Characteristics of a Pilot-Scale Vortexing Fluidized-Bed Combustor

Energy Fuels 2010, 24, 6257–6265 Published on Web 11/29/2010

: DOI:10.1021/ef101164v

Characteristics of a Pilot-Scale Vortexing Fluidized-Bed Combustor with Flue Gas Recirculation (FGR): Effect of Operating Conditions on the Combustion Behavior Fuping Qian,† Chiensong Chyang,*,‡ Chienhao Yeh,‡ and Jim Tso§ †

School of Civil Engineering and Architecture, Anhui University of Technology, Ma’anshan 243002, People’s Republic of China, ‡ Department of Chemical Engineering, Chung Yuan Christian University, Chung-Li 320, Taiwan, Republic of China, and § Department of Chemical Engineering, Feng Chia University, Taichung 407, Taiwan, Republic of China Received August 31, 2010. Revised Manuscript Received October 30, 2010

The effects of operating conditions, such as the fraction of fine coal in the feeding material, the stoichiometric oxygen in the combustion chamber, and the excess oxygen ratio, on the combustion efficiency and combustion proportion were investigated experimentally in a pilot-scale vortexing fluidized-bed combustor (VFBC) with a freeboard inner diameter of 0.75 m and height of 4.6 m. These operating conditions were determined using the response surface methodology (RSM). Coal with particle sizes from 0.394 to 8 mm was used as the fuel. Silica sand was employed as the bed material. The experimental results reveal that the stoichiometric oxygen in the combustion chamber is a dominant factor for the combustion efficiency compared to the fine coal fraction in the feeding material and excess oxygen ratio. The optimal operating condition for combustion efficiency is 20% fine coal fraction in the feeding material, 110% stoichiometric oxygen in the combustion chamber, and 60% excess oxygen ratio. As the fraction of fine coal in the feeding material increases, the combustion proportion in the bubbling bed zone decreases and the combustion proportions in the splash and second air zones increase. Meanwhile, the combustion proportion in the freeboard zone does not change. While the stoichiometric oxygen in the combustion chamber increases, the combustion proportions in the bed and splash zones also increase. As the stoichiometric oxygen in the combustion chamber decreases, the combustion proportions in the second air and freeboard zones increase.

for NOx emission reduction.3 For example, the reduction of NOx and N2O emissions by FGR had been confirmed in the cases of pulverized coal combustion4-6 and fluidized-bed coal combustion.7-9 Although a lot of published papers had studied NOx emission characteristics in the vortexing fluidizedbed combustor (VFBC) with and without FGR,10-12 Chyang et al.13 and Lin14 had also evaluated the combustion performance in a pilot-scale VFBC. However, very little work was about the combustion characteristics, including the combustion efficiency and combustion proportion, in the VFBC with FGR. When a portion of the fresh air is replaced by FGR, the

1. Introduction As well-known to all, NOx is a harmful pollutant to the respiratory organs and is the precursor for acid rain and ground-level ozone. Meanwhile, NOx is also an intermediate in the formation of N2O in combustion systems.1 Moreover, NOx is known to play a role in the formation of photochemical smog. For fluidized-bed combustion, the oxidation of fuel-N is the major source of NOx emissions. It can be attributed to the low operating temperature (700-900 °C) that prevents the formation of the thermal NOx during the combustion process. Altering combustion conditions is widely accepted as an efficient and economically feasible method for reducing NOx emissions;2 for example, the staged combustion technique (air- and fuel-staged combustion) has been proven to cause significant reductions in NOx emissions. However, a fuel-rich zone must usually be created in the application of this technique. In the fuel-rich zone, the combustion of nitrogen chemically bound with char (char-N) is a significant source of NOx production. Therefore, flue gas recirculation (FGR) is used to replace a portion of the primary air to form a fuel-rich condition in the combustion chamber, and the effect of staged combustion can be achieved without decreasing the fluidization quality. Consequently, FGR has been used as a technique

(3) S€anger, M.; Werther, J.; Ogada, T. Fuel 2001, 80, 167–177. (4) Wolsky, A. M. A new method of CO2 recovery. Proceedings of the 79th Annual Meeting on Air Pollution Control Association; Minneapolis, MN, June 22-27, 1986. (5) Nakayama, S.; Miyamae, S.; Maeda, U.; Tanaka, T. J. Japan. Soc. Energy Resour. 1993, 14, 78–84 (in Japanese). (6) Okazaki, K.; Ando, T. Energy 1997, 22, 207–215. (7) Hosoda, H.; Hirama, H.; Azuma, N.; Kuramoto, K.; Hayashi, J.-i.; Chiba, T. Energy Fuels 1998, 12, 102–107. (8) Hirama, H.; Hosoda, H.; Kuramoto, K.; Hayashi, J.-i.; Chiba, T. Fluidization 1998, 9, 765–771. (9) Hayashi, J.-i.; Hirama, H.; Okawa, R.; Taniguchi, M.; Hosoda, H.; Morishita, K.; Azuma, N.; Kuramoto, K.; Chiba, T. Fuel 2002, 81, 1179–1188. (10) Chyang, C. S.; Wu, K. T.; Lin, C. S. Fuel 2007, 86, 234–243. (11) Chyang, C. S.; Qian, F. P.; Lin, Y. C.; Yang, S. H. Energy Fuels 2008, 22, 1004–1011. (12) Qian, F. P.; Chyang, C. S.; Yen, W. S. Energy Fuels 2009, 23, 3592–3599. (13) Chyang, C. S.; Lo, K. C.; Wang, K. L. Korean J. Chem. Eng. 2005, 22, 774–782. (14) Lin, S. P. The study of combustion proportion in a vortexing fluidized bed combustor. Master’s Thesis, Chung Yuan Christian University, Taiwan, 2004 (in Chinese).

