Gentlemen, Your very valid reasons are appreciated. Would the argument hold if the centralised AD were part of a large town's water treatment works. The idea would be to take over the running of the existing works with the intention of combining all the gas generation into one system. The main benefit is that there would be a lot of existing infrastructure that would help reduce the capital cost.
I come back to one of my questions though: does the FFA reside mostly in the liquid or the solid phase prior to digestion? Or is it split? Paul has a feeling that a lot of it is in the liquid and my experience indicates that this is in fact the case. Consequently, local digestion would make far more sense. Alternatively, the mobile unit could come into its own as there are already lagoons at each farm. We also have many chicken farms that are necessarily distributed to keep down the possibility of infection from one location to another. Mobile units make sense for this application as well. Many thanks for the help Rex Rex, Another way to go would be to make a smaller mobile unit that would be transported to the individual sites. A unit that could handle a 1000 cows/pigs, 10000 birds as well as other agricultural concerns, would fit on two flatbeds. This would give a flexibility to demonstrate the success of the process, and share the capital cost, with multiple users in the area. I've been involved in designing multiple mobile batch plants for other industries and this is not as difficult as one might think. On Sun, Jan 8, 2012 at 10:51 PM, Paul Harris <[email protected]>wrote: > G?day Rex,**** > > ** ** > > Apparently a lot of the biogas comes from the fines/solutes in the liquid > ? I noticed that with piggery effluent the raw effluent was almost > impermeable to water (it would hold puddles for quite a while) but after > treatment the solids were pretty free draining (water disappeared in > minutes).**** > > ** ** > > I will also raise my usual point in favour of distributed systems ? why > not put smaller (cheaper!) digesters at each farm and if necessary pipe the > biogas to a central generator (depending on distance)? You will be spending > a lot of energy on trucking waste in and then you have to truck the > digestate out from the central facility again. The total capital cost may > be higher, with more units to maintain, but you can start with one or two > and spread the finances out a bit as well as helping local employment (I > guess people will be working on digesters instead of driving trucks!).**** > > ** ** > > Happy digesting,**** > > HOOROO**** I concur on Paul's preference for smaller systems. I have seen a handful of proposals based on trucking waste from a number of small to medium farms where they have tried to show energy balance (all the trucks are methane powered, etc.), but the numbers never add up. Specifically, de-watering this waste requires relatively well developed systems at each site, and likely still leaves you with a serious amount of waste water treatment for each site to achieve healthy discharge levels (in Mexico it is the water, not the solids that is the treatment challenge). The management system is also more complex and requires a high level of coordination. The alternative de-centralized option will likely have similar initial capital costs, but you can make a better case for farmer investment as they will reap the benefits. You then have much smaller long-term operating costs, and all that energy for trucking can go to good use (or maybe still trucking some effluent fertilizer to local fields). As Paul mentioned, the option to eventually pipe gas a larger distance for centralized use is an option that may make sense (but I have not seen it outside of the European context or large landfill gas context yet). Cheers A Dear Rex, I would suggest contacting Burdens who is distributing the "Portagester" system in the UK and that could be useful for your intended use and scale. Their web is www.bioplex.co.uk/*portagester*.shtml<http://www.bioplex.co.uk/portagester.s html> Best regards, Pablo. _______________________________________________ Digestion mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/digestion_lists.bioenergylists.org for more information about digestion, see Beginner's Guide to Biogas http://www.adelaide.edu.au/biogas/ and the Biogas Wiki http://biogas.wikispaces.com/
