Characteristics of Fluidized-Bed Combustion with Intermittent

Combustion behavior of woodblocks and rubber balls was investigated in a vortex fluidized bed combustor (VFBC) with an intermittent fuel feeding syste...
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Characteristics of Fluidized-Bed Combustion with Intermittent Feeding Using Woodblocks and Rubber. 1. Combustion Behavior HouPeng Wan,† Feng Duan,‡ YunLong Han,§ ChienSong Chyang,*,∥ HsuehJung Chen,∥ and Jim Tso⊥ †

Green Energy and Environment Research Laboratories, ITRI, Hsingchu, Taiwan 31040, R.O.C. School of Metallurgy and Resource, Anhui University of Technology, Ma’anshan 243002, China § School of Civil Engineering and Architecture, Anhui University of Technology, Ma’anshan 243002, China ∥ Department of Chemical Engineering, Chung Yuan Christian University, Chungli, Taiwan 320, R.O.C. ⊥ R&D Center for Environmental Technology, Chung Yuan Christian University, Chungli 320, Taiwan, R.O.C. ‡

ABSTRACT: Combustion behavior of woodblocks and rubber balls was investigated in a vortex fluidized bed combustor (VFBC) with an intermittent fuel feeding system. Effects of various operating parameters on the temperatures within the combustor were studied. Response surface method (RSM) and single factor experimental method were used for statistical analysis. The results show that the combustor temperatures change periodically with the feeding cycles and the highest temperature appears in the splashing region. The mean bed and freeboard temperatures increase with in-bed stoichiometric oxygen ratio (ISOR) for both feedstocks. When the static bed height increases, the mean bed temperature decreases, and the mean freeboard temperature increases for both fuels. When the feeding interval increases, the mean bed and freeboard temperatures decrease for rubber ball burning; on the other hand, both increase in the case of wood burning. According to the experimental results and RSM analysis, the mean bed temperature can be represented by a combinatorial equation of operation parameters, while the mean freeboard temperature can be represented by an equation of static bed height as the only operation parameter.

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INTRODUCTION The utilization of fluidized bed combustion (FBC) systems boomed for the purpose of coal combustion during 1970s. Later its applications extended to the combustion of wastes. FBC has the advantages of high heat transfer efficiency, well mixing of gas and solid phases, and uniform temperatures throughout the bed and freeboard. The success of FBC can be attributed to the great flexibility in handling fuels of various heating values and the reduction of pollutants emitted in the flue gas.1,2 The concept of vortexing fluidized bed combustor (VFBC) was first presented in Soward’s low-pollution incineration of solid waste combustor.3 In order to avoid the elutriation of excessive particles and enhance the combustion efficiency of waste or fuel in the bubbling fluidized bed (BFB), an integration of fluidized bed and cyclone was first designed and operated by Korenberg. In this combustor, the secondary air was introduced tangentially and distributed evenly into the freeboard. The studies of the hydrodynamics, combustion behavior, and emissions of VFBC have been conducted by numerous researchers.4−10 Most FBC systems are operated with a continuous feeding system. For the incineration of hazardous solid wastes such as medical waste, enwrapped in packages in order to prevent hazardous material from emitting to the environment, batch or intermittent feeding is needed. For an intermittent feeding system, fuel is fed to the combustor periodically like a series of feed batches. Although FBCs have been used for many years in different industries, applications with intermittent feeding (e.g., hazardous waste incineration) are uncommon, and knowledge © 2012 American Chemical Society

