RESEARCH ARTICLE

Multi-stage ammonia production for sorption selective catalytic reduction of NOx

  • Chen ZHANG ,
  • Guoliang AN ,
  • Liwei WANG ,
  • Shaofei WU
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  • Institute of Refrigeration and Cryogenics, Key Laboratory of Power Machinery and Engineering of the Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China

Received date: 06 Jul 2021

Accepted date: 21 Oct 2021

Published date: 15 Oct 2022

Copyright

2021 Higher Education Press

Abstract

Sorption selective catalytic reduction of nitrogen oxides (NOx) (sorption-SCR) has ever been proposed for replacing commercial urea selective catalytic reduction of NOx (urea-SCR), while only the single-stage sorption cycle is hitherto adopted for sorption-SCR. Herein, various multi-stage ammonia production cycles is built to solve the problem of relative high starting temperature with ammonia transfer (AT) unit and help detect the remaining ammonia in ammonia storage and delivery system (ASDS) with ammonia warning (AW) unit. Except for the single-stage ammonia production cycle with MnCl2, other sorption-SCR strategies all present overwhelming advantages over urea-SCR considering the much higher NOx conversion driven by the heat source lower than 100°C and better matching characteristics with low-temperature catalysts. Furthermore, the required mass of sorbent for each type of sorption-SCR is less than half of the mass of AdBlue for urea-SCR. Therefore, the multifunctional multi-stage sorption-SCR can realize compact and renewable ammonia storage and delivery with low thermal energy consumption and high NOx conversion, which brings a bright potential for efficient commercial de-NOx technology.

Cite this article

Chen ZHANG , Guoliang AN , Liwei WANG , Shaofei WU . Multi-stage ammonia production for sorption selective catalytic reduction of NOx[J]. Frontiers in Energy, 2022 , 16(5) : 840 -851 . DOI: 10.1007/s11708-021-0797-1

