Study of NH3 removal based on chemical-looping combustion

Mar 4, 2019 - Results indicated that in the reduction reaction between oxidative oxygen carriers (OCs) and NH3, the oxidative OCs were converted to ...
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Study of NH3 Removal Based on Chemical-Looping Combustion Yongjian Wu,† Chunhuan Luo,†,‡ and Qingquan Su*,†,§ †

School of Energy and Environmental Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China ‡ Beijing Key Laboratory of Energy Conservation and Emission Reduction for Metallurgical Industry, Beijing 100083, China § Beijing Engineering Research Center for Energy Saving and Environmental Protection, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China

Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by TULANE UNIV on 03/21/19. For personal use only.

S Supporting Information *

ABSTRACT: The removal characteristics of NH3 based on chemical-looping combustion (CLC), in which NH3 was used as a fuel, were studied. Results indicated that in the reduction reaction between oxidative oxygen carriers (OCs) and NH3, the oxidative OCs were converted to reductive OCs, and at the same time, NH3 was converted to N2 and H2O. Then, the reductive OCs were regenerated to the oxidative OCs with air in the subsequent oxidation reaction. As such, NH3 removal with high N2 selectivity was achieved based on CLC. Compared to MnO2/Al2O3 and Fe2O3/Al2O3, CuO/Al2O3 exhibited the highest NH3 removal activity, and an NH3 removal rate of 90% with a N2 selectivity of 99% was achieved at 250 °C. Among three Cu-based OCs, i.e., CuO/Al2O3, CuO/TiO2, and CuO/SiO2, CuO/Al2O3 using γ-Al2O3 as inert support performed the highest NH3 removal activity and N2 selectivity. Moreover, the required temperature for the complete regeneration of the reductive CuO/Al2O3 with air was as low as 200 °C. However, due to its overly high oxidizability, O2 that existed in the waste gas deteriorated the N2 selectivity. On the basis of these findings, a new process for NH3 deep removal with high N2 selectivity based on CLC coupled with adsorption enrichment was proposed.

1. INTRODUCTION Ammonia (NH3) is a noxious gas and is viewed as one of the important precursors of PM2.5; therefore, NH3 emissions have serious harm.1 According to the amount and concentration of NH3 emissions, technologies for the removal of NH3 include biological methods, absorption, adsorption, catalytic decomposition, selective catalytic oxidation, and others.2−7 For a waste gas containing a high concentration of NH3, the absorption method is commonly adopted.8 However, limited by its absorption efficiency, NH3 is difficult to thoroughly absorb, resulting in low-concentration NH3 emissions. The selective catalytic oxidation (SCO) of NH3 to N2 and H2O (i.e., NH3-SCO) is one of the most promising approaches for waste gas containing low-concentration NH3.2,9,10 A large variety of materials are used as catalysts for NH3SCO.11 An efficient catalyst should have high reactivity and N2 selectivity.12 Platinum, palladium, rhodium, and silver are the catalysts with high activity.13,14 For the catalysts of Pd, Rh, and Pt supported on Al2O3 or ZSM-5, the NH3 oxidation temperatures were 200−350 °C, whereas their N2 selectivities were relatively low, typically less than 80%.11 Considering high cost of noble metals, transition metal oxides, such as MnO2, Fe2O3, CuO, Co3O4, NiO, V2O5, and MoO3, have attracted much attention. 2,15−18 These catalysts show high N 2 © XXXX American Chemical Society

selectivities but require higher operation temperatures (typically 300−500 °C).11,17,19 Song et al.20 reported that a MnOx(0.25)-TiO2 catalyst synthesized by the sol−gel method exhibited high performance; the NH3 removal rate reached 100% at 200 °C, and the operating temperature window for a N2 selectivity greater than 80% was 180−300 °C. Gora-Marek et al.21 showed that a Fe2O3/ZSM-5-P catalyst synthesized by a 2-fold ion-exchange method achieved a N2 selectivity of 90% at 475 °C, with the temperature for the NH3 removal rate of 50% (T50) being 400 °C. Chmielarz et al.12 synthesized a Cu−Mg− Al hydrotalcite-like catalyst by the thermal decomposition method and demonstrated that copper oxide species had vital effects in NH3-SCO. The oxidation of ammonia was observed starting at approximately 200 °C, and the temperature required for an NH3 conversion of 90% (T90) was 415 °C. Tang et al.22 reported that modifying a Cu−Mn/TiO2 catalyst with rare earth oxides (e.g., Ce2O3 and La2O3) was beneficial for the removal of NH3 but also significantly promoted the formation of NO and N2O. Jablonska et al.23 proposed that the gap Received: Revised: Accepted: Published: A

October 30, 2018 March 1, 2019 March 4, 2019 March 4, 2019 DOI: 10.1021/acs.iecr.8b05375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

