Spent Activated Carbon Combustion in a Fluidized-Bed Combustor

Jan 8, 2014 - Schematic diagram of vortexing fluidized-bed combustion system. ... of the combustor was controlled by an automatic control system to ma...
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Spent Activated Carbon Combustion in a Fluidized-Bed Combustor Feng Duan,† Chien-Song Chyang,*,‡ Jing-Ting Wang,‡ and Jim Tso§ †

School of Energy and Environment, Anhui University of Technology, Maanshan, Anhui 243002, People’s Republic of China Department of Chemical Engineering, and §Research and Development Center for Environmental Technology, Chung Yuan Christian University, Chung Li 32023, Taiwan, Republic of China

‡

ABSTRACT: Spent activated carbon (SAC) from wastewater treatment plants is not allowed to be regenerated in Taiwan, and landfill is the only legal way for disposal. Owing its high heating value, SAC can be a potential source of energy from waste. In this study, SAC is used as the fuel in a vortexing fluidized-bed combustor. The combustion characteristics and pollutant emissions are investigated at various operating conditions. Results show that the primary gas flow rate and in-bed stoichiometric oxygen ratio have a significant impact on the bed temperature. The bed temperature increases with the in-bed stoichiometric oxygen ratio, while the bed temperature decreases significantly with the primary gas flow rate. In comparison to the primary gas flow rate and excess oxygen ratio, the in-bed stoichiometric oxygen ratio plays a dominant factor for the combustion fraction in the bed zone and CO/NOx emissions. The combustion fraction in the bubbling-bed region increases, while the CO emission decreases significantly with the in-bed stoichiometric oxygen ratio. In this study, all NOx emission concentrations are within the limit of Taiwan Environmental Protection Agency (EPA) regulations.

1. INTRODUCTION Activated carbon is an adsorbent that has the advantages of low cost and good chemical stability. It has been widely used in the chemical industry, pharmaceutical industry, etc. to absorb pollutants in emissions and sewage.1 The basic structure of activated carbon is similar to the structure of pure graphite, which is composed of layers of fused hexagons. Recent market research estimated the annual global activated carbon production to be around 1.2 million metric tons, and it is expected to grow about 10% each year through 2016 to 1.9 million metric tons. The main treatment method for spent activated carbon (SAC) is regeneration1 or disposal. SAC loses its activity after adsorption. The main treatment method of SAC is regeneration;1 however, SAC from wastewater plants is not allowed to be regenerated in Taiwan, and landfill is the only legal disposal method. SAC has a high calorific value, which makes it a suitable candidate to be converted into energy.2,3 Chen et al. combusted SAC mixed with organic-sludgederived fuel to achieve the goal of waste disposal, combustion enhancement, and pollutant emission reduction.2 Amankwah used microwave energy to combust the SAC from gold ore processing that employed carbon adsorption technology.4 When air is introduced into a microwave combustion chamber, activated carbon is completely oxidized. A fluidized-bed combustor (FBC) has been used for burning of a wide variety of wastes, including agricultural residues,5−7 municipal solid waste,8,9 kitchen waste,10,11 sewage sludge,12,13 and various hazardous materials.14 Most studies concentrate on fluidized-bed behavior and flue gas emission. However, few studies that focused on SAC combustion in FBC are reported. Ludwig et al. used a partial oxygen combustion method to treat solid and liquid waste materials, including spent activated carbon, bone meal, etc., which are suitable for fluidized-bed combustion.15 Oxygen is blown directly at supersonic speed into the fluidized bed. Using this method, specific emissions, © 2014 American Chemical Society