*To whom correspondence should be addressed. Telephone: þ8863-2564119. Fax: þ886-3-4636242. E-mail: [email protected]. (1) Armesto, L.; Boerrigter, H.; Bahillo, A.; Otero, J. Fuel 2003, 82, 1845–1850. (2) Jia, C.; Che, D. F.; Liu, Y. H.; Liu, Y. H. Fuel Process. Technol. 2009, 90, 8–15. r 2010 American Chemical Society

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: DOI:10.1021/ef101164v

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Figure 1. Process flowchart of VFBC system.

oxygen content of the primary gas decreases. The combustion behavior at this time is different from that of the 21% oxygen air, and the accumulation of the CO2 concentration in the fluidizing gas may also influence the overall combustion behavior. If the heat-transfer tubes are installed in the combustor to absorb heat, the arrangement (number and location) of the tubes are determined by the amount of heat released in different zones (which is called combustion proportion). Therefore, it is very important to understand the combustion proportions released in various zones in the combustor. The main purpose of this study is to investigate the effect of different operating conditions, such as the fraction of fine coal in the feeding material, the stoichiometric oxygen in the combustion chamber, and the excess oxygen ratio, on the combustion characteristics in a pilot-scale VFBC with FGR. Meanwhile, these operating conditions were determined by means of response surface methodology (RSM), which enables the studying of parameters with a moderate number of experiments. On the basis of the RSM, the effects of the operating conditions on the combustion efficiency and combustion proportion were presented. The results can be used to optimize the combustion behavior in the VFBC. 2. Experimental Section 2.1. Experimental Apparatus. A process flowchart of the VFBC combustion system used in this study is shown in Figure 1. The total primary gas is made up of the primary air and the FGR. The primary air is supplied by a 15 horsepower (hp) Root’s blower, and the FGR is supplied by a 7.5 hp turbo blower. The secondary air is supplied by a 7.5 hp Root’s blower. To maintain the total amount of primary gas at 3 N m3/min and the total amount of secondary air at 1.5 N m3/min for various operating conditions, a nitrogen/oxygen supply system was installed, which was used to provide the primary gas and secondary air, respectively. Four equally spaced secondary air injection nozzles with 30 mm in diameter are installed tangentially at a level of 2.05 m above the distributor to generate the swirling flow in the freeboard. Figure 2 shows the configuration of the VFBC used in this study. The VFBC can be divided into four parts, i.e., the windbox, the distributor, the combustion chamber, and the freeboard.

Figure 2. Schematic diagram of the VFBC.

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 limit 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, the freeboard inner diameter is 0.75 m. A total of 27 tuyeres with orifices of 5 and 3 mm mounted on a 6 mm stainlesssteel plate were used as the gas distributor (the open area ratio is 0.516%). The temperatures in the VFBC are measured with K-type thermocouples installed in the combustor. The oxygen concentration of flue gas at the outlet of the induced draft (ID) fan is continuously measured by a Novatech oxygen analyzer 1632 (the precision is (1%). In addition, the O2 concentration data were transmitted to the primary gas control system for adjusting 6258

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: DOI:10.1021/ef101164v

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Table 1. Properties of Fuels fuel

Table 2. Experimental Conditions coal

operating parameter coal feeding rate (coal particle, 1-8 mm; fine coal, 0.394 mm) (kg/h) fraction of fine coal in the feeding material (%) superficial velocity (m/s) stoichiometric oxygen in the combustion chamber (%) total primary gas flow rate (N m3/min) secondary air flow rate (N m3/min) excess oxygen ratio (%) bed material average particle size of bed material (μm) density of bed material (kg/m3) static bed height above the gas distributor (cm) bed weight (kg)

Proximate Analysis (wt %) moisture volatile fixed carbon ash

16 32.94 40.49 10.57 Ultimate Analysisa (wt %)

carbon hydrogen oxygen nitrogen sulfur

66.92 4.6 20.095 1.49 0.88 Heating Value (kcal/kg)

LHV (WB) a

On a dry basis.

