permaculture

["Lawrence F. London, Jr." <lflj@intrex.net>]

SANET-MG Archives -- January 2009 (#67)
http://lists.ifas.ufl.edu/cgi-bin/wa.exe?A2=ind0901&L=sanet-mg&T=0&O=D&P=47737
Subject: Re: Mini Methane digesters
From:   David <[log in to unmask]>
Date: 12 Jan 2009 23:41:52 -0800

Randy,

Randy Cutler wrote:
> I am planning on building a 60' x 20' greenhouse. I am going to use it to extend my growing season and produce 
vegetables for sale to the local food coop. I would like to use a methane digester to produce fuel for heater. Can 
anyone refer me to a source for purchasing such a unit? I have 35 ewes, a couple horses and about 50 various poultry as 
a source of manure. I live in Central Wisconsin 45 degrees North lat.So I need a pretty reliable heating source.
>

You've gotten lots of good suggestions from the list, particularly those
that mentioned thermal mass, and the most important suggestion, which
was to put the digester inside the greenhouse. One thing that no one
mentioned is a form of thermal mass: the fact that the earth itself is a
good source of heat. Depending on how much land you have, you may be
able to bury 3"-4" flex pipe of the sort used these days for tiling
fields 4-6 feet deep or deeper, and pull air through the pipe to warm it
and establish a baseline temp above freezing for the greenhouse. (It may
be 60 degrees down there, year round.) A good deal depends on what crops
you want to grow-- or seeds you want to sprout for spring planting-- but
this would be a good adjunct to some of the suggestions already made.

As regards your animals and their manure, a good deal depends on how
they are housed as to whether you can collect their manure, and how
difficult that is. You may want to re-think their barn rather carefully
to make it as easy as possible to do collection, because the more
difficult that is, the less likely it will get done. (Putting the
greenhouse on the south side of the barn may be an option.) It will be
very important to minimize the amount of water that is also collected
with the manure, since it requires a very great deal of energy to heat
water, and the size of the digester is likely to depend on the amount of
mix water-- wash water-- more than any other factor: its volumetric
efficiency will likewise. You can digest manure from any of those
animals with very little added water, with the possible exception of the
poultry, since they drop both feces and urine in the same load, and the
ammonia in the poultry manure can be toxic to the biogas microbes above
a certain concentration. If you put your animals on well-shredded
material such as straw, then that can contribute its share to the process.

If you figure the poultry weigh 5 pounds each, then you can expect 20
cu.ft./day from them. Sheep manure is not reported to be very good at
producing biogas (less than half as good as cow manure on an equal
weight basis). If you figure your sheep weigh 50 lbs ea, then 35 of them
should give you ~50-60 cu.ft/day biogas. The horses, depending how much
they weigh, would give about the same as the sheep. (All this includes a
number of fudge factors, such as the fact that it's late and I'm tired,
and the assumption that you are able to collect all of their manure,
which in practice might easily not be the case.) It would be important,
as well, if you would put your kitchen scraps in the digester. Your own,
uh, contributions and those of your family are less important than you
might think. The rule of thumb is feces from one human yield one cubic
foot of biogas a day. Invite some friends over?



With reference to some of the other comments made:

    "Forgive my ignorance, but I was under the impression that the
    biggest stumbling block to successful methane generation in the
    temperate areas of the planet is a lack of year-round warm temps to
    keep the mesophilic action going.... I'm not sure technology has
    solved that key piece to the puzzle on anything but a large and
    expensive scale."


Biogas can be produced (although rather slowly) at lower temperatures,
but given a good feedstock-- and most importantly where the amount of
additional water is held as low as possible (slurry should be 6-8%
solids, which can be surprisingly difficult to achieve)-- any biogas
digester worth its salt should be able to maintain its own temperature,
assuming sufficient insulation, and where all the biogas is dedicated
thereto. Of course, in many situations, that is precisely the problem--
we want to use the biogas for something else. But in Randy's case, where
the generator is inside the greenhouse, heating the digester heats the
greenhouse, which was the problem in the first place. If the estimates
on the production given above are ballpark correct, and we assume 65%
methane in the biogas, then the animals will produce over 100,000 BTU
per day in methane. (The remainder of the biogas is CO2, and burning the
biogas in the greenhouse will turn the methane into CO2 and heat, all of
which will markedly increase the CO2 concentration inside-- good for
plants, but for humans and continued combustion you may need to consider
how to get more oxygen in there without also lowering the temperature. A
simple heat exchanger may be in order; see previous suggestion about
buried pipes.)

Making a digester larger and more expensive doesn't do a thing to
redress the fundamental physics. You've still got to heat a unit volume
of the digester with the biogas produced by that unit volume. Generators
make more and more sense as they become more and more ecological-- by
which I mean that the "waste" material or heat from one process is used
to provide food or energy for another process. That's ecology (or
permaculture): everything is closely connected to benefit everything,
and all niches are filled. Most farm-scale generators in cold climates
violate this principle. They are built outside, separate from the barn
(or what have you), increasing surface area and losing heat. Not
sensible. In Rudy's case, where the generator is in the greenhouse,
heating one heats the other, the CO2 feeds the plants, etc.; the
principle is illustrated.

At the moment I don't have time or the fire in the belly necessary to
climb through the math, estimate the volume of the generator, make
assumptions about the insulation on the greenhouse, the ambient
temperature outside (and so forth), and demonstrate that my statement
about keeping the generator at a sufficient temperature has a scientific
basis, so for the time being I'll just say that this is a finger in the
wind estimate made by someone who should know. I feel comfortable with it.


As for "Convert your Honda Accord to run on trash", this is via
pyrolysis, and it was done extensively in Europe during WWII, but it's
not used today for the simple reason that it's very hard to haul enough
biomass around to create enough producer gas to power the car to make it
go very far. Besides, depending on the biomass that's used and the
process parameters, it can produce all kinds of nasty chemical species
(creosote?), and probably would damage the Accord. Great for a science
fair, not so fair for the science. That said, for certain stationary
uses such as the greenhouse, it would not make much sense to use
pyrolysis, since the energy in the producer gas that results from
pyrolysis is rather less than would have been produced as heat if you
just burn the stuff in the ordinary manner. Wood heat is indeed an
option, but I can't imagine why, for example, you would want to produce
charcoal first (which is also done by pyrolysis), and then burn the
charcoal to heat the greenhouse. Burn the wood. Directly. If you do need
a source of fuel for an engine, as was suggested (to generate
electricity), then something like that may make sense, but not to heat a
greenhouse.


Please let us know what you come up with, if the prospect is not now too
daunting.



d.
--
David William House
"The Complete Biogas Handbook" |www.completebiogas.com|

"Make no search for water.       But find thirst,
And water from the very ground will burst."
(Rumi, a Persian mystic poet, quoted in /Delight of Hearts/, p. 77)

http://bahai.us/