Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Removal of NOx from Flue Gas Using Yellow Phosphorus and Phosphate Slurry as Adsorbent Shuai Li,†,‡ Jiaqiang Yang,†,‡ Chi Wang,†,‡ Delong Xie,†,‡ Yongming Luo,§ Kai Li,§ Dedong He,*,†,‡,§ and Yi Mei*,†,‡ †
Faculty of Chemical Engineering, ‡The Higher Educational Key Laboratory for Phosphorus Chemical Engineering of Yunnan Province, and §Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, People’s Republic of China S Supporting Information *
ABSTRACT: A composite slurry containing yellow phosphorus and phosphate slurry was used to remove NOx from flue gas, where yellow phosphorus is considered to promote generation of ozone from oxygen. The latter can oxidize NO to form more water-soluble NxOy species, finally converted to HNO2 and HNO3. These acids can react with phosphate slurry to form PO43−. Thus, the final solution containing NO3− and PO43− can be potentially used as raw material for the production of nitrogen phosphorus compound fertilizer. Moreover, effects of various parameters on NOx removal efficiency were optimized, and singlefactor experiments together with response surface optimization were applied for optimizing these parameters. It was indicated that the removal efficiency of NOx can obtain 99.2% under optimal conditions. Subsequently, the corresponding reaction mechanisms were discussed. Therefore, using the mixtures of yellow phosphorus and phosphate slurry as absorbent not only obtains high NOx removal efficiency, but can avoid the need to dispose of spent liquid wastes, which provides an attractive approach for controlling NOx. Moreover, the present slurry system can eliminate NOx and SO2 simultaneously with high removal efficiency.
1. INTRODUCTION Nitrogen oxides (NOx) and sulfur dioxide (SO2) emitted from combustion of coal and fuel oils have brought about significant effects on both the environment and human health, due to the fact that NOx and SO2 contribute considerably to acid rain. More seriously, the presence of NOx in the atmosphere is also responsible for causing the greenhouse effect and photochemical smog, which has received increasing attention recently.1−4 Among various technologies, the wet removal process has been widely developed to control and reduce SO2 and NOx simultaneously.5−8 In most cases, it is easy to attain high SO2 removal efficiency, while high NOx removal efficiency can hardly be obtained at the same time, owing to low water solubility of NO, which accounts for more than 90% of all NOx species.9 Therefore, much more attention has been paid with regard to enhancing NOx absorption ability. A large number of strong oxidants, including ozone,10−13 hydrogen peroxide,14−16 potassium permanganate,17−19 and sodium chlorite,20−23 have been added to wet scrubbing systems to enhance the absorption of NOx during the NOx removal process. It is accepted that NO can be oxidized to form more water-soluble NO2 under the action of oxidants, and high NOx removal efficiency is realized accordingly. Among all the oxidants, ozone is the most promising one because of its high efficiency and environmental friendliness, which highlights its availability and utilizability for the wet NOx removal process. However, in previous works,10−13 ozone was generally provided by an ozone generator, thus resulting in the certain drawback of expensive cost inevitably. Hence, finding a new and cost-effective method to generate ozone in the NOx removal process is meaningful and of great importance. © XXXX American Chemical Society
It is known that China has a rich reserve of phosphate rock resources, and yellow phosphorus (P4) and phosphate rock are recognized as common products or raw materials in phosphorus chemical processes. Herein, a new approach using aqueous mixtures of yellow phosphorus (P4) and phosphate slurry as absorbent to remove NOx is reported. This process, where yellow phosphorus is considered to promote generation of ozone from oxygen, is capable of efficiently removing NOx. Besides, phosphate slurry can react with nitric acid that is formed in the wet NOx removal process to produce phosphoric acid. Thus, the final solution which contains NO3− and PO43− can be potentially applied as raw materials for nitrogen phosphorus compound fertilizer. Hence, using the mixtures of yellow phosphorus and phosphate slurry as absorbent not only obtains high NOx removal efficiency, but also avoids the need for the disposal of the final spent liquid wastes. Actually, as we have demonstrated in our previous papers,24,25 high SO2 removal efficiency can be obtained when phosphate slurry was dedicated as the absorbent or catalyst. Furthermore, an industrial test with high SO2 removal efficiency using an aqueous mixture of phosphate slurry as absorbent has been carried out at Zhonghua Yunlong Co. Ltd. of Yunnan, China (Figure S1, Supporting Information), and the outlet flue gas can reach the emission standards of China. However, to the best of our knowledge, developing yellow phosphorus and phosphate slurry as adsorbent to remove NOx from flue gas has not been reported so far. Based on these considerations, the objective of the present work is to introduce a new process that can remove NOx with Received: December 15, 2017 Revised: March 4, 2018 Published: March 5, 2018 A
