[Digestion] Woody Biomass digestion via compost

Mon Dec 4 04:15:42 CST 2006

Hi
Composting does require a lot of energy even inlarge scale operations if you don't want to wait for ages for the final product. Some years ago I looked into this. The plants use electricity and dieseloil in various proportions depending on technolgy. In the simpliest where composting is done on a concrete slab with the use of contracting machinery for turning they use mainly diesel oil. When you do it in more advanced plants you use more electricity. The total external energy requiremen would be between 20 and 100 kWh of for each ton of mature compost. Remember that 1 litre of diesel oil is approximately 10 kWh. The biggest use of energy consumption in a composting process is however the degrading process of the biological material. For each ton of copmpost dry weight you must use about twice as much raw material dry weigt and the energy content of that material which is mainly converted to CO2 without producing any usefull heat is a fantastic loss of bioebergy and if you related this to the plant nutrious value in the conten of NPK you find that composting is a very expensive and energy consuming way of producing fertiliser. Betwwen 50 and 100 times mor energy consuming than commercial feriliser.

However if you have no need for the heat e g if you have no district heating, or in case of wood if you have no plant where you can burn wood chips and similar for energy, and if you instead need material for improving soil structure and increase the content of soil micro organisms then you have good reasons for composting organic matter. If this is not the case composting is more similar to pretreatment for a very extended kind of landfill. In the composting process you also loose a substantial part of the nitrogen that turn into N2. If you don't look after your process well you may also release amonium fumes, laughing gas and methane all which of have severe environmental effects like eutrophication, acidification, break down of ozone layer, increased green house effect.

Did I swear in the church?

With digesting it is another matter. You can get energy from the process and it is the only way to extract energy if the moisture content is getting high. Below 60-65% moisture content large scale combustion is the most efficient energy process. Above approximately 80% moisture content digestion is the only way for positive energy extraxction. In between you can choose but if you choose combustion it will have to be co-firing with other fuels like e g wood chips followed by flue gas condensation. Then you have the interesting fact that methane is a fuel with a higher commercial
value than just hot water. It has more applications. In addition digesting can be done in rather small scale, however leakage must be limited. 5% methane leakage will make the methane from digesion plants and small scale use not any better than burnig a fossil fuel if you look at the greenhouse effect.

So the technology of running digestion plants is very important. To get out as much gas as possible and to handle it with minimum leakage. Operational problems often result in some leaks. Then you have the task of taking care of the residue and preferably without the problems that you have in composting and that is also something that could be a matter for international discussion. The residue has a firm content that must be handled and also a liquid content that contains most of the nitrogen and it should preferably be used in agriculture without loosing too much in the spreading and cultivating process. The cost of handling is important. The liquid can not be transported very far and that may limit the size of digestion plants. Actually the raw material to the plant might be less bulky with say 25% dry content whereas the liquid could have 5% if not concentrated in a smart way without loosing nitrogen. High density digestion would have logistic advantages even if the the gas production per ton of dry weight of digestible content is less.

Regards
Bjorn Dahlroth
The Stockholm County Assdociation of Local Authorities

-----Ursprungligt meddelande-----
Från: digestion-bounces at listserv.repp.org [mailto:digestion-bounces at listserv.repp.org] För Art Krenzel
Skickat: Saturday, December 02, 2006 8:14 PM
Till: Wake Robin Design WRD; digestion at listserv.repp.org
Ämne: Re: [Digestion] Woody Biomass digestion via compost

Abel,

I have been sitting here waiting for someone else to join in the discussion 
on Jean Pain's work but that does not appear to be in the wings.

I live in the Pacific Northwest and, yes, there are good chipper/shredders 
here.  I would look at my fuel requirements before I bought because there 
are chippers and there are shredders but few chipper shredders.  If you plan 
to move the fuel in some mechanized manner or have it flow in any way - 
chippers are the better answer answer.  If you want to make big pieces of 
wood into little pieces which do not have to be hauled or fed in any way - 
shredders work pretty good.  Middle ground is hard to find.

We have people on this listserve with more years of wood size reduction 
experience than I have who can comment on these items.

I would like to comment on the composting part of Jean Pain's experiment.  I 
have been making compost for a few years now and find it difficult to 
maintain high rate aerobic composting without having the turn or mix the 
pile several times after the initial surge of decomposition.  To extract 
sufficient energy to heat a house and provide for hot water needs as well, 
any compost pile would need to be turned several times and/or new materials 
need to be added for all the time you would be extracting the energy. 
Nicolas Poulain's trip report presents an idyllic lifestyle of free energy 
and no work.  For those of us who have been on the front line of 
composting - that image is a bald faced lie.  Active aerobic composting is 
an energy intensive process!  This is especially true if you only use human 
power.  They say that wood is a fuel that heats twice - once when you cut 
and split it and then when your burn it.  Composting heats more times than 
that because you need to turn it several times to keep the reactivity 
sufficiently high to be able to extract heat energy.

Composting is great but high rate aerobic composting is not idyllic.  Biogas 
is great but works best with soluble oils and food grade feedstocks - not 
wood shavings or wood chips.

Does any of this make sense to anyone?  Has anyone out there had a garden? 
The pictures show that you just need to plant the seeds and then there is 
harvest.  It does not show the weeding, watering, staking, pruning, etc that 
goes on between those two endpoints.  A large garden can eat YOUR lunch. 
There is only so much time in a day and you have to make the best use of it 
if you want to survive to see tomorrow especially where hand labor is 
involved.

As I said, I live in the Pacific Northwest and would like to discuss small 
scale sustainability with you but we need some ground rules as to size, 
support and feedstocks involved.

Art Krenzel

----- Original Message ----- 
From: "Wake Robin Design WRD" <wakerobinlandscaping at yahoo.com>
To: <digestion at listserv.repp.org>
Sent: Sunday, November 26, 2006 6:47 PM
Subject: [Digestion] Woody Biomass digestion via compost

> Greetings,
>  I was wondering if any readers have come across the work of Jean Pain. 
> The man who converted overgrown hardwood forests in southern france into 
> healthy woodlands while simultaneously producing methane, methanol, hot 
> water, and compost from the digestion process?  I am trying to track down 
> his book or a book on the subject which illuminates the specifics of this 
> method.
>
>  What I understand is that instead of creating a slurry, he built piles of 
> wood shavings saturated by water.  I am guessing that by composting the 
> debris he was able to heat the inside of the pile enough to produce 
> methane.  It seems like a simpler approach than heating a tank filled with 
> a slurry.  Does this seem possible?
>
>  I want to eventually apply this method in the Pacific Northwest region of 
> the USA.  What is the feasability of using wood shavings from northwest 
> connifers like douglas fir and western red cedar?  Does anyone have any 
> specifics on the amount of energy produced per ton using this particular 
> material?  How hot do you need to keep it?  Does the collection energy 
> justify the energy retrieved by digestion?
>
>  I am also interested in learning more about the way gas is collected from 
> a compost pile.  This particular bit of information has not been revealed 
> by any articles on Pain
>
>  Finally, any ideas on some good chipper shredders in the PNW?
>
>  If anyone has any thoughts on any of these questions I would be grateful,
>
>  Abel Kloster
>
>
>
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