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Study on the catalysis of CaCO3 in the SNCR deNOx process for cement kilns
Chemical Engineering Journal 262 (2015) 9–17
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Study on the catalysis of CaCO3 in the SNCR deNOx process for cement kilns Shi-long Fu, Qiang Song ⇑, Qiang Yao Key Laboratory of Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, 100084 Beijing, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
CaCO3 inhibits the deNOx
performance of the SNCR deNOx process. CaCO3 catalyzes NH3 conversion through a chain reaction as given in the mechanism. Reaction between NH3 and CaCO3 is the rate-controlling step of NH3 conversion. A kinetic model is proposed to describe the influence of CaCO3.
a r t i c l e
i n f o
Article history: Received 27 July 2014 Received in revised form 12 September 2014 Accepted 13 September 2014 Available online 28 September 2014 Keywords: SNCR CaCO3 NH3 NO Kinetic model
a b s t r a c t The selective non-catalytic reduction (SNCR) deNOx process is influenced by high concentration CaCO3 particles in the pre-calciner when used in cement industry. The influence of CaCO3 on the SNCR deNOx process was investigated using a fixed bed reactor with temperature ranging from 873 K to 1123 K. Experimental results showed that CaCO3 catalyzes NH3 conversion to HNCO and N2 in the absence of O2, and catalyzes NH3 oxidation to NO and N2 in the presence of O2. Both reactions show first order characteristics with respect to NH3 concentration. CaCO3 has no catalytic effect on NO reduction by NH3. The deNOx efficiency of the SNCR deNOx process is inhibited by CaCO3 mainly by catalyzing NH3 oxidation to NO. Mechanism analysis showed that the reaction between NH3 and CaCO3 is the rate-controlling step of NH3 conversion. In the absence of O2, the intermediate product decomposed to CaO, HNCO, H2O and N2, whereas CaO reacted with CO2 to reproduce CaCO3. O2 did not participate in the rate-controlling step of NH3 conversion. However, in the presence of O2, the intermediate product of the reaction between NH3 and CaCO3 can be oxidized to NO by O2 or decomposed into N2 through dimerization reaction. Product selectivity is determined by the competition of these two reactions. A kinetic model was proposed for the CaCO3-involved SNCR deNOx process which well predicted the influence of CaCO3. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxides (NOx) are among the most hazardous air pollutants. In China, the cement industry contributes to 11% of the total
⇑ Corresponding author. Tel./fax: +86 010 62781740. E-mail address: [email protected] (Q. Song). http://dx.doi.org/10.1016/j.cej.2014.09.048 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
NOx emission and has become the third largest industrial NOx emission source [1,2]. Stricter air protection act has been imposed by the Chinese government recently, which means that most of the cement kilns will have to install appropriate deNOx devices. Among all the NOx control technologies, the selective non-catalytic reduction (SNCR) technology is regarded as the most suitable technology in consideration of the deNOx performance and cost [3,4]. The SNCR deNOx technology for utility boilers has been well studied,
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and the deNOx efficiency is approximately 40%. Temperature, reactant composition, and residence time are factors that influence the SNCR deNOx efficiency [5–9]. However, the SNCR deNOx devices that have operated on cement kilns show a diverse range of deNOx efficiency ranging from 15% to 80% [10], which indicates that the SNCR deNOx process used in cement kilns is more complex and different from those used in the utility boilers. Pre-calciner is considered as the most suitable position for the SNCR deNOx process in cement kiln because of its appropriate configuration and temperature distribution [10]. The high concentration of Ca-based particles (mainly composed of CaO and CaCO3) in the pre-calciner (the most suitable position for SNCR process) is the biggest difference from the utility boiler. The concentration of Ca-based particles can be as high as 1 kg/Nm3 in the pre-calciner, and the portion of CaCO3 can be over 60% at the entrance [11]. Studies by Jensen [12], Shimizu [13,14], Li [15,16] and Tang [17] all found that CaCO3 has inhibitive effect on the SNCR deNOx process. Thus, the influence of CaCO3 must be considered when the SNCR deNOx technology is applied in the cement industry. Gas-phase SNCR deNOx process mainly involves the reactions among NH3, NO, and O2. CaCO3 may influence the SNCR deNOx process by catalyzing NH3 decomposition, oxidation, and NO reduction by NH3. Shimizu [13] studied the influence of CaCO3 on NH3 oxidation and found that CaCO3 had a catalytic effect on NH3 oxidation, resulting in the production of NO and N2. Li [16] studied the CaCO3-involved NH3 decomposition, oxidation, and NO reduction by NH3 within the temperature range of 923 K to 1123 K. CaCO3 was found to have an obvious catalytic effect on NH3 decomposition and oxidation, but had no effect on the reaction of NH3 + NO. NO selectivity decreased with increasing temperature during CaCO3-catalyzed NH3 oxidation. Tang’s study [17] in the temperature range of 923 K to 1123 K also showed that CaCO3 catalyzed NH3 decomposition and oxidation. During CaCO3-catalyzed NH3 oxidation, NH3 conversion was not affected by NO or O2; however, NO selectivity increased with increasing temperature, which was in conflict with the result obtained by Li [16]. Abul-Milh [18] studied the interaction between NH3 and CaCO3 in CaCO3 decomposition process using temperature programmed fixed bed reactor and Fourier transform infrared spectroscopy (FTIR). Abul-Milh discovered that HNCO was produced in the reaction between NH3 and CaCO3. The HNCO production rate was in accordance with the CO2 release rate. Thus, Abul-Milh proposed that Eq. (1) took place on the CaCO3 surface.
CaCO3 þ NH3 ! CaO þ HNCO þ H2 O
ð1Þ
Shimizu [14] studied the interaction between NH3 and CaCO3 at 1123 K and 75% CO2 using a fixed bed reactor. Urea deposit was found at the outlet of the reactor, where temperature was low. The production of urea was inhibited in the presence of H2O. Both Abul-Milh [18] and Shimizu [14] found that the products of the reaction between NH3 and CaCO3 were NO and N2 in the presence of O2; no HNCO or urea was observed. Neagle [19] studied NH3 adsorption on the CaCO3 surface using infrared spectroscopy (IR) at room temperature and found no obvious adsorption of NH3. Tang [17] also studied the NH3 adsorption on the CaCO3 surface at 373 K using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and obtained the same results. The temperature programmed desorption of CaCO3 exposed to NH3 by Tang [17] showed that no obvious products were observed below the CaCO3 decomposition temperature. Neagle [19] and Al-Hosney [20] studied the adsorption of H2O, which has similar molecular characteristic as NH3, on the CaCO3 surface. The experimental results showed that Eq. (2) occurred as follows:
CaCO3 þ H2 O ! Ca2þ OH HCO3
ð2Þ
Based on previous studies, the mechanism of CaCO3 in the SNCR deNOx process can be speculated that the first step of NH3 conversion was the reaction between CaCO3 and NH3. The reaction products then reacted with O2 or other species in the reactant gas. However, the complete reaction mechanism and the kinetic model for the CaCO3-involved SNCR deNOx process have not been reported yet. CaCO3 has significant catalytic effect on the SNCR deNOx process, but the reported relative studies are limited and some results are contradictory. Particle concentration, temperature, and reactants distribute non-uniformly in the pre-calciner. When the SNCR deNOx process is applied to the pre-calciner, a reasonable mechanism and an accurate kinetic model for the influence of CaCO3 is urgently required for the SNCR deNOx process design, by which the inhibitive catalytic effect of CaCO3 on deNOx efficiency can be alleviated. Thus, in this work, the influence of CaCO3 on the SNCR deNOx process was experimentally investigated using a fixed bed reactor. The catalytic mechanism of CaCO3 was also analyzed. A kinetic model was proposed to describe the CaCO3-involved SNCR deNOx process. 2. Experimental 2.1. Samples Analytically pure CaCO3 samples were used in the experiment. The properties of the CaCO3 samples are listed in Table 1. 2.2. Setup and gas analysis The fixed bed experimental system is shown in Fig. 1. A quartz reactor consisted of an external and an internal section (25 mm in length and 10 mm in diameter) was used. CaCO3 samples were laid on the bottom of the internal section and distributed by analytically pure quartz wool. The quartz reactor was inserted into the constant temperature zone of an electric furnace. The reactant gas (NH3, NO, and O2) was 500 Nml/min in flow rate and fed into the reactor from different inlets. The experimental system operated at atmospheric pressure. As presented by Khinast [21] and Hu [22], the equilibrium partial pressure of CO2 in CaCO3 decomposition was around 50% at 1123 K and atmospheric pressure. Thus, CO2 was used as balance to prevent CaCO3 from decomposition. The influence of CaCO3 was studied in the temperature range of 873 K to 1123 K, in which the influence of gas-phase SNCR deNOx reactions was negligible [23,24]. Thus, the experimental results just represented the gas–solid reactions in the CaCO3-involved SNCR deNOx process. FTIR (Nicolet 6700) was used in the experiment to analyze the exhaust gas. The FTIR was calibrated using the method presented by Li [16]; the measurement error was <1%. 2.3. Method Experimental conditions are listed in Table 2. NH3 conversion (aNH3 ) is defined by Eq. (3). The products of CaCO3-catalyzed NH3 Table 1 Properties of CaCO3 samples. CaCO3 content (%) Average particle diameter (lm) Specific surface area (m2/g) Specific pore volume (cm3/g) Average pore diameter (nm) Particle density (kg/m3)
99.7 13.2 10.82 0.0173 6.387 2606
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Fig. 1. Schematic of the fixed bed reactor system.
