[Gasification] CO2 recycling

[doug.williams]

Hello Ferruccio

You ask:

> Has anyone got any experience on CO2 recycling within downdraft biomass 
> gasifiers?

I assume you are referring to  biomass gasifiers, in the context of using 
raw fuel, not charcoal, in which case, the direction of flow is irrelevant.

The success of recycling CO2 into a charcoal gasifier, is due to the high 
exothermic temperature available in the absence of pyrolisis gasses, but 
mainly water by way of steam.
If you introduce CO2 into the oxidation zone of a raw fuel gasifier, the 
oxidation temperatures will be suppressed, or quenched by the CO2. The lower 
temperatures will then result in a high tar content to the gas, and the 
reduction zone activity will become limited in it's ability to convert the 
CO2 into CO, and the steam into H2.

If you understand how this affects the behaviour of the oxidation bed char, 
then you can see how the CO2 would dramatically reduce the gas making, by 
suppressing the amount of endothermic heat available to the process. Adding 
exhaust CO2 clearly can be beneficial, and well proven, but only into a 
charcoal gasifier.

Hope this explains the difference.
Doug Williams,
Fluidyne.