Y ¼

the mixing ratio of primary air and FGR to keep the oxygen content in the primary gas at a set value. For some special cases, pure oxygen is injected into the secondary air and nitrogen is injected into primary gas or secondary air to obtain particular operating conditions. The flue gas was sampled at 0.9 m (at the bed surface), 1.5 m (above the bed surface), 2.05 m (beneath the secondary air injection), 3.3 m (in the freeboard), and 4.45 m (outlet of combustor) above the distributor. For a given operating condition, when the temperature profile in the combustor and the concentration of oxygen in flue gas were kept constant, it was considered steady state. Once the steady-state condition was established, the flue gas from five positions were sampled and collected for analysis. The components of the flue gas, such as CO, CO2, O2, and NO, were analyzed by Anapol EU5000 gas analyzers. The values of the concentrations reported in this study were all corrected to 10% residual oxygen on a dry basis. 2.2. Fuels and Bed Materials. The coal is used as the feeding material in this study, which includes coal particles (size = 1-8 mm) and fine coal (mean particle size = 0.394 mm). The total heat (wet base) input is kept at 120 000 kcal/h. The proximate and ultimate analyses of feeding material are listed in Table 1. Silica sand is employed as the bed material in this study. The sand particle size range is 177-840 μm, and the mean size of the sand is 500 μm in diameter. The static bed height is 0.38 m. 2.3. Experimental Conditions. The operating conditions for experiments are shown in Table 2. It should be noted that we controlled the stoichiometric oxygen ratio (Sb) in the combustion chamber by changing the ratio of the primary air to the FGR in this study Q1st 21% þ QFGR CFGR stoichiometric oxygen

23.3 20-80 0.64 80-110 3.0 1.5 40-80 silica sand 500 2500 38 206

The general form of a quadratic model can be represented as

5154

Sb ¼

range

i¼1

Q1st 21% þ QFGR CFGR ¼ QTO

ð3Þ

nX -1

n X

i¼1 j¼iþ1

βi, j Xi Xj þ

n X

βi Xi þ β0

ð4Þ

i¼1

Three values were chosen for EO2, i.e., 40, 60, and 80%. The values of Rc0, Sb0, and EO20 can be seen in Table 3. Three levels were chosen for each factor. The parameter values were coded as the normalized values -1, 0, and þ1. The coded factors, the coded levels, and their corresponding operating parameters and values are summarized in Table 3. The maximum and minimum levels (constraints) of each design variable are determined on the basis of the recommendations from the manufacturer as well as the experience of users. Analysis of variance (ANOVA) was carried out using a commercial package JMP4. Student’s t test was used to examine the main effects, the quadratic effects, and the interaction effects of the parameters. In this work, the combustion efficiency (Ec, %) and combustion proportion at each zone (Yi, %) were assigned as the

where CFGR is the oxygen concentration of the flue gas at the exit of the stack (%), Q1st is the volume flow rate of the primary air (N m3/min), and QFGR is the volume flow rate of the FGR (N m3/min). The flow rate of FGR can be calculated by the following equations: ð2Þ

βi, i Xi 2 þ

where Y is the objective function or response, Xi is the coded operating parameters of factors, and n is the factor number. The coefficient values, β0, βi, βi,i, and βi,j, were chosen by fitting the experimental data using the least-squares method. In this study, Box-Behnken design, which is a common experimental design for RSM, was used. A three-factor Box-Behnken design is illustrated in Figure 3.15 It is a rotatable spherical design that contains no points at the vertices of the cubic region created by the upper and lower limits for each variable. This could present a great advantage when the points at the corners represent factor-level combinations that are cost-consuming or impossible to execute because of the physical process constraints.16 A way to estimate the parameters of eq 4 is to study the response for all (combinations of) factors set at three different levels. This full factorial design would require 33 =27 different experiments. This setup in the Box-Behnken design of three factors requires only 13 experiments, which is a considered reduction compared to the three-level factorial design. 2.4.2. Responses and Factors. The operating parameters included the fraction of fine coal in the feeding material (Rc), the stoichiometric oxygen ratio in the combustion chamber (Sb), and the excess oxygen ratio (EO2) in this study. The values of Rc are 20, 50, and 80%, respectively. Additionally, the definition of Sb can be seen from eq 1, and the values of Sb in this study are 80, 95, and 110%, respectively. For the excess oxygen ratio, it can be defined by the following equation: total oxygen in the combustor - 1 100% ð5Þ EO2 ¼ stoichiometric oxygen

ð1Þ

Q1st þ QFGR ¼ QT1

n X

where QT1 is the volume flow rate of the primary gas, which is kept constant at 3 N m3/min throughout the study, and QTO is the volume flow rate of oxygen in the primary gas that goes into the bed. 2.4. Experimental Design. 2.4.1. RSM. A quadratic polynomial model was used to describe the relationship between the objective function and the operating conditions with the RSM.

(15) Chyang, C. S.; Qian, F. P.; Chiou, H. Y. Chem. Eng. Technol. 2007, 30, 1700–1707. (16) Montgomery, D. C. Design and Analysis of Experiments; Wiley: New York, 1997.

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Figure 3. Three-factor and three-level Box-Behnken design.

