Pyrolysis Behavior of Large Coal Particles in a Lab-Scale Bubbling

Nov 27, 2012 - Thus, the efficient and clean use of coal is an urgent need for sustainable economic development in China. In addition, reserves of oil...
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Pyrolysis Behavior of Large Coal Particles in a Lab-Scale Bubbling Fluidized Bed Cuiguang Yang,†,‡ Songgeng Li,*,† Wenli Song,† and Weigang Lin† †

State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: The results of pyrolysis of coal particles with size ranging from 2 to 14 mm in a bubbling fluidized bed are presented in this paper. The investigation is focused on the effect of the coal particle size and pyrolysis temperature on the product distribution and devolatilization time. Fragmentation of coal particles was observed during pyrolysis. It is found that the extent of fragmentation is more serious for larger coal particles at high temperatures. This leads to a quite different variation trend of the yields of pyrolysis products with temperature between the small and large particles examined in this work. No significant effect of the particle size was noticed at low temperatures because the secondary reaction is not active. The devolatilization times of the coal particles tested in this work can be described with the classic particle size power law relation: t = Adpn. However, the value for the exponent n is much smaller than those obtained by previous researchers, which could be attributed to the fragmentation of coal particles and relative low fluidization velocity employed in this work. small fluidized bed are presented. The influence of coal particle size and pyrolysis temperature on pyrolysis product distribution and composition and devolatilization time are examined in an attempt to understand the mechanism of the pyrolysis of large coal particles.

1. INTRODUCTION Coal is the most abundant fossil fuel energy resource in China. The total coal production in China was nearly 3 billion tons in 2010. The amount is expected to increase to 5 billion tons in 2027.1 With this coal consumption scenario, the environmental impact will result in severe consequences. Thus, the efficient and clean use of coal is an urgent need for sustainable economic development in China. In addition, reserves of oil and natural gas are limited in China. In 2010, the amount of imported oil accounts for over 50% of domestic demand. The production of gas and liquid fuels from coal may be a promising option for Chinese energy systems. A coal “topping” process was proposed by the Institute of Process Engineering, Chinese Academy of Sciences, to cope with the energy situation in China.2 In the process, a downer reactor is integrated into a circulating fluidized-bed boiler as a pyrolyzer. Coal is first pyrolyzed in the downer reactor to produce gas and tar. The remaining char is burned in a circulating fluidized-bed boiler to generate steam and electricity. Because the residence time of coal particles is short (only 1−2 s) in the downer reactor, micrometer-sized coal particles are used in the process. In comparison to the downer reactor, a fluidized-bed reactor allows for relatively larger coal particles with a size ranging from 1 to 15 mm, which is another option as a pyrolyzer to implement the coal topping process. However, very few studies on pyrolysis of millimeter-sized coal particles were performed.3−7 Moreover, the reported data are not consistent. The experimental results from Borah et al.8,9 indicated that the volatile yield increased with the coal particle size, while Stubington et al.10 observed that the volatile yield was constant or decreased. A fundamental cause for the different views is that the pyrolysis behaviors of large coal particles have not been fully understood. In this work, the experimental results on pyrolysis behaviors of large coal particles (with size ranging from 2 to 14 mm) in a © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Coal Samples. A sub-bituminous coal from Fugu, Shanxi, China, is used in this work. The proximate and ultimate analyses of the coal are listed in Table 1. Four different particle sizes (average diameters of 2, 6, 10, and 14 mm) were chosen for pyrolysis experiments. The coal was ground and sieved for different size fractions. The coal particle size with an average diameter of 2 mm ranges from 1.9 to 2.2 mm. Large coal particles were manually selected to obtain the desired diameters (6, 10, and 14 mm). The coal samples were air-dried in a crucible in an oven at a temperature of 105 °C for 4 h prior to experimentation. 2.2. Apparatus and Operation Conditions. The experiments were performed in a lab-scale bubbling fluidized-bed reactor, which is illustrated in Figure 1. The reactor is made of stainless steel with an inner diameter of 36 mm and height of 320 mm. The reactor is electrically heated. The reactor temperature is measured by a Chromel−Alumel thermocouple. Nitrogen is used as a fluidization gas to provide an inert pyrolysis atmosphere, which is introduced through a distributor mounted at the bottom of the reactor and regulated by a mass flow controller. Silica sand is employed as bed material, which has a narrow size range of 105−125 μm with an average diameter of 115 μm. A two-stage valve at the top of the reactor enables a batch of coal particles to be dropped into the hot sand bed instantaneously. The exit of the reactor is connected to a condenser followed by two cool traps immersed in an ice−water bath with acetone as an adsorbent. The gas is further dehumidified in a CaCl4 bottle. After the gas passes through a fabric filter, it is finally collected with gas sample collection bags at preset times to measure the timeReceived: September 6, 2012 Revised: November 27, 2012 Published: November 27, 2012 126

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Table 1. Proximate and Ultimate Analyses of Fugu Sub-bituminous Coal proximate analysis (wt %, dry basis)

a

ultimate analysis (wt %, dry basis)

volatile matter

ash

fixed carbon

C

H

N

S

Oa

34.96

4.50

60.54

77.52

4.81

1.17

0.25

11.77

By difference.