1 Introduction

Nitrogen oxides (NOx, including N2O, NO, NO2, N2O3, N2O4, and N2O5) primarily emitted from factories, power stations and automobiles are regarded as the main cause of acid rain, photochemical smog, greenhouse effects, and PM2.5 [1,2]. It has been reported that with increasing oxygen concentration or increasing temperature the emissions of NOx increase, while higher SO2 levels decrease the emissions of NOx but increase the proportion of CO [3]. To tackle the emission of NOx from gasoline and diesel engines, governments worldwide enact increasingly stringent policies and legislations. However, there is a growing disparity between real-world emissions and tightened standards, such as Euro VI for European Union (EU), Low Emission Vehicle (LEV) III for the USA, and CN VI for China [4,5]. The ever-increasing demand for air purification has created strong momentum for addressing NOx emissions originated from automobiles.
As a large-scale applied NOx reduction technology, selective catalytic reduction of NOx (SCR) technologies reduce NOx with reductants like H2 [6], hydrocarbon (HC) [7], and NH3 [8]. NOx can be reduced without any CO2 formation at the temperature lower than 200°C with H2 as the reducing agent, while H2 has been plagued by the difficulty of synthesis and storage. Hydrocarbon selective catalytic reduction of NOx (HC-SCR) offers the potential to negate external reductant via utilizing un-burnt HC in engine exhaust stream. However, neither has been promoted to practical applications [9]. Aqueous urea solution consisting of 32.5% of urea as the ammonia precursor for the de-NOx reaction, called AdBlue in EU and diesel exhaust fluid (DEF) in the USA, can be decomposed into NH3 for reacting with NOx in a catalytic converter (CC) to convert it into N2 and H2O. Urea selective catalytic reduction of NOx (urea-SCR) systems have been commercially applied in diesel vehicles since three-way catalysts are suitable for NOx reduction in diesel and lean burning gasoline engines [1012]. Furthermore, the nanoparticles addition applied in the fuel of internal combustion engine has a positive effect in the commercial urea-SCR technologies [13]. However, problems of low NOx conversion efficiency and poor activity at a low temperature [14], coking caused by incomplete decomposition [15], urea crystallization [16,17], and low effective ammonia content [18] require further development of better ammonia precursors to replace urea solution.
Alternative reducers should be decomposed into ammonia without producing harmful products and be easy to store and transport at an affordable cost and a wide availability from a technical and commercial standpoint. The ammonia precursors studied can be classified into active carbon (AC), Yzeolite, ammonium carbamate, ammonium formate, methanamide and guanidinium formate, ammonium salt, and ammoniate. The ammonia capture capacities of AC and Y-zeolite with on-site ammonia synthesis are increased compared with non-treated AC and Y-zeolite due to ammonium ion formation and ammine complex formation, respectively [19]. However, their ammonia capture capacities are still too low compared with those of ammoniates. The guanidinium salts with higher decomposition temperatures present larger ammonia densities than ammonium carbamate and urea [20]. The formamide-based mixtures are less efficient compared with urea-based agents but their NH3 slip processes are better prevented [21], while ammonium salts and ammoniates offer better ammonia storage performance than solid urea [22]. The on-board ammonia storage and delivery system (ASDS) with the ammoniate-based SCR technology has the advantages of high density and direct ejecting of ammonia for de-NOx [23]. For instance, after being compressed to 1219 kg/m3, [Mg(NH3)6]Cl2 occupies a volume and weight 3.1 and 2.8 times less than those of AdBlue, respectively [24]. The thermodynamic stability of the monoamine phase ([Sr(NH3)]Cl2), the diamine phase ([Sr(NH3)2]Cl2), and the octopamine phase ([Sr(NH3)8]Cl2) has also been verified to explain the working principle of strontium chloride as the ammonia sorbent for sorption selective catalytic reduction of NOx (sorption-SCR) [25]. To improve the heat and mass transfer performance and avoid agglomeration of ammoniates, expanded natural graphite treated with sulfuric acid (ENG-TSA) is added as the matrix for ammoniates (CaCl2, SrCl2, BaCl2, NH4Cl, and NaBr) to complete sorption-SCR [26,27], with an annual required mass/volume and cost generally lower than those of urea-SCR. Furthermore, due to the advantage of the relatively lower starting temperature for desorbing ammonia compared with AdBlue (over 160°C), the NOx conversion of sorption-SCR with CaCl2/ENG-TSA is around 45% higher than that of the urea-SCR at 50°C [28]. Therefore, it can be concluded that ammoniates with the matrix of ENG-TSA bring a bright future for safe and efficient NH3 selective catalytic reduction of NOx (NH3-SCR).
However, only the single-stage sorption cycle is hitherto adopted for sorption-SCR. The existing single-stage sorption-SCR problems lie in balancing the pressure stability and the proper starting temperature as well as in quick launching and detecting the remaining ammonia in the ASDS. Multi-stage sorption cycles based on ammonia sorption and resorption principles have been utilized and verified in thermal energy conversion applications, such as refrigeration [29], heat transformer [30], and thermal energy storage [31]. To realize compact and renewable ammonia storage and delivery with a low thermal energy consumption and a high NOx conversion, the multifunctional multi-stage ammonia production cycles are first proposed and studied under the working condition of sorption-SCR. After that, the proper ammoniate-ammonia working pairs are selected and tested to obtain their thermodynamic performance. Based on thermodynamic models, the starting temperature, NOx conversion, ammonia storage density, thermal efficiency of ammonia production, and bulk weight and volume of single and multi-stage ammonia production cycles are compared with Adblue as the benchmark material.

2 Working principle

2.1 Mechanism of sorption-SCR

Essential reactions for five NOx formation pathways (the thermal formation pathway, the prompt formation pathway, the N2O formation pathway, the NO2 formation pathway, and the NNH formation pathway) have ever been summarized [32], while the chemistry of the NOx destruction via NH3-SCR can be expressed in Eq. (1), which occurs in a SCR catalytic converter (SCR-CC).
6NOx+4x NH3 (2x+3 )N2+6x H 2O
As shown in Fig. 1(a), the typical sorption-SCR system consists of an ASDS (including ammoniates, electric heaters, inner exhaust pipes, and ammonia charging pipes), a nozzle, an SCR-CC (including carrier, catalyst and encapsulation), an SCR electronic control unit (SCR-ECU), several valves for controlling exhaust flow and ammonia flow, as well as sensors for temperature, pressure, NOx and NH3 detection.
For the on-board de-NOx phase, the ASDS is pre-heated by the electric heater, while the heating source could be switched to the exhaust gas for saving electricity after the vehicle is started. As soon as the pressure of the ASDS reaches around 0.4–0.5 MPa, the valve between the ASDS and the nozzle is opened for ejecting NH3 via the nozzle and mixing it with the exhaust gas. After the mixture is catalyzed in the SCR-CC (Eq. (1)), the products will go through the NH3 sensor and NOx sensor, which return the feedback signal to the SCR-ECU for adjusting the amount of NH3 ejection via the temperature control and the electromagnetic valve control.
If the NH3 stored in the ASDS is almost used up, the NH3 capture and storage phase (Fig. 1(b)) will proceed when connecting the ASDS with an external ammonia tank. When the pressure of the ASDS is lower than that of the ammonia tank, the valve between the ASDS and the ammonia tank is opened for ammonia capture and storage in the ASDS until researching sorption saturation, with the sorption temperature of the ASDS controlled by the external cooling method. The specific working principle of the ASDS can be presented by the Clapeyron diagram.
Fig.1 Working principle of sorption-SCR.