of combustion behavior with intermittent feeding systems is still lacking.11 The combustion behavior and air pollutant emissions changed periodically with the feeding intervals.12,13 When fuel is fed into a FBC, a drying and devolatilization process occurs instantly, and it drives the bed temperature down slightly. After that, the fuel particle sizes and density are changed by devolatization, and a lot of fuel breaks into smaller pieces.14−16 Andrei17 and Adanez18 investigated coal combustion reactivities by burnout time measurements in a batch fluidized bed. Khraisha et al. conducted batch combustion experiments of oil shale particles in a fluidized bed reactor.19 It was found that gas velocity increase leads to a shorter burnout time. They observed that large initial particles created large ash layers that resisted the progression of combustion and resulted in longer burnout time. Meanwhile, as the batch weight increases, the burnout time became longer due to an increase of initial carbon. Oka and Anthong mentioned that for a batch-fed fluidized bed system, the combustion cycle of coal began with evaporation and devolatilization at the early burning stage20 which cooled the bed to the lowest temperature, and then the volatile matter or fixed carbon began to burn and the bed temperature rose to the highest point. The slope of the rising bed temperature was determined by the amount of the volatile matter. Received: May 23, 2012 Revised: August 13, 2012 Published: August 20, 2012 5569

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Figure 1. Flow diagram of the vortex fluidized bed combustion (VFBC) system. the freeboard is 0.75 m in inner diameter. A total of 27 tuyeres with orifices of 5 and 3 mm mounted on a 6 mm stainless-steel plate were used as the gas distributor with an open-area-ratio of 0.52%. The intermittent feeding system (see Figure 2) consists of a 145 mm diameter square steel tube and two pneumatic valves, which are

Skreiberg et al. found that the burnout time increases with rising bed temperature (700−900 °C), with lower inlet O2 concentration and with greater woodblock particle mass in a fixed batch-combustion reactor.21 Hart found that the semibatch feeding condition of combustion process created local oxygen deficiency.22 It is estimated that batch-fed combustion produces about 7−18 times more emission than steady-state combustion. Jangsawang et al. investigated the batch combustion of medical waste in a controlled air incinerator.12 Their results showed that higher per batch feeding weights accelerate the volatile gas release rate, resulting in incomplete combustion in the secondary combustion chamber. Different feeding materials show different combustion characteristics in batch or intermittent feeding combustors; therefore, we chose to use rubber and wood to simulate solid wastes burning such as municipal or medical waste incineration. In this research, we studied the combustion behavior in a pilot scale VFBC. Effects of the feeding intervals, static bed height, and ISOR on the mean bed and freeboard temperatures were also investigated.

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EXPERIMENTAL SECTION

Experimental Apparatus. All the experiments were conducted in a vortex fluidized bed combustor system. A flowchart of the VFBC system used in this study is shown in Figure 1. The VFBC system consists of a fluidized bed combustor, a feeding system, quench and heat recovery sections, an air supply system, and a flue gas treatment system. The primary air is supplied by a 11.2 kW (15 hp) Root’s blower, and the recirculated flue gas is supplied by a 5.5 kW (7.5 hp) turbo blower. The secondary air is supplied by a 5.5 kW (7.5 hp) Root’s blower. All the experiments were conducted at primary gas flow of 3 N m3/min and secondary gas flow of 1.5 N m3/min. Four equally spaced secondary gas injection nozzles of 30 mm in diameter are installed tangentially at a level of 2.05 m above the distributor to generate the swirling flow (vortex) in the freeboard. The VFBC can be divided into four parts, i.e., windbox, distributor, combustion chamber, and freeboard. The combustion chamber with a cross-section of 0.8 × 0.4 m2 is constructed of 6 mm carbon steel lined with 150 mm refractory to reduce heat loss. A windbox with a cross-section of 0.8 × 0.4 m2, connected to an air supply line, is fabricated with 6 mm carbon steel lined with 100 mm refractory. Above the combustion chamber,

Figure 2. Photo of intermittent feeding system. 400 mm apart. Two slide valves cannot be opened simultaneously in order to prevent air from leaking into the combustor. The feeding interval is controlled by adjusting the opening interval period of the lower slide valve. The opening and closing of these pneumatic valves are controlled manually, and they are both closed in regular mode. The feed rate is kept at 24 rubber balls or 12 woodblocks per hour. Due to limited space between the two slide valves, only 2 rubber balls or one woodblock can go in at the same time. The feeding starts with the opening of the first (upper) slide valve and the pouring of the feed (rubber ball or woodblock) into the vertical feeding tube. The second (lower) slide valve is opened after the upper slide valve is closed so the fuel can fall into the VFBC; afterward, the second slide valve will be closed. 5570

