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Review

Removal of Nitrogen Pollutants in the Chemical Looping Process: A Review

by
Yuchao Zhou
1,2,
Xinfei Chen
2,
Yan Lin
2,
Da Song
2,3,
Min Mao
1,
Xuemei Wang
1,
Shengwang Mo
1,2,
Yang Li
2,4,
Zhen Huang
2,3,* and
Fang He
1,*
1
College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541000, China
2
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China
3
School of Energy Science and Engineering, University of Science and Technology of China, Hefei 230026, China
4
College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, China
*
Authors to whom correspondence should be addressed.
Energies 2024, 17(14), 3432; https://doi.org/10.3390/en17143432
Submission received: 12 June 2024 / Revised: 9 July 2024 / Accepted: 10 July 2024 / Published: 12 July 2024
(This article belongs to the Section I2: Energy and Combustion Science)
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Abstract

:
In the process of fuel utilization, traditional combustion technologies result in the conversion of nitrogen elements in fuels into nitrogen oxides, which are released into the atmosphere, posing serious threats to the environment and human health. The chemical looping process (CLP) is an effective technology for reducing nitrogen-containing (N-containing) pollutants during fuel utilization. During the CLP, the oxygen carrier (OC) can oxidize nitrogen oxide precursors (NH3 and HCN) released from the fuel to N2, while the reduced OC can reduce nitrogen oxides to N2. The achievement of efficient nitrogen pollutant removal relies on the development of highly active oxygen carriers (OCs). This review summarizes the recent progress in the removal of nitrogen pollutants within chemical looping processes (CLPs). It delineates the formation pathways of N-containing pollutants (NH3, HCN, NO, NO2 and N2O) and highlights the performance of various OCs. The influence of reaction conditions and feedstock characteristics is also discussed. Ni-based OCs have demonstrated superior performance in the removal of N-containing pollutants, exhibiting strong oxidation capabilities and excellent catalytic properties. Moreover, iron ore, as a cost-effective and environmentally friendly feedstock, holds promise for wide-scale application. Future research should focus on further optimizing OCs strategies and refining reaction conditions to achieve more efficient and economical N-containing pollutant removal, thereby fostering the widespread application of chemical looping technology in the energy sector.