Table 2 Experimental conditions.
a
a
Parameter
Value
Amount of CaCO3 (mg) Temperature (K) Flow rate (Nml/min) NH3 (ppm) NO (ppm) O2 (vol%)
50 1123 (873–1123) 500 500 (100–500) 500 (100–500) 2 (0–4)
Variation within parentheses.
oxidation were NO and N2. No other products were observed. Thus, NO selectivity (SNO ) is defined by Eq. (4).
aNH3 ¼
SNO ¼
C NH3 ; in C NH3 ; out 100% C NH3 ; in
C NO; out 100% C NH3 ; in C NH3 ; out
ð3Þ
ð4Þ
where C NH3 ; in and C NH3 ; out are the NH3 concentrations at the inlet and outlet of the reactor, respectively. C NO; out is the NO concentration at the outlet of the reactor. 3. Results and discussion 3.1. Influence of CaCO3 on NH3 conversion in the absence of O2 The gas-phase reactions of NH3 were found to be very weak and negligible at or below 1123 K, while the presence of CaCO3 could catalyze NH3 conversion. The influence of CaCO3 on NH3 conversion in the absence of O2 at 1123 K as a function of NH3 inlet concentration is shown in Fig. 2. Experimental results showed that NH3 conversion was maintained at approximately 50% with the increase in NH3 inlet concentration from 100 ppm to 500 ppm. The invariant NH3 conversion with increasing NH3 inlet concentration demonstrated that the CaCO3-catalyzed NH3 conversion in the
Fig. 2. Influence of CaCO3 on NH3 conversion in the absence of O2. (50 mg CaCO3, 100–500 ppm NH3, 1123 K).
absence of O2 is a first order reaction with respect to NH3 concentration. Shimizu [14] reported that HNCO and N2 were formed in the reaction between limestone and NH3. Since the infrared characteristic absorption peak positions of HNCO and CO2 are highly coincidental, the change of HNCO was severely concealed by high concentration CO2 which was used as balance gas to prevent CaCO3 decomposition in this work. Thus, the measured FTIR spectrums of inlet and outlet gases in this experiment had little difference. To verify the product of CaCO3-catalyzed NH3 conversion in the absence of O2, a similar experiment was developed with Ar as balance. In this atmosphere, CaCO3 will gradually decompose to CaO. The outlet gas was analyzed by FTIR as a function of time as shown in Fig. 3. NH3 and HNCO were found in the experiment. Before the addition of CaCO3, NH3 had no change. Once CaCO3 was added, the concentration of NH3 dramatically dropped and the concentration of HNCO dramatically increased from zero to nearly 200 ppm. The
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in the absence of O2 at each temperature. As NO did not participate in the reaction process, the products in the CaCO3-catalyzed NH3 + NO system can be speculated to be the same as that in CaCO3-catalyzed NH3 conversion in the absence of O2. In the NH3 + NO system, CaCO3 only catalyzed NH3 conversion, but did not catalyze NO reduction by NH3. NO did not participate in the rate controlling step of CaCO3-catalyzed NH3 conversion, nor reacted with the intermediate products in NH3 conversion. 3.3. Influence of CaCO3 on NH3 oxidation
NH3 and NO outlet concentrations in the CaCO3-involved NH3 + NO system as a function of temperature are shown in Fig. 4. NH3 outlet concentrations in CaCO3-catalyzed NH3 conversion in the absence of O2 at different temperatures were also plotted in Fig. 4. In the CaCO3-involved NH3 + NO system, NH3 concentration decreased from 483 ppm to 249 ppm, as the temperature increased from 873 K to 1123 K. Meanwhile, NO outlet concentration was kept at almost the same value as that of the inlet. NH3 outlet concentration in the CaCO3-involved NH3 + NO system was almost equal to that of the CaCO3-catalyzed NH3 conversion
CaCO3 was found to catalyze NH3 oxidation in the presence of O2. NO was formed in the product gas, while no other nitrogen oxides were found in the product gas. Shimizu [14] studied the reaction between NH3 and CaCO3 in the presence of O2 using gas chromatography, and found that no HNCO was produced in the experiment. This result was also confirmed by the experiments discussed in Section 3.5. Based on the mass balance analysis of N element, the products of CaCO3-catalyzed NH3 oxidation were determined to be NO and N2. NH3 conversion and NO selectivity during CaCO3-catalyzed NH3 oxidation as a function of temperature are shown in Fig. 5. NH3 conversions in CaCO3-catalyzed NH3 conversion in the absence of O2 and the CaCO3-invovled NH3 + NO system are also plotted in Fig. 5. NH3 conversion increased from 4% at 873 K to 52% at 1123 K during CaCO3-catalyzed NH3 oxidation, which was almost the same as the other two conditions. NO selectivity during CaCO3-catalyzed NH3 oxidation decreased from 66.5% at 873 K to 42.3% at 1123 K. Experimental results demonstrated that CaCO3 has a catalytic effect on NH3 oxidation and that NH3 conversion increases, whereas NO selectivity decreases with increasing temperature. NH3 conversions were the same in CaCO3-catalyzed NH3 conversion in the absence of O2, oxidation, and NH3 + NO system, which indicated that the mechanism of CaCO3 in NH3 conversion had the same rate-controlling step in the three reaction processes. Both O2 and NO do not participate in the rate-controlling step of NH3 conversion; thus, O2 and NO have no effect on NH3 conversion. NH3 can be converted to both NO and N2 during CaCO3-catalyzed NH3 oxidation. Product selectivity is determined by the competition of the two pathways. The rate increase in N2 production is larger than that in NO production with increasing temperature; thus, NO selectivity decreases. NH3 conversion and NO selectivity during CaCO3-catalyzed NH3 oxidation as a function of NH3 inlet concentration are shown in Fig. 6. With the increase in NH3 inlet concentration from
Fig. 4. Influence of CaCO3 on the NH3 + NO system. (50 mg CaCO3, 500 ppm NH3, 500 ppm NO, 873–1123 K).
