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Nitrogen oxides pre-ozonation in flue gases from phosphate rock digestion
Chemical Engineering Journal xxx (2016) xxx–xxx
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Nitrogen oxides pre-ozonation in flue gases from phosphate rock digestion Kinga Skalska a,⇑, Stanisław Ledakowicz a, Robertus Louwe b, Radosław Szymczak b a b
Faculty of Process and Environmental Engineering, Lodz University of Technology, ul. Wolczanska 213, 90-924 Lodz, Poland Yara Technology Centre, Hydrovegen 67, 3936 Porsgrunn, Norway
h i g h l i g h t s Pre-ozonation of off gases from phosphate rock digestion is a promising FGT technique. It was proven that it is an efficient and safe way to intensify NOx removal. The required molar ratio O3/NOx equal to 1 was sufficient in pilot scale NOx removal. It was proven in the lab-scale that presence of H2O enhanced NOx conversion to N2O5.
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: NOx Emission abatement Pre-ozonation Pilot scale
a b s t r a c t In this study we demonstrate both laboratory and pilot scale results for NOx ozonation of the simulated and real flue gases from the NPK fertilizer production plant. The off-gases from phosphate rock digestion process contain nitrogen oxides NOx, H2O vapor, hydrogen fluoride (HF) and silicon tetra-fluoride (SiF4). The usefulness of the pre-ozonation as a method for the intensification of NOx removal was proved. The required molar ratio O3/NOx together with the residence time and the influence of water vapor were determined. The O3/NOx ratio turned out to be around 1 for the industrial process. The presence of water vapor enhances the conversion rate from 5% to 63% for residence time lower than 1 s. We had confirmed for the first time that the pre-ozonation of flue gases from phosphate rock digestion process is a viable solution for the NOx removal. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction There are two common ways of producing NPK fertilizers based on phosphate rock digestion. The most popular method utilizes sulfuric acid, however this process produces huge amounts of useless phosphate gypsum. More reasonable method of phosphate rock digestion is the nitrophosphate process where concentrated nitric acid is used. Unfortunately, this technology generates NOx emissions. The most important stage of this technology with respect to the NOx emission is the first step – phosphate rock digestion. The main reaction of phosphates with nitric acid is as follows:
Ca5 FðPO4 Þ3 þ 10HNO3 ! 3H3 PO4 þ 5CaðNO3 Þ2 þ HF
ð1Þ
Apart from the nitrogen oxides liberated from the process, the off-gases contain also water vapor, HF, SiF4 and entrained droplets of HNO3. The NOx (mostly NO2) concentration in the industrial ⇑ Corresponding author. E-mail address: [email protected] (K. Skalska).
off-gases is site dependent but tends to be around 500 mg/Nm3 [1]. These gases are commonly vented to a scrubbing system – an emission control technique. Another technique to reduce the NOx emission from this processes is the urea addition to the digester. This solution has one significant disadvantage – emission of dinitrogen oxide (N2O) which is the greenhouse gas, one with a global-warming potential 300 times that of CO2. Nowadays, various emission abatement technologies (selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), reactive absorption, electron beam flue gas treatment, etc.) are available to reduce the NOx emissions into the atmosphere. Due to the tightening of NOx emission regulations, studies are under way around the world to find new and better solutions. One such state of the art technology is flue gas pre-ozonation process for the intensification of NOx emission control technologies. Various ideas for the utilization of ozone for NOx emission can be found described in earlier literature (Fig.1). Almost 80 years after Tesla’s patent for the first ozone generator, the NO ozonation into NO2 combined with absorption process
http://dx.doi.org/10.1016/j.cej.2016.06.048 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Skalska et al., Nitrogen oxides pre-ozonation in flue gases from phosphate rock digestion, Chem. Eng. J. (2016), http:// dx.doi.org/10.1016/j.cej.2016.06.048
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Fig. 1. Research and application paths in the field of ozone application.
was proposed by Picchierauri et al. [2]. Even though 80 years have passed the production yield of ozone generators of the time was insufficient for industrial applications. This seems to be the main reason why the pre-ozonation was not implemented at that time. Nevertheless, the efforts of many scientists were still focused on the utilization of ozone for flue gas treatment. Ozone can be applied into the industrial exhaust gas stream in order to convert NO to NO2. The attention of a group of researchers have focused on homogeneous ozonation of NO into NO2. The combination of this process with SCR or absorption was studied [3–9]. A different solution is based on the idea that if dinitrogen pentoxide (N2O5) is formed in the reaction of ozone with NO, then in the absorption process only nitric acid will be formed. It was reported in 2001 [10] that this approach can lead to a 90% reduction of NOx emission. The application of ozone for the oxidation of NOx into nitrogen species at higher oxidation state, with higher solubility in water was a subject of several publications already [11–15]. The dependence of the Henry’s constants for these species on the temperature is presented in Table 1. In the room temperature the Henry’s parameters for NO, NO2 and N2O5 are equal to 1.87105, 1.18104 and 2.07 102 kmole m3 kPa1, respectively. The Henry parameter for N2O5 is higher than for NO by 3 orders of magnitude and for NO2 by two orders of magnitude. Therefore, conversion of both NO and NO2 into N2O5 will result in significant increase in the absorption process efficiency. Table 1 Henry’s law Constants (ci = Hpi) [16]. Nitrogen species
Name
Henry’s parameter kmole m3 kPa1
NO
Nitrogen monoxide
1:87 105 exp 1500
NO2
Nitrogen dioxide
1 1 T 298 1 1:18 104 exp 2500 1T 298 1 4 1 3:36 10 exp 2000 T 298
NO3
Nitrogen trioxide
N2O3
Dinitrogen trioxide
5:92 103
N2O4
Dinitrogen tetroxide
1:38 102
N2O5
Dinitrogen pentoxide
HNO3
Nitric acid
1 2:07 102 exp 3400 1T 298 1 4:83 exp 4800 T1 298
In the present work, the employment of the pre-ozonation was proposed as a possible solution to the NOx emission problem during phosphate rock digestion process with nitric acid. The main aim of ozone injection into the flue gas stream is to oxidize NOx into nitrogen oxides of higher oxidation state, mainly dinitrogen pentoxide. This change in the flue gas composition is really beneficial for all absorption processes. Firstly because N2O5 is better soluble in water than NO and even NO2 (Table 1). Secondly as already mentioned, N2O5 in the reaction with water leads to the formation of only HNO3. During NO and NO2 absorption in water equimolar amounts of both HNO2 and HNO3 are formed. This problem has been addressed previously [12,17,18]. The pre-ozonation can be used, therefore as a NOx removal intensification technique. In our previous works we proved that for the flue gas containing only nitrogen monoxide the required O3/NO molar ratio (MR) needed to convert NOx into N2O5 and HNO3 is equal to 1.5 [11–14]. Nitric acid is formed as a result of the reaction (Eq. (2)) that takes place when even a small amount of water vapor is present in the reaction gases.