Figure 1. Schematic diagram of the experimental facility. dependent composition with a micro gas chromatograph (CP-4900, Varian). About a 3 g sample of coal particles is fed into the reactor for each experiment. The reactor temperature is set to 500, 600, 700, and 800 °C. The fluidization velocity is fixed at 1.5 times the minimum fluidization velocity at the specific temperature to ensure that the solids flow in a bubbling mode. A macro thermal balance with a maximum load of 5 g (HCT-1) was also used to characterize the pyrolysis behaviors of coal particles. It was performed at a heating rate of 20 °C/min with a nitrogen flow rate of 60 mL/min. 2.3. Determination of Yields of the Pyrolysis Products. Because the silica sand employed has a narrow size range and the solids flow is operated in the regime of bubbling fluidization, we may assume that all of the bed materials (silica sand and the remaining char) are maintained in the bed without being entrained out during experimentation. On the basis of this assumption, the obtained char can be calculated as the difference of the weight of the bed materials before and after experimentation. A high precision balance with an accuracy of 0.0001 g was used to quantify the bed materials before and after the experiment. Each experiment was conducted 3 times. The relative deviation for the char yield is within 5.1% for each condition. To further verify the results, the char particles were also manually picked up after the experiment to weigh. Almost identical results were obtained. The char yield is calculated as

Ychar =

mchar × 100% mcoal

Vgas, i =

− VN2ti

(3)

where VN2 is the carrier gas volume rate (N L min−1), t stands for sampling time for the bag i (min), and Ci,N2 is the concentration of the carrier gas in the sampling bag (vol %). The volume of component j in the gas sampling bag i is calculated by j Vgas, i = Vgas, iCi , j

(4)

where Ci,j is the concentration of component j in the sampling bag i (vol %). The yield of component j in the gas is described as n

Ygas, j =

∑i = 1

j Vgas, i

22.4

mcoal

Mj

× 100%

(5)

in which Mj is the mole mass of component j. Thus, the total gas yield is obtained as

Ygas =

∑ Ygas, j

(6)

The yield of liquid products is determined by subtracting the gas and char yields. The time for 95% of gaseous volatile evolution during pyrolysis is defined as the devolatilization time because most of the tar was released at an earlier stage.

(1)

3. RESULTS AND DISCUSSION 3.1. Effects of the Particle Size and Bed Temperature on the Volatile Yield. Figure 2 gives the variation of the obtained volatile yield with a coal particle diameter at different temperatures. It is obvious that high temperatures favor the devolatilization of coal. The volatile yield for different particle sizes has the same variation trend with the bed temperature. Higher temperatures lead to higher heating rates when other factors are kept constant. High heating rates generally produce

in which mchar is the mass of char and mcoal is the mass of the coal sample. Thus, the volatile yield is obtained as

Yvolatile = 1 − Ychar

VN2ti Ci ,N2

(2)

The gas yield was determined by the N2 balance based on the chromatography analysis because N2 is introduced as the carrier gas and fuel N is negligible. The gas volume collected in the sample bag i is expressed as 127

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Figure 2. Variation of the yield of volatiles with the particle size at different pyrolysis temperatures. Figure 4. Thermogravimetric analysis curves for the different particle sizes.

a large amount of volatiles. This is in line with previous research.2,11,12 The effect of the coal particle size shows quite different characteristics from the results reported in the literature.8,10,13 At the high temperatures (>500 °C), the volatile yield initially decreases with an increase in the particle size and reaches a minimum at the particle size of 6 mm, over which it significantly increases. No significant effect of the particle size was observed at the relatively low temperature of 500 °C. The decrease of the volatile yield with the particle size may be attributed to the intraparticle secondary reactions. The volatiles produced during primary pyrolysis must pass through the pores within a coal particle to the outer surface. During this migration, they may crack, condense, and polymerize with deposition of some carbon. These reactions are expected to become significant as the particle size increases. The increase of the volatile yield with the particle size further increasing could be explained by particle fragmentation, as shown in Figure 3.

particle size. At low temperatures, secondary reactions are not very active.14−19 The slight increase in the volatile yield could be explained by composition segregation with the particle size, as mentioned above. 3.2. Effects of the Particle Size and Pyrolysis Temperature on the Yields of Gas and Liquid Products. The liquid yields at different pyrolysis temperatures for different particle sizes are depicted in Figure 5. It is found that the

Figure 3. Frame photographs of 14 mm size coal particles at various temperatures of pyrolysis. Figure 5. Variation of the liquid yield with the pyrolysis temperature for different particle sizes.

