Highly Efficient Conversion of Ammonia in Electricity by Solid Oxide Fuel Cells

Abstract

Hydrogen is the fuel for fuel cells with the highest cell voltage. A drawback for the use of hydrogen is the low energy density storage capacity, even at high pressures. Liquid fuels such as gasoline and methanol have a high energy density but lead to the emission of the greenhouse gas CO2. Ammonia could be the ideal bridge fuel, having a high energy density at relative low pressure and no (local) CO2 emission. Ammonia as a fuel for the solid oxide fuel cell (SOFC) appears to be very attractive, as shown by cell tests with electrolyte supported cells (ESC) as well as anode supported cells (ASC) with an active area of 81cm2. The cell voltage was measured as function of the electrical current, temperature, gas composition and ammonia (NH3) flow. With NH3 as fuel, electrical cell efficiencies up to 70% (LHV) can be achieved at 0.35A∕cm2 and 60% (LHV) at 0.6A∕cm2. The cell degradation during 3000 h of operation was comparable with H2 fueled measurements. Due to the high temperature and the catalytic active Ni∕YSZ anode, NH3 cracks at the anode into H2 and N2 with a conversion of >99.996%. The high NH3 conversion is partly due to the withdrawal of H2 by the electrochemical cell reaction. The remaining NH3 will be converted in the afterburner of the system. The NOx outlet concentration of the fuel cell is low, typically <0.5ppm at temperatures below 950°C and around 4ppm at 1000°C. A SOFC system fueled with ammonia is relative simple compared with a carbon containing fuel, since no humidification of the fuel is necessary. Moreover, the endothermic ammonia cracking reaction consumes part of the heat produced by the fuel cell, by which less cathode cooling air is required compared with H2 fueled systems. Therefore, the system for a NH3 fueled SOFC will have relatively low parasitic power losses and relative small heat exchangers for preheating the cathode air flow.

Direct Ammonia Alkaline Anion-Exchange Membrane Fuel Cells

“…From this point of view, to generate electricity directly from ammonia would be more efficient. Ammonia has also been investigated as a fuel for high temperature solid oxide fuel cells ͑SOFCs͒; 9,10 however, SOFCs are in general not suitable for transport application due to the slow startup and brittleness of ceramic components.…”

Ammonia as a Contaminant in the Performance of an Integrated SOFC Reformer System

“…Recently, a number of research groups [9][10][11][12] have been showing great interest in NH 3 as a H 2 carrier or a direct fuel for fuel cells, which are, however, beyond the current scope of this study.…”

“…Based on the experimental results using silver anode and platinum anode with or without ironbased catalyst, Wojcik et al [14] predicted that NH 3 could work very well in an SOFC system based on nickel anodes, although no actual experimental work has been conducted on nickel anode SOFC in their work. NH 3 performance in a SOFC with Ni/8YSZ anode was studied by Dekker et al [10]. In their cell tests, the fuel cell outlet gas was measured and analyzed for NO x and NH 3 to determine the NH 3 conversion.…”

“…At low NH 3 concentrations, reaction mechanism for NH 3 decomposition can be described using the following sequence [10,23]:…”

“…The former studies on NH 3 conversion are limited to either reforming processes or the fuel cell [10,[12][13][14][15]] but none on a system level basis. Very little litereature work can be found to have detailed study on NH 3 reaction at very low partial pressure (<2.6 torr) in the reformer.…”