Surface Morphology and Porosity Evolution of CWS Spheres from a

Apr 15, 2015 - Fluidization–suspension combustion technology is an effective method to utilize coal water slurry (CWS) as a fuel in industrial boile...
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Surface Morphology and Porosity Evolution of CWS Spheres from a Bench-Scale Fluidized Bed Hui Wang,*,† Shuai Guo,† Li Yang,† Yongjun Guo,† Xiumin Jiang,‡ and Shaohua Wu† †

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China School of Mechanical Engineering, Shanghai Jiao Tong University, Minhang District, Shanghai 200240, China

‡

ABSTRACT: Fluidization−suspension combustion technology is an effective method to utilize coal water slurry (CWS) as a fuel in industrial boilers. The evolution of surface morphology and pore structure of CWS spheres under fluidization−suspension combustion is studied. A bench-scale fluidized bed was used for combustion of CWS spheres with a bed temperature of 850 °C, fluidization number of 4, and bed height of 90 mm. After 15, 30, and 45 s of combustion, the samples were removed from the bed for scanning electron microscope (SEM) and N2 adsorption tests. The combustion mechanism of CWS spheres in the fluidization−suspension combustion state is discussed. The results show that after 15 s, the CWS spheres burst due to volatile release, and some particles fragmented to produce a large number of pores. Thus, the specific surface area and volume of pores increased rapidly. After 30 s, combustion occurred mainly at the exterior surface of CWS spheres and appeared as layer by layer inward combustion. This was confirmed by the fact that the specific surface area and volume did not change. After 45 s, as combustion proceeded, the flame front entered the interior surface through pores, and burnt the interior framework to make the pores collapse. Thus, the specific surface area and volume of pores decreased rapidly. In the whole combustion process, the fractal dimension first increased and then decreased, which demonstrates that the pore structure had experienced a process that went from complicated to simple.

1. INTRODUCTION Coal water slurry (CWS) is a low-pollution fuel that has the potential to replace oil because it provides economic benefits by reducing the cost of fuel. However, it is difficult for CWS to burn with high combustion efficiency because it contains a large amount of moisture. Therefore, the efficiency can be improved through technological developments. Among many combustion technologies for CWS, fluidization−suspension combustion technology1 has advantages over other technologies because of eight features: (1) low temperature combustion of CWS, solving the problems of easy slagging, operational instability, and poor safety, and restrains the production and emission of thermal NOx; (2) high combustion efficiency because of circulating combustion; (3) direct desulfurization in a furnace without high cost and or a very complicated system; (4) improved effectiveness of limestone; (5) wide adaptability of rate load of CWS (30−100%); (6) wide adaptability of CWS quality; (7) simple and reliable system; and (8) wide adaptability to the operation of boilers. Considering these advantages, this technology has broad prospects for CWS combustion technology. To date, CWS fluidization−suspension combustion technology has achieved good stability and high combustion efficiency in industrial boilers with a capacity of less than 35 t/h and has tackled the problem of boiler slagging. Long-term operation in these boilers has been achieved with a boiler efficiency of 89−91%.2−7 The combustion characteristics of CWS depend largely on its composition, which is generally composed of 65−70% of pulverized coal, 30−35% of moisture, and 1% of chemical additives. Among them, the additives are neither flammable nor combustion supporting. The major role of these additives is to alter the surface properties of coal particles, promote dispersion © XXXX American Chemical Society

of the particles in water, and bring about good rheological characteristics and stability of the slurry. Moisture is another important part of CWS and greatly influences the combustion characteristics of CWS.3,4 The pulverized coal, which is the most important part of the CWS, shares a similar combustion process with that of common coal. This combustion process of CWS can be summarized as a moisture evaporation period, an ignition period, a visible envelope flame period (devolatilization and combustion), and finally a char combustion period until burnout.5 Within these periods, the uniqueness of CWS combustion lies in the moisture evaporation. During moisture evaporation, CWS is considered to be a colloidal gel that can shrink and swell in response to moisture loss and gain.6 Some studies have been carried out on the physical changes of a CWS upon moisture evaporation. Yavuz et al.5 adopted a suspended single droplet combustion technique to study the combustion characteristics of lignite CWS. It was found that, in the process of slurry drop combustion, intense moisture evaporation was often accompanied by “micro-explosion”, where the slurry drop burst into smaller drops, meaning that it fragmented. The fragmentized slurry drop had a porous and loose structure and experienced volatile release and char combustion. The subsequent processes were considered the same with pulverized coal combustion. Thus, compared to other fuels, moisture evaporation and the drying process of CWS caused it to “burst” and “fragment”, resulting in the formation of char particles with a large number of pores.7 Fragmentized characteristics of fuels during combustion have been studied previously, and the Received: December 31, 2014 Revised: April 14, 2015