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2.2 Single-stage ammonia production

The single-stage ammonia production process is shown in Fig. 2, i.e., the ammonia capture and storage phase and the de-NOx phase, whose process are detailed as follows:
1) When the ammonia stored in the ASDS is not efficient, the ammonia capture and storage phase proceeds. The temperature of the ammonia tank (Ta) is controlled and its state keeps at point 1 in the NH3 line, while the initial state of the ammoniate inside the ASDS is at point 3 in the ammoniate line. After opening the valve between the ammonia tank and the ASDS, the ammoniate state will change into point 2 since its pressure is limited by the pressure of ammonia tank (pa) while its temperature increases because of the release of the sorption heat. After achieving sorption saturation, the valve is closed, and the ammoniate could be cooled back to point 3.
2) During vehicle driving, only the de-NOx phase is required. The ASDS is heated from point 3 to point 4 and then the valve between the ASDS and the nozzle is opened for ejecting ammonia at point 5. It needs to be mentioned that point 5 locates at the right of ammoniate line as the actual desorption pressure has to be lower than the threshold desorption pressure.
The starting temperature is too high for high-temperature ammoniate, while the pressure vibration is so violent that it is difficult to be controlled within the safety pressure for low-temperature ammoniate. Therefore, the ammonia storage material for commercial utilization is chosen for middle-temperature ammoniate.
Fig.2 Single-stage ammonia production process.

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2.3 Double-stage ammonia production

Double-stage ammonia production is proposed to solve the problem of high starting temperature, with the specific principle shown in Fig. 3, i.e., the ammonia capture and storage phase, the ammonia transfer (AT) phase, and the de-NOx phase, whose process are described as follows:
1) The ammonia capture and storage phase of the first stage ammoniate inside the ASDS is the same as that of the single-stage ammonia production process, including points 1, 2, and 3.
2) The initial state of the second stage ammoniate inside AT unit is at point 6 with the temperature set as the ambient temperature. The ASDS is heated from points 3 to 4 and then the valve between the ASDS and the AT unit is opened for ammonia resorption at points 5 and 7 with the same pressure. The temperatures of the ASDS and the AT unit are assumed not to change during the resorption process. After the second stage, the ammoniate achieves the sorption saturation, the valve is closed, and the first and second stage ammoniate will return to points 3 and 6, respectively.
3) The AT unit remains at point 8 and the ASDS is heated from point 3. During the initial stage of vehicle driving, the valve between the AT unit and the nozzle is opened to eject ammonia at point 9. After the ASDS is heated up to point 10, the valve between the ASDS and the AT unit is also opened for ejecting ammonia to the nozzle at point 11.
Since the ammonia desorbed from the ASDS will pass the AT unit during the de-NOx phase, the ammonia filled in the AT unit can remain in saturation, which means the AT phase is only required for the first time.
Fig.3 Double-stage ammonia production process.

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2.4 Triple-stage ammonia production

Triple-stage ammonia production is progressed to help detect the remaining ammonia in the ASDS for multifunctional consideration. The specific principle is depicted in Fig. 4, i.e., the ammonia capture and storage phase, the double step AT phase, the de-NOx phase, and the ammonia warning (AW) phase, whose process are described as follows:
1) The ammonia capture and storage phase of the first stage ammoniate inside the ASDS is the same as that of the double-stage ammonia production process, including points 1, 2, and 3.
2) The initial temperatures of the ASDS and the AW unit remain at the ambient temperature. After the valve between the ASDS and the AW unit is opened, the resorption process will occur, with the state of both the first stage ammoniate in the ASDS and the third stage ammoniate in the AW changing to point 4. If the third stage ammoniate achieves sorption saturation, the valve is closed and the first and third stage ammoniate will return to points 3 and 5, respectively. The AT process between the ASDS and the AT unit is the same as that of the double-stage ammonia production, including points 3, 6, 7, 8, and 9.
3) The de-NOx phase is also the same as that of the double-stage ammonia production, including points 10, 11, 12, and 13.
4) If the detection results of the ammonia sensor before the nozzle present insufficient amount of ammonia output, the AW unit should be heated to point 14. After closing the valve between the AT unit and the nozzle while opening the valve between the AW unit and the nozzle, the ammonia could be ejected to the nozzle at point 15. Since ammonia is inadequate for the ASDS, the ammonia capture and storage phase as well as the AT between the ASDS and the AW unit should proceed before the next driving.
Fig.4 Triple-stage ammonia production process.

