Effects of Acetic Acid Injection and Operating Conditions on NO

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Effects of Acetic Acid Injection and Operating Conditions on NO Emission in a Vortexing Fluidized Bed Combustor Using Response Surface Methodology Fuping Qian,† Chiensong Chyang,*,‡ and Weishen Yen‡ School of CiVil Engineering and Architecture, Anhui UniVersity of Technology, Ma’anshan, 243002 China and Department of Chemical Engineering, Chung Yuan Christian UniVersity, Chung-Li 320, Taiwan ReceiVed January 4, 2009. ReVised Manuscript ReceiVed May 12, 2009

The effects of acetic acid injection and operating conditions on NO emission were investigated in a pilot scale vortexing fluidized bed combustor (VFBC), an integration of circular freeboard and a rectangular combustion chamber. The dimension of the freeboard is 0.75 m I.D. and 4.6 m in height. The cross section of the combustion chamber is 0.8 × 0.4 m2, and the height of the combustion chamber is 1.47 m. The secondary air injection nozzles were installed tangentially at the bottom of the freeboard. Coal was used as the fuel. Silica sand was employed as the bed material. Acetic acid was used as the reductant to reduce NO emission. Operating conditions, such as the stoichiometric oxygen in the combustion chamber, the bed temperature and the injecting location of acetic acid, were determined by means of response surface methodology (RSM), which enables the examination of parameters with a moderate number of experiments. In RSM, NO emission concentration after acetic acid injection and NO removal percentage at the exit of the VFBC are used as the objective function. The results show that the bed temperature has a more important effect on the NO emission than the injecting location of acetic acid and the stoichiometric oxygen in the combustion chamber. Meanwhile, the injecting location of acetic acid and the stoichiometric oxygen in the combustion chamber have a more important effect on the NO removal percentage than the bed temperature. NO emission can be decreased by injecting the acetic acid into the combustion chamber, and NO emission decreases with the height of the acetic acid injecting location above the distributor. On the other hand, NO removal percentage increases with the height of the acetic acid injecting location, and NO emission increases with the stoichiometric oxygen in the combustion chamber and the bed temperature. NO removal percentage increases with the stoichiometric oxygen, and increases first, then decreases with the bed temperature. Also, a higher NO removal percentage could be obtained at 850 °C.

1. Introduction As well known to all, NO is a harmful pollutant causing direct injuries of the respiratory organs and is the precursor for acid rain and ground-level ozone. Meanwhile, NO is also an intermediate in the formation of N2O in combustion systems.1 Moreover, NO is known to play a role in the formation of photochemical smog, too. Compared with other combustors, lower NO emission can be achieved by employing the fluidized bed combustor (FBC). It can be attributed to the lower operating temperature (700∼900 °C) to prevent the formation of the thermal NO and prompt NO during the combustion process. Altering combustion conditions is widely accepted as an efficient and economically reasonable technique for reducing NOx emission.2 However, a fuel-rich zone must usually be created in the application of this technique (e.g., air-staged combustion and fuel-staged combustion). In the fuel-rich zone, the combustion of nitrogen chemically bound with char (char-N) is a * Corresponding Author: Chien-Song Chyang, Department of Chemical Engineering, Chung Yuan Christian University, Chung-Li 320, Taiwan. † Anhui University of Technology. ‡ Chung Yuan Christian University. (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.