dripped onto γ-Al2O3 at 60 °C. The impregnated γ-Al2O3 was calcined at 500 °C for 5 h. The calcined γ-Al2O3 was blended with a polyvinyl alcohol aqueous solution, pressed to flakes, and then calcined. Finally, the calcined flakes were crushed, and the obtained samples with a particle size of 0.71−1 mm were noted to be CuO/Al2O3, Fe2O3/Al2O3, or MnO2/Al2O3. With the same method, SiO2 and TiO2 powder of 250−300 mesh were used as inert supports, and the samples of CuO/ SiO2 and CuO/TiO2 were synthesized, respectively. For all the above OC samples, the mass fractions of the active components were 10%. Additionally, for the blank experiments, samples of γ-Al2O3, SiO2, and TiO2 were prepared and were noted to be Al2O3/B, SiO2/B, or TiO2/B. 2.2. OC Characterization. X-ray diffraction (XRD) patterns were recorded on an Ultima-IV Rigaku X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å, 40 mA, 40 kV). Chemical compositions for OC samples were identified using an X-ray fluorescence analyzer (XRF, Shimadzu XRF-1800 spectrometer). BET surface areas of OC samples were determined with a Quantachrome QuadraSorb-SI instrument. The samples were degassed in nitrogen atmosphere at 300 °C for 4 h prior to nitrogen adsorption at −196 °C. Scanning electron microscopy (SEM, Zeiss EVO-18) was used to obtain the morphology of OC samples. H2-temperature-programmed reduction (H2-TPR) was carried out using an Autochem II 2920 Micromeritics. 200 mg of fresh OC sample was heated in the flow of Ar (50 mL/ min) up to 400 °C and precleaned for 1 h. The sample was cooled to 100 °C before introducing 10% H2/Ar (50 mL/ min). Then, the sample was heated to 500 °C with a heating rate of 10 °C/min, and the signal of TCD detector was recorded. O2-temperature-programmed reduction−oxidation (O2TPRO) was also performed using the same instrument. 200 mg of fresh OC sample was heated in the flow of 10% H2/Ar (50 mL/min) up to 400 °C and prereduced for 1 h. The sample was cooled to 50 °C in the flow of He (50 mL/min) before introducing 10% O2/He (50 mL/min). Finally, the sample was heated to 400 °C with a heating rate of 10 °C/min, and the signal of TCD detector was recorded. 2.3. Apparatus and Methods. A fixed bed reactor with internal diameter of 20 mm and height of 400 mm was used to evaluate the NH3 removal performance of oxidative OCs and the regeneration performance of reductive OCs. The evaluation experiments were carried out at atmospheric pressure. Figure 2 presents a schematic of experimental apparatus. Thermostatic zone of the reactor was filled with OC samples. A K-type thermocouple with diameter of 1.0 mm was used to monitor sample temperatures. Gas mass flow controllers (MFCs, Beijing HORIBA METRON Instruments Co., Ltd.) were used to quantitatively regulate the flow rates of the reactant gases. The NH3, NO, and NO2 concentrations were monitored using online infrared analyzers (Beijing Baifmaihak Analytical Instrument Co., Ltd.). The N2O concentration was monitored using an online gas chromatography− mass spectrometer (Shimadzu GCMS-QP2010 Plus). The O2 and N2 concentrations were determined using a TCD detector of an online gas chromatograph (Agilent GC 6820).29 Gas concentrations refer to volume concentrations. The following several parameters were defined to illustrate performance of OCs.

between CuO/Al2O3 of high conversion temperature and Ag2O/Al2O3 of low N2 selectivity could be bridged by using a Ag−Cu/Al2O3 catalyst, the T50 and T90 of which were 281 and 365 °C, respectively, and the N2 selectivity was maintained higher than 90% at 250−400 °C. There are two separate reactions in chemical-looping combustion (CLC).24,25 In the reduction reactions between oxidative oxygen carriers (OCs) and hydrocarbon fuels, oxidative OCs are reduced to reductive OCs and hydrocarbon fuels are oxidized to H2O and CO2. In the oxidation reactions between the reductive OCs and air, the reductive OCs are regenerated to the oxidative OCs with O2.26 The significant difference between CLC and traditional combustion is that in CLC, the oxidation of fuel can be performed with oxidative OCs rather than molecular oxygen; thus, it is endowed with the characteristics of inherent CO2 capture with almost zero energy-consumption, low NOx emissions, and high energy utilization efficiency.27,28 In addition to CH4, CO, and H2, NH3, can also play the role of fuels in the reduction reaction. In this research, the NH3 removal based on CLC was presented, that is, in the reduction reactions between oxidative OCs and NH3, which is used as a fuel, the oxidative OCs are reduced to reductive OCs, and at the same time, NH3 is converted to N2 and H2O. Then, the reductive OCs are oxidized to the oxidative OCs with air in subsequent oxidation reaction. Schematic of the NH3 removal based on CLC is shown in Figure 1, where MO represents the

Figure 1. Schematic of the NH3 removal based on CLC.

active metal oxides with higher valence states in oxidative OCs and M represents the active metal or metal oxides with lower valence states in reductive OCs. The reduction reactions between oxidative transition metal oxide OCs and NH3 were studied, and the performance of Cubased, Mn-based, and Fe-based OCs on NH3 removal and N2 selectivity was determined. The oxidation reactions between the reductive CuO/Al2O3 and air were studied. According to the results, a new process for NH3 deep removal based on CLC coupled with adsorption enrichment has been proposed.

2. EXPERIMENTAL SECTION 2.1. OC Synthesis. Copper nitrate, iron nitrate, and manganese nitrate (Sinopharm, analytical grade) were used as the precursors of active components. γ-Al2O3 powder of 250− 300 mesh was used as inert support. The nitrate aqueous solution of suitable concentration was prepared and then B

DOI: 10.1021/acs.iecr.8b05375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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blank experiment. MNH3,theo was theoretical NH3 amount needed for the oxidative OC being completely reduced. A cumulative OC oxidation rate XOC,O,t from the beginning to the time point of t min in the oxidation reaction of the reductive OC with O2 was defined as eq 4.30 t

XOC,O, t =

An instantaneous NH3 removal rate XNH3,t was defined as eq

C NH3,in, t

× 100 (1)

An instantaneous N2 selectivity SN2,t was defined as eq 2. Instantaneous N2O, NO, and NO2 selectivities were defined as eqs S1, S2, and S3 (Supporting Information), respectively. SN2, t = C NH3,in, t − C NH3,out, t − 2C N2O,out, t − C NO,out, t − C NO2,out, t C NH3,in, t − C NH3,out, t (2)

