[Digestion] what is igh calorie for a biogas digester?

[ajheggie at gmail.com]

On Mon, 20 Apr 2015 21:44:32 -0400, you wrote:

>
>To: For Discussion of Anaerobic Digestion
>	<digestion at lists.bioenergylists.org>
>Subject: Re: [Digestion] what is igh calorie for a biogas digester?
>From: "Steven Barber (RIT Student)" <stb4703 at rit.edu>
>Date: Mon, 20 Apr 2015 21:44:32 -0400
>Greetings everyone, here is an interesting slide I saw from a 2012 AD conference (Martin, et al., 2012 AgSTAR BMP)


Late to the fray; the slide seems interesting but limited and a bit
ambiguous.

I can see two entries for corn silage with differing yields.

I can understand that feedstocks high in fats, lipids seem easily
digested (and milk has a very high BOD as witnessed by the way it can
de oxygenate a water course) and high energy content, I can also
understand AD Karve's proposition that  foods that humans eat will
have higher yields than manure from an animal that has extracted as
much of the energy as it can before passing it out. Also in cooking we
hydrolyse food like starch to increase its digestibility. Similarly
ensiling hydrolyses forage so I would expect silage to yield biogas
quicker than the whole chopped crop added directly to the digester.
This chart appears to reflect that except that the entries for straw
and switch grass are above green waste and oats which seems strange.

There is no entry for lawn clippings which I thought had a very high
BOD as leachate from these often kills tree roots and therein lies my
interest, see below.
>
>?
>
>On Mon, Apr 20, 2015 at 9:18 PM, Paul Harris <harrisfm at aapt.net.au> wrote:
>G'day Arnand et al,
>
>Since Biological Oxygen Demand (BOD) is often used as a measure of digestibility (carbohydrates and water? (H2O) can provide oxygen) oxygen content is probably not a crucial factor in digestion. Volatile Solids (VS) content is related to BOD (but quicker and easier to assess).
>
>Biomethane potential is probably the best guide (as some VS may not be digestible or there may be inhibitory substances.

OK but what is the relationship between BOD VS and Biomethane
potential.

I'm interested in a couple of aspects of biomethane, first we have
hundreds of tonnes of whole tree woodchip sitting steaming during the
summer months for delivery in the winter, presumably its the volatile
solids that are being respired aerobically and aerobically so
potentially biomethane is being vented to atmosphere?

Second; this place has little agriculture left but a lot of parks and
open spaces as it is densely built up. The grass areas are regularly
cut to waste with gang mowers To my mind the output of these mowers if
collected (often contaminated with dog faeces so of no feed use) and
digested could yield biogas and working from the  chart :

8 tonnes/ha DM
Interpolating grass silage at about 6300cuft/tonne =>180m3 biogas=>
equivalent to 108m3 methane  with calorific value of 1020kWh(t)and a
value to a small business user of GBP40. I suppose capital costs is
what prevent a small municipal digester being used to heat and power
the local authorities facilities.

Or are my calculations ridiculously optimistic?

AJH