Numerical investigation of a direct ammonia tubular solid oxide fuel cell in comparison with hydrogen

https://doi.org/10.1016/j.ijhydene.2020.04.060Get rights and content

Highlights

  • The effect of using different fuel has been investigated on the cell performance.

  • The increasing temperature increases the performance of SOFC for both fuels.

  • The CS-SOFC outperforms the ES-SOFC and AS-SOFC for ammonia fuel.

  • The position of the anode is playing a great roll in performance and polarization.

Abstract

Nowadays, carbon-rich fuels are the principal energy supply utilized for powering human society, and it will be continued for the next few decades. Connecting with this, modern energy technologies are very essential to convert the available limited carbon-rich fuels and other green alternative energies into useful energy efficiently with an insignificant environmental impression. Amongst all kinds of power generation systems, SOFCs running with high temperatures are emerging as a frontrunner in chemical to electrical transformation efficiency, allows the engagement of all-embracing fuel varieties with negligible environmental impact. This study investigates the effect of ammonia usage in tubular SOFC performance. Firstly, the use of ammonia and hydrogen in the electrolyte-supported SOFC (ES-SOFC) has investigated. Then, the effect of using ammonia in anode-supported SOFC (AS-SOFC), ES-SOFC and cathode-supported SOFC (CS–SOFC) on performance has been examined by using COMSOL software. As a result of the study performed, it is found that the ammonia can be used in tubular SOFC's as a carbon-free fuel and CS-SOFC shows better performance compared with ES-SOFC and AS-SOFC. Besides, the findings of this study indicate that the use of ammonia as a fuel for SOFCs is comparable to the use of hydrogen.

Introduction

Energy is one of the ultimate prerequisite tools to bring sustainable economic development and improve citizen's quality of life for any country. A few decades ago, speedy industrializations, growing of inhabitants and urbanizations have a huge pressure on crude oil, coal, and other carbons rich fuels, which is finite. Consequently, fossil fuel consumption is snowballing and the upsurge in carbon-rich fuel demands is leading to rapid fossil fuel reserve depletion [1]. Moreover, carbons rich fuels are the principal source of greenhouse gases which resulting in a destructive impact on the environment. For this reason, finding an alternative green and clean renewable source of energy is unquestionable responsibilities for a researcher to upgrade the energy system, advance their efficiency and diminishing the obliteration of the environment.

Fuel cells are dirt free and efficient electrochemical technologies deployed for direct transformation of fuel's chemical energy into electricity without intermediate products by bringing together the fuel and an oxidant. They can be potentially deployed in miscellaneous applications and works continuously as far as fuel and oxygen or air as oxidant are delivered to its electrodes. This is a unique characteristic of fuel cell that distinguishes its working condition from a battery which is a storage form of energy and becomes discarded after a finite working period.

Solid oxide fuel cells are one type of electrochemical device that changes the fuel's chemical energy directly into electricity efficiently without intermediate products through the combination of numerous fuels (gaseous or gasified, liquid) with an oxidant mainly air [2]. They are the ultimate stimulating of the all-ceramic device for green power generation that runs at a temperature range of 500 to 1000οC [3,4]. Besides, they have high efficiency, essay scalability, the flexibility of fuel, has no movable parts, provide high waste heat quality that can be recuperated through cogeneration, and built-in different sizes with zero pollutants and greenhouse gases emission when carbon-free fuels are applied [[5], [6], [7], [8]]. For these reasons, they are receiving more thoughtfulness.

Nowadays, different fuels (hydrocarbons, ammonia, hydrogen, natural gas, biogas, methanol and ethanol) have been all investigated as possible fuels for fuel cells [9]. Among them, hydrogen is a perfect fuel choice from environmental outlooks and energy density yet production, deposition, and distribution allied snags are still the critical issues that ought to be researched [8,[10], [11], [12], [13], [14], [15], [16]]. That's why, numerous models of ammonia powered SOFCs have been explored whereas most of them were concentrated on planar design. In line with this, Ni [12] has been developed a 2D thermo-electrochemical model for planar SOFC powered with NH3 to examine the electrochemical reactions, thermal breakdown of NH3, fuel flow rate, mass and heat transfer. The finding of the study realized that higher electrical output is achieved when the NH3 fuelled SOFC inlet temperature is raising nonetheless the gradient near to the inlet of SOFC is meaningfully higher. Besides, their work realized that the performance of NH3 powered SOFCs is slightly declining when the velocity of inlet gas increases from 1m/s to 10m/s yet the temperature field is not considerably affected.

