[Bioconversion] Veg Lister - torrified woodcips

Tue Nov 7 03:09:09 CST 2006

Rex & List,

A simple experiment can be done using a propane torch, and introducing
propane into the air intake of the diesel engine.

Provided that the engine governor is working and maintaining a fixed speed,
it will be noticed that the governor acts to close the rack on the injection
pump, thereby reducing the fuel (veg oil) injected.

Details on the Ankur Scientific gasifier site, suggest that the quantity of
pilot fuel can be reduced to about 20%, when wood gas is introduced in this
manner.  As the calorific value of the gas is likely to be variable in
nature, the governor will adjust the position of the injector pump rack to
compensate.

Wood gas should be mixed with air in approximately 50% ratio, using some
sort of a mixing valve.  It should be remembered that the gasifier is
relying on the suction from the engine to draw air through and gas out.  The
mixing valve should ensure that the correct air ratios can be maintained for
efficient gasification.

On the 6hp Lister, the IP rack has sufficient travel to completely cut off
the fuel supply, so closing down to 20% injection should not be a problem.

The whole question of running a gasifier with a diesel engine in dual fuel
mode is a very exciting one, particularly when it forms part of a combined
heat and power system, that has to cope with varying electrical and thermal
loads.   In Northern climates where the heat required might approach 10
times the electricity requirement, this presents an interesting concept,
where the gasifier can be used to provide additional thermal output when
needed.

It should be remembered that the diesel engine rejects approximately 1/3rd
of its heat energy via its exhaust. Efficient utilisation of this heat is
key to maintaining overall system efficiency.

As the exhaust gases are emitted at approximately 300 to 400 C, they are of
the right temperature range to initiate the drying and torrefaction of the
woodchips, prior to them being pyrolised.

Torrefaction is generally done by heating the biomass to around 250 to 300 C
and maintaining it at these temperature for between 30 and 60 minutes.
Torrefaction serves to drive off the volatiles, and reduce the internal
water content of the fuel, resulting in approximately 30% mass loss, but
only 10% energy loss, thus the energy density of the fuel is increased from
around 19MJ/kg  to 22MJ/kg.  The 10% of energy loss driven off with the
volatiles can be recouperated by circulating the torrefaction gases back
through the incandescent charcoal bed. These volatiles will be further
cracked and this will reduce the overall tar content in the final pyrolisis
gases - which is a good thing if you are wanting to run an IC engine.

Torrefaction alters the mechanical nature of the wood, making it more
friable and brittle. This means that it is easier to process, and using a
mechanical grater system such as proposed in the Kalle gasifier of 1943,
torrefied wood could be ground within the gasifier to porduce smaller
particle sizes - and thus more efficient gasification.

Torrified wood is a stable substance and can be stored without re-absorption
of water and the risk of the mosisture content rising, such that would occur
with wood or charcoal.  Torrefied wood is a more homogenous fuel, and if
produced from chips of the right particle size could provide an alternative
fuel for woodstoves and possibly pellet stoves

As only 10% of the energy content is lost in the torrefaction process, it is
a far better fuel for barbeques compared with charcoal, and is easier
toignite and burns with less initial smoke because the volatiles have
already been partly driven off.

So the hot exhaust of the Lister could be used to produce batches of
torrified woodchips, which could either be used immediately within the
gasifier,  or allowed to cool, stored and used much later for barbeque or
heating fuel.

There is approximately 2.5 to 3kW of thermal energy available in the exhaust
of the Lister whilst running at full output.  Whilst mostly consisting of
hot nitrogen, there is CO2, CO, H2O and some excess air O2. There will be
some carbon particulates (soot) and possibly some unburnt fuel. All of these
could be returned into the incandescent reduction zone of the gasifier for
further utilisation.

Any thoughts,

Ken

----- Original Message ----- 
From: "Zietsman, Rex" <Rex at Process.co.za>
To: <bioconversion at listserv.repp.org>
Sent: Tuesday, November 07, 2006 4:33 AM
Subject: [Bioconversion] Veg Lister

> Ken,
>
>
>
> Sorry for the slow brain... If I understand you correctly, the governor
> will cut back the veg oil feed to the engine based on engine speed. I am
> assuming that the fuel injection pump rack can cut back that far when
> gas is being introduced. If so, it is a great win. The trick then is to
> get a link between the governor and the gas feed such that the veg oil
> contribution is about 20%. As you say, if there is no gas available at
> the time, the fuel injection pump rack will automatically be controlled
> by the governor to provide all the energy. This is a particularly clever
> way of running a Lister.
>
>
>
> Rex
>
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