Re: GAS-L: feeding biomass into pressurised systems

["Tom Reed" <tombreed@attbi.com>]

Dear Andrew:

(INTERCOMMENTS IN CAPS)
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.

VALUABLE LESSONS WOULD HAVE BEEN LEARNED IF AN HONEST REPORT WAS EVER
PRODUCED, BUT BUREAUCRATS BURY THEIR DEAD AS SOON AS POSSIBLE.  THE WHOLE
PROJECT WAS PLAGUED WITH DUMB POLITICAL DECISIONS OVERRIDING GOOD SCIENCE
AND ENGINEERING AND THE NEEDS OF MANKIND.

~~~~~~~~~~~~~~~
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.

GLAD TO KNOW THAT I'M NOT ALONE IN THINKING FORCED DRAFT IS WORTH IT WITH
INVERTED DOWNDRAFT GASIFIERS.  WE ARE ABOUT TO GO PUBLIC (IE COMMERCIAL FOR
SALE) WITH OUR WOODGAS CAMPSTOVE (WGCS)THIS MONTH.  BETA TESTS IN PROGRESS.

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.

WOOD AS NORMALLY BURNED IS REALLY TWO FUELS - THE ~ 80% VOLATILE FUEL GASES
COMING OFF AT 300-400 C AND THE REMAINING 20% CHARACOAL.  THE VOLATILE GASES
HAVE AN AIR/FUEL RATIO OF ABOUT 4.5; THE CHARCOAL HAS AN A/F OF 12, SO THE
MIXTURE IS 6 - IF YOU CAN FIGURE OUT HOW TO BURN ALL AT ONCE (AS IN PELLET
HEATING STOVES).

 So there is a need for a cheap
simple method of metering fuel into the combustion chamber.

IN THE WGCS WE RECOMMEND ADDING ADDITIONAL FUEL 1 LAYER AT A TIME WHEN YOU
REACH THE CHARCOAL STAGE.  THE PRODUCES PYROLYSIS GASES AT THE SAME RATE AS
THE INITIAL INVERTED DOWNDRAFT GASIFICATION.

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.

ONE SELDOM NEEDS TO COOK MORE THAN THE 45 MINUTES SUPPLIED BY WOOD PELLETS,
AND OFTEN THE 12 MINUTES FROM CHIPS IS ENOUGH FOR A CUPPA.  HOWEVER,
CONTINUAL FEEDING WOULD BE DESIRABLE FOR SPACE HEATING STOVES, INDUSTRIAL
COOKING ETC.  IN THIS CASE A SIMPLE AUGER INTO THE SIDE OF THE UNIT WILL DO
THE JOB.  CAN BE MOTOR CONTROLLED OR HAND CRANK DEPENDING ON APPLICATION.
SINCE ONLY AIR IS RISING THROUGH THE MASS THIS WON'T INTERFERE WITH THE
GASIFICATION.

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.

ONLY IF YOU NEED A TIGHT SEAL.  THIS TAPERED SCREW METHOD USED FOR FEEDING
PAPER PULP BOILERS.

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 DIFFER TO TOM MILES IN ALL THESE INDUSTRIAL SIZE QUESTIONS...

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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