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Every batch feeding action should be less than 4 s. In other words, when the feeding interval is 20 min (the longest interval), it takes four consecutive feedings to attain the designated feeding amount, and it is done within 30 s. In addition, due to material uniformity of rubber balls, the same feeding amount can be assured as long as the same number of rubber balls is fed. However, the material of woodblock is not uniform hence each batch should be weighed prior to the experiment to ensure identical weight for all batches. The temperatures in the combustor are measured with K-type thermocouples installed in the combustor. The bed temperatures are measured with four K-type thermocouples (at the level of 0.3 m above the gas distributor) in the combustion chamber. The oxygen concentration at the outlet of the induced draft fan is continuously measured by a Novatech oxygen analyzer 1632 (the precision is ±1%). The O2 concentration data were transmitted to the primary gas control system for adjusting the mixing ratio of primary air and recirculated flue gas (FGR) to keep the oxygen content in the primary gas at a set level. Materials. Silica sand is used as the bed material with a mean density of 2,500 kg/m3 and a particle size range of 400−500 μm. The minimum fluidization velocity (Umf) of bed materials is 0.2 m/s at room temperature and 1 atm. The woodblocks are bullet-shaped with the size of Φ105 mm × 280 mm, and its average weight is 1.12 kg. The rubber balls are Nitrile-Butadiene rubber (NBR) with a density of 1460 kg/m3, a diameter of 94 mm, and an average weight of 0.539 kg. The photos of woodblocks and rubber balls used in these experiments are shown in Figure 3. The proximate and ultimate analyses of feedstocks are given in Table 1.

Table 2. Experimental Conditions operating parameter

units

feed rate superficial gas velocity total primary gas flow rate secondary air flow rate in-bed stoichiometric ratio, Cb the time of interval feeding, F −pieces/batch −heat input deviation/h

kg/h m/s Nm3/min Nm3/min % mins/batch pieces/batch %

rubber balls

static bed height, Hb excess oxygen ratio

mm %

woodblocks

12.9 27.2 0.69 3 1.5 100, 120, 140 10, 15, 20 4, 6, 8/4, 6, 8 +0.47, +0.60, +0.70/− 0.88, −1.03, −1.08 260, 380, 500 80

Each experiment will only be conducted after the VFBC furnace start-up has been completed, the preset experimental conditions reached, and the system stabilized. The preheating of bed material of VFBC furnace is done with the heat provided by diesel burner. It takes around 6−7 h to start up the furnace. When the experimental conditions have been reached and the system has been stabilized, the batch feed experiment will begin. After one hour of batch feeding and the confirmation of system stability, we officially start the experiment and collect relevant data. The flow rate ratio between PA and FGR was automatically adjusted to maintain a set O2 concentration in the flue gas. An O2 sensor was inserted into the flue gas pipeline before the stack. Equations 1 and 2 show the calculation of total gas flow rate Q PA + Q FGR = QT1

(1)

Q PA × 21% + Q FGR × OFGR = QTO

(2)

where QPA is the volumetric flow rate of primary air (Nm3/min), QFGR is the volumetric flow rate of FGR (Nm3/min), QT1 is the volumetric flow rate of total primary gas (Nm 3 /min), O FGR is the O 2 concentration of FGR (%), and QTO is the oxygen volumetric flow rate of total primary gas (Nm3/min). Analytic Methods. A quadratic polynomial model was used to describe the relationship between the objective function and the operating parameters by using the response surface methodology (RSM). The general form of a quadratic model can be represented as n

Y=

Figure 3. Features of the feedstocks.

