Solid fuels in Chemical-Looping Combustion – Feeding of fuel and distribution of volatiles

Övrigt konferensbidrag, 2015

Conventional CO2 capture processes have large costs and energy penalties associated with gas separation. Chemical-looping combustion (CLC) uses interconnected fluidized beds and a metal oxide to transfer oxygen from air to fuel. Thus, fuel is oxidized without mixing fuel and air and the combustion products, CO2 and H2O, are recovered in a separate flow. After H2O condensation essentially pure CO2 is obtained, thus avoiding the high costs and penalties of an active gas separation. CLC of solid fuels has important similarities to well-established combustion in circulating fluidized bed (CFB), and a comparison indicates an added cost of 16-26 €/tonne CO2. The major cost besides CO2 compression is oxygen-polishing of the gas from the fuel reactor, indicating that high gas conversion is beneficial. Today, >2000 h of solid-fuel CLC operation in smaller pilots have been accomplished worldwide. The experiences show that the concept works in practice and that high gas conversion is reached with lowvolatile fuels, typically 95%. However, fuels with more volatiles show lower conversion, highlighting the need to feed the fuel in way that provides good contact between volatiles and bed material, i.e. the metal oxide oxygen-carrier. For a larger size CLC the fuel should be fed in a way that make the volatiles enter the bed in the lower part and well distributed over the horizontal cross-section. Here, a system for distribution of volatiles is presented. It is based on a fundamental principle of fluidization, that a box immersed in a fluidized bed with the opening downward becomes empty. Moreover, if such a box has holes in its sides the bed level inside the box will rise but not above the holes and gas added will exit through these holes. Such a box in the form of a system of arms is proposed to distribute volatiles over the cross-section. The paper discusses principles and possible design of such a volatiles distributor and how it can be implemented in a large-scale CLC.