A

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Energy & Fuels Table 1. Proximate and Ultimate CWS Analysis proximate analysis (as received (ar), wt %)

ultimate analysis (ar, wt %)

moisture (Mar, %)

ash (Aar, %)

volatiles (Var, %)

net heat value (Qar,net, kJ/kg)

carbon (Car, %)

hydrogen (Har, %)

oxygen (Oar, %)

nitrogen (Nar, %)

sulfur (Sar, %)

32.9

5.64

43.758

18877

50.57

3.27

6.13

0.93

0.56

results indicate that high volatile fuels were more advantageous for fragmentation compared to low volatile ones.8 Hence, devolatilization is accompanied by a series of processes, including fragmentation and pore formation.2 Yavuz et al.5 pointed out that, for the CWS made from high adhesive coals, slurry drops often swelled before ignition and then burst into large and irregular aggregates. A porous residue was often formed with a large surface area advantageous for oxidation. The resulting swollen coal burnt quickly, shortening the char combustion time and therefore the total combustion time. Kadota et al.9 studied oil−water emulsion and also found that its combustion was often accompanied by unusual “microexplosion” that did not occur in pure fuels. In the combustion process of CWS, due to moisture evaporation and volatile release, its physical properties changed distinctly and a pore structure formed on the interior and exterior surface of CWS particles when the “burst” occurred. A few studies have examined the pore structure formed in dynamic processes. Androutsopoulos et al.10 conducted Greek lignite drying experiments in a fixed bed and employed mercury injection porosimetry (MIP) to test changes in the pore structure in the process of drying. Results showed that with moisture removal of lignite CWS the number of pores with 7.5−150 nm diameters increased and pores with 150−1000 nm diameters decreased dramatically. This was because macropores and partial mesopores shrunk or even collapsed during moisture removal. At the same time, macropores and mesopores gradually changed to micropores. Moreover, it could also be seen that its pore volume decreased slightly but specific surface area distinctly increased. The surface area of CWS was mainly subject to micropores,11 which proved that the number of macropores decreased. Deevi et al.12 studied the pore structure change of North Dakota lignite CWS in the process of moisture evaporation by testing the method of N2 adsorption combined with CO2 adsorption. The results of N2 adsorption showed that mesopores (2−18 nm) decreased with moisture evaporation as did the surface area. However, the surface area measured by CO2 adsorption was contrary to that of N2 adsorption because CO2 adsorption measured mainly micropores below 2 nm. Hence, it can be concluded that in the process of moisture evaporation a lot of micropores are formed. This is because irreversible shrinkage occurs as moisture is evaporated and causes pore collapse and subsequently gradually changes macropores and mesopores to micropores. The research described above only takes moisture evaporation into account but not the influence of volatile release. Takeno et al.13 heated a single coal−water mixture droplet by laser and tested the pore structure changes during moisture evaporation and volatile release. The results showed that the samples first swelled and then shrunk, and the number of micropores increased before decreasing. However, the number of large pores remained unchanged. This is because when moisture was removed from the interior pore structure, a lot of pores were left, the number of pores increased and micropores decreased gradually due to agglomeration as the reaction proceeded.12