i=1

fuel

woodblocks

n

n

βijXiXj +

i=1 j=i+1

∑ βi Xi + β0

(3)

i=1

where Y is the objective function or response; Xi is the coded operating parameters or factors; and n is the factor number. The coefficient values, β0, βi, βii, and βij, were chosen to fit the experimental data using the least-squares method.23 According to Box-Behnken design, which is a common experimental design for RSM, three independent dimensionless parameters such as feeding time interval (F/F0, F0 = 10 min/batch), static bed height (Hb/ Hb0, Hb0 = 26 cm), and in-bed stoichiometric oxygen (Cb/Cb0, Cb0 = 100%) were symbolized as coded factors (X1, X2, X3). Three levels were chosen for each factor as shown in Table 3. Analysis of variance (ANOVA) was conducted using commercial packages of JMP and

Table 1. Proximate and Ultimate Analysis of Feedstocks Proximate analysis (wt.%) moisture 12.30 volatile 69.15 fixed carbon 18.27 ash 0.28 Ultimate analysis (wt.%, dry basis) carbon 49.71 hydrogen 5.41 oxygen 41.79 nitrogen 1.40 sulfur 0.00 Lower heating value (LHV) (MJ/kg, dry basis) 19.20

n−1

∑ βiiXi2 + ∑ ∑

rubber balls 0.31 61.05 36.91 1.73 83.30 3.17 6.38 3.97 0.94

Table 3. Coded Factors, Coded Levels, and Corresponding Operating Parameters and Values coded levels

40.49

−1(low)

Experimental Conditions. The total heat input rate to the VFBC is 145.18 kW (125,000 kcal/h). The experimental conditions are listed in Table 2.The total primary gas flow rate is the total of the primary air (PA) and FGR. The flow from the PA blower and the flow from the FGR blower were mixed in the transportation line. 5571

coded factors

corresponding operating parameters

X1 X2 X3

Cb/Cb0 Hb/Hb0 F/F0

0(center)

1(high)

corresponding operating value 1 1 1

1.2 1.46 1.5

1.4 1.92 2.0

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Minitab. The Student’s t test was used to examine the main effects, quadratic effects, and interaction effects of parameters. The in-bed stoichiometric air ratio (Cb) was Cb =

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(Q PA × 21%) + (Q FGR × OFGR ) stoichiometric oxygen

× 100%

heat loss to the surrounding and cooling by secondary air introduced into the bottom of the freeboard. We found that the bed temperature presents two wave crests in each rubber balls feeding cycle. This can be attributed to the two-staged combustion of rubber balls. The first wave crest has smaller amplitude than the second crest. Rubber ball is composed of hydrocarbon, which has a simple chemical structure. Differences among these hydrocarbon chemical bonds are not obvious. The cracking temperatures of these bonds are relatively consistent.24 Once cracking starts, a great amount of volatile matter releasing and burning creates the first bed temperature wave crest. Afterward, the residual char begins to burn and release heat, which forms the second wave crest of the bed temperature. Moreover, the devolatilization rate of rubber was slower than that of woodblocks as shown in Figure 5. From the derivative thermogravimetric analysis (DGA), as

(4)

RESULTS AND DISCUSSION Temperature Distribution. Heat release from the fuel combustion of VFBC varies with different operating conditions which results in a temperature gradient or localized hot spots in the combustor. In this study, temperature distribution within the combustor for the two feedstocks was investigated at a fixed primary air flow rate 3 N m3/s and secondary air flow rate 1.5 N m3/s as shown in Figure 4.

Figure 5. Mass loss profiles of various samples during pyrolysis and char combustion obtained from DGA.