significant source of NOx production. Therefore, an increasing importance is attached to this subject by the scientists all over the world. The paths of formation and reduction of NO in fluidized bed combustors were complex and it was difficult to explain the variation in NO emission for different operation conditions. When coal was burnt in the FBC, the heterogeneous reduction of NO at the char surface has been considered to be one of the most important reactions of NO destruction. Generally speaking, NO emissions increase with bed temperature and excess air.3 It is generally thought that the conversion of fuel-N to nitric oxides is through the intermediates, hydrogen cyanide (HCN) and ammonia (NH3), formed during fuel combustion. The mechanistic pathways among fuel-N, HCN, NH3, and nitric oxides are strongly dependent on chemical structures of the fuel. It is reported that most nitrogen functional groups in fuels are present as pyrrolic and pyridinic type structures.4 Some research work has demonstrated that pyrrolic and pyridinic function groups are related to emission of nitric oxides. It was investigated the formation of NO from 14 nitrogen-containing model compounds, and found the N2O/NO ratio is a function of the HCN/ (3) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Prog. Energy Combust. Sci. 2003, 29, 89–113. (4) Nelson, P. F.; Kelly, M. D.; Wornat, M. J. Fuel 1991, 70, 403–407.

10.1021/ef9000055 CCC: $40.75  2009 American Chemical Society Published on Web 06/16/2009

Acetic Acid Injection in a VFBC

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Figure 1. Schematic diagram of VFBC system.

NH3 ratio.5 However, the concentration and location of functional groups, such as -OH groups, of the compound may cause much more complicated effects on the emissions of nitric oxides. Most studies, including recent reports,1,6-11 have studied NO emission firing coals and biomass in detail. To improve the performance of FBC, a vortexing fluidized bed combustor (VFBC), an integration of bubbling fluidized bed and cyclone, was developed early in the 70s.12 The first pilot scale VFBC was constructed and operated by Korenberg.13 It was modified and renamed by Nieh and Yang14 later. The concept of the VFBC is to establish a vortex-generating system by injecting the second air tangentially into the freeboard to increase the combustion intensify, the calcium utilization, and the turndown capability. The advantages of low pollutant emissions and high combustion efficiencies by operating a VFBC have been demonstrated for combustion of waste tires in the previous studies. Chyang et al.15-17 have been studied the NO and N2O emission characteristics from the pilot scale vortexing fluidized bed combustor when firing different fuels, for example, rice husk, soybean, and high sulfur subbituminous. We found that NO emission could be reduced by adding soybean to the fuels. Furthermore, NO emission can be decreased by increasing the amount of the soybean. We speculate that the possible reason could be that the carboxyl (-COOH) group in (5) Ha¨ma¨la¨inen, J. P.; Aho, M. J.; Tummavuori, J. L. Fuel 1994, 73, 1894–1898. (6) Svoboda, K.; Pohorˇely, M. Fuel 2004, 83, 1095–1103. (7) Permchart, W.; Kouprianov, V. I. Bioresour. Technol. 2004, 92, 83– 91. (8) Li, Z. W.; Lu, Q. G.; Na, Y. J. Fuel Process. Technol. 2004, 85, 1539–1549. (9) Tarelho, L. A. C.; Matos, Ma. A. A.; Pereira, F. J. M. A. Fuel Process. Technol. 2005, 86, 925–940. (10) Shimizu, T.; Toyono, M.; Ohsawa, H. Fuel 2007, 86, 957–964. (11) Zhao, W.; et al. Bioresour. Technol. 2008, 99, 2956–2963. (12) Iwasaki, Y.; Yamada, Y.; Watanabe, N. US Patent No. 4159000, 1979. (13) Korenberg, J. Proceedings of the Fourth International Conference on Fluidization; Kashikojima: Japan, 1983; pp 491. (14) Nieh, S.; Yang, G. Powder Technol 1987, 50, 121–131. (15) Chyang, C. S.; Wu, K. T.; Lin, C. S. Fuel 2007, 86, 234–243. (16) Chyang, C. S.; Qian, F. P.; Lin, Y. C.; Yang, S. H. Energy Fuels 2008, 22, 1004–1011. (17) Huang, K. S. Emissions of high fuel-nitrogen material during fluidized bed combustion. Master thesis, Chung Yuan Christian University, Taiwan, 1997.

