Sonochemical Removal of Nitric Oxide from Flue Gases - American

Ind. Eng. Chem. Res. 2006, 45, 4475-4485

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Sonochemical Removal of Nitric Oxide from Flue Gases Samuel O. Owusu and Yusuf G. Adewuyi* Department of Chemical Engineering, North Carolina A&T State UniVersity, Greensboro, North Carolina 27411

The absorption of nitric oxide (NO) into water with simultaneous oxidation induced by ultrasonic irradiation at a fixed frequency of 20 kHz has been studied in a bubble column reactor at about room temperature. Factors studied include the flow rate of flue gas, intensity of ultrasound, and effect of sulfur dioxide (SO2) on the fractional conversion of NO. The concentration of NO in the inlet gas studied ranged from 50 to 1040 ppm, while that of SO2 ranged from about 52 to 4930 ppm. The fractional conversions of NO were found to range from 60% to 85%, while complete removal of SO2 was observed for all the inlet gas concentrations studied. In addition, the presence of low to moderate concentrations of SO2 in the inlet gas stream was found to enhance NO removal. Also, increasing the ultrasonic intensity was observed to improve NO removal. Sonochemical oxidation pathways leading to nitrite, nitrate, and sulfate formation are discussed. The results of this study suggest the feasibility of developing an innovative, cost-effective, and low-temperature aqueous sonochemical scrubber to provide an environmentally conscious method for the control of NOx and SO2. This should reduce or eliminate chemical usage, resulting in minimal sludge and disposal problems and associated costs. Introduction The emission of uncontrolled amounts of NOx and SOx into the atmosphere from the combustion of fossil fuel has severe and detrimental effects on plants, animals, and humans. Nitric oxide (NO) and its group of compounds jointly referred to as NOx constitute the main ingredients involved in the formation of ground-level ozone, which can trigger serious respiratory problems. NO reacts with oxygen to form nitrogen dioxide (NO2), which is very poisonous and contributes to the formation of acid rain. It also contributes to nutrient overload that deteriorates water quality, reacts to form toxic chemicals in the environment, and contributes to global warming.1-3 Unlike sulfur dioxide (SO2) and NO2, NO emission from industrial sources is difficult to control by scrubbing due to its low solubility in aqueous solution.4 One of the promising NOx control strategies is the conversion of NO into NO2 (or other more reactive and water-soluble species) followed by simultaneous scrubbing of NO2 and SO2 from flue gases. Numerous reagents have been used in the liquid phase for this purpose using various gas-liquid contactors.4-12 Nevertheless, the additional costs of chemicals and the complexity of some of these methods have discouraged commercial applications. Still, scrubbing of NOx with aqueous solutions promises to be less expensive than competing postcombustion methods for NOx removal such as selective catalytic reduction (SCR) and thermal NOx removal.13-15 Hence, there is a continuing need to identify ways of removing NO and its constituents from industrial flue emissions and the atmosphere by methods that are more efficient, selective, inexpensive, and environmentally responsible or safe. In the atmosphere gas-phase reactions primarily responsible for the effective oxidation of SO2/NOx to sulfuric acid (H2SO4)/ nitric acid (HNO3) both involve the •OH radical.16 Lee et al.17 outlined the photochemical methods to produce homogeneous distribution of •OH radicals at low temperature for postcombustion flue gas treatment. The gas-phase oxidation approaches * To whom correspondence should be addressed. Tel: (336) 3347564 (ext 107). Fax: (336) 334-7417. E-mail: [email protected].

for the control of nitric oxide involving the thermal decomposition of hydrogen peroxide (H2O2) have also been reported.18,19 However, high temperatures are desired for these processes because the principal mode of H2O2 decomposition switches from water and oxygen products to hydroxyl radicals in the temperature range of 673-723 K.17 Low temperature is important because negative apparent activation energies for the reactions of •OH radical toward SO2 and NOx are observed.17 Despite these reported cases, methods such as sonochemical oxidation, which involve efficient production and utilization of •OH radicals at low temperatures and require little to no chemical additions, are yet to be fully investigated for the simultaneous removal of SO2 and NOx.20 Sonochemical techniques utilize ultrasound to produce an oxidative environment via acoustic cavitation due to the formation and subsequent collapse of microbubbles from acoustical wave-induced compression/rarefaction. The collapse of these bubbles leads to local transient high temperatures (g5000 K) and pressures (g1000 atm), resulting in the generation of highly reactive species including hydroxyl (•OH), hydrogen (H•), and hydroperoxyl (HO2•) radicals and hydrogen peroxide.20-24 A number of studies have demonstrated the effectiveness of ultrasound for the oxidation of organics, destruction of pathogenic organisms, and treatment of water and wastewater either as a sole means of treatment or in combination with other oxidation processes such as ozonation, UV irradiation, and photocatalysis.25-31 Despite recent advances in homogeneous sonochemistry, the mechanisms of heterogeneous sonochemistry remain poorly understood. There is a need to improve our understanding of the extent of mass transfer and reaction intensification resulting from ultrasonic irradiation as a function of process and operating parameters in heterogeneous gas-liquid, liquid-liquid, and gas-liquid-solid systems.32-34 Shojaie et al.35 presented experimental results on the sonication of SO2 and NO species in water and NaOH solutions at 20 kHz in the presence of argon and suggested that the yields of H2SO4, nitrous acid (HNO2), and HNO3 resulted from interactions with H2O2 generated from water during the adiabatic collapse of the microbubbles in solution since the formation rate of H2O2

