[Gasification] raising H2 concentration in downdraft gasification

Rex Zietsman rex at whitfieldfarm.co.za
Mon Dec 16 01:56:37 CST 2013

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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