Hi Steve, It has been a long time since we last communicated. I hope you are well. I see that you are still experimenting as ever. As you can see, it appears that I am back in the AD business for my neighbour and another farmer in the district.
You are spot on with respect to being caught between two sizes - hence my use of second hand 3 litre motors driving generators. Brent has come up with a number of suggestions that I believe have merit and should be able to help keep the overall cost down. So I will look at those for sure. The fact is that all of the dairies have existing lagoons and if we can modify them, it makes life a lot easier. I agree with your idea of pre-heating the charge before feeding it to the lagoon. This means that there has to be a large intake pit to take all of the washings in one go, heat them up and let them go to the lagoon. I believe that the cost of this should not be exorbitant and it will allow the lagoon to operate in the mesophilic range. Surprisingly, I have been involved with dry digestion over the past year with the objective of generating biogas for transport purposes. The gas is upgraded to about 92% CH4 before compression, chilling and liquefaction. The bio LNG is then pumped into cylinders in cassettes. The cassettes are mounted on trucks and the gas reduces the diesel consumption. The empty cassettes are swapped out for full ones as required. Clearly this is on a much larger scale than farm digestion and the capital is much larger as well. The requirement for the biomass raw material is that it must be able to ferment. Essentially, the rule of thumb is: "if a cow will eat it, we can ferment it". So your grass attempts are not far off the mark. How well does the fixed film work? Kind regards Rex Hello Rex You seem to be in the classical situation of being too big for ad hoc solutions but too small for automated low labour installations. My suggestion, is there a way in which the dairy farms co-operate to run a central biogas installation? Perhaps each farm has a de-watering plant and transports the 'cake' only to be digested in a dry digestion type facility. On site the fluid faction can be treated in a simple manner to produce biogas(if desired) eg sequencing batch method (fill, react, decant on a daily basis). The digestate is valueable (probably worth the exercise by itself especially for an organic farm) note the fluid should also be used in conjunction, having the N component and (can't remember which) some of the P or K component. In my opinion, heating is the major factor, for installations not located in the tropics. Recovered heat from an engine is possibly the easiest heat source. However if you can keep the quantity of liquid down, pre heating the influent (to make up for the heat loss from the digesting mass) seems to work alright. My suggestion for on farm digesters ie centralised digester is not an option -- most of your efforts will possibly have to be justifying the cost versus benefit, perhaps if you could figure a way for direct burning (on farm)of the gas coupled with the fertiliser benefit, a simple trench covered with a membrane might make an economic match. I mentioned dry digestion. I think I have developed a methodology for digesting grass. Basically, grass and foodscraps are allowed to soften in a vessel and then are leached (flooded method) with effluent from the digester. The leachate spends a day in another vessel where (I believe... hope I am correct) simpler acids are formed and then input into the digester. Perhaps cow manure'cake' can be adapted to this method. The digester is fixed film. Urethane foam cubes as carriers in the intermediate vessel. kia ora stev Rodda _______________________________________________ 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/