Fig. 5. Influence of CaCO3 on NH3 oxidation at different temperatures. (50 mg CaCO3, 500 ppm NH3, 2% O2, 873–1123 K).
Fig. 3. Products of CaCO3-catalyzed NH3 conversion in the absence of O2 during CaCO3 decomposition. (50 mg CaCO3, 900 ppm NH3, 1123 K).
sum of NH3 and HNCO concentrations in the outlet gas was quite close to the inlet NH3 concentration right at the moment of adding CaCO3. During CaCO3 decomposition process, more CaCO3 decomposed to CaO, and CaO only catalyzed NH3 decomposition to N2 and H2 [23]. So the HNCO concentration and the sum of NH3 and HNCO concentrations gradually decreased with increasing time. Combined with Shimizu’s report [14], Fig. 3 demonstrated that HNCO is the main product of CaCO3-catalyzed NH3 conversion in the absence of O2 and a very small amount of N2 is also formed during this process.
3.2. Influence of CaCO3 on the NH3 + NO system
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Fig. 6. Influence of CaCO3 on NH3 oxidation at different NH3 inlet concentrations. (50 mg CaCO3, 100–1000 ppm NH3, 2% O2, 1123 K).
100 ppm to 500 ppm, NH3 conversion changed little, whereas NO selectivity decreased from 98.9% to 28.5%. Experimental results demonstrated that CaCO3-catalyzed NH3 oxidation is a first order reaction with respect to NH3 concentration. Both NO and N2 are produced in CaCO3-catalyzed NH3 oxidation. One nitrogen atom is required to produce NO, whereas two nitrogen atoms are required to produce N2; thus, N2 production is more sensitive to NH3 concentration. N2 production accelerates more than that of NO production with increasing NH3 inlet concentration; thus, NO selectivity decreases. NH3 conversion and NO selectivity during CaCO3-catalyzed NH3 oxidation as a function of O2 concentration are shown in Fig. 7. With the increase in O2 concentration from 0.2% to 4%, NH3 conversion was maintained at approximately 50%, whereas NO selectivity increased from 21.7% to 51.6%. The invariant NH3 conversion with increasing O2 concentration demonstrated that O2 does not participate in the rate-controlling step of NH3 conversion. However, O2 is involved in the subsequent reactions after the rate-controlling step and influences the product selectivity. The increase in O2 concentration accelerates the NH3 conversion to NO; thus, NO selectivity increases. The linear increase in NO selectivity with increasing O2 concentration also manifests that the adsorption of O2 on the CaCO3 surface is weak.
Fig. 7. Influence of CaCO3 on NH3 oxidation at different O2 concentrations. (50 mg CaCO3, 500 ppm NH3, 0.2–4% O2, 1123 K).
13
Fig. 8. Influence of CaCO3 on the SNCR deNOx process. (50 mg CaCO3, 500 ppm NH3, 500 ppm NO, 2% O2, 873–1123 K).
3.4. Influence of CaCO3 on the SNCR deNOx process The influence of CaCO3 on the SNCR deNOx process in the temperature range from 873 K to 1123 K is shown in Fig. 8. In the studied temperature range, the gas-phase reactions were weak and negligible. In the presence of CaCO3, NH3 concentration decreased from 479 ppm at 873 K to 229 ppm at 1123 K, whereas NO concentration increased from 509 ppm to 582 ppm. Experimental results demonstrated that CaCO3 catalyzes the NH3 oxidation to NO and inhibits the deNOx efficiency of the SNCR deNOx process.
3.5. Mechanism of CaCO3 catalysis in the SNCR deNOx process NH3 outlet concentration during CaCO3-catalyzed NH3 conversion in the absence or presence of O2 as a function of time at 1123 K is shown in Fig. 9. Up to 1000 ppm of NH3 was fed into the reactor in stage 1. NH3 concentration decreased gradually until reaching zero when the addition of NH3 was stopped at the end of stage 1, forming a ‘‘NH3 tail’’ in stage 2. Up to 1000 ppm of NH3 with 2% O2 were fed into the reactor in stage 3. At the end of stage 3, the addition of both NH3 and O2 were stopped and the NH3 concentration decreased rapidly until reaching zero. No ‘‘NH3 tail’’ was observed in stage 4. Previous studies [17,19] indicated that the ‘‘NH3 tail’’ was not caused by desorption of NH3 on the CaCO3 surface. Thus, the ‘‘NH3 tail’’ in stage 2 can be described as follows. NH3 first reacts with CaCO3 to produce HNCO. Urea is produced from the reaction between NH3 and HNCO in the low temperature zone of the reactor. Once produced, urea deposits to the surface of the reactor. When the addition of NH3 is stopped, the produced urea decomposes to NH3 and HNCO again. Thus, the ‘‘NH3 tail’’ is formed. In the presence of O2, the disappearance of the ‘‘NH3 tail’’ demonstrated that the reaction pathway of NH3 conversion was changed and HNCO was not produced. Experimental results obtained in this work combined with the previous studies [14,18] showed that the products of CaCO3-catalyzed NH3 oxidation were NO and N2. HNCO was not produced in the reaction in the presence of O2. The morphology of CaCO3 particles before and after catalyzing NH3 oxidation is shown in Fig. 10. The morphology of CaCO3 particles was obviously changed after catalyzing NH3 oxidation. Before the reaction, CaCO3 particles were hexahedron with obvious corners and edges. However, the particles became ellipsoid shaped after the reaction, and the contiguous particles were fused to form
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Fig. 9. NH3 outlet concentration during CaCO3-catalyzed NH3 conversion in the absence or presence of O2. (50 mg CaCO3, 1123 K, Stage 1: CO2 + 1000 ppm NH3; Stage 2: only CO2; Stage 3: CO2 + 1000 ppm NH3 + 2% O2; stage 4: only CO2).
larger particles. The CaCO3 particles in different views showed the same trend. The phenomenon demonstrated that CaCO3 itself participated in the reaction, indicating that the role of CaCO3 particles is not limited to providing a catalytic surface. Thus, the morphology of CaCO3 particles changed and the contiguous particles were fused into larger ones in the reproduction process of CaCO3. The mechanism of CaCO3 in the SNCR deNOx process is summarized below. The first step as well as the rate-controlling step of NH3 conversion is the reaction between NH3 and CaCO3 to produce the intermediate substance. According to the previous studies [19,25], the intermediate substance is speculated to be
CaNH2HCO3. The reaction is shown in Eq. (5). Both O2 and NO do not participate in the reaction; thus, they have no effect on NH3 conversion. Given that the dehydration of bicarbonate radical easily occurs, CaNH2HCO3 decomposes into CaO, HNCO, and H2O in the absence of O2, as shown in Eq. (6). Meanwhile, a very small amount of NH2 radicals in CaNH2HCO3 dimerize to N2H4 and decompose into N2 and H2 subsequently, as shown in Eq. (7). The produced CaO reacts with CO2 in the reactant gas to reproduce CaCO3, as shown in Eq. (8). Thus, the morphology of CaCO3 particles changes and contiguous particles are fused into larger ones after the reaction. Disappearance of the ‘‘NH3 tail’’ in the presence of O2 along with the previous studies [14,18] indicates that O2 inhibits the production of HNCO through the decomposition of CaNH2HCO3, and oxidizes NH2 radicals in CaNH2HCO3 to NO, as shown in Eq. (9). Some NH2 radicals also decompose into N2 and H2 as shown in Eq. (7). The increase in O2 concentration increases the reaction rate of Eq. (9); thus, NO selectivity increases. In the CaCO3-catalyzed SNCR deNOx process, Eq. (5) is the rate-controlling step that determines the rate of NH3 conversion. Eqs. (7) and (9) determine the production of N2 and NO, respectively. Thus, product selectivity is determined by the competition of the two reactions. Temperature and reactant gas composition influence the product selectivity by influencing the relative reaction rates of Eqs. (7) and (9). In the SNCR deNOx process, CaCO3 plays like a catalyst through a chain reaction.