In the combustion process of CWS with moisture evaporation and volatile release, a series of changes occurred, including “bursting” and pore structural changes in terms of physical characteristics. However, the combustion involved in the literature described above is in steady state. Whereas for the fluidization−suspension combustion of CWS, Miccio et al.14,15 and Kijo et al.16,17 conducted a series of research studies on the combustion characteristics of CWS in a fluidized bed. Atesok18 pointed out that, under an appropriate bed temperature, preheated air temperature, and volume of air, the combustion of CWS in the fluidized bed can achieve a very high combustion efficiency and low pollutant emission. Zang et al.19 pointed out that combustion in the fluidized bed produced mainly primary fragmentation and its fragmentation was caused by volatile release. Hence, due to high content of moisture in CWS, its combustion in the fluidized bed not only involves volatile release but also moisture evaporation such that its fragmentation is more complicated in the fluidized bed, and the fragmentation mechanism is not clear. However, it is clear that, at the stage of moisture evaporation and volatile release, a large number of pores are produced; at the char combustion stage, it may be accompanied by pore sintering and closing. Therefore, the whole combustion process of CWS in the fluidized bed is accompanied by fragmentation and changing pores, which also reflects the whole combustion process of CWS. Therefore, studying the fragmentation characteristics of CWS is significant for understanding its combustion mechanism. This paper tries to understand the mechanisms of pore formation in dynamic processes. CWS spheres were prepared by dropping CWS onto red-hot quartz sand to simulate the formation of CWS spheres by dropping CWS from the furnace top onto red-hot bed materials and releasing water in the fluidized bed. Then, single-factor experiments were designed on a bench-scale fluidized bed. After taking out the samples that experienced different fluidization reaction times, we studied the changes of its fragmenting behavior and pore structure by SEM and N2 adsorption, respectively, and then the combustion mechanism of CWS in the fluidization−suspension state was determined on the basis of fractal theory.

2. EXPERIMENTS 2.1. Preparation of CWS Spheres. The CWS used in this study was produced by Shengli Coal Water Slurry Mill in Shandong Shengli Oilfield. The parent coal was Datong bituminous coal from the Shanxi province. The detailed proximate and ultimate analysis is given in Table 1, and the particle size distribution is given in Figure 1. The particle size distribution was measured by a SUCELL CL laser particle size analyzer from Germany SYMPA Co. using the wet method. CWS spheres were prepared in a Muffle furnace with a temperature of 850 °C. The preparation process is detailed as follows: A stainless steel container was filled with quartz sand (see Table 2 for particle size distribution) and placed in the Muffle furnace preheated to 850 °C. Once the temperature of the quartz sand stabilized, 10 CWS drops were quickly dropped onto the red-hot quartz sand. The equivalent diameters of the CWS spheres formed by the drops were controlled within 7−9 mm, and total mass was ∼2.5 g. These CWS drops were then removed and transferred to a dryer to cool. These steps were B

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dilute zone after the dense bed increased gradually from 51 mm at the outlet of the dense zone to 83 mm. This setup was 800 mm in height. Three heating elements were independently controlled and separately used for the air preheater, dense bed, and dilute sections. The distributor plate was rectangle-shaped and made of four layers of a stainless steel mesh screen with 200 net openings per square inch. Air was provided by an air compressor controlled by a pressure regulator and a rotameter. The air preheater with a maximum power of 5 kW was used to heat the air to 800−950 °C. Quartz sand was used as the bed material in the experiments with the particle size distribution the same as that for the preperation of the CWS spheres. 2.3. Experimental Procedures and Testing Method. The operation steps of the tests are as follows:20 (A) First, the quartz sands were fed into the furnace via the feeding entrance (10) of the benchscale fluidized bed as shown in Figure 2. The set air flow was then introduced, and the combustor was heated to the desired temperature. (B) As soon as the desired temperature was reached, the air was stopped. Then, the prepared CWS spheres were quickly introduced to the bed via the feeding entrance. Then, the air was fully opened again, and the vacuum pump was then started immediately. (C) When the fluidization time was reached, the air was switched to high-purity nitrogen with a flow >25 L/min. Then, CWS spheres were sampled by pulling the distributor plate out with all of the quartz sand and CWS spheres on it. (D) Finally, all the CWS spheres and fragmental parts were picked up, put in crucibles with lids, and then placed in a glass desiccator with allochroic silica gel on the bottom for further cooling. The experimental conditions were as follows: bed temperature of 850 °C, fluidization number of 4, bed material height of 90 mm, and primary air volume of 9.83 N m3 h−1. Fluidization combustion experiments of CWS were conducted with four fluidization times of 0, 15, 30, and 45 s according to the steps above. Repetitive tests were performed to avoid further fragmentation and provide optimal samples. After the samples with different fluidization times were cooled, micromorphology and pore structure testing were conducted by SEM and N2 adsorption, respectively, to analyze the combustion mechanism of CWS in the fluidization bed.

Figure 1. Size distribution of coal and CWS.

