo. This makes it a promising alternative fuel to be directly fired/cofired with coal in the existing coalfired furnace.10 On the other hand, the K in bamboo can act as a catalyst and generate synergistic effects during cocombustion with coal.11 However, bamboo ash normally has a high K content,12,13 similar to the straw biomass such as wheat straw and corn stalk. The previous experience from straw biomass combustion14−16 suggests that K can react with Si-containing compounds to form low melting temperature K-containing silicates and thus gives rise to the operation problem associated with the ash sintering and slagging. Niu et al.14 indicated that the initial deformation temperature of a typical wheat stalk obtained in northern China is approximately 700 °C, which implies an expected severe sintering and slagging during the combustion process. Wang et al.15 reported that the deformation temperature of a corn stalk ash was 834 °C and severe sintering was found due to the formation of a mixed K-rich silicates and phosphates melts. Therefore, the knowledge of fusion characteristics of bamboo ash is essential to avoid the sintering Received: January 29, 2018 Revised: March 12, 2018 Published: March 13, 2018 A

DOI: 10.1021/acs.energyfuels.8b00371 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

wheat straw. This implies a low emission of gaseous pollutants of NOx and SOx. Standard Ash Fusion Test. Standard ash fusion test was conducted using an automatic ash fusion temperature analyzer (5EAFII, Cabolite, U.K.). The raw fuel was ashed at 600 °C and then a triangular ash cone was made according to GB/T 219-2008 for the following test. The ash cone was heated up to 1350 °C in a reducing atmosphere with a rate of 10 °C/min. The shape change of the cone was detected using a thermal microscope and recorded in a computer to get the initial deformation temperature (IDT), softening temperature (ST), hemispherical temperature (HT) and flow temperature (FT). Repeated experiments were conducted to verify the reproducibility. Ash Sintering Test. The raw fuels were first ashed at 600 °C. The obtained ashes were then put in a horizontal tube reactor held at a temperature of 700, 800, 900, and 1000 °C as described below: The reactor was first heated up to a desired temperature and maintained for 5 min to have a stable temperature signal. Then the obtained ashes were evenly placed in a Pt crucible and pushed quickly to the center of the reactor. After 1 h in the reactor with air atmosphere, the ash sample was transferred to a desiccator to cool down to room temperature. The ash sintering degree was evaluated visually and graded according to Steenari18 and Wang:19,20 (1) loose ash resembling the original appearance, (2) partial sintering with fragile structures, (3) hard sintering with partial melting, (4) very hard sintered ash with slag formation, and (5) completely melted. This method has been proved to be a simple and reliable way to get valuable information of ash sintering behavior at elevated temperature. Ash Samples Characterizations. The chemical compositions of the collected ash samples after high temperature treatments were analyzed by X-ray fluorescence (EAGLE III, EDAX Inc., U.S.). Each sample was scanned 3 times and the average value was used to minimize the error. The morphologies of the samples were investigated by an environmental scanning electron microscopy with energy dispersive X-ray (Quanta 200, The Netherlands) conducted in BSE mode. The crystalline phases of the ash samples were identified by using X-ray Diffraction instrument (X’Pert PRO, PANalytical B.V., The Netherlands). The obtained pattern figures were analyzed using High Score Plus software package. The detailed operational and pattern figure analyses method was reported in previous works.21,22

and slagging problems. In addition, a certain amount of inorganic species in biomass experience complex physical and chemical reaction to form KOH, KCl, and K2SO4, which can be released to the gas phase and eventually cause PM emissions, ash fouling, and deposition in the combustion system.16,17 These problems are closely related to the ash transformation during the combustion process. However, research on the ash fusion characteristics and transformation behavior during bamboo combustion is fairly limited. In this study, bamboo residues collected from a furniture factory was employed to investigate the fusion and sintering characteristics and ash transformation behavior at elevated temperature. The results are expected to provide background knowledge for the industrial application of bamboo biomass in the production of heat and power.

