SOFC Internal Reforming via Anode Tail Gas Recycling
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Solution Overview
Problem
Existing solid oxide fuel cell (SOFC) systems face inefficiencies due to low fuel processing efficiency, carbon precipitation, and complexity in recycling anode tail gas, particularly requiring external water for internal reforming.
Innovation Solution
Recycling a portion of the anode tail gas directly into the SOFC stack, combined with secondary non-reformed fuel, allows for internal reforming without external water, reducing carbon precipitation and system complexity, and optimizing fuel mixture for efficient endothermic reactions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a CPOx reformer is used to supply reformate to the SOFC stack, then the system is simple and easily controlled, but the fuel processing efficiency is low (70-90%)
Solution Approach 1:
The patent combines the CPOx reformer with an internal reforming section within the SOFC stack itself. The reformate from the CPOx reformer is mixed with unreacted fuel and recycled anode exhaust, then fed into the stack where additional reforming occurs internally using the electrochemical heat generated during operation. This merging allows the system to maintain the simplicity of the CPOx reformer while achieving higher fuel processing efficiency through the added internal reforming capability.
2Productivity
If steam reforming is performed in a heat exchanger with combustor effluent, then fuel processing efficiency is high, but the system complexity increases and durability issues arise
Solution Approach 1:
The patent employs internal reforming within the SOFC stack where the electrochemical reactions themselves provide the heat required for the endothermic reforming processes. The stack generates its own thermal energy through operation, which is then used to drive the reforming reactions internally. This self-service approach eliminates the need for external heat exchangers and combustor effluent, thereby reducing system complexity and potential durability issues while maintaining high fuel processing efficiency.
3Productivity
If internal reforming is performed within the SOFC stack, then fuel processing efficiency is high and system complexity is reduced, but external water injection is required
Solution Approach 1:
The patent recycles the anode exhaust gas, which contains unreacted fuel, water vapor, and carbon dioxide, back to the reformer inlet. This recycled stream provides the necessary water vapor for the reforming reactions without requiring external water injection. By recovering and reusing the water-containing exhaust stream, the system achieves internal reforming while eliminating the need for separate water injection systems.
4Productivity
If anode tail gas is recycled to the reformer inlet, then fuel processing efficiency improves, but carbon precipitation risk increases
Solution Approach 1:
The patent introduces a mixing section where the recycled anode exhaust is combined with fresh reformate before entering the internal reforming zone. This intermediary mixing step ensures proper dilution and distribution of the recycled stream, preventing localized conditions that could lead to carbon precipitation. The mixing also optimizes the composition for efficient reforming while maintaining conditions that prevent carbon deposition.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances fuel processing efficiency, reduces thermal demands, and simplifies system construction and operation by achieving high fuel processing efficiencies and reducing carbon precipitation, while maintaining high oxygen/carbon ratios and water vapor content in the anode tail gas.
Implementation Method 1
endothermic reforming of residual hydrocarbons from the CPOx reformer occurs within the stack
Implementation Method 2
the heat to required for endothermic reforming to occur is supplied by the electrochemical heat released in the SOFC stack
Implementation Method 3
A CPOx reformer is a very simple and easily controlled device with good transient behavior and dynamic range. The CPOx reactions also release heat that serves to maintain the temperature of the reformer
Data Source
AI summary
In an SOFC stack system, wherein a CPOx reformer supplies reformate to the stack, a portion of the anode tail gas is recycled directly into the anode inlet of the stack, such that the fuel reaching the anodes is a mixture of fresh reformate and recycled anode tail gas and is present at a sufficiently high temperature that endothermic reforming of residual hydrocarbons from the CPOx reformer occurs within the stack. Preferably, an amount of secondary non-reformed fuel is also added to optimize the fuel mixture presented for internal reforming. The anode tail gas is hot, at the stack temperature of 750-800° C., which allows for the mixture of anode tail gas and secondary fuel to be mixed and reacted in a clean-up catalyst to react higher hydrocarbons in the secondary fuel, without additional oxygen, prior to being mixed with reformate and sent to the stack.