Invariant properties of solid oxide fuel cell systems with integrated reformers

•Fuel utilization and steam-to-carbon-balance in SOFCs satisfy invariant properties.•Analytically proven to be invariant in SOFCs for a class of hydrocarbon fuels.•Developed notion of conservation of potential hydrogen and steam during reforming.•Existence of invariant properties linked to aforementioned conserved variables.•Derived operating conditions ensuring safe utilization and steam-to-carbon-balance.

In reformer based SOFCs (solid oxide fuel cells) with hydrocarbon fuels, fuel utilization (U) and STCR (steam-to-carbon-ratio) are important variables indicating conditions inside the anode and reformer respectively. However, both variables are difficult to measure due to their dependence on internal species concentrations, temperatures, pressures and flow rates. In contrast to system-specific model-based techniques for estimating U and STCR, this paper proposes a generalized method for formulating and characterizing these variables, that applies to hydrocarbon fuels of the form CaH2bOd, multiple reformer types, and system configurations. The approach takes advantage of the invariance of quantities, such as potential hydrogen and STCB (steam-to-carbon-balance), with respect to reaction pathways, reaction rates, and aforementioned internal conditions. These conserved quantities can be predicted under steady-state conditions in a model-independent fashion. The invariant relationships so obtained are useful for predicting U and STCB using only system level inputs, namely current, supply rate of unreformed fuel, and rate of recirculation, and without requiring expensive and intrusive sensors. They are also useful for maintaining U and STCB at target values and in addressing their transient fluctuations.