× 100

where CNH3,in,t was the instantaneous NH3 concentration of the inlet gas, and CNH3,out,t, CN2O,out,t, CNO,out,t and CNO2,out,t were the instantaneous concentrations of NH3, N2O, NO, and NO2, respectively, in the outlet gas. A cumulative OC reduction rate XOC,R,t from the beginning to the time point of t min in the reduction reaction of the oxidative OC with NH3 was defined as eq 3.30 XOC,R, t = t

× 100 (4)

3. RESULTS AND DISCUSSION 3.1. Elemental Analysis and Structure Characterization. The real loading of transition metals and their form for the obtained fresh OC samples were characterized and identified by XRF and XRD. The XRD results for the fresh OC samples are presented in Figures S1 and S2 (Supporting Information). For the CuO/Al2O3, Fe2O3/Al2O3, and MnO2/ Al2O3 samples, copper, iron, and manganese species existed in the forms of CuO, MnO2, and Fe2O3, respectively. Similarly, the copper species in the CuO/SiO2 and CuO/TiO2 samples also existed in the forms of CuO. Table 1 shows that the average crystallite sizes of the active metal oxides in the above OC samples were estimated by means of the Scherrer equation.14 The actual chemical compositions for OCs identified by XRF are also shown in Table 1. It is apparent that the content values of Cu, Mn, and Fe in the fresh OC samples were nearly in agreement with the design values. Table 1 also shows the specific surface areas (SBET) for fresh OC samples. Among three Cu-based OCs of CuO/Al2O3, CuO/ TiO2, and CuO/SiO2, the crystallite size of CuO in CuO/TiO2 was relatively large and the SBET of CuO/TiO2 was lowest, which might lead to the lower reactivity. 3.2. Reduction Reaction of Oxidative OCs with NH3. To investigate the behaviors of NH3 removal through CLC, in which oxidative OCs play the role of oxidant and NH3 plays the role of fuel, the reduction reaction characteristics of CuO/ Al2O3, MnO2/Al2O3, and Fe2O3/Al2O3 with NH3 were studied. 5.0 g of CuO/Al2O3, 4.5 g of Fe2O3/Al2O3, or 4.6 g of MnO2/Al2O3 was filled in the reactor, and the inlet reactant gas (600 mL/min) was 1000 ppm of NH3/N2. The experiments were performed at 200−350 °C and a gas hourly

1. C NH3,in, t − C NH3,out, t

22.4MO2,theo

where VO2,in,t and VO2,out,t were instantaneous O2 flow rates of inlet and outlet gases, respectively, in the oxidation reaction. MO2,theo was the theoretical O2 amount needed for the reductive OC being completely oxidized. A breakthrough time tb was defined as the reaction time when the XNH3,t decreases to 90%, that is, CNH3,out,t reaches 100 ppm in the reduction reaction of oxidative OC with NH3. Thus, XOC,R,b was the cumulative OC reduction rate from the beginning to the time point of tb.

Figure 2. Schematic of experimental apparatus.

X NH3, t =

∫0 (VO2,in, t − VO2,out, t ) dt

t

′ 3,out, t ) dt ∫0 (VNH3,in, t − VNH3,out, t ) dt − ∫0 (VNH3,in, t − V NH 22.4M NH3,theo (3)

× 100

where VNH3,in,t and VNH3,out,t were instantaneous NH3 flow rates of inlet and outlet gases, respectively, in the reduction reaction. V′NH3,out,t was instantaneous NH3 flow rate of outlet gas in the Table 1. Texture and Composition for Fresh OC Samples

composition (wt %)c sample

SBETa (m2/g)

dMOb (nm)

Cu

CuO/Al2O3 MnO2/Al2O3 Fe2O3/Al2O3 CuO/SiO2 CuO/TiO2

154.45 158.64 164.59 132.80 59.42

23.46 23.30 19.95 23.79 31.13

8.08

Mn

Fe

Al

7.04

47.57 47.66 47.60

6.27 8.11 8.03

Si

Ti

41.88 53.94

a Specific surface area determined by BET characterization. bAverage crystallite size of the active metal oxides in OCs calculated by Scherrer equation. cComposition determined by XRF characterization.

C

DOI: 10.1021/acs.iecr.8b05375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. Outlet concentrations of (a) NH3 and (b) NOx versus time during reduction reactions of OCs with NH3 at 300 °C. Reaction conditions: 1000 ppm of NH3 and balance of N2, total flow rate = 600 mL/min; [CuO/Al2O3] = 5.0 g, [MnO2/Al2O3] = 4.6 g, [Fe2O3/Al2O3] = 4.5 g, [Al2O3/ B] = 4.1 g; GHSV = 5000 h−1.

Figure 4. (a) NH3 removal rate and (b) N2 selectivity versus temperature in the reduction reaction of OCs with NH3. Reaction conditions: 1000 ppm of NH3 and balance of N2, total flow rate = 600 mL/min; [CuO/Al2O3] = 5.0 g, [MnO2/Al2O3] = 4.6 g, [Fe2O3/Al2O3] = 4.5 g; GHSV= 5000 h−1.

