Ind. Eng. Chem. Res. 1994,33, 2749-2756
2749
Characterization of NO2 and SO2 Removals in a Spray Dryer/ Baghouse System William J. O'Dowd,' Joanna M. Markussen, and Henry W. Pennline U.S.Department of Energy, P.O. Box 10940,Pittsburgh, Pennsylvania 15236
Kevin P. Resnik GilbertlCommonwealth, Znc., P.O. Box 618,Library, Pennsylvania 15129
Oxidation of NO to NO2 has been proposed as a method for enhancing NO, removals in conventional flue gas desulfurization (FGD) processes. This experimental investigation characterizes the removals of NO2 and SO2 in a 1.1 m3(standard)/min spray dryerhaghouse system. Flue gas was generated by burning a no. 2 fuel oil, which was subsequently spiked upstream of the spray dryer with NO2 or SO2 or both. Lime slurry was injected via a rotary atomizer into the spray dryer. Variables studied include the approach to the adiabatic saturation temperature, stoichiometric ratio, SO2 concentration, and NO2 concentration. Significant quantities of NO2 are scrubbed in this system, and over half of the total removal (at inlet NO2 > 400 ppm) occurs in the baghouse. Increasing NO2 concentrations enhance the amount of NO, removed in the system. Also, the presence of significant quantities of NO2 enhances the baghouse SO2 removal. Although up to 72% NO2 removals were obtained, concentrations of NO2 that exited the system were greater than 50 ppm for all conditions investigated.
Introduction A number of processes based on the oxidation of nitric oxide (NO)to nitrogen dioxide (NO21 are currently being developed with the objective of achieving nitrogen oxides (NO,) removal in a conventional sulfur dioxide (SOz) removal process. Lyon et al. (1990) has proposed methanol injection into the flue gas around 725 "C to generate hydroperoxyl (OH21 radicals for the oxidation of NO t o NO2. Lee et al. (1990) describe the addition of yellow phosphorus in a wet scrubber system to produce ozone (O3), which subsequently oxidizes NO t o NO2. Researchers at the Pittsburgh Energy Technology Center (PETC) have looked at two different techniques, a hydrogen-oxygen torch (Lee et al., 1989) and ultrasound (Shojaie et al., 19921, to produce hydroxyl (OH) radicals for the oxidation of SO2 and NO. The basic premise for the development of these oxidation processes is that if NO can be readily oxidized to NO2, then the NO2 could be easily removed in a conventional type of scrubbing unit (wet or dry). Physical solubility data indicate that NO2 is more soluble in water than is NO. The Henry's law coefficient for NO2 at 25 "C is 5 times larger than that for NO (Seinfeld, 1986), and the actual solubility of NO2 is further increased due to the reaction of dissolved NO2 with water. Thus, NO, removals are expected to be higher in flue gas systems containing relatively high concentrations of NO2, as compared to typical utility flue gas where greater than 95% of the NO, is present as NO. Although little information is available to verify that increased NO, removal is possible in conventional flue gas desulfurization systems when NO is oxidized t o NO2, some reported process data lend support to the hypothesis. In the E-Beam Process, NO, removals in a conventional lime spray drying system were found t o increase after irradiation (Helfritch and Feldman, 1985). It was speculated that OH radicals formed during irradiation subsequently oxidized NO and that NO2 was more readily removed in the lime slurry spray. A test during hydroxyl radical research by Lee et al. (1989) in a lime spray dryer system indicated that, in a flue gas 0888-5885/94l2633-2749$04.50lO
stream containing relatively high concentrations of NO2 (205 ppm NO2 in 347 ppm NO,), NO, removal (20.3%) was obtainable a t conditions normally showing negligible NO, removal. To help establish a database on the potential removal performance of lime dry-scrubber systems with flue gas streams containing SO2 and relatively high concentrations of NOz, parametric tests have been performed in a small-scale spray dryer facility a t PETC. The effects of parameters such as NO2 concentration, SO2 concentration, approach to the adiabatic saturation temperature (AT,,), and stoichiometric ratio were studied. Some of the results from this research are summarized below.