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2.5 Double-stage triple-effect ammonia production

To pursue a more compact system, double-stage triple-effect ammonia production is designed with the same function of triple-stage ammonia production, and the specific principle is displayed in Fig. 5, i.e., the ammonia capture and storage phase, the AT phase, the de-NOx phase, and the AW phase, whose process are described as follows:
1) Multi-ammoniate is utilized in the ASDS, thus the initial states of middle-temperature ammoniate and high-temperature ammoniate are at points 3 and 4, respectively. After opening the valve between the ammonia tank and the ASDS, the states of two kinds of ammoniates will change to the same point 2. If sorption saturation is achieved, the valve will be closed and the middle- and high-temperature ammoniate will return to points 3 and 4.
2) The overall AT phase between the ASDS and the AT unit is similar to that of the double-stage ammonia production, including points 3, 4, 5, 6, 7, 8, and 9. Since the pressure of point 7 is lower than that of point 8, only the middle-temperature ammoniate can transfer ammonia to the AT unit.
3) The de-NOx phase is also the same as that of the double-stage ammonia production, including points 10, 11, 12, 13, and 14. During this phase, the high-temperature ammoniate does not react because of its much lower partial pressure at point 14.
4) When the ammonia sensor before the nozzle shows that the amount of ammonia output is not sufficient, the ASDS should be heated to point 15 for high-temperature ammoniate ejecting ammonia to the nozzle at point 16. Before the next driving, the ammonia capture and storage phase should proceed again.
Fig.5 Double-stage triple-effect ammonia production process.

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3 Performance assessment

3.1 Materials selection

The complex reaction of ammoniate-ammonia working pairs can be expressed in Eq. (2).
M aX b( NH 3)n+ (mn) NH3 MaXb(NH3)m,
where M represents metal elements, X represents Cl/Br/I, and a, b, n, and m are the reaction constants. The starting temperature of ammoniate can be calculated based on the Clapeyron equation
T st a= ΔHΔS Rlnp sta,
where R is the gas constant, ΔH and ΔS are the reaction enthalpy and entropy change of ammoniate, respectively. Considering the pressure difference, the starting pressure (psta) is set as 0.5 MPa when the output pressure before nozzle (pout) is 0.4 MPa. Therefore, the ammoniates with a starting temperature lower than 150°C include PbCl2 (26.6°C), NH4Cl (29°C), NaBr (32.8°C), BaCl2 (37.1°C), LiCl (45.7°C), CaCl2 (65.4°C), NaI (67.7°C), SrCl2 (73.1°C), BaBr2 (73.6°C), SrBr2 (106.4°C), MnCl2 (125.6°C), and CaBr2 (130.5°C). Utilizing ammonia storage capacity as criterion, NH4Cl/NaBr, CaCl2/SrCl2, and MnCl2 are chosen as the candidate for low, middle, and high temperature ammoniates, respectively.
However, pure ammoniates suffer from swelling and agglomeration during the sorption phase, which causes the severe attenuation of cycle ammonia storage capacity [33]. Therefore, even though the theoretical ammonia storage capacities of pure ammoniates are larger than their related composites, pure ammoniates are not recommended to be utilized in the actual system. It has been proved that ammoniates coupled with the ENG-TSA could prevent swelling and agglomeration and significantly enhance the heat and mass transfer performance over 100 times compared with pure ammoniates [34]. Thus, in this research, ENG-TSA was also applied as the ammoniates matrix to ensure its application feasibility.