1. Introduction

In recent years, renewable energy sources such as solar, wind, and biomass energy have seen rapid development [1,2]. Nevertheless, traditional fossil fuels continue to dominate the global energy mix, accounting for approximately 80% of the total [3]. The extensive extraction and combustion of fossil fuels have led to increased global greenhouse gas emissions, causing a rise in average global temperatures and exacerbating the issue of climate change [4,5]. To address this challenge, researchers have developed various technologies for capturing and storing carbon dioxide from fossil fuel combustion, including membrane separation [6], oxyfuel combustion [7], and chemical looping technology [8]. Chemical looping technology has garnered attention due to its high carbon capture efficiency and the mitigation of the high energy consumption associated with conventional cryogenic air separation for oxygen production [9,10].
As the application of chemical looping technology expands, it has given rise to several subfields, including chemical looping combustion [11,12,13], chemical looping gasification [14,15,16], chemical looping reforming [17,18,19] and chemical looping oxygen uncoupling [20,21,22]. These subfields encompass key energy conversion processes such as fuel combustion, gasification, fuel reforming, and oxygen separation. Among them, chemical looping combustion is one of the more prominent chemical looping technologies in the field of fuel conversion [8,23].
CLPs employ a reaction structure consisting of a fuel reactor and an air reactor. This configuration decomposes the conventional combustion reaction into two-step gas–solid redox reactions [11,24]. Unlike traditional combustion processes where fuel is oxidized in air, the CLP typically involves OC composed of metal oxides that provide lattice oxygen. In the fuel reactor, the fuel reacts with the OC, causing the metal oxide to be reduced to a lower oxidation state, while the fuel is oxidized to CO2 and H2O. Subsequently, the reduced OC is circulated to the air reactor, where it is oxidized by oxygen to regenerate the metal oxide. The regenerated OC is then circulated back to the fuel reactor to continue the process [20,25].
During the combustion and utilization of fuels such as coal, sewage sludge, and biomass, the nitrogen content in these fuels can be transformed into N-containing pollutants like HCN, NH3, NOx, and N2O [26]. The accumulation of these pollutants in the environment can disrupt the nitrogen cycle [27], leading to issues such as photochemical smog, acid rain, ozone layer depletion, and exacerbation of global warming. Excessive inhalation of these pollutants by humans can result in severe diseases such as renal failure, cancer, and blue baby syndrome [28,29]. Consequently, there is an urgent need to reduce the emission of nitrogen-containing pollutants during fuel utilization. As shown in Figure 1, scholars [30,31,32,33] have applied chemical looping technology to remove nitrogen pollutants. Compared to traditional combustion, chemical looping technology reduces the formation of thermal and prompt NOx pollutants through indirect contact combustion of fuel with air and lower reaction temperatures. Moreover, since there is no gaseous molecular oxygen present in the fuel reactor, the oxidizing effect of the reaction atmosphere is significantly weakened, which decreases the probability of fuel-bound NOx formation [34,35,36,37].
The CLP has been validated to markedly enhance the purification of N-containing pollutants during fuel utilization. Researchers have extensively investigated the influence of various OCs and under different reaction conditions on the removal performance of nitrogen pollutants in CLPs. However, a comprehensive and explicit summary is currently lacking. Based on the latest research progress in the removal of nitrogen pollutants in CLPs, this paper summarizes the generation and formation pathways of various N-containing pollutants; analyzes the influence of factors such as OCs types, reaction conditions, and fuel characteristics on the transformation of N-containing pollutants; and provides a reference for the development of processes to reduce N-containing pollutant emissions in the CLP.

2. Formation of Nitrogen Pollutants in the Chemical Looping Process

The nitrogen pollutants generated during the CLP, including HCN, NH3, N2O, NO, and NO2, are derived from N-containing compounds in the fuel, such as inorganic nitrogen (inorganic-N), protein nitrogen (protein-N), pyrrole nitrogen (pyrrole-N), and pyridine nitrogen (pyridine-N) [38,39]. The formation of nitrogenous pollutants involves a series of basic reactions that are closely related to reaction conditions, fuel composition, and the OC, among other factors. To effectively remove nitrogen pollutants, it is necessary to study the formation pathways of reduced nitrogen pollutants (HCN and NH3) and oxidized nitrogen pollutants (N2O, NO and NO2), providing guidance for the subsequent conversion and removal of nitrogen pollutants.

2.1. Formation of Reduced Nitrogen Pollutants

2.1.1. NH3

The formation pathways of NH3 and HCN are depicted in Figure 2. The main sources of NH3 include inorganic-N, protein-N, pyrrole-N, and pyridine-N in the fuel. During the CLP, inorganic-N, being unstable, decomposes directly to form NH3 in the initial stages of the reaction [40,41,42]. Part of the unstable protein-N is converted to NH3 through deamination and hydrogenation reactions, while the remainder undergoes bond cleavage to produce amine-N, which is further converted to NH3 through deamination, decomposition, bimolecular reaction, or reaction with hydroxyl radicals (OH). Additionally, amine-N reacts with hydrogen radicals (H) to form HNCO, which decomposes into NH3 upon hydrolysis [40]. Stable protein-N forms heterocyclic-N through crosslinking, which can be directly hydrogenated to NH3. Pyrrole-N and pyridine-N are also converted to NH3 through hydrogenation reactions [43]. During oxidation, HCN forms CN, which reacts with H2O to form H2NCO. These amide groups dissociate to form amine groups and further hydrogenate to form NH3 [44,45]. Moreover, HCN can also be converted to NH3 by reacting with CO2 and H2O [45,46].

