The results are in
4 tanks x 700 L cattle slurry: 1 control & 3 with 10% biochar (furze, hazel & rush). Gas vol measured over 2 months.
Hazel no effect.
Furze biochar ~25% more gas than control
Rush biochar ~60% less gas than control!
Home brewing (cattle slurry style) during covid lockdown, 3 different biochars in 1,000 L tanks and a control, yes I know not scientific, but no one told the 4 tanks that and some very interesting results. A clue, zoom in on picture and compare the tops of the tanks.