the soybean has the ability for reducing NO emission. Although various chemicals have been employed for NO reducing during the combustion process, we hold this truth as self-evident that all the materials (chemicals) derived from the biomass are environment friendly. Therefore, in order to explain the reason further, the main purpose of this study is to investigate the influence of the carboxyl (-COOH) group on NO emission by injecting the acetic acid (CH3COOH) at different locations into the VFBC. Additionally, the operating conditions, including the stoichiometric oxygen in the combustion chamber and the bed temperature, are also considered in experiments, and these operating conditions were determined by means of response surface methodology (RSM), which enables the examination of parameters with a moderate number of experiments. On the basis of the RSM, the effects of acetic acid injection and the operating conditions on the NO emission were presented. The results can be used to optimize the combustion and decrease NO emission 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 the sum of the primary air and the flue gas recirculation. The primary air is supplied by a 15 hp Root’s blower, and the flue gas recirculation is supplied by a 7.5 hp blower. The secondary air is supplied by a 7.5 hp turbo blower. Four equally spaced secondary air injection nozzles with diameters of 30 mm are installed tangentially at a level of 2.05 m above the distributor to cause the swirling flow in the freeboard. A 20 hp compressor is used for transporting the feed through a pipe into the combustor. The acetic acid is added by the injecting tube. The injecting tube is shown in Figure 2. The injecting location of the acetic acid are 0.90 m (A), 1.45 m (B), and 2.00 m (C) above the gas distributor, respectively. Figure 3 shows the configuration of the VFBC used in this study. The VFBC is made up of four parts: the windbox, the distributor, the combustion chamber, and the freeboard. The combustion chamber, with cross section 0.8 × 0.4 m2, is constructed from a 6 mm stainless steel covered with the ceramic fiber to limit the heat loss. A windbox with cross section 0.6 × 0.4 m2, connected to the air supplied line, is fabricated of 6 mm stainless steel. Above the combustion chamber, the inner diameter of the freeboard is 0.75m. The static bed height is 0.35 m. Twenty-seven nozzles with

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Qian et al. Table 1. The Properties of Fuels fuel

coal Proximate Analysis (wt. %)

moisture volatile fixed carbon ash

13.79 31.85 41.02 13.34

Ultimate Analysisa (wt. %) carbon hydrogen oxygen nitrogen sulfur

Figure 2. Acetic acid injecting tube.

56.46 7.0 18.98 1.13 0.96 Heating Value (kJ/kg)

LHV (WB) a

20975

Dry basis.

Table 2. Experimental Conditions operating parameter

range

coal feeding rate (kg/h) acetic acid feeding rate (mL/min) bed temperature (°C) superficial velocity (m/s) stoichiometric oxygen in the combustion chamber (%) total primary gas flow rate (Nm3/min) secondary air flow rate (Nm3/min) excess air (%) 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) minimum fluidization velocity (cm/s)

23.9 16.1 810-890 0.66-0.71 80-120 3.5 1.17 30-70 silica sand 541 2500 35 175 19

Sb )

Figure 3. Schematic diagram of the vortexing fluidized bed combustor.

diameters of 5 mm and 3 mounted on a 6 mm thickness stainless steel plate are used as the gas distributor (the open area ratio is 0.516%). The temperatures in the VFBC are measured with the K-type thermocouple installed in the combustor. The bed temperature is controlled by the heat-transfer tube immersed in bed. The flue gases are sampled at 0.9, 1.45, 2.00, 3.3, and 4.45 m above the gas distributor. The components of the flue gas, such as CO, CO2, O2, and NO are analyzed by a TSI CA-6203 (the precision of the gas analyzer for NO, CO, and O2 are (5 ppm, (10 ppm, and ( 0.3%, respectively). Gas samples are drawn out from the combustor, cooled, and then passed through the above analyzers. The dry process is carried out within the analyzers. The values of the concentrations reported in this study are all corrected to 6% residual oxygen on dry basis. 2.2. Fuels and Bed Materials. The coal is used as the feeding material in this study. The total heat (wet base) input is kept as 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 541 µm in diameter. 2.3. Experimental Conditions. The operating conditions for experiments are shown in Table 2. It should be noted that we change the stoichiometric oxygen in the combustion chamber by changing the ratio of the primary air and the flue gas recirculation in this experiments. The following definitions were used:

