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NO reduction by methane over iron oxides: Characteristics and mechanisms
Fuel 160 (2015) 80–86
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NO reduction by methane over iron oxides: Characteristics and mechanisms Yaxin Su a,⇑, Bingtao Zhao b, Wenyi Deng a a b
School of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai 201620, China School of Energy and Power Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai 200093, 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
NO reduction by combination of
methane and iron oxides was proposed. The combination of methane and iron oxides was effective to reduce NO. 100% NO reduction was demonstrated by 1.17% methane over iron oxides for 100 h at 1050 °C. More than 95% NO could be reduced by methane over iron oxides at fuel rich condition. Redox of iron between Fe3+ M Fe2+ M Fe due to methane played the critical effect for high NO reduction.
a r t i c l e
i n f o
Article history: Received 28 September 2014 Received in revised form 20 July 2015 Accepted 23 July 2015 Available online 29 July 2015 Keywords: NO reduction Methane Iron mesh roll Iron oxides
a b s t r a c t NO reduction by combination of methane and iron oxides was proposed and investigated in a one-dimensional ceramic tubular reactor at 300–1100 °C in N2 and simulated flue gas atmospheres. Iron mesh rolls were used as iron samples. The combination of methane and iron oxides was proved to have high effectiveness to reduce NO. 100% NO reduction was demonstrated by 1.17% methane over iron oxides for 100 h at 1050 °C in simulated flue gas containing 2.0% O2, 16.8% CO2 and 0.05% NO balanced by N2. More tests proved that more than 95% NO could be reduced by methane over iron oxides at fuel rich condition in simulated flue gas atmospheres. XRD analysis showed that the redox of iron between Fe3+ M Fe2+ M Fe due to methane played the critical effect for high NO reduction. The mechanisms involving in NO reduction by iron and iron oxides reduction by methane were discussed. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nitrogen oxides (NOx) are key air pollutants and are mainly come from combustion process of fossil fuels. Selective catalytic reduction (SCR) of NOx with ammonia (SCR-NH3) is widely applied for the cleanup of flue gas from stationary NOx sources so far. The ⇑ Corresponding author. Tel.: +86 21 67792552. E-mail address: [email protected] (Y. Su). http://dx.doi.org/10.1016/j.fuel.2015.07.077 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
nature of the catalyst is critical to the SCR reaction and several categories of catalysts have been studied in the past three decades including comprise precious metals, metal oxides and zeolites, etc. [1–5]. Although SCR-NH3 method is a well-established technology for NOx removal and has been successfully applied to industries, there are still some disadvantages associated with the use of NH3, for example, expensive reductant, corrosion, toxicity, byproducts, possible NH3 slip, etc. Hydrocarbons including alkanes and alkenes were used as alternative reductant for SCR [6–8].
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2. Experimental setup The experimental setup is showed in Fig 1. A one-dimensional ceramic tube of inner diameter of 2.5 cm was used as the reactor and was electrically heated in the temperature programmed furnace. The total heated length of the reactor was 30 cm. The furnace was heated at a heating rate of 5 °C/min to a given temperature and then this temperate was hold for the reaction. The gases were continuously fed into the reactor. The effluent gases, especially NO, CO, CxHy, etc., were checked by the on-line analyzer. When the effluent NO decreased to a value and did not change any more for at least 10 min, the data was recorded. The reactor was then heated to the next temperature. For each run, half an hour was needed at least. Simulated flue gas consisting 0.05% NO balanced by N2 was fed into the reactor at a flow rate of 1.5 L/min. An online
Reactor tube
Electrically heated furnace Iron mesh roll Simulated flue gas
To gas analyzer
analyzer (ECOM-J2KN, Germany) was used to monitor the gas species. X-ray diffraction (XRD) (RIGAKU, D/Max-2550PC, Japan) was used to detect the iron components evolution and scanning electron microscope (SEM) (JEOL, JSM-5600LV, Japan) was used to detect the iron surface change. Iron meshes were used as iron samples. The size of the basic mesh unit was 6 mm 6 mm and the diameter of the mesh wire was 0.5 mm. The mesh was first rolled into a mesh roll as showed in Fig 2 and then the mesh roll was horizontally set in the center of the heated tube. When the gas flow rate was 1.5 L/min, the reaction time of NO over the iron mesh, i.e., the time that the gas flowed over the mesh roll, was about 0.13 s, 0.13 s and 0.06 s for the iron mesh rolls make of iron mesh of 160 mm 80 mm, 80 mm 80 mm, 160 mm 40 mm respectively. In order to burnout the methane and possible CO produced during the reaction, a second furnace with additional O2 supply was connected to the first furnace. The NO reduction reaction mainly happens in the first furnace as described in the above text. Burnout happens in the second furnace.
3. Results 3.1. NO reduction by iron mesh roll In order to demonstrate that iron mesh roll was effective to reduce NO to N2, experiment was first conducted in N2 atmosphere. NO reduction was defined as the ratio of the difference of inlet and outlet NO to inlet NO. Fig 3 shows the results. NO reduction was rather low below 500 °C. As the temperature increased, e.g., from 500 °C to 700 °C, the NO reduction increased sharply and more than 90% of NO was reduced above 900 °C for all of the three iron mesh rolls. The iron mesh roll of 160 mm 80 mm resulted to the highest efficiency. Gradon and Lasek [15] used iron ball of diameter about 10 mm to reduce NO within the temperature range of 750–1200 °C and found that about 65% of NO was reduced to N2 when they fed 1015 ppm NO balanced by N2 at 850 °C. The present results showed that when iron mesh roll was used as iron sample, a higher NO reduction could be achieved. The reason may due to the increased surface of the iron when a mesh roll was used to replace the iron ball. NO is chemically reduced to N2 by iron according to the following reaction [15,19,20]:
Fe þ NO ! Fex Oy þ N2 Unfortunately, there is not solid conclusion on the final chemical forms of iron oxides so far. XRD analysis was conducted for the iron samples before and after the reaction with NO. The original iron sample and the iron samples after reaction were first ground to be powder of about 10–20 mesh before carrying out the XRD analysis. Fig. 4(a) presents the XRD result of the original iron before experiment. The major component of the original iron is pure iron
6mm×6mm
Width
Cu-ZSM-5 was very active when C2+ hydrocarbon was used as reductant fuel, but the NO reduction efficiency was sharply decreased when there was O2 in the flue gas [6,7]. Only a few elements such as Co, Pd, Ga, Ni, and In can activate methane to reduce NO into N2 [9–10], while copper is unable to perform the reaction with short chain hydrocarbons [9]. Some investigations performed over various cation-ZSM-5 catalysts at 450 °C using methane showed that NO conversions followed this order: Co > Mn > Ni > Cu [11]. Metal oxides are another class of SCR-HC catalysts [12–14]. These researches mainly focused on the cleanup of NO from vehicles with C2+ hydrocarbons. Recently, NO reduction by iron or iron oxides was investigated. Gradon and Lasek [15], Lasek [16] and Su et al. [17,18] demonstrated that metallic iron could reduce NO to N2 while the iron was oxidized to iron oxides. Hayhurst and Ninomiya [19], Hayhurst and Lawrence [20] found that Fe or Fe2O3 could catalytically reduce NO to N2 in a fluidized bed reactor (700–900 °C) when CO gas existed. Methane was used to reduce NO through reburning [21], however, Pilot- and full-scale research over the last three decades has demonstrated a floor, about 60% of NO produced in the primary flames, in either gas or coal reburning, below which NO cannot be reduced further [22]. The major reason lies in the fact that the intermediate products during reburning, i.e., HCN and NH3, will be re-oxidized to form NO in the burnout zone, which limits the overall NO reduction efficiency. Fe2O3 was used to improve reburning efficiency by reducing HCN to N2 while Fe2O3 was reduced to Fe at the same time [23,24]. Results showed that methane could reduce iron oxides to metallic iron via partial oxidation over iron oxides [25–28]. However there was little study on the combination of Fe and methane for NOx reduction, in particular, their effect on characteristics of NO reduction and their interactions were not clearly understood so far. In this paper, we proposed the NO reduction by the combination of methane and iron oxides. The experimental investigation was performed to address NO reduction efficiency by methane over iron oxides under different atmosphere and operating conditions, and further the mechanism was analyzed to explore how methane and iron/iron oxides play the interactive roles on NO reduction.
