Gasification of Wood Chip Waste for Domestic Heat and Power - January 2009.
Kaupp 1984a gives details of gasifier
dimensions, fuel consumptions and gas
production rates. The GEK is somewhere
between the dimensions on line 1 and line 2.
The nozzle diameter is approximately 9mm and
the constriction diameter is about 75mm.
From this we can deduce a gas production
between 5 and 40 Nm3 per hour - bearing in
mind that the Lister at 650 rpm can only draw
in a total of 28m3 per hour or a gas
requirement of about 14Nm3 per hour.
Because the Lister is only a 6hp engine it
might prove necessary to reduce the nozzle
diameter and fit a smaller diameter
constriction (stainless steel or cast iron
reducing ring) to the neck of the reduction
cone. This was a common practice for smaller
engine sizes to lower the tar production.
|For More Information:
Ken Boak can be contacted by email by anyone wishing further information on this project.
Since starting on this Lister project, several other Lister engine enthusiasts and their CS engines have come to
my notice. These links describe a few of them. We have recently started a Lister CS Owners Group,
contactable via Yahoo Groups
On the left is the picture of the partly
assembled GEK gasifier from AllPower
Labs in Berkeley, California.
The GEK provides a common starting point
for gasifier experimentation and research.
It is supplied as a kit, which once painted
and assembled is capable of producing
clean, cool, engine quality wood gas from
woodchips or other biomass.
The woodchips are stored in the 120 litre
drum and descend slowly into the body of
the gasifier. Air is supplied through a ring
of 5 nozzles and the resulting combusting
woodchips supplies the heat energy needed
to convert the bulk of the woodchips into a
flammable mixture of carbon monoxide and
hydrogen, with nitrogen and carbon dioxide.
This dirty, hot raw gas mixture is drawn
through the gas outlet pipe on the right
where the dust is removed in the conical
cyclone and fine filter can above it. The
powerful suction fan on the top of the filter
can provides the initial air suction for
lighting and starting the gasification
The clean, cooled gas emerges from the
pipe on the right of the fan housing and can
be fed into the air intake of the diesel
The Lister diesel engine is run in dual-fuel mode.
This means that a small quantity of waste vegetable oil
is injected as normal, and provides a pilot ignition, but
the bulk of the fuel charge is in the form of flammable
wood gas introduced by way of the air intake.
This technique has been developed in India by gasifier
researchers as a means of reducing the reliance on
diesel fuel. Slow speed Lister engines are still very
common in India for pumping and generating.
The Lister diesel can be started on WVO and when
running the induction stroke provides the necessary
suction to draw air through the gas producer.
The air is directed into the gasifier via a series of
stainless steel corrugated tubes. These tubes are
wrapped in a spiral around th body of the gasifier and
pick up the waste heat from the gasifier and transfer
the heat of the outgoing gas into the incoming air. This
air-preheating warms the air up to about 600C
whereupon it is injected into the woodchip fuel by way
of a ring of 5 cast iron nozzles. This is known as the
combustion or oxidisation zone, where fuel is oxidised
and liberates heat energy.
The woodchip in the immediate area around the nozzle
is combusted and provides the excess heat that drives
the rest of the gasification process.
The heat is conducted through to the woodchips above,
and by a process known as pyrolysis, causes them to
break down into carbon and various gases and volatile
The volatiles, carbon dioxide, carbon monoxide and
water vapour is drawn by the downwards draught
through the oxidisation zone and into the area below
which is full of very hot, incandescent charcoal. This
region is known as the reduction zone. Here the carbon
dioxide, carbon monoxide and water vapour react with
the hot charcoal and form a gaseous mixture consisting
primarily of carbon monoxide and hydrogen, plus the
nitrogen content which was present in the original
combustion air, which passes through the gasifier
unreacted. The chemical reactions occuring in the
reduction zone are endothermic and absorb heat from
the hot charcoal to reduce the water vapour to
hydrogen and the carbon dioxide to carbon monoxide.
The gaseous mixture leaves through the bottom of the
reduction zone at a temperature of about 600C. It will
contain a lot of dust particulates and needs to be cooled
and cleaned before it can be used in the engine.
Introduction to Wood Gasification
Wood gasification is the process by which wood or
other biomass is converted to a flammable gaseous
The wood gas can be burned in a boiler of furnace or
used to run an internal combustion engine for
transport or to generate electrical power.
Converting wood to flammable wood gas releases
more useful energy, in a more usable form, than if the
wood was burned directly.