CaCO3 þ NH3 ! CaNH2 HCO3
ð5Þ
CaNH2 HCO3 ! CaO þ HNCO þ H2 O
ð6Þ
2NH2 ! N2 H4 ! N2 þ 2H2
ð7Þ
CaO þ CO2 ! CaCO3
ð8Þ
Fig. 10. Morphology of the CaCO3 particles before and after catalytic reactions. (a) and (c) before reaction; (b) and (d) after reaction.
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NH2 þ O2 ! NO þ H2 O
ð9Þ
4. Modeling The reaction process may be influenced by the diffusion of the reactants [26]. Thus, the diffusion of the reactants is considered in the kinetic model. For the reaction conditions, Damkohler number (Da) is 0.012, indicating that the influence of external diffusion can be ignored. Thus, only the influence of internal diffusion is considered in the kinetic model. The effectiveness factor, g, is used to evaluate the influence of internal diffusion and is calculated as Eq. (10).
g¼
1 1 1 /S tanhð3/S Þ 3/S
where /S ¼ R3
qffiffiffiffiffiffiffiffiffiffiffi kV De; NH
ð10Þ
is the Thiele modulus for spherical particles; R
3
(m) is the particle diameter; kV (1/s) is the reaction rate constant; De; NH3 (m2/s) is the effective diffusion coefficient of NH3, De; NH3 ¼ se DNH3 ; e is the porosity particle; s is the tortuosity factor
Table 3 Kinetic parameters of the reactions. Rate constant
E/R
A
kNH3 ! NH2 ðm=sÞ k12 (–)
12,989 K 7397 K
4.93 m/s 1.78 105
of CaCO3, e ¼ 0:227 and s ¼ 2; DNH3 (m2/s) is the diffusion coefficient of NH3 and calculated as Eq. (11).
1 1 1 ¼ þ DNH3 Dm; NH3 Dk; NH3
ð11Þ
where Dm; NH3 and Dk; NH3 are the molecular and Knudsen diffusion coefficient of NH3, respectively. In the CaCO3-invovled SNCR deNOx process, NH3 first reacts with CaCO3 to produce CaNH2HCO3. The reaction is first order with respect to NH3. Thus, the reaction rate of NH3 can be expressed using Eq. (12).
dC NH3 _ CaCO3 sCaCO3 kNH3 ¼ m ds
! NH2
gNH3 ! NH2 C NH3
ð12Þ
Fig. 11. Comparisons between the experimental and simulation results. (a) during CaCO3-catalyzed NH3 conversion in the absence of O2, (b) during CaCO3-catalyzed NH3 oxidation at different NH3 inlet concentration, (c) during CaCO3-catalyzed NH3 oxidation at different O2 concentration, and (d) during the CaCO3-catalyzed NH3 + NO + O2 system.
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_ CaCO3 (kg/m3) is the concentration of CaCO3 in the sample where m bed; sCaCO3 (m2/kg) is the specific surface area of CaCO3 particles, and kNH3 ! NH2 (m/s) is the reaction rate constant of the reaction between NH3 and CaCO3. NH2, the intermediate radical of NH3 conversion, can be oxidized to NO by O2 or dimerizes into N2. The completion of the two reactions determines product selectivity. The kinetic analysis of the two reactions is simplified by the quasi-steady assumption. However, the kinetic model still has too many variables and is too complex to be fitted with the experimental data precisely. Thus, in the calculation of product selectivity, the reaction process is simplified and summarized by two overall reactions: NH3 conversion to NO and NH3 conversion to N2. NO selectivity is calculated using Eq. (13). In addition, the two simplified overall reactions do not reflect the real conversion pathways of NH3. Therefore, NH3 conversion must be calculated using Eq. (12).
SNO ¼
kNH3 kNH3
! NO C NH3 C O2
! NO C NH3 C O2
þ
kNH3 ! N2 C 2NH3
¼
k12 C O2 k12 C O2 þ k12 C NH3
ð13Þ
where kNH2 ! NO (m4/mol s) and kNH2 ! N2 (m4/mol s) are the reaction rate constants of NH3 to NO and NH3 to N2, respectively. k12 represents the ratio of kNH2 ! NO to kNH2 ! N2 and defined as Eq. (14). k12 is correlated to the Arrhenius form for convenience; thus, A12 represents the ratio of the two pre-exponential factors and E12 represents the difference between the two activation energies.
kNH2 ! NO ANH2 ! NO ENH2 ¼ exp kNH2 ! N2 ANH2 ! N2 E12 ¼ A12 exp RT
k12 ¼
! NO
ENH2 RT
! N2
Acknowledgments This work was supported by the fund from the National Key Technologies R&D Program (2011BAE29B03) and ‘‘111’’ Project (B13001).
References
ð14Þ
Thus, the expression of NO reaction rate is shown as Eq. (15).
dC NH3 dC NH3 k12 C O2 dC NO ¼ SNO ¼ ds ds ds k12 C O2 þ k12 C NH3
inlet concentration, but increased with increasing O2 concentration. The deNOx efficiency of the SNCR deNOx process was inhibited by CaCO3 mainly by catalyzing NH3 oxidation to NO. Mechanism analysis demonstrated that the rate-controlling step of NH3 conversion on the surface of CaCO3 is the direct reaction between NH3 and CaCO3 that produces CaNH2HCO3. In the absence of O2, N elements in CaNH2HCO3 are converted to HNCO or N2. The produced CaO reacts with CO2 to reproduce CaCO3. The presence of O2 inhibits the conversion of CaNH2HCO3 to HNCO. The amide radical in CaNH2HCO3 can be oxidized to NO by O2, or dimerizes to N2H4 and subsequently decomposes into N2. The competition of the two reactions determines the product selectivity. Temperature, NH3, and O2 concentration influences the product selectivity by influencing the relative reaction rates of the two reactions. A kinetic model was established for the CaCO3-involved SNCR deNOx process based on the mechanism analysis. The experimental results were well predicted by the kinetic model, which proved the accuracy of the kinetic model.
ð15Þ
The reaction rate constants are correlated using the experimental data at different temperatures, and the pre-exponential factors and activation energies are obtained by double-log transformation. The obtained kinetic parameters are listed in Table 3. Fig. 11(a) shows the experimental and simulation results in the interaction between CaCO3 and NH3. Fig. 11(b) and (c) shows the experimental and simulation results in CaCO3-catalyzed NH3 oxidation at different NH3 inlet concentrations and O2 concentrations. The simulation results coincide with the experimental results. In the studied temperature range, the influence of gas phase reactions is negligible. Thus, NH3 and NO outlet concentrations in the CaCO3-involved SNCR deNOx process can be calculated using Eqs. (12) and (15). Fig. 11(d) shows the simulation and experimental results in CaCO3-involved SNCR deNOx process, which are in good agreement. The comparison between experimental and simulation results demonstrates that the proposed kinetic model well predicts the influence of CaCO3 on the SNCR deNOx process. 5. Conclusions CaCO3 had a catalytic effect on NH3 conversion in the absence or presence of O2, but had no effect on NO reduction by NH3. The product of CaCO3-catalyzed NH3 conversion in the absence of O2 was mainly HNCO, while the products in the presence of O2 were NO and N2. Both CaCO3-catalyzed NH3 conversion in the absence or presence of O2 showed first order characteristics with respect to NH3 concentration, and NH3 conversion increased with increasing temperature. In CaCO3-catalyzed NH3 oxidation, NH3 conversion was kept stable with increasing O2 and NH3 concentration; NO selectivity decreased with increasing temperature and NH3
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Study on the catalysis of CaCO3 in the SNCR deNOx process for cement kilns Shi-long Fu, Qiang Song ⇑, Qiang Yao Key Laboratory of Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, 100084 Beijing, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
CaCO3 inhibits the deNOx
performance of the SNCR deNOx process. CaCO3 catalyzes NH3 conversion through a chain reaction as given in the mechanism. Reaction between NH3 and CaCO3 is the rate-controlling step of NH3 conversion. A kinetic model is proposed to describe the influence of CaCO3.
