Multi-fuel processor – a key component in harmonized SOFC system architecture

Solid Oxide Fuel Cells (SOFC) fall under the category of high temperature fuel cells, operating between 600 and 800 oC and can have very high electrical efficiencies of around 60% and a combined heat and power efficiency of around 90%. Utilising the high-quality heat of the SOFC system for downstream processes provides for useful use of residual thermal energy, which would otherwise be dissipated to the environment, and thus provides for increased combined heat and power efficiency.

The SOFC stack itself is capable of multi-fuel operation, fuels such as hydrogen, methane, methanol and ammonia can be used, and this gives the SOFC technology advantages over other fuel cell technologies. Depending on the fuel used in the stack, the SOFC system architecture can vary to incorporate the nuances of fuel reforming, exhaust gas treatment and internal heat utilisation.

The maritime industry is still in its nascent stages when it comes to decarbonisation and emissions reduction and thus there is plenty of scope for several alternate viable technologies to make inroads as main powertrains on-board ships and vessels. When it comes to deep sea shipping or ocean-going vessels, possible alternatives to diesel engines are literally non-existent. However, SOFC technology seems like a promising candidate for this particular category of the maritime industry due to its fuel flexibility and its high electrical efficiency.

One of the goals of the FuelSOME project is to demonstrate an SOFC system with a single architecture that can use multiple fuels – ammonia, methanol and hydrogen. For this to happen, the SOFC stack needs to operate with near similar performances when using these three different fuels and this is exactly what our stack partner Elcogen is trying to do. Elcogen is manufacturing both short stacks and full stacks for the FuelSOME project and will conduct experimental studies on stack performance and operation together with VTT. Other than hydrogen, the other two fuels – ammonia and methanol cannot be directly fed into the SOFC stack and will warrant a fuel processor. Also, the exact degradation mechanisms of the stack components are not very well understood when using ammonia and methanol as fuels. Therefore, it is absolutely vital to design, develop and test a fuel processor which can provide the required fuel quality at its output which in turn can be safely used in the stack. To harmonise with the SOFC system architecture it is also absolutely quintessential that the fuel processor is able to handle multiple fuels so that the fuel processing part in the system architecture can stay the same irrespective of the fuel used in the SOFC system.

VTT has made great strides in the design and development of the multi-fuel processor which can handle both methanol and ammonia. The prototype that is being developed will be used in conjunction with a 6 kWSOFC stack and later incorporated into the system and is able to harness the excess thermal energy coming out from the SOFC stack operation so that the fuel cracking operation in the multi-fuel processor can happen as efficiently as possible. 

For an initial approach, the multi- fuel processor is designed to be a shell and tube heat exchanger type packed bed reformer/cracker. The concept picture below shows the idea of the structure of the fuel processing unit with the fuel and air flows

The arrows in the picture shows directions and temperatures of the flow in the unit. the used catalyst is packed inside the fuel flow channels which will be heated with the air flow from the SOFC outlet. The experimental work with the catalyst is ongoing in a lab test setup to define the correct catalyst loading, heating power and optimal working temperature of the multi-fuel processor.

heated backed bed reformer.

The figure below shows some initial exciting results of the ammonia cracking percentage rate versus the catalyst bed temperature.

The tests were done with a Nickel based commercial catalyst under flow conditions of 4l/min and 100% ammonia concentration together with 8000 GHSV (Gas Hourly Space Velocity) ratio. As seen from the figure, the Nickel catalyst is able to crack almost 99% of the ammonia at 540 oC and this is very promising because the temperature of the residual heat coming out from the SOFC stack is almost 150 oC higher than this and this allows using the thermal energy from the stack.

The next steps will be testing of the same catalyst for methanol reforming and to check if the output gas meets the fuel quality standards of the SOFC stack operation and also to investigate the temperature at which full reforming or more than 99% reforming of methanol can be achieved. Once these tests are complete, the mechanical structure of the multi-fuel processor will be finalised and CFD simulations performed.

VTT believes that this initial prototype designed for a 6 kWe SOFC stack is scalable to higher sizes and believes that this will be a key component to make SOFC system truly multi-fuel compatible. Once the multi0fuel processor is ready and tested, it will be integrated into the system that AVL List GmbH will build.

Author: Saxelin Santeri (VTT), Vikrant Venkataraman (AVL List)