Length Iron mesh Fig. 1. Experimental setup.
Mesh roll
Fig. 2. Iron mesh roll used in the experiment.
Y. Su et al. / Fuel 160 (2015) 80–86
100
100
80
80
NO reduction / %
NO reduction / %
82
60
160x80 mm 80x80 mm 160x40 mm
40
20
400
600
800
60 40 20
1000
0
1200
0
10
20
30
o
Temperature / C
40
50
60
Fig. 3. NO reduction efficiency vs. temperature in N2 atmosphere.
Fig. 5. Continuous reduction of NO by metallic iron in N2 atmosphere (flow rate 1.5 L/min, NO = 0.05% at 800 °C).
2000
+
Fe+2 Fe2 +3O4 Fe2 O3
∗
+
1500
Intensity (CPS)
with a little Ni. The iron surface was oxidized and both Fe3O4 and Fe2O3 were formed after reducing NO in N2 atmosphere, as showed in Fig. 4(b). In a continuous test with the iron mesh roll of 160 mm 80 mm at 800 °C with 0.05% NO balanced by N2, NO reduction decreased from about 99% at the beginning to about 27% after 68.2 h, as showed in Fig 5. XRD analysis of the iron sample after reaction showed that the iron was oxidized to Fe2O3 and Fe3O4, as showed in Fig 6. Iron could reduce NO to N2, while it was oxidized to FexOy. Reducing agent should be added to reduce iron oxides to iron in order to keep the reaction to continue. CO and CH4 were used as reducing agents in the following tests respectively in simulated flue gas.
+
∗ 1000
∗
+
+
500 +
+
∗
+ ∗
∗
+
∗ ∗
10 2000
+ + +∗ +∗ ∗∗ ∗ ∗
∗
30
40
50
60
70
80
90
2θ (°)
Fe
Fig. 6. XRD results of the iron mesh after 68.2 h continuous reaction with NO at 800 °C.
Fe,Ni
Intensity (CPS)
20
+
+
0
1500
3.2. Continuous NO reductions over iron oxides with CO/CH4
1000 500 0
70
reaction time / h
30
40
50
60
70
80
90
2θ (o)
A continuous test was conducted in simulated flue gas containing 2.0 vol.% O2, 0.05 vol.% NO, 16.8 vol.% CO2 balanced by N2 at 1050 °C. Iron mesh roll of 160 mm 80 mm was used as the iron sample. Compared to the NO reduction by iron alone in simulated flue gas, reducing gas, CO, could reduce iron oxides to metallic iron and finally improve the NO reduction [17]. 4.1 vol.% CO was first
2500
Fe Fe3O4
2000
Fe2 O3
100
NO reduction / %
Intensity (CPS)
(a) Original iron mesh
1500 1000 500 0
30
40
50
60
70
80
90
2θ (o)
(b) After reaction with NO in N2 atmosphere (1100°C) Fig. 4. XRD results of the iron mesh.
80 60 stop CO and feed 1.17% CH 4
40 20 0
0
20
40
60
80
100
reaction time / h Fig. 7. Continuous reaction of NO reduction by methane over iron oxides in simulated flue gas at 1050 °C (SR1 = 0.85).
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NO reduction / %
100
60 40
0 200
300
400
500
600
700
800
900 1000 1100
Temperature / oC Fig. 9. NO reduction by methane over iron oxides (flow rate 1.5 L/min, CH4 = 1.17%, NO = 0.05% balanced by N2).
6000 5000
CH4 =1.17%
4000 3000 2000 1000
3.3. NO reduction by methane over iron oxides in N2 atmosphere
0 200
300
400
500
600
700
800
900 1000 1100
Temperature / oC Fig. 10. CO formation during NO reduction over iron oxides (flow rate 1.5 L/min, CH4 = 1.17%, NO = 0.05% balanced by N2).
1000 eO +2 +3 + Fe Fe2 O4 Fe * Fe2 O3
+
800
Intensity (CPS)
A total of 1.5 L/min gas mixture containing 0.05 vol.% NO was fed into the reactor balanced by N2. 1.17 vol.% CH4 was used as the reductant. Fig 9 presents the result. Less than 20% NO reduction was tested below 500 °C, while almost 100% NO was reduced above 850 °C. However, a by-product, CO was formed, as showed in Fig 10. At 1050 °C, the test was stopped since the NO reduction was 100%. The reactor was cooled at 10 °C/min in N2 atmosphere to room temperature. The iron sample was checked by XRD and SEM. Fig 11 and Fig 12 present the results. Fig 11 showed that metallic iron, Fe, and FeO were formed, which indicated that iron oxides (Fe2O3, Fe3O4) were partly reduced to Fe2+ and Fe. When compared to the original iron oxides SEM image showed in Fig 12(a), the SEM image of the iron sample after reaction became porous with many microporous structures, as showed in Fig 12(b). Many fiber-like matters were locally formed. The length was about 15–20 lm and the outer diameter was about 1 lm. The iron sample after the reaction was locally analyzed by X-ray energy dispersive spectroscopy (EDS) and C element was observed on the
80
20
Exit CO / ppm
added into the reactor. At the beginning, NO reduction was 95%; however it decreased to about 20% after 5.05 h, as showed in Fig 7. This showed that CO was not good reductant in long-term application. Lasek [16] recently reported his research on NO reduction over iron sphere when CO was used as the reducing agent and the result showed that no more than 60% of NO was reduced when the air excess ratio was 0.6. When NO reduction efficiency was below 20%, CO was stopped and 1.17% CH4 was fed into the reactor. The corresponding stoichiometric ratio of CH4, as noted SR, was 0.85. The stoichiometric ratio is defined as the ratio of actual oxygen in the flue gas and the oxygen that complete combustion of methane requires. NO reduction efficiency quickly increased to over 98% as methane was fed. A continuous experiment was carried out for about 100 h and there was no sign that NO reduction may decrease, as showed in Fig 7. The result demonstrated that NO could be effectively reduced by methane over iron oxides. The iron sample was checked by XRD after the test and the result showed that iron was not fully oxidized to Fe3+ ion, as showed in Fig 8. This is due to the reducing effect of methane. The redox of iron between Fe3+ M Fe2+ M Fe plays the critical role for high NO reduction. Since CH4 was very effective to reduce NO over iron oxides, it is necessary to carefully investigate the reaction and mechanism between the reaction of NO, CH4 and iron/iron oxides. So the iron mesh roll after the durable reaction with NO for 68.2 h in N2 atmosphere was used as the iron oxides in the following test of NO reduction with CH4.
600
+
+
∗
400
∗
0 10
20
30
+
+
200
40
50
∗
60
70
80
90
2θ (°)
+ Intensity (CPS)
+3
Fig. 11. XRD pattern of iron oxides after reaction (flow rate 1.5 L/min, CH4 = 1.17%, NO = 0.05% balanced by N2, 1050 °C).
+ ++ + + +
fiber-like matters, the locally maximum C was as high as 88% (atom ratio). Previous results suggested that carbon nanotube could be produced by chemical vapor deposition method [29,30]. Catalyzed by transition metals, the hydrocarbon gases decomposed to form carbon atoms at high temperature, which finally formed carbon nanotube by orientated deposition according to the chemical vapor deposition mechanism. Generally, Fe, Ni and Co were most active catalysis during the synthesis of nanotubes by chemical vapor deposition method [29,30]. In the present experiment, methane thermally decomposed over iron oxides at 1050 °C in N2 atmosphere to form the fiber-like matter which was mainly composed of carbon.