a r t i c l e
i n f o
Article history: Received 27 July 2014 Received in revised form 12 September 2014 Accepted 13 September 2014 Available online 28 September 2014 Keywords: SNCR CaCO3 NH3 NO Kinetic model
a b s t r a c t The selective non-catalytic reduction (SNCR) deNOx process is influenced by high concentration CaCO3 particles in the pre-calciner when used in cement industry. The influence of CaCO3 on the SNCR deNOx process was investigated using a fixed bed reactor with temperature ranging from 873 K to 1123 K. Experimental results showed that CaCO3 catalyzes NH3 conversion to HNCO and N2 in the absence of O2, and catalyzes NH3 oxidation to NO and N2 in the presence of O2. Both reactions show first order characteristics with respect to NH3 concentration. CaCO3 has no catalytic effect on NO reduction by NH3. The deNOx efficiency of the SNCR deNOx process is inhibited by CaCO3 mainly by catalyzing NH3 oxidation to NO. Mechanism analysis showed that the reaction between NH3 and CaCO3 is the rate-controlling step of NH3 conversion. In the absence of O2, the intermediate product decomposed to CaO, HNCO, H2O and N2, whereas CaO reacted with CO2 to reproduce CaCO3. O2 did not participate in the rate-controlling step of NH3 conversion. However, in the presence of O2, the intermediate product of the reaction between NH3 and CaCO3 can be oxidized to NO by O2 or decomposed into N2 through dimerization reaction. Product selectivity is determined by the competition of these two reactions. A kinetic model was proposed for the CaCO3-involved SNCR deNOx process which well predicted the influence of CaCO3. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxides (NOx) are among the most hazardous air pollutants. In China, the cement industry contributes to 11% of the total
⇑ Corresponding author. Tel./fax: +86 010 62781740. E-mail address: [email protected] (Q. Song). http://dx.doi.org/10.1016/j.cej.2014.09.048 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
NOx emission and has become the third largest industrial NOx emission source [1,2]. Stricter air protection act has been imposed by the Chinese government recently, which means that most of the cement kilns will have to install appropriate deNOx devices. Among all the NOx control technologies, the selective non-catalytic reduction (SNCR) technology is regarded as the most suitable technology in consideration of the deNOx performance and cost [3,4]. The SNCR deNOx technology for utility boilers has been well studied,
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and the deNOx efficiency is approximately 40%. Temperature, reactant composition, and residence time are factors that influence the SNCR deNOx efficiency [5–9]. However, the SNCR deNOx devices that have operated on cement kilns show a diverse range of deNOx efficiency ranging from 15% to 80% [10], which indicates that the SNCR deNOx process used in cement kilns is more complex and different from those used in the utility boilers. Pre-calciner is considered as the most suitable position for the SNCR deNOx process in cement kiln because of its appropriate configuration and temperature distribution [10]. The high concentration of Ca-based particles (mainly composed of CaO and CaCO3) in the pre-calciner (the most suitable position for SNCR process) is the biggest difference from the utility boiler. The concentration of Ca-based particles can be as high as 1 kg/Nm3 in the pre-calciner, and the portion of CaCO3 can be over 60% at the entrance [11]. Studies by Jensen [12], Shimizu [13,14], Li [15,16] and Tang [17] all found that CaCO3 has inhibitive effect on the SNCR deNOx process. Thus, the influence of CaCO3 must be considered when the SNCR deNOx technology is applied in the cement industry. Gas-phase SNCR deNOx process mainly involves the reactions among NH3, NO, and O2. CaCO3 may influence the SNCR deNOx process by catalyzing NH3 decomposition, oxidation, and NO reduction by NH3. Shimizu [13] studied the influence of CaCO3 on NH3 oxidation and found that CaCO3 had a catalytic effect on NH3 oxidation, resulting in the production of NO and N2. Li [16] studied the CaCO3-involved NH3 decomposition, oxidation, and NO reduction by NH3 within the temperature range of 923 K to 1123 K. CaCO3 was found to have an obvious catalytic effect on NH3 decomposition and oxidation, but had no effect on the reaction of NH3 + NO. NO selectivity decreased with increasing temperature during CaCO3-catalyzed NH3 oxidation. Tang’s study [17] in the temperature range of 923 K to 1123 K also showed that CaCO3 catalyzed NH3 decomposition and oxidation. During CaCO3-catalyzed NH3 oxidation, NH3 conversion was not affected by NO or O2; however, NO selectivity increased with increasing temperature, which was in conflict with the result obtained by Li [16]. Abul-Milh [18] studied the interaction between NH3 and CaCO3 in CaCO3 decomposition process using temperature programmed fixed bed reactor and Fourier transform infrared spectroscopy (FTIR). Abul-Milh discovered that HNCO was produced in the reaction between NH3 and CaCO3. The HNCO production rate was in accordance with the CO2 release rate. Thus, Abul-Milh proposed that Eq. (1) took place on the CaCO3 surface.
CaCO3 þ NH3 ! CaO þ HNCO þ H2 O
ð1Þ
Shimizu [14] studied the interaction between NH3 and CaCO3 at 1123 K and 75% CO2 using a fixed bed reactor. Urea deposit was found at the outlet of the reactor, where temperature was low. The production of urea was inhibited in the presence of H2O. Both Abul-Milh [18] and Shimizu [14] found that the products of the reaction between NH3 and CaCO3 were NO and N2 in the presence of O2; no HNCO or urea was observed. Neagle [19] studied NH3 adsorption on the CaCO3 surface using infrared spectroscopy (IR) at room temperature and found no obvious adsorption of NH3. Tang [17] also studied the NH3 adsorption on the CaCO3 surface at 373 K using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and obtained the same results. The temperature programmed desorption of CaCO3 exposed to NH3 by Tang [17] showed that no obvious products were observed below the CaCO3 decomposition temperature. Neagle [19] and Al-Hosney [20] studied the adsorption of H2O, which has similar molecular characteristic as NH3, on the CaCO3 surface. The experimental results showed that Eq. (2) occurred as follows:
CaCO3 þ H2 O ! Ca2þ OH HCO3
ð2Þ
Based on previous studies, the mechanism of CaCO3 in the SNCR deNOx process can be speculated that the first step of NH3 conversion was the reaction between CaCO3 and NH3. The reaction products then reacted with O2 or other species in the reactant gas. However, the complete reaction mechanism and the kinetic model for the CaCO3-involved SNCR deNOx process have not been reported yet. CaCO3 has significant catalytic effect on the SNCR deNOx process, but the reported relative studies are limited and some results are contradictory. Particle concentration, temperature, and reactants distribute non-uniformly in the pre-calciner. When the SNCR deNOx process is applied to the pre-calciner, a reasonable mechanism and an accurate kinetic model for the influence of CaCO3 is urgently required for the SNCR deNOx process design, by which the inhibitive catalytic effect of CaCO3 on deNOx efficiency can be alleviated. Thus, in this work, the influence of CaCO3 on the SNCR deNOx process was experimentally investigated using a fixed bed reactor. The catalytic mechanism of CaCO3 was also analyzed. A kinetic model was proposed to describe the CaCO3-involved SNCR deNOx process. 2. Experimental 2.1. Samples Analytically pure CaCO3 samples were used in the experiment. The properties of the CaCO3 samples are listed in Table 1. 2.2. Setup and gas analysis The fixed bed experimental system is shown in Fig. 1. A quartz reactor consisted of an external and an internal section (25 mm in length and 10 mm in diameter) was used. CaCO3 samples were laid on the bottom of the internal section and distributed by analytically pure quartz wool. The quartz reactor was inserted into the constant temperature zone of an electric furnace. The reactant gas (NH3, NO, and O2) was 500 Nml/min in flow rate and fed into the reactor from different inlets. The experimental system operated at atmospheric pressure. As presented by Khinast [21] and Hu [22], the equilibrium partial pressure of CO2 in CaCO3 decomposition was around 50% at 1123 K and atmospheric pressure. Thus, CO2 was used as balance to prevent CaCO3 from decomposition. The influence of CaCO3 was studied in the temperature range of 873 K to 1123 K, in which the influence of gas-phase SNCR deNOx reactions was negligible [23,24]. Thus, the experimental results just represented the gas–solid reactions in the CaCO3-involved SNCR deNOx process. FTIR (Nicolet 6700) was used in the experiment to analyze the exhaust gas. The FTIR was calibrated using the method presented by Li [16]; the measurement error was <1%. 2.3. Method Experimental conditions are listed in Table 2. NH3 conversion (aNH3 ) is defined by Eq. (3). The products of CaCO3-catalyzed NH3 Table 1 Properties of CaCO3 samples. CaCO3 content (%) Average particle diameter (lm) Specific surface area (m2/g) Specific pore volume (cm3/g) Average pore diameter (nm) Particle density (kg/m3)
99.7 13.2 10.82 0.0173 6.387 2606
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Fig. 1. Schematic of the fixed bed reactor system.