Q1st × 21% + QFGR × CFGR stoichiometric oxygen

where CFGR is the oxygen concentration of the flue gas at 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 flue gas recirculation, N m3/min. The recirculation flue gas flow rate can be calculated by the following equation:

Q1st + QFGR ) K Q1st × 21% + QFGR × CFGR ) K′ where K is the volume flow rate of the primary air that put into manually, N m3/min; and K′ is the volume flow rate of the oxygen that entered into the bed, N m3/min. 2.4. Experimental Design for NO Emission. 2.4.1. Response Surface Methodology (RSM). A quadratic polynomial model was used to describe the relationship between the objective function and the operating conditions in the response surface methodology. The general form of a quadratic model can be represented as: n

Y)

∑β X

2 ii i

i)1

n-1

+

n

∑ ∑ β XX

ii i j

i)1 j)i+1

n

+

∑βX

i i

+ β0

(1)

i)1

where Y is the objection function or response; Xi is the coded operating parameters of factors; and n is the factor number. The coefficient values, β0, βi, βii, and βij were chosen to fit the experimental data well using the least-squares method. In this study, the Box-Behnken design, which is a common experimental design for RSM, was used. A three-factor Box-Behnken design is illustrated in Figure 4. 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 be a great advantage when the points on the corners represent factor-



Figure 4. A three-factor and three-level Box-Behnken design.18 Table 3. Coded Factors, Coded Levels, and Corresponding Operating Parameters and Values coded factors

corresponding operating conditions

X1 X2 X3

Sb/Sb0a Tb/Tb0a Li/Li0a

coded levels -1 (low)

0 (center)

corresponding operating 1 1.25 1 1.0494 1 1.611

+1(high) value 1.5 1.0988 2.222

Sb0, Tb0, and Li0 are 80%, 810 °C, and 0.9 m above the distributor, respectively. a

Y2 )

(NOb - NOa) × 100% NOb

(2)

where NOb is NO concentration before acetic acid injection, NOa is NO concentration after acetic acid injection.

3. Results and Discussion 3.1. Statistical Analysis for Y1 and Y2. On the basis of the Box-Behnken design, 15 experimental sets were constructed into a fractional factorial experiment. The center point in the experimental design, that is, the combination of all factors at the center-level, was replicated three times to estimate the experimental accuracy. The three-factor and three-level BoxBehnken experimental design and the responses observed in this study are shown in Table 4. It should be noted that -Y2 means that NO emission is increased after acetic acid injection. One obtains second-order response surface models using multiple regressions, as follows:

level combinations that are cost-consuming of impossible to execute due to the physical process constraints.19 A way to estimate the parameters of eq 1 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 results in the Box-Behnken of 3 factors demanding only 13 experiments, which is a considerable reduction compared to the three-level factorial design. 2.4.2. Responses and Factors. The operating parameters included the stoichiometric oxygen in the combustion chamber (Sb), the bed temperature (Tb), and the injecting location of acetic acid (Li) in this paper. Three levels were chosen for each factor. The parameter value was coded as the normalized values -1, 0, and +1. The coded factors, the coded levels, and their corresponding operating geometry parameters and values are summarized in Table 3. The maximum and minimum levels (constraints) of each design variable are normally determined based on the recommendations given by the manufacturer as well as users’ experience. The bed temperature is controlled by the movable heat-transfer tube immersed in bed in this study, and this method can effectively control the bed temperature only at a small range (810∼890 °C). Therefore, Tb0 was set at 810 °C. Additionally, the value Sb in this study is 80, 100, and 120%, respectively, so Sb0 was set at 80%. For the injection points of the acetic acid, three typical locations were chose, that is, 0.9 m above the distributor (in the combustion chamber), 1.45 m above the distributor (near the inlet of the freeboard), and 2.0 m above the distributor (near the secondary air injection). It is noted that the injection points of the acetic acid is determined by the geometry of the setup. The values of Sb0, Tb0, and Li0 can been seen in Table 3. Analysis of variance (ANOVA) was carried out using the commercial package JMP4. The Student’s t test was used to examine the main effects, the quadratic effects, and the interaction effects of the parameters. In this work, NO emission concentration after acetic acid injection and NO removal percentage at the exit of the VFBC were assigned as the objective functions, Y1, and Y2. Y2 can be defined by:

The corresponding ANOVA is tabulated in Tables 5 and 6. The small probability value indicates that the experimental data were fitted well by the regression model. The determination coefficient of the regression models, R2 are 0.978 and 0.948, respectively. A comparison of the Y1 and Y2 experimentally observed with the Y1 and Y2 predicted using the models are shown in Figures 5 and 6, which show that the model fits the experimental data well in most ranges. The effect examinations of the coded factors are tabulated in Tables 7 and 8. The value of prob > t increases with the decreasing absolute t ratio, or the coefficient to standard-error ratio. When the value of prob > t for a factor is greater than 0.05, it means that the influential degree of the factor falls out of 95% confidence interval. For some factors, the standard error was even bigger than the coefficient, resulting in the value of prob > t approaching 1, which means the factor is very uninfluential. Table 7 shows that the probability values of the

(18) Chyang, C. S.; Qian, F. P.; Chiou, H. Y. Chem. Eng. Technol. 2007, 30, 1700–1707.

(19) Montgomery, D. C. Design and Analysis of Experiments; New York: Wiley, 1997.

Y1 ) 126.33333 + 5.375X1 + 32.75X2-12.625X3 + 2.9583333X21 + 9.5X2X1 + 1.7083333X22-17.75X3X1+ 5X3X2 + 6.9583333X23 (3) Y2 ) 19.866667 + 9.8375X1 - 1.7475X2 + 10.0975X3 - 1.745833X21 + 1.01X2X1 - 5.1058333X22 + 12.285X3X1 - 2.895X3X2 - 2.300833X23 (4)

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Table 4. A Three-factor and Three-level Box-Behnken Experimental Designs No.

pattern

X1 (Sb/Sb0)

X2 (Tb/Tb0)

X3 (Li/Li0)

Y1 (ppm)

Y2 (%)

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

--0 -+0 +-0 ++0 0-0-+ 0+0++ -0+0-0+ +0+ 000 000 000

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

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

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

98 155 88 183 126 89 171 154 124 172 136 113 131 126 122

7.94 -1.74 25.74 20.11 -3.09 27.34 3.37 22.22 10.47 5.42 1.65 45.74 25.04 20.09 14.47

Table 5. Analysis of Variance for the Whole Quadratic Model for Y1a source model error total a

DF 9 5 14

sum of squares 12012.317 273.417 12285.733

mean square 1334.70 54.68

F ratio 24.4078

Table 7. Effect Examinations of the Coded Factors for Y1

prob. > F

factor

coefficient

standard error

t ratio

prob > t

0.0013

intercept X1 X2 X3 X12 X2X1 X22 X3X1 X3X2 X32

126.33333 5.375 32.75 -12.625 2.9583333 9.5 1.7083333 -17.75 5 6.9583333

4.2694 2.614463 2.614463 2.614463 3.848385 3.697409 3.848385 3.697409 3.697409 3.848385

29.59 2.06 12.53 -4.83 0.77 2.57 0.44 -4.80 1.35 1.81

t

intercept X1 X2 X3 X12 X2*X1 X22 X3*X1 X3*X2 X32

19.866667 9.8375 -1.7475 10.0975 -1.745833 1.01 -5.105833 12.285 -2.895 -2.300833

