Hydrogen is the primary fuel source for fuel cells. However, the low volume density and difficulty in storage and transportation are major obstacles for the practical utilization. Among various hydrogen carriers, ammonia is one of the promising candidates because of its high hydrogen density and boiling point and ease in liquefaction and transportation. The reaction temperature of ammonia cracking to nitrogen and hydrogen, being about 600°C or higher, is close to the operating temperature of solid oxide fuel cells (SOFCs). The integration of these two devices is beneficial in terms of heat and energy managements and will lead to the development of simplified power generation systems.
Several catalyst materials, i.e., cracking, autothermal cracking, and combustion of ammonia have been investigated and used for the SOFC systems. A demonstration of the stack-level ammonia-fueled SOFC systems is an important step for the actual utilization of ammonia-fueled SOFCs. In this study, 200 W class and 1 kW class SOFC stacks were applied for ammonia fueled generation systems.
Catalytic decomposition of ammonia
Catalysts for decomposition of ammonia has been developed. The activity of Ni/Y2O3 and SrO modified Ni/Y2O3 for ammonia cracking was sufficiently high for combination with SOFCs. The SrO modified Ni/Y2O3 catalyst showed higher performance than Ni/Y2O3 and achieved complete decomposition at 600ºC. The ammonia decomposition completed at lower temperatures than that of the typical operation of SOFC (750ºC). This means the ammonia cracking can proceed without an external heating in the integrated system with SOFCs. This catalyst maintained high activity during the operation for 1000 h at 700ºC.
Autothermal decomposition reactor
Catalyst based on Co-Ce-Zr composite oxide has been developed for autothermal ammonia cracking. Autothermal cracking of ammonia is characterized by fast start-up in the exothermic condition. It is necessary to operate the autothermal ammonia cracker at a suitable air to fuel ratio (AFR) condition, considering the electrical efficiency as well as the amount of heat generated from the cracker. The AFR value for the supply of the autothermally cracked ammonia to the SOFC stack was set to be 0.75, since a sufficient heating value and a high ammonia conversion could be expected. The temperature increased at the initial stage with a honeycomb shaped metal heater until the inlet temperature of the catalyst honeycomb reached at 200ºC. Then, the temperature continuously rose up without electrical heating with the exothermic reaction in the catalyst honeycomb. The start-up time required from the initiation of electrical heating to the achievement of the steady state was 130 s. Therefore, it is concluded that this reactor is suitable for start-up and heating of the SOFC system.
SOFC stack fueled with ammonia
The ammonia fueled SOFC stack systems were evaluated. The cell consisted of a Ni/ZrO2-based fuel electrode, ZrO2-based electrolyte, and perovskite-type oxide air electrode. The planar single cell was 120 mm in diameter. Each cell was connected to metallic separator plate with internal manifolds and gas tightness was ensured with a glass sealant. The stack composed of 10 single cells was evaluated from its I–V and I–P characteristics at 770°C. Four fuel supply systems were connected to the stack, i.e., 1) direct supply of dry ammonia, 2) the ammonia cracker equipped with the Ni/Y2O3 catalyst operated at 600°C, 3) the autothermal ammonia cracker equipped with the catalyst honeycomb operated at an AFR of 0.75, 4) The mixture of hydrogen and nitrogen with a composition of 3 to 1 for comparison. The open circuit voltages of the direct ammonia and cracked ammonia fueled SOFC stacks were almost the same as that of the hydrogen fueled stack. This means, for the direct ammonia fueled stack, the supplied ammonia was almost completely decomposed into hydrogen and nitrogen over the anode. In the case of autothermally cracked ammonia, the open circuit voltage of stack was slightly lower than the other fuel supply systems because of steam in the supply gas. The electrical power achieved with this 10-cell stack was about 250 W. The performances for the SOFC stack was comparable for four fuel supply systems.
One kW-class SOFC stacks and packaged system
The stack consisting of 30 single cells was tested at ca. 750ºC. Since the performance of the cells has been improved, the power per cell in this stack has also been improved as compared with the cells used in 10-cell stack. The power of 1073 W can be achieved with the supply of dry ammonia at the current of 50 A which was almost the same as the supply of 3H2+N2 mixture at the same current. The average cell voltage at this operation point was 0.715 V. The DC efficiency was 52.3% at the fuel utilization of 80%. The 1 kW class stack was successfully operated for 1000 h. The 1kW-class stack was also operated by supplying the reacted gas from autothermal ammonia decomposition reactor.
The packaged SOFC system with automatic start-up and shut-down sequences was tested. Thermally self-sustainable 1 kW system was successfully operated.
Development of catalysts for ammonia combustion
For heating of the system and cleaning of ammonia containing exhaust, ammonia combustion catalysts have to be developed. The activity for catalytic combustion of ammonia was higher over supported Pd catalysts than over Pt or other precious metal catalysts. It is important for combustion catalyst to achieve high selectivity to N2 without producing N2O, NO, NO2, and other NOx species. On most of the supported Pd catalysts, combustion of ammonia started from 200ºC followed by steep enhancement of conversion. The selectivity to N2 and NOx species depended strongly on the support oxide. It was found that Pd supported on zeolite is effective in combustion of ammonia without emitting NOx species and N2O.
This work was supported by the Council for Science, Technology and Innovation (CSTI) Cross-ministerial Strategic Innovation Promotion Program (SIP) “energy carrier” (Funding agency: JST).