Hi List,
This is the second in a series covering RPS experience with our linear
hearth.
We often hear reference to "inappropriate fuels" for gasification and
fuel problems are often cited as the reason for project failures.
The RPS hearth was originally designed with wood chips in mind, but has
proven far more flexible. Over time quite a few "problematic" fuels have
been presented to us, often by companies where there is a significant
disposal cost on the business and no other solution. What one of our
colleagues described as the "thorn covered and snare protected low
hanging fruit of the market"
As we gained experience with our new system we found much of our
knowledge of what works in gasification needed revising as we were
getting results that differed markedly from the literature.
*PLEASE NOTE:* These results ONLY apply to the RPS Gasifiers tested, not
other linear hearths or different gasification systems and results
should not be extrapolated. The RPS development units are the result of
a long period of interest and study in gasification, we are not new
comers in the field, it took us 30 years to get it this "simple" whilst
working this well.
*SPECIAL WARNING NOTE:* Some of these materials shown below are not
suited to ordinary gasifiers. If system temperatures are not high enough
then tyre rubber and some other fossil derived fuels and man made
materials can produce condensates which are potentially harmful to human
health if mishandled. Sewerage sludge and other organic materials have
their own safe handling requirements. We had suitably qualified and
experienced persons present during this testing and have a formal Health
& Safety strategy in place. Some materials were only tested after it was
proven that the system operated at temperatures and under conditions
that were suitable for their safe thermal destruction.
Now read on...
There is a great story from American comedian Bill Cosby who relates
that whilst in Italy he demonstrated his knowledge of Italian by
ordering direct from the restaurant menu...only to discover on being
served that he had ordered whole sparrow complete except for
feathers...he solved his potential embarrassment by ordering bread,
"because an American can eat anything between two slices of bread"!
The RPS Gasifier is a little like Cosby in his story, it seems to be
able to digest most things organic fed to it so long as it is sandwiched
between two wood chips...
Quite a few types of biomass can be gasified directly in the RPS units,
everything else tried so far works well if mixed with wood chips/chunks.
On homogenised mixed feed stocks the system comes up to stable,
consistent operation quite quickly, usually within 30 minutes from a
cold start. General feed stock parameters identified so far:
. Moisture content <25%
. Particle size -- 10mm to 75mm (measured on any given side)
. Fines - <33%
. Very flat thin materials can affect system performance as these can
layer restricting optimal gas flow/heat transfer between pieces.
. Organic liquids such as waste vegetable or mineral oils can be added
by pre-mixing with wood chip or sprayed evenly onto the top of the
fuel column in the upper hopper.
List of materials run so far by type:
. Hardwood & softwood chips & chunks. /Comment: Bread and butter fuels/,
keep t/he moisture content below 25% and particle size within
recommended range and you won't be disappointed./
. Green wood chips at 50% mc. (Positive energy balance achieved with gas
quality sufficient for self sustaining flare). /Comment: Really, gas was
very poor quality and pre drying is the more sensible approach, but it
did demonstrate that inadvertent clumps of high moisture content fuel
finding its way in doesn't result in thermal shock and complete system
shut down./
. Fine ground mixed species garden mulch chips from street scape
management. /Comment:/ /Gas is lower quality with higher condensate
production than for optimum sized wood chips at the same fan pressure./
/Char yields tend to be higher but lower fixed carbon and higher mineral
ash content.
/. Macadamia shell. /Comment: Stand well back!/ /The rocket fuel for our
gasifier. Multiple outputs including high quality gas and //dense //high
grade char/
. Shredded woody residues from composting operations including mixed
plastics. /Comment: Similar results to ordinary wood chips except not
suited to fixed grate models as material generally comes from composting
operations and can contain ground glass and soil residues, forming soft
clinkers on shut down as temperatures in the hearth drop, making the
following days start up problematic without a full system clean out.
