[Gasification] Blue water gas...... Why bother?

[doug.williams]

Hi Drew, and Colleagues,

Drew, there is nothing like the evolution of awareness, and you are right on 
when you suggest that you introduce steam via a riser tube in the reduction 
zone. However, you cannot put it through the centre, because as Tom R, 
say's, it is endothermic, and you will cool the reduction char. If however, 
you had your steam coil or similar device located in the exit gas stream, 
you should be able to put steam in at least 800C to the top of the throat 
tube.

Now, if you do this, it requires the highest possible throat temperature 
(12-1500C), because you are adding water over that found in the fuel. So the 
question remains, can you add more water than the driest biomass (20% oven 
dry, chemically bound), and make more H2? Water by the way will thermally 
crack once you start getting over 850C. This answers your first question,

> The water shift reaction can happen at 500C ?

No it cannot.

 Your second question:

>Am I correct in saying that a secondary function of
> steam injection/water gas shift at this point could be to quench the
> gas, keeping any CO from converting to CO2 and C (soot)?

No, quenching the the gas to prevent CO2 and soot reversion happens after 
the gas is made when reduction stops (850C), to reduce the gas temperature 
down to below 500C, where it then becomes stable, at least, it stops making 
soot.

>    Does this mean that steam at 750 deg, is not H2O as a gas but two
> separate free gasses, and if so what would be the lowest temp at which
> separation occurs?

No, it is just still steam, and stays that way until it is disassociated at 
temperatures over 850C, which means, the hotter the better conversion of H2O 
into H2, and it then seperates into H2 and Oxygen.

Hope this helps you to sort this detail out.

Just as an after thought, it just does not work to use moisture of the fuel 
to control H2 content. There is a limitation to fuel moisture to obtain the 
oxidation temperatures, and that is well know to be fuel around 15% moisture 
content. As Greg stated, atmospheric moisture also has to be taken into 
consideration, and you cannot control that without additional expense. 
Optimising a normal gasifier performance, means ensuring that the fuel is 
consistantly dry (waste heat), adding heat to the gasifier (waste heating of 
the incomIng wood), and limited heating of the incoming air to the gasifier 
(waste heat) depending on the ambiant conditions.

If you follow these basic requirements, the producer gas will be perfectly 
OK for most applications,  about 15-20% H2, 15-20% CO, as the combustable 
gases. This you can expect for a correctly operating air fed gasifier. Over 
this, there are other factors involved that make for more sophisticated 
design and costs. On the other hand, if your gasifier is working correctly, 
only dry fuel makes the biggest difference. Why bother indeed :-)

Doug Williams,
Fluidyne Gasification.



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