Biomass-Ash-Induced Agglomeration in a Fluidized Bed. Part 1

Jul 20, 2016 - Part 1: Experimental Study on the Effects of a Gas Atmosphere ... Department of Chemical and Biochemical Engineering, Technical Univers...
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Biomass-Ash-Induced Agglomeration in a Fluidized Bed. Part 1: Experimental Study on the Effects of a Gas Atmosphere Teng Ma,†,‡,§ Chuigang Fan,† Lifang Hao,† Songgeng Li,† Wenli Song,*,† and Weigang Lin†,∥ †

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, 1 Zhongguancun North Second Street, Beijing 100190, People’s Republic of China ‡ Sino-Danish Center for Education and Research, Beijing 100190, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ∥ Department of Chemical and Biochemical Engineering, Technical University of Denmark, 2800 Kongens Lyngby, Denmark ABSTRACT: Fluidized beds have been widely applied to gasification and combustion of biomass. During gasification, a high temperature is preferable to increase the carbon conversion and to reduce the undesirable tar. However, the high temperature may lead to a severe agglomeration problem in a fluidized bed. Understanding of the agglomeration in various atmospheres is crucial to optimize the design and operation conditions. This study focuses on the effects of gases on agglomeration tendency with different types of biomass, including corn straw, rice straw, and wheat straw. The biomass ash samples are mixed with quartz sand and fluidized by the gas mixtures of N2/CO2, N2/H2, and N2/steam or by air. At 550 °C, the bed temperature is increased at the rate of 3 °C/min until defluidization occurs. In this way, the defluidization temperature can be determined, which represents the agglomeration tendency. The agglomerates are analyzed by scanning electron microscopy−energy-dispersive X-ray spectrometry (SEM−EDS) for morphology and elemental composition. Significant differences are observed on the defluidization temperature (Td) and agglomeration mechanisms in different gas atmospheres. Td in H2 and steam atmospheres are much lower than that in air. It appears that, in a steam atmosphere, the agglomeration of corn straw and rice straw ash is predominantly coating-induced. The agglomeration in both H2 and air atmospheres are melting-induced. In a H2 atmosphere, K2SO4 in the ash samples disappears, caused by decomposition of K2SO4.

1. INTRODUCTION The energy demand is increasing rapidly in China as a result of the fast development of the economy. Although the fossil fuels are still the major energy resource, a transition to renewable energy is urgently needed for the sustainable development. Biomass is one of the most important renewable energy sources, which is CO2-neutral.1 China has a large amount of biomass resource, especially the agricultural residues, such as wheat, corn, and rice straw. Unfortunately, a large fraction of agricultural residues are burnt in the field during the harvest season, causing a severe environmental problem.2 Gasification and combustion in fluidized beds are the promising technologies to convert biomass to syngas and power or heat.3 However, straw normally has a high content of alkali, chlorine, and silicon,4−6 leading to low ash melting temperatures.5−7 The presence of molten ash may lead to bed agglomeration during combustion or gasification in a fluidized bed.8−13 The agglomeration during biomass combustion was classified to “melting-induced” and “coating-induced” agglomeration.14 Melting-induced agglomeration results from direct adhesion of partly molten ash-derived K−Ca/Mg phosphates or K silicates particles/droplets.12,15−18 The coating-induced agglomeration is caused by the chemical reaction or condensation of salts in the gas phase occurring on the bed particle surface.19 The agglomeration tendency is dependent upon the operating parameters. The temperature is one of the important factors for agglomerates.20−22 The agglomeration tendency may be reduced using additives to increase the melting point of © 2016 American Chemical Society