e.g., carbon dioxide and nitrogen oxide mass per ton of incinerated waste material, are reduced. The high moisture content in SAC evidently affects its combustion behavior. For conventional fuels, such as agricultural residues, the released heat is dominated by volatile combustion. In the case of SAC, the volatile content is rather low; hence, the combustion of fixed carbon is more dominant. Increasing excess air would also increase the flow rate of the secondary gas, and the experimental results are influenced by the physical effect of the secondary gas flow rate and the chemical effect of excess oxygen.16−18 It is difficult to distinguish which factor plays a more significant role in a fluidized-bed combustor. Therefore, to investigate the effects of the excess oxygen ratio on the SAC combustion at a given flow rate, the secondary gas is made of air from an air compressor and pure nitrogen from high-pressured nitrogen cylinders. Increasing the primary gas flow rate will affect the degree of fluidization as well as the mixing of the bed material and fuel in the bed zone,19,20 which results in different combustion behaviors and pollutant emission characteristics. The objective of this study is to observe the combustion characteristics and pollutant emissions by varying operating parameters, including the primary gas flow rate (QPRI), excess oxygen ratio (Eo), and in-bed stoichiometric oxygen ratio (Sb), for the combustion of SAC. Experiments were conducted in a lab-scale vortexing fluidized-bed combustor (VFBC). A Box− Behnken experimental design method is used to describe the relationship between the objective functions and the operating parameters with the response surface method (RSM). Received: August 30, 2013 Revised: January 8, 2014 Published: January 8, 2014 1463

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Table 1. Proximate and Ultimate Analyses of FAC and SAC ultimate analysis (%) SAC FAC

proximate analysis (%)

C

H

O

N

S

moisture

volatile matter

fixed carbon

ash

higher heating value (kJ/kg)

55.31 66.07

1.14 1.01

15.33 13.57

0.15 0.07

0 0

24.72 17.41

21.92 16.00

50.01 64.78

3.35 1.81

19528.96 22151.13

The electric heaters are turned on to heat the combustor at the startup. Continuous secondary air injection started from the beginning. First air was injected into the combustor occasionally to stir the sand. The fuel is fed into the combustor via a screw feeder after the bed temperature reaches 500 °C, and the flow rate of first air is increased gradually. The temperature of the combustor was controlled by an automatic control system to maintain the reactor at a fixed temperature. Overall, it takes about 3−5 h for the system to reach steady state. The primary and secondary air are supplied by a NS-100 air compressor. In addition, a nitrogen supply system is used to mix nitrogen into the primary and secondary air. Four equally spaced secondary air injection nozzles of 13 mm in diameter are installed tangentially 796 mm above the distributor. The flue gas generated in the combustor passes through a cyclone to remove most of the unburned char and ash particles and then is cooled by a heat exchanger and cooling tower. Finally, the flue gas is released to the atmosphere, and fly ash is collected in a baghouse. 2.3. Data Acquisition. The temperatures within the combustor are measured with K-type thermocouples. In Figure 1, thermocouples (K type) are positioned at 0.50, 0.65, 1.38, 2.43, 3.47, 4.12, and 5.09 m above the air distributor. Three thermocouples are placed horizontally in the combustor chamber to detect the temperatures at a height of 0.07 m above the distributor. The mean value of the temperatures from these three horizontal thermocouples is taken as the bed temperature. The components of flue gas, such as CO, CO2, O2, and NOx, are analyzed by a HORIBA-PG250. The measurement accuracies of the gas analyzer with O2, CO, CO2, and NOx are 0.5, 0.5, 1, and 0.5%, respectively. In this study, the reported emission values are all calibrated on the basis of 11% residual oxygen on a dry basis. 2.4. Working Conditions. The working conditions for all tests are shown in Table 2. Five electric heaters are employed to control the

2. EXPERIMENTAL SECTION 2.1. Materials. In this study, the SAC comes from the wastewater plant of a printed circuit board (PCB) factory. The main pollutants in the SAC are ferrous sulfate, organic matter, and suspended particles. Table 1 shows the data from the ultimate and proximate analyses of fresh activated carbon (FAC) and SAC. Because of its adsorbed organic matter, the volatile content of SAC is higher than that of FAC. The inert bed material used in the tests is silica sand (99.5% SiO2), with a mean diameter of 0.53 mm and an apparent density of 2500 kg/ m3. 2.2. Vortexing Fluidized-Bed Combustion Test Facility. A process flowchart of the vortexing fluidized-bed combustion system used in this study is shown in Figure 1. The VFBC is assembled with a