N2 O5 þ H2 O ! 2HNO3
ð2Þ
Industrial applications as the one described by Omar (2008) made it clear that for real off-gases the required molar ratio O3/NO will differ based on the off-gas parameters [19]. Although the pre-ozonation is not an entirely new technique for the NOx emission removal, the application of ozone for the NOx emission reduction in phosphate rock digestion was not described in the literature so far. This application of a pre-ozonation process is unique in the sense of flue gas composition. In general for the incineration processes in power plants the flue gases NOx stream compose mostly of 95% NO and 5% NO2. The exhaust gases besides NOx contain also sulfur dioxide, mercury, etc. In the case of the nitric acid production process the ratio NO:NO2 in the flue gases is 1 to 1. Conversely, in the case of the phosphate rock digestion process with nitric acid the off-gases contain mostly NO2 accompanied by water vapor, HF, SiF4 and entrained droplets of HNO3 and dust. Therefore, an experimental assessment of effectiveness of NOx oxidation by means of ozone is necessary for such a complex system.
Please cite this article in press as: K. Skalska et al., Nitrogen oxides pre-ozonation in flue gases from phosphate rock digestion, Chem. Eng. J. (2016), http:// dx.doi.org/10.1016/j.cej.2016.06.048
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In order to check this statement the experiments were carried out both in the laboratory scale and the pilot scale for the real flue gases generated during the digestion process of phosphate rock. Previously the process of NOx ozonation was proven to work for the off-gases from the nitric acid production pilot plant [14]. However, the composition and characteristics of flue gases from both processes differ a lot. This way, the influence of the presence of additional substances (HF, SiF4, H2O, dust, etc.) in the flue gas was verified. The aim of current paper is to analyze the optimal conditions for ozone reaction with NOx in flue gases from the phosphate rock digestion process by the nitrophosphate route.
2. Experimental Three series of experiments were carried out: two at laboratory scale and one for the real off-gases from the phosphate rock digestion process in a pilot plant. Such an approach was adapted since the composition of the flue gas stream from the phosphate rock digestion process is unique and the process had to be first studied with the model gases, then gases from the digestion process, was carried out in the laboratory, and finally with the real off-gases at pilot scale. In the lab-scale experiments we first focused on the determination of the required molar ratio O3/NOx for gases containing only nitrogen dioxide. The second series of the experiments was performed in order to assess the influence of other components of the flue gases on the ozonation process efficiency. In order to scale-up the ozonation process the experiments were performed in Yara’s phosphate rock digestion pilot plant.
2.1. Materials and methods 2.1.1. Lab scale – simulated off gases A series of experiments were conducted with nitrogen oxides commonly occurring in flue gases from chemical industry in particular nitrogen dioxide, the main component of NOx formed in the nitrophosphate process. The experiments were carried out for the NOx concentrations in the range 5.3–23106 mol/L (120–520 ppm) and molar ratio O3/NOx around 0.5, 0.75 and 1. The flow rate of the reaction mixture was set to provide a residence time of 10 s. The overview of the experimental set-up used in these experiments was previously described together with calibration procedure etc. [12].
3
2.1.2. Lab scale – digestion off gases The aim of these experiments was to evaluate the effectiveness of the ozonation process on the batch digestion off-gases. A batch process is by nature an unstable process, therefore stable periods of NOx emission were observed and then chosen to study the NOx ozonation. The digestion of phosphate rock with concentrated nitric acid was performed in a glass reactor (three neck flask with flat bottom) situated over a heated magnetic stirrer (Fig. 2). The temperature of the digestion process was controlled in the range 60–70 °C and three mixing speeds were used: 200, 300 and 350 rpm. It enabled to obtained quasi-stable NO2 concentrations in the gas phase around 130 and 250 ppm. The Gasmet portable DX-4000 analyzer equipped with the gas cell with optical path: 2.5 m was used. 2.1.3. Pilot plant off gases The pilot scale tests were conducted in the NPK production pilot plant located in Yara’s Technology Centre in Porsgrunn, Norway. The series of experiments were conducted on real off-gases from a continuous phosphate rock digestion process. The digestion process parameters were manipulated to obtain a stable NO2 concentration around 400 ppm. Different flow rates as well as different sampling points were used in order to verify ozonation process efficiency for various residence times. The off-gases from the reactor were vented into the absorption columns (scrubbers) that are part of the pilot plant. The modification was introduced into the off-gas line. An additional segment of venting line was introduced with the whole length of 100 m. Three measuring points were installed (Fig. 3): P0 P1 P2 P3 P4
– measurement point for flue gas velocity, – before ozone injection (residence time 0 s), – after ozone injection (residence time around 3 s), – after ozone injection (residence time around 7 s), - after ozone injection (residence time around 10 s).
The ozone generation equipment was supplied by Redox AS. It consisted mainly of an oxygen generator, an ozone generator (OZAT CFS-7 2G – nominal O3 production from O2 up to 500 g/h) and a control system. The total flue gas flow rate was kept between 49 and 73 m3/h. The flow rate of oxygen-ozone mixture stream was not changed and was equal to 2 m3/h. The ozone-oxygen mixture was injected through a Venturi mixer (Mazzei Injector Company) (0.5 m after first measuring point P1). An HACH Indigo Method No. 8311 using AccuVac Ampuls was used to analyze the
Fig. 2. Experimental set-up 2. 1 – air gas cylinder, 2 – oxygen gas cylinders, 3 – mass flow meters, 4 – mass flow meter regulator, 5 – inlet concentration ozone analyzer, 6 – ozone generator, 7 – magnetic stirrer, 8 – digester, 9 – reactants addition, 10 – ozonation reactor, 11 – flue gas conditioner, 12 – FTIR spectrometer Gasmet portable DX-4000.
Please cite this article in press as: K. Skalska et al., Nitrogen oxides pre-ozonation in flue gases from phosphate rock digestion, Chem. Eng. J. (2016), http:// dx.doi.org/10.1016/j.cej.2016.06.048
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Fig. 3. Simplified pilot scale trial set-up. Table 2 Process parameters in the pilot scale. Day
No
Off-gas velocity m/s
NO2 initial ppm
Ozone flow rate m3/h
CinO3 g/m3
O3/NO2
T o C
Total flow rate m3/h
Residual ozone sample
3
3.1.1 3.1.2 3.2.1 3.2.2 3.2.3 3.2.4 4.1.1 4.1.2 4.1.3 4.1.4 4.1.5 4.1.6 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6 4.2.7 4.2.8
9 9 7.7 7.7 7.7 7.7 7.2 7.2 7.2 7.2 7.5 7.5 7.1 6.7 7 7 7 7 7 7
400 403 341.9 341.9 341.9 341.9 361.3 361.3 361.3 361.3 361.3 361.3 423.5 423.5 423.5 423.5 423.5 423.5 423.5 423.5
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
31.4 29 9.9 14.3 21.3 14.8 4.7 8.8 12 15 21.1 23.7 5.1 8.2 6 8.5 13.1 19.5 19.9 26
0.94 0.93 0.44 0.64 0.95 0.66 0.22 0.4 0.55 0.69 0.97 1.09 0.2 0.32 0.23 0.33 0.51 0.75 0.77 1
27.7 27.7 26.1 26.1 26.1 26.1 35 35 35 35 35 35 39.8 39.8 39.8 39.8 39.8 39.8 39.8 39.8
73.5 73.5 63.2 63.2 63.2 63.2 59.2 59.2 59.2 59.2 61.6 61.6 58.4 55.2 57.6 57.6 57.6 57.6 57.6 57.6
+ +
4
residual ozone concentration in the reaction gases. The process parameters for this trials were gathered in Table 2, together with the data about the residual ozone sample collection.
