Characterization of a novel radial reactor for a solid oxide fuel cell system with anode off-gas recirculation

Content Provider	Semantic Scholar
Author	Bosch, Timo
Copyright Year	2018
Abstract	This study is part of a project in which a novel gas processor for a natural gas (NG) fueled solid oxide fuel cell (SOFC) system with a net electric power of 1 kW and anode off-gas recirculation (AOGR) has been developed. The main topic of this work is the experimental characterization of the novel reactor of this gas processor in the form of a prototype. This reactor operates as prereformer and processes NG with the aid of recirculated anode off-gas during SOFC power operation. It has the shape of a hollow cylinder with a volume of approximately 1 l, is of the type radial reactor with centrifugal z-flow and is equipped with two different packages of precious metal wire-mesh catalyst for reforming as well as with an internal electric heater. The reforming capability of the reactor is investigated in a special reactor test setup. There the reactor is tested as if it would operate within the total SOFC system with AOGR. For the tests it is assumed that the SOFC system runs on CH4 instead of NG. The experiments focus on reactor operation during the startup process of the SOFC system. For this purpose the startup procedure of the SOFC system, especially of the anode gas processor, is derived and a test rig is developed to emulate SOFC system operation with AOGR of the stand-alone prototype. The reactor prototype is equipped with 20 thermocouples to make the internal radial and axial temperature distribution transparent as well as with a differential pressure transducer to study the pressure loss characteristics during reforming. Moreover, the reactor is embedded into heating sleeves for thermal loss compensation. The composition of the product gas is investigated by means of non-dispersive infrared for CO, CO2 and CH4, a thermal conductivity sensor for H2, a paramagnetic sensor for O2 and a dew point mirror together with an absolute pressure sensor for H2O. Reforming experiments of the startup process cover reactor operation points from an oxygen to carbon ratio (ϕ) of 1.2 to 2.4. At low ϕ air is supplied to the reactor inlet in addition, which is not done at large ϕ. A Monte Carlo simulation is used to evaluate whether the reactor product gas is in equilibrium. The simulation covers the total error chain of the test rig from the gas conditioning system to the gas analysis. The evaluation shows that 23 of the 37 experiments are likely to be in equilibrium and further 13 are close to it with a volume fraction offset of less than ±0.3%. Besides the startup experiments, sensitivity tests are carried out in terms of volume flow variations at the reactor inlet and temperature variations at the reactor outlet. Furthermore, a long-term test of 75 h duration and some dynamic tests are presented. No catalyst degradation is measurable in the long-term test. A dynamic test shows that mean temperatures of the outer catalyst package above 460 °C are necessary to reach equilibrium at ϕ = 2.4 with AOGR. The catalytic ignition temperature is investigated in another set of experiments. In these experiments CH4 volume fractions at the reactor inlet ranging from approximately 10 to 30% are covered. The CH4 is supplied together with air which is partly diluted with N2. The measured ignition temperature is between 310 and 360 °C. Reactor operation after ignition shows symmetrical reactor heating-up. Chemical equilibrium is achieved at mean temperatures of the outer catalyst package above 420 °C if 3.2 slpm CH4 and 9.2 slpm air are supplied to the reactor inlet. Carbon deposits formation, especially at low ϕ when the chemical equilibrium calculation predicts its formation, is checked with an oxidation procedure. Minor carbon deposits can be measured in terms of CO and CO2 but the simultaneously measured O2 consumption does not coincide with this. Further tests indicate that reactor internal oxidation and reduction is the reason for this, most probably due to the active materials of the catalyst. A pressure loss characterization shows that the pressure losses across the catalyst packages contribute to less than 10% of the total losses of the prototype. When supplying 34 slpm dry air of 22 °C the packages show losses lower than 9 Pa. The measurements coincide with a developed pressure loss model. Furthermore from the pressure loss characterization it can be deduced that the gas flow in radial direction is homogeneously distributed over the whole flow area. Measurements of the residence time distribution of the reactor show neither dead spaces nor channeling. The distribution can be approximated by a cascade of five equal vessels, the tank-in-series model. During SOFC system startup the reactor operates as air preheater using its internal electric heater. The homogeneity of heat input of this internal heater is validated by using an infrared camera in addition to the reactor internal thermocouples.
File Format	PDF HTM / HTML
DOI	10.17185/duepublico/47703
Alternate Webpage(s)	https://duepublico2.uni-due.de/servlets/MCRFileNodeServlet/duepublico_derivate_00046782/Diss_Bosch.pdf
Alternate Webpage(s)	https://doi.org/10.17185/duepublico%2F47703
Language	English
Access Restriction	Open
Content Type	Text
Resource Type	Article