Therefore, it is difficult to calculate the composition of the flue gas, and usually, NOx generated in the reaction is negligible. It is assumed that the consumed oxygen in each zone only reacts with carbon and hydrogen molecules, the majority of fixed carbon is burned in the bed, and most volatile is burned in the zones above the bed surface. In a steady state, the oxygen concentration in each zone is considered constant; consequently, the combustion proportion of each zone can be calculated by the consumed oxygen obtained from flue gas analysis with the following formula: QC , O 2 Yi ¼ 100% ð7Þ Qt

Table 3. Coded Factors, Coded Levels, and Corresponding Operating Parameters and Values coded levels corresponding coded factors operating conditions X1 X2 X3 a

a

Rc/Rc0 Sb/Sb0a EO2/EO20a

-1 (low)

0 (center)

þ1 (high)

corresponding operating value 1 1 1

2.5 1.1875 1.5

4.0 1.375 2.0

Rc0, Sb0, and EO20 are 20, 80, and 40%, respectively.

objective functions. Because the consumed oxygen can be used to account for the burning of carbon and hydrogen, the combustion efficiency is calculated by the consumed oxygen in the combustion process. Baron et al.17 had used the consumed oxygen in the combustion process to verify if the value of the theoretical oxygen demand was correct. Combustion efficiency Ec can be defined by O2, in - Fgas CO2 , outlet 100% Ec ð%Þ ¼ O2, theo requ

where Qt is the total consumed oxygen (kg mol/h) and QC,O2 is the consumed oxygen in each zone of the VFBC (kg mol/h), which can be calculated by the following equation: QC, O2 ¼ Qf ðCO2 , in - CO2 , out Þ where Qf is the flow rate of flue gas (kg mol/h) and CO2,in and CO2,out are the oxygen concentrations in and out of each zone (ppm), respectively.

ð6Þ

3. Results and Discussion

where O2,in is the total oxygen that entered into the bed (kg mol/h), Fgas is the flow rate of the flue gas (kg mol/h), CO2,outlet is the oxygen concentration at the outlet of VFBC, and O2,theo requ is the rate of theoretical oxygen demand (kg mol/h). Combustion proportion can be defined as the ratio of the combustion at a section of the VFBC to the total combustion in the VFBC; this can help to understand the combustion behavior at each section of the VFBC. The combustion proportion in this study is calculated by the consumed oxygen, and the following basic assumptions should be made to simplify the mathematical calculation procedure: (1) The VFBC is divided into four zones, i.e., the bed zone, the splash zone, the secondary air zone, and the freeboard zone (see Figure 2). Among them, the bed zone is the sand bed, the splash zone is referred to the zone that the bed materials move up and down turbulently, the secondary air zone is referred to the zone for the secondary air injection, and the freeboard zone is from above the secondary air injection to the exit of the VFBC. Each zone can be seen as a continuous stirred tank reactor (CSTR),13 which is in a stable state with no temperature and concentration gradients, and the calculation of the oxygen concentration balance is performed only accounting for the input and output of each zone, regardless of their reaction extent at various points. (2) The fuel is fed overbed into the VFBC, and the drying process of the feed takes place above the bed surface. (3) The composition of the flue gas is varied according to the fuel properties, operating conditions, and the extent of burning.

3.1. Statistical Analysis for Tb, Tf, Ec, and Yi. On the basis of the Box-Behnken design, 15 experimental sets were constructed in a fractional factorial experiment. The center point in the experimental design, i.e., the combination of all factors at the center level, was replicated 3 times to ensure experimental accuracy. The three-factor and three-level Box-Behnken experimental design and the responses observed in this study are shown in Table 4. The results of Table 4 are used to estimate the secondorder models for each response. The fitted responses are shown in eqs 8-14 Tb, coded ¼ 924:913 - 24:576X1 þ 25:445X2 þ 8:886X3 - 17:905X1 2 - 8:348X2 2 - 7:33X3 2 - 2:833X1 X2 þ 3:6027X1 X3 - 0:995X2 X3

ð8Þ

Tf , coded ¼ 823:64 þ 10:194X1 þ 6:886X2 þ 8:37X3 þ 9:456X1 2 - 6:924X2 2 - 5:621X3 2 þ 15:853X2 X1 þ 12:825X3 X1 - 6:98X3 X2

ð9Þ

Ec, coded ð%Þ ¼ 88:1233 - 4:3413X1 þ 5:585X2 - 1:6462X3 - 0:0729X1 2 þ 1:0646X2 2 - 12:4529X3 2 þ 4:035X1 X2

(17) Baron, J.; Bulewicz, E. M.; Kandefer, S.; Pilawska, M.; Zukowski, W.; Hayhurst, A. N. Fuel 2006, 85, 2494–2508.

