Elsevier

Energy

Volume 230, 1 September 2021, 120767
Energy

Process design and exergy cost analysis of a chemical looping ammonia generation system using AlN/Al2O3 as a nitrogen carrier

https://doi.org/10.1016/j.energy.2021.120767Get rights and content

Highlights

  • A chemical looping ammonia generation (CLAG) system was developed.

  • Reaction kinetics was transformed and imported into the simulation model.

  • Both exergy and exergy cost analyses were conducted for the CLAG system.

  • The exergy efficiency of the proposed CLAG system reached to 26%.

Abstract

Chemical looping ammonia generation (CLAG), in which the N2 fixation and hydrolysis reactions occur via the circulation of nitrogen carriers, has the advantages of low-pressure, low energy consumption and high ammonia yield. Therefore, CLAG is considered as a promising alternative to conventional Haber–Bosh technology. In this work, a model for the CLAG system with a capacity of 300,000 t/a is first established. For the simulation, the N2 fixation and hydrolysis reactors are modeled as the kinetics-based mixed flow reactor, and AlN/Al2O3 is used as the nitrogen carrier. The optimal operation conditions of the CLAG system are then determined by sensitivity analyses. The distribution of the exergy loss is gained from exergy analysis. The results showed that the exergy efficiency of the system reached to about 26%. Finally, exergy cost analysis is conducted to evaluate the cost formation of the system. Generally, the unit exergy cost of heat exchangers is larger than those of the other components. The distillation tower in air separation unit, the N2 fixation reactor, and the compressor in compression and purification unit should be primarily considered in system improvement because of the significant effects of their irreversibilities on other components.

Introduction

Ammonia is an important raw material for the production of industrial and civil chemicals, and is widely used in chemical fertilizers, refrigeration, and pharmaceutical industries [1]. Due to its high energy density, easy liquefaction, and convenient storage and transportation, ammonia is also considered as an important carrier of hydrogen [[2], [3], [4]]. Nowadays, over 90% of ammonia is produced through the Haber–Bosch synthesis (HBS) process [5]. However, HBS has three main limitations [5]. First, although ammonia synthesis is an exothermic process, a large amount of heat is still required to activate the nitrogen, because of its high dissociation energy [5]. Secondly, a catalyst is required to lower the activation energy. Finally, a high pressure (approximately 30 MPa) is necessary to drive the thermodynamic equilibrium of the ammonia synthesis reaction towards the ammonia generation side, and, despite this high pressure, the one-pass yield hardly exceeds 25% [5,6]. In addition, the acquisition of raw materials (hydrogen and nitrogen) for HBS consumes significant energy and release large quantities of pollutants [5].

For the above reasons, many researchers have attempted to develop new ammonia production methods with mild operating conditions, low energy consumption, and high yields. Taking advantage of the characteristics of metal nitrides (A-N) that can react with H2O to form ammonia at low pressure, Gálvez et al. [5,7] first proposed a two-step process for ammonia production by circulating AlN/Al2O3 intermediates. In the first step, nitrogen is fixed by reaction with carbon and Al2O3 to produce AlN. In the second step, the generated AlN is hydrolyzed by steam to produce ammonia and regenerate Al2O3. The reaction steps are represented by R1 and R2.3C+N2+Al2O3=2AlN+3COΔH25°C0=708.1kJ/mol2AlN+3H2O=Al2O3+2NH3ΔH25°C0=274.1kJ/mol

The proposed two-step process for ammonia production can be regarded as a special case of chemical looping [[8], [9], [10]]. The concept of “chemical looping combustion” was first proposed by Richter and Knoche [11], aiming to reduce exergy loss in fossil fuel combustion. The terminology of “chemical looping combustion” was first formally presented by Ishida et al. [12], with a further understanding of its essential advantages of inherent CO2 separation and low NOx emission. Actually, the main characteristic of chemical looping is that it splits one chemical reaction into two or multiple sub-reactions via the circulation of solid intermediate. The nitrogen carrier of AlN/Al2O3 in the proposed ammonia production process is just such one kind of solid intermediates. This process is, thus, called chemical looping ammonia generation (CLAG) [13,14].

