The heat needed to drive endothermic conversion of

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Unformatted text preview: e coal gasification are autothermal processes [68]. The heat needed to drive endothermic conversion of the coal to syngas by steam is provided by the exothermic in-situ oxidation of part of the coal by oxygen. However, it is only competitive where oil and natural gas is expensive, e.g. South Africa and China [16]. 1.3.1. Steam reforming Steam reforming of hydrocarbons (STR) is up to date the dominant process for the hydrogen and syngas production [11]. Methane, light and heavier paraffins as well as naphtha can be used as feedstock. It is the most hydrogen efficient process, it is well established and it has been optimised over the last decades. Nevertheless it faces several drawbacks especially the large net energy input needed to drive the endothermic reaction. The heat required for the reaction is provided by external burning of fuel leading to a complex and bulky reactor installation and gaseous emissions of NOx and SO2 [11, 69]. Industrial catalysts used in the steam reforming are based on supported Ni [10, 11, 17, 18]. Co and several noble metals are also active catalysts, but more expensive. Even though coke formation, being the main reason for catalyst deactivation, is significantly smaller over noble metals than over Ni catalysts, Ni catalysts are still the most cost effective. This trend should increase due to the dwindling world reserves of the noble metals [70]. The Ni catalysts are activated by reduction prior to the reaction. The reaction conditions in the steam reformer are 24 1. Introduction ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff generally reaction temperatures -40 bar and steam to • ƒ& SUHVVXUHV DERXW  carbon ratios of 2-5 [10, 11, 17, 18, 71]. Due to the severe reaction conditions only thermoresistant supports are used, commonly α-alumina, magnesium oxide, magnesia aluminium spinel or zirconia [17]. The supports and the catalysts need to be silica f...
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This document was uploaded on 10/07/2013.

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