- 0:5775X1 X3 þ 1:16X2 X3 6260

ð10Þ

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Table 4. Three-Factor and Three-Level Box-Behnken Experimental Designs X1

X2

X3

Yi (%)

number

pattern

Rc/Rc0

Sb/Sb0

EO2/EO20

Tb (°C)

Tf (°C)

Ec (%)

bed zone

splash zone

secondary air zone

freeboard zone

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

--0 þ-0 -þ0 þþ0 -0þ0-0þ þ0þ 0-0þ0-þ 0þþ 000 000 000

-1 1 -1 1 -1 1 -1 1 0 0 0 0 0 0 0

-1 -1 1 1 0 0 0 0 -1 1 -1 1 0 0 0

0 0 0 0 -1 -1 1 1 -1 -1 1 1 0 0 0

883 859 944 908 930 854 938 877 875 923 897 942 917 931 927

826 819 801 858 825 815 814 856 784 819 817 824 817 827 826

95.4 75.5 94.7 90.9 80.9 76.5 75.8 69.2 70.6 83.3 67.9 85.2 87.4 88.4 88.6

63.1 52.9 54.0 38.9 70.9 36.8 69.7 34.9 65.7 53.1 56.1 58.3 55.1 52.5 60.1

3.7 8.8 15.0 16.9 9.2 18.4 7.6 16.7 7.4 14.4 9.2 11.9 10.4 10.8 7.5

31.6 35.4 30.7 43.1 7.8 40.5 16.0 46.4 25.2 31.7 28.1 27.3 32.5 31.2 30.3

1.7 2.9 0.3 1.1 12.0 4.3 6.8 2.1 1.8 0.9 6.6 2.6 2.0 5.5 2.1

Table 5. Effect Examinations of the Coded Factors for Tb

Table 6. Effect Examinations of the Coded Factors for Tf

factor

coefficient

standard error

t ratio

probability > t

factor

coefficient

standard error

t ratio

probability > t

intercept X1 X2 X3 X1 2 X2 2 X3 2 X2X1 X3X1 X3X2

924.913 -24.576 25.445 8.886 -17.905 -8.348 -7.330 -2.830 3.602 -0.995

7.724 4.730 4.730 4.730 6.962 6.962 6.962 6.689 6.689 6.689

119.749 -5.196 5.380 1.879 -2.572 -1.199 -1.053 -0.423 0.539 -0.149

0 0.003 0.003 0.119 0.050 0.284 0.341 0.690 0.613 0.888

intercept X1 X2 X3 X1 2 X2 2 X3 2 X 2 X1 X3 X1 X3 X2

823.64 10.194 6.886 8.370 9.456 -6.924 -5.621 15.853 12.825 -6.980

3.737 2.288 2.288 2.288 3.368 3.368 3.368 3.236 3.236 3.236

220.407 4.455 3.009 3.658 2.807 -2.055 -1.669 4.898 3.963 -2.157

0 0.007 0.030 0.015 0.038 0.095 0.156 0.004 0.011 0.084

Table 7. Effect Examinations of the Coded Factors for Ec

Yi, b, coded ð%Þ ¼ 49:2667 - 11:74X1 - 0:5437X2 - 1:4237X3 - 3:3008X1 2 þ 0:6667X2 2 - 5:3533X3 2 þ 1:1125X1 X2 þ 0:1375X1 X3 þ 3:44X2 X3

ð11Þ

Yi, spl, coded ð%Þ ¼ 8:4133 - 2:085X1 - 3:77X2 - 0:6225X3 þ 1:4183X1 2 þ 0:0983X2 2 - 0:1217X3 2 - 0:49X1 X2 - 0:225X1 X3 þ 0:72X2 X3

ð12Þ

Yi, sec, coded ð%Þ ¼ 27:6133 þ 6:42X1 þ 3:0262X2 þ 0:6137X3 ð13Þ

Yi, f , coded ð%Þ ¼ 2:82 - 1:1088X1 - 0:6713X2 - 0:2175X3 þ 0:5975X1 2 - 2:1375X2 2 þ 1:475X3 2 þ 0:025X1 X2 þ 0:6875X1 X3 - 0:4475X2 X3

coefficient

standard error

t ratio

probability > t

intercept X1 X2 X3 X1 2 X2 2 X3 2 X2 X1 X3 X1 X3 X2

88.1233 -4.3413 5.5850 -1.6462 -0.0729 1.0646 -12.4529 4.0350 -0.5775 1.1600

2.127 1.303 1.303 1.303 1.917 1.917 1.917 1.842 1.842 1.842

41.427 -3.333 4.287 -1.264 -0.038 0.555 -6.495 2.190 -0.313 0.630

0.000 0.021 0.008 0.262 0.971 0.603 0.001 0.080 0.767 0.557

Table 8. Effect Examinations of the Coded Factors for Yi in the Bed Zone

þ 1:2158X1 2 þ 2:4433X2 2 - 8:4467X3 2 þ 3:3825X1 X2 - 1:1775X1 X3 - 1:11X2 X3

factor

ð14Þ

where Tb and Tf are the bed and freeboard temperatures, Yi,b, Yi,spl, Yi,sec, Yi,f are the combustion proportions of the bed, splash, secondary air, and freeboard zones, respectively. The effect examinations of the coded factors are tabulated in Tables 5-11. The value of probability > t increases with a decreasing absolute t ratio or the coefficient to standard error ratio. When the value of probability > t for a factor is greater than 0.05, it signifies that the influential degree of the factor falls out of the 95% confidence interval. When the value of probability > t for a factor is greater than 0.1, it means that the influential degree of the factor falls out of the 90% confidence level. For some factors, the standard error was even bigger than the coefficient, resulting in the value of