Experimental Section A schematic of the PETC spray dryer test facility is shown in Figure 1. The furnace produces approximately 1.1m3(standard)/min of flue gas by burning 0.073 kgl min of a no. 2 fuel oil with enough excess air to obtain 0 2 concentrations in the range of 3-4%. The average fuel oil composition is shown in Table 1. The flue gas flow rate is measured with an annubar downstream of the combustor (before the spray dryer). The inlet spray dryer temperature is controlled by the use of both a water-cooled and an air-cooled heat exchanger. Wetbulb and dry-bulb measurements are taken before the spray dryer with the measured wet-bulb temperatures ranging from 56 to 58 "C for dry bulb temperatures ranging from 162 to 178 "C. To achieve the desired concentrations of NO, and SO2 in the inlet flue gas, a gas-spiking system is installed upstream of the spray dryer. The gas-spiking system consists of mass flow controllers, for accurate and reproducible gas flows, and approximately 9.1 m of 0.64cm-diameter stainless steel tubing t o the flue gas duct. SO2 and NO are injected separately as relatively pure gases while mixtures of NO and NO2 are formed by mixing pure 0 2 with NO. When the NO2 is formed, a substantial temperature rise is recorded approximately 0.6 m from where the gases are mixed, indicating that the reaction is initially vigorous. In an attempt to 0 1994 American Chemical Society
2750 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994
Air Blower
, IIj'
Induced
o1
c,qdr,, I
Water Cooler Combustion Air Blower
z p
Clean FIue Gas k
Baghouse
(fabric filter)
2 Slurry
No2 Fuel Oil
Spent Material
Slurry Feed Pump
@ jl
Gas Analysis Sample Point Solid Sample Point
Figure 1. Flow diagram of the spray dryer test facility. Table 1. Fuel Oil Properties composition (wt %) carbon hydrogen oxygen sulfur kinematic viscosity (at 40 "C) (cSt) specific gravity (g/mL) heating value ( J k g )
86.7 13.0 0.27 0.36 2.6 0.839 9.32 x lo6
quantify the conversion of NO to NO2 in the spiking system by eliminating the influence of the system and sample gas conditioning system, the spiking gas was routed directly into a Beckman 864 infrared NO analyzer (10% full range). The measured NO concentrations for the typical NO and 0 2 flow rates employed in this test program during NO2 generation indicate at least 96% NO conversions. Reagent slurry is prepared by mixing known weights of water and a Mercer hydrated lime in a small mixing tank. The Mercer hydrated lime is sampled periodically and found to have an average available calcium hydroxide (Ca(OH)2)content of approximately 85%. The slurry is pumped through a mass flow meter to a rotary atomizer in a 0.79-m-diameter Niro spray dryer. To obtain the desired spray dryer exit temperature, water is added separately into the slurry line entering the rotary atomizer. Slurry droplets with a calculated Sauter mean diameter of 37 to 43 pm are produced (Masters, 1985). Reported AT,, values refer to the difference between the spray dryer exit temperature and the wet-bulb temperature. A pulse-jet baghouse containing three Nomex filter bags is located downstream of the spray dryer. The baghouse typically operates at air-to-cloth ratios between 0.70 and 0.85 d m i n . The cleaning of the bags is operated manually and requires modification of system operating conditions (O'Dowd et al., 1994). Therefore, cleaning of the bags is kept to a minimum and is performed at the beginning and end of each test
period. The solid sample collected from the baghouse is representative of removals occurring during the entire test period. While a sample collected from the baghouse is representative of the system removals, obtaining a solid sample which represents the removals occurring in the spray dryer is desirable. Therefore, a solid sample system was installed between the spray dryer and baghouse. The solid sample system consists of a 1.3cm-diameter eductor and uses nitrogen (heated to > 149 "C) as the motive fluid. The solid sample system is maintained above 149 "C when a solid sample is collected. The sampling is initiated and terminated by opening and closing a valve located between the eductor and the flue gas duct. During the sampling period, the flue gas passes through a high-volume filter where the solids are separated from the gas phase. Upon completion, the collected solid sample is isolated from the atmosphere, cooled, and withdrawn the following day for analysis. The collected solids are used to verify the removals determined from the gas phase measurements. The analyses necessary for the material balance closure are total Ca (atomic absorption), total nitrogen (LECO CHN), and total sulfur (LECO sulfur). Also, additional analyses of the solids, consisting of speciation of the byproduct, are performed to obtain an insight into the removal chemistry. Ion chromatography is used to determine the sulfitelsulfate, nitritehitrate, and carbonate present in the solid byproduct. An initial laboratory investigation was conducted to determine an adequate gas sampling system for measuring S02, NO2, and NO in a simulated flue gas. The known interaction of , 9 0 2 , NO2, NO, and H2O in the formation of sulfuric acid, such as in the Lead Chamber Process (Brasted, 19611, implies that traditional condensation type flue gas conditioning systems may not be adequate. Two gas sampling systems were investigated: a condensation trap (ice bath) and a permeation