The chemistry of wood gasification was formalised in
the 1850's, having been studied from observations
known for centuries to blacksmiths and ironworkers,
from their workings with charcoal forges and furnaces.
Gasification was extensively deployed in WWII
Europe for running road vehicles when petroleum fell
in short supply. Up to 1 million European vehicles
were fitted with wood and charcoal fuelled gasifiers.
For my project, I have selected a Gasifier
Experimenter's Kit or GEK, supplied by AllPower
Labs of Berkeley, California. This is an open source
design intended for self build or purchased ready
built. It provides a well thought out, common starting
point for those interested in experimental wood
Energy disruptions and price hikes in the early 21st
Century has seen a renaissance in interest in wood
gasification technology, with information widely
being shared by way of internet groups, such as the
Yahoo Woodgas Group and for GEK users there's now
the GEKgasifiers Forum on pbwicki.
Cross sectional drawing of GEK downdraft reactor -
showing principal dimensions - Courtesy All Power
Labs. Note nozzles are 9mm dia, constriction is 75mm.
More data on gasifier dimensioning from Kaupp 1984a
The GEK is assembled from a series of nesting cylinders
fabricated from 16 gauge mild steel. The flanges are 10
The woodchips are supplied by gravity from a suitable
hopper above which is flange mounted to the main
The air is introduced near the top and follows the route
shown in red, contained within the stainless steel
air-preheater tubes. It emerges about 3.25"above the
reduction cone through 5 inward pointing radial nozzles.
The hot wood gas is drawn through the incandescent
charcoal contained in the reduction cone and passes out
through the bottom and up the annular gap created
between the outer wall and the inner reaction vessel.
Here it sheds a lot of its heatload to the incoming air,
cooling the gas by some 400C and reducing its volume.
The shaded area is a layer of ash used as insulation to
help retain the heat within the main reaction vessel.
Here is a shot of the inside of the GEK reactor vessel.
Two of the air pipes are visible which then supply air
to the inward pointing nozzles, not fitted at this stage.
The top of the reduction inverted cone can be seen in
the centre and just visible is the perforated metal sheet
below it which forms the support grate.
The annular gap between the inner vessel and the
middle vessel is filled with insulation material, such as
mica, perlite, lipor or plain wood ash. This helps keep
the heat of reaction within the core of the reactor
In the foreground are the capped air inlets, and also the
central facing lighting port.
FIRST A SAFETY WARNING!
Wood gasification involves the production of toxic, explosive gas mixtures and the use of high temperatures.
A principal component of wood gas is CARBON MONOXIDE which is a toxic gas!
Carbon Monoxide Poisoning due to misuse and misunderstanding leads to many thousand deaths world wide
The wood gasifier should always be operated in a well vented area, preferably outside and a carbon monoxide
monitoring alarm used at all times. Test that the alarm works and has a fresh battery!
Carbon Monoxide poisoning can overcome an otherwise healthy person without warning leading to fatal
results. Do not work alone and do not take any unnecessary risks.
The use of flammable gases and sources of ignition also presents explosion hazard and risk of severe burns. Wear
appropriate safety clothing including gloves and safety goggles.
The GEK is a very comprehensive kit and is
supplied with a full range of accessories and
instrumentation. (Pictured right).
In addition to the basic gasifier reactor, the kit
comes with dust removal cyclone, fine filter
can, suction blower with fan and motor, gas
hose (blue opposite) and swirl burner.
Instrumentation includes a 2 channel digital
thermometer with three K type thermocouple
probes and a U-Tube manometer for measuring
the pressure drop across the gasifier.
Also shown in this picture are the five stainless
steel flexy air lines used for air pre-heating and
the steel impeller and 3 speed 12V dc motor
used in the suction fan.
The woodchip fuel is shown below. It is a real
mix of chips, leaves twigs and bark. After
several months drying undercover the
moisture content is estimated at about 20%.
The density of this fuel is 180kg/m3 (11.2
lbs/cuft). The calorific value will be
approximately 3kWh/kg. (4650 BTU/lb).
From an estimate of the woodchip fuel
density it can be calculated that the fuel
hopper (30 gallon drum) and the upper part of
the reactor vessel will hold a total of 25 kgs
of woodchips. A maximum woodchip
consumption of about 18kg per hour is
The woodchips will be up to about 20%
moisture content, so a single batch burn will
be around 20kg of chips with the penalty of
having to drive off about 5kg of water.