10.1021/ie0509692 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/16/2006

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Ind. Eng. Chem. Res., Vol. 45, No. 13, 2006

Figure 1. Experimental setup of the sonochemical bubble column scrubber.

limited the yields of oxidation products. However, to the best of our knowledge, no study has yet looked into the possibility of developing an advanced sonochemical aqueous scrubber of any kind in the open literature. In this study, we determine the feasibility of removing NOx from flue gas emission using an innovative aqueous sonochemical bubble column scrubber, which may prove to be affordable and easily adapted to retrofit existing flue gas wet scrubbing systems. The approach used in this study is appropriate for relatively small scale operation involving limited volumes of waste gases, which allow sufficient residence time in the scrubber for NOx to react in minimum scrubbing solutions, though larger volumes could be treated using several ultrasonic probes. The approach is desirable for small operations that wish to avoid new capital expenditures or have limited space to build a new SO2 scrubber and an SCR catalytic unit for NOx control, and may also have application as an emergency system when the ordinary deNOx system is out of order or in chemical industries where the gas flow rates are normally smaller than from oil- and coal-fired power plants. In this paper, we present for the first time the results of the aqueous removal of NO induced by ultrasound using a laboratory-scale aqueous sonochemical scrubber and the effects of SO2 in the inlet gas stream. Experimental Section Reagents. The reagent gases used were separate mixtures of NO and SO2 in ultrapure nitrogen obtained from Air Products and Chemicals Co. The NO cylinders contained gases ranging from 50 to 1040 ppm NO, while the SO2 cylinders consisted of varying amounts of gases from about 52 to 5000 ppm SO2, all in ultrapure nitrogen as the carrier gas. Millipore water was used for all the sonication experiments to reduce the presence of impurities. The Milli-Q system included electrodeionizing (ELIX) and reverse osmosis (RiOs) water purification systems

from the Millipore Corp. The combination system is capable of producing purified water with a resistivity of e18.2 MΩ‚ cm at 25 °C and reducing the total organic carbons (TOC), silicates, and heavy metals to very low part per billion levels, if any. Samples of the water were analyzed prior to experimental use with a Dionex-DX 500 ion chromatograph (IC) and found to be free of trace amounts of sulfates, nitrites, and nitrates. Apparatus and Procedure. The sonochemical bubble column scrubbing system used for this work consists of a jacketed bubble column, a digital sonifier, a flue gas blending system, and an analytical train. The jacketed bubble column unit is approximately 10 cm i.d. by 30 cm long and is made of Pyrex glass. Flue gas was introduced into the reactor through a gas dispersion tube fitted at the bottom of the scrubber. The sparger dispersion tube was designed by Ace Glass Inc., and had an 8 mm o.d. by 150 mm long tube connected to a 25 mm diameter disk at the discharge end. The disk had a porosity of grade “C” (25-50 µm, Ace Glass). The reactor was designed to allow the gas to flow continuously in an upward direction, while the liquid phase can be operated either in batch mode or continuously. In the semibatch mode of run the liquid phase was stationary while the gas phase flowed continuously, while in the continuous mode of run both the liquid and gas phases flowed continuously in countercurrent mode. The ultrasonic system consists of a digital sonifier, model 450, obtained from Branson with a frequency of 20 kHz and power output of 400 W. It has a variable amplitude that increases directly with the power. According to the manufacturer’s recommendation, the amplitude was not to exceed 70%. As a result, the maximum amplitude used for this work was set at 70%, corresponding to a power output of about 110 W. The entire setup used for this work was similar to that used by the same authors in a previous gas-scrubbing work,5 except the current system has its scrubber fitted with a sonicator at the

Ind. Eng. Chem. Res., Vol. 45, No. 13, 2006 4477

Figure 2. Semibatch absorption of 490 and 960 ppm NO in ultrasonic irradiated water (gas flow rate 0.1 slpm, I ) 55.2 W/cm2).