biomass ash. For example, by adding kaolin, potassium salts in the biomass ash can be transformed to kalsilite (KAlSiO4) with a high melting point.23−25 CaCO3 could react with SiO2 to form CaSiO3, which competes with the reaction between SiO2 and potassium salts, inhibiting the formation of low-meltingtemperature K silicates.26 The reaction atmosphere in gasification is different from that in combustion, which may influence the agglomeration tendency. Although agglomeration in biomass gasification and combustion has been compared,8,12,27 a comprehensive understanding is still scarce. The differences may be due to the burning temperature of biomass, which may exceed the bed temperature by 40−600 °C during combustion.28 In addition, H2, CO, CO2, and steam are the predominant gaseous compounds in gasifiers. The transformation of alkali compounds may be affected by steam.29−31 Thermodynamic equilibrium calculations show that steam has significant effects on the form of sodium species and causes more NaCl to be released to the gas phase.29 It is also reported that, with the addition of steam, nearly all KCl remains in the bed; however, the effect on defluidization is not clear.30 It was proposed that steam has a hindering effect on evaporation of KCl. KCl may be converted to KOH, and KOH reacts with SiO2 to form K2O· nSiO2 with low melting temperatures, as shown in eqs 1 and 2.30,31 Received: January 22, 2016 Revised: June 5, 2016 Published: July 20, 2016 6395

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distributor. The signals of temperatures and pressure drop are logged to a computer by a data acquisition system. To avoid a temperature overshoot by the burning of char particles at oxidizing conditions, biomass ash is used in the defluidization experiments. To study the effect of the ratio of biomass ash to quartz sand, 1, 2, 3, and 4 g of biomass ash are mixed with 20 g of quartz sand. The mixture is added to the reactor before heating. Air and three types of gas mixtures, including 15 vol % CO2 in N2, 5 vol % H2 in N2, and 25 vol % steam in N2, are used as fluidizing gas. The mixture of N2/H2 is used to simulate the reducing atmosphere in a gasifier. Air is for simulation of the oxidizing atmosphere in a combustor. Individual gas is applied to study its effect on the agglomeration tendency in this study. In the low-temperature range, a heating rate of 15 °C/min is applied until 550 °C. Then, the heating rate is decreased to 3 °C/min for determining the defluidization temperature accurately. As the bed temperature is gradually increased, biomass ash may experience chemical reactions and interactions with the bed material. Severe agglomeration may result in defluidization. When defluidization occurs, the bed pressure drop suddenly decreases, which is a general phenomenon reported in the literature.9 At the same time, the slope of the curve of the temperature ramp is varied as a result of the poor mixing and heat transfer caused by defluidization. The defluidization temperature (Td) is defined as a temperature at which the bed pressure suddenly decreases, as illustrated in Figure 2.

2KCl(g) + nSiO2(s) + H 2O(g) = K 2O·nSiO2(l) + 2HCl(g) (n = 1, 2, and 4)

(1)

2KOH(g) + nSiO2(s) = K 2O·nSiO2(l) + H 2O(g) (n = 1, 2, and 4)

(2)

However, the experimental results could not provide evidence for the hypothesis.30 The main objective of this work is to study the effect of CO2, H2, O2, and steam on the agglomeration induced by various straw ashes in a laboratory-scale fluidized bed. The defluidization temperature is used to indicate the agglomeration tendency. Different methods are applied to analyze the agglomerate samples to reveal the mechanisms of bed agglomeration in varied gas atmospheres.

2. EXPERIMENTAL SECTION 2.1. Apparatus and Procedures. 2.1.1. Fluidized-Bed Experiments. Defluidization experiments were carried out in a laboratoryscale bubbling fluidized bed (BFB), schematically shown in Figure 1.

Figure 2. Illustration of the definition of the defluidization temperature from a typical pressure drop and temperature curves in an agglomeration experiment. Figure 1. Schematic diagram of the BFB setup: (1) furnace, (2) preheating section, (3) dense bed section, (4) free board section, (5) distributor, (6) filter, (7) constant flow pump, (8) gas cylinders, (9) mass flow controllers, (10) measuring thermocouple, (11) controlling thermocouple, (12) pressure sensor, and (13) data acquisition.