Table 2. Working Conditions operating parameter SAC feeding rate (kg/h) superficial gas velocity in-bed stoichiometric oxygen primary gas flow rate total gas flow rate excess oxygen ratio bed material average particle size of bed material (μm) density of bed material static bed height

Figure 1. Schematic diagram of vortexing fluidized-bed combustion system.

symbol

unit

conditions

F vs Sb QPRI QT Eo

kg/h m/s % NL/min NL/min %

2.5 0.78 70−90 260−300 500 40−60 silica sand 530

kg/m3 mm

2500 200

dp ρ H

freeboard temperature independently. The freeboard wall temperature is fixed at 730 °C, and the feeding rate of SAC is 2.5 kg/h. The total gas flow rate is fixed at 500 NL/min, and the total primary gas ranges between 260 and 300 NL/min. To interpret the combustion characteristics of SAC in the VFBC, the combustion fraction is calculated through the oxygen consumption. As seen in Figure 1, the boundary between the bed zone and freeboard zone is located at z = 0.796 m. The combustion fraction can be calculated by the following equation:

windbox, distributor, combustion chamber, and freeboard. The crosssection of the combustion chamber is 0.22 × 0.11 m2, and its height is 678 mm. The freeboard section is fabricated with a SUS310 pipe with an inside diameter of 154 mm and height of 4000 mm. The combustion chamber and freeboard are equipped with an electric heating system and enclosed in a 25 mm ceramic fiber to maintain the set temperature. The feeding system is made of a feedstock hopper and screw feeder, with the feeding point at 500 mm above the distributor.

Yi = 1464

Q in,O − Q out,O 2

2

QTO

× 100% (1)

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where Qin,O2 is the oxygen flow rate of the inlet in each zone (NL/ min), Qout,O2 is the oxygen flow rate of the outlet in each zone (NL/ min), and QTO is the total oxygen consumed in the combustor (NL/ min). 2.4. RSM Design. The full factorial design of three parameters and three levels requires 33 = 27 different tests. Using the Box−Behnken design, only 13 tests are required, which is a considerable reduction compared to the full factorial design. The general form of this method can be represented by eq 2 n

Y=

n−1

n

∑ βi , iXi 2 + ∑ ∑ i=1

Table 4. Results Based on the Box−Behnken Experimental Design

n

βi , jXiXj +

i=1 j=i+1

∑ βi Xi + β0

(2)

i=1

where Y is the objective function, Xi is the coded operating parameters of factors, and n is the factor number. The coefficient values, β0, βi, βi,i, and βi,j, are determined by fitting the experimental data using the leastsquares method. The operating parameters include the primary gas flow rate (QPRI), excess oxygen ratio (Eo), and in-bed stoichiometric oxygen ratio zone (Sb) with the initial values of 260 NL/min, 40%, and 70%, respectively. The values of the primary flow rate are 260, 280, and 300 NL/min. The in-bed stoichiometric oxygen ratio, Sb, is the ratio of the flow rate of oxygen in the primary gas to the theoretical oxygen required for complete combustion, and the values in this study are 70, 80, and 90%. Three values of the excess oxygen ratio are 40, 50, and 60%. The coded factors, coded levels, and values of these three parameters are given in Table 3. The maximum and minimum levels of each

X1

X2

X3

number

QPRI

Eo

Sb

Tb (°C)

CO (ppm)

NOx (ppm)

bed zone Yi (%)

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

0 1 1 1 0 0 −1 −1 0 0 0 −1 0 −1 1

−1 0 1 0 1 0 −1 1 0 1 −1 0 0 0 −1

1 1 0 −1 −1 0 0 0 0 1 −1 1 0 −1 0

956 918 904 882 901 935 963 955 967 964 907 997 965 887 909

140 202 380 602 558 249 195 299 273 153 403 114 309 562 307

79 82 71 70 67 66 58 60 56 75 60 72 51 68 59

82.5 83.5 74.1 53.3 55.2 63.7 75.1 62.1 63.7 76.7 52.1 80.5 63.7 58.2 61.3

Yb,coded (%) = 0.636061 − 0.004464X1 − 0.000617X 2 + 0.131270X3 + 0.034431X12 + 0.117X 2 2 + 0.01836X32 + 0.064341X1X 2