top as shown in Figure 1. An extender allows the probe of the sonicator to extend about 0.15 m below the surface of the solution in the reactor. The flow rate of the gas mixturesswith the concentrations described earliersranged from 5 standard liters per minute (slpm) during preliminary testing to 0.1 slpm for the actual experiments (i.e., a range of superficial velocity, Vg, of 0.17-0.0034 m/s). The total volume of liquid water used was 1.2 L for both semibatch and continuous runs, corresponding to a liquid height of 0.24 m. For continuous runs, the aqueous phase was circulated at approximately 0.475 L/min. The water used for all the scrubbing experiments was not buffered and had an initial pH of 6.8 ( 0.1, which decreased rapidly during sonication to about 3.2 ( 0.20 at the time of completion of each experimental run depending on the solute concentration and/or acid product composition. All the experiments were carried out using a fixed frequency of 20 kHz at or near room temperature (23 ( 2 °C), with the heat generated from the acoustic excitation removed by the use of a water jacket. The upward flow of the gases through the liquid produces bubbles that promote mixing of the liquid and the flue gas. At the same time ultrasonic waves, which also promote mixing, are emitted from the sonifier into the solution. The gas exiting from the reactor is passed through a condenser that is cooled to about 1 °C or less. The cold trap was used to remove any moisture carried in the gas mixture before it exited the reactor into the analyzer. The concentrations of input and output gases were analyzed using a chemiluminescence NO-NO2-NOx analyzer and a fluorescence SO2 analyzer. An Orion pH meter was used to determine the pH of the scrubbing solution before and after each run, and a Dionex-DX 500 IC was used to measure the concentration of ions in solution. Determination of nitrite (NO2-)/nitrate (NO3-) produced by NOx oxidation was obtained by subtracting the amount produced in a controlled reaction with only N2 flow from the amount obtained with NOcontaining N2 flow. Results and Discussion We have assessed the effectiveness of ultrasonic irradiation for the control of NOx and SO2 using a laboratory-scale sonochemical bubble column reactor. The efficiency (Ef) of the

sonochemical scrubber is determined from the percent removal of the feed gas, NO or SO2, i.e., the fractional conversion defined as

Ef ) 1 - ([gas]out/[gas]in)

(1)

where [gas]in and [gas]out ) [gas]in(1 - conversion) are steadystate gas concentration values as recorded from the analyzers and corrected for with a calibrating curve. Preliminary NO Absorption and Sonochemical Oxidation Test. In a previous study in a bubble column scrubber, the result of a blank test to determine the solubility of NO in Milli-Q water with no chemical additives (and in the absence of ultrasonic irradiations) using 1003 ppm NO in N2 bubbled at a flow rate of 1.7 slpm indicated that the NO was not absorbed and NO gas breakthrough was instantaneous.4 To determine the optimum flow rates and mode of operation for the ultrasonic experiments, approximately 500 ppm NO gas flowing at 5 slpm was initially absorbed in water sonicated at an intensity of 55.2 W/cm2 in semibatch mode. The effectiveness of the sonochemical removal of pollutants was evaluated by measuring the breakthrough times (i.e., the time when the absorption capacity of the sonicated solution is exhausted and a significant amount of the gas passes through without being removed). At this flow rate, the initial absorption of NO possibly due to mixing was followed by instantaneous breakthrough. Nitrogen dioxide was produced briefly as the NO gas was absorbing into the water. The flow rate was then decreased to 1.0 slpm. The initial absorption of NO was almost complete before breakthrough occurred in about 11/2 min. Again, nitrogen dioxide production was seen to peak instantaneously and then decrease after breakthrough occurred. Next, approximately 490 ppm NO and 960 ppm NO were separately absorbed in water irradiated with ultrasound at an intensity of 55.2 W/cm2 in semibatch mode at a lower flow rate of 0.1 slpm (Vg ) 0.0034 m/s). Contact times for commercial bubble columns and packed columns range from 0.01 to 1.0 s.36-38 The results of the output concentrations of NO with time are shown in Figure 2. Breakthrough occurred at 5 and 9 min for the 960 ppm NO and 490 ppm NO, respectively, in comparison with previous runs at high gas flow rates where breakthrough was instantaneous. The delay in breakthrough time

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Figure 3. Continuous absorption of 490 and 1040 ppm NO in irradiated water (gas flow rate 0.1 slpm, liquid flow rate 0.475 L/min, I ) 86.8 W/cm2).

with a decrease in the gas flow rate could be attributed to the increase in contact time for reaction between the solute gas molecules and possibly reactive •OH radicals generated from sonolysis of water. The sonolysis of water results in the formation of •H and •OH as primary free radical intermediates, attributable to the thermal dissociation of water vapor present in the cavities during the compression phase of the wave. The formation of these active species and their subsequent recombination at low scavenger concentrations to produce typical molecular products of aqueous sonolysis, hydrogen peroxide (H2O2) and hydrogen, are represented by the following reactions:20-26 )))

H2O 98 •H + •OH

(2)

•H + •H f H2

(3)

•OH + •OH f H2O2

(4)

Nitric oxide undergoes gas-phase reactions either in the acoustic cavities or with hydroxyl radicals in the interfacial zone and in bulk solution, ultimately resulting in the formation of nitrous and nitric acids:20

