[Stoves] why can't we use wood gas directly in an i.c. engine?

ajheggie at gmail.com ajheggie at gmail.com
Sat Oct 5 06:55:00 MDT 2013


[Default] On Sat, 5 Oct 2013 06:41:23 +0530,Anand Karve
<adkarve at gmail.com> wrote:

>Dear Stovers,
> fuel in the form of a combustible gas seems to be a pre-requisite for an
>internal combustion engine. Even liquid fuels are atomised before
>introducing them into the cylinder of an i.c.engine. Wood gasifies when
>heated, but this gas is used in internal combustion engines only after
>filtering out the tar. Why is it necessary to filter out the tar? 

This is more a question for the sister [gasification] list, it's
complex. Cylinder walls and valves are cold relative to the offgas so
tars condense on the surfaces, they then contaminate the oil. We are
used to very high mileages/hours between oil changes with expensive
oil, running on dirty gas reduces the service interval and increases
costs. There was a large pyrolysis plant in Germany that distilled
pyrolysis offgas of beech to produce acetic acid and the gases left
over were used in a big reciprocating engine  but it had to be
stripped and cleaned regularly.

On the other hand a reciprocating engine will run quite happily on
biogas from a digestion plant, a friend has a 200hp MAN running on
biogas which has clocked 27000 hours from new and is just approaching
its first major service.

In UK town gas (basically the offgas from pyrolysis of bitumous coal)
was used to run reciprocating engines but the gas was thoroughly
cleaned before being injected into the network. As the town gas plants
became redundant and ripe for redevelopment there were problem from
the soil being heavily contaminated with nasty organic compounds from
the cleaning process.

You would have less problem running the engine on char in a down
draught gasifier.

There is a class of ic engine that will run quite happily on pyrolysis
offgas but it all becomes a trade off of capital cost verses operation
and maintenance costs, on the large scale a steam turbine has a fifth
of the O&M costs of a large diesel, so even though the diesel may be
25% better at converting the heat energy to motion if the fuel is
relatively cheap the steam system wins. ( generally coal is far
cheaper than liquid or gaseous fuels).

AJH



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