space velocity (GHSV) of 5000 h−1. Additionally, to estimate the saturated NH3 adsorption amount for OCs, the blank experiments using Al2O3/B were also performed under the same conditions. Figure 3 shows the relationship between the outlet NH3 and NOx concentrations and the reaction times for Al2O3/B, CuO/ Al2O3, Fe2O3/Al2O3, and MnO2/Al2O3 at 300 °C. As shown in Figure 3a, for Al2O3/B, NH3 began to escape at 11 min and the NH3 concentration in the outlet gas reached 1000 ppm near 20 min, meaning that NH3 adsorption on γAl2O3 occurred. The curve of NH3 concentration in the outlet gas for Fe2O3/Al2O3 almost overlapped with that for Al2O3/B. For MnO2/Al2O3, the initial time for the NH3 escape was prolonged to 16 min, and for CuO/Al2O3, it was further prolonged to 70 min. Meanwhile, from Figure 3b, the NOx concentration in the outlet gas for CuO/Al2O3 was only 30 ppm in the initial reaction stage and finally decreased to 0 ppm, whereas there was little NOx in the outlet gas for MnO2/Al2O3 and Fe2O3/Al2O3. Furthermore, the corresponding NH3 consumption for reactions in Figure 3a was estimated by the time integral of

the difference between the inlet NH3 concentration and the outlet NH3 concentration (Supporting Information, Table S1). For Al2O3/B, the NH3 consumption, which was attributed to NH3 adsorption on γ-Al2O3, was appropriately 7.5 mL. For Fe2O3/Al2O3, the experimental NH3 consumption was appropriately 8.1 mL. After subtraction of the NH 3 consumption on Al2O3, the experimental NH3 consumption by Fe2O3 was far less than its theoretical value. It indicated that only a little of Fe2O3 was reduced to Fe3O4. Similarly, for MnO2/Al2O3, only a small part of MnO2 was reduced to Mn2O3. In contrast, for CuO/Al2O3, the experimental NH3 consumption by CuO was approximately equal to the theoretical NH3 amount needed for CuO being reduced to Cu2O, indicating that most of CuO might be reduced to Cu2O. The above results indicated that in reduction reactions of oxidative OCs with NH3, the oxidative OCs were converted to reductive OCs, and at the same time, most NH3 was converted to N2 and H2O. As the reaction proceeded, the oxidative OC was gradually consumed; as a result, the substantial GHSV of the reaction increased continuously (actually, the GHSV of 5000 h−1 can be viewed as an initial GHSV), and the outlet D

DOI: 10.1021/acs.iecr.8b05375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research NH3 concentration finally reached the inlet NH3 concentration. To evaluate the performance of NH3 oxidation by the oxidative OCs more veritably, the NH3 removal rate and selectivities of N2 and NOx were determined with the outlet concentrations of NH3 and NOx at the time point of ta min in the reduction reactions. ta referred to the time when the outlet NH3 concentration reached the inlet NH3 concentration in the blank experiment at the same temperature. By this method, the contribution of NH3 adsorption on inert carriers to NH3 removal could be minimized. Figure 4 provides the NH3 removal rate and N2 selectivity in the above reduction reactions of CuO/Al2O3, Fe2O3/Al2O3, and MnO2/Al2O3, and at the same time, selectivities of N2O, NO, and NO2 are shown in Figure S3 (Supporting Information). From Figure 4a, the NH3 removal rate increased with elevating reaction temperature. For CuO/Al2O3, T50 and T90 were 215 and 250 °C, respectively. For MnO2/Al2O3, T50 was 325 °C, whereas for Fe2O3/Al2O3, the NH3 removal rate was only 27.1%, even when the temperature was elevated to 350 °C. The results indicated that in the reduction reactions of OCs with NH3, the NH3 removal activity of CuO/Al2O3 was much higher than that of MnO2/Al2O3 and Fe2O3/Al2O3. This should be because the NH3 removal activity of OC was related to its reduction reaction characteristics, and CuO/Al2O3 was more easily reduced compared to MnO2/Al2O3 and Fe2O3/ Al2O3. From Figure 4b, the N2 selectivity decreased slightly with the increasing reaction temperature and was maintained at 95% even at 350 °C, indicating that OCs exhibited high N2 selectivity. The CuO/Al2O3 samples after reacting with NH3 at various temperatures were characterized by XRD to examine the state transformation of copper species in the above reduction reactions, as presented in Figure 5.

For the CuO/Al2O3 sample obtained at 300 °C, diffraction peaks assigned to CuO disappeared, whereas those assigned to Cu2O became stronger. The diffraction peaks corresponding to Cu were observed. This indicated that all of CuO was reduced and a part of CuO was further reduced to Cu at 300 °C. Furthermore, for the CuO/Al2O3 sample obtained at 350 °C, diffraction peaks assigned to CuO and Cu2O both disappeared, indicating all of CuO was reduced to Cu at 350 °C. The above results indicated that the state transformations of copper species during the reduction reaction of CuO/Al2O3 with NH3 were relevant with reaction temperatures. Figure 6 shows maximum outlet concentrations of N2O, NO and NO2 during the above reactions between CuO/Al2O3 and NH3 at different temperatures.

Figure 6. Maximum outlet concentrations of N2O, NO, and NO2 during reactions between CuO/Al2O3 and NH3 at 200−350 °C. Reaction conditions: 1000 ppm of NH3 and balance of N2, total flow rate = 600 mL/min; [CuO/Al2O3] = 5.0 g; GHSV = 5000 h−1.

It is apparent that the outlet concentrations of N2O, NO, and NO2 were 0 ppm when the temperature was below 225 °C; a small amount of N2O and NO was detected when the temperature was 225−300 °C, and a small amount of NO2 was detected when the temperature was above 300 °C. On the basis of thermodynamic data from HSC Chemistry 6 (Supporting Information, Tables S2−S9) and the above XRD results, it can be inferred that when the temperature was below 225 °C, only reaction 5 occurred, reactions 6−9 could also occur as the temperature increased, whereas reaction 10 could occur when the temperature was above 300 °C.

Figure 5. XRD patterns of CuO/Al2O3 samples after reacting with NH3 at (a) 250 °C, (b) 300 °C, and (c) 350 °C. Reaction conditions: 1000 ppm of NH3 and balance of N2, total flow rate = 600 mL/min; [CuO/Al2O3] = 5.0 g; GHSV = 5000 h−1.

From Figure 5, for the CuO/Al2O3 sample obtained at 250 °C, two species related to copper were observed; one was Cu2O, and the other one was CuO. It indicated that CuO/ Al2O3 was able to be reduced with NH3, and copper species with 2+ valence (CuO) in the sample was only able to be reduced to copper species with 1+ valence (Cu2O) at 250 °C.