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2751 (Perma Pure) dryer. The results of this initial investigation, for a simulated flue gas containing 150 ppm NO2, 2300 ppm SO2, and 17% H20, indicated that not only was a substantial portion of the NO2 (250%) being scrubbed in the ice bath but the presence of NO2 was also enhancing the scrubbing of SO2 in the condensation trap. However, the influence of the Perma Pure dryer on the gas phase measurements of NO2 and SO2 was not severe. This information was the basis for the design of the flue gas sampling system employed for this experimental test program. The flue gas is continuously sampled before the spray dryer, after the spray dryer, and after the baghouse. Dual-stage vacuum pumps with extended heads transport the sampled flue gas in 0.64-cm-diameter stainless steel lines, maintained at about 149 "C, t o either an ice bath or a permeation dryer. A dual system was designed to minimize the amount of sampled flue gas that the Perma Pure dryer needs to condition. The ice traps remove water from the sampled flue gas that feeds paramagnetic 0 2 analyzers, which are used for air inleakage determination, and Beckman infrared (IR) C02 analyzers. Perma Pure dryers are used to remove water from the sampled flue gas that supplies Horiba PIR 2000 SO2 analyzers and Thermoelectron (TECO) Model 10 chemiluminescent NO, analyzers. Also, an Anarad ultraviolet (W)NO2 analyzer was installed on the sampling stream before the spray dryer and was used as a check on the NO2 reading obtained from the TECO (by difference). The entrance of each Perma Pure dryer (Model No. PD 750-24SS) is located in a heated enclosure to eliminate condensation. While the flue gas enters the Perma Pure a t 149 "C, the purge nitrogen enters countercurrently at room temperature and at lower pressure. During each test day, a computerized data acquisition system continuously records gas concentrations, system temperatures, and system pressures. Spiking gas flows are initiated early during warmup to allow the NO2 gas concentrations to approach steady state, which takes at least 1h after initiating the spiking gas flows. The lag time in reaching steady state gas concentration appears to result from conditioning (sorptioddesorption of NO21 on the Perma Pure dryers. Once steady state is achieved for a test condition, the baghouse is pulsed to remove the spent lime present and the solid sample system collection is initiated. Typically, a test consists of a t least 1 h of slurry injection. At the end of the test, the baghouse is pulsed again and the solid sample system is shut off in an attempt to obtain a representative solid sample for both the system and the spray dryer removals. Also, during the test, the analyzers are periodically switched from the NO, mode to the NO mode, allowing the NO2 concentrations to be determined by difference. Overall, the flue gas generated from the combustor was well characterized and consistent. There is good agreement between the measured operating parameters of the system with the values which can be theoretically calculated. From the known oil flow rate, oil composition, and 0 2 concentration of the flue gas leaving the combustor, a stoichiometric calculation (STOCAL)of the total gas flow rate is possible. Results from the theoretical calculation agree reasonably well with the calibrated annubar measurements. The differences between the measured flow rates and the STOCAL flow rates are always less than 4%, and they are less than 2% in over 75% of the tests. Also, the measured CO2
70 C a / ( S 0 2 + l /2(NOx))=1 .25 NOx=560 ppm N 0 2 = 4 2 0 pprn 50,=2050 p p m Spray Dryer 0
60
50
K
E E x
30
0 Z
20
0 10
0
0
5
15
10
A
20
25
30
T O S SCo
Figure 2. Effect of the approach to the adiabatic saturation temperature on NO, removal.
concentrations are always within 3% of the theoretical CO2 concentrations (relative to measured COZ)with over 60% of the tests within 1%.There is also good agreement between the measured wet-bulb/dry-bulb temperatures and the theoretical moisture calculation.