Table 2. Size Distribution of Quartz Sand as the Bed Material size range (mm)

mass percent (%)

0.88−1.6 1.6−2.5 2.5−3.01 3.01−4

7.5 47.5 35 10

employed to simulate the process of slurry sphere formation in a fluidized bed by dropping CWS from the furnace top onto red-hot bed materials. 2.2. Bench-Scale Fluidized Bed Combustor. Experiments were carried out in a bench-scale fluidized bed combustor as shown in Figure 2, which can also be seen in reference 20. The setup was composed of five major parts in sequence: an air preheater, dense bed, dilute bed, cyclone separator, and bag-type gas−solid separator. The inner diameter of the dense bed was 51 mm. The inner diameter of the

3. RESULT AND DISCUSSION 3.1. Micromorphological Changes. SEM photos with 80×, 500×, and 5000× resolution were taken by an S-2150 SEM of Japan HITACHI Company for CWS with different fluidization times, as shown in Figures 3−5. The sample at 0 s was fresh slurry spheres after being split. The rest of the samples were the fragments of slurry spheres taken from the furnace chamber found to be covered with ashes upon removal.

Figure 2. Schematic diagram of the experimental system: 1, air compressor; 2, pressure reducing valve; 3, rotameter; 4, throttle valve; 5, pressure regulator; 6, air preheater; 7, distributor plate; 8, materials outlet; 9, thermocouple; 10, feeding entrance; 11, temperature controller; 12, fluidized bed body; 13, cyclone separator; 14, heat exchanger; 15, bag-type separator; 16, control valve; 17, vacuum pump; and 18, chimney.

Figure 3. Photos (80×) of CWS spheres of reference time after fluidization times of (a) as prepared (0 s), (b) 15 s, (c) 30 s, and (d) 45 s. C

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tion. In 15 s case (Figure 3b), the compact structure at the exterior layer is clear, which means it impedes the release of internal gas and produces great pressure, resulting in the production of intense fragmentation as well. Moreover, the surface looks pretty smooth, which is also a product of intense fragmentation. After fragmentation, the former internal surface burns more intensely than the external surface, and the influence of attrition becomes more significant. The 45 s photo (Figure 3d) illustrates a through-hole on the exterior wall. The formation of the through-hole decreases the pressure difference, but the intense release of gas at this through-hole makes this position the one with the lowest strength. Thus, fragmentation occurs easily and large fragments are formed near this position. From Figure 3b, the sphere particles also appear as hollow shells, and it is presumed that slurry spheres have entered the so-called constant-diameter porous rigid-shell period,21,22 which means it is in a constant-diameter burning mode by the presence of a continuously thickening, rigid porous shell with a regressing inner surface. Antaki22 predicted that the diameter of the inner surface would vary cubically with time, which is the so-called d3-law. Before this period, the change in the diameter will go through a so-called regressing-diameter period (i.e., what we call layer by layer inward flaking-off combustion20 stage, which could be described well by the classical d2-law).21 Figure 4 illustrates the pore structure of the slurry spheres with an amplification of 500×. At 0 s (Figure 4a), coal particles can be seen clearly, and some pore structures are formed. At 15 s (Figure 4b), large pores are formed, and their surfaces are smoother than those at 0 s. Large cracks exist in partial regions, and small fragments are produced when colliding with bed materials or the inner wall of the furnace chamber. At 30 s (Figure 4c) and subsequent cases, an obvious honeycomb structure is formed with lots of large pores that are not significantly different. This could be taken as a sign that percolation combustion has occurred. This pore structure will last for a while until it enters the disruptive period.21 Then the “framework”, which plays the connecting role in the particle network structure, will burn out and be destroyed; the protruding parts on the surface will fall off due to attrition by the bed materials. Figure 5 illustrates the micropores of slurry spheres at 5000× amplification. At 0 s (Figure 5a), the space among pulverized coal particles is obviously visible, and at 15 s (Figure 5b), the pores formed on coal particles can be seen. This shows that evaporation of the external residual moisture in the slurry spheres enlarges the pores of the particles and weakens the connection among the particles. A great number of pores in the particles are formed due to moisture evaporation and volatile release, which improves the reactivity of the coal. The photos after 30 s (Figure 5c) show that some small grains on the particle surfaces have melted to balls after combustion, which causes blockage of some micropores and partially impedes hot flue gas from entering the interior. It can be concluded from Figures 3−5 that CWS spheres burst as soon as they are put into the furnace chamber and some of the particles fragment; because the outer layer of the particle is compact, combustion occurs mostly at the exterior surface and shows a layer by layer inward flaking combustion; moreover, the outer ash layer maintains nearly the same thickness at the earlier stage because burning and attrition proceed at nearly the same speed. Some deep cracks on the particles are formed. The flame front enters into the cracks during combustion, burns off the framework, and causes

Figure 4. Photos (500×) of CWS spheres of reference time after different fluidization times of (a) as prepared (0 s), (b) 15 s, (c) 30 s, and (d) 45 s.