Table 3. Coded Factors of RSM

+ 0.01972X1X3 − 0.022265X 2X3

coded levels coded factors

corresponding parameters

−1 (low)

0 (center)

1 (high)

X1 X2 X3

QPRI Eo Sb

260 40 70

280 50 80

300 60 90

(4)

COcoded (ppm) = 260.958 + 40.045X1 + 42.882X 2 − 189.097X3 + 43.995X12 − 9.523X 2 2 + 65.285X32 − 7.361X1X 2 + 9.786X1X3 − 35.59X 2X3

parameter are determined on the basis of the recommendations from user experience. The bed temperature, CO and NOx emissions, and combustion fraction of each zone are the objective functions. Analyses of variance are carried out using commercial software MiniTab. The main effects, quadratic effects, and interaction effects of the parameters are also examined.

NOx,coded (°C) = 40.3832 + 2.5261X1 + 4.5054X 2 + 2.9184X3 + 2.5979X12 + 0.8665X 2 2 + 10.5900X32 + 1.9923X1X 2 + 1.4603X1X3 − 2.4482X 2X3

3. RESULTS AND DISCUSSION

(6)

The effect examinations of the coded factors are shown in Tables 5−8. As seen in these tables, the value of probability > t is the discrimination index on the influential degree of the factor. For example, when the value of a factor is greater than

3.1. Statistical Analysis. A total of 15 experimental tests are performed on the basis of the Box−Behnken design. The combination of all factors at the center level in the experimental design is replicated 3 times to ensure accuracy. The results based on the Box−Behnken experimental design are given in Table 4. The results of Table 4 are used to estimate the second-order models for each objective function. The fitted responses are shown in eqs 3−6.

Table 5. Effect Examinations of the Coded Factors for the Bed Temperature

Tb,coded (°C) = 964.123 − 23.332X1 − 1.716X 2 + 31.351X3 − 19.866X12 − 11.007X 2 2 − 23.171X32 + 0.056X1X 2 − 18.104X1X3 + 3.687X 2X3

(5)

(3) 1465

factor

coefficient

standard error

t ratio

probability > t

constant X1 X2 X3 X12 X22 X32 X1X2 X1X3 X2X3

964.123 −23.332 −1.716 31.351 −19.866 −11.007 −23.171 0.056 −18.104 3.687

4.406 2.681 2.687 2.687 3.991 3.946 3.946 3.783 3.783 3.792

218.827 −8.702 −0.639 11.667 −4.978 −2.789 −5.871 0.015 −4.786 0.973

0 0.001 0.551 0.001 0.004 0.038 0.002 0.989 0.005 0.375

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Table 6. Effect Examinations of the Coded Factors for the Combustion Fraction of the Bed Zone factor

coefficient

standard error

t ratio

probability > t

constant X1 X2 X3 X12 X22 X32 X1X2 X1X3 X2X3

0.363939 0.004464 0.000617 −0.131270 −0.034431 −0.011170 −0.018360 −0.064341 −0.019720 0.022265

0.02628 0.01599 0.01603 0.01603 0.02381 0.02354 0.02354 0.02257 0.02257 0.02262

13.848 0.279 0.039 −8.190 −1.446 −0.475 −0.780 −2.851 −0.874 0.984

0 0.791 0.971 0.001 0.208 0.655 0.471 0.136 0.422 0.370

Table 7. Effect Examinations of the Coded Factors for the CO Emission Concentration factor

coefficient

standard error

t ratio

probability > t

constant X1 X2 X3 X12 X22 X32 X1X2 X1X3 X2X3

260.958 40.045 42.882 −189.097 43.995 −9.523 65.285 −7.361 9.786 −35.59

18.61 11.33 11.35 11.35 16.86 16.67 16.67 15.98 15.98 16.02

14.020 3.536 3.778 −16.658 2.609 −0.571 3.916 −0.461 0.612 −2.222

0 0.017 0.013 0.001 0.048 0.593 0.011 0.664 0.567 0.077

Figure 2. Effect of the excess oxygen ratio on the bed temperature and bed zone combustion fraction.