3.2 Thermodynamic test

The thermodynamic measurement was conducted to study the relation between sorption capacity and sorbent temperature under equilibrium conditions at a constant pressure. The thermodynamic properties of composites (with NH4Cl/ENG-TSA, CaCl2/ENG-TSA, and MnCl2/ENG-TSA as examples) were tested by the Rubotherm balance (TA Instruments, America) demonstrated in Fig. 6(a). The sample was put in a steel basket suspending in a sealed steel chamber, whose temperature was controlled through thermal radiation via the circulation oil whose temperature was controlled by the thermostatic bath (SE-6, JULABO, Germany) with a temperature accuracy of 0.01 K. A PT100 temperature sensor with a precision of 0.1 K was located beneath the basket to monitor the temperature of the sorbent. The measuring chamber was connected to an ammonia tank which acted as the condenser/evaporator, whose temperature was controlled by the thermostatic glycol water bath (F32-ME, JULABO, Germany) with a temperature accuracy of 0.01 K. The working pressure of the measuring chamber was under the saturated pressure of ammonia and was detected by an absolute pressure sensor (DPI-282, Druck, UK) with a precision of 0.04%. As presented in Fig. 6(b), the initial temperature was set as low as possible for the desorption process and then moved forward to the equilibrium state that followed by increasing the temperature of thermostatic oil bath (Toil) with 5°C per step, until desorption was completed. Only after both the weight and temperature data were made stable for each step, could the temperature be increased. On the contrary, the initial temperature should be set high enough for the sorption process and Toil would also be decreased by 5°C per step. The pressure was controlled within 1.0 MPa and Toil kept higher than 20°C.
Fig.6 Experimental test unit and test procedure.

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As manifested in Fig. 7, the thermodynamic sorption results under the condition of 0.87 MPa (corresponding to 20°C saturated ammonia) show that CaCl2/ENG-TSA and MnCl2/ENG-TSA have notable hysteresis and obvious single-point sorption characteristic, while NH4Cl/ENG-TSA has negligible hysteresis and presents multi-point sorption characteristic which leads to a higher sorption capacity.
Fig.7 Thermodynamic sorption results of NH4Cl/ENG-TSA, CaCl2/ENG-TSA, and MnCl2/ENG-TSA at 0.87 MPa.

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The reaction enthalpy and entropy change of composites can be calculated with threshold reaction temperature at 0.87 MPa, 0.63 MPa and 0.44 MPa, and the relationship between Ts and sorbent pressure (ps) can be ensured based on Eq. (3). Figure 8 indicates that the starting temperature of NH4Cl/ENG-TSA, CaCl2/ENG-TSA, and MnCl2/ENG-TSA at 0.5 MPa are 30.9°C, 61.5°C, and 126.9°C, respectively, all of which are close enough to their theoretical value (29°C, 65.4°C, and 125.6°C). Therefore, the theoretical thermodynamic properties of sorbents are chosen for further calculation and evaluation without error bars.
Fig.8 Ts versus ps of NH4Cl/ENG-TSA, CaCl2/ENG-TSA, and MnCl2/ENG-TSA, with experimental value shown in solid lines and theoretical value shown in dash lines.