2.1.2. HCN

The formation pathway of HCN is depicted in Figure 3, with the main precursors being protein-N, pyrrole-N, and pyridine-N in the fuel. Unstable protein-N can be converted to NH3 through direct deamination and dehydrogenation reactions or through bond cleavage to form amine-N. Amine-N can further cyclize to form heterocyclic nitrogen, which can then be converted to HCN through ring-opening reactions [40,41,42]; alternatively, amine-N can be directly converted to HCN through dehydrogenation reactions. Stable protein-N forms heterocyclic nitrogen through crosslinking reactions [41]. Heterocyclic-N, pyrrole-N, and pyridine-N can be converted to HCN through hydrogenation and ring-opening reactions [43]. Additionally, N-containing compounds formed during the reaction may also participate in secondary reactions, further promoting the formation of HCN. Under conditions with a low N2O/NO to CH4 ratio (less than 0.2), N2O and NO can react with CHi to generate HCN [47].

2.2. Formation of Oxidized Nitrogen Pollutants

2.2.1. NOx

NH3 and HCN, as precursors of nitrogen oxides, may undergo secondary reactions during the CLP, leading to their oxidation and transformation into nitrogen oxides such as NO and NO2 [48,49,50,51,52]. As shown in Figure 4, volatile HCN can interact with water vapor and free radicals (H, OH, O) in other reactive gases to generate intermediates such as NCO and HNCO. These intermediates then react with free radicals to form NO or NHi. Similarly, NH3, another NOx precursor, can react with H· or OH· to form NH or NH2, which are then oxidized by O and OH· to generate NO [41,45,53]. Thus, the formation of NO is the result of a series of basic reactions between nitrogen oxide precursors and free radicals. In addition to free radical reactions, there are also reactions between NOx and NH3 [50,51,54]. Under the presence of oxygen free radicals, NO is easily oxidized into NO2, although the formation of NO2 is less than that of NO [45,55].

2.2.2. N2O

The formation pathway of N2O is depicted in Figure 5. HCN is one of the primary precursors of N2O. During the CLP initial stages of the reaction., HCN reacts with O· to form NCO, which further reacts with NO to generate N2O. NH3 can also react with free radicals, thereby indirectly producing N2O. NOx, particularly NO, can be reduced by the OC to N2 and N2O [51]. Additionally, the direct reaction of N· on the surface of char with NO is another pathway for the formation of N2O [45,56].