Heating of the reactor is stopped after defluidization occurs. When the temperature of the reactor approaches the room temperature, the gas flow is switched off and the bed materials are collected. The elutriated ash is collected in the filter. Agglomerates are defined as the particles with a diameter larger than 0.3 mm. The morphology and element distribution of agglomerates formed in different gas atmospheres are examined by scanning electron microscopy (SEM, JSM-7001F) combined with energy-dispersive X-ray spectrometry (EDS, INCA X-MAX). Carbon and oxygen have been excluded in the EDS results, and the contents of the other elements are normalized. To elucidate the mechanism of interaction between sand particles and biomass ash, SEM−EDS analysis is mainly performed on the surface of sand particles and agglomerate necks, as shown in Figure 3a. 2.1.2. Fixed-Bed Experiments. The transformation of ash components in different atmospheres is studied in a fixed bed. A schematic diagram of the fixed bed is illustrated in Figure 4. The reactor is made of quartz, with an inner diameter of 32 mm. N2/H2 (5 vol % H2), N2/steam (25 vol % steam), and air are chosen as purge gas to simulate the reducing, steam, and oxidizing atmospheres, respectively. The gas is introduced to the reactor from the bottom. The superficial gas velocity is set to 0.004 m/s at the ambient temperature to prevent the fine ash particles from entrainment. A total of 1 g of ash sample is used for each test. The ash sample is loaded into the reactor, and the reactor is heated in different gas atmospheres. The reactor is heated from room temperature to 550 °C,

The setup consists of a reactor, gas-dosing system, and data acquisition/control system. The reactor is made of quartz with a total height of 900 mm, which is divided into three section: gas preheating, dense bed, and freeboard sections. A porous quartz distributor is located just above the preheating section. The dense bed section has a dimension of 32 mm in inner diameter and 240 mm in height. The freeboard section is 120 mm in inner diameter and 150 mm in height. A filter is installed at the exit of the reactor to collect the elutriated ash. The reactor is electrically heated, with a maximum temperature of 1020 °C. The flow rate of the gas mixture and air are controlled by mass flow controllers. The gas is introduced into the reactor from the bottom. Water is evaporated to steam in the preheating section. The flow rate of water is controlled by a constant flow pump. The bed pressure is measured by a differential pressure transducer. Two thermocouples are used to measure and control the reactor temperature. The measuring thermocouple is located in the bed 20 mm above the gas distributor. The controlling thermocouple is situated between the heating element and reactor close to the 6396

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Figure 3. Typical SEM image. Three types of biomass, rice straw, wheat straw, and corn straw, are used in this study. The results of the proximate analysis of the biomass (GB 28731-2012) are shown in Table 1.

Table 1. Proximate Analysis of Biomass (wt %, on a Wet Basis)

with a heating rate of 15 °C/min. After the temperature reaches 550 °C, the heating rate is decreased to 3 °C/min. When the temperature reaches 900 °C, the heating of the reactor is stopped. The ash samples before and after heating in the fixed bed are carefully weighed and analyzed by X-ray fluorescence (XRF, AXIOS-MAX) and X-ray diffraction (XRD, Empyrean). The composition of the molten part of ash samples is analyzed by EDS equipped in SEM. As shown in Figure 3b, the smooth part of the ash is assumed to be the molten part. The release behaviors of the main elements are characterized by the “release ratio”, βX, which is defined as the fraction of element X in the ash released to the gas phase during heating to 900 °C in the fixed bed. The release ratio of each element is calculated by eq 3

m0X 0 − m TX T × 100% m0X 0

volatile matter

fixed carbon

ash

moisture

68.03 66.39 66.1

15.03 15.37 16.63

9.81 10.45 9.49

7.13 7.79 7.78

The ashing process is based on the standard procedure: a corundum crucible loaded with the biomass sample is placed in a muffle furnace and heated from room temperature to 250 °C at a heating rate of 5 °C/min and kept for 1 h. After that, the sample is heated from 250 to 550 °C at a heating rate of 5 °C/min and kept for 2 h for complete combustion of the biomass sample. In the context, RS, WS, and CS are used to denote the ash from rice straw, wheat straw, and corn straw, respectively. The compositions of biomass ash and quartz sand are analyzed by XRF (AXIOS-MAX) and summarized in Table 2. The components of the crystalline phase in the ash are analyzed by XRD (Empyrean), with the High Score Plus software package. The results are shown in Figure 5, which indicate that the main crystalline components in the biomass ash are KCl, K2SO4, and SiO2. The ash particle size distribution has been measured using a laser particle size analyzer (Mastersizer 2000). Table 3 shows the accumulated particle size distribution of different ashes. 2.3. Assessment of Agglomeration Tendency. The defluidization temperature (Td) is used to evaluate the agglomeration tendency in different atmospheres. The fraction of agglomerates (A) in the bed material after defluidization is used as another parameter to evaluate the agglomeration tendency. The value of A is the weight fraction of agglomerates in the bed material of the fluidized bed. A higher value of A indicates that more agglomerates are formed in the fluidized bed.

