Catalytic reforming of higher hydrocarbon fuels to hydrogen

This thesis discusses the investigation of the catalytic partial oxidation on rhodium-coated honeycomb catalysts with respect to the conversion of a model surrogate fuel and commercial diesel fuel into hydrogen for the use in auxiliary power units. CPOx on monolithic catalysts allows the auto thermal production of large quantities of hydrogen at millisecond contact times. Various logistic fuels can be converted efficiently to hydrogen, allowing the supply of fuel cells by means of the today’s infrastructure.

A new fuel feeding concept was developed and is introduced for the management of high-boiling logistic fuels. Defined transition of the liquid fuel into the gaseous phase even for temperatures above the auto-ignition point and mixing with a multitude of reactants allows the investigation of defined technical reformer parameters under accurate boundary conditions with time-resolved resolution. The set-up allows transient and steady-state experiments under varying reaction conditions with computer-assisted reproducible precision and reliability.

Two different problems arising during the CPOx reaction of higher hydrocarbons have been evaluated. Beginning with isooctane as model fuel, the significance of gas-phase reactions in the catalytic partial oxidation at short contact times and high temperatures was identified and studied in experimental and numerical terms. Special attention was given to the formation of coke precursors downstream the oxidizing catalyst zone. Two different kinds of carbon deposition zones deriving from different carbon formation pathways were identified during the slightly fuel-rich operated CPOx reaction, initiated by thermal cracking reactions of unconverted fuel as a function of temperature, feed composition, and residence time. Furthermore, die influence of certain amounts of both water and carbon dioxide were investigated over a broad range of concentrations for a CPOx reformer operating with isooctane. The experiments investigate the change in the chemical behavior for the reformer unit, when tail-gas is partly recycled from a fuel cell’s exhaust gas due to practical issues. The specific impact of water and/or carbon dioxide on the reformer’s behavior can be interpreted by the water-gas shift chemistry and, in parts, by steam reforming. Syngas composition is not dramatically affected but the formation of soot precursors can significantly be reduced, preventing the reformer from coking, even at decreased fuel conversion. However, tail-gas recycling shifts the occurrence of soot precursors towards lower C/O ratios, which is untypical for isooctane CPOx.

Commercial diesel fuel was investigated for most interesting C/O regimes in CPOx operation. Pulse-free fuel feeding without pre-combustion allows the detailed and time-resolved investigation of a logistic hydrocarbon fuel more with respect to the chemical reaction. The results revealed a rather close operation window for diesel fuel and coke formation was assigned to methane cracking even at reaction conditions, where no soot precursors were present, at temperatures close to 1450 K.