[Gasification] 500 kW Gasifier concept

James Joyce james at jamesjoyce.com.au
Mon Feb 1 14:13:48 CST 2016

OK I hear what you are saying. I have grossly underestimated the impact of the bed on airflows and what that does to maintenance of the reaction region. Back to the drawing board !


-----Original Message-----
From: Doug Williams [mailto:doug.williams.nz at gmail.com] 
Sent: Monday, 1 February 2016 6:30 PM
To: Discussion of biomass pyrolysis and gasification <gasification at lists.bioenergylists.org>
Cc: James Joyce <james at jamesjoyce.com.au>
Subject: Re: [Gasification] 500 kW Gasifier concept

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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