[Gasification] 500 kW Gasifier concept

Doug Williams doug.williams.nz at gmail.com
Mon Feb 1 02:30:16 CST 2016

Hi James,

I reply within the text.

> Thanks for your thorough and detailed reply Doug. 
> Seven Years ago I selected the updraft rotary hearth approach we use in the BIGCHAR systems for many of the reasons you addressed in your reply. I might yet regret trying to do something simpler in recognition of an easier handling fuel.
> One thing I am trying to achieve is a "pull through" system, to reduce the risk of fugitive emissions and reduce the potential for over-pressure or even explosions that can happen when fuelling positively pressurised gasifiers.

While suction gasifiers have less risk of emissions, these are the ones
at risk of internal hopper explosions. As you stated that the system
was hoped to run 24x6, refuelling via air tight lock hoppers or fuel
locks will be required. Stop start refuelling isn't a long term option
for most clients. 
> I was also looking to do a downdraft charcoal fuelled unit as a future prospect for engine co-fuelling. The logic is that updraft designs cannot produce a gas that is worth trying to clean up .. even when fuelled on charcoal.

Not sure how you arrived at that conclusion. Most charcoal gasifiers
used for WW2 engine operation were up draft and the fuel in the hopper
acted as both a filter of fine dust and ash, plus provided a degree of
gas cooling. They only required a cotton bag filter to get very clean
gas. Steam was added to the incoming air nozzle/s to make the H2, which
in turn made the more abundant CO more responsive to load change by
increasing the flame speed/pressure in the cylinder.

A couple of comments to your replies:

> I have seen these designs presented in a number of versions and at best are not as simple as one might expect. As a concept design it will fail in the way it is perceived to work, as the oxidation zone shown at the bottom cannot be made to stay in place.
> > That was indeed my concern. If it was this simple somebody would already be doing it. Some have, at much smaller scales, but most systems are using Tuyeres, as you imply.

As I am sure you know, up-scaling gasifier output is not about
conversion calculations, but by maintaining a gas making phenomena
controlled by the availability of free oxygen in the bed oxidation
area. Nozzles just limit the area of oxidation, but this leads use to
your next comment. 

> > Fair enough. I will look at the idea of running a rotatable pipe down from the top to the desired oxidation zone. With a small bore cross pipe at the end, some blades and a rotating mechanism that could achieve the bed agitation and fixed location air injection that will be required.

What you describe is similar to that used by a Buck Rogers gasifier and
a number of other designs tried years back in the 1970-80s. Stirred
beds require huge torque and disrupt the exothermic heat production.

> >I did not want to have to add this, but sounds like either do this or go back to our standard rotary hearth (which will not produce a gas suitable for engine use). I will size it so that much of the air still has to come down through the bed, to bring moisture into the hot zone. 

Unless you blow the air into the central tube to create a complete
oxidation zone via the air nozzles, the bed will continue to move
upwards against the incoming air. If the oxidation zone could stay in
place, then it's temperature to reliably create the H2 will have to be
at least 1,000-1,200C. The endothermic heat consumption rapidly
increases to handle moisture, resulting in a rapid drop to the
reduction zone making the CO. If it drops to below 850C, then you see a
decline in the amount of combustible gas and an increase in CO2. 

> >Fortunately we have access to materials that can tolerate the possible peak temperatures in the hot zone.

Unless you have tried them in a packed carbon bed in the presence of
free oxygen, then you might be in for a surprise. Most austentic metals
will melt and burn, or at best erode very fast. We operate our systems
between 12-1500C, but take great care to keep air and gas in motion
away from any metals. 
> 9. Suction fans are not reliable when flow resistance is a bed issue, causing both a drop in gas flow and temperature of the char/air interface. A Roots blower with constant displacement is the better choice for suction.

> > I will see if I can better manage the bed resistance with addition of a central agitation shaft first ... 

Agitation of the upper bed is not likely to be helpful. Agitation of
the oxidation zone will not allow the exothermic heat production to
remain stable and maximise oxidation temperatures. An agitation paddle
located in the reduction zone will be really hot 1,000C> and at risk.

In offering the above comments James, they are from personal experience
acquired over the years, and I don't like to see anyone waste money and 
time. See how you go with any rethink and let us know how you go.


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