Running+engines+on+biogas

=Running (small) engines on biogas.= The following is extracted from the Anaerobic Digestion Discussion List Archives at [], hosted by [|REPP], and is provided as information only.

No recommendations should be taken solely from this record and the Authors disclaim all responsibility and liability for any actions taken on the basis of this information.

Initial Request
Hi all,

I am Andre Daud from Indonesia. Currently we (I and my colleagues) are working to encourage the use of biogas, especially to smallholder dairy cattle farmer. There is no problem so far, but it seems some of the farmers were more interested to the use of biogas as fuel for generator due to poor electricity infrastructure in village areas. We, together with them, tried to modify common genset (gasoline fuel, 900-1200 watts output) to work on biogas. But we have not fully successful because our limited knowledge in engineering. Oh ya.. maybe we have enough background on animal-related knowledge but not engineering. Our main problem here is to determine how best to modify carburettor (mixing air and biogas) and how to determine the effect of engine compression. Perhaps this mailing list could give us best references or solution to our problem.

Thank you very much.

Andre Bandung, Indonesia.

Replies
Andre,

Biogas is a very appropriate technology for rural electricity. Can I ask 900-1200 watts if you are trying to run a 2-cycle engine? These have a couple of important draw-backs, 2-cycle engines are lubricated by mixing with the fuel. With biogas, some other kind of lubrication system I am not aware of would be necessary.

A small 4-cycle engine---possibly out of an old motor scooter or something? is ideal. You can advance the fuel/air mixture and timing to find the "sweet spot" for ignition. Also need to keep enough weight on the gas bag to produce sufficient fuel pressure. There's a number of different ways you can modify the carburettor for the biogas fuel line, but I've seen people just take the rubber fuel line and stick it down the carburettor throat---the four cycle has a separate value for exhaust and will keep fuel from flowing in during the power and exhaust strokes.

Here's a little 1 kW biogas generator made in China. They're about $450 US dollars. []

Note the mechanical fuel/air mixture advance. It appears to be a 4-cycle, note the oil filler cap at the bottom.

If you have a use for the waste heat, a steam boiler and turbine are the simplest and most powerful prime mover for a generator---90% of the electricity in the world is produced with steam---however, the trade-off is even low-pressure steam can be very dangerous.

Warren Weisman, USA

Apologies, I don't know if they're available elsewhere, but in the US we have 2-cycle lawnmower engines that one does not need to use fuel mix with and that oil themselves. If that is the case---as in the picture of the Chinese generator it has an oil filler cap---it would work with biogas (only much louder than a 4-cycle engine and not last as long).

In fact, it would have the advantage of magneto ignition, which is more reliable.

Warren

G'day All,

While I do believe in the KISS principle and really like Albert Einstein's quotation "Things should be as simple as possible, but no simpler!" I think the "hose down the carburettor" is "too simple".

The problem arises when the engine cuts out (granted this should not happen on a generator, but we have to allow for it). Biogas will still be flowing so there is a growing risk (likelihood!) of an explosion at restarting. The solution is a solenoid held open by generator output, or maybe a valve opened by engine rotation - you may need a starting arrangement as well. There is also a possibility of trouble with the simple hose during starting, so be careful.

Happy digesting, HOOROO

Mr. Paul Harris,

Andre, if you want to dual fuel in the simplest way, do not modify your carburettor at all. Instead remove the air cleaner off the top of it and insert a tee made from a smooth bore copper pipe fitting. You will need to make flanges at either end to fit the carburettor and the air cleaner. Then to the outlet on the side of the tee you place a butterfly valve of an old carburettor, or easier but not so nice looking a gate or globe type valve and then continue the pipe work onto your gas supply. You start the engine on petrol and then turn off the tap on the petrol fuel line and when the engine begins to falter as it runs out of petrol you gently open the gas valve until the engine picks up power again. With a little bit of practice you will get the engine running on gas and still kind of controlled by the governor and throttle valve on the carburettor. Now this set up will only work with a constant load on the generator. If people are switching electrical load off and on the engine finds it hard to follow the load. And, running uncleaned biogas through a carburettor sized throttle you will get a lot less power than you would on petrol. However, for demonstration purposes you can show how a duel fuel conversion works without too much sweat. Ken C.