+2
+ Fe Fe2 O 4 FeO
3000
2000
+ 0
+
+
1000
10
20
+ 30
40
+
+
+ 50
60
70
80
90
2θ (°) Fig. 8. XRD pattern of iron oxides after continuous reaction with CH4 and NO at 1050 °C.
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(a) iron oxides before reaction
(b) iron oxides after reaction
Fig. 12. SEM image of iron oxides (flow rate 1.5 L/min, CH4 = 1.17%, NO = 0.05% balanced by N2, 1050 °C).
3.4. NO reduction by methane over iron oxides in flue gas atmosphere
100
NO reduction / %
80 60 40 20 0
SR 1=0.7, SR 2 =0.7 SR 1=0.7, SR 2 =1.2 SR 1=0.8, SR 2 =0.8 SR 1=0.8, SR 2 =1.2 SR 1=0.9, SR 2 =0.9 SR 1=0.9, SR 2 =1.2 SR 1=1.0, SR 2 =1.0 SR 1=1.0, SR 2 =1.2 SR 1=1.1, SR 2 =1.1 SR 1=1.1, SR 2 =1.2 SR 1=1.2 CH 4 =0, Fe
200
400
80
NO reduction / %
In real conditions, there are O2 and CO2 in the flue gas. The NO reduction by methane over iron oxides was tested in simulated flue gas of 1.5 L/min containing 2.0 vol.% O2, 0.05 vol.% NO, 17.0 vol.% CO2 balanced by N2. The amount of methane was controlled by the stoichiometric ratio (SR). The combustion conditions in the first and second furnace are noted as SR1 and SR2 respectively. When SR1 equals to SR2, additional O2 is not fed into the second furnace. In the test, the temperatures in the two furnaces were kept the same. When SR1 < 1, i.e., fuel-rich condition, NO reduction by methane over iron oxides was more than 90% over 900 °C, as showed in Fig 13. When SR1 exceeded 1, i.e., fuel-lean condition, NO reduction was lower than 35%. The difference of NO reduction was small when there was and was not a burnout process, as noted in Fig 13 by SR2 = 1.2 and SR2 = SR1. When iron was used alone to reduce NO, the NO reduction was no more than 30%, as noted CH4 = 0, Fe in Fig 13. When iron oxides were not used, it was actually reburning of methane. NO reduction after burnout decreased by about 20% when SR1 = 0.7, SR2 = 1.2 (with burnout) compared to the NO reduction when SR1 = 0.7, SR2 = 0.7 (without burnout), as showed in Fig 14. It was known that an intermediate product, HCN is produced during reburning of CH4 [21]. HCN is oxidized by O2 to form
100
60 40
without Fe2 O3, SR 1=0.7, SR 2 =0.7 with Fe2 O3, SR 1=0.7, SR 2 =0.7 without Fe2 O3, SR 1=0.7, SR 2 =1.2 with Fe2 O3, SR 1=0.7, SR 2 =1.2 without Fe2 O3, SR 1=1.0, SR 2 =1.0 with Fe2 O3, SR 1=1.0, SR 2 =1.0 without Fe2 O3, SR 1=1.0, SR 2 =1.2 with Fe2 O3, SR 1=1.0, SR 2 =1.2 without Fe2 O3, SR 1=1.2, SR 2 =1.2 with Fe 2 O3, SR 1=1.2, SR 2 =1.2
20 0 300
400
500
600
700
800
900
1000
1100
Temperature / oC Fig. 14. Comparison of NO reduction by methane in simulated flue gas atmosphere when iron oxides was or was not used (CH4 conversion rate was 100% above 800 °C) (flow rate 1.5 L/min, O2 = 2.0 vol.%, NO = 0.05 vol.%, CO2 = 17.0 vol.% balanced by N2).
new NO, resulting to the decrease of NO reduction after burnout. When iron oxides were used, NO reduction after burnout was almost the same to that without burnout, e.g., about 95% above 900 °C when SR1 = 0.7, SR2 = 1.2 (with burnout) and SR1 = 0.7, SR2 = 0.7 (without burnout), as showed in Fig 14. This result verified that iron oxides could reduce the intermediate product, HCN, during methane reburning. In fuel lean conditions, i.e., SR1 P 1:0, methane was burned by O2 and could not reduce NO and iron oxides any more. NO reduction was lower than 40% when SR1 P 1:0 whatever SR2 was and whether iron oxides was used or not. 3.5. Mechanism of NO reduction by methane over iron oxides Iron could reduce NO to N2 [15,19,20]:
Fe þ NO ! Fex Oy þ N2
600
800
1000
1200
Temperature / oC Fig. 13. NO reduction by methane over iron oxides in simulated flue gas atmosphere (flow rate 1.5 L/min, O2 = 2.0 vol.%, NO = 0.05 vol.%, CO2 = 17.0 vol.% balanced by N2).
The iron oxides were finally oxidized to Fe2O3, although Fe3O4 and FeO could be produced as the intermediate forms. The lattice oxygen provided by iron oxides (Fe2O3 and Fe3O4) at high temperature could partially oxidize methane to CO/CO2 and the iron oxides would be reduced to metallic iron at the same time by methane above 570 °C according to the sequence Fe2O3 ? Fe3O4 ? FeO ? Fe. [25–27,31]. Wang and Wei [28] suggested that Fe2O3 could completely oxidize methane to CO2 below 570 °C, while it could partially oxidize methane above 570 °C. The following reactions describe the main and secondary reactions.
Y. Su et al. / Fuel 160 (2015) 80–86
Main reactions:
4. Conclusions
3Fe2 O3 þ CH4 ðgÞ ! 2Fe3 O4 þ COðgÞ þ 2H2
ðR1Þ
Fe3 O4 þ CH4 ðgÞ ! 3FeO þ COðgÞ þ 2H2
ðR2Þ
FeO þ CH4 ðgÞ ! Fe þ COðgÞ þ 2H2
ðR3Þ
Secondary reactions:
12Fe2 O3 þ CH4 ðgÞ ! 8Fe3 O4 þ CO2 ðgÞ þ 2H2 OðgÞ
ðR4Þ
4Fe3 O4 þ CH4 ðgÞ ! 12FeO þ CO2 ðgÞ þ 2H2 OðgÞ
ðR5Þ
4FeO þ CH4 ðgÞ ! 4Fe þ CO2 ðgÞ þ 2H2 OðgÞ
ðR6Þ
(R1)–(R6) could go on spontaneously once the temperature is above 700 °C. According to thermodynamics Gibbs free energy and reaction heat calculated by Wang and Wei [28], when the temperature is above 700 °C, reaction (R4) is easier to go on than reaction (R1) and Fe2O3 is firstly reduced to Fe3O4, then Fe3O4 is reduced to FeO according to reaction (R2) and CO is produced. FeO is then reduced to Fe according to reaction (R3) and CO is produced again. The reducing route of iron oxides is Fe2O3 ? Fe3O4 ? FeO ? Fe. The produced CO and H2 during the reactions (R2) and (R3) will further reduce the un-reacted iron oxides to iron and water vapor will be produced. In the test, water was observed to condensate on the effluent tube. In addition, methane decomposes to C and H2 at high temperature:
CH4 ðgÞ ! C þ 2H2 ðgÞ
ðR7Þ
The cracking reaction of methane begins at 550 °C, but it goes on very slowly at 600–850 °C and no more than 3.4% methane could decompose below 850 °C [28]. When there is oxygen carrier, like Fe2O3, methane first reacts with lattice oxygen rather than cracking. Only when there is not enough lattice oxygen, methane decomposes to carbon and hydrogen. The carbon due to the decomposition of methane was very active and reacted with iron oxides immediately to reduce iron oxides to metallic iron [32,33].