test runs with varying X2 but constant X1 and X3. They are runs 1 and 10, runs 3 and 15, runs 5 and 11, and runs 7 and 8. The deltas between experimental data obtained in these test runs are less than 10 °C for each pair, which implies that the excess oxygen introduced into the VFBC via the secondary gas injection does not enhance the combustion in the bed zone. During the test, the maximum temperature deltas between three horizontal thermocouples are within 10 °C. This is good evidence of well mixing of bed material and fuel from fluidization. Also in Figure 2, the bed zone combustion fraction decreases with an excess oxygen ratio. The hydrodynamic behavior of the fluidized bed can be affected by various factors, such as distributor plate, superficial velocity, and internal baffle; in our case, this behavior does not change from run to run. The primary gas flow rate in this test is fixed, and the hydrodynamic behavior of the bubbling bed has little impact on the combustion fraction. In this figure, the bed combustion fraction decreases from 75.1 to 63.1% when an excess oxygen ratio rises from 40 to 60%, while the freeboard combustion fraction increases by 12% because of the injection of extra oxygen in the secondary gas. Figure 3 shows the temperature distributions at various excess oxygen ratios. In this figure, bed and freeboard

Table 8. Effect Examinations of the Coded Factors for the NOx Emission Concentration factor

coefficient

standard error

t ratio

probability > t

constant X1 X2 X3 X12 X22 X32 X1X2 X1X3 X2X3

40.3832 2.5621 4.5054 2.9184 2.5979 0.867 10.5900 1.9923 1.4603 −2.4482

1.749 1.065 1.067 1.067 1.585 1.567 1.567 1.502 1.502 1.505

23.084 2.373 4.223 2.735 1.6939 0.553 6.758 1.326 0.972 −1.626

0 0.064 0.008 0.041 0.162 0.604 0.001 0.242 0.376 0.165

0.05, it means that the influential degree of the factor falls out of the 95% confidence level. Table 5 shows the probability values of the regression models. In this table, X1, X3 (main effect), X12, X22, X32 (quadratic effect), and X1X3 (interaction effect) are lower than 0.05, which suggests that they have a significant influence on the bed temperature. For the bed combustion fraction, X3 (main effect) is the key influence factor (Table 6). X1, X2, X3 (main effect), X12, and X32 (quadratic effect) have a significant influence on the CO emission (Table 7), while X2, X3 (main effect), and X32 (quadratic effect) have a significant influence on the NOx emission (Table 8). 3.2. Effect of the Excess Oxygen Ratio. The bed temperature and combustion fraction can be used as indexes of the combustion behavior for fluidized-bed combustion. Figure 2 shows the effect of the excess oxygen ratio on the bed temperature and bed zone combustion fraction. One can find that the bed temperature is slightly affected by excess oxygen ratios. From the data shown in Table 4, there are four pairs of

Figure 3. Temperature distribution within the combustor at various excess oxygen ratios. 1466

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temperatures increase slightly with Eo. The bed temperature stays around 900 °C, while the splashing zone temperature decreases sharply from 900 to 750 °C because of the cooling effect of the secondary gas and lower freeboard zone combustion fraction. The freeboard temperatures are all below 750 °C, which is unfavorable to the complete combustion. Figure 4 shows the effect of the excess oxygen ratio on CO and NOx emissions. As a comparison between Figures 4 and 2,

Figure 5. Effect of the in-bed stoichiometric oxygen ratio on the bed temperature and bed zone combustion fraction.

increases with Sb, which can be attributed to the higher oxygen content in the primary gas and in the bubbling region, which is favorable for complete combustion. Figure 6 shows the effect of the in-bed stoichiometric oxygen ratio on CO and NOx emissions. In this figure, the CO Figure 4. Effect of the excess oxygen ratio on CO and NOx emissions.