2.946137 1.804133 1.804133 1.804133 2.655612 2.551429 2.655612 2.551429 2.551429 2.655612

6.74 5.45 -0.97 5.60 -0.66 0.40 -1.92 4.81 -1.13 -0.87

0.0011 0.0028 0.3772 0.0025 0.5400 0.7085 0.1125 0.0048 0.3080 0.4259

by the movable heat-transfer tube immersed in bed in this study, and this method can effectively control the bed temperature only at a small range. Therefore, the bed temperatures in experiments are set between 810 and 890 °C. The bed temperature and the primary air and the secondary air flow rate are fixed when investigating the temperature changing at the different locations in the VFBC. The temperature distributions at different locations with different stoichiometric oxygen in the combustion chamber (Sb) in the combustor are shown in Figures 7 and 8. Owing to the coal used in this study with a high level fixed carbon, and large particle sizes (1∼8 mm), most of the combustion reactions will occur in the combustion chamber, and the maximum temperature of the region can be seen in the bed surface. These indicate that a large number of fixed carbons are burned and released the volatile compounds in the bed surface and above the bed surface. The cracking reaction is increased with the proportion of shorting of oxygen in the chamber, which makes more volatiles rise to above the bed surface and burn. From Figures 7 and 8, it can be found that the temperature at the freeboard is decreased with the stoichiometric oxygen in the combustor. The reason is that, when the stoichiometric oxygen in the combustor is equal to 80%, the flue gas recirculation makes the chamber short of oxygen, resulting in more volatile matter cracking up to the freeboard.



Figure 10. Main effects of Y1 on each factor with any other two factors fixed at center level for Y1: X1 ) -1, 0 and -1, Sb/Sb0 ) 1, 1.25, and 1.5; X2 ) -1, 0, and 1, Tb/Tb0 ) 1, 1.0494, and 1.0988; X3 ) -1, 0, and +1, Li/Lio ) 1, 1.611, and 2.222.

Figure 7. Temperature distribution of coal in the VFBC with various Sb (bed temperature ) 850 °C; secondary air ) 50% stoichiometric air).

Figure 11. Main effects of Y2 on each factor with any other two factors fixed at center level for Y2: X1 ) -1, 0, and -1, Sb/Sb0 ) 1, 1.25, and 1.5; X2 ) -1, 0, and 1, Tb/Tb0 ) 1, 1.0494, and 1.0988; X3 ) -1, 0, and +1, Li/Lio ) 1, 1.611, and 2.222.

Figure 8. Temperature distribution of coal in the VFBC with various Sb (bed temperature ) 890 °C; secondary air ) 50% stoichiometric air).

Figure 9. Temperature distribution of coal in the VFBC with various Tb (Sb ) 80%; secondary air ) 50% stoichiometric air).

Wang and Thomas20 pointed out that the content of volatile matter was increased with the pyrolysis temperature by the coal pyrolysis experiment. When the stoichiometric oxygen and primary air and secondary air flow rate are fixed, the temperature distributions at different locations with different bed temperatures in the combustion chamber are shown in Figure 9. The higher average bed temperature facilitates volatile matter release in the combustor chamber, which increases the combustion proportion of the volatile matter above the bed surface and the freeboard. Therefore, the temperature of the freeboard and the exit of the VFBC are increased with the bed temperature. (20) Wang, W. X.; Thomas, K. M. Energy Fuels 1996, 10, 409–416.