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3.3 Thermodynamic models

The thermal efficiency of ammonia production ( ηN H3) is defined as
η NH3= mNH3Qin =mNH3Qsen+ Qdes,
where mNH3 is the total mass of ammonia production, Qin is the input heat including sensible heat (Qsen) and desorption heat (Qdes). The ammonia storage density (ρNH 3) at the material level is
ρNH3= m NH3 Vs,
where Vs is the volume of composite sorbent. For various ammonia production principles, expressions of these parameters are summarized as follows.
1) Single-stage ammonia production
mNH 3=βASDSxt_ASDSms_ASDS,
where βASDS (0.8) is the mass proportion of ammoniate inside composite sorbent, xt_ASDS is the theoretical sorption capacity of ammoniate and ms_ASDS (1 kg) is the mass of composite sorbent for the ASDS.
Qsen=Nms_ASDScs_ AS DSΔ Tde_ ASDS,
where N is the cycle index for per kilogram composite sorbent, cs_ASDS is the specific heat of composite sorbent and ΔTde_ASDS is the temperature increment of the ASDS during the de-NOx phase.
N= m NH3D vtc ,
where D (2000 km/kg) represents the valid driving distance with per kilogram ammonia, v (60 km/h) is the average driving speed, and tc (1 h) is the average driving time for each de-NOx phase.
cs_ASDS=β AS DSct_ AS DS+(1βASDS)c ENG,
where ct_ASDS and cENG are the specific heat of ammoniate of the ASDS and ENG-TSA, respectively.
Qdes= mNH3ΔHASDS,
where ΔHASDS is the reaction enthalpy change of ammoniate of the ASDS.
Vs=ms_ AS DSρs_ AS DS,
where ρs_ AS DS (1 g/cm3) is the density of composite sorbent of the ASDS.
2) Double-stage ammonia production
Qdes of double-stage ammonia production is the same as that of single-stage ammonia production.
mNH 3=βASDSxt_ASDSms_ASDS=mNH3_ASDS+ mNH3_AT,
where mNH 3 _ASDS and mN H3_AT are the ammonia stored in the ASDS and the AT unit after the AT phase.
Qsen= ms_ ASDS cs_ASDS[ΔTtr_ASDS+(N 1)Δ Tde_ASDS],
where ΔTtr_ASDS is the temperature increment of the ASDS during the AT phase.
Vs=ms_ AS DSρs_ AS DS+ms_ ATρs_ AT,
where ms_AT and ρs_AT (1 g/cm3) are the mass and density of composite sorbent of the AT unit, respectively.
3) Triple-stage ammonia production
m NH3=β AS DSxt_ASDSms_ASDS= mNH 3_ASDS+mNH 3_AT+mNH 3_AW,
where m NH3_AW is the ammonia stored in the AW unit after the AT phase.
Q se n= ms_ASDS cs_ASDS[ΔTtr_ASDS+(N2)ΔTde_ASDS]+m s_AWc s_AWΔ Tde_AW,
where ms_AW, cs_AW, and ΔTde_AW are the mass, the specific heat of ammoniate, and the temperature increment during the de-NOx phase of the AW unit.
Q de s= (mNH3_ASDS+ mNH 3_AT)ΔHASDS+ mNH 3_AWΔHAW,
where ΔHAW is the reaction enthalpy change of ammoniate of the AW unit.
Vs = ms_ASDS ρs_ASDS+ ms_AT ρs_AT+ m s_AWρs_AW,
where m s_AW and ρs_AW (1 g/cm3) are the mass and density of composite sorbent of the AW unit, respectively.
4) Double-stage triple-effect ammonia production
m NH3= βASDS(x t_ASDS_mms_ASDS_m+ xt_ASDS_hms_ASDS_h),
where xt_ASDS_m and xt_ASDS_h are the theoretical sorption capacity of middle-temperature and high-temperature ammoniate respectively, and ms_ASDS_m and ms_ASDS_h are the mass of composite sorbent with middle-temperature and high-temperature ammoniate for the ASDS respectively.
Q se n= ms_ASDS c ¯ s_ASDS[ΔTtr_ASDS+(N 2) ΔTde_ASDS_m+ΔTde_ASDS_h],
where c¯s _ASDS is the average specific heat of ammoniate of the ASDS, and ΔTde_ASDS_m and ΔTde_ASDS_h are the temperature increment of the ASDS with middle-temperature and high-temperature ammoniate during the de-NOx phase, respectively.
Q de s= mNH 3_mΔHASDS_m+mN H3_hΔHASDS_h,
where m NH3_m and mNH3_h are the total mass of ammonia production of middle-temperature and high-temperature ammoniate respectively, and ΔHASDS_m and ΔHASDS_h are the reaction enthalpy change of ammoniate of the ASDS with middle-temperature and high-temperature ammoniate respectively.
Vs = ms_ASDS_m+ms_ASDS_h ρs_ASDS+ m s_ATρs_AT.

4 Results and Discussion

4.1 NOx conversion

Low-temperature catalysts have been studied in Refs. [35,36]. Therefore, in this section, the NOx conversion of sorption-SCR and urea-SCR is theoretically calculated with the NOx conversion data of existing high-performance low-temperature catalyst Co-Mn-O-10 [37], as presented in Fig. 9. It is assumed that the temperatures of the CC (Tcat) and the AdBlue box/ASDS (Ts) are the same, which are both driven by co-heating of the electric heater and exhaust gas. Since the initial pyrolysis temperature of AdBlue is over 150°C [15], its practical NOx conversion remains zero within 150°C even though the theoretical NOx conversion capacity of Co-Mn-O-10 could reach almost 100%. Sorption-SCR can be divided into low starting temperature (single-stage with NH4Cl and NaBr, double-stage, triple-stage and double-stage triple-effect), middle starting temperature (single-stage with CaCl2 and SrCl2), and high starting temperature (single-stage with MnCl2) driven sorption-SCR. Except for the single-stage ammonia production with MnCl2, sorption-SCR presents an overwhelming advantage over urea-SCR considering the matching characteristics with low-temperature catalysts.
Fig.9 Comparison of sorption-SCR and urea-SCR on NOx conversion at various driven temperatures.