Figure 4. Schematic view of the fixed bed.

βX =

biomass rice straw wheat straw corn straw

3. RESULTS AND DISCUSSION 3.1. Fixed-Bed Test. The fixed-bed tests are aimed to elucidate the effect of gas atmospheres on the transformation of ash components during heating to 900 °C. The results of the element release ratio are summarized in Table 4. The results indicate that the release ratio of K in CS is the highest among the three types of biomass ash. This may be attributed to the composition and existing form of K in biomass ash. In comparison to WS and RS, CS contains more Ca, which could contribute to the release of K.26 Furthermore, CS may

(3)

where m0 and mT represent the weight of the original ash and remaining ash after heating to 900 °C, respectively, and X0 and XT represent element X fraction in the original ash and remaining ash after heating to 900 °C analyzed by XRF, respectively. 2.2. Fuel and Bed Particles. Quartz sand, with a particle density of 2600 kg/m3, is used as the bed material in the fluidized-bed experiments. The quartz sand particles have a narrow size distribution ranging from 0.2 to 0.3 mm, with a mean size of 0.28 mm. 6397

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Energy & Fuels Table 2. Compositions of Ash Samples and Quartz Sand (wt %) sample

Na2O

MgO

Al2O3

SiO2

P2O5

SO3

K2O

CaO

Fe2O3

Cl

RS WS CS quartz sand

1.36 0.30 0

5.51 1.86 5.06

1.12 0.85 2.23 1.05

38.66 41.14 25.99 98.33

1.49 0.70 4.48

6.88 11.61 5.49

25.95 24.72 34.65 0.62

8.09 6.87 9.86

0.69 0.39 1.52

10.23 11.55 10.72

sample in a H2 atmosphere is higher than that in an air or a steam atmosphere, which is the result of eq 4. K 2SO4(s) + nSiO2(s) + 4H 2(g) = K 2O·nSiO2(l) + H 2S(g) + 3H 2O(g)

Table 3. Distribution of the Ash Particle Size RS (%)

WS (%)

CS (%)

300 150 100 50 30 10

100 91.12 74.44 44.95 25.65 11.09

100 83.35 63.51 33.92 20.14 7.94

100 85.16 67.12 40.02 24.06 10.22

Table 4. Release Ratio of Elements in the Biomass Ashes (%) ash sample

Na

Si

S

K

Ca

Cl

WS in H2 WS in steam WS in air RS in H2 RS in steam RS in air CS in H2 CS in steam CS in air

4.57 11.07 11.57 4.03 28.58 25.82

0.35 0.35 0.35 0.27 0.28 0.28 0.42 0.21 0.24

80.62 −3.4 1.06 76.72 5.03 1.41 56.57 12.60 13.82

1.11 4.18 4.25 13.12 20.18 16.31 24.22 31.74 22.48

16.1 20.15 21.23 8.16 18.32 19.15 14.95 11.23 13.55

20.74 34.16 27.37 20.87 26.05 15.03 19.64 38.05 12.78

(4)

