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
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