Design and Testing of a Hot Gas Blower for Solid Oxide Fuel Cell Applications

The study focuses on the design and testing of a centrifugal blower to be used for recirculating hot anode off-gas in solid oxide fuel cells (SOFC). SOFCs operate at high temperatures of up to 800 °C and typically, the anode off-gas is treated as exhaust gas and led into a stack. However, nowadays it is commonly known that the efficiency of the SOFC system can be improved by recirculating the anode off-gas.

Up until recent years, the problem with the recirculation has been that there are no suitable blowers feasible enough to sustain the heat of the anode off-gas. In this study, a 500 W centrifugal blower having a design pressure ratio of 1.126 was designed and tested using a 50 kW SOFC application as a basis of the design, but the designs can also be applied to other applications, e.g. recirculation needs of hot air.

The blower was based on the principle of high-speed technology where one characteristic feature is that the rotational speed is much greater than the synchronous speed corresponding to the frequency of 50 Hz or 60 Hz of the electric grid. The compressor impeller and the electric motor were coupled on a common shaft such that there is no need for a gearbox. In this project, the idea was to use contactless oil-free bearings, and the use of gas bearings and active magnetic bearings were considered.

In the gas blower design, different geometries of 2D and 3D impellers were analyzed computationally using the rotational speeds of 60000 rpm and 90000 rpm.  Based on the results of the analysis, a 2D impeller design was adopted and the lower rotational speed of 60000 rpm was selected as a basis of the blower design when also the results of the computational analyses related to rotor dynamics and mechanical design were considered.

One of the most important phases in the design process was the constructional design entailing the bearings and the cooling arrangements where the special requirements related to hot gas or hot air applications must be taken into account. An external cooling fan was used. The target was to prevent hot gas or hot air leakages from the blower as well as to minimize heat transfer from the hot blower parts to the electric motor in order to keep the temperatures of the electric motor parts low enough.

The blower was tested first with air at room temperature and then with air at about 300 °C. In the design phase, cooling was found one of the main challenges when hot inlet air is used. The results obtained from the test runs were well-matched with the design values and the blower operated successfully for several hours at the inlet air temperature of about 300 °C.