Sunday, August 3, 2008
Biogas generator part II
I proved to myself that it will run on one form of gas fuel by putting a propane torch in the carb entrance and slowly cutting off the gasoline -- do not try this at home! With a little manipulation of air and propane, I could keep it running. This step, apart from being dangerous for beginners, is completely unnecessary. We already know gasoline engines can be converted to run on methane. But it gave me some ideas for how to build a methane-injection manifold to replace the carb, reminded me that I would need a throttle valve/choke to control air supply, and taught me that the engine speed governor would have to be disabled.
Then the cutting and fitting of a Lexan top to the plastic barrel which will become our digester vessel. Lexan because this is a science project and so it's nice to see the inside, even if it is just a mess of pig's poo. I used a machine bandsaw for this, an expensive tool, but if you don't have one, you could use a coping saw or hacksaw blade. Warning -- Lexan doesn't tolerate anything except the finest toothed-blades. And be sure to wear eye protection for all cutting operations.
This assembly will have to be sealed up completely with a silicon gasket for anaerobic decomposition to begin. I also fitted the gas pipe, using a brass pipe fitting which, give a hole just slightly smaller than itself, was willing to cut it's own tread in the Lexan. All sealed up with plumber's TFE paste, which, like Lexan and the plastic tub itself, resists the corrosive properties of biogas. A vice grip placed on the outlet tube should serve as a gas valve for now.
Today I'll fill it two-thirds full with manure-and-bedding from our pigsty, add water, fit the gasket, clamp the hose shut and wait for decomposition to begin. According to all the bulletins I've read, the first few days and weeks digesters give off only CO2, before settling into methane production, so once filled and sealed, we'll set it aside in a warm, sunny, outside spot and bleed off any gas that forms every few days. Since one objective is to have everything ready for the Common Ground Fair, the timing of the switch from CO2 to methane production should be good.
I'll be looking around in the next few days trying to spy material for an engine-and-alternator carrying frame, which will have to be heavy steel, but could be railroad ties or similar improvised materials. I will need pulleys and belts. Also the two differentially sized oil drums that are used to make a "gasometer" for storing methane and removing carbon dioxide. One fits upside down inside the other, which is filled with water. A pipe from the digester feeds gas into the assembly, which floats on water, but is contained by the drum. Excess CO2 dissolves in the water to form a mild carbolic acid.
This material scrounging project might require a trip to Jackson, Maine's famous Bog Road junkyard. Always a fun excursion. You never quite know what you'll find.
What does this experiment prove? Not much, since this is a replication of hundreds of similar science projects around the world, since the Chinese and others have used biogas for on-farm electricity generation for years now, and since even in the west the commercial technology and engineering of methane recovery is now well known and becoming widespread. But it is still fairly arcane to most folk, and so making a project like this, particularly if it works well, is well-constructed and lasts, will help to educate Unity students to the value of biogas systems and their basic configuration.
Biogas generation is also one answer to a farm-and-climate change problem, the recovery of methane from livestock operations. Livestock farming is too valuable a source of protein for humans and fertilizer for agriculture to phase out, and regular readers will know I advocate and practice the small-scale, rotational grazing, low intensity livestock operations that reduces climate emissions. But I still have a pigsty and two pigs, very useful for compost processing, but also producers of methane, albeit in much smaller amounts than feedlot cattle. Delicate readers unfamiliar with the porcine race will be delighted to know that pigs experience flatulence quite similar in sound to that of humans. One of my porksters let a good one rip just the other day while I was in the barn, and it sounded quite similar to some British servicemen I knew when I was in the mob, a good old-fashioned Land Rover-emptying cannonade...
Potty humour aside, one fine day, most farms will either have on-site methane recapture built-in to farm compost-and-fertilizer systems, or, if they lack the necessary economy of scale, may be part of some local methane recovery cooperative, and ship their manure small distances to a plant of suitable scale.
Methane is too valuable a fuel and too dangerous a climate agent for us to just release it to the atmosphere the way we do now.