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4.2 Ammonia storage density and thermal efficiency

The ρ NH3 and ηNH3 of various ammonia production processes are exhibited in Fig. 10. For single-stage ammonia production, NH4Cl (s1) has both the largest ρ NH3 (0.76 g/cm3) and η NH3(0.65 g/kJ) with a starting temperature of 29.0°C. However, the pressure variation of NH4Cl is violent for its sole utilization. For instance, if the temperature of NH4Cl rises from 30°C to 50°C, its pressure will dramatically increase up to around 1 MPa. For double-stage, triple-stage, and double-stage triple-effect ammonia production processes, the starting temperature is decided by the ammoniate of the AT unit, i.e., 29.0°C for NH4Cl based and 32.8°C for NaBr based AT unit, while the results suggest that the type of AT unit has almost no influence on their ρ NH3 and ηNH3. Compared with SrCl2, CaCl2 is more suitable as the candidate ammoniate filled in the ASDS caused by a larger ρNH3. Therefore, the optimal ammoniate pairs of various principles are CaCl2-NH4Cl (d1) with ρ NH3 and ηNH3of 0.71 g/cm3 and 0.31 g/kJ; CaCl2-NH4Cl-MnCl2 (t1) with ρ NH3 and ηNH3 of 0.66 g/cm3 and 0.31 g/kJ; CaCl2/MnCl2-NH4Cl (dt1) with ρ NH3 and ηNH3 of 0.69 g/cm3 and 0.30 g/kJ.
Fig.10 Ammonia storage density and thermal efficiency of various ammonia production processes (s1: NH4Cl; s2: NaBr; s3: CaCl2; s4: SrCl2; s5: MnCl2; d1: CaCl2-NH4Cl; d2: SrCl2-NH4Cl; d3: MnCl2-NH4Cl; d4: CaCl2-NaBr; d5: SrCl2-NaBr; d6: MnCl2-NaBr; t1: CaCl2-NH4Cl-MnCl2; t2: SrCl2-NH4Cl-MnCl2; t3: CaCl2-NaBr-MnCl2; t4: SrCl2-NaBr-MnCl2; dt1: CaCl2/MnCl2-NH4Cl; dt2: SrCl2/MnCl2-NH4Cl; dt3: CaCl2/MnCl2-NaBr; dt4: SrCl2/MnCl2-NaBr).

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4.3 Bulk weight and volume

The bulk weight and volume of different sorption-SCR types are compared in Figs. 11 and 12 with urea-SCR as the benchmark. The least required mass of sorbents (ms) of different types of sorption-SCR is about 26% of mAdBlue (54.5 kg) to obtain the same amount of ammonia (10.0 kg). Even for the worst candidate working pair of each type of sorption-SCR, the required ms is less than half of mAdBlue. As presented in Fig. 12, the density of compressed composite sorbents ( ρs) has a significant impact on the Vs, i.e., as the density increases, the volume decreases. Since 20% ENG-TSA is added as the matrix of composite sorbents, ρs can achieve as high as 1.0 g/cm3 with a sufficient heat and mass transfer performance for saving 60%–70% of volume. The triple-stage and double-stage triple-effect ammonia production can keep Vs smaller than that of urea-SCR (V Ad Bl ue) with 50 L standard AdBlue, while both single-stage and double-stage with MnCl2 filled in the ASDS will occupy around 20% larger volume than VAdBlue if ρs decreases to 0.4 g/cm3.
Fig.11 Comparison of bulk weights of different types of sorption-SCR with urea-SCR as the benchmark.

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Fig.12 Comparison of bulk volumes of different types of sorption-SCR with urea-SCR as the benchmark (The solid lines are the best candidates and the dash lines are the worst candidate of different type of sorption-SCR).

