[Gasification] dioxin

[Kevin Chisholm]

Dear Toby

Toby Seiler wrote:
> Doug, Kevin and list,
>    
>   When you mention the formation of dioxin, you say;
>    
>   "it is that easy, but can be overcome "IF" you have a high performance tar cracking gasifier system. If output gas contains CH4 in amounts over say 3%, you can use that as a marker, that the gas will carry dioxin over to it's combustion phase."
>   


I think that what Doug is driving at is that if all the tars and carbon 
are burned, that a residual CH4 of 3% or greater would be an indicator 
that there was sufficient relatively free C and H to react with any Cl 
present, to form dioxins. BUT, that if the CH4 level was below 3%, then 
the likelihood of dioxin formation would be very small.
>    
>   I did not intend on implying that a torrefaction pellet system should use any plastic for commodity fuel production, but that if it did make a performance blend, the burner had better be high temperature.  I've noted that a small content of plastic may increase the energy content by around 30% or so.  However that fuel would be limited to use in certain "high performance" burners or gasifiers.  And are there other constituents that could make the fuel blend even more high performance, even somewhat explosive or promoting of rapid oxidation??
>       
>   What would be the difference of "high performance tar cracking" and other high temperature combustion?  Is there a threshold temp that the dioxins break down?    
>   Are you saying in a high temp system that the ash will contain dioxin and that it is unavoidable if the plastics contain chlorine?  Or is this mainly a problem with lower combustion or gasification temperatures?  Please clarify this for me.
>   

As I understand it, dioxins are formed after combustion, as the gases 
are cooling. The intensity of combustion, or lack thereof,  is not so 
much a concern as whether or not there are dioxin forming reactants in 
the cooling gases. As I understand it, dioxins are formed at 
temperatures somewhere in the range of about 200 to 300C.  As I further 
understand it, combustion gases containing HCl or Cl can be passed 
through a lime bed, to trap teh HCl or Cl, and prevent formation of dioxins.

Hopefully someone more knowledgeable than I am could confirm or correct 
my impressions.

Best wishes,

Kevin
>    
>   Regards,  Toby Seiler, STC   
>
>
>        
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