Chemical simulation of hydrogen generation in a plasma fuel reformer

Abstract

A model for a plasma fuel reformer or plasmatron has been developed. The model was based in a series of experiments realized at the Plasma Science and Fusion Center with such a plasmatron. The device is set up to produce syngas (hydrogen and carbon monoxide gas mixture) from partial oxidation of any hydrocarbon. We studied the behavior of methane as fuel and used the GRI methane combustion mechanism in our simulations. The goal was to characterize the reactor to be able to understand and predict its performance for a wide range of operating conditions, such as different flow rates, air to fuel flow ratio or power supply. The simulation tool used for this purpose was CHEMKIN 3.7. The fuel reformer was designed as a reactor where combustion is initiated by an electric discharge due to ohmic heating of the arc region. Two different types of reactors were used to model the plasmatron. The Plug Flow Reactor (PLUG) assumed a homogenous zero-dimensional closed system. The Partially Stirred Reactor (PASR) considered random mixing determined by a frequency mixing parameter, which is directly dependant of the system fluid dynamic properties. Experimental results with methane generated 6%-7% molar of hydrogen and 5% of carbon monoxide. Hydrogen and oxygen balance evidenced that water and carbon dioxide are important co-products, obtaining respectively 10% and 3% at the exhaust. Also 15%- 20% of methane and 3%-5% of oxygen remained unreacted. From discharge observations, energy estimations and model simulations, it was found that the electric arc initiates combustion by locally rising the temperature and then propagating the reaction by heat and mass transfer/mixing to the surroundings.

(cont.) Simulation results demonstrated that there is an optimum characteristic mixing time for each residence time, depending on the initial temperature reached at the arc. It was also found that the more spread the energy is, or the more mass is heated to a moderate temperature, the better the performance results.