[Gasification] 500 kW Gasifier concept

Doug Williams doug.williams.nz at gmail.com
Sun Jan 31 18:49:48 CST 2016

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. 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.

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.

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 combustion

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.

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.

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(:-) 

Doug Williams,

 > Hello list. I have been asked to design a 500 kW heat gasifier, for
operation on damp nut shell charcoal. They are using the heat from the
producer gas burner to heat water, so I have the luxury of using a
water jacket to keep the barrel cool.
> Presuming the attachment gets through, any comments on the viability of this rather simple downdraft design ? Has anyone down a similar unit ?
> Given the down-draft design it can be run as a batch fuelled unit or semi-continuous fulling. The end user wants to be able to run 24 hours/day 6 days a week at rates from 50 to 500 kW heat output. They will be drawing the gas out to a burner tube using a suction fan on the cool side of a fin tube heat exchanger.
> If it works well for heat, then further down the track they will look to pull some of the producer gas off to cooler/filter and into a small diesel engine generator as a co-fuel.
> Regards,
> James

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