Figure 5. Photos (5000×) of CWS spheres of reference time after different fluidization times of (a) as prepared (0 s), (b) 15 s, (c) 30 s, and (d) 45 s.

Because the ashes were brittle and easily fell off during operation, the ashes covering the surface can not be seen in the photos. It can also be observed that although some slurry spheres did not fragment, wide and deep cracks were formed; some were formed inside cavities, and some were concentric balls. Furthermore, there was a distinct spherical crack between the inner spheres center and the outer spheres shell, some of which were partially connected and others separated completely. Figure 3 illustrates the cross section of the exterior wall of theslurry spheres. With the amplification at 80×, we can see a large range of pore structures. The prepared CWS spheres (see Figure 3a) form a clear pore structure. The outer surface looks smooth, and structure is more compact than the interior. However, there are a lot of micropores seen in SEM photos of 15−45 s, illustrating the fragments of the burst slurry spheres of which the honeycomb structure is obvious. The micropores on the exterior surface of the slurry spheres during preparation are not sufficient to balance the pressure due to internal moisture evaporation and volatile releasing, thus resulting in fragmentaD

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Figure 6. N2 adsorption−desorption isotherms of different samples.

similar pore structures. This also agrees with the surface micromorphology (panels (c) and (d) in Figures 3−5). It can be seen from the adsorption isotherm of each sample that, in the low pressure zone (0 < P/P0 < 0.2), adsorption capacity increases quickly with relative pressure P/P0, which shows that the samples contain a certain amount of micropores; in the medium pressure zone (0.2 < P/P0 < 0.8), the adsorption capacity increases slower than in the low pressure zone as relative pressure P/P0 rises. In the 0 and 15 s cases in particular, the adsorption capacity remains almost unchanged as the relative pressure rises, which demonstrates that few of the pores are mesopores, far less than the number of micropores. It can be seen from the pore size distribution in Figure 7 that the most probable pore size of all of the samples is ∼2 nm, which also proves conclusions described above. It can be seen from Figure 6 that adsorption and desorption isotherms do not enclose, which is named “desorption hysteresis”. The cause of which is “capillary condensation” in the adsorption process, related primarily to factors like pore size, pore shape, and the interaction strength between adsorbent and adsorbate. On the basis of these results, we know that the adsorption and desorption isotherms illustrated in Figure 6 do not belong to the six categories of isotherms defined by the International Union of Pure and Applied Chemistry (IUPAC). However, it can be regarded as a combination of isotherms of categories I, II, and IV and possesses characteristics of all three categories.

secondary fragmentation. For the small fragmented particles, the former exterior layer is still more compact than the interior layer, and the fragments wrapped by flame are burnt layer by layer. The interior layer burns faster than the exterior layer, which may possibly cause the hollow sphere shell shape.21 Thus, the strength of the sphere shell is weakened and burst into smaller fragments due to bed material impact or percolation combustion. Subsequently, it may be burned to ash particles at this size, and then burst into smaller ash particles and carried by hot flue gas out of the furnace chamber. 3.2. Pore Structural Changes. Pore structures of CWS spheres were tested at different fluidization times using an ASAP 2010 M+C specific surface area analyzer. The samples were degassed at 100 °C in a vacuum pressure of 266.644 Pa for 15 h. Then, N2 adsorption at 77.8 K was measured at a relative pressure of 0.01−0.995. The specific surface area of the sample is calculated by the Brunauer−Emmett−Teller (BET) equation using the linear part (0.05 < P/P0 < 0.25) of the adsorption by assuming a closely packed BET monolayer. The pore size distribution of the samples is calculated by the Barrett−Joyner−Halenda (BJH) equation. 3.2.1. Pore Type and Shape. Figure 6 illustrates N2 adsorption−desorption isotherms of CWS spheres at different fluidization times. At the fluidization times of 0, 15, 30, and 45 s, the development of pores in the samples is observed, which is demonstrated in Figures 3−5. The change rules of samples of 30 and 45 s are similar, showing that these two samples have E

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Figure 7. Pore-size distribution of different samples with adsorption branch.

Figure 8. Pore-size distribution of different samples with the desorption branch.