2. EXPERIMENTAL SECTION Fuels. The bamboo residue used as a fuel in this study was collected from a furniture factory in Hubei Province, China. The bamboo is a type of phyllostachys pubescens and is widely grown up in the local region. A typical woody biomass, poplar, and a wheat straw, collected in the central part of China, were also used in the experimental test for comparison. The as-received fuels were dried in air to a constant weight and then milled and sieved to a particle size of less than 150 μm for the following analysis. The proximate and ultimate analyses were measured by means of the SDTGA-2000 industrial analyzer (Las Navas, Spain) and EL-2 type elemental analyzer (Vario, Germany), respectively. The heating values were measured using an automatic calorimeter (model 6300, U.S.). The proximate and ultimate analyses and heating values of the fuels are presented in Table 1. The N content of the bamboo fuel is comparable to that of poplar but much lower than that of wheat straw. Additionally, it has the lowest S content compared to poplar and

Table 1. Proximate and Ultimate Analyses, Heating Value, and Ash Chemical Composition of the Fuels a

high heating values (MJ/kg, db ) moisture contents, (wt % adb) proximate analysis (wt % dba) moisture contents, wt % volatile matter fixed carbon ash ultimate analysis (wt % dba) C H O, by difference N S ash composition (wt % dba) K2O CaO MgO Al2O3 SiO2 Cl SO3 P2O5 MnO Fe2O3 a

bamboo

wheat straw

poplar

19.42 4.6

17.18 6.73

20.62 6.8

4.6 72.83 21.84 0.73

6.73 67.36 16.31 9.6

6.8 79.7 12.2 1.3

48.37 6.11 39.84 0.27 0.08

39.69 5.49 37.78 0.53 0.18

41.39 5.27 39.13 0.25 0.27

49.93 4.97 6.74 1.18 13.58 4.12 10.94 6.35 1.58 0.62

50.16 4.48 1.22 1.53 19.20 16.77 5.32 0.93 0.04 0.37

8.30 35.18 4.18 6.92 27.12 1.51 5.58 7.11 0.10 3.80

3. RESULTS AND DISCUSSION Ash Fusion and Sintering Tests. Although the standard ash fusion test cannot precisely predict the ash sintering behavior in the combustion application, it is still useful to compare the fusion characteristics of different fuels.19,23 Figure 1 shows the ash fusion characteristic temperatures, IDT, ST, HT, and FT for the three fuels, bamboo, wheat straw, and poplar. A clear temperature difference between IDT and HT

db = dry basis. bad = as received.

Figure 1. Ash fusion characteristic temperatures for different fuels. B


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sintering behavior at elevated temperature. Slightly sintered structure can be found at 700 °C, and the sintering degree increases at 800 °C with clear melting. The ash is completely melted with a smooth surface at 900 °C and is hard to separate from the crucible. The melting and sintering are greatly enhanced at 1000 °C, and the generated ash sample cannot be separated from the crucible. Unlike wheat straw, the appearance of bamboo ash changes slightly with no sintering structure at 700 °C. Clear sintering of bamboo ash can be observed at 900 °C with a fragile structure. As the temperature increases to 1000 °C, the sintering degree becomes enough high leading to the formation of slags and smooth surface. However, the structure is still not hard and can be crushed with hand. In summary, ash melting temperature of bamboo is higher than that of wheat straw but lower than that of poplar. This should be attributed to the differences of the fuel properties and ash compositions and will be discussed in the following sections. Although the ash fusion temperature of bamboo is lower than 900 °C, it shows only slight sintering at 900 °C and the structure is fragile. With the further increase of temperature to 1000 °C, the sintering is enhanced but the sintered ash still can be broken manually. This implies that the sintered ash can be easily removed after combustion. With respect to the ash sintering result and the low ash content (i.e., 0.73 wt %), the bamboo residue studied is expected to be a good quality biofuel which can be fired solely or cofired with other fuel in the boiler with no severe sintering. Ash Chemical Composition. The chemical compositions of the raw fuel ashes are presented in Table 1. It should be mentioned that no further analyses on chemical composition and crystalline phase of wheat straw ash at 1000 °C were conducted due to that the corresponding ash cannot be separated from the crucible. The bamboo ash is mainly composed of K, Si, and S with a less content of Mg, P, and Ca. This is consistent with the previous results of bamboo chips,24 Phyllostachys nigra, Phyllostachys bambusoides, and Phyllostachys bissetii.2 Clear difference in composition between bamboo and the other two fuels can be seen. K, Si, and Cl are abundant in wheat straw ash, similar to the previous report.19,26 The abundance of K in wheat straw ash together with the high Si content favors the formation of alkali-rich silicates and thus decreases the ash melting temperature. Poplar ash is mainly composed of Ca and Si. The presence of large amount of Ca

