(13a) Optimal Design of an Absorbent-Enhanced Ammonia Synthesis Process for Solar Thermochemical Energy Storage

The utilization of renewable energy is increasingly recognized as essential in our efforts to reduce our reliance on fossil fuels and mitigate the emissions of carbon dioxide, a major contributor to the greenhouse effect. Solar energy, in particular, holds great promise as an energy source due to its abundance and freely available nature [1]. Concentrating Solar Power (CSP) systems play a pivotal role in harnessing solar energy by concentrating sunlight using mirrors onto a receiver. This receiver then converts solar energy into heat, which can be used to generate electricity through a power conversion block such as a steam turbine [2]. However, the intermittent nature of sunlight necessitates effective energy storage solutions. Ammonia-based thermochemical energy storage (TCES) systems have emerged as a promising option. A typical operation of an ammonia-based TCES system comprises a solar cycle and a heat recovery cycle (HRC). During the solar cycle, solar energy is employed to dissociate ammonia into hydrogen and nitrogen gas via an endothermic reaction. In the HRC, this gaseous mixture undergoes ammonia synthesis via an exothermic reaction, releasing energy that can drive a steam Rankine cycle [3]. The utilization of ammonia in TCES systems holds significant promise due to its carbon-free nature, aligning with green energy initiatives. Moreover, the ammonia reversible reaction is free from side reactions, simplifying the management of reactors, and the system benefits from over a century of industrial experience with the Haber–Bosch process and on-the-shelf catalyst [4].

The HRC faces two primary challenges. Firstly, to improve overall conversion in the ammonia synthesis process, an energy-intensive condensation system is needed to separate unreacted gases, leading to significant costs. Secondly, it is uncertain whether this technology can efficiently achieve power capacities compatible with modern Gen3 CSP systems requiring supercritical steam at temperatures above 700 °C [5]. Continuous efforts are underway to enhance the efficiency of ammonia-based TCES systems, particularly the HRC. Proposed solutions include replacing condensation with an enhanced absorption system using metal halides for selective absorption of ammonia immediately after synthesis [6]. However, there has been no investigation into the design and performance of an entire HRC at an industrial scale, including ammonia separation while meeting current power requirements. Moreover, the utilization of absorption for ammonia separation within an HRC remains unexplored.

The current study addresses the optimal design of an ammonia synthesis process, integrating ammonia separation through exothermic absorption within a heat recovery cycle for solar thermochemical energy storage. The objective is to increase the temperature of supercritical steam from 350°C to above 700°C using the heat of reaction. The proposed absorbent-enhanced ammonia synthesis process includes a heat recovery reactor, an autothermal reactor, and two heat exchanging absorbers alternating between absorption and desorption operations. Additional processes such as heat exchanging, compression, and storage were incorporated, resulting in a closed-loop system with heat integration. All process units were mathematically described using first-principles modeling, considering transient absorption operation. The effect of materials and operating conditions on the size of each process unit was taken into account. Due to the disparity between steady-state and transient models, a nested optimization/simulation approach was devised for optimal design.

The results lead to optimal dimensions for each process unit and associated operating conditions such as temperature, composition, flow rate, and pressure, minimizing the total capital cost. These findings demonstrate that the proposed absorbent-enhanced ammonia synthesis process can generate the required thermal energy to increase the temperature of supercritical steam from 350°C to 720°C, generating around 40.5 MWt. Various operating pressures were examined, resulting in minor differences among the corresponding designs. However, higher pressures tend to produce lower thermal and discharging efficiencies. The process units with the highest costs include the storage tank, compressor, and absorber. Nevertheless, opportunities exist to mitigate these costs, necessitating further techno-economic and efficiency studies. Future research aims to integrate the solar cycle into the optimal design and explore options to enhance system efficiency and reduce overall costs.

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