Figure 4. Temperature distribution within the VFBC. (EA = 80%, Cb = 120%, Hb = 380 mm, Bt = 15 min/batch).

shown in Figure 5(a), the highest pyrolysis rate of woodblocks was −22%/min at 410 °C; however, for rubber it was −6%/min at 450 °C as shown in Figure 5(b). Uniformity of the Bed Temperature. The amplitude of temperature fluctuation in the bed zone is smaller than that of other zones. The uniformity of the bed temperature can be calculated by the deviation method, which is given in eq 5

It can be seen that the temperatures within the combustor change periodically, i.e., rise at first and fall later with the feeding period. The lowest temperature in the combustor (zones of splashing region, freeboard, and exit zone) appeared at the time of feedstocks being injected into the combustor. This can be attributed to feeding material absorbing heat by devolatilization and pyrolysis once it is injected to the bed. A great deal of volatile matter is released during the devolatilizing process and ignites instantly. Therefore, the highest temperature appears in the splashing zone. The temperatures of the freeboard and exit zones are lower than splashing zone due to

(

∑ T− DT = 5572

∑T N

N−1

2

)

(5)

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Figure 6 shows the deviation of bed temperatures for rubber balls and woodblocks combustion varies with time under the

Figure 6. Bed temperature deviation of two feedstocks. (EA = 80%, Cb = 120%, Hb = 380 mm, Bt = 15 min/batch).

same operating conditions. We found that the bed temperature for woodblocks is more uniform than that for rubber balls. This can be attributed to the loose structure of woodblocks that is easy to break into fragments. Wood pieces distributed evenly in bed leads to a more uniform bed temperature. On the other hand, the heat transferred from the bed material will lead to the drying, volatilization, and fragmentation of woodblocks and rubber balls during VFBC combustion. The fluidic behavior of the sand in the fluidized bed can be affected by various parameters such as distribution board, bubble size, and fluidization number. There has been prior research related to the VFBC fluidics behavior in this study such that the fluidization of the sand bed in this system has been very strong and is influenced by the high thermal conductivity and high mass transfer coefficient.25 The rubber balls are significantly smaller than woodblocks, so they can be heated more uniformly after entering the furnace. This leads to higher burning rates and more rapid cracking, and the fragments are relatively small. In Figure 7, we can see the brightness and uniformity of the flame of burning rubber balls are obviously higher than that of burning woodblock. Figure 7 clearly shows the generation of bright particles during the combustion in photographs of burning woodblocks and rubber balls. With the combustion temperature in the VFBC furnace bed and splash zone at around 750−875 °C, we believe these particles are mainly fine burning fragmented fuel particles. In the meantime, there is a huge amount of char particles (which were originally burning in the bed) thrown into the splash zone or even into the freeboard zone and continued to burn. Analysis of Variance of Temperature with Operation Parameters. (1). Mean Bed Temperature. The analysis of variance of all the mean bed temperatures shows that the Pvalue is 0.001 (lower than standard value 0.1) and R-Square (R2) is 0.983; it shows that the experimental results and regression analysis model were highly correlated with the three operation parameters X1, X2, and X3. Table 4 shows the coded factors examination. The mean bed temperature of both fuels can be represented by the relation equation of operation parameters as shown in eq 6. The impact of all operation parameters on the mean bed temperature are in the order of X1 = X2 > X3 > X1 × X3 > X2 × X3 > X2 × X2. This indicates the in-

Figure 7. Photographs of combusting particles of different feedstocks taken from above the combustor.

bed stoichiometric oxygen (X1) and static bed height (X2) have an important effect on the mean bed temperature. We can simplify eq 6 to create eq 7 by removing the less significant terms Tb(°C) = 774.98 + 66.62 × X1 − 66.67 × X 2 − 30.57 × X3 + 0.15 × X1 × X1 + 18.84 × X 2 × X 2 − 15.87 × X3 × X3 − 17.3 × X1 × X 2 − 25.79 × X1 × X3 + 25.49 × X 2 × X3

(6)

TbS(°C) = 318.66 + 9.94 × X1 − 13.81 × X 2 + 8.68 × X3 + 0.14 × X 2 × X 2 − 0.26 × X1 × X3 + 0.43 × X 2 × X3

(7)