Results and Discussion Although the initial laboratory investigation indicated a Perma Pure dryer would be adequate for moisture removal in the gas sampling system, unanticipated subtleties were observed during operations. As discussed previously, there is a substantial lag time in achieving steady state NO2 concentrations (due to possible NO2 sorption). Also, an interaction between SO2 and NO2 is observed in the gas sampling system. The reaction between NO2 and SO2 results in a slight loss of SO2 and a reduction of NO2 to NO. The variables which influence this reaction are the SO2 concentration, NO2 concentration, and the age (sampling time) of the Perma Pure dryer. While the SO2 concentration in the flue gas influences the measured NO2 concentration, the total measured NO, concentration is independent. Therefore, the discussion of the test results focuses on NO, removals. The reported values of inlet NO2 concentration are based upon the inlet NO, concentration and the amount of NO leaving the system (at lowest steady state SO2 concentration). A detailed characterization of the on-line gas sampling systems for measuring NO2 and SO2 is given by ODowd et al. (1994). NO, Removal. The effect of the AT,, on the NO, removal for the baseline NO2 injection case (420 ppm NO2) is shown in Figure 2. The baghouse inlet temperature was generally maintained within 5 "C of the spray dryer exit temperature. The equivalence ratio, the ratio of the molar feed rate of available Ca(OH12 to the molar feed rate of SO2 and NO, in the flue gas (Ca/ (SO2 VzNO,)), was maintained around 1.25. With a
+
2752 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 40
,
I
'
~
i
I
Ca/(S0,+~/2(NOx))=1.25
1
NOw=560 pprn NO,=420 p p d S0,=2050 p p m
75
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I
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Baghouse
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51
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0-
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80
90
Baghouse Outlet Temperature,
0
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100
Figure 3. Effect of baghouse temperature on baghouse NO,
0
100
200
300
400
500
Inlet NO, Concentration, p p m
removal.
Figure 4. Influence of NO2 concentration on NO2 removal and lime utilization for the nitrogen species.
decreasing AT,,, the NO, removal increases in the spray dryer as well as in the baghouse. In all figures, the least squares method is used to determine regression curves through the data points. A number of tests with NO injection, instead of NO2, at similar NO, concentrations and over the same range of AT,, resulted in negligible NO, removals in both the spray dryer and the baghouse. The spray dryer inlet temperature was also varied and showed a modest influence on the NO, removals. Spray dryer NO, removals increased from 18.6% to 23.1% for a corresponding increase in the inlet temperature from 168 to 177 "C at a AT,, = 14 C". The effect of the inlet spray dryer temperature on the spray dryer NO, removal, at a constant AT,,, can be directly attributed to the drying time of the slurry droplets in the spray dryer. The influence of the baghouse temperature, shown as the baghouse outlet temperature, on the baghouse NO, removal is shown in Figure 3. The baghouse inlet temperature is within 4 "C of the baghouse exit temperature for the reported removals. As seen in Figure 3, increasing baghouse temperature results in a significant decrease in the baghouse NO, removal. The slurry feed rate t o the spray dryer was constant, so the rate of solids buildup and the resulting increase in pressure drop were relatively constant for these tests. The tests shown in Figure 3 represent a range of AT,, exiting the spray dryer, which results in a slight variation in the moisture content of the flue gas entering the baghouse. However, the parameter affecting NO, removal in the baghouse appears to be temperature, which is related directly to the relative humidity. This effect is shown in Figure 3 by the decrease in NO, removals seen in going from a baghouse temperature of 73 to 91 "C. Both tests were run at the same spray dryer exit condition of ATa, = 14 C",and the flue gas was reheated to obtain the higher baghouse temperature test. The relative humidity at a baghouse temperature of 91 "C is about half of the relative humidity at 73 "C, corresponding to