for bamboo and wheat straw is found, which implies the coexistence of both high-melting and low-melting substances. However, for poplar, a small temperature difference is observed probably due to the dominated roles of high melting temperature substances in the ash. The IDT of the three fuels varies considerably, from 770 °C for wheat straw to 1088 °C for poplar, while only slight differences are found with respect to the ST, HT, and FT. This suggests that the high melting temperature compounds generated in the fuel combustion act as skeleton structure in the ashes, which are similar for all the biomass fuels.12,14 To evaluate the ash fusion characteristics, IDT has been used frequently in previous research14,15,19 and is employed in this work. It can be observed from Figure 1 that the IDT or the ash fusion temperature of bamboo (862 °C) is slightly higher than wheat straw but clearly lower than poplar. This value is closed to Fusco’s results24 (910 °C) but lower than Fang’s results of 1002 °C.25 It has been well-known that,12 for the woody biomass such as poplar, the ash has a high melting temperature and no severe sintering can be observed in the combustion process. For bamboo biomass, the ash fusion and sintering tendency can be predicted from Figure 2 in comparison with wheat straw.

Figure 2. Sintering degree of bamboo and wheat straw ashes at elevated temperatures

Clearly, the different sintering characteristics can be seen for wheat straw and bamboo ashes. Wheat straw ash shows a severe

Table 2. Chemical Compositions of the Biomass Ash Samples at Different Temperatures wt % samples

bamboo

wheat straw

poplar

T, °C

MgO

Al2O3

SiO2

P2O5

SO3

Cl

K2O

CaO

MnO

Fe2O3

600 700 800 900 1000

6.74 8.71 9.56 11.07 14.11

1.18 1.12 1.44 1.73 3.11

13.58 14.61 17.65 16.84 20.22

6.35 8.14 9.25 10.71 8.44

10.94 14.55 12.83 15.83 13.94

4.12 1.06 0.45 0.48 0.62

49.93 42.64 39.04 33.83 30.16

4.97 6.33 6.68 7.00 6.39

1.58 1.71 2.11 1.45 1.62

0.62 1.12 1.00 1.05 1.39

600 700 800 900

1.22 1.87 1.67 1.63

1.53 1.72 2.42 2.31

26.2 31.48 42.18 47.57

0.93 1.43 1.66 1.19

5.32 9.22 13.23 12.83

16.77 8.72 2.30 0.74

43.16 38.87 26.48 23.27

4.48 5.95 9.18 8.83

0.04 0.05 0.05 0.07

0.37 0.69 0.82 1.55

600 800 1000

4.18 2.83 3.49

6.92 7.10 7.21

27.12 23.75 29.97

7.11 7.27 6.62

5.58 9.39 10.18

1.51 0.52 0.62

8.3 11.72 5.82

35.18 30.84 32.79

0.1 0.09 0.10

3.8 6.47 3.18

C


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Energy & Fuels can effectively increase the ash melting temperature. Although bamboo can be categorized as a woody biomass, it has a much higher K content compared to poplar and other woody biomasses.27 Therefore, a lower ash melting temperature can be anticipated and is validated by the ash fusion tests. However, the S and P contents in bamboo ash are obviously high in comparison to wheat straw. According to the thermodynamic reaction order,16 K would not react with Si under the ideal condition, when there is a competition for K among the acid component, until P and S were consumed due to the higher reactivity than Si. This reduces the possibility of formation of alkali silicates via reaction between K and Si-containing components. Additionally, alkali earth metal contents especially Mg in bamboo are much higher than that of wheat straw. In this case, Mg could further react with the generated alkali silicate to form high melting temperature K−Mg silicates and thus increase the ash melting temperature.16 This agrees well with the ash fusion temperatures presented in Figure 1. The ash compositions at elevated temperatures are presented in Table 2 for the three fuel ashes used in the test. As the temperature increases, Cl decreases dramatically with K2O in a less noticeable amount for bamboo and wheat straw, while ash composition varies slightly with temperature for poplar. It should also be noted that Cl content decreased sharply when the temperature is higher than 700 °C. Although the melting temperature of pure KCl is 770 °C, the value can be substantially reduced with the presence of NaCl and/or alkali sulfates.28 Therefore, the significant release of Cl at 700 °C can be attributed to the evaporation of KCl to gas phase and similar results have been reported elsewhere.29,30 Meanwhile, the contents of refractory species Mg, Ca, Fe, Al, Si as well as S and P are nearly constant or slightly increased with temperature. The increment should be attributed to the increase of the relative content of these elements with the release of the volatile species. The increment of Mg, Ca, P, S contents in the ash implies more high melting temperature substances are formed.16 Crystalline Phases. Ash samples were analyzed by the XRD technique to study the mineral phase transformation behavior at elevated temperature. It should be mentioned that part of the ash components are probably present as the amorphous phase, which cannot be identified by XRD analysis. The X-ray diffraction patterns of bamboo ash at elevated temperatures are shown in Figure 3, and the corresponding crystalline phases are shown in Table 3. K2SO4, KCl, CaCO3, and CaMgSiO4 are found in bamboo ash at 600 °C. The intensity of KCl disappears at 700 °C, indicating the