It should be noted that in the 15, 30, and 45 s cases, the adsorption and desorption isotherms of the samples never enclose, unlike cases for general samples in which a “hysteresis loop” appears, in which adsorption and desorption isotherms match together when P/P0 < 0.45. Similar results also appear in the literature.23,24 The most likely reasons for the above phenomenon are that (1) there are many micropores in the sample and N2 cannot easily get out due to the lack of a driving force but can by capillary works when desorption happens; (2) samples contain pores that have an ink bottle shape in which the adsorbate come easily enter but barely exit; (3) pore structure of the samples collapse due to swelling during desorption and the N2 molecules are “buried” inside. Based on the pore size distribution of the samples illustrated in Figure 7, there are many micropores in the samples of 15, 30, and 45 s; therefore, reason 1 seems to be the most likely. 3.2.2. Changes in Pore Structure during Combustion. The pore size distribution of samples is calculated by the BJH equation. Calculations can be conducted by the data of either the adsorption or desorption branch. Different pore structure should be calculated by data from different branches based on GB/T 21650.2-2008 “Pore Size Distribution and Porosity of Solid Materials by Mercury Porosimetry and Gas Adsorption − Part 2: Analysis of Mesopores and Macropores by Gas Adsorption”. Special attention should be paid to the “critical value” of pore size when using the desorption branch. This means that if the pore size is too small, desorption is difficult such that the desorption branch cannot genuinely reflect the pore size distribution of the samples; the “critical value” is different for different experimental systems (N2@77K, Ar@87K, Ar@77K, and so forth). Taking the most commonly used N2@77K, for example, the critical values are a relative pressure of 0.4−0.5 and pore size of ∼5 nm. If the pore size is smaller than the value, the results calculated by desorption branch will show a fake peak; thus, the desorption branch should not be used for calculations in this case. It is known from section 3.2.1 that the samples contain a significant number of micropores; thus, the fake peak is likely to arise. It is also known from Figure 8 that the pore size distribution calculated by the desorption contains a fake peak. Therefore, the pore size distribution is better calculated by the adsorption branch. Figure 7 shows the results of the pore size distribution. Table 3 provides BET specific surface area, specific surface area of micropores, BJH average

Table 3. Pore Structure Parameters of Different Samples sample (s)

BET-specific surface area (m2/g)

microporespecific surface area (m2/g)

BJH average pore size (nm)

specific pore volume (cm3/g)

0 15 30 45

2.1157 43.1313 43.2126 16.7243

0.1096 30.2021 21.0811 0.0209

39.0732 6.3403 5.4946 5.5

0.011836 0.028475 0.032953 0.016828

pore size, and specific pore volumes of the samples at different fluidization times. Figures 9 and 10 illustrate the transition of the BET surface area and pore volume of the samples.

Figure 9. BET-specific surface area and pore volume at different fluidization times.

Figure 11 provides the mass percentage of unburned carbon at different fluidization times. It can be seen that the unburned carbon amount decreases as the fluidization time increases, which shows that CWS spheres have truly gone through the burning process. The detailed changing process of CWS spheres in the fluidized bed is as follows: First, before ignition (0s), samples contain some but not many micropores. In addition, the pore size distribution is uniform with a BJH average pore size of ∼39 F

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Second, the specific surface area and volume of the pores first increases and then decreases as combustion proceeds. Specifically, ignition occurs during 0−15 s and a larger number of pores appear in the samples due to moisture evaporation and volatile release. The BET-specific surface area and volume of the pores increase rapidly during the process of stable combustion from 15 to 30 s. Combustion proceeds layer by layer from the outer surface to the interior; therefore, the pore structure and its BET surface area remain unchanged. At the same time, the pore volume increases slightly and continues to increase until 30 s, which demonstrates the intense combustion state of CWS spheres. As combustion proceeds from 30 to 45 s, the BET-specific surface area and volume of the pores decrease rapidly. There are two reasons for this. On one hand, high temperature causes pores to sinter, and spherical molten grains as illustrated in Figure 5c and d are produced by sintering. On the other, the flame front enters the interior surface from pores and burns out the framework. Consequently, the internal pores collapse, which causes a decrease in the number of pores. Finally, it can be seen from the results presented above that CWS spheres produce a number of micropores after ignition, which decrease constantly as combustion proceeds until they disappear. That is to say, changing of the pore structure during combustion mainly lies in the micropores. Figure 10 illustrates the changing of the micropore-specific surface area. Therefore, CWS spheres experience a process of first producing micropores and then destroying them. Therefore, the number of micropores reflect the combustion process. Specifically, there are mainly mesopores and macropores before and after combustion, and micropores only exist during combustion. 3.2.3. Fractal Dimension of Pore Structure. Pore structure parameters of CWS spheres at different combustion times can be acquired by the N2 adsorption method. Thus, we determined macroscopic characteristics of pore structure but cannot quantitatively describe the roughness of the pore surface or the complexity of the pores, which greatly influences the surface reactivity as well as diffusion ability of gas molecules, so it is a combustion characteristic. Fractal theory can quantitatively describe the physical structure, which is characteristic scale-free, with self-similarity. In 1983, Pfeifer and Avnir25 introduced fractal theory to porous materials for describing pore surface roughness and the complexity of pore structure. Cheng et al.26 studied the fractal characteristics of CWS. Zhou et al.27 also studied the pore structure of primary CWS using fractal dimensions but did not describe the changing process of fractal dimensions during combustion as much. Here, we employ fractal theory to analyze changes in the pore surface roughness and structure complexity during combustion of CWS. The Frenkel−Halsey−Hill (FHH) equation is an effective approach for calculating fractal dimensions based on the gas adsorption approach. The FHH equation is presented as follows28