an observed decrease in NO, removal from 28%to 9%. The elevated temperature substantially inhibited the NO, removal, highlighting the importance of relative humidity on solid particle sorption in the baghouse. The influence of NO2 concentration on the NO2 removals a t constant lime molar flow rate, SO2 concentration, and AT,, is shown in Figure 4. The NO2 removal efficiency, based upon the inlet NO2 concentration, decreases in the spray dryer with increasing NO2 concentration. The baghouse NO2 removals, system removal minus spray dryer removal, increase with increasing NO2 concentration. At low inlet NO2 concentrations of 120 ppm only about 1/3 of the NO2 entering the baghouse (75 ppm) is removed. At high inlet NO2 concentrations of 430 ppm, over half of the NO2 entering the baghouse is removed. Although up to 72% NO2 removals were obtained, concentrations of NO2 that exited the system were greater than 50 ppm for the range of inlet NO2 concentrations investigated. The lime utilized by the NO,, determined from the gas phase mass balance, is also shown in Figure 4. Since the molar flow rate of lime is constant for these tests, increasing the NO2 concentration results in an increasing Ca utilization (for the nitrogen species). The importance of the stoichiometric feed of hydrated lime on the NO, removal for the baseline spiking case of 2050 ppm SO2 and -430 ppm NO2 is shown in Figure 5. The NO, removal increases in both the spray dryer and baghouse with an increasing lime feed rate. Lime utilized by the nitrogen species decreases in the spray dryer with increasing equivalence ratios. However, the utilization (NO,) across the entire system initially increases with an increasing lime feed rate before decreasing at the higher stoichiometric feed rates. This may be a direct result of high utilization of lime by the sulfur species at the lower feed rates (84% utilization for the sulfur species at an equivalence ratio of 0.91, resulting in the low availability of lime for the NO, species. In an attempt to increase the equivalence ratio
"1
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2753
SO2=2O50 p p m NOx=550 p p m N02=430 pprn
60
M
50
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-
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NOx Removal Spray Dryer 0-
Ca Utllization D....
lo
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I ,
0
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1000
SO,
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3000
Concentration, ppm
Figure 7. Effect of SO2 concentration on NO, removal.
TaI=14 Co SO2=18O p p m NOx=540 pprn N 0 2 = 4 4 0 p p
I
0
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0
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1
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2
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a
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C a / ( S O , + 1 /2(NOx))
Figure 6. Effect of stoichiometric feed rate on NO, removal.
as well as determine the impact of the presence of SOz, a series of tests was conducted over a wide range of lime molar feed rates (similar to Figure 5) without SO2 spiking. The results, shown in Figure 6, indicate that increasing equivalence ratios above 3.8does not result in any noticeable increase in NO, removal in the spray dryer and across the system. However, the system data are scattered and the regression fit is poor. The influence of SO2 concentration, for a constant stoichiometric feed rate, on the NO, removal is shown
in Figure 7. The least squares fit of the gas phase measurements indicate that as the SO2 concentration increases, slightly higher NO, removals occur in both the spray dryer and across the system. However, the fit of the least squares regression is poor. The data in Figure 7 suggest that there is a significant decrease in the Ca utilization for the nitrogen species with increasing SO2 concentration (at a constant equivalence ratio). In an attempt to further identify the influence of the sulfur species in the system on the NO, removal, data from Figures 5-7 are combined to make Figure 8. In Figure 8, lime utilizations (NO,) are plotted as a function of the hydrated lime molar feed rates for all tests with the baseline NO2 injection (550 ppm NO, and 430 ppm NO21 which cover a range of SO2 concentrations. The utilization of lime in the spray dryer, for the NO, species, is unaffected by the presence of the sulfur species over the range of equivalence ratios and SO2 concentrations investigated. While there is scatter in the results for the system utilizations, there seems t o be an increase in the lime utilization (NO,) in the baghouse when SO2 is present. SO2 Removal. The effect of the approach to the adiabatic saturation temperature on the SO2 removal for the different flue gas spiking conditions is shown in Figure 9. Decreasing the ATas significantly increases the SO2 removals in the spray dryer as well as the baghouse for all the conditions studied. Lower AT,, values result in longer droplet drying times in the spray dryer and increased relative humidities in the baghouse. Decreasing the spray dryer inlet temperature results in decreased spray dryer ,902 removals. A decrease in the spray dryer inlet temperature at a constant stoichiometric ratio requires less water to be injected into the spray dryer, thereby decreasing the slurry droplet drying time. The influence of the drying time and relative humidity on , 9 0 2 removals in a spray dryer and baghouse is well established in the literature (Huang et al., 1988).