NO + •OH f HNO2

(5)

NO2-/HNO2 + •OH f NO2 + OH-/H2O

(6)

NO + •OH f NO2 + •H

(7)

2NO2(aq) + H2O f HNO2 + HNO3

(8)

It is also well-known that the •OH radical reacts with NO2- to produce NO2 (k ≈ 1 × 1010 M-1 s-1).39,40 The NO2 can also react directly with •OH to form nitrate:20

NO2 + •OH f HNO3

(9)

Moisy et al.41 also suggested the absorption of N2O3 formed from NO and NO2 (in the cavitation bubbles) to yield HNO2 as a possible mechanism. The NO2 formed may also equilibrate with its dimer N2O4 (k ) 4.5 × 108 M-1 s-1, K ) 1.53 × 10-5 M), which then disproportionates to NO2- and NO3-.41

The effectiveness of the sonochemical scrubber was also investigated by operating the system in continuous countercurrent mode with the gas flowing upward and the liquid circulated at approximately 0.475 L/min. The experiments were again carried out by absorbing 490 and 1040 ppm NO separately in the sonochemically irradiated water at a gas Vg of 0.0034 m/s. The results of these experiments are shown in Figure 3. As shown in both Figures 2 and 3, there were initial dips in the concentration of NO partly attributable to mixing and dilution effects of the purge gas (i.e., N2). Similar observations were made in our previous studies.4,5 These initial dips lasting about 5-10 min and representing almost complete removal of NO are followed by actual absorption induced by ultrasound. However, enhanced absorption was observed for the case of continuous countercurrent flow. In cases where the experiments were carried out in semibatch mode, the NO gas concentration approached the initial gas concentration values at breakthrough. On the other hand, in the case of continuous runs, a steady state was observed where the gas concentration remained almost constant at a value far less than the initial concentration throughout the duration of the experiment. As illustrated in Figure 3, fractional conversion for initial NO concentrations of 490 and 1040 ppm were 78.6% and 64.9%, respectively, in the continuous operation. The improved NO absorption could be attributed to the fact that liquid circulation leads to more dispersion of the bubble gas, resulting in increased residence time in the reaction. That is, the cumulative effect of the liquid circulation and more importantly the interfacial turbulence created by ultrasonic irradiation improves the breakup of bubbles into smaller bubbles, which spend more time in the reactor and increase gas holdup. Also, the continuous accumulation of reactive radicals available for reaction with NO gas improves as the liquid is continuously recirculated. On the basis of these results, it was established that more effective NO scrubbing could better be achieved by running both the liquid and gas phases in continuous mode at low flow rates for all subsequent experiments. Finally, it is important to note that, despite the thermostatic control of the experiments, slight increases in bulk liquid temperatures occurred with the progression of all experiments run in both the semibatch and continuous flow modes. The increasing liquid temperature would be expected to increase the transformation of NOx to product due to the increased amount of water vapor in the bubble, which can promote the


Figure 4. Effect of different ultrasonic powers on the removal efficiency of 1040 ppm NO (gas flow rate 0.1 slpm, liquid flow rate 0.475 L/min).

formation of free radicals from the dissociation of water molecules. On the other hand, increasing the liquid temperature, leading to less violent collapse, can result in lower internal temperatures at the end of bubble collapse. The effects of temperature on sonochemical reaction rates and mechanisms are discussed in more detail elsewhere.20,28 Effect of Ultrasonic Intensity on NO Fractional Conversion. The effect of increasing the ultrasonic intensity of the ultrasound generated on the removal of NO was studied. Figure 4 illustrates the effect of increasing the ultrasonic power on the removal efficiency of approximately 1040 ppm NO. Two settings of the ultrasonic power, 70 W (55.2 W/cm2) and 110 W (86.8 W/cm2) are shown here. It was observed that increasing the ultrasonic power increased the scrubbing efficiency of the irradiated water, thereby resulting in an increase in the fractional removal of NO by about 3-fold. At the higher acoustic powers, the increase in ultrasonic energy emitted into the solution results in higher cavitational activity, leading to the formation of larger numbers of smaller cavitating bubbles and a subsequent increase in the amount of radicals (e.g., •OH) produced. Thus, it could also be concluded that the higher NO removal at higher intensities resulted from an increased number of cavitation events. The result is consistent with other studies which reported that the rates of degradation of pollutants are higher at higher power dissipation into the system due to a higher number of cavitating bubbles and hence free radicals.42,43 Also, the liquid circulation velocity due to acoustic streaming or fluid turbulence increases with an increase in the extent of power dissipation. Vichare et al.42 also reported that the liquid circulation velocity due to acoustic streaming increases with an increase in the extent of power dissipation. The intensity, I (i.e., energy transmitted per unit of time per unit area of fluid), is described by the equation

