Friday, September 18, 2009
Still experimenting with small engines on biogas
After a week of cold days (averaging 15 degrees) with clouds and rain we finally got a couple of sunny days to fill our 200 liter biogas tank. We wanted to see how many minutes we could run the generator on this amount. Unfortunately a little over half of the gas was lost experimenting with the in-line oilers (which didn't work) so by the time we got the engine going we had less than 100 liters to work with. We ended up getting 3 minutes of run time from this (and we had to oil the crankshaft through the spark plug hole.)
All in all it may have been a godsend, because when we pulled the spark plug to see if it was still oily we found it dry. Had the engine run dry it might have been ruined.
We still feel the need to find a way to use two-stroke engines with biogas because 2-strokes are cheaper, more robust, smaller and more often found in "developing countries" and among the "poor" than 4-stroke engines. Obviously 4-stroke engines are much better suited for biogas because they are "self-oiling". But it would be nice to figure out how to get the right amounts of oil into a two-stroke running on our biogas.
As you can see in the video, we had put too much oil in through the spark plug port, leading the engine to smoke like crazy. Note that this smoke has nothing to do with the biogas. Biogas burns absolutely cleanly with no smoke or smell. This shows also that the major problems with two stroke engines (and why motorcycles, lawn mowers and rickshaws produce so much pollution) is almost completely due to the oil associated with the fuel.
This will be our last test of two stroke engines for now, until we can solve the oiling problem (maybe using motorkote will help? Since biogas is not a solvent like gasoline, if a better lubricant were in the motor maybe it would last longer.)
In Cairo at Hanna's next month we will convert a 4 stroke engine to run on "tri-fuel" (principally biogas, but with the option to revert to other fuels if necessary -- 4 stroke engine conversion kits give you hybrid tri-fuel possibilities!) and figure out how long we can run it on a 1000 liters of gas.
Our conservative estimate from this experiment, where we got about 3 minutes from about 100 liters is that we will get about 30 minutes -- a half hour -- from our Cairo systems. If the generator is a 1 KW generator and we can run it for 30 minutes we can "bank" about 500 Wh in a battery system. Then that 500 W can be used to power 5 twenty-watt light bulbs (100 watts worth) for about 5 hours (or maybe 4, considering losses). That would be a good result if each day or two a family could produce and store enough electricity from their garbage to run their lights for the evening.
Today's experiment is therefore encouraging.
Other encouraging things:
We had had concerns that this open tank design led to losses and that we would need to create totally airtight containers but now that we have covered the digester with plastic and seen it does not fill with gas we feel more confident that it does not lose a lot from the sides and we can save money and time continuing to use open telescoping digester designs.
In practice we haven't seen any filling of the plastic bag with gases evolving from the open gaps on the outside of the tank. This suggests that methane losses are negligible. Perhaps the anaerobic bacteria avoid these areas because of the possible air exposure and build their biofilms inside the total anerobic chamber inside.
One very useful comment we got on youtube was to try replacing the bricks with a 30 liter water tank (given that we need 23 to 25 kg to run the engine) which would allow us to vary the weight by adding or removing water and would distribute the weight more evenly. While the current design of the "cage" surrounding the gas collection vessel precludes this option for the moment, we will incorporate this suggestion into future designs.
Note we've had to abandon the air compressor oilers completely and placed the regulator where it should be, below the engine. Something that is not shown in the video is just how many times we tried to start the engine and were unable. Either there was too much oil in the line, or not enough gas, or the air-fuel mixture was wrong. Finally, by removing the in-line oilers, draining some of the oil that was stuck in the feed tube and priming, then starting (as usual for biogas with the choke wide open) we got it to start. But by the time we were able to get the gas flowing properly we had wasted more than half our gas, so we were running on a bit less than 100 liters which lasted the 3 minutes shown.
The very conservative estimate now, considering that we get about 12 to 15 minutes cooking on the same amount of gas, is that running the engine consumes about 4 to 5 times as much gas. In the 1000 liter systems where we get about 2 hours of cooking gas we thus estimate half an hour of electricity generation. But these are very conservative estimates based on a single flawed sample. The literature says that 1 cubic meter (1000 liters) of biogas should give about 2 KW of power. This might be for more pure gas however (our mixture of home made biogas is said to be about 60 percent CO2). We will be optimistic and hope for an hour of electricity with a 1 KW motor, but would still be happy with 500 Watts worth. That would be just enough to make this worthwhile at the home level.
We'll report back when we know more.
Suggestions and comments are welcome.