3. Removal of Nitrogen Pollutants in the Chemical Looping Process

3.1. Effect of OCs

Compared to conventional pyrolysis without OCs, the presence of the OC in the CLP effectively converts NH3 and HCN and significantly decreases the emission of nitrogen oxides [57]. This beneficial effect is attributed to the fact that NH3 and HCN, acting as reductive precursors, can be oxidized by the lattice oxygen in the OC upon contact, converting into N2 and thereby suppressing the formation of NOx [58,59].
In recent years, researchers have explored the effects of various types of OCs on N-containing pollutants in CLPs, including Fe-based, Ni-based, Cu-based, Mn-based, perovskite, and ilmenite OCs, as shown in Table 1. These OCs differ in their ability to release lattice oxygen, leading to significant variations in the distribution of nitrogen compounds during the CLP [42].
In terms of capacity for the transformation and removal of N-containing pollutants using single-component OCs, Ni-based OCs exhibit superior performance compared to Fe-based, Cu-based, and Mn-based OCs [71]. Ni-based OCs have shown exceptional performance in the removal and conversion of nitrogen pollutants due not only to their oxidation capabilities but also to their ability to catalyze the partial decomposition of NH3 into N2 and H2 [67,72]. The differences in performance between Fe-based and Mn-based OCs and Ni-based carriers may be related to their reductive properties. Fe, FeO, and Fe3O4 have been proven to reduce NO [31], and the Fe2O3/Al2O3 OC has shown high activity towards HCN, significantly promoting the oxidation of HCN [47]. In contrast, MnO cannot reduce NO, and Cu-based and Mn-based OCs have poorer removal capabilities for NOx, which is influenced not only by their reactivity but also by their oxygen release characteristics. Normann et al. [67] conducted a laboratory-scale fluidized bed reactor study in 2019 and found significant differences in the oxidation of NH3 by three different OCs based on Fe, Mn, and Ni oxides. The Ni-based OC did not observe the formation of NO during the experiment, while the NO conversion rates for the Fe-based and Mn-based OCs were approximately 40% and 60%, respectively. In 2022, Huang et al. [57] further investigated this and found that the Mn-based OC had the highest NO production under complete oxidation conditions, whereas the NO production by the Fe-based OC was significantly less than that of the Mn-based OC.
In addition to the traditional research on single-component OCs, researchers have increasingly focused on the development of bimetallic composite OCs in recent years. In perovskite OCs, especially the CaFe combination, the formation of NH3 and HCN is enhanced [43]. The reasons for this phenomenon include the following: (1) Ca2Fe2O5 was formed in char at temperatures above 873 K, which improved the transformation of CaCxNy and FeNx to N2. With an increasing temperature, Ca2Fe2O5 became the predominant mineral compound, and the ratio of Ca2Fe2O5 was higher in char. (2) The presence of Fe3+ and Ca2+, respectively, facilitates ring-opening reactions and the conversion of amine nitrogen to NH3 [73]. Furthermore, compared to perovskite OCs, titanate OCs exhibit superior performance, which can be attributed to the higher oxidizability of Ti4+ ions in titanate OCs compared to Fe2+ ions in perovskite OCs. This enhanced oxidizability results in the release of a greater amount of free oxygen, which in turn promotes the selective catalytic reduction (SCR) of NO [56]. A similar reduction in nitrogen oxide emissions has also been observed on NiFe OCs [41]. On one hand, the Fe of OC strongly promotes the formation of interstitial iron nitrides from NH3, which then convert to N2, cutting off the formation of NOx [74,75]. On the other hand, the catalytic action of Fe promotes the direct conversion of protein-N to N2, thereby inhibiting the generation of NH3/HCN and reducing the yield of NOx [76].
By loading or doping a certain proportion of metals into the OC, such as alkali and alkaline earth metals (K+, Na+, Ca2+), these metals not only serve as good catalysts for the reforming of hydrocarbons [77,78] but also play a crucial role in modifying OCs, enhancing reactivity and significantly promoting the oxidation conversion of NOx precursors tar-N and char-N to N2, thereby reducing nitrogen pollutant emissions [42]. Moreover, the superior performance of Ni and Cu-modified iron-based carriers may be due to the formation of oxygen-bearing complexes from modified crystal phase structures by foreign active metals (ions), which exhibit better oxidation and catalytic cracking performance than single-component carriers [79,80,81,82]. In particular, the K-modified iron-based carrier had a relatively low HCN formation, which may be related to the formation of highly active solid solutions (such as KFeO2). Different metal ions have different modification effects on OCs; transition metal-modified Fe-based carriers, such as NiFe2O4 and CuFe2O4, can be completely reduced to alloy phases (e.g., Fe-Ni/Fe-Cu), while the active solid solutions derived from K modification (KFeO2) cannot be completely reduced to alloy phases (e.g., K-Fe/Ca-Fe/Na-Fe), thus acting as catalysts [53]. This conclusion has also been confirmed by previous studies [83,84]. Doping on composite OCs can also improve the removal and conversion capacity of nitrogen pollutants. Huang et al. [59] found in their experiments that when modifying Fe-based OCs with metal ions K/Na/Ca/Ni/Cu/Mn, the total emissions of NH3 and HCN for all modified carriers except for those modified with Ni and Cu were greater than those for the initial carrier. Adding metal-ion-modified OCs, due to their high activity, promotes the release of nitrogen, leading to an increase in the amount of NOx precursors [53]; however, this addition also significantly promotes the oxidation conversion of NOx precursors tar-N and char-N to N2 [42]. However, excessive addition of foreign metal ions can inhibit the release of lattice oxygen due to the formation of inert components, thus suppressing lattice oxygen release [85]. Huang et al. [58] modified the OC by adding different masses of metal K and found that the OC with 10 wt% K loading exhibited the highest performance. Furthermore, OCs with excessive high content loads are prone to sintering. Therefore, selecting the appropriate metal ions and an appropriate mass of metal ions for modification is very important.
In addition to introducing metal ions to modify OCs for enhanced removal capacity, increasing the mass of the OC is also an effective strategy. With the increase in the mass of the OC, the amount of lattice oxygen gradually increases, producing more oxygen radicals. These radicals interact with radicals brought by water vapor, accelerating the oxidation of tar-N and char-N and increasing the formation of nitrogen oxide precursors [40]. However, the large amount of lattice oxygen in the nitrogen oxide precursors will oxidize them to generate N2, thereby reducing the content of NH3 and HCN [53]. Meanwhile, the increase in radicals prolongs the contact time between NH3, HCN, and the OC, further reducing the emission of NH3 and HCN in the fuel reactor [59]. As shown in Figure 6, experiments by Huang et al. [69] indicate that by increasing the lattice oxygen of the OC, the yield of N2 can be significantly improved from 52.49% to 91.98%.