I ) PA2(2Fc)-1

(10)

where PA is the maximum pressure amplitude for a given intensity, F is the density of the fluid (e.g., water), c is the speed of sound in the fluid (1500 m/s in water), and the term Fc represents the acoustic impedance (Z) of the medium.20,25 An increase in ultrasonic intensity (i.e., acoustic power per area of the probe tip) results in an increase in the acoustic amplitude, which favors more violent and faster cavitation bubble collapse since the bubble collapse time, the transient temperature, and the internal pressure in the cavitation bubble during collapse are all dependent on the acoustic amplitude. The energy that is

required to expand a population of bubbles in solution can be estimated by25

E ) (4/3)πRm3PmN

(11)

where Rm is the maximum radius of the bubble before it collapses, Pm is the magnitude of the hydrostatic pressure (Ph) and acoustic pressure (Pa ) PA sin 2πft at any given time, t), i.e., Pm ) Ph + Pa, f is the frequency of the sound wave, and N is the number of bubbles in solution. This equation indicates that the number of bubbles will increase linearly with the power density. As observed by other investigators,32 dissipating more power appears to be beneficial for the mass-transfer operations, and hence, in the large-scale applications requiring higher power dissipation, it seems that there will not be any mass-transfer limitations for transfer of gaseous reactants. Higher power dissipation into the system creates more turbulence and hence more breakage of the bubbles and higher rates of liquid circulation and hence more dispersion of the bubble gas, resulting in increased residence time for the reaction.32 The fractional conversions of varying initial concentration of NO (about 50-1000 ppm) were also evaluated at ultrasonic powers of 70, 90, and 110 W, corresponding, respectively, to ultrasonic intensities of 55.2, 71.0, and 86.8 W/cm2. The results depicted in Figure 5 indicate that, at all initial NO concentration levels, increasing the ultrasonic power from 70 to 90 W significantly improves the fractional conversions, the improvement becoming less pronounced from 90 to 110 W. The maximum size or radius, Rmax, of a cavitation bubble is dependent on the density of the liquid, the applied frequency, the hydrodynamic pressure and the acoustic pressure as follows:

Rmax )

[ ][

2 4 (P - Ph) 3ωa A FPA

0.5

1+

2 (P -Ph) 3Ph A

]

0.33

(12)

where ωa ()2πf) is the applied or acoustic frequency.25 For a bubble in an ultrasonic field under constant pressure, the maximum radius of the bubble is related to the collapse time by the expression

τ ) 0.915Rmax

[ ]( ) F Pm

0.5

1+

Pv T < Pm 2

(13)

where τ ) time of cavitation bubble collapse, T ) ultrasonic period, Pv ) vapor pressure in the bubble, and Pm ) pressure in the liquid at the moment of transient collapse, which is usually

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Figure 5. Fractional removal of NO as a function of the initial NO concentration (51, 490, and 1040 ppm) for different ultrasonic intensities (gas flow rate 0.1 slpm, liquid flow rate 0.475 L/min, I ) 55.2, 71, and 86.8 W/cm2).

Figure 6. Continuous absorption of 960 ppm NO in the presence of 990 ppm SO2 (gas flow rate 0.1 slpm, liquid flow rate 0.475 L/min, I ) 55.2 W/cm2).

taken to be equal to Pv assuming no influx of gas into the cavity. As the above equations predict and the experimental results show, there is an optimum power density (or acoustic intensity) that can be applied during sonochemical irradiation to obtain maximum reaction rates before a point of diminishing return is reached.25 For an ultrasonic probe system this effect can also be explained by bubble shielding, whereby at high intensities a saturation power is reached due to the formation of clouds of cavitation near the transducer, which block the energy transmitted from the probe to the fluid.22 The results illustrated in Figure 5 also indicate that while the fractional conversion decreases slightly with an increase in initial NO concentration at the lower ultrasonic power setting, the fractional conversion undergoes a maximum at the medium initial concentration of approximately 490 ppm NO for the higher power ultrasonic settings. This observation is indeed to be expected because, when the NO concentration is higher than nominal, at a particular ultrasonic power, there will be too many solute (i.e., NO) molecules competing for the sonolytically generated hydroxyl radicals. However, in the presence of a lean concentration of NO and at high ultrasonic powers it appears

that the tendency for the hydroxyl radicals (formed at a greater quantity) to recombine predominates over interaction with the fewer solute molecules. It has been reported that, at the surface of the collapsed bubble, the •OH concentration is a maximum and is estimated to be between 4 × 10-3 and 10-2 M.30,44-46 At a low scavenger (i.e., NO, NO2-, HNO2, or NO2) concentration, it is expected that a considerable portion of the •OH radicals generated on sonolysis of aqueous media will recombine outside the bubbles and form H2O2 according to eq 4 since the second rate constant of this reaction, k ) 6.0 × 109 M-1 s-1, is comparable to those of the reaction of •OH with NO, NO2-, and HNO2 (eqs 5 and 6), which are 1.0 × 1010, 1.0 × 1010, and 2.6 × 109 M-1 s-1, respectively.47-52 Absorption and Sonochemical Oxidation Pathways for NO and the Effect of SO2. The effects of different concentrations of SO2 on the conversion of approximately 490 ppm NO and 960 ppm NO were also evaluated at ultrasonic powers of 70 W (I ) 55.2 W/cm2) and 110 W (I ) 86.8 W/cm2). The results of these tests are illustrated in Figures 6-8. The effect of SO2 on the removal of NO was first studied by simultaneously scrubbing approximately 490 and 960 ppm NO in the presence of about