2754 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 '
N O X = 5 5 0 p p m N0;=430
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ppm
Ca Utilization 0 Spray Dryer System
2.0
1
0
2 4 5 3 4 Ca(OH), Feed Rate ( x 1 0 ), moles/min
Figure 8. Effect of flue gas composition and lime feed rate on Ca utilization for the nitrogen species. 100
90 80 70 60 50 0 CY
0" vr
OC
0
0
-0
To,= 14 Co 50,=2040 pprn N O x = 5 5 0 pprn NO,=420 Removal
S.D.
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40
SO2=2O50 p p m Ca/S=l.4 0 Spray Dryer Inlet Ternperature=l77 C
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NO, Spiking ( 4 3 0 p p m NO,) Without NOx Spiking NO Spiking ( 5 5 0 p p m )
10 0
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20
25
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Co
Figure 9. Effect of NO2 and the approach to the adiabatic saturation temperature on SO2 removal.
The influence of NO2 on the SO2 removal can also be seen in Figure 9. The SO2 removal in the spray dryer is not influenced by the presence of NO2. However, the presence of NO2 seems to enhance the baghouse SO2 removal. The influence of the stoichiometric feed rate on the SO2 removal and resulting lime utilization (SO21 is
Figure 10. Effect of lime stoichiometric feed rate on SO2 removal and Ca utilization for the sulhr species.
shown in Figure 10. As expected, increasing the lime feed rate results in increased removals and decreased lime utilizations in the spray dryer and across the system. Also, it was found that increasing the inlet SO2 concentration, while maintaining a constant stoichiometric feed rate, results in decreased relative removals in the spray dryer. A more detailed discussion of the variables influencing SO2 removal in these tests is given by O'Dowd et al. (1994). Solids. The solids sampling and analysis are conducted for two purposes: to verify the gas phase measurements and also t o obtain insight into the removal chemistry of the system. Comparison of the Ca utilization determined from the nitrogen captured in the solids to the Ca utilization determined from the gas phase mass balance for NO, is shown in Figure 11. While there is scatter in the data, there is reasonably good agreement between the collected solids and the gas phase measurements. The comparison of the Ca utilization determined from the sulfur captured in the solids to the gas phase mass balance for the SO2 shows similar behavior t o Figure 11, with 80% of the points falling within 3~20%.The largest deviation between the solids and gas phase measurements is seen at the low gas phase utilization values (for both the nitrogen and sulfur species) which occw when no or very low concentrations of NO2 and SO2 are present in the flue gas. Under these conditions, a large variation is observed in the solids data, especially the baghouse solids data. This probably results from the mixing of solids from one test into another test because of the difficulty in fully cleaning the baghouse at any given time. Total nitrogen is determined by LECO CHN analysis, and the nitrogen speciation is determined by ion chromatography (IC). The comparison of the two techniques for all the samples collected showed average closures between the techniques (ICLECO) of 64% from solids collected before the baghouse and 68% for the solids collected from the baghouse. This discrepancy between
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2765 0
, Spray Oryer System
100
,'
Case A S0,=2050 ppm B S0,=2050 ppm, NO,=420 ppr C S02=180 ppm, N 0 2 = 4 4 0 pprn
7.h 9.
90 Spray Dryer Baghouse
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-0
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70
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,,y 60 0" v,
50
OS 0
40
2
4
6
8
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10
Ca Utilization (Gas Phase), %
Figure 11. Panty plot of lime utilizations for the nitrogen species.
