GAS-L: RE; Engine Experience

["Graeme Williams" <graeme@powerlink.co.nz>]

Dear Tom M,

Here is an attempt to answer your questions which I can only provide from
experience we have acquired over the years. I appreciate you ask specific
questions, but if you want to do it right, then the whole exercise begins
with where the installation is to be located.

Given we are talking about a gasifier that delivers tar free gas without
some sort of exotic clean up system, almost any engine built as a generator
set will do the job.

Engine:
The choice of engine should be one that is the most popular used locally, so
that the servicing and spare parts are available if required. Outside of the
USA, spark ignition gas engines as used for natural gas are not thick on the
ground, making it necessary to carry consumable spares, usually two years
supply.  Spark plugs can be a major problem if dirty gas gets to the engine,
or the engine is converted from diesel where the plug is deep in the head
causing it to cook.  Ceramic H.T ignition connectors can also be a problem
and they don't like rough handling.

Maintenance:
Even though the gas can be very clean, the presence of moisture will ensure
carbon blacks reach the engine.  These are not harmful, but they do reach
the oil, as does the moisture.  Time or distance has never been the criteria
for oil changes, and with producer gas, it is more important to monitor the
oil condition and change it before the failure of its additive package
causes acid erosion and excessive friction from loss of boundary
lubrication.

Because we used to make them, the addition of a bypass oil filter will
control the moisture and keep the insoluble levels down; all of which can be
monitored using the Blotter Spot Test.  Unless excessive heating becomes a
problem, oil filters are changed at the interval recommended by the
manufacturer.  Oil filter replacement should ensure the engine maker
supplies them as replacement brands often have less filtering surface
shortening service life.  Air cleaner life can be monitored and determined
by the pressure drop, which should not exceed 10" W.G.  This is a choice you
can opt for in places where dust isn't a problem.

Best Choice of Engine:
Dual Fuel:
Where the genset is the only source of power, my choice is to recommend dual
fuel diesels as there will always come a time for the gasification to stop.
Where this option is preferred, the engine pistons should have austentic
steel crowns for the top rings.  Aluminium pistons do suffer from top groove
hammering caused by reduced diesel flows on dual fuel.  Valve seat recession
can be a problem with cheap engines.  Inlet manifolds with entry at one end
will cause uneven gas flows to the cylinders and this results in unnatural
bearing wear on the crankshaft.

With appropriate modifications, this would be the cheapest option if you
were buying new engines.

Spark Ignition:
Any of the engines made for natural gas are appropriate for producer gas,
but most rely on turbo charging to get the power up and this introduces a
potential problem in the after cooler if fitted.  If dirty gas reaches the
engine, the fine tubes on the cooler block with condensed tar.  Not a
problem 'if' your gas quality never changes.  The same applies if the engine
is a C.H.P system where the exhaust heat exchanger also blocks.

Engine Life:
Because producer gas derates the engine, it can never be pushed to reach the
maximum output levels established with other fuels.  Under these
circumstances, the engine life should at least be equal to fossil fuel use,
given the oil is monitored as previously mentioned.

Engine Size:
For 200kWe, you also must remember to add 50 or 60 cycle as one runs at
1500rpm and the other 1800rpm.  This also highlights the need to ensure the
ancillary electrical system of the gasifier matches the electrical frequency
if the two are from separate supply.  These are figures for naturally
aspirated engines, as turbo engines were not considered appropriate for
remote installations.

Spark Ignition     50 Cycle      43 litre cylinder volume
                             60 Cycle      36 litre cylinder volume

Diesel Dual Fuel 50 Cycle     35 litre cylinder volume
                              60 Cycle     29 litre cylinder volume

As you can see, the cylinder volumes are considerable for a singular engine,
so twin engines could offer a cost alternative.  If you can take a risk,
turbo engines seem to be the way to go, but until my associates show me
their results from current testing, I will stick to that which I know best.

The Dilemma:
There are no engines specifically built for producer gas, and I would avoid
any that have been modified internally for the purpose.  The standard
engine, other than pistons and valve seats is perfectly okay, if we just
size them larger for our needs.  Any internal modifications to a standard
engine makes it an orphan and difficult to service if parts are required.

In a do it yourself project, there are many opportunities to use old
engines, but if you implement a commercial project, junking a new engine
will kill your project and most likely your reputation.

The engine remember, is part of an integrated system and it will only be as
good as the gasifier.  To interface the gasifier and engine is another
system again - not so hard with a base load, but tricky with stand alone
systems.  For me to skip over the main points regarding engines fuelled with
producer gas just doesn't do the subject justice.  It should I hope indicate
that there are plenty of engines to choose from for every size of project
and they can work like 'Swiss Watches'.

8,000 Hours:
I have not heard of any ordinary gasification system where the engine
operates 24 hours a day, but there shouldn't be a problem if the gasifier
can deliver an uninterrupted gas supply.  This amounts to a 100% duty cycle
for the gasifier, not impossible, but pretty demanding on components.
Realistically I think you could certainly operate an engine to the maximum
less the servicing requirement, with possibly improved life from never
starting from cold.

Fuel Moisture:
The danger of high moisture levels in the fuel is that it can slow down the
oxidation zone temperature and watery hydrocarbons (tar) enters the gas
coolling and cleaning system (pretty messy).

If the gasifier is of reasonable design, the oxidation temperature will stay
high enough to handle say, up to 30% M.C.  All this does is increase the
amount of water condensate without the tar coming through the system.

The engine might slow if the condensing moisture is carried through an
undersized gas cooler in which case saturation of the filter system will
reduce gas flows.  It's better to have good fuel quality!

Finally, if we accept that engines can do a great job generating power from
a gasifier, given we do have to pay attention to detail, what's the problem?

Without a doubt, it's the people we attempt to help to use their own
resources.  They don't follow operating procedures, change fuel
specifications and become greater experts than those who supply the
equipment.  We do our best and they do their darndest!

Hope this helps

Regards
Doug Williams
FLUIDYNE GASIFICATION





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