Partial oxidation (fuel-rich combustion) with air as oxidizer is the simplest way to convert hydrocarbon fuels to hydrogen-rich syngas, which is able to be directly used by Solid Oxide Fuel Cells (SOFC). Hydrogen (H2) and carbon monoxide (CO) are the major equilibrium state products in an oxygen-deficient (fuel-rich) environment. However, the energy released from the exothermic partial oxidation reaction itself is typically not sufficient to adequately raise the reaction temperature to a sufficient extent needed to drive the reforming reactions within the available residence time. Therefore, catalysts are needed to accelerate the reaction rate and enable the reformate to reach its chemical equilibrium state, where the optimal H2 and CO concentrations are achieved. Moreover, the swiss roll combustor is insensitive to fuel contaminants such as sulfur which is a component in military logistic fuels, JP-8, and a notorious catalyst poison. Expensive catalysts and complicated subsystems are used to address the sulfur poisoning issue in catalytic-based systems but can be avoided in swiss-roll combustors. In addition, coking and degradation of catalysts are also technical challenges for catalytic-based fuel reformers.
As mentioned, non-catalytic partial oxidation (Thermal Partial Oxidation, TPOX) can avoid the catalyst issues and efficiently generate the necessary reformate composition. For TPOX systems, the use of pure oxygen as an oxidizer is one option to increase the reaction temperature needed to achieve near-equilibrium syngas yields (due to the absence of the additional thermal mass, i.e. nitrogen in the air). While such a strategy has been used in other industries for large-scale reformate production, air/oxygen separation is hardly practical at the portable scale. Other methods that allow air as an oxidizer typically require external energy sources, such as plasma, have therefore also been developed to promote the reforming reactions. However, the electricity needs for such a system are subject to efficiency loss during the conversion process and the need for an additional power source increases the size, weight, and system complexity.
Figure 1. The flare of liquid fuel n-heptane reformate. The blue color indicates no visible soot formation in the reformate.
An alternative compact, air-breathing TPOX reformer can be achieved with the Swiss-roll heat recirculating combustor. The combustor effectively recuperates the heat released from the exothermic partial oxidation to the reactants and significantly increases the reaction temperature. This enables the reformates to reach their chemical equilibrium state without the need for catalysts or external energy sources. The long spiral inlet and outlet channels provide additional benefits for the reforming reaction including enhanced mixing of reactants and increased residence time of the reaction. The Swiss-roll reformer concept has been experimentally and numerically demonstrated at ACT (Figure 2 and Figure 3). Reforming liquid fuels such as n-heptane (Figure 1) and JP-8 was also demonstrated.
Figure 2. Tests with a rich propane-air mixture. Left: Without reforming reaction in the center, the downstream flare is yellow due to the soot formation of non-premixed combustion of propane. Right: With reforming reaction in the center, the downstream flare is blue due to the absence of soot formation in an H2 and CO flame.
Figure 3. Comparison of H2 mole fraction between CFD (FLUENT) and 1D reactor (CHEMKIN) modeling. Left: CFD mole fraction contour. Right: CHEMKIN PFR mole fraction profile of the outlet channel.
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