All,

To add to Paul's mention of the critical importance of safety, please let me add a point about efficiency. Small engine/generators will convert biogas to electricity at direct efficiencies which are rather poorer than larger gensets. Thus, you might expect 20-25% conversion efficiency with smaller generators vs. perhaps 35% with larger generators. While either figure may seem quite similar, 20% vs. 35% is a loss of nearly half.

But the more important fact is that the picture of the generator provided that started this thread was of an air-cooled genset, which means that the energy wasted as heat is expressed as diffuse hot air.

Where one is working with a water-cooled genset, /system /efficiencies might reach 80%, because a very good portion of the 65% of the energy in the biogas which is not provided back as electricity is poured into the cooling water, and can therefore be far more easily used to heat the digester. By contrast, hot air cannot easily be gathered and used to heat the digester.

Thus, when using a small genset with poor direct conversion efficiency which is also air-cooled, one tends to lose twice.

d. -- David William House

David (//Paul, actually//) I too am an Einstein Fan! My latest one is "The only thing more dangerous than Ignorance is Arrogance!" You are talking about small petrol engines which normally have very simple filter type air cleaners. If you replace those with an oil bath air cleaner and pipe in the gas underneath that, you have a low pressure non return valve to atmosphere. And you can balance the suction on the gas line against the several Inches water gauge of suction needed to work an oil bath filter. Larger engines, particularly diesels, normally have them anyway. Ken C.

G'day All,

Ken may be talking about larger diesel engines, but the "oil bath" air cleaners used on tractors around the 1960s pass air over the oil surface (to reduce pressure drop) and through a mesh/wire wool filter rather than through the oil as Ken indicates. This "over oil" type would not stop gas flow, so still be careful.

If you don't like Wikipedia <[]> try "Tractors and their Power Units" by Barger et al, 2nd Ed, John Wiley & Sons (1963),pp 193-194.

Happy digesting, HOOROO

Mr. Paul Harris,

Paul et al Hi! Brought up on a N.Z farm in the 50s and 60s and drove more than a few tractors in me time!!! The Air intake is a tube down the middle and the coarse wire net was around the outside. The mark to fill the oil was just above the bottom of the central tube. When the engine started, the air being sucked in down that central tube splattered the oil up into the wire gauze around it and kept it wet and sticky and washed down so that all the dust ended up as sludge in the oil bath itself which could be easily removed.

When the engine is not running the oil covers the end of the inlet tube, and if you try to push gas in below the air cleaner you have to push all the oil in the wider bath back up that central tube, several inches of it before you could start to push bubbles of gas back up through the column of oil the that gas pressure itself was holding up! According to Wikipedia "Lighter and smaller particles are trapped by the filtration media in the insert, which is wetted by oil droplets aspirated there into by normal airflow." And I trust that that is how I described it! But they missed out on the increased pressure required for reverse flow!! Ken C.

//(I suspect that there may be several different designs of oil bath air cleaner – if you are using Ken’s suggestion make sure the air has to pass through the actual oil and that the intake line will hold a back pressure. Paul Harris)//

Response
Hi all,

Thanks for advices. If I am correct, I don't need to make any major changes in carburettor. We have tried it. It seems that there should be no air flow at once we start to ignite the engine. It works. One more question: we don’t have a chance to use iron sponge (to scrub water contents?), so what will happen to the genset? Or maybe, because of that, our genset sometimes go unstable (not stationary) and knocking hard.

I agreed with Mr. Warren. We don’t have abundant source of technology, especially biogas. Our main source of biogas technology is just from old book published by GTZ around 1980-90. I am sure that the biogas tech has changed dramatically over ten years. FYI: we don’t do this for making money, we just jump to the remote area, hope we could do something to share with people there, especially to smallholder cattle or dairy farmer. As you aware, along with global trade, it seems there is no more incentive to small farmer to keep farming (everything imported was cheaper!). Maybe biogas can be their main reason to stay keeping cattle in future.

Thanks. Andre Bandung, Indonesia.

For Further Information
“Engines for Biogas” [] "The Complete Biogas Handbook" |[|www.completebiogas.com]|

Compiled/Edited by Paul Harris – paul.harris@adelaide.edu.au