Table 2 Experimental conditions.
a
a
Parameter
Value
Amount of CaCO3 (mg) Temperature (K) Flow rate (Nml/min) NH3 (ppm) NO (ppm) O2 (vol%)
50 1123 (873–1123) 500 500 (100–500) 500 (100–500) 2 (0–4)
Variation within parentheses.
oxidation were NO and N2. No other products were observed. Thus, NO selectivity (SNO ) is defined by Eq. (4).
aNH3 ¼
SNO ¼
C NH3 ; in C NH3 ; out 100% C NH3 ; in
C NO; out 100% C NH3 ; in C NH3 ; out
ð3Þ
ð4Þ
where C NH3 ; in and C NH3 ; out are the NH3 concentrations at the inlet and outlet of the reactor, respectively. C NO; out is the NO concentration at the outlet of the reactor. 3. Results and discussion 3.1. Influence of CaCO3 on NH3 conversion in the absence of O2 The gas-phase reactions of NH3 were found to be very weak and negligible at or below 1123 K, while the presence of CaCO3 could catalyze NH3 conversion. The influence of CaCO3 on NH3 conversion in the absence of O2 at 1123 K as a function of NH3 inlet concentration is shown in Fig. 2. Experimental results showed that NH3 conversion was maintained at approximately 50% with the increase in NH3 inlet concentration from 100 ppm to 500 ppm. The invariant NH3 conversion with increasing NH3 inlet concentration demonstrated that the CaCO3-catalyzed NH3 conversion in the
Fig. 2. Influence of CaCO3 on NH3 conversion in the absence of O2. (50 mg CaCO3, 100–500 ppm NH3, 1123 K).
absence of O2 is a first order reaction with respect to NH3 concentration. Shimizu [14] reported that HNCO and N2 were formed in the reaction between limestone and NH3. Since the infrared characteristic absorption peak positions of HNCO and CO2 are highly coincidental, the change of HNCO was severely concealed by high concentration CO2 which was used as balance gas to prevent CaCO3 decomposition in this work. Thus, the measured FTIR spectrums of inlet and outlet gases in this experiment had little difference. To verify the product of CaCO3-catalyzed NH3 conversion in the absence of O2, a similar experiment was developed with Ar as balance. In this atmosphere, CaCO3 will gradually decompose to CaO. The outlet gas was analyzed by FTIR as a function of time as shown in Fig. 3. NH3 and HNCO were found in the experiment. Before the addition of CaCO3, NH3 had no change. Once CaCO3 was added, the concentration of NH3 dramatically dropped and the concentration of HNCO dramatically increased from zero to nearly 200 ppm. The
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in the absence of O2 at each temperature. As NO did not participate in the reaction process, the products in the CaCO3-catalyzed NH3 + NO system can be speculated to be the same as that in CaCO3-catalyzed NH3 conversion in the absence of O2. In the NH3 + NO system, CaCO3 only catalyzed NH3 conversion, but did not catalyze NO reduction by NH3. NO did not participate in the rate controlling step of CaCO3-catalyzed NH3 conversion, nor reacted with the intermediate products in NH3 conversion. 3.3. Influence of CaCO3 on NH3 oxidation
NH3 and NO outlet concentrations in the CaCO3-involved NH3 + NO system as a function of temperature are shown in Fig. 4. NH3 outlet concentrations in CaCO3-catalyzed NH3 conversion in the absence of O2 at different temperatures were also plotted in Fig. 4. In the CaCO3-involved NH3 + NO system, NH3 concentration decreased from 483 ppm to 249 ppm, as the temperature increased from 873 K to 1123 K. Meanwhile, NO outlet concentration was kept at almost the same value as that of the inlet. NH3 outlet concentration in the CaCO3-involved NH3 + NO system was almost equal to that of the CaCO3-catalyzed NH3 conversion
CaCO3 was found to catalyze NH3 oxidation in the presence of O2. NO was formed in the product gas, while no other nitrogen oxides were found in the product gas. Shimizu [14] studied the reaction between NH3 and CaCO3 in the presence of O2 using gas chromatography, and found that no HNCO was produced in the experiment. This result was also confirmed by the experiments discussed in Section 3.5. Based on the mass balance analysis of N element, the products of CaCO3-catalyzed NH3 oxidation were determined to be NO and N2. NH3 conversion and NO selectivity during CaCO3-catalyzed NH3 oxidation as a function of temperature are shown in Fig. 5. NH3 conversions in CaCO3-catalyzed NH3 conversion in the absence of O2 and the CaCO3-invovled NH3 + NO system are also plotted in Fig. 5. NH3 conversion increased from 4% at 873 K to 52% at 1123 K during CaCO3-catalyzed NH3 oxidation, which was almost the same as the other two conditions. NO selectivity during CaCO3-catalyzed NH3 oxidation decreased from 66.5% at 873 K to 42.3% at 1123 K. Experimental results demonstrated that CaCO3 has a catalytic effect on NH3 oxidation and that NH3 conversion increases, whereas NO selectivity decreases with increasing temperature. NH3 conversions were the same in CaCO3-catalyzed NH3 conversion in the absence of O2, oxidation, and NH3 + NO system, which indicated that the mechanism of CaCO3 in NH3 conversion had the same rate-controlling step in the three reaction processes. Both O2 and NO do not participate in the rate-controlling step of NH3 conversion; thus, O2 and NO have no effect on NH3 conversion. NH3 can be converted to both NO and N2 during CaCO3-catalyzed NH3 oxidation. Product selectivity is determined by the competition of the two pathways. The rate increase in N2 production is larger than that in NO production with increasing temperature; thus, NO selectivity decreases. NH3 conversion and NO selectivity during CaCO3-catalyzed NH3 oxidation as a function of NH3 inlet concentration are shown in Fig. 6. With the increase in NH3 inlet concentration from
Fig. 4. Influence of CaCO3 on the NH3 + NO system. (50 mg CaCO3, 500 ppm NH3, 500 ppm NO, 873–1123 K).
Fig. 5. Influence of CaCO3 on NH3 oxidation at different temperatures. (50 mg CaCO3, 500 ppm NH3, 2% O2, 873–1123 K).