The results in Table 4 show a high release ratio of chlorine in a steam atmosphere. As indicated in Figure 5, chlorine mainly exists in the form of KCl in the biomass ash samples. At a high steam partial pressure, KCl may be transformed to HCl and KOH,31 resulting in a high release ratio of chlorine. However, the hypothesis is not supported by the XRD results in Figure 6, in which the crystalline phases of ash samples heated in a steam atmosphere are almost the same as those heated in air. 3.2. Agglomeration Test in a Fluidized Bed. The effects of gas atmospheres on bed agglomeration in a fluidized bed are studied in the agglomeration test. To select the adequate operating parameters for the agglomeration test, the effects of superficial gas velocity (Ug) and the weight ratio of biomass ash to sand (B/q) is examined for one type of biomass ash (RS) in an inert atmosphere. Then, the characteristics of the agglomeration induced by biomass ash are studied in different atmospheres with a fixed gas velocity and ratio of biomass ash to sand. For each sample, EDS measurements are conducted at more than six different spots of the sand particle surface and agglomerate necks. The given results are the average value with deviation. 3.2.1. Determination of Ug and B/q. RS is chosen to study the effect of Ug and B/q in the fluidized bed, with N2 as the fluidization gas. Table 6 summarizes the ranges of parameters and their influence on the elutriation of fine ash particles. It is shown that the fraction of elutriated ash increases with superficial gas velocity and ash to sand ratio increasing. Figure 7a shows the effects of Ug on the defluidizaiton temperature (Td) and the fraction of agglomerates (A). When Ug is 0.0249 m/s, Td is 820 °C and the value of A is 15.79%. As gas velocity is increased to 0.0373 m/s, Td is apparently increased and the value of A is decreased to 6.2%, which is in agreement with the trend reported in other literature.1,20,22 A higher gas velocity can lead to a better mixing of bed particles and more frequent collision of the particles, which can enhance the breakage of formed agglomerates. Furthermore, with the increase of Ug, the fraction of elutriated ash is increased significantly. The decrease of the amount of ash remaining in the bed reduces the amount of potential molten ash, which may lower the tendency of agglomeration. It is noticed that there is no significant change of the defluidization temperature when Ug is further increased from 0.0373 to 0.0622 m/s. Thus, 0.0373 m/s is chosen as Ug to study the effects of gas atmospheres on agglomeration. Figure 7b shows the effect of B/q on the defluidizaiton temperature (Td) and the fraction of agglomerates (A) at a constant gas velocity. When B/q is lower than 5%, no agglomeration occurs at up to 1020 °C. In comparison to the case of 10% for the ash to sand ratio, when B/q is increased to 15%, Td is decreased by 30 °C and A is increased by 16.8%.

Figure 5. Crystalline components in ash samples from XRD.

sieve (μm)

(n = 2 and 4)

contain more water-soluble K, which has a high tendency to release to the gas phase.32 As shown in Figure 6, after heating to 900 °C in a H2 atmosphere, the K2SO4 peak disappears from the XRD spectra in all three types of biomass ash. The results are in agreement with the observation in the literature.33 A significant amount of sulfur present in the ash is in the form of K2SO4, as indicated by XRD of the ashes. The results in Table 4 also show that a large amount of sulfur is released to the gas phase in a H2 atmosphere, while most sulfur remains in the ash in an air or a steam atmosphere. In a H2 atmosphere, sulfur may be released to the gaseous phase as H2S as a result of eq 4. This hypothesis may be supported by the results in Table 5. It is shown that the content of K in the molten part of the ash 6398

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Figure 6. Crystalline phase of biomass ash heated to 900 °C in different atmospheres from XRD: (a) WS, (b) RS, and (c) CS.

Table 5. Composition of the Molten Part of the Biomass Ash (mol %) ash sample

Na

WS in H2 WS in steam WS in air RS in H2 RS in steam RS in air CS in H2 CS in steam CS in air

5.33 1.38 9.47 3.17 6.48

Mg

Si 34.12 61.77 60.83 61.13 73.27 61.43 58.45 65.75 62.24

3.63

2.45

3.52

1 2 3 4 5 6 7

Ug (×10−2, m/s)

B/q (wt %)

K

Ca

0.25 1.14

56.54 27.65 24.75 27.48 23.56 22.53 35.73 27.48 23.89

3.13 9.13 6.41 1.92

0.52 2.1

6.83 5.82 6.25 8.25

which may result in a larger amount of agglomeration formed in the bed. Therefore, the fraction of agglomerates (A) is increased significantly as B/q is increased. With reference to the range reported in the literature,8,28 a value of 10% for B/q is chosen to study the effects of gas atmospheres. 3.2.2. Effects of Gas Atmospheres. To study the effects of gas atmospheres, Ug and B/q are fixed as 0.0373 m/s and 10%, respectively. The summary of the agglomeration tests in different gas atmospheres is listed in Table 7. The value of Td in the agglomeration tests has a good repeatability, with a standard deviation of ±8 °C. As illustrated in Figure 8, Td in a CO2 atmosphere and air are almost the same. The mechanism of agglomeration in those two atmospheres are thought to be the same. However, Td in both H2 and steam atmospheres are much lower than that in air. To gain insights into the differences, the agglomerates formed in air, H2, and steam atmospheres are further analyzed by SEM−EDS.