Fe2 O3 þ 1:5C ! 2Fe þ 1:5CO2
ðR8Þ
Li and Shen [34] concluded that iron reduced by methane from iron oxides was not easy to be re-oxidized to iron oxides by O2 when the temperature was above 900 °C. This implies that when the temperature is above 900 °C, iron oxides could be easily reduced to metallic iron by methane, and then the iron is involved in NO reduction more than oxidation by O2 in the flue gas. In fuel rich conditions, methane also reduces NO via reburning [21]. Reburning reaction of methane with NO needs O radical [21]. Iron oxides provide lattice oxygen. In N2 atmosphere, O radical provided by iron oxides is not enough to make reburning the dominant mechanism. In the simulated flue gas atmosphere, however, reburning reaction could not be neglected at fuel rich condition. In the reburning of methane, an intermediate product, HCN, is produced, which forms new NO in the burnout process [22]. Su et al. [23] experimentally proved that Fe2O3 could reduce HCN and Tan et al. [24] proposed the following reaction scheme:
Fe2 O3 þ 3HCN ! 2Fe þ 3CO þ 1:5N2 þ 1:5H2
85
ðR9Þ
In the present experiments, when iron oxides was used, NO reduction by methane did not decrease after burnout (when SR2 was higher than 1 as noted in Fig 14). However, when iron oxides was not used, NO reduction after burnout, e.g., SR1 = 0.7, SR2 = 1.2 in Fig 14, was about 20% lower than that when iron oxides was used. This implied that the possible reburning intermediate product, HCN was reduced by iron oxides other than forming new NO.
NO reduction by methane over iron oxides was experimentally investigated. Results showed that metallic iron could reduce NO to N2 when the temperature was higher than 900 °C, but the durable performance was poor because that iron was oxidized to iron oxides. Methane was a good reducing agent and was very effective to reduce NO over iron oxides. In N2 atmosphere, 100% NO reduction efficiency was achieved above 850 °C. In simulated flue gas, NO reduction by methane over iron oxides could exceed 95% when the temperature was above 900 °C at fuel rich condition. 1.17% methane could continuously reduce 100% NO over 100 h at 1050 °C in simulated flue gas which contained 2.0% O2, 16.8% CO2 and 0.05% NO balanced by N2. Acknowledgment This work was supported by National Natural Science Foundation of China (Nos. 51278095 and 50806049), which are gratefully acknowledged. References [1] Centi G, Perathoner S. Introduction: state of the art in the development of catalytic processes for the selective catalytic reduction of NOx into N2. Stud Surf Sci Catal 2007;171:1–24. [2] Brandenberger S, Kröcher O, Tissler A, Althoff R. The state of the art in selective catalytic reduction of NOx by ammonia using metal-exchanged zeolite catalysts. Catal Rev 2008;50:492–581. [3] Long RQ, Yang RT. Characterization of Fe-ZMS-5 catalyst for selective catalytic reduction of nitric oxide by ammonia. J Catal 2000;194:80–90. [4] Apostolescu N, Geiger B, Hizbullah K, Jan MT, Kureti S, Reichert D, et al. Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts. Appl Catal B: Environ 2006;62:104–14. [5] Krishna K, Seijger GBF, van den Bleek CM, Makkee M, Mul G, Calis HPA. Selective catalytic reduction of NO with NH3 over Fe-ZSM-5 catalysts prepared by sublimation of FeCl3 at different temperatures. Catal Lett 2003;86:121–32. [6] Iwamoto M. Zeolites in environmental catalysis. Stud Surf Sci Catal 1994;84:1395–410. [7] Tabata T, Kokitsu M, Okada O. Study on patent literature of catalysts for a new NOx removal process. Catal Today 1994;22:147–69. [8] Armor JN. Catalytic reduction of nitrogen oxides with methane in the presence of excess oxygen: a review. Catal Today 1995;26:147–58. [9] Li Y, Battavio PB, Armor JN. Effect of water-vapor on the selective reduction of NO by methane over cobalt-exchanged ZSM-5. J Catal 1993;142:561–71. [10] Cant NW, Liu IOY. The mechanism of the selective reduction of nitrogen oxides by hydrocarbons on zeolite catalysts. Catal Today 2000;63:133–46. [11] Choi BC, Foster DE. State-of-the-art of de-NOx technology using zeolite catalysts in automobile engines. J Ind Eng Chem 2005;11:1–9. [12] Iliopoulou EF, Evdou A, Lemonidou AA, Vasalos IA. Ag/alumina catalysts for the selective catalytic reduction of NOx using various reductants. Appl Catal A: Gen 2004;274:179–89. [13] Kotsifa A, Kondarides DI, Verykios XE. A comparative study of the selective catalytic reduction of NO by propylene over supported Pt and Rh catalysts. Appl Catal B: Environ 2008;80:260–70. [14] Liu Z, Wang K, Zhang X, Wang J, Cao H, Gong M, Chen Y. Study on methane selective catalytic reduction of NO on Pt/Ce0.67Zr0.33O2 and its application. J Nat Gas Chem 2009;18:66–70. [15] Gradon B, Lasek J. Investigation of reduction of NO to N2 by reaction with Fe. Fuel 2010;89:3505–9. [16] Lasek J. Investigations of the reduction of NO to N2 by reaction with Fe under fuel-rich and oxidative atmosphere. Heat Mass Trans 2014;50:933–43. [17] Su Y, Su A, Cheng H. Experimental study on effect of CO on NO reduction by iron mesh roll. J Basic Sci Eng 2013;21(4):638–46 [in Chinese]. [18] Su Y, Su A, Cheng H. Experimental study on direct catalytic reduction of NO by metallic iron. J China coal Soc 2013;38(s1):206–10 [in Chinese]. [19] Hayhurst AN, Ninomiya Y. Kinetics of the conversion of NO to N2 during the oxidation of iron particles by NO in a hot fluidised bed. Chem Eng Sci 1998;53:1481–9. [20] Hayhurst AN, Lawrence AD. The reduction of the nitrogen oxides NO and N2O to molecular nitrogen in the presence of iron, its oxides, and carbon monoxide in a hot fluidized bed. Combust Flame 1997;110:351–65. [21] Smoot LD, Hill SC, Xu H. NOx control through reburning. Prog Energy Combus Sci 1998;24:385–408. [22] Johnson DK, Engelhardt DA, Harvilla J, Beecy DJ, Watts JU. Reburning technologies for the control of nitrogen oxides emissions from coal-fired boilers, Washington (DC): United States Dept. of Energy Clean Coal Technology Topical Report, 1999.
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[23] Su Y, Gathitu BB, Chen WY. Efficient and cost effective reburning using common wastes as fuel and additives. Fuel 2010;89:2569–82. [24] Tan HZ, Wang XB, Niu YQ, Liu HY, Wang CL, Xu TM. Studies of interaction mechanism between iron and HCN. Asian J Chem 2010;22:4017–25. [25] Takenaka S, Hanaizumi N, Son VTD, Otsuka K. Production of pure hydrogen from methane mediated by the redox of Ni- and Cr-added iron oxides. J Catal 2004;228:405–16. [26] Nakayama O, Ikenaga N, Miyake T, Yagasaki E, Suzuki T. Production of synthesis gas from methane using lattice oxygen of NiO–Cr2O3–MgO complex oxide. Ind Eng Chem Res 2010;49:526–34. [27] Tamaura Y, Wada Y, Yoshida T, Tsuji M. The coal/Fe3O4 system for mixing of solar and fossil energies. Energy 1997;22:337–42. [28] Wang H, Wei Y. Partial oxidation of methane by lattice oxygen to produce synthesis gas. Beijing: Metallurgical Industry Press; 2009. [29] Ivanov V, Nagy JB, Lambin P, Lucas A, Zhang XB, Zhang XF, et al. The study of carbon nanotubes produced by catalytic method. Chem Phys Lett 1994;223:329–35.