Fig. 3. Products of CaCO3-catalyzed NH3 conversion in the absence of O2 during CaCO3 decomposition. (50 mg CaCO3, 900 ppm NH3, 1123 K).
sum of NH3 and HNCO concentrations in the outlet gas was quite close to the inlet NH3 concentration right at the moment of adding CaCO3. During CaCO3 decomposition process, more CaCO3 decomposed to CaO, and CaO only catalyzed NH3 decomposition to N2 and H2 [23]. So the HNCO concentration and the sum of NH3 and HNCO concentrations gradually decreased with increasing time. Combined with Shimizu’s report [14], Fig. 3 demonstrated that HNCO is the main product of CaCO3-catalyzed NH3 conversion in the absence of O2 and a very small amount of N2 is also formed during this process.
3.2. Influence of CaCO3 on the NH3 + NO system
S.-l. Fu et al. / Chemical Engineering Journal 262 (2015) 9–17
Fig. 6. Influence of CaCO3 on NH3 oxidation at different NH3 inlet concentrations. (50 mg CaCO3, 100–1000 ppm NH3, 2% O2, 1123 K).
100 ppm to 500 ppm, NH3 conversion changed little, whereas NO selectivity decreased from 98.9% to 28.5%. Experimental results demonstrated that CaCO3-catalyzed NH3 oxidation is a first order reaction with respect to NH3 concentration. Both NO and N2 are produced in CaCO3-catalyzed NH3 oxidation. One nitrogen atom is required to produce NO, whereas two nitrogen atoms are required to produce N2; thus, N2 production is more sensitive to NH3 concentration. N2 production accelerates more than that of NO production with increasing NH3 inlet concentration; thus, NO selectivity decreases. NH3 conversion and NO selectivity during CaCO3-catalyzed NH3 oxidation as a function of O2 concentration are shown in Fig. 7. With the increase in O2 concentration from 0.2% to 4%, NH3 conversion was maintained at approximately 50%, whereas NO selectivity increased from 21.7% to 51.6%. The invariant NH3 conversion with increasing O2 concentration demonstrated that O2 does not participate in the rate-controlling step of NH3 conversion. However, O2 is involved in the subsequent reactions after the rate-controlling step and influences the product selectivity. The increase in O2 concentration accelerates the NH3 conversion to NO; thus, NO selectivity increases. The linear increase in NO selectivity with increasing O2 concentration also manifests that the adsorption of O2 on the CaCO3 surface is weak.
Fig. 7. Influence of CaCO3 on NH3 oxidation at different O2 concentrations. (50 mg CaCO3, 500 ppm NH3, 0.2–4% O2, 1123 K).
13
Fig. 8. Influence of CaCO3 on the SNCR deNOx process. (50 mg CaCO3, 500 ppm NH3, 500 ppm NO, 2% O2, 873–1123 K).
3.4. Influence of CaCO3 on the SNCR deNOx process The influence of CaCO3 on the SNCR deNOx process in the temperature range from 873 K to 1123 K is shown in Fig. 8. In the studied temperature range, the gas-phase reactions were weak and negligible. In the presence of CaCO3, NH3 concentration decreased from 479 ppm at 873 K to 229 ppm at 1123 K, whereas NO concentration increased from 509 ppm to 582 ppm. Experimental results demonstrated that CaCO3 catalyzes the NH3 oxidation to NO and inhibits the deNOx efficiency of the SNCR deNOx process.
3.5. Mechanism of CaCO3 catalysis in the SNCR deNOx process NH3 outlet concentration during CaCO3-catalyzed NH3 conversion in the absence or presence of O2 as a function of time at 1123 K is shown in Fig. 9. Up to 1000 ppm of NH3 was fed into the reactor in stage 1. NH3 concentration decreased gradually until reaching zero when the addition of NH3 was stopped at the end of stage 1, forming a ‘‘NH3 tail’’ in stage 2. Up to 1000 ppm of NH3 with 2% O2 were fed into the reactor in stage 3. At the end of stage 3, the addition of both NH3 and O2 were stopped and the NH3 concentration decreased rapidly until reaching zero. No ‘‘NH3 tail’’ was observed in stage 4. Previous studies [17,19] indicated that the ‘‘NH3 tail’’ was not caused by desorption of NH3 on the CaCO3 surface. Thus, the ‘‘NH3 tail’’ in stage 2 can be described as follows. NH3 first reacts with CaCO3 to produce HNCO. Urea is produced from the reaction between NH3 and HNCO in the low temperature zone of the reactor. Once produced, urea deposits to the surface of the reactor. When the addition of NH3 is stopped, the produced urea decomposes to NH3 and HNCO again. Thus, the ‘‘NH3 tail’’ is formed. In the presence of O2, the disappearance of the ‘‘NH3 tail’’ demonstrated that the reaction pathway of NH3 conversion was changed and HNCO was not produced. Experimental results obtained in this work combined with the previous studies [14,18] showed that the products of CaCO3-catalyzed NH3 oxidation were NO and N2. HNCO was not produced in the reaction in the presence of O2. The morphology of CaCO3 particles before and after catalyzing NH3 oxidation is shown in Fig. 10. The morphology of CaCO3 particles was obviously changed after catalyzing NH3 oxidation. Before the reaction, CaCO3 particles were hexahedron with obvious corners and edges. However, the particles became ellipsoid shaped after the reaction, and the contiguous particles were fused to form
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Fig. 9. NH3 outlet concentration during CaCO3-catalyzed NH3 conversion in the absence or presence of O2. (50 mg CaCO3, 1123 K, Stage 1: CO2 + 1000 ppm NH3; Stage 2: only CO2; Stage 3: CO2 + 1000 ppm NH3 + 2% O2; stage 4: only CO2).
larger particles. The CaCO3 particles in different views showed the same trend. The phenomenon demonstrated that CaCO3 itself participated in the reaction, indicating that the role of CaCO3 particles is not limited to providing a catalytic surface. Thus, the morphology of CaCO3 particles changed and the contiguous particles were fused into larger ones in the reproduction process of CaCO3. The mechanism of CaCO3 in the SNCR deNOx process is summarized below. The first step as well as the rate-controlling step of NH3 conversion is the reaction between NH3 and CaCO3 to produce the intermediate substance. According to the previous studies [19,25], the intermediate substance is speculated to be
CaNH2HCO3. The reaction is shown in Eq. (5). Both O2 and NO do not participate in the reaction; thus, they have no effect on NH3 conversion. Given that the dehydration of bicarbonate radical easily occurs, CaNH2HCO3 decomposes into CaO, HNCO, and H2O in the absence of O2, as shown in Eq. (6). Meanwhile, a very small amount of NH2 radicals in CaNH2HCO3 dimerize to N2H4 and decompose into N2 and H2 subsequently, as shown in Eq. (7). The produced CaO reacts with CO2 in the reactant gas to reproduce CaCO3, as shown in Eq. (8). Thus, the morphology of CaCO3 particles changes and contiguous particles are fused into larger ones after the reaction. Disappearance of the ‘‘NH3 tail’’ in the presence of O2 along with the previous studies [14,18] indicates that O2 inhibits the production of HNCO through the decomposition of CaNH2HCO3, and oxidizes NH2 radicals in CaNH2HCO3 to NO, as shown in Eq. (9). Some NH2 radicals also decompose into N2 and H2 as shown in Eq. (7). The increase in O2 concentration increases the reaction rate of Eq. (9); thus, NO selectivity increases. In the CaCO3-catalyzed SNCR deNOx process, Eq. (5) is the rate-controlling step that determines the rate of NH3 conversion. Eqs. (7) and (9) determine the production of N2 and NO, respectively. Thus, product selectivity is determined by the competition of the two reactions. Temperature and reactant gas composition influence the product selectivity by influencing the relative reaction rates of Eqs. (7) and (9). In the SNCR deNOx process, CaCO3 plays like a catalyst through a chain reaction.