2NH3 + 6CuO = 3Cu 2O + N2 + 3H 2O

(5)

2NH3 + 8CuO = 4Cu 2O + N2O + 3H 2O

(6)

NH3 + 5CuO = 2.5Cu 2O + NO + 1.5H 2O

(7)

2NH3 + 3Cu 2O = 6Cu + N2 + 3H 2O

(8)

2NH3 + 4Cu 2O = 8Cu + N2O + 3H 2O

(9)

NH3 + 7CuO = 3.5Cu 2O + NO2 + 1.5H 2O

(10)

To further investigate reduction characteristics of CuO/ Al2O3, Fe2O3/Al2O3, and MnO2/Al2O3, H2-TPR characterizations were implemented. As shown in Figure 7, for MnO2/ E

DOI: 10.1021/acs.iecr.8b05375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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activity. From Figure 8b, the N2 selectivity for CuO/Al2O3 was almost the same as that of CuO/SiO2 and was better than that of CuO/TiO2. To examine the morphology difference of the above three Cu-based OCs, SEM characterization was implemented (Supporting Information, Figure S4). As shown in Figure S3, compared with CuO/SiO2, the particle surface of CuO/TiO2 was relatively smooth, which could lead to a lower specific surface area of CuO/TiO2. However, compared with CuO/ SiO2, the particle surface of CuO/Al2O3 was much rougher and more porous, which could contribute to a higher specific surface area of CuO/Al2O3 and its higher reactivity. To further examine the influence of inert supports on reduction characteristics of Cu-based OCs, H2-TPR characterizations on CuO/TiO2 and CuO/SiO2 were added, as shown in Figure 9. The reduction peaks for CuO/TiO2 and CuO/ Figure 7. H2-TPR results for CuO/Al2O3, Fe2O3/Al2O3, and MnO2/ Al2O3.

Al2O3, two obvious peaks appeared at around 305 and 370 °C. The low-temperature peak was probably ascribed to the reduction of MnO2 to Mn2O3, while another peak was probably assigned to the further reduction of Mn2O3 to Mn3O4.31 Only a broad peak was observed at around 390 °C for Fe2O3/Al2O3. However, for CuO/Al2O3, the reduction peak was detected at approximately 215 °C, which was assigned to the reduction of CuO to Cu.32 Thus, the results showed that compared with MnO2/Al2O3 and Fe2O3/Al2O3, CuO/Al2O3 was much easier to be reduced. 3.3. Influence of Inert Support on NH3 Removal Performance of Cu-Based OCs. An OC consists of active component and inert support. It is necessary to explore influence of inert support on NH3 removal performance of OCs. CuO/Al2O3, CuO/TiO2, and CuO/SiO2 were compared under the same experimental conditions as presented in section 3.2. The results are presented in Figure 8. As shown in Figure 8a, T50 for CuO/Al2O3, CuO/TiO2, and CuO/SiO2 was 215, 235, and 250 °C, respectively, indicating that compared to CuO/TiO2 and CuO/SiO2, CuO/Al2O3 using γ-Al2O3 as inert support exhibited higher NH3 removal

Figure 9. H2-TPR results for CuO/SiO2 and CuO/TiO2.

SiO2 shifted to the higher temperature side with 25 and 40 °C, respectively. Thus, the H2-TPR results also supported the conclusion that CuO/Al2O3 had higher NH3 removal activity than CuO/SiO2 and CuO/TiO2.

Figure 8. Comparisons of (a) NH3 removal rate and (b) N2 selectivity in the reduction reaction between the three different Cu-based OCs and NH3 at 200−300 °C. Reaction conditions: 1000 ppm of NH3 and balance N2, total flow rate = 600 mL/min; [CuO/Al2O3] = 5.0 g, [CuO/SiO2] = 4.6 g, [CuO/TiO2] = 6.5 g; GHSV = 5000 h−1. F

DOI: 10.1021/acs.iecr.8b05375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 3.4. Influence of GHSV on NH3 Removal Performance of of CuO/Al2O3. GHSV is an important factor that affects the efficiency, compactness, and cost of a gas processing system. Moreover, as mentioned in section 3.2, the continuously increasing substantial GHSV during the reduction reaction of CuO with NH3 resulted in the cessation of NH3 removal. Therefore, additional experiments on the influence of GHSV were performed over CuO/Al2O3 with a gas (1000 ppm of NH3/N2, 600 mL/min) at 300 °C and 5000−20 000 h−1. From Figure 10, the influence of GHVS on tb and XOC,R,b was significant. When GHSV was 5000 h −1 , t b was

necessary to investigate the oxidation characteristics of reductive OCs. Experiments on the oxidation regeneration of the reductive CuO/Al2O3 (i.e., Cu/Al2O3) with air were performed at 90 h−1 and 100−300 °C. The oxidation reaction is as follows: 2Cu + O2 = 2CuO

(11)