Figure 7. Comparison of the absorption of 1040 and 990 ppm NO in 990, 2520, and 4930 ppm SO2 (gas flow rate 0.1 slpm, liquid flow rate 0.475 L/min, power 110 W, I ) 86.8 W/cm2).

Figure 8. Effect of fractional removal of 490 and 1040 ppm NO in the presence of varying concentrations of SO2 (gas flow rate 0.1 slpm, liquid flow rate 0.475 L/min, I ) 86.8 W/cm2).

990 ppm SO2 in aqueous solutions subjected to an ultrasonic intensity of 55.2 W/cm2. In both cases, the presence of SO2 was found to enhance the sonochemical removal of NO. For example, using a feed gas containing 1040 ppm NO in the presence of 990 ppm SO2, the fractional NO conversion at an ultrasonic intensity of 86.8 W/cm2 was determined to be about 82.2% at breakthrough compared with a fractional conversion of 64.9% in the absence of SO2 (Figures 3 and 8). The concentration-time graph for the simultaneous removal of NO and SO2 for the 490/990 ppm (NO/SO2) system at I ) 55.2 W/cm2 is also depicted in Figure 6, indicating a fractional conversion of about 81.3%. To further elucidate the increased NO removal in the presence of SO2, 1040 ppm NO was scrubbed simultaneously with 990, 2520, and 4930 ppm SO2. On the basis of the results shown in Figures 7 and 8, it is obvious that the presence of a moderate amount of SO2 improves the conversion of NO. However, in the presence of high concentrations of SO2 (i.e., 2520 and 4930 ppm), the fractional conversion of 490 ppm NO decreased from 78.6% (without SO2) to about 74.1% (with 2520 ppm SO2) and 74.5% (with 4930 ppm SO2) as shown in Figure 8. The fractional conversions were, respectively, 82.2%, 83.1%, and 83.8% for 1040 ppm NO in the presence of 990,

2520, and 4930 ppm SO2 as compared to fractional conversion of 66.3% for 1040 ppm NO in the absence of SO2. As shown in Figure 8, the results of varying the concentration of SO2 on fractional conversions of the two different concentrations of NO (490 and 1040 ppm) indicate that, for approximately 490 ppm NO, the removal efficiency was highest when the SO2 concentration was approximately 1000 ppm (i.e., a NO/SO2 ratio of 1/2), while for 1040 ppm NO, the fractional conversion was about the same for all the various concentrations (approximately 1000, 2500, and 5000 ppm) of SO2 used. It appears that while the presence of SO2 enhances removal of NO, the SO2 also effectively competes in the reaction with oxidative species (most likely •OH in the bubble-liquid interface) when the initial concentration of NO is significantly smaller than that of SO2 (i.e., in a leaner NO environment). The competition of solute (i.e., SO2) molecules for hydroxyl radicals is evident from the fractional conversions of 490 ppm NO obtained in the presence of varying amounts of SO2, as shown in Figure 8, indicating a sharp decrease in fractional conversion as the SO2 concentration is increased above 990 ppm. On the other hand, the sustained fractional conversions obtained with 1040 pppm NO as the concentrations of SO2 increased above 990 ppm might be due

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Figure 9. Amount of products as a function of the run duration during absorption of 1040 ppm NO in the presence of 2520 ppm SO2 (total time 60 min, gas flow rate 0.1 slpm, liquid flow rate 0.475 L/min, I ) 86.8 W/cm2).

to the interactions of sufficient NOx species with bisulfite species to form N-S intermediates as discussed in more detail in other papers involving aqueous systems in the absence of ultrasound.5 Finally, it is important to discuss the possible effects of the more drastic drops in solution pH (3.2 ( 2), observed with SO2 in the feed stream, on the NO removal efficiency. In general, it is difficult to generalize the effects of pH on sonochemical reactions since they depend on a variety of parameters (e.g., the state or chemical structure of the contaminant, solution ionic strength, presence of H2O2 or other oxidants, and reaction operating conditions).20,28,44-46 For example, in the ionic state, the molecule does not vaporize into the cavitation bubble and oxidation reaction occurs with •OH radicals outside the bubble film. However, in the molecular state, reactions occur by both thermal cleavage inside the bubble and oxidation with •OH radicals outside, leading to more effective decomposition.20,28 Also, the well-known detrimental effect of alkaline solution pH on sonochemical reaction is partly ascribed to the rapid dissociation of •OH by reaction with OH- to produce the oxide radical (O•-), which is known to react more slowly with the substrate than •OH.25,28 The decreasing solution pH in the presence of SO2 is expected to have the beneficial effect of increasing the transformation of nitrite to nitrate. Kruus et al.46 studied the formation of nitrite and nitrate ions in water under irradiation with 900 kHz (27 W) ultrasound as a function of time, temperature, and gas (oxygen/nitrogen) composition. They also found that the differences in the NO2-/NO3- ratio found between various studies could be explained through a mechanism where HNO2 and HNO3 were formed in the gas phase of the imploding cavity, and then dissolved in the water and dissociated to ions, with NO2- species substantially favored initially as considerably more NO was formed than NO2, and the NO2- subsequently reacting with hydrogen peroxide to give NO3- via eq 14.