the values may result partially from the existence of sulfur-nitrogen compounds, which could not be measured by ion chromatography. Results indicate the possible presence of sulfur-nitrogen compounds when solids data from the baseline NO2 injection tests without SO2 spiking are compared with data from the baseline NO2 injection tests with SO2 spiking (2050 ppm SO2). When significant quantities of SO2 are not present, the closure between the nitrogen species quantified by IC and total nitrogen by LECO is 84%. When significant quantities of SO2 (2050 ppm) are present in the flue gas, the nitrogen closure drops to 65%. The predominant nitrogen species in the collected solid samples during NO2 injection, with and without SO2 injection, is nitrite. The average nitrite concentration is between 50 and 60 mol % for all tests with NO2 injection. The comparison between the total sulfur (LECO)and the ion chromatography results shows good agreement. For all the solids collected, the average closure of ion chromatography values with LECO results was 94% for the spray dryer solids and 93% for the baghouse solids. Excluding five samples from each group (which showed >60% closure), the sulfur closure (ICLECO) was 100% from the spray dryer and 103%from the baghouse. The sulfur content is usually at least an order of magnitude greater than nitrogen content and may account for some of the improved agreement of the sulfur closures versus the nitrogen closures. Varying the ratio of the spiking gases results in very noticeable trends in the sulfur speciation found in the solid byproducts. Three different spiking gas variations and the resulting influence on the amount of sulfite present in the solids are shown in Figure 12. When SO2 is injected and NO2 is not present (case A), the measured molar ratio of sulfite to the total sulfur in the solid byproduct is 91% in the spray dryer and 85% in the baghouse. When NO2 is introduced into the system a t the baseline SO2 spiking of 2050 ppm (case B), the ratio of sulfite to total sulfur decreases to 68% in the spray dryer and 63% in the baghouse. Case C shown in
C
B
Figure 12. Speciation of captured sulfur for three different flue gas compositions. 50 Cote A S O 2 = 2 O 5 0 ppm B SO2=2O50 ppm, N O Z = L 2 C ppm C S02=180 ppm, N 0 2 = 4 4 0 ppm 40
@
Spray Dryer Baghouse
30
-a0
I
+ N O
y 20 II
n
0 0
10
0
A
B
C
Figure 13. Carbonate content of spent lime for three flue gas compositions.
Figure 12 is for NO2 injection without SO2 spiking. With SO2 present from the combustion of the fuel oil, the sulfite composition decreases to 50% in the spray dryer and 47% in the baghouse. Clearly, increasing the ratio of NO2 to SO2 in the system results in decreasing sulfite concentrations in the byproduct solids. Correspondingly, the sulfate concentration is found t o increase proportionally. The carbonate content of the spent lime for the different spiking gas ratios is shown in Figure 13. For case A and case B where baseline SO2 spiking (2050
2756 Ind. Eng. Chem. Res., Vol. 33, No. 11,1994
ppm) and baseline SO2 and NO2 (430 ppm) spiking occur, respectively, the carbonate content was around 1 3 % in both the baghouse and the spray dryer solids. However, for case C where only NO2 is being injected, the carbonate content of the lime increases to 33%.
Summary and Conclusions A series of tests was conducted to characterize the potential for NO2 removal in a spray dryerhaghouse system. The results of this test program indicate the following: 1. Substantial removal of NO, can be obtained in a lime spray dryerhaghouse system when the NO, in the flue gas stream is predominately NO2. 2. System NO, removals increase significantly at lower approaches to the adiabatic saturation temperature. 3. For a flue gas containing substantial concentrations of NO2 ('400 ppm), over half of the NO, removal occurs in the baghouse. 4. The NO, capture in the baghouse is strongly influenced by the baghouse temperature and the relative humidity of the flue gas. 5. Increasing the NO2 concentration (at a constant lime feed rate) results in a decreasing spray dryer NO2 removal and increasing baghouse NO2 removal. 6 . Increasing the lime feed rate (equivalence ratio ranging from 0.9 to 1.8), with significant quantities of SO2 present, results in a substantial increase in baghouse NO, removal. 7. The presence of significant quantities of SO2 in the inlet flue gas does not influence lime utilizations for the NO, in the spray dryer, while slightly enhancing baghouse utilizations for the NO, species. 8. The presence of NO2 in the flue gas enhances baghouse SO2 removals. 9. As the ratio of NO2 to SO2 increases in the flue gas, the formation of sulfate in the spent lime increases. 10. The NO, captured in solids is 50-60 mol % nitrite, when NO2 is present in significant quantities. 11. In a flue gas containing NO2 and low concentrations of SO2 (180 ppm), CO2 is readily absorbed and converted to carbonate in the spent lime.
Hreha for the solids analysis. Reference in this paper to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply endorsement or favoring by the United States Department of Energy.
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Received for review September 29, 1993 Revised manuscript received J u n e 8, 1994 Accepted July 5 , 1994@
Acknowledgment The authors would like to acknowledge the efforts of Chuck Perry, Jack Thoms, and Dave Benson for operations, Joe Niedzwicki for instrumentation, and Deborah
Abstract published in Advance ACS Abstracts, September 15, 1994. @