For each oxidation experiment, the oxidative CuO/Al2O3 was first heated in the flow of 20% H2/N2 (100 mL/min) up to 400 °C and then completely prereduced for 1 h. Subsequently, the OC sample was cooled to the reaction temperatures, and air (30 mL/min) was introduced. From Figure 11a, the outlet concentration of O2 was 0% in the first 13 min even at 100 °C, indicating that the temperature required for the oxidation of Cu/Al2O3 with air was quite low. In Figure 11b, the XOC,O,t increased linearly with the reaction time before O2 in the outlet gas escaped and then approached its maximum value. For 100 °C, the maximum XOC,O,t was approximately 52.6%, whereas when the temperature was elevated to 200 °C or higher, it approached 100%, indicating that Cu/Al2O3 could be completely oxidized and regenerated to CuO/Al2O3 above 200 °C. To further examine the oxidation characteristics of the reductive CuO/Al2O3, an O2-TPRO characterization was implemented. As shown in Figure 12, a sharp oxidation peak near 95 °C was detected, and another oxidation peak appeared at approximately 209 °C. The peak at lower temperature was ascribed to the oxidation of Cu0 to Cu+, corresponding to the oxidation rate of 52.6% at 100 °C. The other peak was attributed to the oxidation of Cu+ to Cu2+. Thus, the conclusion that the reductive CuO/Al2O3 could be completely regenerated above 200 °C was supported by the O2-TPRO results. 3.6. Influence of O2 on NH3 Removal Performance of Oxidative OCs. Considering that O2 is usually contained in waste gas, the influence of O2 on NH3 removal performance of oxidative OCs was investigated. 5% O2 was introduced into the reduction reactions of CuO/Al2O3, Fe2O3/Al2O3, and MnO2/ Al2O3 with 200 ppm of NH3/N2 at 50 000 h−1 and 200−350 °C. Figure 13 presents the NH3 removal rate and N2 selectivity at the steady reaction stage, and at the same time, selectivities

Figure 10. tb and XOC,R,b versus GHSV in the reduction reaction of CuO/Al2O3 with NH3 at 300 °C. Reaction conditions: 1000 ppm of NH3 and balance of N2, total flow rate = 600 mL/min; [CuO/Al2O3] = 1.2−5.0 g.

approximately 78 min and the corresponding XOC,R,b was approximately 37.2%, whereas when GHSV was elevated to 10 000 h−1, tb decreased to 30 min and the corresponding XOC,R,b decreased to 30%. Therefore, in order to obtain a reasonable NH3 removal rate, a GHSV should not exceed 10 000 h−1. 3.5. Oxidation Reaction of Reductive OCs with Air. For the regeneration of reductive OCs to oxidative OCs, it is

Figure 11. (a) Outlet concentration of O2 and (b) XOC,O,t versus time during reactions between Cu/Al2O3 and air at 100−300 °C. Reaction conditions: air, flow rate = 30 mL/min; [CuO/Al2O3] = 14.2 g; GHSV = 90 h−1. G

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Figure 12. O2-TPRO profile for CuO/Al2O3.

Figure 14. New process for NH3 deep removal with high N2 selectivity based on CLC coupled with adsorption enrichment.

of N2O, NO, and NO2 are shown in Figure S5 (Supporting Information). From Figure 13a, T50 for CuO/Al2O3, MnO2/Al2O3, and Fe2O3/Al2O3 was 225, 235, and 290 °C, respectively, and T90 for each was 255, 270, and 335 °C, respectively. From Figure 13b, with temperatures increasing, the N2 selectivity decreased significantly, and when the temperature was elevated to 300 °C, it decreased to 51.0%, 23.4%, and 71.4% for CuO/Al2O3, Fe2O3/Al2O3, and MnO2/Al2O3, respectively. The results showed that due to its high oxidizability, O2 in the waste gas would deteriorate the N2 selectivity. 3.7. NH3 Removal Process Based on CLC. On the basis of the above findings, to create an atmosphere containing less or no O2 for the reduction reaction of oxidative OCs with NH3, we proposed to introduce an adsorption process with an adsorption tower, in which NH3 was absorbed and separated from O2, prior to the reduction reaction. As shown in Figure 14, for a waste gas containing low-concentration NH3, an NH3 deep removal with high N2 selectivity process based on CLC coupled with adsorption enrichment was presented. As illustrated in Figure 14, the waste gas is introduced into adsorption tower 1, where NH3 is absorbed and separated from O2 by the adsorbent at atmospheric temperature. Once

the NH3 concentration in the cleaned gas reaches the given value, three-way valves 3 and 4 are switched, and circulating fan 5 and heater 6 begin to work. Then, the adsorbed NH3 is gradually desorbed, and the desorption gas is circulated between adsorption tower 1, circulating fan 5, heater 6, and CLC reactor 2. As the oxidative CuO/Al2O3 in CLC reactor 2 is heated to 200−300 °C, NH3 is oxidized to N2 and H2O, and at the same time, the oxidative CuO/Al2O3 is gradually converted to reductive CuO/Al2O3. After the regeneration of the adsorbent is completed, three-way valves 3 and 4 are switched to their initial states, and adsorption tower 1 is gradually cooled to atmospheric temperature, and then the waste gas is reintroduced. Meanwhile, electric valves 7 and 8 are opened, and air is introduced into CLC reactor 1 to oxidize and regenerate the reductive CuO/Al2O3 at 200−300 °C. When the regeneration of the reductive CuO/Al2O3 is completed, circulating fan 6 is stopped and electric valves 7 and 8 are closed. In this proposed process, the gas held in adsorption tower 1 and CLC reactor 2 is used as the heating medium and desorption gas. This is a great difference between the new process and the conventional adsorption enrichment-NH3SCO process, in which a large amount of desorption gas is

Figure 13. (a) NH3 removal rate and (b) N2 selectivity versus temperature for CuO/Al2O3, Fe2O3/Al2O3, or MnO2/Al2O3 at the steady reaction stage. Reaction conditions: 200 ppm of NH3, 5% O2, and balance of N2, total flow rate = 1000 mL/min; [CuO/Al2O3] = 0.8 g, [Fe2O3/Al2O3] = 0.8 g, [MnO2/Al2O3] = 0.8 g; GHSV = 50 000 h−1. H

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Fundamental Research Funds for the Central Universities (Grant FRF-SD-12-013A).

introduced into and discharged from the adsorption tower, resulting in significant heat loss. Moreover, the heat released in the reduction reaction of the oxidative CuO/Al2O3 with NH3 and the oxidation regeneration reaction of the reductive CuO/ Al2O3 with air is effectively used for heating adsorption tower 1 and CLC reactor 2, thereby supplying NH3 desorption heat. Furthermore, unlike the conventional NH3-SCO system, the outlet gas from CLC reactor 2 is circulated inside the closed system such that even if some NH3 escapes from CLC reactor 2 in one pass, it will be removed in the next pass.