View into gasifier reactor vessel showing the 5 radially inwards
pointing air nozzles. These are made from 1/2" BSP cast iron
Above and left: View of the woodchips fuel - with a 1" self
tapping screw for scale comparison.
This wood chip waste is direct from the tree surgeon's chipper.
As well as wood chips it contains leaves, bark, twigs, dirt and
possibly other foreign objects.
Test have been conducted to prove that it burns furiously with a
jet of preheated air. However I suspect that it may present some
challenges for gasification. Fuel handling may cause problems
so additional shaking and augering may be necessary to make it
feed successfully. This is as yet a big unknown.
This wood chip fuel is a locally available "free" resource. It is
often dumped to rot. Despite its poor consistency and low
energy density, if it can be utilised in a gasifier and provide
heat and power without pollution, it could provide
considerable economic savings. This is what I aim to discover -
can you run an engine on this trash?
This is the wood chip fuel feed auger.
It's made from a 6" post hole auger
bit contained within a 6.5" diameter
tube made from 2 fire extinguisher
cans. The auger bit has 5 complete
cycles so is approximately 30" long
The necks of the fire extinguisher
canisters provide suitable mountings
for plain bearings for the auger shaft.
A geared motor is on the right,
which makes 1 turn every 50
seconds. Woodchips flow into the 5"
square hole towards the right hand
end of the tube, and are transported
to a similar sized exit hole,
underneath at the left hand side. It's
currently intended to use this
horizontally to transfer wood from a
larger hopper to the gasifier.
The Project Plan
The Lister generator already provides up to 3kW of
electricity and 6kW of hot water for domestic heating.
It currently burns 3 litres of waste vegetable oil every
WVO is in short supply and getting expensive. Much of
the original supplies are being bought up by biodiesel
manufacturers. Health and safety and waste licensing
regulations in the UK has made it more difficult for
the individual to obtain sufficient supplies.
Diesel engines can be run in "dual fuel mode". A
small amount of fuel oil is injected in the normal way,
but the bulk of the fuel charge is introduced into the
air intake in the form of a flammable gas and air
The gas is generated by the thermal decomposition of
woody biomass, or woodchips, by a process known as
pyrolysis or gasification. This process liberates wood
gas or producer gas, which has a calorific value
approximately 1/6th that of natural gas. ~ About
5.2MJ/m3 or 140BTU/cuft.
(Wood gas can vary between 5 and 7 MJ/Nm3.)
To obtain an explosive mixture for the engine, the
wood gas needs to be mixed with air in a ratio of
The woodchipper waste is not the ideal fuel for
gasification because of its small particle size and
varied consistency. The best fuel would be uniform
wood blocks about 2cm cubes.
However, being able to utilise this trashy fuel is a
key part of the strategy, and key to the economics, in
both time and money, of running this gasifier system.
Small twigs may give rise to bridging within the fuel
hopper or cause an obstruction between the air
nozzles. These could be removed by screening the
fuel first through a sieve.
This sample of woodchips contains a fair percentage
of leafy green material. As this material is
practically paper thin and dry, it is likely to be
consumed within the oxidisation zone first (small,
thin material burns better) and give rise to a greater
percentage of fine ash or char-ash.
It will be interesting to note whether this material is
consumed preferentially in the oxidisation zone,
allowing the larger wood chips to pyrolise and char
and pass into the reduction zone.
The small chip size of the fuel, should show a rapid
pyrolysis and the finely divided char should provide
a large surface area for reaction within the
reduction zone. Hopefully the fuel problems will be
overcome and not present insurmountable
Energy Balance - Some Lister Specifics:
The 6hp Lister currently uses 3 litres of veg oil in 2
hours when generating a 2.5kW electrical load. It's
about 18.5 to 20% overall efficient in this respect.
About 6kW of waste heat is produced as hot water for
The calorific value of veg oil is about 9kWh per litre.
So each hour, the fuel energy input requirement of the
Lister is 13.5kWh.
We need to produce sufficient wood gas to
approximately match this energy requirement and
make sure that the Lister can draw sufficient capacity
in each stroke.
The Lister has a swept volume of 1.433 litres. At
650rpm, it will draw 27.94m3 of air in per hour. To
meet the stoichiometric ratio, half of this must be
woodgas - approximately 14.0m3.
The energy in 14m3 of woodgas is 72.8MJ or
20.22kWh. So the induction stroke of the Lister
exceeds the conditions of energy flow needed for
engine operation by approximately 50%.