factor

coefficient

standard error

t ratio

probability > t

intercept X1 X2 X3 X1 2 X2 2 X3 2 X2 X1 X3 X1 X3 X2

49.2667 -11.7400 -0.5437 -1.4237 -3.3008 0.6667 -5.3533 1.1125 0.1375 3.4400

3.227 1.976 1.976 1.976 2.909 2.909 2.909 2.795 2.795 2.795

15.267 -5.941 -0.275 -0.720 -1.135 0.229 -1.840 0.398 0.049 1.231

0 0.002 0.794 0.503 0.308 0.828 0.125 0.707 0.963 0.273

probability > t approaching 1, which means the factor is very uninfluential. Table 5 shows that the probability values of the regression model for those terms such as the intercept, X1 (the main effect), X2 (the main effect), and X12 (the quadratic effect) are lower than 0.05. This suggests that these factors have a significant influence on the objective function Tb. Table 6 shows that the probability values of the regression model for those terms such as the intercept, X1 (the main effect), 6261

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: DOI:10.1021/ef101164v

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Table 9. Effect Examinations of the Coded Factors for Yi in the Splash Zone factor

coefficient

standard error

t ratio

probability > t


8.41300 2.08500 3.77000 -0.62250 1.41833 0.09833 -0.12167 -0.49000 -0.22500 -0.72000

1.2720 0.7789 0.7789 0.7789 1.1466 1.1466 1.1466 1.1016 1.1016 1.1016

6.6140 2.6770 4.8400 -0.7990 1.2370 0.0860 -0.1060 -0.4450 -0.2040 -0.6540

0.001 0.044 0.005 0.460 0.271 0.935 0.920 0.675 0.846 0.542

Table 10. Effect Examinations of the Coded Factors for Yi the Secondary Air Zone factor

coefficient

standard error

t ratio

probability > t


27.6133 6.4200 3.0262 0.6137 1.2158 2.4433 -8.4467 3.3825 -1.1775 -1.1100

3.568 2.185 2.185 2.185 3.216 3.216 3.216 3.090 3.090 3.090

7.740 2.939 1.385 0.281 0.378 0.760 -2.627 1.095 -0.381 -0.359

0.001 0.32 0.225 0.790 0.721 0.482 0.047 0.324 0.719 0.734

Figure 4. Main effects of each factor on the bed temperature (°C) with any other two factors fixed at the center level.

three factors on Tb can be seen from Table 5, and the sequence of importance is X2 = X1 > X12; that is, the fraction of fine coal in the feeding material (X1) and the stoichiometric oxygen in the combustion chamber (X2) have an important effect on the bed temperature in the VFBC. The main effects of the three factors on the bed temperature are shown in Figure 4, and this figure analyzes the effect of one factor on the bed temperature without taking the impact of the other two factors into account. It can be seen that the bed temperature decreases with the fraction of fine coal in the feeding material from Figure 4. Because of the light weight of the fine coal fed into the VFBC, it is blown up above the bed surface by the primary air and burned. Furthermore, when the fraction of fine coal in the feeding material increases, the coal particles burned in the bed decrease, which leads to the decline of the bed temperature. Figure 4 also shows that the bed temperature increases with the stoichiometric oxygen in the combustion chamber. Because the total amount of the primary air is fixed at 3 N m3/min in all experiments, the stoichiometric oxygen in the combustion chamber becomes low when the proportion of FGR increases. Consequently, the combustion chamber is becoming a fuel-rich zone. This results in a drop in the bed temperature because the fuel-rich state is conducive to the pyrolysis reaction rather than the combustion reaction. For an excess oxygen ratio, Figure 4 shows that the bed temperature increases with it, but the trend is not obvious as it approaches 80%. From the experimental data shown in Table 4, runs 6 and 8, runs 9 and 11, and runs 10 and 12 are operated each at the same fraction of fine coal in the feeding material and stoichiometric oxygen in the combustion chamber, with different excess oxygen ratios; when the excess oxygen ratio is higher, the average bed temperature has an increase of about 20 °C. We speculate that the experiments were operated at a lower excess oxygen ratio at first and then at a higher excess oxygen ratio later, which causes heat accumulated in the bed and increases the bed temperature. 3.2.2. Effect of the Operating Conditions on the Freeboard Temperature (Tf). The influence of the three factors on Tf can be seen from Table 6, and the sequence of importance is X1X2 > X1 > X1X3 > X3 > X2 > X12; that is, the fraction of fine coal in the feeding material (X1), the stoichiometric oxygen in the combustion chamber (X2), and the excess