CaCO3 þ NH3 ! CaNH2 HCO3
ð5Þ
CaNH2 HCO3 ! CaO þ HNCO þ H2 O
ð6Þ
2NH2 ! N2 H4 ! N2 þ 2H2
ð7Þ
CaO þ CO2 ! CaCO3
ð8Þ
Fig. 10. Morphology of the CaCO3 particles before and after catalytic reactions. (a) and (c) before reaction; (b) and (d) after reaction.
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NH2 þ O2 ! NO þ H2 O
ð9Þ
4. Modeling The reaction process may be influenced by the diffusion of the reactants [26]. Thus, the diffusion of the reactants is considered in the kinetic model. For the reaction conditions, Damkohler number (Da) is 0.012, indicating that the influence of external diffusion can be ignored. Thus, only the influence of internal diffusion is considered in the kinetic model. The effectiveness factor, g, is used to evaluate the influence of internal diffusion and is calculated as Eq. (10).
g¼
1 1 1 /S tanhð3/S Þ 3/S
where /S ¼ R3
qffiffiffiffiffiffiffiffiffiffiffi kV De; NH
ð10Þ
is the Thiele modulus for spherical particles; R
3
(m) is the particle diameter; kV (1/s) is the reaction rate constant; De; NH3 (m2/s) is the effective diffusion coefficient of NH3, De; NH3 ¼ se DNH3 ; e is the porosity particle; s is the tortuosity factor
Table 3 Kinetic parameters of the reactions. Rate constant
E/R
A
kNH3 ! NH2 ðm=sÞ k12 (–)
12,989 K 7397 K
4.93 m/s 1.78 105
of CaCO3, e ¼ 0:227 and s ¼ 2; DNH3 (m2/s) is the diffusion coefficient of NH3 and calculated as Eq. (11).
1 1 1 ¼ þ DNH3 Dm; NH3 Dk; NH3
ð11Þ
where Dm; NH3 and Dk; NH3 are the molecular and Knudsen diffusion coefficient of NH3, respectively. In the CaCO3-invovled SNCR deNOx process, NH3 first reacts with CaCO3 to produce CaNH2HCO3. The reaction is first order with respect to NH3. Thus, the reaction rate of NH3 can be expressed using Eq. (12).
dC NH3 _ CaCO3 sCaCO3 kNH3 ¼ m ds
! NH2
gNH3 ! NH2 C NH3
ð12Þ
Fig. 11. Comparisons between the experimental and simulation results. (a) during CaCO3-catalyzed NH3 conversion in the absence of O2, (b) during CaCO3-catalyzed NH3 oxidation at different NH3 inlet concentration, (c) during CaCO3-catalyzed NH3 oxidation at different O2 concentration, and (d) during the CaCO3-catalyzed NH3 + NO + O2 system.
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S.-l. Fu et al. / Chemical Engineering Journal 262 (2015) 9–17
_ CaCO3 (kg/m3) is the concentration of CaCO3 in the sample where m bed; sCaCO3 (m2/kg) is the specific surface area of CaCO3 particles, and kNH3 ! NH2 (m/s) is the reaction rate constant of the reaction between NH3 and CaCO3. NH2, the intermediate radical of NH3 conversion, can be oxidized to NO by O2 or dimerizes into N2. The completion of the two reactions determines product selectivity. The kinetic analysis of the two reactions is simplified by the quasi-steady assumption. However, the kinetic model still has too many variables and is too complex to be fitted with the experimental data precisely. Thus, in the calculation of product selectivity, the reaction process is simplified and summarized by two overall reactions: NH3 conversion to NO and NH3 conversion to N2. NO selectivity is calculated using Eq. (13). In addition, the two simplified overall reactions do not reflect the real conversion pathways of NH3. Therefore, NH3 conversion must be calculated using Eq. (12).
SNO ¼
kNH3 kNH3
! NO C NH3 C O2
! NO C NH3 C O2
þ
kNH3 ! N2 C 2NH3
¼
k12 C O2 k12 C O2 þ k12 C NH3
ð13Þ
where kNH2 ! NO (m4/mol s) and kNH2 ! N2 (m4/mol s) are the reaction rate constants of NH3 to NO and NH3 to N2, respectively. k12 represents the ratio of kNH2 ! NO to kNH2 ! N2 and defined as Eq. (14). k12 is correlated to the Arrhenius form for convenience; thus, A12 represents the ratio of the two pre-exponential factors and E12 represents the difference between the two activation energies.
kNH2 ! NO ANH2 ! NO ENH2 ¼ exp kNH2 ! N2 ANH2 ! N2 E12 ¼ A12 exp RT
k12 ¼
! NO
ENH2 RT
! N2
Acknowledgments This work was supported by the fund from the National Key Technologies R&D Program (2011BAE29B03) and ‘‘111’’ Project (B13001).
References
ð14Þ
Thus, the expression of NO reaction rate is shown as Eq. (15).
dC NH3 dC NH3 k12 C O2 dC NO ¼ SNO ¼ ds ds ds k12 C O2 þ k12 C NH3
inlet concentration, but increased with increasing O2 concentration. The deNOx efficiency of the SNCR deNOx process was inhibited by CaCO3 mainly by catalyzing NH3 oxidation to NO. Mechanism analysis demonstrated that the rate-controlling step of NH3 conversion on the surface of CaCO3 is the direct reaction between NH3 and CaCO3 that produces CaNH2HCO3. In the absence of O2, N elements in CaNH2HCO3 are converted to HNCO or N2. The produced CaO reacts with CO2 to reproduce CaCO3. The presence of O2 inhibits the conversion of CaNH2HCO3 to HNCO. The amide radical in CaNH2HCO3 can be oxidized to NO by O2, or dimerizes to N2H4 and subsequently decomposes into N2. The competition of the two reactions determines the product selectivity. Temperature, NH3, and O2 concentration influences the product selectivity by influencing the relative reaction rates of the two reactions. A kinetic model was established for the CaCO3-involved SNCR deNOx process based on the mechanism analysis. The experimental results were well predicted by the kinetic model, which proved the accuracy of the kinetic model.
ð15Þ
The reaction rate constants are correlated using the experimental data at different temperatures, and the pre-exponential factors and activation energies are obtained by double-log transformation. The obtained kinetic parameters are listed in Table 3. Fig. 11(a) shows the experimental and simulation results in the interaction between CaCO3 and NH3. Fig. 11(b) and (c) shows the experimental and simulation results in CaCO3-catalyzed NH3 oxidation at different NH3 inlet concentrations and O2 concentrations. The simulation results coincide with the experimental results. In the studied temperature range, the influence of gas phase reactions is negligible. Thus, NH3 and NO outlet concentrations in the CaCO3-involved SNCR deNOx process can be calculated using Eqs. (12) and (15). Fig. 11(d) shows the simulation and experimental results in CaCO3-involved SNCR deNOx process, which are in good agreement. The comparison between experimental and simulation results demonstrates that the proposed kinetic model well predicts the influence of CaCO3 on the SNCR deNOx process. 5. Conclusions CaCO3 had a catalytic effect on NH3 conversion in the absence or presence of O2, but had no effect on NO reduction by NH3. The product of CaCO3-catalyzed NH3 conversion in the absence of O2 was mainly HNCO, while the products in the presence of O2 were NO and N2. Both CaCO3-catalyzed NH3 conversion in the absence or presence of O2 showed first order characteristics with respect to NH3 concentration, and NH3 conversion increased with increasing temperature. In CaCO3-catalyzed NH3 oxidation, NH3 conversion was kept stable with increasing O2 and NH3 concentration; NO selectivity decreased with increasing temperature and NH3
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