There will however still be a small quantity of fuel
oil injected into the cylinder and this is compression
ignited in the usual manner and serves as pilot
ignition to detonate the main gas air charge.
Dual fuelling allows the amount of diesel fuel or
WVO to be cut to about 20 to 25% of its previous
consumption. About 1.5kg of wood is needed to
produce 1hp per hour at the engine. This suggests that
the Lister will use between 9 and 10kg of woodchips
per hour. The hopper of woodchips should last for 2.5
hours before refilling.
Most gasifiers built today follow the basic design
principles of the Imbert downdraught gasifier
devised by Jacques Imbert in the late 1930s. This
design was used extensively and copied and
manufactured widely during WWII.
This design provided much of the basis for new
research in the 1980s, and again now, 20 years later.
Until recently, if you wanted a gasifier, you would
have to design and build your own. In order to
achieve gas of suitable quality for running an engine,
the gas has to be cleaned, cooled and free for tars that
would otherwise quickly damage the engine valves.
The downdraught gasifier is one of the few designs
that meets the requirements of low tar gas. For this
reason it is the prime candidate choice for engine
The downdraught gasifier is based on a 70 year old
design known as the Imbert Gasifier. A description
of this type can be found in The Handbook of Biomass
Downdraft Gasifier Engine Systems - by Dr. Tom
Reed, and Agua Das, notable gasifier scientists.
Recently, following the efforts of Jim Mason and his
colleagues at Allpower Labs, you can now purchase a
ready made downdraught gasifier - below.
This is my 1953 6hp Lister diesel Startomatic
generator set installed in the renewable energy
workshop. It supplies heated water and power
to my house. Currently it runs on waste veg oil.
The aim is to run this engine using wood gas
produced from woodchip waste from tree
surgery, plus about 20% veg oil in dual fuel
Engines have been powered on wood gas for
just over 100 years, with main developments in
Europe and Scandinavia occurring during
wartime fuel shortages in the 1940s.
These pages look at my wood gas project.
Woodgas and Slow Speed Diesels
The Lister CS is a slow speed diesel at just 650rpm
working. The engine construction is heavy and is built
to last. No-one in their right minds would build a 6hp
engine that weighs 330kg these days (- apart from Indian
Wood gas has a slow flame speed, and this is well suited
to the relatively slow piston speed and long power
stroke of the Lister.
Work by H.S Mukunda has shown that wood gas does not
suffer from pre-ignition problems in diesel engines up
to about 17:1 compression ratio. This should suit the
Lister just fine.
One of the key innovations of Jim Mason's GEK
design is the use of flexible stainless steel
corrugated gas-lines to provide a gas to air heat
exchanger. This not only cools the exiting gas, but
warms the incoming air to about 600C.
Whilst waiting for the paint to cure, I decided to
have a trial fitting of the air-lines. Some adjustment
of position will be needed before I finally seal the
joints with PTFE tape.
These lines are 1.8m long and present considerable
surface area for heat exchanging.
Saturday 17th January 2009
The weather was above freezing this weekend, so I
took the opportunity of painting the GEK with high
temperature stove paint. The paint is manufactured
in Oregon under the "Stove Bright" name and I
chose a Forest Green shade to match the usual
mid-Brunswick green traditionally used on Lister
engines. The colour, to me, looks a sort of
camouflage green - ideal for any clandestine
gasification activities in the woods.
The Stove Bright paint should be good for 650C
(1200F) which is well suited to the exit temperature
of the gas adjacent to the grate.
The cyclone and filter can were done in a matt black
high temperature paint obtained from an automotive
accessories shop. Minor parts such as the grate
rotating handle and the legs were done in high
temperature black enamel - good for 250C.
If the metalwork is visible and exposed to air and
moisture - paint it.
Text Box -place
Shown above is the access hatch in the bottom of the
main gasifier housing - just below the grate. The kit
is supplied with a malleable weather seal strip,
3.2mm diameter which is used as a gasket seal to
make all the metal to metal joints gas tight.
On the right is the painted cyclone and filter can.
Another view of the newly painted GEK with my
Chinese mill/lathe in the background.
Hopefully there will be some time tomorrow to
complete the assembly, seal all joints and mating
surfaces and perhaps even have an initial first run.
On the right is the reduction cone with it's 76mm
diameter opening. A short piece taken off the old
cylinder liner above it will slide in, up to the
shoulder, and reduce this aperture to 65mm. This
will make the reduction cone dimensions more in
keeping with Kaupp's table of measurements.