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(1) Geng, Q. J.; Guo, Q. J.; Cao, C. Q.; Zhang, Y. C.; Wang, L. T. Investigation into Photocatalytic Degradation of Gaseous Ammonia in CPCR. Ind. Eng. Chem. Res. 2008, 47, 4363. (2) Jablonska, M.; Palkovits, R. Copper based catalysts for the selective ammonia oxidation into nitrogen and water vapour-Recent trends and open challenges. Appl. Catal., B 2016, 181, 332. (3) Baquerizo, G.; Maestre, J. P.; Sakuma, T.; Deshusses, M. A.; Gamisans, X.; Gabriel, D.; Lafuente, J. A detailed-model of a biofilter for ammonia removal: Model parameters analysis and model validation. Chem. Eng. J. 2005, 113, 205. (4) Kubota, M.; Yamanouchi, R.; Matsuo, K.; Matsuda, H. Ammonia absorption characteristics with nickel chloride for ammonia fixation. J. Chem. Eng. Jpn. 2016, 49, 257. (5) Amid, H.; Maze, B.; Flickinger, M. C.; Pourdeyhimi, B. Dynamic adsorption of ammonia: apparatus, testing conditions, and adsorption capacities. Meas. Sci. Technol. 2017, 28, 055901. (6) Schuth, F.; Palkovits, R.; Schlogl, R.; Su, D. S. Ammonia as a possible element in an energy infrastructure: catalysts for ammonia decomposition. Energy Environ. Sci. 2012, 5, 6278. (7) Atsumi, R.; Noda, R.; Takagi, H.; Vecchione, L.; Di Carlo, A.; Del Prete, Z.; Kuramoto, K. Effects of steam on Ni/Al2O3 catalysts for ammonia decomposition. Ind. Eng. Chem. Res. 2014, 53, 17849. (8) Zhu, J.; Liu, Z. H.; Bai, J.; Yang, Y. F.; Peng, Q.; Ye, S. C.; Chen, M. Z. Modeling and experimental studies of ammonia absorption in a spray tower. Korean J. Chem. Eng. 2016, 33, 63. (9) Lippits, M. J.; Gluhoi, A. C.; Nieuwenhuys, B. E. A comparative study of the selective oxidation of NH3 to N2 over gold, silver and copper catalysts and the effect of addition of Li2O and CeOx. Catal. Today 2008, 137, 446. (10) Olofsson, G.; Hinz, A.; Andersson, A. A transient response study of the selective catalytic oxidation of ammonia to nitrogen on Pt/CuO/Al2O3. Chem. Eng. Sci. 2004, 59, 4113. (11) Cui, X. Z.; Zhou, J.; Ye, Z. Q.; Chen, H. R.; Li, L.; Ruan, M. L.; Shi, J. L. Selective catalytic oxidation of ammonia to nitrogen over mesoporous CuO/RuO2 synthesized by co-nanocasting-replication method. J. Catal. 2010, 270, 310. (12) Chmielarz, L.; Jablonska, M.; Struminski, A.; Piwowarska, Z.; Wegrzyn, A.; Witkowski, S.; Michalik, M. Selective catalytic oxidation of ammonia to nitrogen over Mg-Al, Cu-Mg-Al and Fe-Mg-Al mixed metal oxides doped with noble metals. Appl. Catal., B 2013, 130, 152. (13) Li, J. Y.; Tang, X. L.; Yi, H. H.; Yu, Q. J.; Gao, F. Y.; Zhang, R. C.; Li, C. L.; Chu, C. Effects of copper-precursors on the catalytic activity of Cu/graphene catalysts for the selective catalytic oxidation of ammonia. Appl. Surf. Sci. 2017, 412, 37. (14) Zhang, Q. L.; Wang, H. M.; Ning, P.; Song, Z. X.; Liu, X.; Duan, Y. K. In situ DRIFTS studies on CuO-Fe2O3 catalysts for low temperature selective catalytic oxidation of ammonia to nitrogen. Appl. Surf. Sci. 2017, 419, 733. (15) Nassos, S.; Svensson, E. E.; Nilsson, M.; Boutonnet, M.; Jaras, S. Microemulsion-prepared Ni catalysts supported on ceriumlanthanum oxide for the selective catalytic oxidation of ammonia in gasified biomass. Appl. Catal., B 2006, 64, 96. (16) Lietti, L.; Ramis, G.; Busca, G.; Bregani, F.; Forzatti, P. Characterization and reactivity of MoO3/SiO2 catalysts in the selective catalytic oxidation of ammonia to N2. Catal. Today 2000, 61, 187. (17) Chmielarz, L.; Kustrowski, P.; Rafalska-Lasocha, A.; Dziembaj, R. Selective oxidation of ammonia to nitrogen on transition metal containing mixed metal oxides. Appl. Catal., B 2005, 58, 235. (18) Zhang, T.; Chang, H. Z.; You, Y. C.; Shi, C. N.; Li, J. H. Excellent activity and selectivity of one-pot synthesized Cu-SSZ-13 catalyst in the selective catalytic oxidation of ammonia to nitrogen. Environ. Sci. Technol. 2018, 52, 4802.

