Production of hydrogen by unmixed steam reforming of methane

Abstract

Unmixed steam reforming is an alternative method of catalytic steam reforming that uses separate air and fuel–steam feeds, producing a reformate high in H 2 content using a single reactor and a variety of fuels. It claims insensitivity to carbon formation and can operate autothermally. The high H 2 content is achieved by in situ N 2 separation from the air using an oxygen transfer material (OTM), and by CO 2 capture using a solid sorbent. The OTM and CO 2 sorbent are regenerated during the fuel–steam feed and the air feed, respectively, within the same reactor. This paper describes the steps taken to choose a suitable CO 2 -sorbent material for this process when using methane fuel with the help of microreactor tests, and the study of the carbonation efficiency and regeneration ability of the materials tested. Elemental balances from bench scale experiments using the best OTM in the absence of the CO 2 sorbent allow identifying the sequence of the chemical reaction mechanism. The effect of reactor temperature between 600 and 800 ∘ C on the process outputs is investigated. Temperatures of 600 and 800 ∘ C under the fuel–steam feed were each found to offer a different set of desirable outputs. Two stages during the fuel–steam feed were characterised by a different set of global reactions, an initial stage where the OTM is reduced directly by methane, and indirectly by hydrogen produced by methane thermal decomposition, in the second stage, steam reforming takes over once sufficient OTM has been reduced. The implications of these stages on the process desirable outputs such as efficiency of reactants conversion, reformate gas quality, and transient effects are discussed.