3.2. Effect of Reaction Conditions on the Conversion of Nitrogen-Containing Pollutants

3.2.1. Effect of Reaction Atmosphere

The reaction atmosphere in the CLP, such as CO2 and steam, has a significant impact on the conversion of N-containing compounds, as shown in Figure 7.
Compared to the inert atmosphere, the addition of CO2 intensifies the oxidative properties of the reaction atmosphere, which may lead to an increase in the formation of nitrogen oxides [57]. Moreover, during the char gasification stage, CO2 participates in (1), resulting in the production of a significant amount of CO [47,86]. CO, being a reducing gas, can promote the reduction of NO to N2 [60], as shown in (2). Additionally, the presence of CO2 facilitates the gasification of char pores, allowing more char nitrogen to enter the gas phase and increasing the generation of HCN (3) [55]. While CO2 reacts with HCN to form NH3, it also generates CO through (4), which promotes the reduction of NOx and reduces the production of nitrogen oxides [46]. Therefore, CO2 plays a significant role in the removal of nitrogen pollutants during the CLP.
C + CO 2 → 2 CO
2 NO + 2 CO → N 2 + 2 CO 2
Char − N + CO 2 → HCN + CH x O y
2 NH 3 + 3 CO 2 → N 2 + 3 CO + 3 H 2 O
The addition of steam to the reaction atmosphere leads to the generation of a large number of H, O, and OH radicals, which can promote the hydrogenation and cracking of nitrogen compounds in tar-N and char-N. The HNCO produced by the cracking of amine-N can further hydrolyze to form NH2COOH, and N-containing heterocyclic compounds are activated and decomposed under the action of free radicals. These processes ultimately result in an increase in the production of NH3 and HCN [87,88,89]. Additionally, the enhancement of the reducing atmosphere aids in the reduction of nitrogen oxides, promoting the conversion of NOx to N2 [51,89].
As CO2 or steam are introduced, the gas purge rate increases, leading to a reduction in the average residence time of the gas within the reactor. The reduced gas residence time leads to insufficient contact between nitrogen pollutants and the OC, resulting in incomplete reactions that decrease the oxidation of NOx precursors and increase the production of HCN and NH3 [90]. Huang et al.′s [58] experiments showed that after increasing the gas purge flow rate, the NH3 content increased from 26.64% to 42.91%, while the HCN content remained constant at 18.23%. This indicates that shorter residence times are not conducive to the oxidation of nitrogen pollutants. The increase in steam content gradually reduces the N2 content and gradually increases the HCN content, with the growth trend slowing down. The significant increase in NH3 remains relatively constant, mainly due to the fact that HCN readily hydrolyzes in the presence of H2O to form NH3. Therefore, the shorter contact time between HCN, NH3, and the OC reduces opportunities for oxidation. At the same time, this aids in the cleavage of functional groups in the feedstock, thus forming HCN and NH3 [59].