(2). Mean Freeboard Temperature. Analysis of variance for the whole quadratic model for the mean freeboard temperature with operation parameters shows that the P-value is 0.054 and R-Square (R2) is 0.892. Table 4 shows the coded factors examination; the freeboard mean temperature with operation parameters relation equation is shown in eq 8. Equation 8 can be simplified by removing the less significant terms to create eq 9 Tf (°C) = 801.78 + 4.35 × X1 + 35.58 × X 2 − 3.11 × X3 − 10.25 × X1 × X1 + 15.03 × X 2 × X 2 − 2.48 × X3 × X3 + 1.43 × X1 × X 2 − 6.59 × X1 × X3 − 0.03 × X 2 × X3 TfS(°C) = 690.35 + 2.97 × X 2 5573

(8) (9)

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Table 4. Effect Examinations of the Coded Factors for the Temperaturesa mean bed temperature

a

mean freeboard temperature

factor

coefficient

standard error

t ratio

prob.

factor

coefficient

standard error

t ratio

prob.

constant X1 X2 X3 X1*X1 X2*X2 X3*X3 X1*X2 X1*X3 X2*X3

774.98 66.62 −66.67 −30.57 0.15 18.84 −15.87 −17.30 −25.79 25.49

10.158 6.221 6.221 6.221 9.156 9.156 9.156 8.797 8.797 8.797

76.291 10.709 −10.718 −4.915 0.016 2.057 −1.733 −1.967 −2.931 2.898

0.000 0.000 0.000 0.004 0.988 0.095 0.144 0.106 0.033 0.034

constant X1 X2 X3 X1*X1 X2*X2 X3*X3 X1*X2 X1*X3 X2*X3

801.78 4.35 35.58 −3.11 −10.25 15.03 −2.48 1.43 −6.59 −0.03

9.811 6.008 6.008 6.008 8.844 8.844 8.844 8.497 8.497 8.497

81.72 0.7239 5.921 −0.517 −1.159 1.699 −0.28 0.168 −0.775 −0.003

0.000 0.502 0.000 0.114 0.548 0.112 0.164 0.156 0.236 0.134

X1: in-bed stoichiometric air ratio; X2: static bed height; X3: interval feeding time.

Effect of Operation Parameters. (1). Effect of ISOR on the Temperature. Figure 8 shows the mean bed temperatures

Figure 9. Effect of static bed height on the mean bed temperature. (Bt = 10 min/batch, Cb = 120%, EA = 80%). Figure 8. Effect of ISOR on the mean bed temperature. (Hb = 380 mm, Bt = 10 min/batch, EA = 80%).

with increasing static bed height. The bubbles at the bed surface burst more intensively for higher static bed height because bubble sizes increase with the bed height. Larger bubbles have bigger bubble wake and more momentum, which causes more solids to be carried to the freeboard. Thus the bed temperature decreases due to a lower residence time of combusting particles in the bed zone. The freeboard temperature increases with static bed height as shown in Figure 10. When the static bed height increases, more combustible matters were carried to freeboard by the higher momentum of bigger bubbles’ wake. This is more obvious for smaller rubber balls (94 mm diameter) than larger woodblocks (Φ105 mm × 280 mm). Moreover, the total heat input is fixed in this study. That is, the feeding rate of woodblocks (LHV: 19.20 MJ/kg (4,591 kcal/kg), moisture: 12.3%) is almost twice that of rubber balls (LHV: 40.49 MJ/kg (9,681 kcal/kg), moisture: 0.31%). The mean bed temperatures of woodblocks and rubber balls are similar (Figure 9); however, the mean freeboard temperature of rubber balls is much higher than woodblocks (Figure 10). (3). Effect of Feeding Interval. Figure 11 shows the mean bed temperature vs the length of feeding interval. Longer feeding interval of rubber balls decreases the mean bed temperature, but its effect on woodblocks is not obvious. As the feeding interval lengthens, the quantity per feeding batch increases; this creates a temporary fuel-rich condition in the

