[Gasification] 500 kW Gasifier concept

[James Joyce]

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.

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.

I have added a couple of responses in your message below.

Regards,

James



>From Doug Williams

Hi James,

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.


I will list issues and you can then see how the bed behaves.

1. Ignition of the char at the bottom would for a very short time make gas, but not from over the full diameter of the bed.
> Not getting oxidation across the full diameter is indeed one of my concerns.

2. The air entering the top hatch will take the shortest route through the bed towards the gas outlet on the bottom, effectively cutting your hopper diameter in half. All the oxidation activity will take place within that diagonal air flow.
> It might not be clear on the drawing but the gas outlet is a radial belt design, ie. from slots around the full diameter... still I understand what you are saying ... the air will quite likely try to bypass as much of the bed as it can.

3. Once ignition is initiated, the top of the combusting char known as the char/air interface,(for it will not be incandescent oxidation/reduction temperatures), will move upwards towards the incoming air following that diagonal line of air flow.

4. As the combustion zone moves upwards, the char under the very narrow fire band becomes finer and begins to collect ash and fines that will increase the bed resistance to gas flow.

5. A certain amount of positional movement will take place of the rising combustion zone until some sort of equilibrium of the pressure drop is reached, and the combustion bed will be then burning on the top of the fuel pile.

6. The combusting char exposed to the incoming air will then oxidise completely to ash on the air side of the char and then begin to smother the combustion at the air/char interface.

7. The combustion temperature will drop and the exothermic heat generation is lost, thus preventing any reduction of the combustionCO2>CO.

8. The only reliable way to hold the oxidation zone in place at the bottom as shown in your design would be to add air at an appropriate height from the bottom.

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

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 ... 

9. Alternately fit a sealed lid and blow the air in the bottom and make a conventional up draft charcoal gasifier out of your design. That way you will also get better heating of your water jacket, and it will assist to cool the gas. You would have to ensure your hopper fuel is kept at a fairly high level to ensure you maintain the bed stability.

10. You will need steam to enter with the air if you go up draft, as char moisture will be blown away with the gas stream. There would not have been much H2 in the down draft concept, in fact the damp char will have further inhibit exothermic heat generation as the moisture was pulled through the combustion zone..

Hope this might help before you get to caught up in the simplicity(:-) 

Regards,
Doug Williams,
Fluidyne.