3.2.2. Effect of Reaction Temperature

The reaction temperature exerts a substantial influence on the reactivity of the OC and the transformation of N-containing intermediates. Higher temperatures enhance the oxidative capacity of the OC and increase the reactivity of nitrogen oxide precursors with the OC. As the temperature rises, the heterogeneous reaction between char-N and NO becomes significantly stronger. This results in a greater conversion of char, reducing the release of fuel-N from the air reactor and consequently lowering NO emissions. With increasing temperature, the OC produces more CO2 while oxidizing HCN to form N2. CO2 reacts with NH3 to generate HCN, leading to an increase in HCN. Simultaneously, NH3 is converted to N2 and H2 at high temperatures, thus reducing the release of NH3. As depicted in Figure 8, Huang et al.’s [59] study compared the impact of different temperatures on the release of NH3 and HCN, finding that with increasing temperature, the release of NH3 decreases while the release of HCN increases. Nevertheless, the total release continues to decline, reaching a minimum at 900 °C, indicating that high temperatures can improve the removal and conversion efficiency of nitrogen pollutants in the CLP.
However, high temperatures also promote the transformation of non-gaseous nitrogen in char and tar into gaseous nitrogen (NH3/HCN) [58]. Additionally, in high-temperature conditions, reducing gases such as CO and H2 can be converted by the OC, leading to a weakening of the reducing gas atmosphere. This combined effect may lead to an increase in NO emissions [47]. Previous studies [33,91] have also reached similar conclusions. Therefore, when selecting the reaction temperature, it is crucial to consider the characteristics of the reaction system and the catalyst to achieve optimal nitrogen pollutant removal [92].

3.2.3. Effect of the Cycle Number

Increasing the number of cycles has been shown to reduce the overall emission of N-containing pollutants [11]. With the increase in cycle count, the ash content in the reactor bed accumulates gradually, significantly extending the average residence time of the gas [93]. Under these conditions, the contact time between N-containing pollutants and the OC is prolonged, leading to more thorough gas–solid reactions that facilitate the oxidation of NH3 and HCN into N2, thereby effectively reducing the emission of nitrogen pollutants. Furthermore, the accumulation of ash in the bed increases the contact area between the gas and the solid particles, promoting more gas–solid reactions between nitrogen oxides and the solid surface. As the cycle count increases, the activity of the OC may gradually stabilize, and the stable OC activity aids in enhancing the reaction efficiency between nitrogen oxides and the OC, further reducing the overall emission of N-containing pollutants. Huang et al.′s [69] experimental findings suggest that the prolonged average residence time of the gas–solid reaction results in an increase in the activity of OC, leading to the oxidation of more NH3 and HCN. More detailed experiments and studies conducted in 2022 revealed that after eight cycles, the average removal rates of NH3 and HCN were 61.37% and 92.67%, respectively, with an overall average removal rate of 59.69% for nitrogen-containing pollutants, as illustrated in Figure 9 and Table 2 [55]. The metal elements such as Fe, Ca, K, etc. present in the ash also contribute to the decomposition and reduction of NOx by these metals [94,95]. With the accumulation of ash, the concentration of these metal elements increases, further enhancing the removal efficiency of nitrogen pollutants.