+
+ +
Although small increases in the conversion rate can be observed for smaller NOx initial concentrations, the obtained NOx conversion depends both on the applied molar ratio O3/NOx and initial
3. Results and discussion 3.1. Simulated flue gas lab-scale tests The laboratory studies with simulated flue gas (composed of NO2, O2 and N2) were conducted for different molar ratios O3/NOx and different initial NO2 concentrations. The stoichiometric-theoretical molar ratio is equal to 0.5 for NO2 ozonation into N2O5, according to summary reaction (3):
2NO2 þ O3 $ N2 O5 þ O2
ð3Þ
As it can be seen in Fig. 4 the stoichiometric molar ratio O3/NOx (equal 0.5) is insufficient for the complete conversion of nitrogen dioxide for the applied residence time (around 10 s). The increase in the NOx initial concentration does not lead to the NOx conversion increase, the observed conversion is around 60% at the stoichiometric O3/NOx ratio equal to 0.5. On the other hand, for a molar ratio around 1, even at lower NO2 concentrations the conversion rate reached 90%.
Fig. 4. NOx conversion versus NOx initial concentration for molar ratios O3/NOx: 0.5, 0.75 and 1.
Please cite this article in press as: K. Skalska et al., Nitrogen oxides pre-ozonation in flue gases from phosphate rock digestion, Chem. Eng. J. (2016), http:// dx.doi.org/10.1016/j.cej.2016.06.048
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concentration of nitrogen dioxide. However, the impact of the NOx initial concentration is not as significant as the influence of O3/NOx ratio applied. For the almost complete NOx conversion a molar ratio O3/NOx equal to 1 was necessary. The reaction (Eq. (3)) is a summary reaction of the NO2 ozonation process, which follows:
NO2 þ O3 ! NO3 þ O2
ð4Þ
NO2 þ NO3 $ N2 O5
ð5Þ
The possible reason for the increased O3 consumption in respect to theoretical one is the reversibility of reaction (Eq. (5)) The typical FTIR spectra for nitrogen dioxide before and after the reaction with ozone were presented in Fig. 5. The formation of the reaction products in respect to molar ratio O3/NOx (FTIR spectra for the nitrogen dioxide ozonation with various molar ratios O3/NOx: 0, 0.5, 0.75, 1) was shown. The main products observed were dinitrogen pentoxide (N2O5) and nitric acid (HNO3). The absorption bands at 1245 cm1 and 745 cm1 can be assigned to N2O5, whereas peaks at 1316 cm1 and 887 cm1 are connected with the presence of HNO3. The peak at around 1700 cm1 can be attributed both to the presence of N2O5 and HNO3 formed during ozonation of NO. The presence of residual ozone was also detected and analyzed based on the peak height (1051 cm1). The analysis of selectivity of NO2 ozonation into N2O5 and HNO3, calculated according to the formulas:
SNO2intoN2O5 ¼
2ð½N2 O5 t ½N 2 O5 0 Þ ½NO2 0 ½NO2 t
ð6Þ
SNO2intoHNO3 ¼
½HNO3 t ½HNO3 0 ½NO2 0 ½NO2 t
ð7Þ
contrast, in the process when carried out with O3/NOx molar ratio 0.5, the ozone residual concentration remains almost constant at the level on around 1106 mol/L, clearly pointing to ozone deficiency. Hence, incomplete NO2 ozonation took place.
3.2. Lab scale – digestion process flue gas The second series of experiments was performed on the real flue gases coming from a batch digestion process (NO2 initial concentrations around 130 ppm and 250 ppm). The required molar ratio O3/NOx turned out to be higher (1.7) than observed for the simulated flue gases (Table 3). It is important to note that in these series of experiments the residence time was around 5 s. Therefore, the observed behavior can be explained firstly by the shorter
showed that their values were almost independent on the applied molar ratio O3/NOx in the range 0.5–1 (Fig.6). Above the NOx initial concentrations equal to 7.5106 mol/L the selectivity SNO2intoN2O5 did not change significantly with the increasing NOx concentration. Around 30% of NO2 was oxidized into dinitrogen pentoxide. The selectivity SNO2intoHNO3 grew slightly from 6% to 10% with the increasing NOx concentration. The residual ozone concentration in the off-gases was also measured. As expected, the amount of the residual ozone rises with the increase of the molar ratio of O3/NOx used. Similar conclusions were previously drawn for the nitrogen monoxide ozonation [11]. The increase in residual ozone concentration for experiments carried out with O3/NOx molar ratio around 0.75 and 1 proves that the NOx conversion reached its maximum values since ozone is no longer consumed in the process (Fig. 7). In
Fig. 6. Product selectivity versus NOx initial concentration.
Fig. 7. Residual ozone concentration versus initial NOx concentration.
Table 3 NOx removal efficiency for the batch digestion process.
Fig. 5. FTIR spectra of reaction gases before and after the reaction with ozone.
Initial NO2 concentration ppm
Molar ratio O3/NOx mole/mole
Obtained NOx removal %
140 190 138 145
0.5 0.8 1.1 1.7
43 50 83 100
Please cite this article in press as: K. Skalska et al., Nitrogen oxides pre-ozonation in flue gases from phosphate rock digestion, Chem. Eng. J. (2016), http:// dx.doi.org/10.1016/j.cej.2016.06.048
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Fig. 8. Influence of humidity in flue gases on the NO2 ozonation for various molar ratios O3/NOx.