However, the secondary air addition can capture the unburned carbon and provide oxygen for the second combustion; consequently, the temperature above the secondary air injection is declined at first, and then increased slightly. This can be attributed to the fact that the cooling effect is greater than that of the secondary combustion due to the secondary air addition. Therefore, on the whole, the temperature of the exit of the VFBC is decreased with the stoichiometric oxygen and is increased with the bed temperature. 3.3. Effect of Acetic Acid Injection and Operating Conditions on the NO Emission. The general dependencies of Y1 and Y2 on each factor are shown in Figures 10 and 11, with the other two factors fixed at the center level. The short vertical bar on the curve represents the 95% confidence interval for the Y1 and Y2 value. The influence of three factors on the Y1 value can be seen from Figure 10, that is, X2 > X3 > X1. The NO emission concentration (Y1 value) increases with the bed temperature (X2), which is consistent with other researchers’ results. The influence of three factors on the Y2 value can be seen from Figure 11, that is, X1, X3 > X2. It can be seen that NO concentration has the lowest value when the stiochiometric oxygen in the combustion chamber (Sb) is 80% from Figure 10. When the bed is the fuel-rich condition, the oxygen is less than the stoichiometric air, which can decrease NOx emission.21 Meanwhile, the flue gas is also recirculated to the chamber, thus increases the chance of NO reduction.22 Figure 11 shows that Y2 (the NO removal percentage) increases with Sb. Because Y2 can be expressed by the eq 2 (which is shown in Section 2.4.2), it is obviously that NO concentration before acetic acid injection will effect the calculation of Y2, and the NO concentration before acetic acid injection increases with Sb greatly. Therefore, Y2 in Figure 11 increases with Sb. 3.3.1. Effect of the Acetic Acid Injection on the NO Emission. Figure 10 indicates that NO emission concentration is decreased with increasing the height of the acetic acid injecting location above the gas distributor, that is, when the (21) Turner, D. W.; Siegmund, C. W. Combustion 1973, 44, 21–30. (22) Sa¨nger, M.; Werther, J.; Ogada, T. Fuel 2001, 80, 167–177.

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acetic acid injecting location is higher (2.05 m above the gas distributor), the NO emission concentration has the lowest value. The NO emission removal effect when injecting the acetic acid can be seen in Figure 11, which shows that the acetic acid can reduce the NO emission concentration. This phenomenon can be explained as follows: (1) Ha¨ma¨la¨inen et al.5 has demonstrated that pyrrolic and pyridinic function groups are related to emission of nitric oxides. It was investigated that the formation of NO from 14 nitrogencontaining model compounds, and found that the -OH in the compound with the carboxyl (-COOH) hardly might convert HCN to NH3. Therefore, the intermediate, ammonia (NH3), hardly appears for the compounds with carboxyl and without phenolic during their combustion. It is generally thought that the conversion of fuel-N to nitric oxides is through the intermediates, hydrogen cyanide (HCN) and ammonia (NH3), formed during fuel combustion. (2) Nimmo et al.23,24 found that Calcium Magnesium acetate (CaMg2(CH3COO)6) could be decomposed to the reductive gas, CHi, which can be reacted with NO: CaMg2(CH3COO)6 f CaO + MgO + CHi Low O2

NO + CHi f HCN 98 N2

(3A) (4A)

From these two points of view, the acetic acid can reduce the NO emission concentration. However, it should be noted that when the acetic acid injecting location is lower (0.90 m above the gas distributor), the reductive reaction also occurs, but the O, H, and OH radical liberated from the acetic acid at high temperature reacted with HCN and NH3 liberated from the volatile, thus forming NO, and this effect is higher than the above reductive reaction. Therefore, the NO emission concentration is decreased with increasing the injection location of the acetic acid. 3.3.2. Effect of the Operating Conditions on the NO Emission. Figures 10 and 11 also shows the effects of the operating conditions on the NO emission, and Figure 11 indicates that NO removal percentage is increased with the stoichiometric oxygen. From eq 2, it can be seen that NO emission concentration when no acetic acid injection into the combustor will influence the calculation of the removal effect. Meanwhile, Figure 10 indicates that NO emission concentration when acetic acid injection into the bed is increased slightly with the stoichiometric oxygen, and on the other hand, NO emission concentration is increased obviously with the stoichiometric oxygen when no acetic acid injection into the bed. Therefore, NO removal percentage calculated from eq 2 in Figure 11 is increased greatly with the stoichiometric oxygen. Figure 10 shows that NO emission concentration is increased greatly with the bed temperature, and the effect of the reduction reaction producing NO due to acetic acid injection is smaller than NO concentration increasing due to raising the bed temperature. Therefore, NO removal percentage is decreased with the bed temperature. From Figure 11, it can be found that NO removal percentage is increased first and then decreased with the bed temperature, and a higher removal percentage occurred at 850 °C. The possible reason is that the effect of the NO (23) Nimmo, W.; Patsias, A. A.; Hampartsoumian, E.; Gibbs, B. M.; Fairweather, M.; Williams, P. T. Fuel 2004, 83, 149–155. (24) Nimmo, W.; Patsias, A. A.; Hampartsoumian, E.; Gibbs, B. M.; Fairweather, M.; Williams, P. T. Fuel 2004, 83, 1143–1150.

