biogas generating energy

I am an interested beginner.  The idea of biogas in general is incredible to me and I often wonder about other energy generating ideas that could be implemented around a house or a farm.  For instance, would it be possible to run a generator on a "hot" compost bed if you had a radiant heat type tubing system under the pile of compost, would an oil in the radiant tube move through the tubes with sufficient pressure to run a generator? 
Is biogas comparable to normal gasoline as far as octane, etc?  See, I don't know anything, I really need that book!
Thanks,
Ed

Edmund wrote:
I am an interested beginner.  The idea of biogas in general is incredible to me and I often wonder about other energy generating ideas that could be implemented around a house or a farm.  For instance, would it be possible to run a generator on a "hot" compost bed if you had a radiant heat type tubing system under the pile of compost, would an oil in the radiant tube move through the tubes with sufficient pressure to run a generator? 
Is biogas comparable to normal gasoline as far as octane, etc?  See, I don't know anything, I really need that book!

The last problem is definitely the easiest to solve(!)

Not to toot the horn, but in fact, the Biogas Handbook starts with several chapters that lay down the fairly simple basis that many of us need in order to think clearly about alternative sources of energy. One of these areas-- a crucial one-- is physics.

There is a hierarchy of types of energy, with sunlight being the best or highest, and heat being the worst or lowest. Of course, if we can concentrate sunlight either mechanically or biologically then those secondary types of energy are among the most useful (such as fossil fuel, aside from its other disadvantages). Sunlight can also be "concentrated" as wind, or as water from a waterfall (for example).

In any of those cases, it is fairly easy to make use of the energy to do difficult things, such as transport, or very useful things, such as generating electricity. (You can't run a computer by feeding it grass, but you can burn the grass, generate steam, use the steam to run a turbine, produce electricity... then use that to run the computer.)

So again, in this hierarchy of energy, heat is the worst or lowest, because it is hard to take diffuse heat and make it concentrated, whereas it is far easier to take diffuse sunlight and make it concentrated.

The upshot is that the 140-160 degree F temp that a compost pile produces is not enough to do very much with. Unless we can produce steam with some heat source, then we are unlikely to be able to get much work out of it.

The summation is that compost pile may help heat the greenhouse, and it may produce some moderately hot water, but it will not run a generator.


As far as the octane rating of biogas (or its cetane rating, if we are trying to replace diesel), I don't think most folks actually know what that is.

Spark engine fuels are rated by their octane number. Essentially, the octane rating of a fuel is a measure of how well it avoids predetonation. Methane has an octane number of 120 or more. This means that it can easily be used in high compression engines, because it very rarely predetonates.

And the cetane number? As it says in The Complete Biogas Handbook

Diesel engines do not have spark plugs. What happens in a diesel engine is that air alone is compressed and when the piston reaches the right place in the cylinder, the diesel fuel is squirted (injected) into the cylinder and the heat which was developed by compressing the air, ignites the whole mixture. Diesel fuels do not have octane ratings, they have cetane ratings. The word is different for diesel because the qualities needed for diesel fuel are very different than the qualities needed for gasoline type fuels. In the spark engine, we want the fuel to wait to burn until we torch it off with the spark. In the diesel engine, we want the injected fuel to burn as soon as it enters the cylinders. Ergo, cetane numbers are all about how easily the fuel spontaneously ignites in the cylinder.

Biogas has a poor cetane rating, but as the book explains, nevertheless it makes a good replacement fuel for a diesel engine....



d.

Great explanation, David! 

Edmund, if you're interested in reading more of the same, I highly recommend the book "Warmth Disperses and Time Passes," by Hans Christian Von Baeyer. If you have less time, this recent blog post (by a very different author) is also worthwhile.

Hot compost has been used to keep biogas digesters at the proper temperature, notably by Jean Pain.

It is also theoreticaly possible to skew the C:N ratio of a hot compost pile so very low that the ammonia it emits is enough to power a spark-ignition engine, but in almost every situation I can imagine, there are better uses for all that nitrogen.