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Newbie advice to Newbies

Page history last edited by jim mason 11 years, 3 months ago

A running summary of important things for beginner orientation

Comments (11)

Garret Krampe said

at 5:10 pm on May 16, 2009

Summary of my wood gas knowledge and all of this is from reading.

Instead of me saying.

"you have to discover it for your self"
"It's like sex - you can't learn anything from a book"

What I have learnt so far from reading.

Wood Gas Contains

CH4 ~2%
CO ~18-20%
Tars C1 - C6
H2 ~18-20%
N2 ~50%

CO is odourless, and highly toxic

Wood Tar is highly carcinogenic and causes genetic mutations.

Full of soot and ash.

Octane Rating around 130 to 180

Fuel quality is highly variable.

Timing ADVANCE needs to be around 12 to 28 degrees.
(Now this seems to be contradictory to what I know about H2 mixes. Where
H2 is used the timing is usually about 2 - 10 degrees AFTER TOP DEAD

If the flame was slow it would limit the RPM due to slow flame front.
Flame propagation and burn rate from reading are very high.

Peak IMEP (Indicated Mean Effective Pressure) target at 17 - 19 Degrees

The system can be prone to leaks and therefore safer to run in a suction

Air entry can lead to catastrophic explosions and fire.

Garret Krampe said

at 5:11 pm on May 16, 2009

The wood is heated in an O2 depleted environment to extract the gas from
it. O2 or air is only used to heat the wood by burning the charcoal.

The lower the combustor temp the higher the tar content, and higher the

The higher the combustor temp the higher the NOX, the lower the

The higher the combustor temp the more likely of slag formation from the

Wood gas needs to be cooled and cleaned prior to being introduced to an
IC engine.

Wood gas engines seems to like high compression ratios around 15-17:1
17:1 produced 8% more efficiency than 11:1 compression.

It is possible to convert direct injection diesels to spark ignition for
this purpose.

Don't use wet or green wood. Use 20% moisture or under.
You need to shake the can to break up bridging.

The size and type of the fuel is important.
Some fuels like rice husks produce lots of slag, yet this (rice husk)
slag is a nearly pure form of silica used in semiconductors.

Garret Krampe said

at 5:11 pm on May 16, 2009

When the gasifier is not running at full power for some time it
"shrinks" it's performance back so getting going is a matter of time,
and during this time tars are made.

I believe that this is because of the lack of penetration of the pryo
zone into the fuel, and the temp dropping off so that gases are just not
available for extraction.

RPM is limited, in part due to flame propagation rate and in part due to
the gasifiers lack of ability to supply the necessary flow rate, whilst
being able to supply gas at a rate that will support a stable idle.

I believe that keeping the wood at a high temp, with deep penetration of
the pryo zone, will improve the lack of performance starting off after a
period of idle.

Lack of idle seems to be related to the amount of air drawn into the
combustor by the suction provided by idle conditions. The lack of flow
through the nozzles drops the temp of the pyro zone, shrinks it, and
eventually the combustion area is too small to sustain it's self due to
heat loss.

Garret Krampe said

at 5:12 pm on May 16, 2009

Tar, particulate removal and cooling.

Tars are possibly the largest problem. I believe that an "after burner"
made from ceramic foam, maintaining about 1100 C, through which the gas
flows, will destroy most of the tar. I think this is better done inside
the reactor, thereby using less of the produced gas to power the heater.

I believe that cooling should be done first, and as part of this process
the heavy particulate and tar will be removed.

I believe that multiple small cyclones of no larger than 2" in external
diameter , possibly with water cooling will perform around 85% of the
task in one operation.

I believe that cyclone with a top co-axial entry and exit with air ramp
are most appropriate with the cone and body section able to be removed
for cleaning.

A Ranque-Hilsch vortex tube cooling effect occurs if a side entry is
used and the cone is inverted and placed say 3/4 from the bottom of the
tube. The HOT gas can be recycled to the top of the gasifier to assist
in efficiency gain. The cold gas can be filtered and or cascaded into
another "classic style" cyclone.

Garret Krampe said

at 5:12 pm on May 16, 2009

Wood gas is very forgiving in it's mixture ratios. Carbon monoxide is
explosive from 12 % to 75% mixtures.

Wood gas engines should be build from slow reving large capacity engines
if high compression ratios are not achievable.

Wood gas engines produce 16% to 30% less power than diesel engines.
Mostly dependent on compression ratio.

Wood gasifiers can be built for emergencies from basic materials at hand
and still provide operational capability, at some sacrifice to longevity
of the engine being gassed.

