GAS-L: feeding biomass into pressurised systems

[AJH <andrew.heggie@dtn.ntl.com>]

Some time back Tom Reed criticised an expensive US government
sponsored gasification system in Hawaii, Tom Miles refuted the money
was wasted as valuable lessons were learned( and by inference many
researchers wages paid), though appeared to acknowledge few of the
lessons won at such large expense had yet been applied.


I was intrigued by the feed method and hoped Tom Miles would be able
to explain more. My quest is not at an industrial gasification scale
but in the micro scale of cook stoves. Whilst there is a lot of
current expertise in direct combustion of biomass for cooking most of
the work is on natural draught with (in some cases) draught assisted
by chimney effect. At least Tom Reed and myself, and presumably some
others are tinkering with "blown" stoves achieving near complete
combustion without chimneys. This to my mind has several advantages in
cost, stability and ability to burn "poor" or "green/fresh" fuels.

I think that if I slightly pressurise the primary combustion I can use
the power in the off gases to entrain and turbulently combine with
secondary air.

This is OK when batch loaded but leads to problems of cleanly burning
the residual char at the end of a run. So there is a need for a cheap
simple method of metering fuel into the combustion chamber. At the
moment I have to rely on a low powered fan for combustion air, so I
can accept a similar electrical method of fuel metering.

In the previous discussion a plug screw system was mentioned, I take
it this is a variable pitch auger such as in a meat mincer, the
biomass becomes temporarily formed into a plug where the auger flights
become closer and hence form a seal. If I am correct does the friction
make this a power hogging device? If not how does it function.

The other methods seem to be rotary valves, which seem to need good
engineering tolerances and bell hoppers (which I currently favour) as
well as under stoking from sealed bins and "jacking" the fuel up as
demonstrated by Peter Verhaart on the stoves list.

Any other methods I have missed?

I believe I missed some discussion on "chunking" fuel for a gasifier,
the idea is that chunking requires less power, leaves sufficient
interstitial spaces for good airflow and drying as well as affecting
the superficial velocities in the gasifier. I had wondered some while
back at making a chunker from a redundant tracked undercarriage, in
essence mount one opposed to the others with the track shoes moving in
synchronisation. The grouser plates then forming shear surfaces that
met at the outfeed end, the tracks converging from a feed end to this
shear. The idea being that the woody biomass would be loaded onto the
lower track and drawn forward until it was also gripped by the upper
track converging on it, the grousers biting into the stems until they
were sheared of into chunks the length of the track pitch at the
outfeed, any comments?

AJH

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