Table 3. Crystalline Phases of the Biomass Ash Samples at Different Temperatures samples

T, °C

bamboo

600 700 800 900 1000

wheat straw

600 700 800 900

poplar a

600 800 1000

main crystalline phasesa K2SO4; K2SO4; K2SO4; K2SO4; K2SO4;

KCl; CaCO3; CaMgSiO4 KCaPO4; MgAl2(SiO4)3 K3CaH(PO4)2; Ca15(PO4)2(SiO4)6 K0.9Al0.9Si0.1O2; CaMgSi2O6 K0.9Al0.9Si0.1O2; K2P2O8; CaMgSi2O6

KCl; K2SO4; SiO2 KCl; K2SO4 K2SO4; KCl K2SO4 CaO, SiO2, CaCO3, K2Ca2(SO4)3, KAlSi3O8, Ca2SiO4 CaO, K2Ca2(SO4)3, SiO2, KAlSi3O8 SiO2, CaSO4, Ca2P2O7, KAlSi3O8, KAlSiO4, Ca2SiO4, K2Ca2(SO4)3

In decreasing order.

evaporation of KCl to gas phase and transformation into other compounds at elevated temperature, e.g., K2SO4, KCaPO4, K3CaH(PO4)2, K2P2O8 and K-containing silicates as K0.9Al0.9Si0.1O2. K2SO4 are the main K phases in most cases due to the high S content and its intensity increases clearly at 800− 900 °C. This could be due to (1) the relative increase of K2SO4 content because of the reduction of the degree of crystallization at elevated temperature, and (2) the transformation of KCl and/or K2CO3 to K2SO4 via the sulfation reaction.31,32 On the other hand, Mg mainly exists in complex silicates compounds, e.g., CaMgSiO4, MgAl2(SiO4)3, and CaMgSi2O6, which remain stable under the current operating temperature ranges.28 The raw wheat straw ashes are mainly composed of KCl, K2SO4, and SiO2. The intensity of KCl decreases dramatically, as the temperature increases, with the disappearance of SiO2 at 700 °C. The disappearance of SiO2 suggests the formation of amorphous alkali silicates, which cannot be detected by XRD, via the reaction between K-containing compounds and SiO2. Moreover, KCl either transforms into gas phase or incorporates into the silicates structure via the reaction with Si-containing compounds. This can be reflected from the gradual decrease of the KCl peak and the disappearance at 900 °C in the diffraction spectrum. Crystalline phases of Ca-containing (CaO, CaCO3, CaSO4, K2Ca2(SO4)3, Ca2SiO4) and Si-containing compounds (SiO2, KAlSi3O8, KAlSiO4, Ca2SiO4) are abundant in poplar ashes due to the dominated roles of Ca and Si in the ash. These phases mostly are stable at elevated temperatures28 and only vary slightly with temperature. Ash Morphology and Microchemistry. The results of the morphologies and EDX analyses of wheat straw ashes at elevated temperatures are presented in Figure 4 and Table 4, respectively. Raw wheat straw ash presents a heterogeneous structure with a wide variation of particle sizes, colors, shapes, and structures. White particles (spot 1, Figure 4a) with irregular shapes can be observed and are identified as KCl according to the EDX analyses. In addition, the round or cluster shape white particles (spot 5, Figure 4a) are identified as K2SO4. Particles with smooth surface (spot 4, Figure 4a) or slightly smooth surface (spot 2, Figure 4a) generally are composed of mainly K and Si. According to the literature,20,33 the melting temperature of binary K2O−SiO2 mixtures can be as low as 540−600 °C with certain K/Si molar ratios. These particles therefore suggest