DOI: 10.1021/acs.energyfuels.7b03964 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Schematic diagram of experimental apparatus. 1, N2 cylinder; 2, NO cylinder; 3, O2 cylinder; 4, relief valve; 5−7, flowmeters; 8, gas mixing tank; 9, thermostatic water bath; 10, thermometer; 11, round-bottom flask with four necks; 12, rotor; 13, water removal using allochroic silica; 14, dust removal using cotton wool; 15, flue gas analyzer; 16, aerator; 17, tee joint. where Cinlet is the concentration of NOx in inlet flue gas, mg/m3; Coutlet is the concentration of NOx in outlet flue gas, mg/m3. Meanwhile, the solid content of phosphate slurry is analyzed as follows:
high efficiency from flue gas, where phosphate slurry was used as absorbent, while yellow phosphorus was devoted to promoting production of ozone from oxygen. The produced ozone can oxidize NO into NO2, thus increasing the water solubility of NOx and improving its removal efficiency. Effects of process parameters on NOx removal efficiency were investigated in detail. Furthermore, single-factor experiments and response surface optimization were used to optimize these parameters, and the optimal conditions were acquired accordingly. In addition, reaction processes as well as the corresponding mechanism during oxidation and absorption processes were discussed subsequently.
solid content of phosphate slurry =
where M1 is the quality of phosphate, g; M2 is the quality of water and yellow phosphorus, g. 2.3. Possible Reaction Mechanism. Herein, the reaction between yellow phosphorus and oxygen is considered to generate ozone, while phosphate slurry is used as absorbent. The removal of NOx from flue gas is based on the reaction between the simulated flue gas and the formed ozone. Some important reactions are summarized in the Supporting Information.12,26−30
2. EXPERIMENTAL SECTION 2.1. Materials. Standard gases including N2 (≥99.99%) and O2 (≥99.50%) were supplied by the Gas Co. Ltd. of Messer, Kunming, China, and NO/N2 (0.3%NO, v/v) was provided by the Gas Co. Ltd. of Jin Kexing, Chengdu, China. Moreover, yellow phosphorus and phosphate rock were offered by the Co. Ltd. of Nanlin, Yunnan, China and the Co. Ltd. of Zhonghua Yunlong, Yunnan, China, respectively. Besides, other chemical reagents, such as sulfuric acid (95−98%, AR), sodium hydroxide (≥96%, AR) and calcium carbonate (≥99%, AR) were purchased from the Co. Ltd. of Keyi, Yunnan, China and used without further purification. 2.2. Experimental Setup. Figure 1 shows a schematic diagram of the experimental apparatus, which includes a gas supply system, a gasphase mixing tank, an absorption reactor, a flue gas analyzer system, and a tail gas treatment system. The simulated flue gas composed of N2, O2, and NO was supplied from gas cylinders, and the flow rate was controlled by mass flow controllers (Beijing Seven-Star Electronics Co., Ltd., China). The feed aqueous mixtures in the absorption reactor were prepared by mixing 500 mL of deionized water with a certain amount of yellow phosphorus and phosphate rock with vigorous stirring. A magnetic stirrer was used to control the reaction temperature and stirring speed. In addition, a pH electrode (DHS-3C, INESA Scientific Instrument Co., Ltd., Shanghai, China) was inserted into the reactor to measure the pH value, and H2SO4 or NaOH was added to adjust the pH of aqueous mixtures. The concentrations of ozone and outlet gas were measured by an ozone detector (ZX-O3, Technology Co,. Ltd., of Zhen Xiong, Guangzhou, China) and a flue gas analyzer (Ecom-J2KN, RBR Co., Ltd., Germany), respectively. It is to be noted that removing water and the formed dust (such as solid particles of P2O5) in the outlet gas through the tail gas treatment system was needed before conducting the analysis. The removal efficiency of NOx is analyzed as follows:
removal efficiency (%) =
M1 × 100% M1 + M 2
3. RESULTS AND DISCUSSION 3.1. Generation of Ozone between Yellow Phosphorus and Oxygen. From Figure 2, it is confirmed that yellow
Figure 2. Ozone concentration and aqueous mixture pH as a function of time. Concentration of O2, 12%; concentration of N2, 88%; concentration of yellow phosphorus, 1.6 g/L; solid content of phosphate slurry, 9.09%; reaction temperature, 65 °C; gas flow rate, 500 mL/min; stirring intensity, 1000 rpm.