Kishimoto et al. [17] have been investigated the dissemination of temperature, species, flow and electrochemical amounts within the cell using a 2D numerical model for NH3 fuelled planar SOFC. Their outcomes disclosed that SOFC fuelled with direct and pre-decomposed NH3 has comparable performance. Besides, Tan [18] has been developed a 3D numerical simulation for planar SOFCs employing internal cracking of NH3 as the fuel to investigate its performances. The finding of the study disclosed that the underprivileged distribution of current density is mainly caused by the underprivileged distribution of gas concentration in the air electrode conduit.

Farhad and Hamdullahpur [19] have been studied on ammonia supplied anode supported 3D planar SOFC. The experimental outcomes revealed that 0.8 liters of NH3 can offer a continuous power nearly for ten hours using a 100 W SOFC at the operating voltage of 0.73 V and 800 °C. This is for the reason that ammonia is easily cracked at raised temperatures. Moreover, Ma et al. [20] have been fabricated SOFC supplied with direct commercial liquefied NH3 to investigate the cell performance at a temperature of 600 °C and 750 °C. The outcomes of the study disclosed that the uppermost power density of the cell supplied with ammonia was recorded as 355 mW/cm2 at 700 °C and with hydrogen was 371 and 324 mW/cm2 with pure oxygen and air, respectively at the same temperature. Additionally, Ma et al. [21] have been also performed an experimental investigation of AS-SOFCs powered with liquid ammonia using 30 μm thin-film YSZ electrolyte. The conclusion of the study is confirming that the power densities were 299 mW/cm2 at 750 °C which is lowered than the value reported by [20] and 526 mW/cm2 at 850 °C which is sensitive to high temperature. Moreover, Dekker and Rietveld [22] have been developed AS- and ES-SOFC supplied with NH3 to examine the cell voltage as a function of current, temperature and NH3 flow. The outcome of the study realized that electrical cell efficiency powered with ammonia is increased by 13% compared with dry hydrogen. Ammonia powered SOFCs are comparatively suitable than carbon-containing fuels. This is often as a result of fuel humidification is not compulsory to protect carbon deposition and less cathode flow is possible due to the endothermic nature of ammonia cracking. Connecting with this, Shy et al. [23] have been examined the performance of power and electrochemical impedance of AS-SOFC fuelled with pressurized NH3 at diverse pressures and temperatures. Their results proved that the power densities of ammonia powered SOFCs are improving linearly and virtually equivalent to hydrogen when the temperatures and pressures of ammonia are increasing.

Liu et al. [24] have been examined the performance of SOFC powered with pure liquid methanol in comparison with NH3 and H2 fuels. The result disclosed that the peak power densities were found as 870 mW/cm2, 698 mW/cm2, and 467 mW/cm2 for H2, methanol, and NH3, respectively at 650 °C. Differently, Meng et al. [25] have been conducted a performance comparative study on a nickel-based AS-SOFC powered with H2 and NH3. The uppermost power density for hydrogen and direct ammonia as the fuel were found as 1190 mW/cm2 and 1872 mW/cm2, correspondingly at 650 °C. Even though the cathode component and electrolyte thickness are different, the performance of liquid ammonia supplied SOFC is superior to the methanol which is differing from the report of Liu et al. [24]. As well, contrary to the previous work, this study revealed that hydrogen has better performance with a significant difference over ammonia at the corresponding temperature. Besides, the power density variation of ammonia is considerably large at lowered working temperatures over hydrogen supplied cells. This is often because cracking of ammonia is an endothermic reaction that makes the actual cell temperature is dropped than the thermocouple reading. However, up to nowadays, the power densities were the maximum among the informed in the literature. This is may be interconnected with electrolyte thickness and its ionic conductivities.