residence time. Hence, in the series of coupled and subsequent reactions that take place in the system the obtained conversion rate is smaller. Further to the above, based on the kinetic equations of NO2 ozonation [12] the rate of reaction is dependent also on the initial NO2 concentration not only the amount of the oxidant supplied. Nevertheless, this experiment enabled to determine the influence of other process gas components (HF, SiF4,CO2, HNO3g, H2O). It was observed that apart from water vapor none of the extra contaminants consumes large quantities of ozone. The influence of water vapor was confirmed by tests with the use of 836 ppm of NO2 gas stream (RT = 0.75 s.). It was shown that presence of water vapor in flue gases increases the NO2 conversion rate (Fig. 8). The influence of the water vapor content was growing with the increasing O3/NOx ratio. The main explanation is that in the presence of water vapor the reaction (Eq. (2)) occurs, therefore N2O5 is consumed and the equilibrium of reaction (Eq. (5)) shifts to the right in accordance with Le Chatelier’s principle. Since ozone can react in water to form hydroxyl radicals in series of chain reactions [20]: O3 þ OH ! O 2 þ HO2
ð8Þ
HO2 $ Hþ þ O 2
ð9Þ
pkA ¼ 4:8
reactions are caused by direct ozonation and not indirect (through hydroxyl radicals). Furthermore, as we proved previously O3 solubility in the acid solutions decreases with increasing acid concentration [17]. Therefore, we believe that the NO2 removal enhancement in the presence of water vapor is mainly a result of the already mentioned equilibrium shift to the right of reaction (Eq. (5)). 3.3. Pilot plant off gases In the final series of experiments performed on the real flue gases the efficiency of the ozonation process was evaluated in gases that contained water vapor, HF, SiF4 and post-digestion dust. Exemplary FTIR spectra are presented in Fig. 9. The peaks that correspond to HNO3 (1700, 1316 and 887 cm1) can overlap with the peaks characteristic for the presence of HF in the off-gases. Therefore, nitric acid readings cannot be considered as a reliable representation of the ozonation process effectiveness. The broad peaks around 4000–3500 cm1 and 2000–1200 cm1 confirm that water vapor is present in the tested gases. The peaks around 1026 cm1 can be assigned to SiF4. The peaks around
Then the radical exchange reactions occur: O3 þ O 2 ! O3 þO2
ð10Þ
þ O 3 þ H ! HO3
ð11Þ
HO3 ! O2 þ HO
ð12Þ
and the hydroxyl radicals are formed. If they get in contact with NO2 molecules an additional set of reactions can also take place increasing overall conversion rate of NO2, therefore more NO2 can be removed from the off-gases, according to reactions [21]:
NO2 þ OH þ M ! HNO3 þ M
ð13Þ
NO2 þ HO2 þ M ! HNO2 þ O2
ð14Þ
However, importance of the chain reactions in which radicals are formed will definitely decrease in the real off-gases from phosphate rock digestion process where concentrated HNO3 is used, because the off-gases contain acid droplets. It is well known fact in the water chemistry that in acidic conditions the ozonation
Fig. 9. Exemplary FTIR spectra for off-gases before and after ozonation (spectra selected from experiments conducted on day 3).
Please cite this article in press as: K. Skalska et al., Nitrogen oxides pre-ozonation in flue gases from phosphate rock digestion, Chem. Eng. J. (2016), http:// dx.doi.org/10.1016/j.cej.2016.06.048
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Fig. 10. NO2 concentration profile during experiment 3.2.1-3 (residence time = 3 s).
1630 cm1 and 2920 cm1 that correspond to NO2 presence are visible for off-gases before ozonation. What is more, this peak clearly decreases in the spectrum taken for the off-gases after ozonation. Therefore, in order to verify the effectiveness of ozonation processes the analysis of only the NO2 concentrations was done. Nitric oxide is present in the gases in the concentrations as low as 5 ppm. Therefore, NOx stream composed mainly of NO2. In the presence of ozone-oxygen mixture NO concentrations dropped to 0. The examination of the obtained results showed that for all the measured species (except NO2, NO, HNO3) no influence of the ozonation process on their concentrations is observed. Therefore, it can be concluded that they do not react with ozone. However, we cannot exclude the inhibitory action of these gas components. Fig. 10 presents results of the series of experiments with the same residence time (RT) but increasing O3/NO2 ratio (0.44, 0.64, 0.95). Some variation in NO2 concentration was observed during ozonation of real flue gases. Few sharp peaks can be observed, they correspond well with the times when antifoaming agent was added into the digestion process (normal operation procedure). This figure shows the measured NO2 concentrations before and during ozonation for NO2 average concentration 341.9 ppm (verified before and after the experiment by switching FTIR unit from the measuring point P3 to P1). It is clear that NO2 removal efficiency rises with the increasing O3/NO2 ratio from 44 to 95 %. In order to analyze the ozonation process in wider range of O3/NO2 ratio (0.2–1) additional two series of experiments were performed with different RTs (3 s and 7 s) (Fig. 11). For under-stoichiometric ratios O3/NO2 (0.2 and 0.3) the conversion degree was low, as expected, for both RTs. For stoichiometric O3/NOx ratios around 0.5 the conversion was around 66%. Further increasing MR to around 1 yields almost 100% NO2 removal. Better results were obtained for longer residence time (7 s). Whereas, in the case of the pre-ozonation process tested for the off-gases from nitric acid pilot plant the required O3/NOx ratio was 1.4 [14]. As previously stated the NOx stream in the nitric acid production plant comprises of NO and NO2 in the ratio 1:1. These results indicate the importance of off-gases composition on the effective/required dose of ozone. It is clear that effectiveness of NO2 removal from ozonated flue gases is dependent on the amount of ozone added, more precisely
Fig. 11. Influence of reaction gases residence time on the NO2 conversion efficiency.
on molar ratio of ozone to nitrogen dioxide. Pilot scale results suggest that for compete NO2 removal from the flue gases, a molar ratio O3/NO2 around 1 should be used. This is still higher than would be expected based on reaction stoichiometry (O3/NO2 = 0.5), but lower than it was concluded from the digestion lab-scale experiments. It is important to remember that in the off-gases from phosphate rock digestion many reactions are possible, e.g. ozone decomposition due to the presence of water and dust and N2O5 reverse reaction or possible ozone reaction with other gas impurities etc. This would result in the increase of required O3/NO2 molar ratio. On the other hand presence of H2O enhances N2O5 removal from the reaction mixture by N2O5 absorption into H2O and shifting the reversible reaction (5) to the right side. Further data analysis also proves that the residence time is an important process parameter and increasing the residence time could allow the use of lower O3/NO2 for the complete NO2 removal. Summarizing the results obtained in the pilot scale the surface response plot is presented in Fig. 12, wherein the NO2 concentration in the outlet was plotted versus residence time (RT) and the ozone
Please cite this article in press as: K. Skalska et al., Nitrogen oxides pre-ozonation in flue gases from phosphate rock digestion, Chem. Eng. J. (2016), http:// dx.doi.org/10.1016/j.cej.2016.06.048
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respect to the future application of the pre-ozonation followed by the absorption process in scrubbers. Acknowledgement Dr Kinga Skalska thanks The Kos´ciuszko Foundation for 10-month fellowship in 2015. References
Fig. 12. Surface response plot ([NO2in] = 397.8 ppm).