varying with in-bed stoichiometric oxygen. We found that the mean bed temperatures of both feedstocks increase with stoichiometric oxygen. This can be explained by the fact that more oxygen molecules can diffuse through the gas film of combusting particles at higher ISOR. Borah et al. studied devolatilization dynamics, and the results confirmed that fuel particles devolatilization process is limited by gas film diffusion.26 Oxygen concentration at the fuel particle surface has a significant impact on the overall devolatilization rate. We also found that rubber ball’s fixed carbon content is twice as woodblock’s, but the mean bed temperatures are about the same. A probable explanation is that rubber’s devolatilization rate is much lower than wood’s (Figure 5). We observed that rubber’s volatile matter burned slowly at the surface. Meanwhile, oxygen molecules diffuse slowly through the particle surface and cannot react efficiently with fixed carbon as the volatile matter of rubber was coming out. This leads to a decrease in the combustion reaction rate of rubber particles. Based on eq 9, freeboard temperature is only affected by static bed height; therefore, the effect of the ISOR on mean freeboard temperature is negligible. (2). Effect of the Static Bed Height. Figure 9 shows the mean bed temperatures varying with the static bed height in the VFBC. The mean bed temperatures of both feedstocks decrease 5574

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Figure 10. Effect of static bed height on the mean freeboard temperature. (Bt = 10 min/batch, Cb = 120%, EA = 80%).

Figure 12. Effect of feeding interval on the mean freeboard temperature. (Hb = 380 mm, Cb = 120%, EA = 80%).

1. The temperatures in the VFBC change cyclically with the feeding intervals. The highest temperature appears in the splashing zone. The bed temperature presents two wave crests for rubber balls due to two combustion stages, i.e., the combustion stages of volatile matter and fixed carbon. 2. For rubber combustion, the mean bed temperature increases with ISOR, while it decreases with static bed height and feeding interval. The mean freeboard temperature increases with ISOR and static bed height, while it decreases with feeding interval. 3. For woodblock combustion, the mean bed temperature increases with ISOR, while it decreases with static bed height. The mean freeboard temperature increases with ISOR. The effect of the feeding interval on the bed and freeboard temperatures is not obvious. 4. According to the experimental results and RSM analysis, the mean bed temperature can be represented by a combinatorial equation of operation parameters, while the mean freeboard temperature can be represented by an equation of static bed height as the only operation parameter.

Figure 11. Effect of feeding interval on the mean bed temperature. (Hb = 380 mm, Cb = 120%, EA = 80%).

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combustor, which causes incomplete combustion. Thus, rubber ball’s mean bed temperature decreases. For wood, combustion is enhanced by woodblock breakage and easier oxygen diffusion through its loose structure. The quantity per feeding batch increases when the feeding interval changes from 10 to 15 min. The higher collision probability of woodblocks due to higher fuel quantity results in wood breaking into pieces easily; therefore, the combustion of fixed carbon is accelerated and the mean bed temperature rises. The collision possibility of woodblocks increases further as the feeding interval rises to 20 min; however, overabundant woodblocks also results in fuel-rich and incomplete combustion. The mean bed temperature decreases. Figure 12 shows that the effect of feeding interval on the mean freeboard temperature is not obvious for both fuels. This agrees with our RSM model that feeding interval is not a critical factor in determining mean freeboard temperature (eq 9).

AUTHOR INFORMATION

Corresponding Author

*Phone: +886 3 2654119. Fax: +886 3 4636242. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work by the National Science Foundation under grant NSC 95-2221-E-033-064 and the Bureau of Energy, Ministry of Economic Affairs, Taiwan is gratefully acknowledged.

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REFERENCES

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CONCLUSIONS In this study, fuels were fed into a VFBC periodically to simulate the combustion behavior of solid wastes. We reached the following conclusions: 5575

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dx.doi.org/10.1021/ef3008882 | Energy Fuels 2012, 26, 5569−5576