The digester has been producing gas reliably since mid July.   We used a good inoculum (cattle slurry and digestate from another digester) and it was no surprise that the fresh green biomass produced burnable biogas.

It is quite difficult to see any gas flame in daylight as even when it is not sunny the polytunnel is very bright.  By making a dark background and blocking out some light we have this picture.


The picture shows the biogas flame heating a kettle.

The amount of gas we have been producing has been linked to the amount of grass we have fed.  Initially we wanted to make as much gas as possible but we are still developing the kitchen to make it useful so we have been in the situation of having to burn the gas it off when we tend the digester.  This has led us to ease off the feeding to reduce the gas production, not an ideal situation.  The aim of this project was not to just produce biogas but to make micro AD useful to food producers.  So we are in the process of bringing the gas to the kitchen where it can be used.  This requires a bit of setting up and training the  people who will be using the equipment.

We have been testing the biogas on all three of our gas rings as well as our tea urn, which has the added feature of a thermocouple flame detector.  These are standard on tea urns as they are unsupervised appliances; the flame heats the end of a thermocouple which allows gas through to the burner head, if the flame goes out the gas is shut off.  It is all quite simple but we anticipated that the moist gas might not hold a steady flame or might gum up the gas valve.  We have been using it for some weeks and it seems to work reliably.

The overall energy balance; energy out compared with energy in is more important than being able to produce biogas.  We have been investigating this both in the workshop and now in the field , this will be dealt with in detail in the next post but it is clear that the heat loss of the digester is the key to the energy balance.  Reducing heat loss dramatically increases the net energy gain.