inlet concentration (O3 in). As can be seen the increase of RT results in the growth of the NO2 removal degree. For molar ratio O3/NO2 equal to 1 (NO2 initial concentration 400 ppm and O3 initial concentration 400 ppm) and residence time around 11 s, the complete conversion of NO2 was be observed. 4. Conclusions Based on the laboratory experiments with simulated off-gases, the recommended molar ratio O3/NOx for the complete removal of the NO2 is around 1, for a residence time around 10 s. The lab-scale results revealed that water content in off-gases has an significant influence on ozonation reaction. Therefore, high humidity of the off-gases will be beneficial, while the presence of other gas components (in digestion off gases) had no effect on the process effectiveness. In the pilot scale studies a residence time around 7 s was sufficient for almost 100% NO2 conversion provided the O3/NOx molar ratio is around 1. The application of ozone to NO2 removal from off-gases liberated by nitrophosphate route was proved to be an effective and a safe solution. The required O3/NOx is dependent on many factors and without doubt it should be determined in situ for the specific industrial gases. The best solution in this case would be the online analysis of flue gas composition. The process parameters and the programmed correction of the O3/NOx ratio should be appropriately adapted. The presented results of the experimental work both in the lab and the pilot scale provided the theoretical basis for the industrial application of the pre-ozonation technology. The above findings are crucial with
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Nitrogen oxides pre-ozonation in flue gases from phosphate rock digestion Kinga Skalska a,⇑, Stanisław Ledakowicz a, Robertus Louwe b, Radosław Szymczak b a b
Faculty of Process and Environmental Engineering, Lodz University of Technology, ul. Wolczanska 213, 90-924 Lodz, Poland Yara Technology Centre, Hydrovegen 67, 3936 Porsgrunn, Norway
h i g h l i g h t s Pre-ozonation of off gases from phosphate rock digestion is a promising FGT technique. It was proven that it is an efficient and safe way to intensify NOx removal. The required molar ratio O3/NOx equal to 1 was sufficient in pilot scale NOx removal. It was proven in the lab-scale that presence of H2O enhanced NOx conversion to N2O5.
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: NOx Emission abatement Pre-ozonation Pilot scale
a b s t r a c t In this study we demonstrate both laboratory and pilot scale results for NOx ozonation of the simulated and real flue gases from the NPK fertilizer production plant. The off-gases from phosphate rock digestion process contain nitrogen oxides NOx, H2O vapor, hydrogen fluoride (HF) and silicon tetra-fluoride (SiF4). The usefulness of the pre-ozonation as a method for the intensification of NOx removal was proved. The required molar ratio O3/NOx together with the residence time and the influence of water vapor were determined. The O3/NOx ratio turned out to be around 1 for the industrial process. The presence of water vapor enhances the conversion rate from 5% to 63% for residence time lower than 1 s. We had confirmed for the first time that the pre-ozonation of flue gases from phosphate rock digestion process is a viable solution for the NOx removal. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction There are two common ways of producing NPK fertilizers based on phosphate rock digestion. The most popular method utilizes sulfuric acid, however this process produces huge amounts of useless phosphate gypsum. More reasonable method of phosphate rock digestion is the nitrophosphate process where concentrated nitric acid is used. Unfortunately, this technology generates NOx emissions. The most important stage of this technology with respect to the NOx emission is the first step – phosphate rock digestion. The main reaction of phosphates with nitric acid is as follows:
Ca5 FðPO4 Þ3 þ 10HNO3 ! 3H3 PO4 þ 5CaðNO3 Þ2 þ HF
ð1Þ
Apart from the nitrogen oxides liberated from the process, the off-gases contain also water vapor, HF, SiF4 and entrained droplets of HNO3. The NOx (mostly NO2) concentration in the industrial ⇑ Corresponding author. E-mail address: [email protected] (K. Skalska).
off-gases is site dependent but tends to be around 500 mg/Nm3 [1]. These gases are commonly vented to a scrubbing system – an emission control technique. Another technique to reduce the NOx emission from this processes is the urea addition to the digester. This solution has one significant disadvantage – emission of dinitrogen oxide (N2O) which is the greenhouse gas, one with a global-warming potential 300 times that of CO2. Nowadays, various emission abatement technologies (selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), reactive absorption, electron beam flue gas treatment, etc.) are available to reduce the NOx emissions into the atmosphere. Due to the tightening of NOx emission regulations, studies are under way around the world to find new and better solutions. One such state of the art technology is flue gas pre-ozonation process for the intensification of NOx emission control technologies. Various ideas for the utilization of ozone for NOx emission can be found described in earlier literature (Fig.1). Almost 80 years after Tesla’s patent for the first ozone generator, the NO ozonation into NO2 combined with absorption process
http://dx.doi.org/10.1016/j.cej.2016.06.048 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.
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Fig. 1. Research and application paths in the field of ozone application.
was proposed by Picchierauri et al. [2]. Even though 80 years have passed the production yield of ozone generators of the time was insufficient for industrial applications. This seems to be the main reason why the pre-ozonation was not implemented at that time. Nevertheless, the efforts of many scientists were still focused on the utilization of ozone for flue gas treatment. Ozone can be applied into the industrial exhaust gas stream in order to convert NO to NO2. The attention of a group of researchers have focused on homogeneous ozonation of NO into NO2. The combination of this process with SCR or absorption was studied [3–9]. A different solution is based on the idea that if dinitrogen pentoxide (N2O5) is formed in the reaction of ozone with NO, then in the absorption process only nitric acid will be formed. It was reported in 2001 [10] that this approach can lead to a 90% reduction of NOx emission. The application of ozone for the oxidation of NOx into nitrogen species at higher oxidation state, with higher solubility in water was a subject of several publications already [11–15]. The dependence of the Henry’s constants for these species on the temperature is presented in Table 1. In the room temperature the Henry’s parameters for NO, NO2 and N2O5 are equal to 1.87105, 1.18104 and 2.07 102 kmole m3 kPa1, respectively. The Henry parameter for N2O5 is higher than for NO by 3 orders of magnitude and for NO2 by two orders of magnitude. Therefore, conversion of both NO and NO2 into N2O5 will result in significant increase in the absorption process efficiency. Table 1 Henry’s law Constants (ci = Hpi) [16]. Nitrogen species
Name
Henry’s parameter kmole m3 kPa1
NO
Nitrogen monoxide
1:87 105 exp 1500
NO2
Nitrogen dioxide
1 1 T 298 1 1:18 104 exp 2500 1T 298 1 4 1 3:36 10 exp 2000 T 298
NO3
Nitrogen trioxide
N2O3
Dinitrogen trioxide
5:92 103
N2O4
Dinitrogen tetroxide
1:38 102
N2O5
Dinitrogen pentoxide
HNO3
Nitric acid
1 2:07 102 exp 3400 1T 298 1 4:83 exp 4800 T1 298
In the present work, the employment of the pre-ozonation was proposed as a possible solution to the NOx emission problem during phosphate rock digestion process with nitric acid. The main aim of ozone injection into the flue gas stream is to oxidize NOx into nitrogen oxides of higher oxidation state, mainly dinitrogen pentoxide. This change in the flue gas composition is really beneficial for all absorption processes. Firstly because N2O5 is better soluble in water than NO and even NO2 (Table 1). Secondly as already mentioned, N2O5 in the reaction with water leads to the formation of only HNO3. During NO and NO2 absorption in water equimolar amounts of both HNO2 and HNO3 are formed. This problem has been addressed previously [12,17,18]. The pre-ozonation can be used, therefore as a NOx removal intensification technique. In our previous works we proved that for the flue gas containing only nitrogen monoxide the required O3/NO molar ratio (MR) needed to convert NOx into N2O5 and HNO3 is equal to 1.5 [11–14]. Nitric acid is formed as a result of the reaction (Eq. (2)) that takes place when even a small amount of water vapor is present in the reaction gases.
