Abstract
When using solid fuel in a chemical looping system, the char fraction of the fuel needs to be gasified before syngas react with the oxygen carrier. This can be done inside the fuel reactor with fuel and oxygen carriers well mixed, and, since this gasification is comparably slow, this will be the time limiting step of such a system. An option is to use an oxygen carrier that is able to release gas-phase oxygen which can react with the fuel by normal combustion giving a significantly faster overall fuel conversion. This last option is generally referred to as Chemical Looping combustion with Oxygen Un-coupling (CLOU). In this work, an overview is given of parameters that affect the fuel conversion in laboratory CLC and CLOU experiments. The main factor determining the fuel conversion, in both CLC and CLOU, is the fuel itself. High-volatile fuels are generally more rapidly converted than low volatile fuels. This difference in fuel conversion rate is more pronounced in CLC than in CLOU. However, the fuel conversion is also, both for CLC and CLOU, increased by increasing temperature. Increased steam and SO<sub>2<sub/> fraction in the surrounding gas will also enhance the fuel conversion in CLC. CO<sub>2<sub/> gasification in CLC appears to be very slow in comparison to steam gasification. H<sub>2<sub/> can inhibit fuel gasification in CLC whereas CO did not seem to have any effect. Possible deactivation of oxygen carriers due to SO<sub>2<sub/> or ash also has to be considered.
Highlights
Chemical Looping Combustion (CLC) is a two-step cyclic process where an oxygen carrier is cycled between two reactors
The oxygen carrier is usually a metal oxide which first is oxidized with air in the air reactor, and transferred to the second reactor, the fuel reactor, where the oxygen carrier reacts with a fuel
The oxidation of the oxygen carriers in the air reactor in the case of regular CLC is generally fast at all temperatures of interest
Summary
Chemical Looping Combustion (CLC) is a two-step cyclic process where an oxygen carrier is cycled between two reactors. The CO2 can be transported to a suitable storage facility such as a saltine aquifer [1] By this arrangement the nitrogen from the combustion air is never mixed with the fuel or the flue gases; costly and energy-intensive gas separation steps can be avoided. It is possible to avoid the generally very slow gasification step in reaction (1) and (2) by using an oxygen carrier that is able to release gas-phase oxygen in the fuel reactor according to reaction (7): 2MeXOY ⇒ 2MeXOY–1 + O2. H Leion et al / Chemical Looping Combustion of Solid Fuels in a Laboratory Fluidized-bed Reactor just as regular CLC, equal to the amount of heat released when burning the same fuel with air. In this work an overview is given of the laboratory research at Chalmers concerning CLC and CLOU with a focus on parameters that affect solid fuel conversion
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