Analysis of Ammonia Decomposition Reactor to Generate Hydrogen for Fuel Cell Applications

Abstract  

In this paper, reaction engineering principles are utilized to analyze process conditions for producing sufficient hydrogen in an ammonia decomposition reactor for generating net power of 100 W in a fuel cell. It is shown that operating the reactor adiabatecally results in a sharp decrease in temperature due to endothermic reaction, which results in low conversion of ammonia. For this reason, the reactor is heated electrically to provide heat for the endothermic reactions. It is observed that when the reactor is operated non-adiabatically, it is possible to get over 99.5% conversion of ammonia. The weight of absorbent to reduce ammonia to ppb levels is calculated. An energy balance on the reactor exit gas indicates that there is sufficient heat available to vaporize enough water to achieve 100% relative humidity in the fuel cell. A suitable fuel cell stack is designed and it is shown that this stack is able to provide the necessary power to electrically heat the reactor and produce net power of 100 W.

Keywords: Ammonia decomposition Hydrogen generation Fuel cell

Introduction

There is current interest in the development of technologies that provide alternatives to conventional batteries in the 100 W range to power portable devices in remote areas, where access to the power grid is limited. In particular, the development of power systems based on polymer electrolyte membrane (PEM) fuel cells that utilize hydrogen to produce power can provide thermodynamic and environmental advantages [1]. Fuel cells can operate for longer durations as compared to batteries and are only limited by the size of the fuel tank for generating power continuously. While the ultimate goal is to use renewable resources to generate hydro- gen for use in a fuel cell stack to produce power, there are currently many barriers to the hydrogen economy because the issue of efficient storage and transport of hydrogen is not yet resolved. For this reason, there is considerable interest in utilizing fuel processing technologies to generate hydrogen in situ on an “as needed” basis. Fuel processing technologies convert a hydrogen containing mate- rial into a hydrogen rich stream [2]. One popular fuel processing technology involves steam reforming of hydrocarbons and there is a considerable amount of literature in this area that describes the fabrication of small reactors for mobile power applications. Pattekar and Kothare [3] fabricated a radial flow micro-packed-bed reactor via deep reactive ion etching that utilizes methanol to generate sufficient hydrogen for a 20 W power application. Sohn et al. [4] developed a plate-type integrated fuel processor-PEM fuel cell where methanol is reformed to produce up to 150 W    of power. Tan et al. [5] developed a methane processing system for producing high-purity hydrogen for 10–1500 W power applications. Lindstrom et al. [6] developed a diesel fuel reformer for generating hydrogen for an auxiliary power unit in a truck. Chu et al. [7] developed a compact reformer that utilizes natural gas or propane to deliver up to 3 kW of power. Kolb et al. [8] developed a microstructure reactor that uses iso-octane as a hydrocarbon source for producing hydrogen for mobile auxiliary power units. A review of reformers that convert hydrocarbon fuel to hydrogen for fuel cell applications is available in Kundu et al. [9].

The use of hydrocarbons for producing hydrogen typically leads to the production of carbon monoxide, which poisons  the  PEM fuel cell catalyst. Furthermore, sulfur compounds in the hydro- carbons also present operational difficulties in the fuel cell. To alleviate these problems, it is necessary to add multiple processing steps such as water–gas shift reactions, methanation, oxidation and desulfurization. A viable alternative is to use ammonia as a source of hydrogen. Pure ammonia has an energy density of 8.9 kWh kg−1, which is higher than methanol (6.2 kWh kg−1), but less than diesel or JP-8 (13.2 kWh kg−1) [10]. It is an inexpensive fuel that has an extensive distribution system [2]. Ammonia decomposition to hydrogen occurs in a single reaction step and there is no carbon monoxide or sulfur in the product stream. It has a strong odor, which makes leak detection simple [11]. Powell et al. [12] fabricated an integrated 50 W, 100 Wh ammonia cracker-PEM fuel cell prototype. While the potential for using ammonia as a hydrogen carrier has been recognized and small-scale reactors have been fabricated, the literature is very sparse in the area of modeling and analysis of reactors that utilize ammonia to produce hydrogen for fuel cell applications. In this paper, a packed-bed reactor is analyzed in which decomposition ammonia occurs to produce hydrogen. In particular, the reactor is modeled as a non-isothermal, non-isobaric packed-bed reactor. The reactor is heated electrically Reactor conditions are analyzed to generate sufficient hydrogen that can be used in a polymer electrolyte (PEM) fuel cell to produce sufficient power to heat the reactor and a net usable power of 100 W. A schematic of the process under consideration is shown in Fig. 1.

Fig. 1. Schematic of reformer and fuel cell system.