Table 11. Effect Examinations of the Coded Factors for Yi in the Freeboard Zone factor

coefficient

standard error

t ratio

probability > t


2.82000 -1.10875 -0.67125 -0.21750 0.59750 -2.13750 1.47500 0.02500 0.687500 -0.44750

1.6022 0.9811 0.9811 0.9811 1.4442 1.4442 1.4442 1.3875 1.3875 1.3875

1.760 -1.130 -0.684 -0.222 0.414 -1.480 1.021 0.018 0.495 -0.323

0.139 0.310 0.524 0.833 0.696 0.199 0.354 0.986 0.641 0.760

X2 (the main effect), X3 (the main effect), X12 (the quadratic effect), X2X1, and X1X3 (the interaction effect) are lower than 0.05, which also suggests that these factors have a significant influence on the objective function Tf. Table 7 shows that the probability values of the regression model for terms such as the intercept X1 (the main effect), X2 (the main effect), X32 (the quadratic effect), and X1X2 (the interaction effect) are less than 0.1. This suggests that these factors have a significant influence on the objective function Ec. The probability values of the regression model in Table 8 show that only the intercept and X1 (the main effect) have a significant influence on the objective function Yi in the bed zone. Table 9 shows that the probability values of the regression model for those terms such as the intercept, X1 (the main effect), and X2 (the main effect) are lower than 0.1; this suggests that these factors have a significant influence on the objective function Yi in the splash zone. From Tables 10 and 11, it is seen that the three factors do not have a significant influence on the objective functions Yi in the secondary air and freeboard zones. 3.2. Effect of the Operating Conditions on the Temperature Distribution in the VFBC. 3.2.1. Effect of the Operating Conditions on the Bed Temperature (Tb). The influence of 6262

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Figure 6. Main effects of each factor on the combustion efficiency (%) with any other two factors fixed at the center level.

Figure 5. Main effects of each factor on the freeboard temperature (°C) with any other two factors fixed at the center level.

Figure 6 shows that the combustion efficiency increases with the stoichiometric oxygen in the combustion chamber. For the combustion reaction at high temperature, the oxygen mass-transfer rate is the controlling factor for the burning rate. Therefore, the rate of the oxygen transferred to the surface of the particles increases with higher stoichiometric oxygen in the combustion chamber,19 which increases the combustion efficiency. Figure 6 also shows that the optimal combustion efficiency is achieved when X3 is 60%. The correlation between X3 and the combustion efficiency cannot be clearly seen from this figure; the possible reason may be that the further increase of X3 no longer has any impact when they are higher than 60%. On the basis of the above experimental results and analysis using the RSM, it can be determined that the operating conditions for the optimal combustion efficiency are 20% (X1), 110% (X2), and 60% (X3), respectively. Figure 6 shows that the combustion efficiency decreases with an increasing X1, and Figure 7 shows that the fly ash increases with an increasing X1. Lin20 burned coal with three different particle sizes of 0.698, 2.362, and 6.66 mm in his study, and he found that the combustion efficiency increased in tandem with the coal particle size. Therefore, when X1 is low, the ratio of the fixed carbon burned in the bed is high accordingly, which will increase the bed temperature and the combustion efficiency. This is consistent with the results shown in Figures 6 and 7. 3.4. Effects of the Operating Conditions on the Combustion Proportion (Yi) in the VFBC. 3.4.1. Effect of X1 on Yi. Figure 8 compares the calculated values based on eqs 11-14 and experimental data of the combustion proportions in the VFBC when X2 and X3 are fixed at 95 and 60%, respectively. The smaller the coal particle size fed into the VFBC, the more likely the particles are entrained and burned above the bed surface. Therefore, when X1 increases, the coal particles burned in the bed decrease, which reduces the combustion proportion in the bed zone; on the other hand, the combustion proportions in the splash and secondary air zones increase. Additionally, the secondary air injection in the

oxygen ratio (X3) all have important effects on the freeboard temperature in the VFBC. Figure 5 shows the main effects of the three factors on the freeboard temperature, and this figure analyses the effect of one factor on the freeboard temperature without taking into account the impact of the other two factors. It can be seen from this figure that the freeboard temperature increases with the fraction of fine coal in the feeding material because most fine coal particles were entrained by the primary gas and burned in the freeboard zone. Figure 5 also shows that the freeboard temperature increases with increasing stoichiometric oxygen (X2) in the combustion chamber, but the trend is not obvious at higher X2. Wang and Thomas18 pointed out that the content of volatile increased with the pyrolysis temperature by coal pyrolysis experiments. The bed temperature increases with increasing stoichiometric oxygen in the combustion chamber, and the higher average bed temperature facilitates volatile release in the combustor chamber, which increases the combustion of the volatile above the bed surface and the freeboard. Consequently, the temperature of the freeboard increases with an increasing bed temperature. In addition, the energy carried by the primary gas moving up to the freeboard zone increases because of the higher bed temperature, which also raises the freeboard temperature. Lastly, Figure 5 shows that the freeboard temperature increases with an increasing excess oxygen ratio (X3). When the oxygen entering the VFBC increases, the coal reacts with oxygen more easily in the system; consequently, more heat is released into the freeboard, which increases the freeboard temperature. 3.3. Effects of the Operating Conditions on the Combustion Efficiency of the VFBC (Ec). The influence of the three factors on the Ec value can be seen from Table 7, and the sequence of importance is X32 >X2 >X1 >X1X2; that is, the stoichiometric oxygen in the combustion chamber (X2) and the fraction of fine coal in the feeding material (X1) have important effects on the combustion efficiency of the VFBC. The main effects of the three factors on the combustion efficiency are shown in Figure 6, and this figure analyses the effect of one factor on the combustion efficiency without considering the other two factors.