the CO emission at different excess oxygen ratios has an inverse trend to the bed combustion fraction. Unburned particles and volatile released from SAC combust with oxygen from the secondary air in the freeboard zone at a higher primary gas velocity. The lower freeboard temperatures are unfavorable to the complete combustion of fine SAC particles, resulting in a higher CO emission. This result disagrees with the studies by Khan and Gibbs21 and Okasha.22 This can be explained by the fact that the characteristics of SAC particles are different from fossil fuels. The high specific surface of porous SAC enhances the reactivity, which enables the combustion of very fine fixed carbon particles at lower temperatures. As seen in Figure 4, the NOx emission increases slightly from 36.7 to 45.8 ppm as the excess oxygen ratio increases because a higher combustor temperature (from higher Eo) is conducive to the creation of NH3 and HCN (major precursor of NOx). Meanwhile, the CO emission in this test is higher (>100 ppm), which increases the chance of the reduction reaction of NOx with CO. From the compromise between the two competing reactions, the NOx emission barely changes with different excess oxygen ratios. 3.3. Effect of the In-Bed Stoichiometric Oxygen Ratio. Figure 5 shows the effect of the in-bed stoichiometric oxygen ratio on the bed temperature and combustion fraction. In this figure, the bed temperature rises from 904 to 965 °C as Sb increases from 70 to 90%. The combustion in the bed zone is fuel-rich because Sb is smaller than 100%, resulting in a lower bed temperature. As Sb approaches the theoretical stoichiometric ratio of 1.0, the bed temperature increases because of more complete combustion. Also in this figure, the bed zone combustion fraction increases significantly with the in-bed stoichiometric oxygen ratio. As discussed above, X3 (main effect) is the only key factor for the bed combustion fraction. In comparison to QPPI and Eo, Sb has a significant effect on the bed combustion fraction. In this figure, the bed combustion fraction

Figure 6. Effect of the in-bed stoichiometric oxygen ratio on CO and NOx emissions.

emission decreases with Sb, which can be attributed to the fact that the bed temperature increases significantly with Sb, as shown in Figure 5. This leads to an increase in the combustion reaction rate of SAC particles. Also in Figure 6, the NOx emission goes down and then up with the rising Sb, which can be attributed to the competition between the temperature effect and the reduction reaction effect. The reduction reaction is the dominant factor at lower Sb, while the temperature is the dominant factor at higher Sb. The optimal condition for NOx emission reduction is found at Sb = 80%. NOx emission data in this test are all below 100 ppm, well below the Taiwan Environmental Protection Agency (EPA) regulation. 3.4. Effect of the Primary Gas Flow Rate. According to the ultimate analysis data shown in Table 1, the fixed carbon has by far the highest content among all materials in SAC. This is similar to anthracite; hence, their combustion behaviors are similar. Increasing the primary gas flow rate enhances the mixing of the bed material and SAC particles. 1467

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Figure 8 shows the interaction effects on the bed temperature. The third factor is fixed when the interaction effect of the two factors is analyzed. When the three contour plots in this figure are compared, the interaction of the primary gas flow rate and in-bed stoichiometric oxygen ratio has a significant effect on the bed temperature. In Figure 8C, the bed temperature increases with higher Sb and lower QPRI at 60% excess oxygen ratio. This can be attributed to the fact that the lower QPRI would prolong the residence time in the bed zone and the higher Sb would increase the oxygen concentration in the bed zone, which favors complete combustion. Figure 9 shows the effect of the primary gas flow rate on CO and NOx emissions. The CO emission increases from 252.9 to

The influence of the three factors on the bed temperature can be seen in Table 5. In comparison to X22, X1, X3 X12, X32, and X1X3 are the dominant factors on the bed temperature. Figure 7 shows the effect of the primary gas flow rate on the

Figure 7. Effect of the primary gas flow rate on the bed temperature and bed zone combustion fraction.

bed temperature and bed zone combustion fraction, while X2 and X3 are kept constant; the bed temperature decreases with the increase of the primary gas flow rate. Because the Eo, Sb, and total gas flow rate are fixed, a higher primary gas rate means that more nitrogen is introduced into the windbox with the primary air. This results in more heat being carried up from the bed zone. More fine particles are entrained in the flow from the bubbling zone into the splashing zone at higher primary gas velocity, resulting in a lower bed temperature. As shown in Figure 7, the combustion fraction of the bed zone decreases with an increasing primary gas flow rate at a given Sb, while less combustible SAC remained in the bed. A probable explanation for the effect of QPPI is that a lower primary air rate prolongs the residence time of reactants in the bed zone.