phosphorus promotes generation of ozone from oxygen, and the concentration of ozone can achieve about 200 mg/m3. Furthermore, it is seen from Figure 2 that the pH of aqueous mixtures decreases with reaction time, which may be ascribed to the fact
C inlet − Coutlet × 100% C inlet B
DOI: 10.1021/acs.energyfuels.7b03964 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 3. Effects of (A) dispersion medium, (B) stirring intensity, (C) concentration of yellow phosphorus, and (D) reaction temperature on the concentration of produced ozone. Concentration of O2, 12%; concentration of N2, 88%; concentration of yellow phosphorus, 1.6 g/L; solid content of phosphate slurry, 9.09%; reaction temperature, 65 °C; gas flow rate, 500 mL/min; stirring intensity, 1000 rpm.
yellow phosphorus and oxygen. Besides, it should be noted that, more importantly, increasing the stirring intensity can increase the dispersion and the holdup of O2, which is beneficial for the generation of ozone. However, the amount of the generated ozone at the stirring speed of 1600 rpm is lower than of other speeds after 160 min of exposure time, which may be attributed to the fast consumption of yellow phosphorus during the reaction, and not enough yellow phosphorus can be offered. 3.2.3. Effect of Concentration of Yellow Phosphorus. The influence of the concentration of yellow phosphorus on the production of ozone was investigated, and the results are recorded in Figure 3C. It is found that the produced ozone is proportional to the concentration of yellow phosphorus in the aqueous mixtures. When increasing the concentration of yellow phosphorus from 0.62 to 2.8 g/L, the amount of the produced ozone is increased. It is accepted that increasing the concentration of yellow phosphorus can facilitate reactions 1−3 in the Supporting Information and the production rate of ozone increases accordingly. 3.2.4. Effect of Reaction Temperature. The effect of reaction temperature on the generation of ozone was explored. From Figure 3D, it is revealed that the concentration of ozone is increased when the reaction temperature is raised from 45 to 65 °C, but further increasing the reaction temperature, the concentration of ozone begins to decrease. The amount of the produced ozone at the reaction temperature of 85 °C displays results similar to those at 45 °C. The above phenomenon can be explained as follows: the increased reaction temperature can enhance the dispersion degree of yellow phosphorus, thus promoting the reaction rate as expected. However, decomposition of ozone is also promoted with the rise of reaction temperature.31 Therefore, the concentration of ozone decreases with further increasing the reaction
that phosphorus oxides are obtained and dissolved into water, and finally converted into the corresponding acid solution. Besides, the produced nitric acid and phosphoric acid during the reaction can cause the amount of acid in the slurry to be accumulated, and as a result, the pH of the slurry decreases with reaction time. 3.2. Optimization of Conditions for Ozone Generation. 3.2.1. Effect of Dispersion Medium. Phosphate rock was added into the absorption reactor to obtain aqueous mixtures, and the formed phosphate slurry was applied as the absorbent. However, it is to be noted that some phosphate rock in solid particle phase still existed and worked as the dispersion medium. In fact, calcium carbonate was also widely reported for this purpose.26 Therefore, a comparison study for ozone generation through different dispersion mediums between phosphate rock and calcium carbonate was conducted, and the results are presented in Figure 3A. It is shown that the phosphate rock exhibits a better promoted effect on the generation of ozone than calcium carbonate, and its removal efficiency of NOx is higher than that of calcium carbonate (Figure S2 in the Supporting Information). Thus, phosphate rock is a good candidate and is considered to be a replacement for calcium carbonate to be the dispersion medium. Therefore, the composite slurry of yellow phosphorus and phosphate slurry was chosen in our present research. 3.2.2. Effect of Stirring Intensity. Comparison of stirring intensities on the generation of ozone was performed, and the results are displayed in Figure 3B. It is seen that the amount of the generated ozone increases with increasing the stirring speed from 500 to 1600 rpm at the initial 140 min of reaction time, which owns to the fact that high stirring intensity promotes the dispersion of aqueous mixtures, thus increasing the contact area of C
