Syngas, a combustible gaseous mixture of hydrogen, carbon monoxide, and other species, is a promising fuel for efficient energy conversion technologies. Syngas is produced by breaking down a primary fuel into a hydrogen-rich mixture in a process called fuel reforming. The motivation for the utilization of syngas rather than the primary fuel is that syngas can be used in energy conversion technologies that offer higher conversion efficiencies, e.g. gas turbines and fuel cells. One approach for syngas production is partial oxidation, which is an oxygen starved combustion process that does not require a catalyst. Efficient conversion to syngas occurs at high levels of oxygen depletion, resulting in mixtures that are not flammable in conventional combustion applications. In non-catalytic partial oxidation, internal heat recirculation is used to increase the local reaction temperatures by transferring heat from the product stream to pre-heat the fuel/air mixture before reactions occur, thus increasing reaction rates and allowing for combustion outside the conventional flammability limits. As peak temperatures lie above the adiabatic equilibrium temperature predicted by thermodynamic calculations, the combustion regime used for non-catalytic fuel reforming is referred to as 'superadiabatic'. Counter-flow heat exchange is an effective way to transfer heat between adjacent channels and is used for a novel, heat-recirculating fuel reformer design. An analytical study predicts that combustion zone locations inside adjacent flow channels adjust to operating conditions, thus stabilizing the process for independent variations of flow velocities and mixture compositions. In experiments, a reactor prototype with four channels with alternating flow directions is developed and investigated. Tests with methane/air and propane/air mixtures validate the operating principle, and measurements of the resulting syngas compositions verify the feasibility of the concept for practical fuel-reformer applications. Results from a two-dimensional numerical study with detailed reaction chemistry are consistent with experimental observations. Details of the reaction zone reveal that reactions are initiated in the vicinity of the channel walls, resulting in "tulip"-shaped reaction layers. Overall, results confirm the viability of the non-catalytic reactor design for fuel reforming applications. / text

In the effort to introduce fuel cell technology in the field of decentralized and mobile power generators, a hydrocarbon reformer to syngas seems to be the way for the market uptake. In this thesis, a potential technology is developed and investigated, in order to convert commercial liquid fuel (diesel, kerosene and biodiesel) to syngas. The fundamental concept is to oxidise the fuel in a oxygen depleted environment, obtaining hydrogen and carbon monoxide as main products of the reaction. In order to extend the flammability limit of hydrocarbon/air mixtures, the rich combustion experiments have been carried out in a two-layer porous medium combustor, which stabilises a flame at the matrix interface and recirculates the enthalpy of the hot products in order to enhance the reaction rates at ultra-rich equivalence ratio. This thesis demonstrates the feasibility of the concept, by exploring characteristic parameters for a compact, reliable and cost effective device. Specifically, a range of equivalence ratios, thermal loads and porous materials have been examined. n-heptane was successfully reformed up to an equivalence ratio of 3, reaching a conversion efficiency (based on the lower heating value of H2 and CO over the fuel input) up to 75% for a packed bed of alumina beads. Thermal loads from P=2 to 12 kW at phi=2.0 demonstrated that heat losses can be reduced to 10%.Similarly, diesel, kerosene and bio-diesel were reformed to syngas in a Zirconia foam burner with conversion efficiency over 60%. The effect of different burners, thermal loads and equivalence ratios have also been assessed for these commercial fuels, leading to equivalent conclusions. A preliminary attempt to reduce the content of CO and hydrocarbons in the reformate has been also performed using commercial steam reforming and water-gas shift reaction catalysts, obtaining encouraging results. Finally, soot emission has been assessed, demonstrating particle formation for all the fuels above phi=2.0, with biodiesel showingthe lowest soot formation tendency among all the fuels tested.

Combustion with a high surface area continuous solid immersed within the flame, referred to as combustion in porous media, is an innovative approach to combustion as the solid within the flame acts as an internal regenerator distributing heat from the combustion byproducts to the upstream reactants. By including the solid structure, radiative energy extraction becomes viable, while the solid enables a vast extension of flammability limits compared to conventional flames, while offering dramatically reduced emissions of NOx and CO, and dramatically increased burning velocities. Efforts documented within are used for the development of a streamlined set of design principles, and characterization of the flame's behavior when operating under such conditions, to aid in the development of future combustors for lean burn applications in open flow systems. Principles described herein were developed from a combination of experimental work and reactor network modeling using CHEMKIN-PRO. Experimental work consisted of a parametric analysis of operating conditions pertaining to reactant flow, combustion chamber geometric considerations and the viability of liquid fuel applications. Experimental behavior observed, when utilizing gaseous fuels, was then used to validate model outputs through comparing thermal outputs of both systems. Specific details pertaining to a streamlined chemical mechanism to be used in simulations, included within the appendix, and characterization of surface area of the porous solid are also discussed. Beyond modeling the experimental system, considerations are also undertaken to examine the applicability of exhaust gas recirculation and staged combustion as a means of controlling the thermal and environmental output of porous combustion systems. This work was supported by ACS PRF "51768-ND10 and NSF IIP 1343454.