Much more prone to bridging due to material shapes. On the Mark 3. model
with active grate this was much less of a problem./
. Sawdust. /Comment: Similar results to fine ground garden mulch only
more severe. Mixed blend with wood chips presented no problems./
. Cotton gin trash /Comment: Excellent fuel once it is in the hearth
zone, getting it there past the normal gravity feed upper hopper fitted
to development units has presented problems. Currently trialling some
different (including novel!) approaches to ensure the in feed works
reliably. Alternatively pelleting or briquette also solves the material
handling issue...for additional plant cost./
. Sugar cane trash. /Comment: Similar handling problems to cotton trash
in that getting it past the upper hopper is a bigger problem than
gasifying it when it reaches the hearth. Lower gas quality and lower gas
volume than wood chips at equivalent fan pressures. High ash. Not suited
to fixed grate models./
. Rice straw. /Comment: As for Sugar Cane Trash./
. Oil Mallee residues (leaf and stem). /Comment: As for fine ground
garden mulch./
. Sawdust briquettes. /Comment: Good fuel similar to quality wood
chips./ /Higher ash and lower quality char though./
. Pyrethrum briquettes (horticultural residues). /Comment: Good fuel
similar to wood briquettes only higher ash and a low quality char co
product, not ideally suited to fixed grate systems as fine ash builds
very quickly, then requiring very high fan pressures to maintain flow.
Works well in active grate model./
. Cattle manure briquettes (40% ash) /Comment: Poor fuel due to 40% ash
content (due to collection on clay pans with a skid steer loader!),
think pyre thrum briquettes with ground glass added./
. Glycerine/acid oil waste from Bio-diesel production (in a blend with
wood chips). /Comment: Awesome wood chip additive //increases gas energy
content./
. Paulownia chunks. /Comment: Think grass in wood chunk form. Worked
well but lower quality gas and char compared to wood chips./
. Grape Marc (in a blend with wood chips). /Comment: Mixed in blend with
wood chips worked fine. Subsequent tests with 100% grape marc pellets
gave equivalent result to standard wood chips but with higher char
yield, although with lower fixed Carbon and higher ash./
. Chipped Industrial Hemp plants (the fibre variety!) /Comment: May well
be a law enforcement must have, worked similar to fine ground woody
garden mulch, good flare but higher ash, lower quality char./
. Sewerage sludge (composted & fresh out of centrifuge) in blend with
wood chips. /Comment: Worked well in blend with dry wood chips./
Non renewable:**
. Anthracite briquettes /Comment: Worked well despite quite high ash
contents, best suited to active grate models. Had a similar condensate
analysis to the wood sample in the previous post, only with the pyridine
replaced by benzene./
. Lignite briquettes /Comment: As above only better as they were much
lower ash./
. Low quality high ash black coal straight from the mine (Run Of Mine).
/Comment: Not every black looking piece is actually made of coal, the
high ash content seemed concentrated in large rocks, nonetheless where
piece sizes do not exceed 30 mm and the active grate model is used it
looks like a good fuel. Just add water for really good gas as the mc
was below 2%!./
. Mixed electrical transformer waste including compressed cardboard,
Bakelite insulators, lump tar, oil soaked timber, aluminium coated paper
rolls, polyethylene cable sheathing. /Comment: As additives to a
standard wood chip feed in low volumes these went through with no problem./
. Car tyre rubber chips./Comment: Mixed in a blend with wood chips makes
an awesome fuel, when used alone fan pressure has to be substantially
increased or you get a bright orange flame and copious quantities of
condensate the equivalent to crude oil. If running only on rubber chips
then the system has to have wood chips through it immediately before
shut down to replace the rubber otherwise the cooling fuel pile bonds
into a solid vulcanised mess that takes a lot of breaking up the next
day (Ahh the voice of experience...) /
Now, some of these were preliminary tests with only 100-200kgs of
material so cannot be called definitive. Testing was done under our
"Suck it and see!" approach, developed after it became apparent that we
could not rely on the experience of others or published literature to
determine how our system would actually respond to different fuels.
Much more work remains to be done before commercial models of the system
can be finalised and certified for particular fuels, nonetheless we are
quietly excited.
2012 looks like developing as the year of validation for RPS, with much
more formal, larger scale and in some cases independently over-sighted
trials in the works. Where confidentially agreements allow we will
report on these projects as results become available.
Roll on Year of the Dragon!
Cheers,
Peter & Kerry Davies
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