[Digestion] small generators

[Leslie Gornall]

Warren,  you could take a practical approach to this problem.   For a start biogas will generally derate an engine,  take an engine with about twice the output you require (this engine can be derated by 50% for biogas).  Then start it on liquid fuel.  The engine is now running on an air fuel mixture that will most likely be controlled by the control system on the generator.   If you bleed biogas (at constant pressure) into the air intake the total hydrocarbon fuel inducted into the cylinder will increase (and the amount of oxygen inducted will decrease.)  Initially as the flow rate of biogas in the fuel increases,  the engine will work harder.  The reaction for most generators is that the carburetta will close down and starve the engine of liquid fuel. However, the engine output will still stay high. Progressively increasing the biogas should close down the liquid fuel consumption to about 5%-10% of normal running.   Now as the liquid fuel is reduced the large volume of the biogas in each cylinder full of mixed fuel will prevent oxygen from entering the cylinder just by displacement.  For each engine there is an optimum setting for the biogas.   The benefit of running the engine like this is that the liquid fuel acts as an igniter so that very poor quality biogas will still power the engine. 

Electronic ignition circuits are generally better for biogas as the spark is fatter and poor quality biogas can be difficult to ignite.

The system is used on diesel engines and the diesel fuel is the igniter.   I have seen several 1 megawatt units started on diesel and the biogas manually increased until the engines are at full output and 5% diesel consumption.  If you can live with using a small amount of puchased fuel then this is a good way to go for small units.   You could make your own biodiesel and then extend the running time for each gallon by 10x with a biogas amendment.  The second fuel is also useful if your biogas plant runs low.   The end user always demands reliable service of the system.

Old slow running Lister engines are great for this kind of work.  Does anyone remember the work of Fry in South Africa in the 1970s - look up his approach.

Slow running engines give the flame time to propagate across the face of the piston but require heavy flywheels.  

I hope this is helpful.  

Best Regards
Les Gornall



> Date: Sun, 30 Mar 2008 10:13:33 -0700
> From: weiswar at yahoo.com
> To: digestion at listserv.repp.org
> Subject: [Digestion] small generators
> 
> 
> Is anyone familiar with an "off the shelf"
> spark-ignited four cycle generator that could be
> converted to biogas smaller than the Honda EU1000 ?
> Possibly something in the 250-500 watt range? 
> 
> Not sure the Honda has high enough compression, as
> over 8:1 is necessary for biogas---though, maybe Honda
> makes some different piston shapes to improve this? I
> noticed the Chinese are taking old diesel rigs and
> punching spark plug holes in them to run biogas, so a
> tiny diesel generator might work, too? 
> 
> Thanks, 
> 
> Warren Weisman
> USA
> 
> 
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