elutriated ash (wt %)

Effect of Superficial Gas Velocity 2.49 10 3.73 10 4.97 10 6.22 10 Effect of the Ratio of Ash to Quartz Sand 3.73 5 3.73 15 3.73 20

Cl 0.89 1.2 1.86

0.29

Table 6. Scheme of Operating Parameters and Their Effects on the Ratio of Elutriated Ash run

S

11 24.5 28.4 30.5 11 25 27

Although the fraction of elutriated ash is increased with the increase of B/q, the amount of the ash remaining in the fluidized bed is still increased from 0.89 to 2.92 g. With the increase of B/q, the amount of potential molten ash increases, 6399

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Figure 7. Effects of operating parameters on Td and A.

3.2.2.2. Steam Atmosphere. As shown in Figure 8, Td for all three types of biomass ash in a steam atmosphere is lower than that in air. The morphology of agglomerates formed in a steam atmosphere is illustrated in Figure 11, and the composition of the sand particle surface and agglomerate necks measured by EDS are shown in Figure 12. In the case of RS and CS, as shown in panels a and b of Figure 12, the content of K on the surface of sand particles in a steam atmosphere is much higher than that in air. Furthermore, the composition of agglomerate necks is similar to that of the sand particle surface. These results indicate that the coatings are formed on the surface of sand particles and agglomeration can be caused by the molten coatings in a steam atmosphere. Thus, the coating-induced agglomeration may be the predominant mechanism in a steam atmosphere, which is quite different from that in air. The coatings, mainly consisting of Si and K, are probably K silicates, which agree with the report in the literature.15,17,18 At a high partial pressure of steam, K silicates may be formed by eq 1 between KCl and SiO2, which results in the release of HCl to the gas phase. This hypothesis is partly confirmed by the results in the fixed-bed experiment that a larger amount of Cl in the biomass ash is released to the gas phase in a steam atmosphere. For WS, as shown in Figure 12c, the surface of sand particles mainly contains Si, indicating that little coating is formed on the surface of sand particles. The agglomerate necks consist of Si, K, and Ca, with a small amount of Na, Mg, and Al. The composition of agglomerate necks resembles that of the ash sample. This indicates that bed agglomeration is caused by the molten ash, which is quite different from the agglomeration mechanism of RS and CS. K in the biomass ash may be released to the gas phase in the form of KCl, which could react with SiO2 to form K silicates on the surface of quartz sand particles in a steam atmosphere.30 As observed in the results of the fixedbed experiment, the release ratios of K in both RS and CS are much higher than that in WS. Thus, in the case of WS, the release ratio of KCl is not enough to react with quartz sand particles to form the coating on the bed particle surface. The detailed agglomeration mechanisms in a steam atmosphere needs further investigation. The present study shows that the agglomeration tendency increases significantly in a steam atmosphere, i.e., at a high partial pressure of steam. The presence of steam may change the ash chemistry with involvement of varied K species, such as KCl, KOH, and silicates. The detailed reaction schemes are yet to be studied. 3.2.2.3. H2 Atmosphere. The results in Figure 8 show that the defluidization temperature in a H2 atmosphere is lower than

Table 7. Scheme of Agglomeration Tests in Different Gas Atmospheres run

biomass ash

1 2 3

RS WS CS

4 5 6

RS WS CS

7 8 9

RS WS CS

10 11 12

RS WS CS

fluidization gas Effect of Air air air air Effect of a CO2 Atmosphere 85% N2 + 15% CO2 85% N2 + 15% CO2 85% N2 + 15% CO2 Effect of a Steam Atmosphere 75% N2 + 25% H2O(g) 75% N2 + 25% H2O(g) 75% N2 + 25% H2O(g) Effect of a H2 Atmosphere 5% H2 + 95% N2 5% H2 + 95% N2 5% H2 + 95% N2

elutriated ash (wt %) 24.8 20.8 25.2 25 20.5 24.5 19 20.5 18.5 23 20 21.8

Figure 8. Defluidization temperatures in different gas atmospheres.