Figure 3. X-ray diffraction patterns of bamboo ash at elevated temperatures. D


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and continuous structure. Three different zones can be distinguished in the figure according to the brightness: dark gray zones (spot 1), light gray zones (spots 2 and 4), and bright white particles. The dark gray zones are small quartz particles based on the EDX results. The light gray zones are mainly composed by Si and K which indicate the formation of viscous K rich silicates. This confirms that the interactions between K salts and Si-containing compounds play the major role in the formation of low melting temperature K silicates and eventually cause the melting and sintering of the wheat straw ash. Some white particles (spot 3, Figure 4c) are incorporated in the melts, and a closer view of the zone confirms the existence of KCl in the molten phase. For the ash after high temperature process at 900 °C, the melting and sintering degree is strengthened and brightly white particles can hardly be seen in the molten ash. This suggests KCl was either released to the gas phase or reacted with SiO2/silicates by forming viscous K-rich silicates in the residual ash, which is also consistent with the disappearance of KCl phase at 900 °C as shown in Table 3. Over 1000 °C, the ash particles cannot be scraped from the Pt crucible and therefore were not analyzed. The morphologies of bamboo ash at elevated temperatures and the EDX analysis results are presented in Figure 5 and Table 5, respectively. Unlike wheat straw, the raw bamboo ash is prone to bond together and shows a heterogeneous and fluffy structure. Ash grains with filament appearance can be observed. It can be seen that the particles represented by spots 1−3 in Figure 5a are mainly composed of K, Mg with a small amount of Si, P, S, and Ca, which suggests basically a mixture of K−Mg silicates, phosphates, and sulfates. A very small quantity of particles with a smooth surface and dark color represented by spot 4 in Figure 5a can also be observed, which is mainly composed of Si and K as seen in Table 5. These particles therefore suggest the possible formation of molten K-silicates in the raw bamboo ash obtained at 600 °C. The morphology and chemical composition change slightly at 700 °C, while the bonding between ash particles starts to weaken. A large quantity of individual ash particles/aggregates can be observed

Figure 4. SEM images of wheat straw ash at elevated temperatures: (a) 600 °C, (b) 700 °C, (c) 800 °C, and (d) 900 °C.

the formation of molten K-silicates in wheat straw ash during the standard ashing process, similar to the result reported elsewhere.20 Particles (spot 3, Figure 4a) composed of Si, Ca, Al, and K can also be observed which indicates the existence of complex silicates in wheat straw ash. After heating up to the temperature 700 °C, a large amount of round shape and smooth surface particles (spots 2 and 4, Figure 4b) can be observed, which indicates the occurrence of melting and shrinking at higher temperatures. Spot 2 and spot 1 in Figure 4b are identified as the particles mainly composed of KCl and K2SO4, respectively. Wheat straw ash appears as a molten phase with smooth surface at the elevated temperatures of 800 and 900 °C. The ashes after high temperature process were embedded in a mixture of resin and hardener and then polished using silicon carbide grinding paper to get the cross section for SEM/EDX analysis. From Figure 4c, it can be observed that the cross section of the molten wheat straw ashes present a dense

Table 4. Chemical Compositions of the Spots Presented in Figure 4, Analyzed by Energy Dispersive X-ray Detector at % temperature, °C