Figure 10. Micropore-specific surface area at different fluidization times.

Figure 11. Mass percentage of unburned carbon at different fluidization times.

nm and within the mesopore range. Figure 3a shows obvious pore structure, which is presumably caused by moisture evaporation by the red-hot quartz sand during the preparation process. Once CWS spheres are ignited (at 15 s), the percentage of micropores in the pore size distribution of the samples increases rapidly with the BJH average pore size decreasing to ∼6 nm. It can also be seen from Figure 3b that a large number of micropores appear on the surface. As combustion proceeds (at 30 and 45 s), the pore size distribution and BJH average pore size do not change dramatically. As panels c and d in Figure 3 show, the morphology of the two samples does not change distinctly. Once combustion of the CWS spheres starts (at 15 s), moisture is evaporated from the samples and leaves a great number of micropores,13 which provide passage for volatile release. Furthermore, as combustion proceeds (30 s) until burnout (at 45s), the surface of samples does not have a lot of mesopores or macropores, just micropores. This can be explained by the fact that the combustion of CWS spheres starts at the compact exterior surface and proceeds layer by layer with little influence on the interior pores. Thus, the interior pores of the samples maintain their original state.

⎡ ⎛ ⎛ P ⎞⎞⎤ ln(Q ) = A⎢ln⎜ln⎜ 0 ⎟⎟⎥ + constant ⎣ ⎝ ⎝ P ⎠⎠⎦

(3-1)

where Q is the gas adsorption amount (mL/g) when the equilibrium pressure is P, A is the power factor relevant to fractal dimension D and the adsorption mechanism, P0 is the saturation pressure (Pa) of gas adsorption, and P is the equilibrium pressure (Pa) of gas adsorption. On the basis of eq 3-1, the slope of the fitting line is A, with ln [ln (P0/P)] and ln G

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Energy & Fuels (Q) as the horizontal and vertical coordinates. Fractal dimension D relates to A namely by A=D−3

(3-2)

The fractal dimension can be calculated by equation 3-2. It is critical to acquire the correct fractal dimension by choosing the proper relative pressure range. For this, Wu et al.29 have pointed out that the mechanisms obey single-layer adsorption in the pores of the samples and subsequently obey multilayer adsorption. As the layers increase, the surface of the samples gradually become smooth, and the fractal dimension calculated by the data of multilayers cannot accurately characterize the fractal features of the sample surface. Therefore, it is critical to determine the single-layer adsorption range. The research of Tang et al.30 shows that it can be regarded as single-layer if the adsorption layer n is within the range of (1.0 ± 0.5)−(2.0 ± 0.5). Here, the gas adsorption layer n of the samples can be calculated by eq 3-3. n = (Q /Q mono)1/(3 − D)

Figure 12. DR curves of the samples at different reaction periods. (3-3)

Figure 12 illustrates the DR curves of the samples at different reaction periods. Because tests in the case of P/P0 < 0.01 cannot be conducted due to restrictions of the testing instrument, such values are provided in the DR curves. It can be seen from Figure 12 that, because DR curves contain two linear parts, pores with diameters of three- or four-layer molecules can be regarded as single-layer gas adsorption when P/P0 is within 0.01−0.05 and this data can be used for the calculation of fractal dimensions. Above all, the fractal dimension of mesopores in the primary CWS spheres and micropores during combustion of CWS was calculated in this study. The calculation process and reliability analysis are shown in Figures 13 and 14, and the results are shown in Figure 15.