Figure 12. The relationship between the concentration of CO and NO within the flue gas with and without adding the acetic acid.

reduction reaction due to acetic acid injection is greater than NO concentration increasing due to raising the bed temperature when the temperature is in the range of 810-850 °C; consequently, NO removal percentage has a rising tendency at this temperature range. When the bed temperature is up to 850 °C, these two effects achieve a balance condition. However, when the bed temperature is in the range of 850-890 °C, the effect is just the opposite, so NO removal percentage is decreased gradually with the bed temperature at this time. Furthermore, when the bed temperature is higher, the amount of acetic acid, or the free radicals that can be reacted with NO, declined, and results in the decrease of the reduction effect. 3.4. Relationship between NO and CO Emissions. Many studies25,26 have reported that above the bed surface, CO on the surface of char could be the catalyzed to reduce NO, and the probability of reduction of NO may increase with increasing the CO concentration. In the previous study,15 the presence of CO enhanced the reduction of NO during the combustion process. In this study, it is found that NO emission can be reduced by the injection of acetic acid into the freeboard; however, the concentration of CO in the flue gas is increased. The relationship between the concentration of CO and NO within the flue gas with and without adding acetic acid is shown in Figure 12, which can be seen that it is in agreement with the result found by Furusawa et al.25 and Chyang et al.15 Our goal is to find out the optimum operating conditions to get not only low NO but also CO emission. From this view, a more sophisticated study is needed in the future. 4. Conclusions NO emission characteristics in a pilot scale vortexing fluidized bed combustor was studied using response surface methodology (RSM). The effects of acetic acid injection and operating conditions on the NO emission in this fluidized bed were investigated comprehensively. The following results can be concluded: (1) The bed temperature has a more important effect on the NO emission than the other two factors, and the effect order is (25) Furusawa, T.; Honda, T.; Takano, J.; Kunii, D. J. Chem. Eng. Jpn. 1978, 11, 377–383. (26) Berger, A.; Rotzoll, G. Fuel 1995, 74, 452–455.



the bed temperature, the injecting location of acetic acid, and the stoichiometric oxygen in the combustion chamber. On the other hand, the injecting location of acetic acid and the stoichiometric oxygen in the combustion chamber have more important effect on the NO removal percentage than the bed temperature.

(3) NO emission is increased with the stoichiometric oxygen in the combustion chamber and the bed temperature. NO removal percentage is increased with the stoichiometric oxygen in the combustion chamber, and is increased first, then decreased with the bed temperature. Also, a higher NO removal percentage occurred at 850 °C.

(2) NO emission can be decreased by injecting the acetic acid into the combustion chamber. NO emission decreases with the height of the acetic acid injecting location above the distributor, and NO removal percentage increases with the height of the acetic acid injecting location.

Acknowledgment. This research was supported by the project of the specific research fields in the Chung Yuan Christian University, Taiwan, under grant CYCU-97-CR-CE. EF9000055

Effects of Acetic Acid Injection and Operating Conditions on NO

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