Below: About 9 hours after shaking the first can of
Stove Bright spray paint, the GEK is almost fully
assembled. I rather wish I'd chosen gloss instead of
matt black for the cyclone and filter can.
Whilst most of Jim's GEKs are bright red - this one
is a slightly tamer Forest Green.
The instrumentation needs to be added, as do the
PTFE sealed joints - but hey, tomorrow is just
Having spent a day painting and
assembling the GEK, I must say that
I am very impressed with the kit and
the quality of the workmanship.
The best part was wrestling with the
serpentine air-lines - which
reminded me somewhat of a prop
from Dr. Who and alien mothership
The use of weather seal strip for all
gas tight joints is inspirational, and
very easy to do in practice.
There was only one bolt hole in the
blower fan motor plate that needed
filing to make it fit - so I am very
pleased with the fit and finish.
Gasifiers are functional chemical
reactors and as such undergo
extremes of temperature and harsh
oxidising and reducing atmospheres,
not to mention the general problems
of dust and tars. Moreover the GEK
is an experimental tool to learn the
basics of gasification and it is
expected to have a hard life.
The wood chip fuel I propose to use is probably
about 3kWh per kg useful calorific value. The
gasification process is about 70% efficient.
The density of the woodchip is 180kg per m3 - so a
208 litre standard drum will contain about 37.5 kg.
Ideally, I would be able to fill a drum with
woodchips in the morning, start the Lister on diesel
or WVO and then leave the engine charging
batteries for 5 or 6 hours without having the need
for further attention.
Being able to batch load a gasifier like this would
really improve the user friendliness of the biomass
energy route. Mechanical fuel stirring or transport
might be necessary - but if it allows several hours of
unattended operation - just like we take for granted
with liquid fuelled engines - then this will be a real
breakthrough for biomass.
Text Box -place
Text Box -placeholder
The annular gap around the reactor vessel was filled
with vermiculite, as was the gap around the
reduction cone. This will help keep the heat
released during oxidisation within the reactor
Tuesday 20th January.
The GEK was ready for its first burn. Simon my
gasification friend came over to witness the event.
The reduction cone was filled with charcoal pieces and
this was ignited using a propane blow torch with the
blower turned on. Once the charcoal was well alight, a
few handfuls of the wood chip fuel was added on top
and the air -nozzle caps removed - so that the air was
directed via the nozzles into the wood fuel.
After about 5 minutes, flammable gas was produced
and the burner flare was lit. The flames were fairly
light in colour where they emerged from the gas-jet
tube, becoming more pink/orange at the tips.
Below are some photos of typical flames from the
swirl-burner. As you can see, they range from
orange-pink to a much lighter blue colour, which is
indicative that the wood fuel has been converted to
charcoal and the gas is mainly carbon monoxide.
As the fuel load is consumed, to the point where there
is just charcoal remaining in the reduction tube, the
flame colour changes to a distinctive blue colour.
In 3 trial runs I burned a total of 60 litres of wood chip
fuel and has flarable gas for almost exactly 3 hours.
The GEK will run for about 1 hour on the integral fuel
container and the char that remains in the reduction
cone. Its best to leave this char in the cone - it makes
lighting much easier the next time.
The 24" thermocouple probe was used to check
temperatures. In the oxidisation zone by the nozzles the
temperature peaked at around 1100 degrees C. In the
reduction zone it was between 650 and 850C.
The fine fuel tended to make very small pieces of
charcoal - about pea-sized. This is good for providing a
large reactive surface area, but at the disadvantage that
is can lead to a reduction in gas flow in the reaction
zone. Regular grate shaking is needed to maintain the
Additionally the wood chip fuel is prone to bridging, so
regular poking and stirring was needed. This will be
eventually automated using a windscreen wiper motor
and agitator/stirrer arm.
The first three trial runs have proven that the GEK can
use the trashy wood chip fuel provided that some
means of mechanical grate shaking and fuel stirring is
The GEK is producing an estimated 10 to 12kW
thermal power, based on the fuel consumption of 20
litres (3.6kg) per hour.
This should be sufficient to offset about 75% of the
vegetable oil needed to fuel the Lister.
With outside temperatures close to freezing, Simon
finds the warmest place is next to the gasifier.
View of the swirl burner with a typical flare.
Camera flash obscures the real flame colour.