Zhang et al. [26] have been manufactured AS-(NiO-YSZ) tube by extrusion method with 15 μm thin YSZ electrolytes by coating onto the anode substrate by a vacuum-assisted dip-coating method. The results realized that the uppermost power density of straight ammonia supplied cells was 200 mW/cm2 at 800 °C without NOx emissions which are virtually the same with the hydrogen-powered cell (202 mW/cm2). In line with this, Wojcik et al. [10] have been conducted an experimental work on direct ammonia fed tubular SOFCS using a YSZ electrolyte, Ag, and Pt electrodes with or without packed bed iron catalyst at 800 °C. Their studies realized that the achievement of NH3 powered SOFCs using Pt electrode without any catalyst is virtually the same that of H2 powered SOFCs. Nevertheless, Pt is awfully overpriced thence it is unviable for commercialization. As well, Hajimolana et al. [27] have developed a dynamic model of ammonia powered tubular SOFCs to investigate the ammonia cracking reaction, electrochemical reactions, diffusion, transport (heat and mass transfer), electrodes activation, and concentration overpotential. They were also considered the effects of design parameters on cell performance by considering the fuel-cell-tube temperature, dynamic output voltage and efficiency connected with the pressure of inflow fuel using different values. The outcome of their study disclosed that the inside cell tube diameter has the sturdiest impacts on cell efficiency among the considered parameters in the study. On the opposite hand, cathodic porosity has a higher impact on cell performance compared with anodic porosity.

Milad et al. [28] have been made a techno-economic comparison of AS, ES and CS-SOFCs. Their findings illustrate that when power is taken as the only objective function the CS has a maximum power density whereas when considering the materials cost as an objective function, the ES-SOFC is obtained to be the optimal choice.

Several studies regarding experimental and numerical modeling of SOFC's are mentioned above. There are also numerous studies on mathematical modeling of AS-, ES- and CS–SOFC using hydrogen and hydrocarbon fuels. Among them, tubular SOFC has higher volumetric power density, mechanical stability, high thermal shocking resistance, and no sealing issue problems. However, there is very little done on tubular SOFC powered with direct ammonia as a green fuel source. Therefore, in this work, a numerical investigation of a tubular solid oxide fuel cell running on direct ammonia in comparison with hydrogen was studied to investigate the performance of the ammonia fuelled cells. In addition, AS-, ES- and CS-SOFCs have been developed to determine which types of self-supported cell is superior in performance to use ammonia as a carbon-free fuel in tubular SOFCs. And also, the best-performed configuration is carefully chosen for further experimental investigations.

Section snippets

Modeling

In this paper, a 3D numerical model was developed to use ammonia as the primary fuel in tubular SOFC using COMSOL Multiphysics 5.3a software according to the Batteries and Fuel Cells Module present in the program. Besides, the performance of ammonia is comparing with that of hydrogen in anode-, electrolyte- and cathode supports of tubular SOFC configuration. The geometry used in the SOFC analysis is shown in Fig. 1 and the geometrical, input parameters and operating conditions are given in

Results and discussions

This study presents the performance of tubular SOFC powered by ammonia in comparison with hydrogen. This model is validated with experimental results of Wojcik et al. [10] on tubular SOFC running with direct ammonia using Pt and Ag anode electrodes with and without iron bed packed catalyst at 800 °C and Fuerte et al. [14] tested a 200 μm thick electrolyte at 800 °C. This is because there is very limited tubulated experimental work performed for ammonia powered tubular SOFC configuration in

Conclusions

Today, the most resounding challenges we are fronting are the production of pollution-free energy and combating climate changes. Whilst introducing the economy of hydrogen has abundant merits for stationary, mobile and transportation applications yet production, onboard hydrogen storing and transportation infrastructure-related problems are still the most barriers for implementation. On the contrary, hydrogen can be deposited in ammonia, methanol, ethanol and other related hydrogen contained

Acknowledgements

We gratefully acknowledge the Nigde Omer Halisdemir University, Department of Mechanical Engineering for allowing us to use their computer ansd software facilities.

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