spot

Mg

Al

Si

P

S

Cl

K

Ca

Fe

600

1 2 3 4 5

2.0 2.9 1.5 3.8

1.8 19.6 1.0 -

62.2 31.8 59.8 6.1

3.0

6.0 28.7

47.3 4.6 2.4 -

50.8 31.4 17.8 31.7 58.4

20.5 -

4.9 -

700

1 2 3 4

1.5 -

1.0 -

3.2 2.9 59.8 66.0

-

24.2 6.0 -

3.9 46.3 1.3

64.5 50.8 31.7 28.2

3.8 4.5

-

800

1 2 3 4

4.4

-

100.0 66.0 58.5

-

-

49.0 1.6

26.8 51.0 26.2

7.2 9.3

-

1 2 3

2.5 2.0

-

100.0 62.6 64.9

-

-

0.9

26.9 24.0

8.0 8.2

-

900

E


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content and the particles by spot 3 high K, Mg and S contents. When the temperature is raised to 900 °C, large ash particles/ agglomerates with molten structure are formed. The cross section of the ash grains presents a dense and uncontinuous phase with a large amount of irregular shape and white tiny spots/particles embedded in the ash aggregates. They are mainly composed of K−Mg silicates as represented by spots 3 and 4 in Figure 5d and K−Ca/Mg silicates and phosphates by spots 1 and 2. The ash melting and sintering are enhanced to a great extent when the temperature is raised to 1000 °C. Small ash grains are glued or connected together by the bridges and necks, leading tothe formation of the large ash agglomerates with a homogeneous main body and a heterogeneous and uncontinuous layer. The main body of the molten ash is mainly composed of K, Si, and Mg represented by spots 1 and 3 in Figure 5e or K, P, Ca, and Si by spot 4. In contrast, the heterogeneous and uncontinuous layer represented by spot 5 in Figure 5e generally has a high Mg content. Meanwhile, it can be observed that ash grains with high P content (spot 4, Figure 5e) generally have a comparatively darker color with a large amount of black spots on the surface. Ash Transformations Analysis. Bamboo, wheat straw, and poplar used as the fuels in the experiment test show different ash transformation behaviors. Like other woody biomasses,21,29 the poplar ash is mainly composed of Ca, Si plus a less content of K and dominated by refractory Cacontaining and Si-containing compounds which are relatively stable under high temperature. Although bamboo is also categorized as a woody biomass, the bamboo ash contains a high K and Si content similar to wheat straw. However, the bamboo ash melting temperature is higher than the wheat straw

Figure 5. SEM images of bamboo ash at elevated temperatures: (a) 600 °C, (b) 700 °C, (c) 800 °C, (d) 900 °C, and (e) 1000 °C.

at 800 °C, which are mainly presented as K−Mg/Ca silicates based on the EDX results in Table 5. In addition, the particles represented by spot 2 in Figure 5c have a relatively high P

Table 5. Chemical Compositions of the Spots Presented in Figure 5, Analyzed by Energy Dispersive X-ray Detector at % temperature, °C

spot

Na

Mg

Al

Si

P

S

K

Ca

Mn

Fe

600

1 2 3 4

2.0 2.1 1.9

18.6 21.9 23.8 7.8

3.0

4.4 6.9 4.5 38.0

4.7 8.2 12.9 1.1

13.1 7.6 6.2 2.0

-

47.0 44.7 47.6 35.3

7.5 7.9 10.9

2.9 3.0 -

2.7 -

700

1 2 3 4

1.3 2.1 2.6

28.8 23.8 21.0 16.7

-

3.7 4.5 5.8 13.9

5.5 12.9 8.3 8.1

7.0 6.2 7.6 3.2

1.0 1.9 2.4

37.2 47.6 51.1 50.3

8.5 -

6.9 3.0 4.3 2.8

-

800

1 2 3 4

-

13.1 29.0 19.0 19.3

0.0 1.0 -

26.6 5.0 1.8 27.1

2.2 10.4 3.6 4.3

2.2 6.3 23.2 4.0

-

19.0 31.4 52.4 45.3

36.9 13.9 -

3.1 -

-

900

1 2 3 4

-

6.1 9.7 10.6 13.8

0.6 0.8 -

19.5 16.1 33.2 38.3

16.3 19.4 3.8 2.3

2.6 1.4 0.0

1.6 -

38.9 25.1 45.1 43.5

19.2 24.2 5.1 2.1

-

-

1000

1 2 3 4 5

2.0 1.1 2.0

14.9 4.7 13.6 6.1 34.6

1.7 1.0 0.6

37.2 2.0 35.8 16.3 40.8

1.7 19.5 1.1

1.1 -

0.3 -

44.2 12.4 41.8 38.9 17.3

0.7 5.6 19.2 3.6

70.5 -

-

F

Cl


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Energy & Fuels

compounds. Ash sintering occurred when the temperature is higher than 800 °C, for wheat straw, due to the formation of the low melting temperature K-rich silicate. Bamboo has a low ash melting temperature of 862 °C, much lower than that of poplar. In spite of the high K content in bamboo ash, no severe melting and sintering were observed under the temperature lower than 1000 °C. This can be explained by that bamboo ash has relatively high Mg, Ca, and P content compared to that of wheat straw, which facilitates the formation of high melting temperature compounds of K−Ca/Mg silicates and phosphates,. Moreover, the ash content in bamboo is low. As a conclusion, bamboo is a good quality biofuel which can be fired in biomass combustion plants without severe sintering.