N2 O5 þ H2 O ! 2HNO3
ð2Þ
Industrial applications as the one described by Omar (2008) made it clear that for real off-gases the required molar ratio O3/NO will differ based on the off-gas parameters [19]. Although the pre-ozonation is not an entirely new technique for the NOx emission removal, the application of ozone for the NOx emission reduction in phosphate rock digestion was not described in the literature so far. This application of a pre-ozonation process is unique in the sense of flue gas composition. In general for the incineration processes in power plants the flue gases NOx stream compose mostly of 95% NO and 5% NO2. The exhaust gases besides NOx contain also sulfur dioxide, mercury, etc. In the case of the nitric acid production process the ratio NO:NO2 in the flue gases is 1 to 1. Conversely, in the case of the phosphate rock digestion process with nitric acid the off-gases contain mostly NO2 accompanied by water vapor, HF, SiF4 and entrained droplets of HNO3 and dust. Therefore, an experimental assessment of effectiveness of NOx oxidation by means of ozone is necessary for such a complex system.
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In order to check this statement the experiments were carried out both in the laboratory scale and the pilot scale for the real flue gases generated during the digestion process of phosphate rock. Previously the process of NOx ozonation was proven to work for the off-gases from the nitric acid production pilot plant [14]. However, the composition and characteristics of flue gases from both processes differ a lot. This way, the influence of the presence of additional substances (HF, SiF4, H2O, dust, etc.) in the flue gas was verified. The aim of current paper is to analyze the optimal conditions for ozone reaction with NOx in flue gases from the phosphate rock digestion process by the nitrophosphate route.
2. Experimental Three series of experiments were carried out: two at laboratory scale and one for the real off-gases from the phosphate rock digestion process in a pilot plant. Such an approach was adapted since the composition of the flue gas stream from the phosphate rock digestion process is unique and the process had to be first studied with the model gases, then gases from the digestion process, was carried out in the laboratory, and finally with the real off-gases at pilot scale. In the lab-scale experiments we first focused on the determination of the required molar ratio O3/NOx for gases containing only nitrogen dioxide. The second series of the experiments was performed in order to assess the influence of other components of the flue gases on the ozonation process efficiency. In order to scale-up the ozonation process the experiments were performed in Yara’s phosphate rock digestion pilot plant.
2.1. Materials and methods 2.1.1. Lab scale – simulated off gases A series of experiments were conducted with nitrogen oxides commonly occurring in flue gases from chemical industry in particular nitrogen dioxide, the main component of NOx formed in the nitrophosphate process. The experiments were carried out for the NOx concentrations in the range 5.3–23106 mol/L (120–520 ppm) and molar ratio O3/NOx around 0.5, 0.75 and 1. The flow rate of the reaction mixture was set to provide a residence time of 10 s. The overview of the experimental set-up used in these experiments was previously described together with calibration procedure etc. [12].
3
2.1.2. Lab scale – digestion off gases The aim of these experiments was to evaluate the effectiveness of the ozonation process on the batch digestion off-gases. A batch process is by nature an unstable process, therefore stable periods of NOx emission were observed and then chosen to study the NOx ozonation. The digestion of phosphate rock with concentrated nitric acid was performed in a glass reactor (three neck flask with flat bottom) situated over a heated magnetic stirrer (Fig. 2). The temperature of the digestion process was controlled in the range 60–70 °C and three mixing speeds were used: 200, 300 and 350 rpm. It enabled to obtained quasi-stable NO2 concentrations in the gas phase around 130 and 250 ppm. The Gasmet portable DX-4000 analyzer equipped with the gas cell with optical path: 2.5 m was used. 2.1.3. Pilot plant off gases The pilot scale tests were conducted in the NPK production pilot plant located in Yara’s Technology Centre in Porsgrunn, Norway. The series of experiments were conducted on real off-gases from a continuous phosphate rock digestion process. The digestion process parameters were manipulated to obtain a stable NO2 concentration around 400 ppm. Different flow rates as well as different sampling points were used in order to verify ozonation process efficiency for various residence times. The off-gases from the reactor were vented into the absorption columns (scrubbers) that are part of the pilot plant. The modification was introduced into the off-gas line. An additional segment of venting line was introduced with the whole length of 100 m. Three measuring points were installed (Fig. 3): P0 P1 P2 P3 P4
– measurement point for flue gas velocity, – before ozone injection (residence time 0 s), – after ozone injection (residence time around 3 s), – after ozone injection (residence time around 7 s), - after ozone injection (residence time around 10 s).
The ozone generation equipment was supplied by Redox AS. It consisted mainly of an oxygen generator, an ozone generator (OZAT CFS-7 2G – nominal O3 production from O2 up to 500 g/h) and a control system. The total flue gas flow rate was kept between 49 and 73 m3/h. The flow rate of oxygen-ozone mixture stream was not changed and was equal to 2 m3/h. The ozone-oxygen mixture was injected through a Venturi mixer (Mazzei Injector Company) (0.5 m after first measuring point P1). An HACH Indigo Method No. 8311 using AccuVac Ampuls was used to analyze the
Fig. 2. Experimental set-up 2. 1 – air gas cylinder, 2 – oxygen gas cylinders, 3 – mass flow meters, 4 – mass flow meter regulator, 5 – inlet concentration ozone analyzer, 6 – ozone generator, 7 – magnetic stirrer, 8 – digester, 9 – reactants addition, 10 – ozonation reactor, 11 – flue gas conditioner, 12 – FTIR spectrometer Gasmet portable DX-4000.
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Fig. 3. Simplified pilot scale trial set-up. Table 2 Process parameters in the pilot scale. Day
No
Off-gas velocity m/s
NO2 initial ppm
Ozone flow rate m3/h
CinO3 g/m3
O3/NO2
T o C
Total flow rate m3/h
Residual ozone sample
3
3.1.1 3.1.2 3.2.1 3.2.2 3.2.3 3.2.4 4.1.1 4.1.2 4.1.3 4.1.4 4.1.5 4.1.6 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6 4.2.7 4.2.8
9 9 7.7 7.7 7.7 7.7 7.2 7.2 7.2 7.2 7.5 7.5 7.1 6.7 7 7 7 7 7 7
400 403 341.9 341.9 341.9 341.9 361.3 361.3 361.3 361.3 361.3 361.3 423.5 423.5 423.5 423.5 423.5 423.5 423.5 423.5
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
31.4 29 9.9 14.3 21.3 14.8 4.7 8.8 12 15 21.1 23.7 5.1 8.2 6 8.5 13.1 19.5 19.9 26
0.94 0.93 0.44 0.64 0.95 0.66 0.22 0.4 0.55 0.69 0.97 1.09 0.2 0.32 0.23 0.33 0.51 0.75 0.77 1
27.7 27.7 26.1 26.1 26.1 26.1 35 35 35 35 35 35 39.8 39.8 39.8 39.8 39.8 39.8 39.8 39.8
73.5 73.5 63.2 63.2 63.2 63.2 59.2 59.2 59.2 59.2 61.6 61.6 58.4 55.2 57.6 57.6 57.6 57.6 57.6 57.6
+ +
4
residual ozone concentration in the reaction gases. The process parameters for this trials were gathered in Table 2, together with the data about the residual ozone sample collection.