where Qmono is the gas adsorption amount of single-layer adsorption (mL/g). It can be seen from the section above that the pores of the CWS spheres are mesopores before ignition and micropores during combustion. The calculation method for fractal dimensions of different pore sizes is different. Thus, the fractal dimensions of micropores and mesopores are calculated separately below. First, as for the adsorption process of micropores, the relative pressure of single-layer adsorption can be determined by the Dubinin−Radushkevich (DR) equation. The DR equation links the gas adsorption amount with the adsorption potential under a certain P0/P, namely 2⎤ ⎡ ⎛ V A ⎞⎥ ⎢ θ= = exp −⎜ ⎟ ⎢⎣ ⎝ βE0 ⎠ ⎥⎦ V0

(3-4)

where θ is the micropore filling rate, V is the filled micropore volume under a certain P/P0 (mL/g), V0 is the total volume (mL/g) of micropores, A is the Gibb’s adsorption free energy A = RT ln (P0/P), and βE0 is the characterisitic adsorption energy. Taking the logarithm of the DR equation, namely ⎛P ⎞ ln (Q ) = − D ln 2 ⎜ 0 ⎟ ⎝P⎠

(3-5)

where D is a constant relevant to the adsorption heat. See Figure 12 for DR curves of CWS spheres at 15, 30, and 45 s. Kaneko et al.30 pointed out that the filling process of gases in micropores was different at different stages. Specifically, when P/P0 < 0.01, N2 first fills in the pores with the strongest potential field and with the diameter of single- or double-layer molecules. As P/P0 increases, single-layer adsorption occurs in the pores with the diameter of three- or four-layer molecules; as P/P0 increases continuously, multilayer adsorption occurs in the pores with the diameter of three- or four-layer molecules. The results described above show that when P/P0 is extremely low N2 agglomerates rapidly within micropores, which is not proper for describing the irregularity of the pore surface. When the micropores with diameter of three- or four-layer molecules complete single-layer adsorption, they can describe the irregularity of the pore surface such that the data in this section can be employed to calculate the fractal dimensions.

Figure 13. Fitting line of ln (ln (P0/P)) and ln Q at 0 s.

It can be seen from Figures 13 and 14 that adsorption layer n is in the range of (1.0 ± 0.5)−(2.0 ± 0.5), and the linear correlation coefficient is >95%, which guarantees the accuracy of the calculated results. Figure 15 illustrates the change in the fractal dimension of the CWS spheres during combustion; from the figure, it can be seen that the fractal dimensions of the CWS spheres are between 2−3. As combustion proceeds, the fractal dimension increases and then decreases. On the basis of the H

DOI: 10.1021/ef502923t Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

surface. During combustion, the flame front enters the cracks produced upon bursting, burns out the framework, and causes secondary fragmentation. The small fragmented particles still burn layer by layer from the exterior surface. Because the former interior layer burns faster than the exterior layer, it may form a hollow shell, which weakens the impact intensity. Smaller fragments are produced due to impact and percolation combustion within the furnace, which then become ash particles. The ash particles are then fragmented into smaller particles by impact and carried out of the furnace by hot flue gas. (B) During combustion of the CWS spheres, the specific surface area and volume of pores increases before decreasing. This means samples produce a great number of pores due to moisture evaporation and volatile release after ignition. Subsequently, pores decrease due to “framework” burn out and pore sintering. (C) CWS spheres produce a large number of micropores after ignition. As combustion proceeds, micropores gradually decrease until they disappear altogether. Thus, the number of micropores reflects the progress of combustion. (D) The fractal dimension increases before decreasing over the whole combustion, which parallels the pore structure first becoming more complicated and then back to being simple over the same time frame.

Figure 14. Fitting line of ln (ln (P0/P)) and ln Q during 15−45s.

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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 We gratefully acknowledge the financial support provided by the National Natural Science Funds for Young Scholars of China (Grant No. 51106038) and the financial support by Key Technologies Research and Development Program of China (Grant No. 2012BAA02B01-04).

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Figure 15. Fractal dimensions of CWS during combustion.

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DOI: 10.1021/ef502923t Energy Fuels XXXX, XXX, XXX−XXX