[Gasification] raising H2 concentration in downdraftgasification

[Rex Zietsman]

Thanks Kevin.  I am pleased that I have not missed something while I was not
paying attention. One item missing on the list is to use oxygen enriched
air. A while back one of the membrane guys suggested that getting to 28% O2
was economically feasible as the pressures required for separation are
relatively small. 27% does not sound like much of an improvement until you
realize how much nitrogen reduction has taken place. In air, the N:O ratio
is roughly 79:21. With slight enrichment, this goes to 72:28. If we start
with 1Nm3 of air, we reduce this to 75% air or we lose 25% of the starting
nitrogen. This reduces the dilution significantly with all the energy
benefits resulting from this. Then, as you have noted, the H2 concentration
also rises purely as a result of this.

Cheers
Rex 

-----Original Message-----
From: Kevin [mailto:kchisholm at ca.inter.net] 
Sent: 20 December 2013 10:49 AM
To: Discussion of biomass pyrolysis and gasification
Subject: Re: [Gasification] raising H2 concentration in
downdraftgasification

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
loads".

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,

Kevin

----- 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
downdraftgasification


> 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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