+
+ +
Although small increases in the conversion rate can be observed for smaller NOx initial concentrations, the obtained NOx conversion depends both on the applied molar ratio O3/NOx and initial
3. Results and discussion 3.1. Simulated flue gas lab-scale tests The laboratory studies with simulated flue gas (composed of NO2, O2 and N2) were conducted for different molar ratios O3/NOx and different initial NO2 concentrations. The stoichiometric-theoretical molar ratio is equal to 0.5 for NO2 ozonation into N2O5, according to summary reaction (3):
2NO2 þ O3 $ N2 O5 þ O2
ð3Þ
As it can be seen in Fig. 4 the stoichiometric molar ratio O3/NOx (equal 0.5) is insufficient for the complete conversion of nitrogen dioxide for the applied residence time (around 10 s). The increase in the NOx initial concentration does not lead to the NOx conversion increase, the observed conversion is around 60% at the stoichiometric O3/NOx ratio equal to 0.5. On the other hand, for a molar ratio around 1, even at lower NO2 concentrations the conversion rate reached 90%.
Fig. 4. NOx conversion versus NOx initial concentration for molar ratios O3/NOx: 0.5, 0.75 and 1.
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concentration of nitrogen dioxide. However, the impact of the NOx initial concentration is not as significant as the influence of O3/NOx ratio applied. For the almost complete NOx conversion a molar ratio O3/NOx equal to 1 was necessary. The reaction (Eq. (3)) is a summary reaction of the NO2 ozonation process, which follows:
NO2 þ O3 ! NO3 þ O2
ð4Þ
NO2 þ NO3 $ N2 O5
ð5Þ
The possible reason for the increased O3 consumption in respect to theoretical one is the reversibility of reaction (Eq. (5)) The typical FTIR spectra for nitrogen dioxide before and after the reaction with ozone were presented in Fig. 5. The formation of the reaction products in respect to molar ratio O3/NOx (FTIR spectra for the nitrogen dioxide ozonation with various molar ratios O3/NOx: 0, 0.5, 0.75, 1) was shown. The main products observed were dinitrogen pentoxide (N2O5) and nitric acid (HNO3). The absorption bands at 1245 cm1 and 745 cm1 can be assigned to N2O5, whereas peaks at 1316 cm1 and 887 cm1 are connected with the presence of HNO3. The peak at around 1700 cm1 can be attributed both to the presence of N2O5 and HNO3 formed during ozonation of NO. The presence of residual ozone was also detected and analyzed based on the peak height (1051 cm1). The analysis of selectivity of NO2 ozonation into N2O5 and HNO3, calculated according to the formulas:
SNO2intoN2O5 ¼
2ð½N2 O5 t ½N 2 O5 0 Þ ½NO2 0 ½NO2 t
ð6Þ
SNO2intoHNO3 ¼
½HNO3 t ½HNO3 0 ½NO2 0 ½NO2 t
ð7Þ
contrast, in the process when carried out with O3/NOx molar ratio 0.5, the ozone residual concentration remains almost constant at the level on around 1106 mol/L, clearly pointing to ozone deficiency. Hence, incomplete NO2 ozonation took place.
3.2. Lab scale – digestion process flue gas The second series of experiments was performed on the real flue gases coming from a batch digestion process (NO2 initial concentrations around 130 ppm and 250 ppm). The required molar ratio O3/NOx turned out to be higher (1.7) than observed for the simulated flue gases (Table 3). It is important to note that in these series of experiments the residence time was around 5 s. Therefore, the observed behavior can be explained firstly by the shorter
showed that their values were almost independent on the applied molar ratio O3/NOx in the range 0.5–1 (Fig.6). Above the NOx initial concentrations equal to 7.5106 mol/L the selectivity SNO2intoN2O5 did not change significantly with the increasing NOx concentration. Around 30% of NO2 was oxidized into dinitrogen pentoxide. The selectivity SNO2intoHNO3 grew slightly from 6% to 10% with the increasing NOx concentration. The residual ozone concentration in the off-gases was also measured. As expected, the amount of the residual ozone rises with the increase of the molar ratio of O3/NOx used. Similar conclusions were previously drawn for the nitrogen monoxide ozonation [11]. The increase in residual ozone concentration for experiments carried out with O3/NOx molar ratio around 0.75 and 1 proves that the NOx conversion reached its maximum values since ozone is no longer consumed in the process (Fig. 7). In
Fig. 6. Product selectivity versus NOx initial concentration.
Fig. 7. Residual ozone concentration versus initial NOx concentration.
Table 3 NOx removal efficiency for the batch digestion process.
Fig. 5. FTIR spectra of reaction gases before and after the reaction with ozone.
Initial NO2 concentration ppm
Molar ratio O3/NOx mole/mole
Obtained NOx removal %
140 190 138 145
0.5 0.8 1.1 1.7
43 50 83 100
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Fig. 8. Influence of humidity in flue gases on the NO2 ozonation for various molar ratios O3/NOx.
residence time. Hence, in the series of coupled and subsequent reactions that take place in the system the obtained conversion rate is smaller. Further to the above, based on the kinetic equations of NO2 ozonation [12] the rate of reaction is dependent also on the initial NO2 concentration not only the amount of the oxidant supplied. Nevertheless, this experiment enabled to determine the influence of other process gas components (HF, SiF4,CO2, HNO3g, H2O). It was observed that apart from water vapor none of the extra contaminants consumes large quantities of ozone. The influence of water vapor was confirmed by tests with the use of 836 ppm of NO2 gas stream (RT = 0.75 s.). It was shown that presence of water vapor in flue gases increases the NO2 conversion rate (Fig. 8). The influence of the water vapor content was growing with the increasing O3/NOx ratio. The main explanation is that in the presence of water vapor the reaction (Eq. (2)) occurs, therefore N2O5 is consumed and the equilibrium of reaction (Eq. (5)) shifts to the right in accordance with Le Chatelier’s principle. Since ozone can react in water to form hydroxyl radicals in series of chain reactions [20]: O3 þ OH ! O 2 þ HO2
ð8Þ
HO2 $ Hþ þ O 2
ð9Þ
pkA ¼ 4:8
reactions are caused by direct ozonation and not indirect (through hydroxyl radicals). Furthermore, as we proved previously O3 solubility in the acid solutions decreases with increasing acid concentration [17]. Therefore, we believe that the NO2 removal enhancement in the presence of water vapor is mainly a result of the already mentioned equilibrium shift to the right of reaction (Eq. (5)). 3.3. Pilot plant off gases In the final series of experiments performed on the real flue gases the efficiency of the ozonation process was evaluated in gases that contained water vapor, HF, SiF4 and post-digestion dust. Exemplary FTIR spectra are presented in Fig. 9. The peaks that correspond to HNO3 (1700, 1316 and 887 cm1) can overlap with the peaks characteristic for the presence of HF in the off-gases. Therefore, nitric acid readings cannot be considered as a reliable representation of the ozonation process effectiveness. The broad peaks around 4000–3500 cm1 and 2000–1200 cm1 confirm that water vapor is present in the tested gases. The peaks around 1026 cm1 can be assigned to SiF4. The peaks around
Then the radical exchange reactions occur: O3 þ O 2 ! O3 þO2
ð10Þ
þ O 3 þ H ! HO3
ð11Þ
HO3 ! O2 þ HO
ð12Þ
and the hydroxyl radicals are formed. If they get in contact with NO2 molecules an additional set of reactions can also take place increasing overall conversion rate of NO2, therefore more NO2 can be removed from the off-gases, according to reactions [21]:
NO2 þ OH þ M ! HNO3 þ M
ð13Þ
NO2 þ HO2 þ M ! HNO2 þ O2
ð14Þ
However, importance of the chain reactions in which radicals are formed will definitely decrease in the real off-gases from phosphate rock digestion process where concentrated HNO3 is used, because the off-gases contain acid droplets. It is well known fact in the water chemistry that in acidic conditions the ozonation
Fig. 9. Exemplary FTIR spectra for off-gases before and after ozonation (spectra selected from experiments conducted on day 3).
