My final trip to the RTC this time around was to run an experiment on the diesel engine running on biogas to try to ascertain just how much gas was being used as apposed to diesel fuel. I bought a few odds and sods at the Chinese market including a large syringe to meter the amount of diesel going into the engine.
Unlike the propane I used last year the biogas will not self ignite, therefore it will always need some diesel fuel for both ignition and lubrication of the injectors. There are also some issues of the detrimental effect that the biogas (which contains I unknown amount of sulphur) will have on the engine.
The plan was to disconnect the diesel fuel tank and fill the syringe with diesel directly into the fuel inlet. We can now measure how much fuel the engine was consuming for a set speed. We could then run the engine with the biogas and see how much less fuel was being used. The gas was regulated by the valve at the opposed hose connection end to the mixing chamber, as apposed to the valve at the mixing chamber end. The reason for this was that we wanted to take a measurement of the flow of gas which would have been difficult with the mixing chamber. The valve was cracked open slowly until the engine started spluttering as the governor ran through its confusing cycles of not really knowing what to set itself to and then the valve was closed slightly. At this point we reckoned that we were running the maximum amount of biogas that we could get away with.
We ran the experiment at a number of different speeds and we concluded that we were running the engine on biogas however it was not a large percentage about 15% at the higher revs. And I have to admit to being a little disappointed. However we have to look at these things in perspective.
1. We know we have issues with the digester, there is manure/ water mix oozing out of the containment plates and the discharge chute is still bubbling with methane, therefore not all of the biogas is being extracted. We have no idea of the composition of the gas and no means of testing (there could be as little as 30% methane in it). I am bringing a sample back with me for testing and I will be looking for a testing kit of some form to take with me on my next trip. If anybody has any ideas I would be more than welcome to listen to them. Clearly improving the digester function will improve the biogas quality.
2. The gas was not scrubbed in anyway, so there is C02, water vapour and sulphur in various amounts in there, the C02 and the water vapour should cause too much of a problem but the sulphur could be an issue. My next step will be to construct a crude charcoal scrubber to try to remove as many impurities as I can prior to ignition.
3. The engine itself is in very poor state of repair and will soon be destined for the scrap yard. As I have mentioned in previous blogs the low compression caused by the bad condition of the engine can be seen as helping the process and did allow the engine to run on 100% propane. However, logic would dictate that as the tolerances in the engine start to degrade the running efficiency will also degrade, this maybe a benefit or a hindrance to the running on biogas, at the present I don’t know however it is a variable.
On the conclusion of the experiment we measured the pressure of the gas using a manometer ( yes there was tall man holding the open end) and the pressure measured at 37”H2O ( about 9.25 Kpa )and the flow rate was 6 litres/ min. We measured this by filling a 1 ½ Litre Pepsi bottle full of water and holding upside down in a bucket of water and inserting the biogas pipe into the bottle, turning on the valve to the set point and counting how long it took the gas to replace the water.