With the development of CLAG, three kinds of technical route have been proposed, and they are (1) ammonia generation by the hydrolysis of metal nitrides (H2O-CLAG) [2,5,6,[13], [14], [15], [16], [17], [18], [19]], (2) ammonia production via the reaction of metal nitrides with H2 (H2-CLAG) [20,21], and (3) ammonia formation by the hydrogenation of alkaline earth metal imides (A-NH-CLAG) [22]. In H2O-CLAG, a metal nitride is hydrolyzed to produce ammonia and metal oxide/hydroxide. Subsequently, the metal nitride is regenerated in a N2 fixation process. Experimental results have shown that the metal nitride/metal oxide pairs of AlN/Al2O3 [2,5,[13], [14], [15], [16], [17]], Cr/Cr2N/Cr2O3 [18], Li/Li3N/LiOH [19], and Mn5N2/MnO [6] can be used for H2O-CLAG. Compared with the HBS process, H2O-CLAG has the following three advantages: the reactions proceed at low pressure without a catalyst; the raw materials (H2O and carbon) are cheap and easily available; and the yield of ammonia is improved. However, one critical disadvantage of H2O-CLAG is that the regeneration of the metal nitride occurs at high-temperature conditions, resulting in an energy-intensive process. H2-CLAG consists of two steps: N2 fixation and hydrogenation. A metal is converted to a metal nitride by N2 fixation reaction and, then the metal nitride is reduced by H2 to regenerate the metal and release ammonia. The main advantage of H2-CLAG is the mild temperature conditions needed for the hydrogenation reaction. For example, the hydrogenation reactions of Mn6N2.58, Ca3N2, and Sr2N2 can take place at 550 °C [20], and the hydrogenation temperature of manganese nitrides could be as low as 400 °C in the presence of Li [21]. Unfortunately, the low ammonia yield and the requirement of an energy-intensive raw material (in this case, hydrogen) hinder its development. Recently, Gao et al. [22] proposed an A-NH-CLAG process utilizing alkaline earth metal hydride/imide pairs. Nitrogen is first fixed through the reaction of the alkaline earth metal hydrides with N2 to form imides. Then, the imides are hydrogenated to produce ammonia and regenerate the metal hydrides. This process proceeds at low temperatures and low pressures, making it a promising cost-effective process.

To date, the three CLAG processes remain at the fundamental research stage, and research has mainly focused on screening of appropriate nitrogen carriers and studying the corresponding reaction processes. However, no system level study has been reported in the literature. At the system level, exergy analysis is able to detect and quantify in detail where irreversibility occurs and, thus, it is useful in the search for new improvements in energy-intensive systems. Exergy cost analysis goes a step further than exergy analysis by introducing the concept of exergy cost, and is able to analyze the process of cost formation resulting from the irreversibility in the different components of the system.

To provide guidance for experimental research and the industrialization of CLAG, we propose a CLAG system in this paper and then establish its corresponding model in Aspen Plus (schematically shown in Fig. 1). The system consists of an air separation unit (ASU), a chemical looping ammonia generation unit (abbreviated CLAG), and a compression and purification unit (CPU). Because the technical route of H2O-CLAG has been studied in more detail (i.e., the necessary reaction kinetics can be obtained from the literature), it is chosen as the object for system design and modeling in this work. The kinetics-based mixed flow reactor (MFR) model is used to simulate the hydrolysis reactor, since the equilibrium reactor model is not applicable here because the decomposition of ammonia cannot reach thermodynamic equilibrium. Based on the established model, sensitivity analyses of the key parameters of the system are subsequently conducted to determine the optimal operating conditions. Then, exergy analysis is carried out to determine the distribution of the exergy loss and the exergy efficiency of the system. Finally, exergy cost analysis is conducted to understand the cost formation process in this system.

Section snippets

Model of the CLAG system in Aspen Plus

A model of the CLAG system with a capacity of 300,000 t/a is first established in Aspen Plus, as shown in Fig. 2. The model can be simply described as follows: in the ASU, high-purity nitrogen is produced by cryogenic air separation and introduced into the CLAG unit as the reactant of the N FIXATION reactor. Then, in the CLAG unit, the mixture containing ammonia is produced by circulating the nitrogen carrier: AlN/Al2O3. Finally, the generated ammonia in the CLAG unit is compressed and purified

Reactor model for CLAG

The reactor model RGIBBS is not applicable for modeling the hydrolysis reactor because, in practice, the overall hydrolysis process cannot reach its thermodynamic equilibrium (i.e., ammonia decomposes nearly completely) in most cases. That is to say, the decomposition rate of ammonia is slower than its generation rate under such conditions, and the ammonia generated by AlN hydrolysis discharges from the reactor rapidly before its complete decomposition. As the reaction kinetics of N2 fixation,

Conclusions

In this work, a CLAG system model with a capacity of 300,000 t/a was first established in Aspen Plus. Based on the established model, sensitivity analysis of the key operating parameters was conducted to obtain the optimal operating conditions. Then, exergy analysis over the CLAG system under the optimal operating condition was carried out to calculate the exergy efficiency of the system and to discover the distribution of exergy loss in the system. Finally, to understand the cost formation

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by “National Key R&D Program of China (2018YFB0605403)” and “National Natural Science Foundation of China (52025063)”.

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1

X. Wang and M. Su contributed equally to this work.

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