Large particle tends to fragment into small pieces at high temperatures because of inner pressure and thermal stress. The extent of the fragmentation depends upon the particle properties, particle size, and temperature. A high temperature leads to a large temperature gradient within the particle and, thus, high thermal stress. Large particles contain a relatively large amount of volatiles, which leads to high inner pressure within the particle during pyrolysis. The extent of fragmentation for larger particles is even worse. The breakage of particles into smaller fragments and the formation of cracks in the particle enable the volatiles released within the particle to escape immediately, without further intraparticle secondary reactions. Another possible reason is the variation from particle to particle in the maceral/chemical compositions, as indicated in the literature.10 It is evidenced by the macro thermogravimetric analyses for the different particle sizes given in Figure 4. It is shown that the weight loss increases with an increase in the

variation trend of the liquid yield with the temperature for the tested particle sizes appears to follow a different pattern. For the particle sizes of 2 and 6 mm, the liquid yield increases with the temperature to a maximum value and then decreases because of the rapid increase of the secondary cracking reaction rate. This variation trend agrees well with the observations of other researchers.2,12,20 It is worth mentioning that the peak position for a 6 mm size particle shifts toward a lower temperature in comparison to that for a 2 mm size particle. The peak temperature for the maximum liquid yield can be considered as an indication as to whether the secondary cracking reaction of tar has taken place at a high rate or not. An increase in the particle size increases the residence time of primary volatiles within the particle, thus augmenting their 128

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compositions. It is seen that the pyrolysis gas consists mainly of CO, CO2, and CH4, with small amounts of H2 and CxHy. The yields of CO, CH4, H2, and C2H4 show a similar pattern as a function of the temperature, as presented in Figures 7−10. They are produced in small quantities at a temperature of 500 °C, which is considered mainly from the primary decomposition because the secondary reactions are insignificant at low temperatures. They increase with an increase in the temperature. The increase is more pronounced at higher temperatures for particle sizes of 2 and 6 mm. In contrast, the yields for particle sizes of 10 and 14 mm become nearly invariant with the temperature increasing from 700 to 800 °C, where the fragmentation of particles occurs. This phenomenon suggests that these gases are good indicators of the extent of secondary reactions. The CO2 yield depends upon the amount of carboxyl groups in coal. CO2 is mainly formed via the decomposition of carboxyl groups in coal. The carboxyl group can be converted to CO2 at lower temperatures. Owing to the lack of availability of the carboxyl groups in coal, no significant effect of the temperature is observed, except for the points at a temperature of 600 °C for particle sizes of 10 and 14 mm (shown in Figure 11). Further experimentation is needed to give an explanation. CH4 is the dominant product of the gaseous hydrocarbons. At high temperatures, the yields of C2H4 and C3H6 are much higher than those of C2H6 and C3H8. It is known that alkenes are more stable than the corresponding alkanes. At high temperatures, the alkanes are decomposed into alkenes, releasing H2.21 The cracking of tar is the main route to produce alkenes (particularly C2H4) when temperatures are higher than 600 °C.15,18 The crack of C4 and long-chain aliphatic hydrocarbons may form alkenes as well. 3.4. Ratio of H/C of the Liquid Products. The ratio of H/C is a significant index of liquid product quality. Elemental balances of hydrogen and carbon are conducted to obtain the ratio of H/C in the liquids because the liquid sample contains a large amount of acetone, which causes difficulty quantifying the tar chemical compositions by instrument. The results are shown in Table 3. As expected, the ratio of H/C follows a decreasing trend with an increase in the temperature. For a particle size effect, it is seen that the 6 mm size particle has the highest ratio of H/C in the liquids, followed by 14, 10, and 2 mm size particles. It is known that the average heating rate decreases with an increase in the particle size.22−24 A low heating rate favors the production of high-quality liquid products.25,26 For the particle sizes of 10 and 14 mm, fragmentation occurs, which enhances the heating rate. Therefore, their H/C is lower than that of the 6 mm size particle. It should be pointed out that the liquids contain both tar and pyrolysis water formed during pyrolysis, although the coal samples have been air-dried prior to experimentation. Thus, conclusions cannot be reached from it for the tar quality. Nevertheless, it can still give us a hint of the tar quality to some extent. 3.5. Effects of the Particle Size and Pyrolysis Temperature on the Devolatilization Time. The devolatilization time is assessed from gas evolution, which is defined as the time corresponding to evolution of 95 wt % of the total gases.10 The correction for the time of traveling from the bed to the gas sampling point has been made by injecting CO2 tracer gas into the bed, which was found to be about 4 s. The coal devolatilization time is generally controlled by chemical kinetics of pyrolysis, heat transfer to and within the