DOI: 10.1021/acs.energyfuels.7b03964 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 4. Effects of (A) injected yellow phosphorus concentration, (B) reaction temperature, (C) pH of phosphate slurry, (D) solid content of phosphate slurry, (E) inlet NOx concentration, (F) O2 concentration, (G) stirring intensity, and (H) gas flow on NOx removal efficiency. Concentration of yellow phosphorus, 1.6 g/L; reaction temperature, 65 °C; pH 4.5; solid content of phosphate slurry, 4.76%; concentration of NO, 750 mg/m3; concentration of O2, 10%; stirring intensity, 1500 rpm; gas flow rate, 510 mL/min. D
DOI: 10.1021/acs.energyfuels.7b03964 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
slurry increases from 9.09 to 16.67%, and further increasing the solid content of phosphate slurry to 23.08%, NOx removal efficiency begins to decline. This is because an excess of the solid content of slurry can lead to the decrease of the dispersion degree of yellow phosphorus and cause the decrease of solubility of oxygen; thus the concentration of ozone decreases accordingly. Considering this result, the optimal solid content of slurry in the current work is chosen as 9.09%. 3.3.5. Effects of NOx and O2 Concentrations. Effects of inlet NOx and O2 concentrations on NOx removal efficiency were investigated, and the results are displayed in parts E and F, respectively, of Figure 4. From Figure 4E, the NOx removal efficiency decreases from 95 to 67% as the NOx concentration increases from 274 to 1110 mg/m3, because not enough ozone can be produced and provided to oxidize NO. Accordingly, the NOx removal efficiency is decreased. As for Figure 4F, it is observed that NOx removal efficiency increases as the O2 content increases from 4 to 14%, due to the promotion of the reaction between yellow phosphorus and oxygen when increasing O2 concentration, thus resulting in the increase of NOx removal efficiency. Considering that the O2 concentration from industrial
temperature. Furthermore, oxygen dispersion and solubility in the slurry can also be affected by temperature. Hence, the decreased solubility of oxygen at high temperature can inhibit the formation of ozone. 3.3. Optimization of Conditions for NOx Removal. From these results, it is obtained that different operation conditions show different effects on the generation of ozone, and the latter is considered to be related to NOx removal efficiency. Herein, optimization of conditions for NOx removal was studied. 3.3.1. Effect of Yellow Phosphorus Concentration. Figure 4A indicates that NOx removal efficiency increases from 59 to 98% when the concentration of added yellow phosphorus increases from 0.45 to 1.85 g/L, and the NOx removal efficiency can maintain at about 98% when the yellow phosphorus concentration is 1.85 g/L. This result is consistent with the result as shown in section 3.2.3, where the increase in the concentration of yellow phosphorus is conducive to the formation of ozone, and the increased ozone amount can facilitate the reaction rate of NOx oxidation and accelerate the dissolution of NOx (according to reactions 4 and 5 in the Supporting Information). As a result, the NOx removal efficiency increases with the increase of concentration of yellow phosphorus. Note that yellow phosphorus concentration in the present study is selected as 1.6 g/L, since high NOx removal efficiency can be obtained, and the outlet flue gas can reach the emission standards. 3.3.2. Effect of Reaction Temperature. Temperature was varied from 45 to 85 °C to investigate the effect of reaction temperature on NOx removal efficiency, and the results are displayed in Figure 4B.The NOx removal efficiency increases with the rise of reaction temperature from 45 to 75 °C, and the highest NOx removal efficiency of 87% is seen at 75 °C. But further increasing the reaction temperature to 85 °C, the NOx removal efficiency decreases slightly. As analyzed in section 3.2.4, high reaction temperature can lead to the decomposition of ozone and cause the decrease of solubility of oxygen, thus resulting in the reduction of ozone content and decreasing the NOx removal efficiency. Besides, it is noted that spontaneous combustion of yellow phosphorus can occur when the reaction temperature is at or above 75 °C, which causes the rapid consumption of yellow phosphorus and inhibits the formation of ozone. Hence, 65 °C is considered as the optimal reaction temperature in this study. 