3.3. Effect of Fuel

The diversity of fuel compositions leads to differences in the types of major N-containing pollutants. In the reaction process of high-grade fuels (e.g., coal), HCN is the predominant form of nitrogen release. In contrast, in low-grade fuels (e.g., biomass), NH3 is the main nitrogen species in the volatiles [56]. The stability of nitrogen functional groups and the availability of H radicals are two important factors affecting nitrogen evolution [96]. The pre-baking and drying of the fuel can consume a certain amount of hydrogen radicals, reducing the number of unstable nitrogen functional groups (such as pyridine-N and pyrrole-N) [40]. Upon pre-baking and drying, these unstable nitrogen functional groups are transformed into more stable inorganic nitrogen and protein nitrogen, thereby increasing the stability of nitrogen functional groups in the fuel. This change reduces the number of unstable nitrogen functional groups, making the fuel structure more stable, which in turn affects the release and transformation of nitrogen during the CLP.

4. Conclusions and Outlook

Chemical looping technology has demonstrated great potential for the removal of nitrogen pollutants during fuel conversion processes. The transformation pathways and removal method of nitrogen pollutants is crucial for nitrogen pollutant control in the CLP. Fuels contain nitrogen compounds that lead to nitrogen pollutant formation during the CLP. These compounds form intermediates like NH3 and HCN, which can convert to N2, NOx, and N2O. Nickel-based OCs demonstrate superior performance due to their strong oxidation capabilities and catalytic properties. Bimetallic composite OCs, such as NiFe2O4, also show promising results. Developing novel, highly active OCs and optimizing their regeneration strategies are essential for enhancing nitrogen removal efficiency. Reaction atmosphere, temperature, and cycle number all play critical roles in nitrogen pollutant removal. The addition of CO2 and steam can promote NOx reduction and N2 formation, respectively. Optimizing these parameters can balance pollutant removal efficiency and system performance. Furthermore, the addition of CO2 and steam can promote NOx reduction and N2 formation, respectively. Higher temperatures improve OC activity and NOx conversion but may increase HCN and NH3 formation. Optimizing these parameters can balance pollutant removal efficiency and system performance. Pre-treatment methods like torrefaction can reduce the formation of unstable nitrogen functional groups, thus decreasing pollutant emissions. The interactions between fuels, carriers, and reaction conditions need further study to enhance nitrogen pollutant removal efficiency.

Funding

This research was funded by the National Natural Science Foundation of China grant no. 52306289, 52106285, 52076209, 22179027; the Foundation and Applied Foundation Research of Guangzhou grant no. 2024A04J4601, the Foundation and Applied Foundation Research of Guangdong Province grant no. 2022B1515020045, 2023A1515011930 and 2023B1515120062, and the Funder Grant Number Young Talent Support Project of Guangzhou Association for Science and Technology grant no. QT-2023-042.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

Full nameabbreviation
Chemical looping processCLP
Chemical looping processesCLPs
nitrogen-containingN-containing
oxygen carrierOC
oxygen carriersOCs
NO and NO2NOx
fuel nitrogenfuel-N
inorganic nitrogeninorganic-N
protein nitrogenprotein-N
pyrrole nitrogenpyrrole-N
pyridine nitrogenpyridine-N
amine nitrogenamine-N
heterocyclic nitrogenheterocyclic-N
NH3, NH2 and NHNHi