(19) Winter, F.; Michael, E. P.; Hermann, H. Combust. Flame 1997, 108, 302–314. (20) Lin C. H. The study of coal combustion in a vortexing fluidized bed combustor. Master’s Thesis, Chung Yuan Christian University, Taiwan, 1992 (in Chinese).

(18) Wang, W. X.; Thomas, K. M. Energy Fuels 1996, 10, 409–416.

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Figure 7. Effect of X1 on fly ash with any other two factors at the center level.

Figure 9. Effect of X2 on the combustion proportion (%) (X1, 20%; X3, 80%; primary air flow rate, 3 N m3/min; and secondary air flow rate, 1.5 N m3/min).


secondary air zone (1) increases the oxygen content in this zone and (2) creates a vortex, which captures the unburned elutriated particles, and, hence, increases the combustion proportions in this zone. Lastly, because most of the fine coal is burned in the splash and secondary air zones, the combustion proportion of the freeboard zone is insignificant and not impacted by the change of X1. 3.4.2. Effect of X2 on Yi. Figure 9 compares the calculated values based on eqs 11-14 and experimental data of the combustion proportion in the VFBC when X1 and X3 are fixed at 20 and 80%, respectively. When the value of X1 is small, coal particles make up a higher proportion in the fed fuel. Meanwhile, if X2 is high, the high oxygen content intensifies the combustion reaction of the coal particles in the bed and the fine coal above the bed surface and, thus, increases the combustion proportions of the bed and splash zones.


On the other hand, a low X2 value signifies that the proportion of the FGR in the primary gas is higher. The combustion chamber is in a fuel-rich state, which is conducive to the pyrolysis reaction that produces more volatile. The volatile rises to the secondary air zone and burns, resulting in a higher combustion proportion of this zone; meanwhile, a small portion of the volatile continues to rise and be burned 6264

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in the freeboard zone. For this reason, when X2 is low, the secondary air and freeboard zones have higher combustion proportions. 3.4.3. Effect of X3 on Yi. Figure 10 compares the calculated values from eqs 10-14 and experimental data of the combustion proportion in the VFBC when X1 and X2 are fixed at 20 and 80%, respectively. Because the excess oxygen is injected in the secondary air zone while the other two factors are kept constant, it has no significant impact on the combustion proportions (Yi) of the other two (bed and splash) zones. However, it should intensify the combustion reaction and increase the combustion proportion in the secondary air zone. Also noted, because the majority of the reaction occurred in the bed and secondary air zones, X3 has little impact on the Yi of the freeboard and splash zones.

operating conditions on the freeboard temperature is the fraction of fine coal in the feeding material, the excess oxygen ratio, and the stoichiometric oxygen in the combustion chamber. (3) The stoichiometric oxygen in the combustion chamber and the fraction of fine coal in the feeding material have a stronger influence on the combustion efficiency of the VFBC than the excess oxygen ratio. (4) The operating conditions for the optimal combustion efficiency are 20% (the fraction of fine coal in the feeding material), 110% (the stoichiometric oxygen in the combustion chamber), and 60% (the excess oxygen ratio). (5) The combustion proportion in the bed zone decreases with the increasing fraction of fine coal in the feeding material, while the opposite is true in the splash and secondary air zones. No significant variation is observed in the combustion proportion of the freeboard zone. (6) The combustion proportions in the bed and splash zones increase with increasing stoichiometric oxygen in the combustion chamber. In addition, in the secondary air and freeboard zones, they decrease with increasing stoichiometric oxygen in the combustion chamber. (7) The combustion proportions in the bed, splash, and freeboard zones have no significant correlation with the excess oxygen ratios, and in the secondary air zone, it increases with an increasing excess oxygen ratio.

4. Conclusions The effects of operating conditions (i.e., the fraction of fine coal in the feeding material, the stoichiometric oxygen in the combustion chamber, and the excess oxygen ratio) on the combustion characteristics (including the combustion efficiency and the combustion proportion) were investigated experimentally in a pilot-scale VFBC. The following conclusions were obtained: (1) The fraction of fine coal in the feeding material and the stoichiometric oxygen in the combustion chamber have a more important effect on the bed temperature than the excess oxygen ratio. (2) The effects order of the

Acknowledgment. This research was sponsored by the National Science Foundation and specific research programs in the Chung Yuan Christian University under Grants NSC962221-E-033-053-MY2 and CYCU-98-CR-CE.

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