Novel Metal Oxide Bed Materials for Efficient Solid Fuel Gasification and Gas Clean-up in Fluidized Beds

Doktorsavhandling, 2015

In this work the utilization of some novel metal oxide bed materials in dual-fluidized bed gasification systems has been investigated. For this purpose, metal oxide based bed materials can be 1) directly utilized in the dual fluidized bed gasifier (DFB) or in the Chemical Looping Combustion (CLC) process in order to achieve higher gasification rates, or 2) employed in downstream hot gas conditioning processes, e.g. Chemical Looping Reforming (CLR), in order to reform hydrocarbons in the raw gasification gas. When utilizing such bed materials directly in the primary loop of a DFB gasifier or a CLC fuel reactor, it was shown in this work that the char gasification rate can be increased by different mechanisms. First, the gasification rate can be increased by reducing hydrogen inhibition of the steam gasification reaction. This decrease is caused by the rapid reaction of the metal oxide bed materials with hydrogen produced by the steam gasification. Thus, hydrogen is effectively removed from the vicinity of the gasifying char particle. Second, the gasification rate can be enhanced by the introduction of a potent catalyst for the gasification reaction via the bed material, which is then transferred onto the gasifying char and catalyzes the steam-carbon gasification reaction. This sequence of events has been demonstrated with a Manganese ore that contained Potassium as an effective catalyst for char gasification. Low-temperature gasification processes such as the DFB process commonly produce a raw gasification gas that is heterogeneous in its composition and that contains, among other contaminants, significant quantities of condensable and non-condensable hydrocarbons. Active metal oxide bed materials can therefore be employed in a hot gas conditioning process such as the CLR process for the purpose of conditioning the raw gasification gas by reforming tars and lighter hydrocarbons. In this work, a large number of transition metal oxides were screened as potential bed materials. Based on the screening results, two promising groups of bed materials for this application were developed. The first group, Cu-impregnated alumina materials, exhibited high degrees of ethylene conversion, but did not reform monoaromatic compounds. The second group, Fe,Sr-doped lanthanum zirconates, achieved high degrees of ethylene and benzene reforming while retaining methane in the gas. The benzene and ethylene conversion could be further improved by co-feeding O2 with the gasification gas. A benzene conversion of up to 80 % and an ethylene conversion of about 95 % could be achieved at a temperature of 850°C and at a rather high Gas Hourly Space Velocity (GHSV) of 6800 h-1. Considering the expected lower costs and lower toxicity of this material compared to Precious metal- and Nickel-containing catalysts, these metal oxide bed materials are highly promising candidates for application in gasification gas conditioning with the CLR process, in particular when methane is the desired end-product.