[Gasification] Making char vs producer gas

[Peter Davies]

Fair comment Tom, my concern was with the negative preamble framing the 
question. I can't help but feel within the industry we continue to talk 
down our passion, highlighting problems that are not universal but 
irrespectively taken as given, and in doing so only harm ourselves.

In regard your description of "most gasifiers" this is not our 
experience with our own system, which I guess qualifies it then as 
"unique". Tars are not increased with increasing char outputs, rather 
the system can reach a point where gas production shuts down, the cut 
off being sharply demarcated, not a linear decline. For our system this 
appears to be around the 15% char recovery mark at the moment for 
consistent performance, highest gas heating value appears to be around 
5-10% char, coming back to one of Tom Reeds earlier posts on the same 
subject and reflects the higher H2/CO ratio and lower nitrogen dilution 
when running under such conditions, though the system is still quite 
efficient in a conventional gasifier sense at 1% char outputs.  Part of 
the development path we followed was understanding why with our early 
pilot systems gas the quality was higher in the period immediately after 
start-up when the system had first settled, then declined to a lower 
equilibrium, conventional wisdom being that this was a bed porosity 
change over time (ash/fine particle build up). We found this was only 
part of what was going on, hence the advances we have made.

In Australia we are only offered 4c/kWh for export power, for most 
projects we are involved with retaining the energy in the char as even 
low value agricultural soil amendment co-products provides superior 
economic outcomes once initial on site energy needs are met (offsetting 
retail energy costs). This clearly is not the case in other parts of the 
world, so horses for courses as they say. We are nothing if not flexible...

Cheers,
Peter


On 3/12/2014 9:58 AM, Tom Miles wrote:
> Peter,
>
> No harm done with this question. In most gasifiers it is a trade-off between
> char yield and gas quantity and quality. Normally the higher the gas quality
> the lower the char yield. Compare a typical downdraft gasifier making engine
> quality gas at less than 5% char, more typically about 2%, to a downdraft
> making 25% or more char and a tarry gas. The engine quality gas needs
> cooling and dry filtering with a final polishing step for light oils if
> necessary. The tarry gas needs scrubbing with liquid and filtering before
> use in an engine. There are costs associated with handling the black soup
> from a scrubber that are not incurred with dry cleaning. If you have aunique
> design that makes a high quality gaas an dproduces variable quantiites of
> char then the choice is which carbon is has the higher net value, so it is a
> cost/revenue decision rather than just a cost issue.
>
> Tom
>

-- 
Peter Davies
Director
ID Gasifiers Pty Ltd
Delegate River, Victoria
Australia
Ph: 0402 845 295