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Fig. 10. NO2 concentration profile during experiment 3.2.1-3 (residence time = 3 s).
1630 cm1 and 2920 cm1 that correspond to NO2 presence are visible for off-gases before ozonation. What is more, this peak clearly decreases in the spectrum taken for the off-gases after ozonation. Therefore, in order to verify the effectiveness of ozonation processes the analysis of only the NO2 concentrations was done. Nitric oxide is present in the gases in the concentrations as low as 5 ppm. Therefore, NOx stream composed mainly of NO2. In the presence of ozone-oxygen mixture NO concentrations dropped to 0. The examination of the obtained results showed that for all the measured species (except NO2, NO, HNO3) no influence of the ozonation process on their concentrations is observed. Therefore, it can be concluded that they do not react with ozone. However, we cannot exclude the inhibitory action of these gas components. Fig. 10 presents results of the series of experiments with the same residence time (RT) but increasing O3/NO2 ratio (0.44, 0.64, 0.95). Some variation in NO2 concentration was observed during ozonation of real flue gases. Few sharp peaks can be observed, they correspond well with the times when antifoaming agent was added into the digestion process (normal operation procedure). This figure shows the measured NO2 concentrations before and during ozonation for NO2 average concentration 341.9 ppm (verified before and after the experiment by switching FTIR unit from the measuring point P3 to P1). It is clear that NO2 removal efficiency rises with the increasing O3/NO2 ratio from 44 to 95 %. In order to analyze the ozonation process in wider range of O3/NO2 ratio (0.2–1) additional two series of experiments were performed with different RTs (3 s and 7 s) (Fig. 11). For under-stoichiometric ratios O3/NO2 (0.2 and 0.3) the conversion degree was low, as expected, for both RTs. For stoichiometric O3/NOx ratios around 0.5 the conversion was around 66%. Further increasing MR to around 1 yields almost 100% NO2 removal. Better results were obtained for longer residence time (7 s). Whereas, in the case of the pre-ozonation process tested for the off-gases from nitric acid pilot plant the required O3/NOx ratio was 1.4 [14]. As previously stated the NOx stream in the nitric acid production plant comprises of NO and NO2 in the ratio 1:1. These results indicate the importance of off-gases composition on the effective/required dose of ozone. It is clear that effectiveness of NO2 removal from ozonated flue gases is dependent on the amount of ozone added, more precisely
Fig. 11. Influence of reaction gases residence time on the NO2 conversion efficiency.
on molar ratio of ozone to nitrogen dioxide. Pilot scale results suggest that for compete NO2 removal from the flue gases, a molar ratio O3/NO2 around 1 should be used. This is still higher than would be expected based on reaction stoichiometry (O3/NO2 = 0.5), but lower than it was concluded from the digestion lab-scale experiments. It is important to remember that in the off-gases from phosphate rock digestion many reactions are possible, e.g. ozone decomposition due to the presence of water and dust and N2O5 reverse reaction or possible ozone reaction with other gas impurities etc. This would result in the increase of required O3/NO2 molar ratio. On the other hand presence of H2O enhances N2O5 removal from the reaction mixture by N2O5 absorption into H2O and shifting the reversible reaction (5) to the right side. Further data analysis also proves that the residence time is an important process parameter and increasing the residence time could allow the use of lower O3/NO2 for the complete NO2 removal. Summarizing the results obtained in the pilot scale the surface response plot is presented in Fig. 12, wherein the NO2 concentration in the outlet was plotted versus residence time (RT) and the ozone
Please cite this article in press as: K. Skalska et al., Nitrogen oxides pre-ozonation in flue gases from phosphate rock digestion, Chem. Eng. J. (2016), http:// dx.doi.org/10.1016/j.cej.2016.06.048
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respect to the future application of the pre-ozonation followed by the absorption process in scrubbers. Acknowledgement Dr Kinga Skalska thanks The Kos´ciuszko Foundation for 10-month fellowship in 2015. References
Fig. 12. Surface response plot ([NO2in] = 397.8 ppm).
inlet concentration (O3 in). As can be seen the increase of RT results in the growth of the NO2 removal degree. For molar ratio O3/NO2 equal to 1 (NO2 initial concentration 400 ppm and O3 initial concentration 400 ppm) and residence time around 11 s, the complete conversion of NO2 was be observed. 4. Conclusions Based on the laboratory experiments with simulated off-gases, the recommended molar ratio O3/NOx for the complete removal of the NO2 is around 1, for a residence time around 10 s. The lab-scale results revealed that water content in off-gases has an significant influence on ozonation reaction. Therefore, high humidity of the off-gases will be beneficial, while the presence of other gas components (in digestion off gases) had no effect on the process effectiveness. In the pilot scale studies a residence time around 7 s was sufficient for almost 100% NO2 conversion provided the O3/NOx molar ratio is around 1. The application of ozone to NO2 removal from off-gases liberated by nitrophosphate route was proved to be an effective and a safe solution. The required O3/NOx is dependent on many factors and without doubt it should be determined in situ for the specific industrial gases. The best solution in this case would be the online analysis of flue gas composition. The process parameters and the programmed correction of the O3/NOx ratio should be appropriately adapted. The presented results of the experimental work both in the lab and the pilot scale provided the theoretical basis for the industrial application of the pre-ozonation technology. The above findings are crucial with
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Please cite this article in press as: K. Skalska et al., Nitrogen oxides pre-ozonation in flue gases from phosphate rock digestion, Chem. Eng. J. (2016), http:// dx.doi.org/10.1016/j.cej.2016.06.048