[30] Flahaust E, Govindaraj A, Peigney A, Laurent C, Rousset A, Rao CNR. Synthesis of single-walled carbon nanotubes using binary (Fe Co, Ni) alloy nanoparticles prepared in situ by the reduction of oxide solid solutions. Chem Phys Lett 1999;300:236–42. [31] Chen G. The kinetics of the gas-based reduction of iron oxide. Dalian: Dalian University of Technology; 2011. [32] Zenkov VI, Pasichnyi VV. Reduction kinetics of iron oxides used for hydrogen production in various gas media. Powder Metall Met Ceram 2010;49:231–7. [33] Zenkov VS, Pasichnyi VV, Redko VP. Reduction of iron-containing metallurgical waste to obtain hydrogen with iron vapor method. Powder Metall Met Ceram 2008;47:733–42. [34] Li R, Shen S. Study on lattice oxygen used in the conversion of methane to synthesis gas – redox performance of Fe2O3 Catalyst. J Molecular Catal (China) 2001;3:181–6.
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Fuel journal homepage: www.elsevier.com/locate/fuel
NO reduction by methane over iron oxides: Characteristics and mechanisms Yaxin Su a,⇑, Bingtao Zhao b, Wenyi Deng a a b
School of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai 201620, China School of Energy and Power Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai 200093, 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
NO reduction by combination of
methane and iron oxides was proposed. The combination of methane and iron oxides was effective to reduce NO. 100% NO reduction was demonstrated by 1.17% methane over iron oxides for 100 h at 1050 °C. More than 95% NO could be reduced by methane over iron oxides at fuel rich condition. Redox of iron between Fe3+ M Fe2+ M Fe due to methane played the critical effect for high NO reduction.
a r t i c l e
i n f o
Article history: Received 28 September 2014 Received in revised form 20 July 2015 Accepted 23 July 2015 Available online 29 July 2015 Keywords: NO reduction Methane Iron mesh roll Iron oxides
a b s t r a c t NO reduction by combination of methane and iron oxides was proposed and investigated in a one-dimensional ceramic tubular reactor at 300–1100 °C in N2 and simulated flue gas atmospheres. Iron mesh rolls were used as iron samples. The combination of methane and iron oxides was proved to have high effectiveness to reduce NO. 100% NO reduction was demonstrated by 1.17% methane over iron oxides for 100 h at 1050 °C in simulated flue gas containing 2.0% O2, 16.8% CO2 and 0.05% NO balanced by N2. More tests proved that more than 95% NO could be reduced by methane over iron oxides at fuel rich condition in simulated flue gas atmospheres. XRD analysis showed that the redox of iron between Fe3+ M Fe2+ M Fe due to methane played the critical effect for high NO reduction. The mechanisms involving in NO reduction by iron and iron oxides reduction by methane were discussed. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nitrogen oxides (NOx) are key air pollutants and are mainly come from combustion process of fossil fuels. Selective catalytic reduction (SCR) of NOx with ammonia (SCR-NH3) is widely applied for the cleanup of flue gas from stationary NOx sources so far. The ⇑ Corresponding author. Tel.: +86 21 67792552. E-mail address: [email protected] (Y. Su). http://dx.doi.org/10.1016/j.fuel.2015.07.077 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
nature of the catalyst is critical to the SCR reaction and several categories of catalysts have been studied in the past three decades including comprise precious metals, metal oxides and zeolites, etc. [1–5]. Although SCR-NH3 method is a well-established technology for NOx removal and has been successfully applied to industries, there are still some disadvantages associated with the use of NH3, for example, expensive reductant, corrosion, toxicity, byproducts, possible NH3 slip, etc. Hydrocarbons including alkanes and alkenes were used as alternative reductant for SCR [6–8].
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2. Experimental setup The experimental setup is showed in Fig 1. A one-dimensional ceramic tube of inner diameter of 2.5 cm was used as the reactor and was electrically heated in the temperature programmed furnace. The total heated length of the reactor was 30 cm. The furnace was heated at a heating rate of 5 °C/min to a given temperature and then this temperate was hold for the reaction. The gases were continuously fed into the reactor. The effluent gases, especially NO, CO, CxHy, etc., were checked by the on-line analyzer. When the effluent NO decreased to a value and did not change any more for at least 10 min, the data was recorded. The reactor was then heated to the next temperature. For each run, half an hour was needed at least. Simulated flue gas consisting 0.05% NO balanced by N2 was fed into the reactor at a flow rate of 1.5 L/min. An online
Reactor tube
Electrically heated furnace Iron mesh roll Simulated flue gas
To gas analyzer
analyzer (ECOM-J2KN, Germany) was used to monitor the gas species. X-ray diffraction (XRD) (RIGAKU, D/Max-2550PC, Japan) was used to detect the iron components evolution and scanning electron microscope (SEM) (JEOL, JSM-5600LV, Japan) was used to detect the iron surface change. Iron meshes were used as iron samples. The size of the basic mesh unit was 6 mm 6 mm and the diameter of the mesh wire was 0.5 mm. The mesh was first rolled into a mesh roll as showed in Fig 2 and then the mesh roll was horizontally set in the center of the heated tube. When the gas flow rate was 1.5 L/min, the reaction time of NO over the iron mesh, i.e., the time that the gas flowed over the mesh roll, was about 0.13 s, 0.13 s and 0.06 s for the iron mesh rolls make of iron mesh of 160 mm 80 mm, 80 mm 80 mm, 160 mm 40 mm respectively. In order to burnout the methane and possible CO produced during the reaction, a second furnace with additional O2 supply was connected to the first furnace. The NO reduction reaction mainly happens in the first furnace as described in the above text. Burnout happens in the second furnace.
3. Results 3.1. NO reduction by iron mesh roll In order to demonstrate that iron mesh roll was effective to reduce NO to N2, experiment was first conducted in N2 atmosphere. NO reduction was defined as the ratio of the difference of inlet and outlet NO to inlet NO. Fig 3 shows the results. NO reduction was rather low below 500 °C. As the temperature increased, e.g., from 500 °C to 700 °C, the NO reduction increased sharply and more than 90% of NO was reduced above 900 °C for all of the three iron mesh rolls. The iron mesh roll of 160 mm 80 mm resulted to the highest efficiency. Gradon and Lasek [15] used iron ball of diameter about 10 mm to reduce NO within the temperature range of 750–1200 °C and found that about 65% of NO was reduced to N2 when they fed 1015 ppm NO balanced by N2 at 850 °C. The present results showed that when iron mesh roll was used as iron sample, a higher NO reduction could be achieved. The reason may due to the increased surface of the iron when a mesh roll was used to replace the iron ball. NO is chemically reduced to N2 by iron according to the following reaction [15,19,20]:
Fe þ NO ! Fex Oy þ N2 Unfortunately, there is not solid conclusion on the final chemical forms of iron oxides so far. XRD analysis was conducted for the iron samples before and after the reaction with NO. The original iron sample and the iron samples after reaction were first ground to be powder of about 10–20 mesh before carrying out the XRD analysis. Fig. 4(a) presents the XRD result of the original iron before experiment. The major component of the original iron is pure iron
6mm×6mm
Width
Cu-ZSM-5 was very active when C2+ hydrocarbon was used as reductant fuel, but the NO reduction efficiency was sharply decreased when there was O2 in the flue gas [6,7]. Only a few elements such as Co, Pd, Ga, Ni, and In can activate methane to reduce NO into N2 [9–10], while copper is unable to perform the reaction with short chain hydrocarbons [9]. Some investigations performed over various cation-ZSM-5 catalysts at 450 °C using methane showed that NO conversions followed this order: Co > Mn > Ni > Cu [11]. Metal oxides are another class of SCR-HC catalysts [12–14]. These researches mainly focused on the cleanup of NO from vehicles with C2+ hydrocarbons. Recently, NO reduction by iron or iron oxides was investigated. Gradon and Lasek [15], Lasek [16] and Su et al. [17,18] demonstrated that metallic iron could reduce NO to N2 while the iron was oxidized to iron oxides. Hayhurst and Ninomiya [19], Hayhurst and Lawrence [20] found that Fe or Fe2O3 could catalytically reduce NO to N2 in a fluidized bed reactor (700–900 °C) when CO gas existed. Methane was used to reduce NO through reburning [21], however, Pilot- and full-scale research over the last three decades has demonstrated a floor, about 60% of NO produced in the primary flames, in either gas or coal reburning, below which NO cannot be reduced further [22]. The major reason lies in the fact that the intermediate products during reburning, i.e., HCN and NH3, will be re-oxidized to form NO in the burnout zone, which limits the overall NO reduction efficiency. Fe2O3 was used to improve reburning efficiency by reducing HCN to N2 while Fe2O3 was reduced to Fe at the same time [23,24]. Results showed that methane could reduce iron oxides to metallic iron via partial oxidation over iron oxides [25–28]. However there was little study on the combination of Fe and methane for NOx reduction, in particular, their effect on characteristics of NO reduction and their interactions were not clearly understood so far. In this paper, we proposed the NO reduction by the combination of methane and iron oxides. The experimental investigation was performed to address NO reduction efficiency by methane over iron oxides under different atmosphere and operating conditions, and further the mechanism was analyzed to explore how methane and iron/iron oxides play the interactive roles on NO reduction.