H2O2 + NO2- f NO3- + H2O

(14)

They observed that this conversion was favored at lower pH values, and the pH decreased with the time of sonication, suggesting that the conversion occurs in the aqueous phase rather than in the imploding cavity. Jiang et al.44 studied the initial rates of formation of H2O2 (at 610 kHz, 25 W, and 15 ( 1 °C) as a function of pH in the range of 2-9. They reported the

maximum rate of H2O2 formation at pH ≈ 3.2, decreasing with increasing (and decreasing) pH of the solution, with a more rapid decrease of the H2O2 yield in the alkaline region. The effect of the ultrasonic irradiation time on the product distribution of NO was also studied by conducting repeated experiments for time periods of up to 20, 30, and 60 min using a feed gas containing 1040 ppm NO in the presence of 2520 ppm SO2, and observing the fractional NO conversion at various time intervals as well as the products formed. As the experiments proceeded the products formed were observed to be nitrite, sulfite, nitrate, and sulfate, with nitrate and sulfate forming in significant amounts when the solutions were ultrasonically irradiated for extended durations. The amounts of sonolysis products formed in solution as a function of irradiation time are shown in Figure 9 for up to the 60 min experimental run. Nitrite forms in significant amounts initially and then decreases as the nitrate concentration begins to increase after a steady state (i.e., when NO fractional conversion remains unchanged) has been achieved and as the reaction progresses with time. The results suggest that nitrite is formed as an intermediate that is ultimately oxidized to nitrate. This observation is consistent with the sonolysis results of air- and nitrogen-saturated water.53-55 Mead et al.54 studied the sonolysis of water at 447 kHz and 25 °C in the presence of air, N2, Ar, and O2 to ascertain kinetic data on the formation rates of nitrous and nitric acids and hydrogen peroxide. The initial rate data indicated that HNO2 was the major acid component formed during ultrasonic irradiation of water saturated with air and nitrogen. They suggested that the aqueous-phase second-order reaction of •OH radicals with NO (formed in the same cavitation) was the predominant source of HNO2. It was also shown that, for the air- and nitrogen-saturated water at an irradiation time greater than 15-20 min, the production of HNO2 passed through a maximum while the concentration of HNO3 showed a greater increase. The results suggest HNO2 is scavenging •OH radicals, a scenario that could lead to the suppression of H2O2 formation. Table 1 shows the material balances for nitrogen in liquid samples collected for some typical experimental conditions for run times of 10, 20, and 30 min. These typical balances for N(NO), N(NO2-), and N(NO3-) (i.e., NO, NO2-, and NO3reported as nitrogen) indicate that the scrubbing systems exhibit good nitrogen recoveries.


SO2(g) f SO2(aq)

(22)

