It started with a 1 lb. cylinder of propane, a gas grill regulator, and a needle valve. The engine was a yamaha 50cc (2 cycle) moped engine.
I simply pryed open the rubber than fits between the air cleaner and carburator. Then shoved the 1/4" clear plastic tubing down into the airway. Upstream is the needle valve and regulator, then the LP bottle.
The engine is started on gasoline. The needle valve is turned on slightly. It doesn't take much to smother the engine with too much propane replacing the air.
So once you get the engine to start and run on gasoline with just a little propane (running the engine quite rich)i shut the gasoline supply off. First the float bowl must run ampty before the engine truely runs on propane but it soon does. Now i have achieved running the engine, but at only one speed. This is gas grill regualtor, not a regulator designed for engine throttling. They do make them. I don't have one yet. That type of regulator can completely shut off if there is no vacuum across the output ventury. This experiment, combine with what i've learned about engine specific regulators was enough to satisfy me.
You need to know your engine displacement(divided by 2 four a 4 cycle)and the intended r.p.m. and know that it takes 10% gas to air. From that you can derive your cubic footage of gas production needed for a given time. (remember you're calculating for 1 atmosphere) If i take a 20# tank and compress to 250lbs. i can run a 50cc moped for 7.4hrs @ 2000rpm. or an 80" v-twin for 33 minutes @ 2000rpm. More than enough for the days commute.
Mind, you need to know something about propane and natural gas. Propane is liquified and the vapor above the gas can reach 250lbs. Natural gas (roughly the same properties as biogas) liquifies at much higher pressure(and lower temp). I'm not going to liquify the biogas, but for the experiment i used propane. This is the reason for the regulator. Bringing the cylinder pressure down to inches of water collumn instead of psi. Know your regulators, and know your stuff, when messing with gaseous fuel. In my estimation they never would have legalized gasoline by today's safety standards. I'm much more comfortable working with natural gas, be it petroleum or bio. Propane expands more than 600 to 1 yet is heavier than air, so it can linger on the ground unnoticed waitng for a spark. Methane is lighter than air, so it disipates up. Also i am not going to compress it that much so the expansion is less as well.
I can see by the combination of technologies why people have not adopted biogas, but it's nothing new. It's more of a lost technology. This is old fuel waiting to be harvested. The only reason we're burning gasoline today is purely political.
I'm an electronic engineer by trade. Electricity is a fine thing that has made much possible, But we've also spent way too much to simply extract it and pump it. The fine folks that build wind generators, pumps and mills have a grasp as well on an old energy source. I'm glad there are other people that ask questions.
P.S. on filteration - bubbling through water cleans most of the acid out of the gas. CO2 of course, can't completely be removed but is heavier than methane, hence the reason for the valve on the top of the holding tank. This gas, biogas, isn't 100% methane, but neither is petroleum natural gas. It's "good enough" gas.