3.2.2.1. Air. Figure 9 illustrates morphology of the agglomerates formed in air. The element distribution of the sand particle surface and agglomerate necks are shown in Figure 10. For all three types of biomass ash, the surface of the sand particles consists of more than 90% Si, while the agglomerate necks mainly contain Si, K, and Ca, with a little Al, Na, and Mg, the composition of which resembles that of biomass ash. This means that little coating is formed on the surface of bed particles and bed agglomeration is mainly caused by molten ash.34 In a fluidized bed, the sand particles may stick together upon collision in the presence of the viscous molten ash. 6400

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Figure 9. Surface morphology of agglomerates formed in air: (a) RS, (b) CS, and (c) WS.

Figure 10. Composition of agglomerate necks and the sand particle surface sampled in air: (a) RS, (b) CS, and (c) WS.

that in air but higher than that in a steam atmosphere. The results may suggest that the agglomeration tendency may be higher for gasification than for combustion. SEM images of the agglomerates formed in a H2 atmosphere are shown in Figure 13. The EDS results of the elemental distribution of the sand

particle surface and agglomerate necks are presented in Figure 14. For all three types of biomass ash, the surface of sand particles is mainly composed of Si, while the agglomerate necks consist of Si, K, Ca, and Na. This indicates that the melting6401

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Figure 11. Surface morphology of agglomerates formed in a steam atmosphere.

Figure 12. Composition of agglomerate necks and the sand particle surface sampled in a steam atmosphere: (a) RS, (b) CS, and (c) WS.

samples in a H2 atmosphere is lower than that in air. As revealed in the fixed-bed test, K2SO4 in the ash samples is transformed to K silicates in reducing conditions (H2 atmosphere), leading to a larger amount of molten ash present

induced agglomeration is the predominant mechanism for the formation of agglomerates in a H2 atmosphere. Although the agglomeration mechanism in a H2 atmosphere is similar to that in air, the value of Td for all three types of ash 6402

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Figure 13. Surface morphology of agglomerates formed in a H2 atmosphere: (a) RS, (b) CS, and (c) WS.

Figure 14. Composition of agglomerate necks and the sand particle surface sampled in a H2 atmosphere: (a) RS, (b) CS, and (c) WS.

in the fluidized bed. In contrast, in oxidizing conditions, K2SO4 has a much higher melting temperature, resulting in a much lower agglomeration tendency.5,30,35 These facts may partly explain why the defluidization temperature in a H2 atmosphere is lower than that in air and CO2 atmospheres.

4. CONCLUSION Agglomeration/defluidization phenomena of biomass in different atmospheres have been studied in a fluidized bed to partly simulate the combustion and gasification conditions. Attempts are made to reveal the mechanism of agglomeration by 6403

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Energy & Fuels elemental transformation in a fixed bed and by varied analytical methods. On the basis of the studies, the following conclusions can be drawn: (1) The defluidization temperature, which is an indicator of the agglomeration tendency, in CO2 and air atmospheres is almost the same. In contrast, the defluidization temperature is significantly lower in H2 and steam atmospheres than that in air for all three types of biomass studied. (2) In a steam atmosphere, the agglomeration induced by corn straw and rice straw ash may be coating-induced. It seems that agglomeration induced by wheat straw ash is melting-induced in a steam atmosphere. The detailed mechanisms of the influence of steam on agglomeration need further studies. (3) It appears that the agglomeration in both H2 and air atmospheres are melting-induced. However, the defluidization temperature in a H2 atmosphere is much lower than that in air. This is partly attributed to the transformation of K2SO4 to K silicates in a H2 atmosphere.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is supported by the International S&T Cooperation Program of China (2013DFG62640) funded by the Ministry of Science and Technology (MOST), the Sino-Danish collaboration project (DANCNGAS) funded by the Innovation Fund Denmark, and the National Natural Science Foundation of China (51104137).

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DOI: 10.1021/acs.energyfuels.6b00164 Energy Fuels 2016, 30, 6395−6404