reactivity with coal particle internals and with other volatiles. For particle sizes of 10 and 14 mm, the liquid yield has an inverse trend with the temperature, which decreases with the temperature to a minimum value and then increases. The formation of cracks and fissures in the particles are responsible for this phenomenon. Primary fragmentation is caused by inner pressure and thermal stress, the extent of which depends upon coal properties, temperature, particle size, and residence time of the particles in the bed. As shown in Figure 3, no obvious cracks are observed at temperatures of 500 and 600 °C. Cracks are developed as the temperature further increases. At a temperature of 800 °C, the large particle even fragments into small pieces. The formation of cracks and fragments limits the secondary reactions. Thus, the liquid yield for particle sizes of 10 and 14 mm decreases with the temperature in the relatively low temperature range and increases with the temperature further increasing. The different variation trends between the small and large particles well support the findings in volatile yield previously mentioned. As shown in Figure 5, the temperature for obtaining the highest pyrolysis liquids is 700 °C with a coal particle size of 2 mm in this work. For the larger coal particles, a higher pyrolysis temperature should be employed to achieve maximum amount of liquid products in consideration of the primary fragmentation. Figure 6 gives the variation of the gas yield with the pyrolysis temperature for different particle sizes. In general, the gas yield

Figure 6. Variation of the gas yield with the pyrolysis temperature for different particle sizes.

increases with an increase in the pyrolysis temperature. The increased gas yield may be the result of small molecular compounds evolved from secondary decomposition of tar and more volatiles released from char at elevated temperatures. It should be noted that the gas yields for particle sizes of 10 and 14 mm increase initially with the temperature and then start to level off after 600 °C. This result indicates that the effect of secondary reactions becomes insignificant, which is rational because small fragments and surface fissures are formed in the particles as mentioned previously. The formation of small fragments and surface fissures improves the intradiffusion of volatile matter during pyrolysis, thus reducing the secondary reactions. 3.3. Effects of the Particle Size and Pyrolysis Temperature on Gas Compositions. Table 2 gives the gas 129

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Table 2. Gas Compositions yields (wt % of dried coal) coal size (mm)

2

6

10

14

pyrolysis temperature (°C)

H2

CO

CO2

CH4

C2H4

C2H6

C3H6

C3H8

500 600 700 800 500 600 700 800 500 600 700 800 500 600 700 800

0.03 0.04 0.28 0.74 0.03 0.12 0.42 0.97 0.01 0.11 0.44 0.50 0.03 0.13 0.61 0.62

0.56 0.72 2.41 7.42 0.60 1.55 3.09 5.94 0.52 1.99 4.11 4.69 0.55 2.31 4.49 4.51

5.73 7.37 6.41 7.78 8.20 7.48 8.68 11.54 11.20 15.86 9.19 10.47 8.11 14.50 8.91 8.95

0.88 1.13 2.10 5.10 0.77 1.85 2.63 4.17 0.66 2.52 3.59 4.09 1.02 2.57 3.82 3.84

0.09 0.12 0.42 1.76 0.10 0.38 0.87 1.77 0.16 0.60 1.44 1.64 0.19 0.52 1.87 1.88

0.38 0.49 0.36 0.45 0.50 0.44 0.37 0.30 0.39 0.68 0.58 0.66 0.51 0.66 0.64 0.64

0.21 0.27 0.61 0.46 0.22 0.35 0.34 0.37 0.22 0.47 0.87 0.99 0.24 0.40 1.56 1.57

0.16 0.21 0.08 0.03 0.27 0.16 0.10 0.01 0.25 0.22 0.15 0.17 0.24 0.23 0.19 0.19

Figure 7. Variation of the H2 yield with the temperature for different particle sizes.

Figure 9. Variation of the CH4 yield with the temperature for different particle sizes.

Figure 8. Variation of the CO yield with the temperature for different particle sizes.

Figure 10. Variation of the C2H4 yield with the temperature for different particle sizes.

particle, and mass transfer of volatiles within the particle. For very small particles, the rate of pyrolysis is controlled by

chemical kinetics. As the particle size increases, a critical size is reached where heat and/or mass transfer becomes limiting, and this size appears to be