Length Iron mesh Fig. 1. Experimental setup.
Mesh roll
Fig. 2. Iron mesh roll used in the experiment.
Y. Su et al. / Fuel 160 (2015) 80–86
100
100
80
80
NO reduction / %
NO reduction / %
82
60
160x80 mm 80x80 mm 160x40 mm
40
20
400
600
800
60 40 20
1000
0
1200
0
10
20
30
o
Temperature / C
40
50
60
Fig. 3. NO reduction efficiency vs. temperature in N2 atmosphere.
Fig. 5. Continuous reduction of NO by metallic iron in N2 atmosphere (flow rate 1.5 L/min, NO = 0.05% at 800 °C).
2000
+
Fe+2 Fe2 +3O4 Fe2 O3
∗
+
1500
Intensity (CPS)
with a little Ni. The iron surface was oxidized and both Fe3O4 and Fe2O3 were formed after reducing NO in N2 atmosphere, as showed in Fig. 4(b). In a continuous test with the iron mesh roll of 160 mm 80 mm at 800 °C with 0.05% NO balanced by N2, NO reduction decreased from about 99% at the beginning to about 27% after 68.2 h, as showed in Fig 5. XRD analysis of the iron sample after reaction showed that the iron was oxidized to Fe2O3 and Fe3O4, as showed in Fig 6. Iron could reduce NO to N2, while it was oxidized to FexOy. Reducing agent should be added to reduce iron oxides to iron in order to keep the reaction to continue. CO and CH4 were used as reducing agents in the following tests respectively in simulated flue gas.
+
∗ 1000
∗
+
+
500 +
+
∗
+ ∗
∗
+
∗ ∗
10 2000
+ + +∗ +∗ ∗∗ ∗ ∗
∗
30
40
50
60
70
80
90
2θ (°)
Fe
Fig. 6. XRD results of the iron mesh after 68.2 h continuous reaction with NO at 800 °C.
Fe,Ni
Intensity (CPS)
20
+
+
0
1500
3.2. Continuous NO reductions over iron oxides with CO/CH4
1000 500 0
70
reaction time / h
30
40
50
60
70
80
90
2θ (o)
A continuous test was conducted in simulated flue gas containing 2.0 vol.% O2, 0.05 vol.% NO, 16.8 vol.% CO2 balanced by N2 at 1050 °C. Iron mesh roll of 160 mm 80 mm was used as the iron sample. Compared to the NO reduction by iron alone in simulated flue gas, reducing gas, CO, could reduce iron oxides to metallic iron and finally improve the NO reduction [17]. 4.1 vol.% CO was first
2500
Fe Fe3O4
2000
Fe2 O3
100
NO reduction / %
Intensity (CPS)
(a) Original iron mesh
1500 1000 500 0
30
40
50
60
70
80
90
2θ (o)
(b) After reaction with NO in N2 atmosphere (1100°C) Fig. 4. XRD results of the iron mesh.
80 60 stop CO and feed 1.17% CH 4
40 20 0
0
20
40
60
80
100
reaction time / h Fig. 7. Continuous reaction of NO reduction by methane over iron oxides in simulated flue gas at 1050 °C (SR1 = 0.85).
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NO reduction / %
100
60 40
0 200
300
400
500
600
700
800
900 1000 1100
Temperature / oC Fig. 9. NO reduction by methane over iron oxides (flow rate 1.5 L/min, CH4 = 1.17%, NO = 0.05% balanced by N2).
6000 5000
CH4 =1.17%
4000 3000 2000 1000
3.3. NO reduction by methane over iron oxides in N2 atmosphere
0 200
300
400
500
600
700
800
900 1000 1100
Temperature / oC Fig. 10. CO formation during NO reduction over iron oxides (flow rate 1.5 L/min, CH4 = 1.17%, NO = 0.05% balanced by N2).
1000 eO +2 +3 + Fe Fe2 O4 Fe * Fe2 O3
+
800
Intensity (CPS)
A total of 1.5 L/min gas mixture containing 0.05 vol.% NO was fed into the reactor balanced by N2. 1.17 vol.% CH4 was used as the reductant. Fig 9 presents the result. Less than 20% NO reduction was tested below 500 °C, while almost 100% NO was reduced above 850 °C. However, a by-product, CO was formed, as showed in Fig 10. At 1050 °C, the test was stopped since the NO reduction was 100%. The reactor was cooled at 10 °C/min in N2 atmosphere to room temperature. The iron sample was checked by XRD and SEM. Fig 11 and Fig 12 present the results. Fig 11 showed that metallic iron, Fe, and FeO were formed, which indicated that iron oxides (Fe2O3, Fe3O4) were partly reduced to Fe2+ and Fe. When compared to the original iron oxides SEM image showed in Fig 12(a), the SEM image of the iron sample after reaction became porous with many microporous structures, as showed in Fig 12(b). Many fiber-like matters were locally formed. The length was about 15–20 lm and the outer diameter was about 1 lm. The iron sample after the reaction was locally analyzed by X-ray energy dispersive spectroscopy (EDS) and C element was observed on the
80
20
Exit CO / ppm
added into the reactor. At the beginning, NO reduction was 95%; however it decreased to about 20% after 5.05 h, as showed in Fig 7. This showed that CO was not good reductant in long-term application. Lasek [16] recently reported his research on NO reduction over iron sphere when CO was used as the reducing agent and the result showed that no more than 60% of NO was reduced when the air excess ratio was 0.6. When NO reduction efficiency was below 20%, CO was stopped and 1.17% CH4 was fed into the reactor. The corresponding stoichiometric ratio of CH4, as noted SR, was 0.85. The stoichiometric ratio is defined as the ratio of actual oxygen in the flue gas and the oxygen that complete combustion of methane requires. NO reduction efficiency quickly increased to over 98% as methane was fed. A continuous experiment was carried out for about 100 h and there was no sign that NO reduction may decrease, as showed in Fig 7. The result demonstrated that NO could be effectively reduced by methane over iron oxides. The iron sample was checked by XRD after the test and the result showed that iron was not fully oxidized to Fe3+ ion, as showed in Fig 8. This is due to the reducing effect of methane. The redox of iron between Fe3+ M Fe2+ M Fe plays the critical role for high NO reduction. Since CH4 was very effective to reduce NO over iron oxides, it is necessary to carefully investigate the reaction and mechanism between the reaction of NO, CH4 and iron/iron oxides. So the iron mesh roll after the durable reaction with NO for 68.2 h in N2 atmosphere was used as the iron oxides in the following test of NO reduction with CH4.
