[Gasification] raising H2 concentration in downdraftgasification

Kevin kchisholm at ca.inter.net
Fri Dec 20 02:49:02 CST 2013

Dear Rex

In principle, the way to increase the Hydrogen content of "Wood Gas" is to 
add external heat to the gasifier inputs, and to reduce the "input heat 

Actions that accomplish these goals would be :
1: Dry the wood.
2: Preheat the wood.
3: Preheat the air.
4: Add superheated steam to the air

Best wishes,


----- Original Message ----- 
From: "Rex Zietsman" <rex at whitfieldfarm.co.za>
To: <gasification at lists.bioenergylists.org>
Sent: Friday, December 20, 2013 3:32 AM
Subject: Re: [Gasification] raising H2 concentration in 

> Hi All,
> The deafening silence on this post is quite interesting. At the very least 
> I
> would have thought that someone would have suggested adding steam to the
> reduction zone. What about a charcoal gasifier with steam addition?
> Any comments on a 1.6MW/2MW thermal gasifier?
> Kind regards
> Rex
> -----Original Message-----
> From: Rex Zietsman [mailto:rex at whitfieldfarm.co.za]
> Sent: 16 December 2013 09:57 AM
> Subject: raising H2 concentration in downdraft gasification
> Hi All,
> We are looking at a system that will hydrotreat bio oils from pyrolysis,
> catalytic cracking and vegetable oil. For this we need in the region of 
> 30kg
> hydrogen/ton oil. At small scale ie 1 ton oil/hour, this works out at, you
> guessed it, 30kg of hydrogen/hour. As this is a small amount in the 
> overall
> scheme of things, we are looking at gasifying wood chips and to recover 
> the
> hydrogen using pressure swing absorption. What I would like to know is
> whether we can increase the H2 concentration in syngas by tweaking the
> gasifier. Clearly we can look at the water gas shift reaction but, as the
> syngas has to be cooled, washed, pressurised and reheated, it is quite an
> additional investment for the scale we are looking at. If we could simply 
> up
> the H2 content, we would go straight from washing to PSA. Residual gas 
> would
> be piped to a diesel generator where CO and the like will be combusted 
> prior
> to exhaust to atmosphere.
> For easy mental arithmetic, let us assume a 33% H2 concentration in dry
> syngas. 30kg/hour of H2 is 15kmol/hour or 15/0.33 = 45kmol/hour of syngas. 
> A
> kmol of gas has a volume of 22.4 Nm3. So, to get 30kg/hour requires 22.4 x
> 45 = 1000 Nm3 syngas/hour (mental arithmetic here, go with the flow).
> Assuming an 80% PSA recovery this means that we need 1250Nm3/hour of 
> syngas.
> Not a bad sized downdraft gasifier! Assuming 6MJ/Nm3, this is around a 2MW
> thermal unit. If we can get the H2 concentration up to say 40%, then the
> syngas requirement would be 37.5kmol/hour or 37.5/45 x 1250 = roughly 80% 
> of
> 1250 or roughly 1000 Nm3 syngas/hour. This reduces the size of gasifier to
> 1.6MW thermal and more sensible in size.
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