4. CONCLUSIONS (1) In the reduction reactions of CuO/Al2O3, MnO2/Al2O3, and Fe2O3/Al2O3 with NH3, oxidative OCs were converted to reductive OCs, and at the same time, NH3 was converted to N2 and H2O. Since the reductive OCs can be regenerated to the oxidative OCs with air, deep NH3 removal with a high N2 selectivity could be achieved based on CLC. (2) For the NH3 removal based on CLC, the NH3 removal activity of CuO/Al2O3 was significantly higher than that of MnO2/Al2O3 and Fe2O3/Al2O3. For CuO/Al2O3, T90 was 250 °C, and a N2 selectivity of 96.6% was maintained even when the reaction temperature was elevated to 350 °C. Compared to CuO/SiO2 and CuO/TiO2, CuO/Al2O3 also exhibited a higher NH3 removal activity and N2 selectivity. (3) The reductive CuO/Al2O3 could be completely regenerated to the oxidative CuO/Al2O3 with air above 200 °C. (4) Due to its too high oxidizability, O2 in the waste gas deteriorated the N2 selectivity. For CuO/Al2O3, Fe2O3/Al2O3, or MnO2/Al2O3, the N2 selectivity decreased to less than 80% when the reaction temperature was elevated to 300 °C or higher. (5) For a waste gas containing low-concentration NH3, a new NH3 deep removal process with high N2 selectivity based on CLC coupled with adsorption enrichment was proposed.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b05375.

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REFERENCES

Definitions for N2O, NO, and NO2 selectivities; XRD patterns of the fresh OC samples; theoretical and experimental NH3 consumption; thermodynamic data of reactions 5−10; SEM images of the fresh Cu-based OC samples; N2O, NO, and NO2 selectivities for Figures 4 and 13 (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +86 10 62333542. E-mail: [email protected]. ORCID

Yongjian Wu: 0000-0002-0256-5094 Qingquan Su: 0000-0002-9272-7666 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the financial support from the Beijing Science and Technology Program (Grant Z131100005613045), the I

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Industrial & Engineering Chemistry Research (19) Chmielarz, L.; Jablonska, M. Advances in selective catalytic oxidation of ammonia to dinitrogen: a review. RSC Adv. 2015, 5, 43408. (20) Song, D. D.; Shao, X. Z.; Yuan, M. L.; Wang, L.; Zhan, W. C.; Guo, Y. L.; Guo, Y.; Lu, G. Z. Selective catalytic oxidation of ammonia over MnOx-TiO2 mixed oxides. RSC Adv. 2016, 6, 88117. (21) Gora-Marek, K.; Brylewska, K.; Tarach, K. A.; Rutkowska, M.; Jablonska, M.; Choi, M.; Chmielarz, L. IR studies of Fe modified ZSM-5 zeolites of diverse mesopore topologies in the terms of their catalytic performance in NH3-SCR and NH3-SCO processes. Appl. Catal., B 2015, 179, 589. (22) Duan, K. J.; Tang, X. L.; Yi, H. H.; Ning, P.; Wang, L. D. Rare earth oxide modified Cu-Mn compounds supported on TiO2 catalysts for low temperature selective catalytic oxidation of ammonia and in lean oxygen. J. Rare Earths 2010, 28, 338. (23) Jablonska, M.; Beale, A. M.; Nocun, M.; Palkovits, R. Ag-Cu based catalysts for the selective ammonia oxidation into nitrogen and water vapour. Appl. Catal., B 2018, 232, 275. (24) Monazam, E. R.; Breault, R. W.; Tian, H. J.; Siriwardane, R. Reaction kinetics of mixed CuO-Fe2O3 with methane as oxygen carriers for chemical looping combustion. Ind. Eng. Chem. Res. 2015, 54, 11966. (25) Zhang, Y. W.; Langorgen, O.; Saanum, I.; Chao, Z. X.; Jakobsen, H. A. Modeling and simulation of chemical looping combustion using a copper-based oxygen carrier in a double-loop circulating fluidized bed reactor system. Ind. Eng. Chem. Res. 2017, 56, 14754. (26) Lin, L. M.; Liu, D. Y.; Jin, J.; Cheng, Q.; Li, W.; Feng, L. High iron and calcium coal ash as the oxygen carrier for chemical looping combustion. Ind. Eng. Chem. Res. 2018, 57, 9725. (27) Diglio, G.; Bareschino, P.; Mancusi, E.; Pepe, F. Technoeconomic evaluation of a small-scale power generation unit based on a chemical looping combustion process in fixed bed reactor network. Ind. Eng. Chem. Res. 2018, 57, 11299. (28) Adanez, J.; Abad, A.; Garcia-Labiano, F.; Gayan, P.; de Diego, L. F. Progress in chemical-looping combustion and reforming technologies. Prog. Energy Combust. Sci. 2012, 38, 215. (29) Wu, Y. J.; Luo, C. H.; Wu, W.; Su, Q. Q. Denitration of the gasfired boiler flue gas based on chemical-looping combustion. Chem. Eng. J. 2019, 361, 41. (30) Zheng, X. M.; Su, Q. Q.; Mi, W. L.; Zhang, P. K. Effect of steam reforming on methane-fueled chemical looping combustion with Cubased oxygen carrier. Int. J. Hydrogen Energy 2014, 39, 9158. (31) Yu, C. L.; Huang, B. C.; Dong, L. F.; Chen, F.; Yang, Y.; Fan, Y. M.; Yang, Y. X.; Liu, X. Q.; Wang, X. N. Effect of Pr/Ce addition on the catalytic performance and SO2 resistance of highly dispersed MnOx/SAPO-34 catalyst for NH3-SCR at low temperature. Chem. Eng. J. 2017, 316, 1059. (32) Ribeiro, M. F.; Silva, J. M.; Brimaud, S.; Antunes, A. P.; Silva, E. R.; Fernandes, A.; Magnoux, P.; Murphy, D. M. Improvement of toluene catalytic combustion by addition of cesium in copper exchanged zeolites. Appl. Catal., B 2007, 70, 384.

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