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Figure 1. Schematic diagram of nitrogen pollutant removal in the chemical looping process.
Figure 1. Schematic diagram of nitrogen pollutant removal in the chemical looping process.
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Figure 2. Formation pathway of NH3 in the chemical looping process.
Figure 2. Formation pathway of NH3 in the chemical looping process.
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Figure 3. Formation pathway of HCN in the chemical looping process.
Figure 3. Formation pathway of HCN in the chemical looping process.
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Figure 4. Formation pathway of NOx in the chemical looping process.
Figure 4. Formation pathway of NOx in the chemical looping process.
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Figure 5. Formation pathway of N2O in the chemical looping process.
Figure 5. Formation pathway of N2O in the chemical looping process.
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Figure 6. Distribution of nitrogen-containing species at different oxygen equivalent ratios [69].
Figure 6. Distribution of nitrogen-containing species at different oxygen equivalent ratios [69].
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Figure 7. Distribution of nitrogen oxides under different atmospheric conditions [57].
Figure 7. Distribution of nitrogen oxides under different atmospheric conditions [57].
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Figure 8. Effect of temperature on the release of NH3 and HCN during lignite CLC [59].
Figure 8. Effect of temperature on the release of NH3 and HCN during lignite CLC [59].
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Figure 9. The generation rates of (a) NH3 and HCN and (b) NOx vs. the number of CLC cycles [69].
Figure 9. The generation rates of (a) NH3 and HCN and (b) NOx vs. the number of CLC cycles [69].
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Table 1. Summary of N-containing pollutant transformation in the chemical looping process.
Table 1. Summary of N-containing pollutant transformation in the chemical looping process.
No.OC TypeFuelNOx CharacteristicsRef.
1NiO/Al2O3CoalXNO: 16.98–8.85% (AR); XN2: 100% (FR)2012 [60]
2Iron oreCoalXNO: 3.8–7.1% (AR); XNO: 0.5–0.8% (FR)2015 [31]
3Fe-BasedCoalXNO: 10–15% (FR)2013 [61]
4IlmeniteCoalXNO: 11%; XN2: 62% (FR)2014 [62]
5IlmeniteCoalXN2: 100% (FR)2014 [33]
6CuO/Fe2O3/MgAl2O4CoalXNO: 20% (FR)2016 [63]
7CuO/CementCoalXNOX: 6–18% (FR)2015 [45]
8HematiteBiomassXN2: 94% (FR)2019 [64]
9NiONH3 + Natural GasXNO: <0.01% (AR); XN2: 99.99% (FR)2014 [65]
10IlmeniteNH3XNO: 18% (FR)2015 [66]
11NiO/Al2O3CoalXNO: 45%, 63% and 0% for Fe/Mn/Ni-based OC (FR)2019 [67]
12HematiteCoalXNO: 21.2% XN2: 35.9%2021 [47]
13Iron oreCoalXN2: 92.91% and 99.99% for perovskite/Cu-based (FR)2020 [68]
14HematiteCoalXN2: 76% for blank and 94–99% for others2022 [59]
15IlmeniteCoalXN2: 68.10–75.33%2021 [69]
16IlmeniteCoalXN2: 39.74–80.40%2021 [42]
17IlmeniteCoalXN2: 91–98%2021 [70]
Table 2. The distributions of N-containing pollutants in the pyrolysis and CLC processes, and the N removal rate via CLC, at 900 °C [55].
Table 2. The distributions of N-containing pollutants in the pyrolysis and CLC processes, and the N removal rate via CLC, at 900 °C [55].
N-Containing PollutantGeneration Rate (%)Removal Rate (%)
PyrolysisCLC
NH311.7 ± 0.004.52 ± 2.1661.37
HCN9.93 ± 0.000.73 ± 0.1292.64
NO03.10 ± 0.93/
Total N-containing pollutants21.638.7259.69
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Zhou, Y.; Chen, X.; Lin, Y.; Song, D.; Mao, M.; Wang, X.; Mo, S.; Li, Y.; Huang, Z.; He, F. Removal of Nitrogen Pollutants in the Chemical Looping Process: A Review. Energies 2024, 17, 3432. https://doi.org/10.3390/en17143432

AMA Style

Zhou Y, Chen X, Lin Y, Song D, Mao M, Wang X, Mo S, Li Y, Huang Z, He F. Removal of Nitrogen Pollutants in the Chemical Looping Process: A Review. Energies. 2024; 17(14):3432. https://doi.org/10.3390/en17143432

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Zhou, Yuchao, Xinfei Chen, Yan Lin, Da Song, Min Mao, Xuemei Wang, Shengwang Mo, Yang Li, Zhen Huang, and Fang He. 2024. "Removal of Nitrogen Pollutants in the Chemical Looping Process: A Review" Energies 17, no. 14: 3432. https://doi.org/10.3390/en17143432

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