partially due to the reaction of NO2 (from the sonochemical oxidation of NO) with HSO3- ion to form N-S complexes. In aqueous chemical systems containing S(IV) and NOx species (without the influence of ultrasound irradiation) the possibility of other side reactions between HSO3- and NO2(aq), NO2-, or HNO2 in the pH range 3-8 leading to the formation of N-S intermediates such as hydroxylamine disulfonate (HON(SO3)22or HADS) and nitrososulfonic acid (ONSO3- or NSS) has been reported.5 It is also well-known that NO2 (from the oxidation of NO) is a better oxidizing agent compared to O2 in oxidizing SO2 to SO3.57 It should be noted that, in a flue gas system containing typically 2-15% O2 resulting from excess air introduced into the fossil fuel combustion system to ensure complete combustion, the presence of O2 should enhance the sonochemical removal rates of NOx due to the generation of more reactive species such as HO2• and ozone (O3).20,25 Wakeford et al.55 observed a dramatic improvement in the yields of nitrite and nitrate from the ultrasonic irradiation of air-saturated water compared to yields from nitrogen-saturated water, and concluded that the presence of molecular oxygen enhanced product yield. Fossil fuel combustion also results in the formation of 1012% CO2, which is then present in the exhaust gas. It is reported that irradiation of ultrasound has hardly any sonochemical effect under a CO2 atmosphere.58,59 However, in an argon atmosphere, the main products of the sonolysis of CO2 are CO and a small amount of formic acid, about 30 times smaller than the CO yield.58 Haradi59 observed that dissolved CO2 was deoxidized to carbon monoxide (CO2 f CO + O) by sonolysis under argon and showed that argon was the most favorable atmosphere for reducing CO2 compared with other gases in the order Ar > He > H2 > N2. Since hydrogen is also obtained from the sonolysis of water and both CO and H2 are fuel gases, which also react to produce C1 compounds such as methanol, they proposed sonication as a useful and potential technique to reduce greenhouse gas, CO2, and produce fuel. The final oxidation products using an ion chromatograph to analyze the scrubbing solutions showed that the only anions in solution after the sonochemical oxidations of NOx and SO2 were mainly NO3-, NO2-, and sulfate (SO42-). In practice, appropriate treatment methods may be needed for spent solutions that may contain high levels of sulfate, nitrate, and nitrite. Nitrates, the final products of the sonochemical oxidation, are not very toxic themselves compared to nitrites, the intermediate products. Treatment processes that have been applied full scale for removal of soluble nitrate ion include evaporation (or distillation), ion exchange, biological denitrification, reverse osmosis, and catalytic liquid-phase methods.60 Catalytic liquid-phase methods are also under development to remove both nitrite and nitrate, which use palladium (Pd) to reduce them to N2.61-63 Alternatively, the nitrogen-sulfur can be removed from the scrubbing liquor as alkaline-metal salt precipitates of commercial value. For example, sulfate and sulfite ions can be precipitated as calcium salts by reaction with lime/limestone, and sodium nitrite and nitrate can be manufactured by the addition of sodium hydroxide.64 Hence, the liability of the pollution problem is advantageously converted into assets.

SO2(aq) + H2O f H+ + HSO3-

(23)

Conclusions

HSO3- + H2O2 f HSO4- + H2O

(24)

HSO4- + H+ f H2SO4

(25)

The absorption-oxidation of NO and SO2 in aqueous solutions enhanced by ultrasonic irradiation has been successfully demonstrated. The concentration of NO studied ranged from 50 to 1040 ppm, while that of SO2 ranged from about 52 to 4930 ppm. The fractional removal of NO ranged anywhere

Table 1. Material Balance Summary for NO parameter

run 1

run 2

run 3

flue gas flow rate (slpm) superficial gas velocity (m/s) inlet NO concn (ppm) inlet SO2 concn (ppm) power setting (W) irradiation time (min) ultrasonic intensity (W/cm2) fractional conversion (%) for NO amt of N(NO) in inlet gas (mmol) amt of N(NO) converted (mmol) amt of N(NO) in outlet gas (mmol) amt of N(NO2-) (mmol) amt of N(NO3-) (mmol) total amt of N in solution (mmol)

0.1 0.0034 1040 4930 110 10 86.8 83.8 0.0716 0.0600 0.0120 0.0767 0.0000 0.0767

0.1 0.0034 1040 4930 110 20 86.8 83.8 0.1432 0.1200 0.0230 0.0880 0.0059 0.0939

0.1 0.0034 1040 4930 110 30 86.8 83.8 0.2148 0.1800 0.0350 0.1382 0.0088 0.1470

Although several side reactions can possibly occur in the sonochemical oxidation of NO as discussed earlier, on the basis of our results, the following reaction pathways appear most probable in the bubble-liquid interface:

As indicated earlier, the oxidation of the nitrite to nitrate by sonochemically generated H2O2 also proceeds in the liquidbubble interface or in the bulk solution. A similar mechanism for the oxidation of SO2 by •OH radical in the bubble-liquid interface is expected to proceed by56-57

The overall reaction for the simultaneous oxidation of both NO and SO2 by •OH in the bubble-liquid interface may then be summarized by

SO2 + NO + 5•OH f H2SO4 + HNO3 + H2O (21) The formation of sulfate could also result from the oxidation of bisulfite ion (HSO3-) in the bubble-liquid interface or bulk solution by sonochemically generated H2O2. Shojaie et al.35 presented experimental results on the sonication at 20 kHz (45 W) of SO2 and NO in which gases containing 1960 ppm SO2 in argon or 514 ppm NO in argon were bubbled simultaneously in water and NaOH solutions using both batch and continuous flow treatment systems. They observed that the formation rate of H2O2 limited the yields of oxidation products, suggesting that the yields of H2SO4, HNO2, and HNO3 resulted from interactions with H2O2 generated from water during the adiabatic collapse of the microbubbles in solution. They proposed the following reaction sequence for the oxidation of SO2 in water:

The observed enhanced NO removal effect could also be

4484


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ReceiVed for reView August 25, 2005 ReVised manuscript receiVed April 15, 2006 Accepted April 18, 2006 IE0509692

Sonochemical Removal of Nitric Oxide from Flue Gases - American

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