ash due to the different ash composition and transformation behaviors at elevated temperature. For wheat straw, the molar ratio of Cl/K is 0.44, which implies that a large amount of K will be presented in the form of KCl. The large existence of Cl can greatly facilitate the transformation of K to the gas phase and gives rise to a high potential of fouling and erosion.30 Meanwhile, the reaction rate of K salt with quartz/silicates in the ash can also be enhanced because of the shift of solid−solid reactions to gas−solid reactions.34,35 This favors the formation of low melting temperature K silicates.16,36 The Cl content in bamboo is ignorable and thus K mostly presents as KOH, K2CO3, K2SO4, and other K-containing minerals.16 This means the fouling tendency caused by the evaporation of KCl is low. Compared to wheat straw, the molten phases of bamboo ash have a much higher Mg and Ca content. The presence of these alkali earth metals has proved to be effective in increasing the ash melting temperature of the K-silicates.16,37 Mg/Ca containing compounds can react with the generated molten K silicates and form K−Mg/Ca silicates. These silicates have a high melting temperature, prevent the further reaction of K salts with Si containing compounds, and therefore avoid the intensification of melting and sintering. Meanwhile, bamboo ash contains a higher P content which facilitates the formation of high melting temperature K−Ca/Mg phosphates (e.g., KCaPO4, K3CaH(PO4)2). Therefore, large sintered particles can hardly be seen at an elevated temperature over 900 °C. Although large agglomerates are formed at 1000 °C, it can be observed from the SEM images in Figure 5e that the bridges and necks connecting the ash grains are not hard. This can also be supported by the aforementioned fragile-structure of the sintered bamboo ash. In summary, the fuel properties and ash compositions of bamboo are different from straw biomass such as wheat straw and woody biomass such as poplar, leading to different ash fusion characteristics and transformation behaviors during the thermochemical conversion process. The standard ash fusion test shows that the ash fusion temperature of bamboo is lower than 900 °C. However, ash sintering tests at elevated temperature shows only slight sintering at 900 °C and the structure is fragile. No severe melting is observed at 1000 °C. The generated ash can be manually broken which suggests the sintered ash can be easily removed after combustion. On the other hand, Baxter et al.38 indicated that the sintering and slagging properties were sensitive to the variation of total ash content and ash forming elements of the fuels. Considering both the ash content of the fuel and the results from the ash sintering tests, we proposed that bamboo can be fired in biomass combustion plant without severe sintering at a temperature lower than 1000 °C.

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AUTHOR INFORMATION

Corresponding Author

*Phone: + 86 27 87542417. Fax: + 86 27 87545526. E-mail: [email protected]. ORCID

Haiping Yang: 0000-0002-8323-8879 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was financially supported by Natural Science Foundation of China (Grants 51021065 and 50930006), Provincial Key Research Project of higher education institutions in Henan (Grant 15600097), and the Doctoral Research Foundation of Zhengzhou University of Light Industry (Grant 13100368). The authors are also grateful for the assistance on the sample testing provided by the Analytical and Testing Center of Huanzhong University of Science and Technology.

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4. CONCLUSIONS In this work, the bamboo ash fusion and sintering characteristics were studied by using standard ash fusion tests and ash sintering tests at elevated temperature, respectively. A woody biomass, poplar, and a wheat straw were used as the reference fuels for comparison. Ash samples at elevated temperatures were analyzed by means of SEM/EDX, XRF, and XRD to illustrate the ash transformation behavior at high temperature. Standard ash fusion tests indicated that the melting temperatures of bamboo, wheat straw, and poplar ashes are 862 °C, 770 °C, and 1088 °C, respectively. No severe sintering can be observed for poplar due to the large existence of refractory G


Article

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