600
+
+
∗
400
∗
0 10
20
30
+
+
200
40
50
∗
60
70
80
90
2θ (°)
+ Intensity (CPS)
+3
Fig. 11. XRD pattern of iron oxides after reaction (flow rate 1.5 L/min, CH4 = 1.17%, NO = 0.05% balanced by N2, 1050 °C).
+ ++ + + +
fiber-like matters, the locally maximum C was as high as 88% (atom ratio). Previous results suggested that carbon nanotube could be produced by chemical vapor deposition method [29,30]. Catalyzed by transition metals, the hydrocarbon gases decomposed to form carbon atoms at high temperature, which finally formed carbon nanotube by orientated deposition according to the chemical vapor deposition mechanism. Generally, Fe, Ni and Co were most active catalysis during the synthesis of nanotubes by chemical vapor deposition method [29,30]. In the present experiment, methane thermally decomposed over iron oxides at 1050 °C in N2 atmosphere to form the fiber-like matter which was mainly composed of carbon.
+2
+ Fe Fe2 O 4 FeO
3000
2000
+ 0
+
+
1000
10
20
+ 30
40
+
+
+ 50
60
70
80
90
2θ (°) Fig. 8. XRD pattern of iron oxides after continuous reaction with CH4 and NO at 1050 °C.
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Y. Su et al. / Fuel 160 (2015) 80–86
(a) iron oxides before reaction
(b) iron oxides after reaction
Fig. 12. SEM image of iron oxides (flow rate 1.5 L/min, CH4 = 1.17%, NO = 0.05% balanced by N2, 1050 °C).
3.4. NO reduction by methane over iron oxides in flue gas atmosphere
100
NO reduction / %
80 60 40 20 0
SR 1=0.7, SR 2 =0.7 SR 1=0.7, SR 2 =1.2 SR 1=0.8, SR 2 =0.8 SR 1=0.8, SR 2 =1.2 SR 1=0.9, SR 2 =0.9 SR 1=0.9, SR 2 =1.2 SR 1=1.0, SR 2 =1.0 SR 1=1.0, SR 2 =1.2 SR 1=1.1, SR 2 =1.1 SR 1=1.1, SR 2 =1.2 SR 1=1.2 CH 4 =0, Fe
200
400
80
NO reduction / %
In real conditions, there are O2 and CO2 in the flue gas. The NO reduction by methane over iron oxides was tested in simulated flue gas of 1.5 L/min containing 2.0 vol.% O2, 0.05 vol.% NO, 17.0 vol.% CO2 balanced by N2. The amount of methane was controlled by the stoichiometric ratio (SR). The combustion conditions in the first and second furnace are noted as SR1 and SR2 respectively. When SR1 equals to SR2, additional O2 is not fed into the second furnace. In the test, the temperatures in the two furnaces were kept the same. When SR1 < 1, i.e., fuel-rich condition, NO reduction by methane over iron oxides was more than 90% over 900 °C, as showed in Fig 13. When SR1 exceeded 1, i.e., fuel-lean condition, NO reduction was lower than 35%. The difference of NO reduction was small when there was and was not a burnout process, as noted in Fig 13 by SR2 = 1.2 and SR2 = SR1. When iron was used alone to reduce NO, the NO reduction was no more than 30%, as noted CH4 = 0, Fe in Fig 13. When iron oxides were not used, it was actually reburning of methane. NO reduction after burnout decreased by about 20% when SR1 = 0.7, SR2 = 1.2 (with burnout) compared to the NO reduction when SR1 = 0.7, SR2 = 0.7 (without burnout), as showed in Fig 14. It was known that an intermediate product, HCN is produced during reburning of CH4 [21]. HCN is oxidized by O2 to form
100
60 40
without Fe2 O3, SR 1=0.7, SR 2 =0.7 with Fe2 O3, SR 1=0.7, SR 2 =0.7 without Fe2 O3, SR 1=0.7, SR 2 =1.2 with Fe2 O3, SR 1=0.7, SR 2 =1.2 without Fe2 O3, SR 1=1.0, SR 2 =1.0 with Fe2 O3, SR 1=1.0, SR 2 =1.0 without Fe2 O3, SR 1=1.0, SR 2 =1.2 with Fe2 O3, SR 1=1.0, SR 2 =1.2 without Fe2 O3, SR 1=1.2, SR 2 =1.2 with Fe 2 O3, SR 1=1.2, SR 2 =1.2
20 0 300
400
500
600
700
800
900
1000
1100
Temperature / oC Fig. 14. Comparison of NO reduction by methane in simulated flue gas atmosphere when iron oxides was or was not used (CH4 conversion rate was 100% above 800 °C) (flow rate 1.5 L/min, O2 = 2.0 vol.%, NO = 0.05 vol.%, CO2 = 17.0 vol.% balanced by N2).
new NO, resulting to the decrease of NO reduction after burnout. When iron oxides were used, NO reduction after burnout was almost the same to that without burnout, e.g., about 95% above 900 °C when SR1 = 0.7, SR2 = 1.2 (with burnout) and SR1 = 0.7, SR2 = 0.7 (without burnout), as showed in Fig 14. This result verified that iron oxides could reduce the intermediate product, HCN, during methane reburning. In fuel lean conditions, i.e., SR1 P 1:0, methane was burned by O2 and could not reduce NO and iron oxides any more. NO reduction was lower than 40% when SR1 P 1:0 whatever SR2 was and whether iron oxides was used or not. 3.5. Mechanism of NO reduction by methane over iron oxides Iron could reduce NO to N2 [15,19,20]:
Fe þ NO ! Fex Oy þ N2
600
800
1000
1200
Temperature / oC Fig. 13. NO reduction by methane over iron oxides in simulated flue gas atmosphere (flow rate 1.5 L/min, O2 = 2.0 vol.%, NO = 0.05 vol.%, CO2 = 17.0 vol.% balanced by N2).
The iron oxides were finally oxidized to Fe2O3, although Fe3O4 and FeO could be produced as the intermediate forms. The lattice oxygen provided by iron oxides (Fe2O3 and Fe3O4) at high temperature could partially oxidize methane to CO/CO2 and the iron oxides would be reduced to metallic iron at the same time by methane above 570 °C according to the sequence Fe2O3 ? Fe3O4 ? FeO ? Fe. [25–27,31]. Wang and Wei [28] suggested that Fe2O3 could completely oxidize methane to CO2 below 570 °C, while it could partially oxidize methane above 570 °C. The following reactions describe the main and secondary reactions.