Wood gas MIXING into an engine is done using a "choke" method.

The air fuel ratio by volume is 1.2 to 1.5 : 1
so 1.2-15 L of air to 1 L of producer gas.

Example " Carby "

Engine Peak Flame temp is around 1800 K +- 50 K
1527 C +- 50 C

Energy Density is 4.5 to 5.5 MJ / Kg

Garret Krampe said

at 5:12 pm on May 16, 2009

Essential reading:
Handbook of biomass downdraft gasifier engine systems
by Tomas B Reed and Agua Das

::Also add to this the World Bank Technical Paper #296. If you study how how the scientific/engineered gasification projects failed versus the sucsessful local home grown applications you may then fully appreciated the absolute Necessity for on hands, local to you, your climate, your fuels experience and involment.

the main problem is this:
there is significantly more tar gas produced in pyrolysis than can be fully burned in combustion and reduced over the amount of available char. we calculate the excess tar gas to be 2.2x what is needed for stoich combustion to feed reduction and consume all the char. the rest of the tar gas has to be thermally cracked. this involves both plumbing the tar to where adequate temps are, as well as maintaining adequate temps and residence times once it is there.

if the world was redesigned for gasifiers, biomass would not average 20% fixed carbon and 80% volatiles. that 80% portion becoming tar gasses is the killer. this is what we are always fighting until we give up and go to charcoal or coal as a fuel.

biomass does vary a bit in its composition. i find it informative to look at the varying volatile to fixed carbon ratios in various fuels in the phyllis database

but in general, it all stays way above the desired ratios of volatiles to fixed carbon. way too much tar gas to char produced.

IBPP at 17:1 compression is 5400 (Indicated Brake Peak Pressure)
IBPP at 11.5:1 compression is only 3400 that is a 40% drop.
Any higher than 17:1 and you risk dieseling the engine.
It has bee said that 16.9:1 is a safe ratio I think this was Kevin's Listeroid Pages.


Garret Krampe said

at 5:13 pm on May 16, 2009

Some corrections are comming.
11:32 AM Sunday 17h May 2009

Garret Krampe said

at 2:38 am on May 19, 2009

i find most of this drop to be from the nozzle sizes, not the fuel. the only time fuel seems to matter is when it is packed somehow. the pressure drop across the reactor seems mostly from nozzle size.

Garret Krampe said

at 3:04 am on May 19, 2009

a gasifier can be a giant counterflow heat transfer system. this is clearly what i'm doing in all this tube within tube and varied stage heat exchange systems.

i would only refine that there are places in the reactor where we want temp spikes that will be hindered by allowing heat loss to the next lower temp stage. maintaining the total heat budget through good exchange is important, but max temps are also important. those are separate variables and solutions. insulation to prevent losses around these spikes are important.

thernally, the ideal gasifier is a sphere witin a sphere within a sphere within a sphere. combustion would be in the middle, and heat would propagate outwards though the successively lower temp layers: reduction, pyrolysis and drying. all heat from each stage drives the next stage. zero loss to any vessel walls. unfortunately, this is a difficult gasifier to realize in actuality. cylinders are the best buildable approximation. square are worse.

the real problem, however, is that the ideal chemical relationships in a gasifier are nowhere near the same as the desired thermal relationships. the desired pathway of various gasses and solids internally in a gasifier violate the desired chemical relationships in multiple locations.

thus you get the well known situations like an updraft being thermally optimized but a chemical mess. or a downdraft being chemically optimized but a thermal mess.

i therfore find gasifier design to be a three dimensional thermo mechanical puzzle where you are trying to solve these two very different problems simultaneously. the puzzle is to establish the correct chemical relationships, while maintaining as much of the thermal relationships as possible.

Garret Krampe said

at 5:01 am on May 19, 2009

with any engine throttle, the gasifier is not contributing any extra vacuum to the cylinders on intake. once you reach the point that you have zero vacuum in the intake (full throttle or close thereof), then the gasifier restriction is relevant. at least on an otto heat engine. diesel is different.

John Murray said

at 2:05 pm on Dec 3, 2009

Thanks for all of this.
As a newby I have some questions:
Part of my thinking on energy outputs would be not just the mech' power created y this process, but also the waste stream the GEK produces.
In this regard, is the waste heat of any use after other GEK processes (i.e. drying of feed material) for cogeneration, and perhaps more importantly, what is the biomass conversion to charcoal- itself a very useful fuel?

I have seen others say they get from 10%- 25% charcoal from their differing units. What does the GEK output?

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