3.3.3. Effect of pH of Solution. Figure 4C shows that the NOx removal efficiency increases with the increase of the pH of solution. A 72% NOx removal efficiency is seen when the pH of solution is 2.3, while the highest NOx removal efficiency of 89% is observed with solution pH 4.9. Therefore, it is concluded that increasing the pH of solution is beneficial to eliminating NOx, due to the good absorbing ability of slurry solution with high pH for acidic NOx. Condidering that slurry pH values 4.5 and 4.9 show similar NOx removal results, the outlet flue gas can reach the emission standards under both conditions. Thus, a pH of slurry of 4.5 is preferred in the present work. 3.3.4. Effect of Solid Content of Phosphate Slurry. Experiments toward the effect of the solid content of phosphate slurry on NOx removal efficiency were performed (calculation of the solid content of phosphate slurry is described in section 2.2). From Figure 4D, it can be seen that NOx removal efficiency increases with increasing the solid content of phosphate slurry from 2.44 to 9.09%, which can be explained as that the increase of the solid content of slurry increases the dispersion degree of yellow phosphorus and promotes the reaction between yellow phosphorus and oxygen. Furthermore, NOx removal efficiency maintains at about 83% when the solid content of phosphate
Table 1. Independent Variables and Their Coded Levels for the BBD coded variable level variable
code
−1
0
+1
P4/NOx slurry temp, °C slurry pH gas flow rate, mL/min
X1 X2 X3 X4
666 60 3.2 350
1666 70 4.1 550
2666 80 5.0 750
Table 2. Experimental Design Matrix and Response Results
E
run
X1
X2
X3
X4
R1, %
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
666 666 1666 666 2666 1666 1666 666 1666 666 2666 1666 2666 1666 1666 1666 1666 2666 2666 1666 1666 666 1666 1666 1666 2666 1666 1666 1666
70 70 70 80 70 70 70 70 70 60 70 60 70 70 70 60 80 60 80 70 80 70 80 60 60 70 70 80 70
5.0 4.1 4.1 4.1 5.0 4.1 5.0 4.1 5.0 4.1 3.2 4.1 4.1 3.2 4.1 4.1 4.1 4.1 4.1 3.2 3.2 3.2 4.1 3.2 5.0 4.1 4.1 5.0 4.1
550 350 550 550 550 550 750 750 350 550 550 350 750 750 550 750 350 550 550 350 550 550 750 550 550 350 550 550 550
73.47 84.24 94.03 71.06 98.13 97.63 85.19 61.14 99.62 47.73 98.92 93.10 94.55 88.25 97.63 70.78 99.18 92.87 99.03 99.37 97.18 69.47 95.61 81.17 78.85 99.14 96.86 96.56 96.86
DOI: 10.1021/acs.energyfuels.7b03964 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels coal-fired flue gas is around 10%, hence, the O2 concentration is selected as 10% in this study. 3.3.6. Effect of Stirring Intensity. The effect of stirring intensity on NOx removal efficiency was investigated. From Figure 4G, it is seen that the NOx removal efficiency increases as stirring intensity increases, and this is mainly due to the fact that increasing stirring intensity promotes the dispersion of aqueous emulsion. Besides, increasing the stirring intensity also enhances the gas− liquid transfer rate and increases the dispersion and holdup of O2. As a result, NOx removal efficiency increases, and the optimal stirring intensity is considered as 1500 rpm. 3.3.7. Effect of Gas Flow Rate. The gas flow rate was changed from 148 to 1050 mL/min to examine its effect on NOx removal efficiency, and the results are displayed in Figure 4H. It is found that the NOx removal efficiency decreases as the gas flow rate
increases, which may be ascribed to the fact that increasing the gas flow rate can decrease the contact time of yellow phosphorus and oxygen, thus leading to the decrease of ozone amount during the reaction. Furthermore, the increased gas flow rate can also reduce the residence time of NO in the solution, and a part of NO cannot be oxidized by ozone. All these should cause the decrease of NOx removal efficiency. However, a too-low gas flow rate means that the handling capacity of NOx will decrease. Based on this result, a gas flow rate of 535 mL/min is chosen in our experiment. 3.4. Response Surface Results. 3.4.1. Response Surface Design. Response surface methodology (RSM) with a Box− Behnken design (BBD) is a well-known method to optimize multiple variables during a reaction, using a combination of mathematical and statistical techniques. Table 1 shows the ranges and levels of the dependent variables in this study: P4/NOx (X1),
Table 3. Analysis of Variance (ANOVA) for Response Surface Quadratic Model
a
source
sum of squares
degree of freedom
mean of square
F-value
p-value
remarksa
model X1 X2 X3 X4 X1X2 X1X3 X1X4 X2X3 X2X4 X3X4 X12 X22 X32 X42 residual lack of fit pure error cor total CV% R2 R2adj
5152.04 2644.19 778.60 0.96 495.75 78.41 3.86 52.64 0.72 107.64 2.74 807.53 303.82 8.90 15.19 95.16 86.30 8.86 5247.20 2.96 0.9819 0.9637
14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 14 10 4 28
368.00 2644.19 778.60 0.96 495.75 78.41 3.86 52.64 0.72 107.64 2.74 807.53 303.82 8.90 15.19 6.80 8.63 2.22 −
54.14 389.02 114.55 0.14 72.94 11.54 0.57 7.74 0.11 15.84 0.40 118.81 44.70 1.31 2.23 − 3.90 − −