Y. Su et al. / Fuel 160 (2015) 80–86
Main reactions:
4. Conclusions
3Fe2 O3 þ CH4 ðgÞ ! 2Fe3 O4 þ COðgÞ þ 2H2
ðR1Þ
Fe3 O4 þ CH4 ðgÞ ! 3FeO þ COðgÞ þ 2H2
ðR2Þ
FeO þ CH4 ðgÞ ! Fe þ COðgÞ þ 2H2
ðR3Þ
Secondary reactions:
12Fe2 O3 þ CH4 ðgÞ ! 8Fe3 O4 þ CO2 ðgÞ þ 2H2 OðgÞ
ðR4Þ
4Fe3 O4 þ CH4 ðgÞ ! 12FeO þ CO2 ðgÞ þ 2H2 OðgÞ
ðR5Þ
4FeO þ CH4 ðgÞ ! 4Fe þ CO2 ðgÞ þ 2H2 OðgÞ
ðR6Þ
(R1)–(R6) could go on spontaneously once the temperature is above 700 °C. According to thermodynamics Gibbs free energy and reaction heat calculated by Wang and Wei [28], when the temperature is above 700 °C, reaction (R4) is easier to go on than reaction (R1) and Fe2O3 is firstly reduced to Fe3O4, then Fe3O4 is reduced to FeO according to reaction (R2) and CO is produced. FeO is then reduced to Fe according to reaction (R3) and CO is produced again. The reducing route of iron oxides is Fe2O3 ? Fe3O4 ? FeO ? Fe. The produced CO and H2 during the reactions (R2) and (R3) will further reduce the un-reacted iron oxides to iron and water vapor will be produced. In the test, water was observed to condensate on the effluent tube. In addition, methane decomposes to C and H2 at high temperature:
CH4 ðgÞ ! C þ 2H2 ðgÞ
ðR7Þ
The cracking reaction of methane begins at 550 °C, but it goes on very slowly at 600–850 °C and no more than 3.4% methane could decompose below 850 °C [28]. When there is oxygen carrier, like Fe2O3, methane first reacts with lattice oxygen rather than cracking. Only when there is not enough lattice oxygen, methane decomposes to carbon and hydrogen. The carbon due to the decomposition of methane was very active and reacted with iron oxides immediately to reduce iron oxides to metallic iron [32,33].
Fe2 O3 þ 1:5C ! 2Fe þ 1:5CO2
ðR8Þ
Li and Shen [34] concluded that iron reduced by methane from iron oxides was not easy to be re-oxidized to iron oxides by O2 when the temperature was above 900 °C. This implies that when the temperature is above 900 °C, iron oxides could be easily reduced to metallic iron by methane, and then the iron is involved in NO reduction more than oxidation by O2 in the flue gas. In fuel rich conditions, methane also reduces NO via reburning [21]. Reburning reaction of methane with NO needs O radical [21]. Iron oxides provide lattice oxygen. In N2 atmosphere, O radical provided by iron oxides is not enough to make reburning the dominant mechanism. In the simulated flue gas atmosphere, however, reburning reaction could not be neglected at fuel rich condition. In the reburning of methane, an intermediate product, HCN, is produced, which forms new NO in the burnout process [22]. Su et al. [23] experimentally proved that Fe2O3 could reduce HCN and Tan et al. [24] proposed the following reaction scheme:
Fe2 O3 þ 3HCN ! 2Fe þ 3CO þ 1:5N2 þ 1:5H2
85
ðR9Þ
In the present experiments, when iron oxides was used, NO reduction by methane did not decrease after burnout (when SR2 was higher than 1 as noted in Fig 14). However, when iron oxides was not used, NO reduction after burnout, e.g., SR1 = 0.7, SR2 = 1.2 in Fig 14, was about 20% lower than that when iron oxides was used. This implied that the possible reburning intermediate product, HCN was reduced by iron oxides other than forming new NO.
NO reduction by methane over iron oxides was experimentally investigated. Results showed that metallic iron could reduce NO to N2 when the temperature was higher than 900 °C, but the durable performance was poor because that iron was oxidized to iron oxides. Methane was a good reducing agent and was very effective to reduce NO over iron oxides. In N2 atmosphere, 100% NO reduction efficiency was achieved above 850 °C. In simulated flue gas, NO reduction by methane over iron oxides could exceed 95% when the temperature was above 900 °C at fuel rich condition. 1.17% methane could continuously reduce 100% NO over 100 h at 1050 °C in simulated flue gas which contained 2.0% O2, 16.8% CO2 and 0.05% NO balanced by N2. Acknowledgment This work was supported by National Natural Science Foundation of China (Nos. 51278095 and 50806049), which are gratefully acknowledged. References [1] Centi G, Perathoner S. Introduction: state of the art in the development of catalytic processes for the selective catalytic reduction of NOx into N2. Stud Surf Sci Catal 2007;171:1–24. [2] Brandenberger S, Kröcher O, Tissler A, Althoff R. The state of the art in selective catalytic reduction of NOx by ammonia using metal-exchanged zeolite catalysts. Catal Rev 2008;50:492–581. [3] Long RQ, Yang RT. Characterization of Fe-ZMS-5 catalyst for selective catalytic reduction of nitric oxide by ammonia. J Catal 2000;194:80–90. [4] Apostolescu N, Geiger B, Hizbullah K, Jan MT, Kureti S, Reichert D, et al. Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts. Appl Catal B: Environ 2006;62:104–14. [5] Krishna K, Seijger GBF, van den Bleek CM, Makkee M, Mul G, Calis HPA. Selective catalytic reduction of NO with NH3 over Fe-ZSM-5 catalysts prepared by sublimation of FeCl3 at different temperatures. Catal Lett 2003;86:121–32. [6] Iwamoto M. Zeolites in environmental catalysis. Stud Surf Sci Catal 1994;84:1395–410. [7] Tabata T, Kokitsu M, Okada O. Study on patent literature of catalysts for a new NOx removal process. Catal Today 1994;22:147–69. [8] Armor JN. Catalytic reduction of nitrogen oxides with methane in the presence of excess oxygen: a review. Catal Today 1995;26:147–58. [9] Li Y, Battavio PB, Armor JN. Effect of water-vapor on the selective reduction of NO by methane over cobalt-exchanged ZSM-5. J Catal 1993;142:561–71. [10] Cant NW, Liu IOY. The mechanism of the selective reduction of nitrogen oxides by hydrocarbons on zeolite catalysts. Catal Today 2000;63:133–46. [11] Choi BC, Foster DE. State-of-the-art of de-NOx technology using zeolite catalysts in automobile engines. J Ind Eng Chem 2005;11:1–9. [12] Iliopoulou EF, Evdou A, Lemonidou AA, Vasalos IA. Ag/alumina catalysts for the selective catalytic reduction of NOx using various reductants. Appl Catal A: Gen 2004;274:179–89. [13] Kotsifa A, Kondarides DI, Verykios XE. A comparative study of the selective catalytic reduction of NO by propylene over supported Pt and Rh catalysts. Appl Catal B: Environ 2008;80:260–70. [14] Liu Z, Wang K, Zhang X, Wang J, Cao H, Gong M, Chen Y. Study on methane selective catalytic reduction of NO on Pt/Ce0.67Zr0.33O2 and its application. J Nat Gas Chem 2009;18:66–70. [15] Gradon B, Lasek J. Investigation of reduction of NO to N2 by reaction with Fe. Fuel 2010;89:3505–9. [16] Lasek J. Investigations of the reduction of NO to N2 by reaction with Fe under fuel-rich and oxidative atmosphere. Heat Mass Trans 2014;50:933–43. [17] Su Y, Su A, Cheng H. Experimental study on effect of CO on NO reduction by iron mesh roll. J Basic Sci Eng 2013;21(4):638–46 [in Chinese]. [18] Su Y, Su A, Cheng H. Experimental study on direct catalytic reduction of NO by metallic iron. J China coal Soc 2013;38(s1):206–10 [in Chinese]. [19] Hayhurst AN, Ninomiya Y. Kinetics of the conversion of NO to N2 during the oxidation of iron particles by NO in a hot fluidised bed. Chem Eng Sci 1998;53:1481–9. [20] Hayhurst AN, Lawrence AD. The reduction of the nitrogen oxides NO and N2O to molecular nitrogen in the presence of iron, its oxides, and carbon monoxide in a hot fluidized bed. Combust Flame 1997;110:351–65. [21] Smoot LD, Hill SC, Xu H. NOx control through reburning. Prog Energy Combus Sci 1998;24:385–408. [22] Johnson DK, Engelhardt DA, Harvilla J, Beecy DJ, Watts JU. Reburning technologies for the control of nitrogen oxides emissions from coal-fired boilers, Washington (DC): United States Dept. of Energy Clean Coal Technology Topical Report, 1999.
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