Figure 9. Effect of the primary gas flow rate on CO and NOx emissions.

344.9 ppm as the primary gas flow rate increases from 260 to 300 NL/min, while the NOx emission changes slightly between 40.4 and 45.5 ppm. The fluidization behavior of the bubbling bed is influenced by the primary gas flow rate significantly. With a higher primary gas flow rate, bubble amalgamation becomes

Figure 8. Interaction effects of the factors on the bed temperature. 1468

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(10) Zhang, D. P.; Li, X. D.; Yan, J. H.; Chi, Y.; Cen, K. F. J. Fuel Chem. Technol. 2003, 31, 322−322. (11) Zhang, Y.; Li, Q.; Meng, A.; Chen, C. Waste Manage. Res. 2011, 29, 294−308. (12) Deng, W.; Yan, J.; Li, X.; Wang, F.; Chi, Y.; Lu, S. J. Environ. Sci. (Beijing, China) 2009, 21, 1747−1752. (13) Duan, Y.; Zhao, C.; Wang, Y.; Wu, C. Energy Fuels 2010, 24, 220−224. (14) McFee, J. N.; Rasmussen, G. P.; Young, C. M. J. Hazard. Mater. 1985, 12, 129−142. (15) Ludwig, P.; Gross, G. Proc. Int. Conf. Fluid. Bed Combust. 2003, 17th, 43−52. (16) Duan, F.; Liu, J.; Chyang, C.-S.; Hu, C.-H.; Tso, J. Energy 2013, 57, 421−426. (17) Madhiyanon, T.; Sathitruangsak, P.; Soponronnarit, S. Fuel 2011, 90, 2103−2112. (18) Wan, H.; Chyang, C. Korean J. Chem. Eng. 1999, 16, 654−658. (19) Duan, F.; Cong, S. Chem. Eng. Commun. 2012, 200, 575−586. (20) Kaewklum, R.; Kuprianov, V. I. Chem. Eng. Sci. 2008, 63, 1471− 1479. (21) Khan, W. Z.; Gibbs, B. M. Fuel 1995, 74, 800−805. (22) Okasha, F. Exp. Therm. Fluid Sci. 2007, 32, 52−59.

more complete. The higher number of big bubbles and bed height makes it easy for unburned particles to be entrained to the freeboard of a low temperature (750 °C) and, thus, a higher CO emission. During the combustion of SAC particles with different primary gas flow rates, the creation of NOx and its reduction to N2 proceed simultaneously. The bed temperature decreases, while the freeboard zone combustion fraction increases with the primary gas flow rate. Unburned char of smaller particles and CO are carried up to the freeboard zone; more freeboard combustion increases NOx creation, and more CO increases NOx reduction to N2. Therefore, the NOx emission barely changes with the primary gas flow rate.

4. CONCLUSION The effect of various operating parameters, such as the primary gas flow rate, in-bed stoichiometric oxygen ratio, and excess oxygen ratio, on the combustion characteristics and pollutant emissions were investigated experimentally in a lab-scale VFBC. We reached the following conclusions: (1) SAC from wastewater treatment can be used as a fuel in fluidized-bed combustion and burned well by adjusting the operating conditions. (2) Both the primary gas flow rate and in-bed stoichiometric oxygen ratio have significant impact on the bed temperature. The bed temperature increases with the in-bed stoichiometric oxygen ratio, while the bed temperature decreases significantly with the primary gas flow rate. (3) The in-bed stoichiometric oxygen ratio is a dominant factor for the bed combustion fraction and CO emission. A higher in-bed stoichiometric oxygen ratio increases the bed combustion fraction but decreases the CO emission significantly. (4) All NOx emission concentrations in this study are within the limits of the Taiwan EPA regulation.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is sponsored by the National Science Council (NSC) under Grant NSC 102-2221-E-070.

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dx.doi.org/10.1021/ef401743j | Energy Fuels 2014, 28, 1463−1469