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4.4 Overall evaluation

The parameters of the single and multi-stage ammonia production cycle driven sorption-SCR are summarized in Table 1, where cNOx represents the NOx conversion at 75°C with Co-Mn-O-10 as the catalyst, and Vs/VAdBlue is the bulk volume ratio with ρ s ranging from 1.0 to 0.4 g/cm3. The symbols of de-NOx agents have coincident definitions with those in Fig. 10, where s1, d1, t1, and dt1 represent the best candidate for the corresponding type of sorption-SCR, where s5, d6, t4, and dt4 are the worst. It is shown that the common ground of sorption-SCR includes a lower starting temperature, higher NOx conversion at a low temperature, a higher thermal efficiency, a larger ammonia storage capacity, and a more compact system in comparison with urea-SCR. Based on the above results, the overall evaluation of sorption-SCR types with urea-SCR as the benchmark is concluded as tabulated in Table 2. Urea-sorption only has the advantage of pressure stability over sorption-SCR. Compared with the single-stage and double-stage sorption-SCR, the triple-stage and double-stage triple-effect sorption-SCR could realize both a quick launch and an insufficient AW with the cooperation of low temperature and high temperature sorbent. Especially for the double-stage triple-effect sorption-SCR, the application of multi-ammoniate as sorbent in the ASDS does not require the AW unit and thus makes the system more compact than the triple-stage sorption-SCR.
Tab.1 Parameters of types of sorption-SCRs with urea-SCR as the benchmark
De-NOx agent Tsta/°C c NOx/% ηNH3/(g·kJ−1) ρNH3/(g·cm−3) ms/mAdBlue Vs/VAdBlue
s1 29 95.5 0.65 0.76 0.26 0.26–0.66
s5 125.6 0 0.21 0.43 0.46 0.46–1.16
d1 29 95.5 0.31 0.71 0.28 0.28–0.71
d6 32.8 95.5 0.21 0.42 0.48 0.48–1.21
t1 29 95.5 0.31 0.66 0.30 0.30–0.76
t4 32.8 95.5 0.31 0.54 0.37 0.37–0.93
dt1 29 95.5 0.30 0.69 0.29 0.29–0.73
dt4 32.8 95.5 0.30 0.57 0.35 0.35–0.88
AdBlue 160 0 0.07 0.20 1 1
Tab.2 Overall evaluation of types of sorption-SCR with urea-SCR as the benchmark
SCR type Pressure stability NOx conversion efficiency Thermal efficiency Ammonia storage capacity Quick launch AW
Single-stage sorption-SCR ☆-☆☆ ☆☆-☆☆☆ ☆☆-☆☆☆ ☆☆-☆☆☆ x x
Double-stage sorption-SCR ☆☆ ☆☆☆ ☆☆ ☆☆-☆☆☆ x
Triple-stage sorption-SCR ☆☆ ☆☆☆ ☆☆ ☆☆☆
Double-stage triple-effect sorption-SCR ☆☆ ☆☆☆ ☆☆ ☆☆☆
Urea-SCR ☆☆☆ x x

Note: ☆ low, ☆☆ middle, ☆☆☆ high, ‘x’ has no function, ‘√’ has that function.

5 Conclusions

In this paper, various multi-stage ammonia production cycles to solve a relatively high starting temperature with the AT unit and help detect the remaining ammonia in the ASDS with the AW unit are proposed and compared with the single-stage sorption SCR and urea-SCR. The following conclusions could be reached.
Since the initial pyrolysis temperature of AdBlue is over 150°C, its practical NOx conversion remains zero within 150°C even though the theoretical NOx conversion capacity of Co-Mn-O-10 could reach almost 100%. Except for the single-stage ammonia production cycle with MnCl2, other sorption-SCR strategies have overwhelming advantages over urea-SCR considering the much higher NOx conversion driven by a heat source lower than 100°C and better matching characteristics with a low-temperature catalyst.
For single-stage ammonia production, NH4Cl has both the largest ammonia storage density ( ρNH3, 0.76 g/cm3) and thermal efficiency ( ηNH3, 0.65 g/kJ) with a starting temperature of 29.0°C. However, the pressure variation of NH4Cl is violent for its sole utilization. For double-stage, triple-stage, and double-stage triple-effect ammonia production processes, the starting temperature is decided by the ammoniate of the AT unit, while the type of the AT unit has almost no influence on their ρ NH3 and ηNH3. Compared with SrCl2, CaCl2 is more suitable as the candidate ammoniate filled in the ASDS because of the larger ρ NH3.
The least required ms of different types of sorption-SCR is about 26% of mAdBlue to obtain the same amount of ammonia. Even for the worst candidate working pair of each type of sorption-SCR, the required ms is less than half of mAdBlue. If ρs is controlled as 1.0 g/cm3, 60%–70% of the bulk volume of sorption-SCR (Vs) could be saved compared with urea-SCR (VAdBlue). Even if ρs decreases to 0.4 g/cm3, triple-stage and double-stage triple-effect ammonia production can keep the Vs smaller than the VAdBlue, while both the single-stage and double-stage with MnCl2 filled in the ASDS will occupy around a 20% larger volume than VAdBlue.
In summary, the advantages of sorption-SCR include a lower starting temperature, higher NOx conversion at a low temperature, a higher thermal efficiency, and a larger ammonia storage capacity in comparison with urea-SCR. In addition, by applying multi-ammoniate as sorbent, the double-stage triple-effect sorption-SCR could realize both a quick launch and an insufficient AW with a compact structure, which brings a bright potential for the efficient commercial de-NOx technology.

Acknowledgments

This work was supported by the National Natural Science Foundation of China for the Distinguished Young Scholars (Grant No. 51825602).
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