[Gasification] Gasification Operational Principles

[doug.williams]

Hi Oscar,

I will refine my comment:

>" I do not use that measure to establish the correct
>operating parameters, as flow rates are controlled by resistance set by the
>fuel size...."

This comment relates specifically the downdraft gasifiers with multi air 
nozzles and throat, and having had the dubious benefit if spending thousands 
of hours upto my elbows in charcoal, ( I really am a slow learner) found 
that if you use calculations only, you are in for a rude shock, dealing with 
the variables of biomass fuels.

>As far as my modest knowledge let me see it, I think both are
>linked or strongly interrelated in such a way that fixing one of them the
>other gets automatically fixed.

I agree absolutely, but what I say is that you must establish the fact 
before you can measure the velocity, not the other way around, but in fixing 
either, that does not automatically give you a working gasifier.

>Superficial velocity let us get an estimate
>of gasifier diameter along with its height so it should bear influence on
>gas flow rates and its relationship with the resistance sets out by fuel
>size which at the same time should match reactor dimensions.

If what you suggest is possible, then I would wonder why so few gasifiers 
built to calculations by students cannot make a tar free gas. It is also 
important to ensure that you are applying the correct calculations to the 
appropriate gasifier, because I see students crossing open cores with 
information relating to Swedish designs similar to Imbert.

>At the same
>time superficial velocity is a variable which can be quantified for
>predicting gasifier sizing and operating parameters,
..
A gasifier is certainly designed working from a known parameter, but if 
there is little or no understanding of how packed carbon beds behave in a 
high temperature environment, what do you change to maintain the tar free 
gas making, or indeed keep it making even dirty gas?  Can you specifically 
state which gasifiers can be built to your understanding of applying these 
principles, and how variations are controlled, or are you referring to it as 
a prediction tool, rather than one for proven design?

> however I don't see how
>easy it could be having a number or quantified estimates of flow rates
.controlled by resistance

At this point, I should say that not all information of how gasifiers work, 
or should be designed is in the public domain, and commercial gasification 
knowledge has to move on from prior art. It is one thing to make a gasifier 
that can work in a laboratory situation for an hour or two, using 
calculations, but this knowledge alone rarely makes it fit for commercial 
application.

"IF" you are aiming for tar free gas making, all sizing, including output, 
revolves around the fuel size in it's raw state, it's then structural 
integrity as a charcoal in the oxidation phase, it's size in the reduction 
zone, and finally, the amount of heat available for the endothermic demand 
of the reduction zone. Flow resistance is more than just about the 
interstitial spacing of the fuel, and is the point where gasifiers work or 
fail, at least in their ability to make clean gas continuously.

 If I may step back to the 1970's, when gasifiers of European origin (built 
using all the known calculations) were sent out to developing countries, and 
researchers in developing countries tried to build their own from 
literature, not one real success story was reported for tar free  wood 
gasifiers at the 1985, 2nd International Gasification Conference, held in 
Bandung, Indonesia. If anything, it highlighted the need for other types of 
gasifiers, away from the traditional European systems, and the variations 
you see today originated from that time. What has not changed, is the 
ability to make tar free gas easily despite all the knowledge of 
calculation, so emphasis has changed to gas cleaning, rather than gas 
making.

>Can you please further a little bit more on these aspects. I would really
>appreciate your comments.

My ability to not satisfy everyone with these answers, is clearly evident 
from private criptic e-mails that this Forum never sees.  Gasification has 
many variations to how it might be presented as a written technology, and 
even that has value to someone trying to give explanation to their own work. 
I have said many times, that I can share only so much, being restricted by 
commercial obligations, and contribute only as time allows.

Doug Williams,
Fluidyne.