Various international studies prove an increased gas yield through the addition of biochar to the biogas process. With approx. 9,200 biogas plants (Statista 2018), Germany in particular offers great potential for optimisation. The study investigates which location (main fermenter or secondary fermenter) is most suitable for improved gas yield in the fermentation process.
Optimum quantity of plant coal
In order to determine the optimum amount of plant carbon to be introduced, 3 variants were used with 1%, 2.5% and 5% admixture quota in relation to the amount of water used. The 2.5% mixture turned out to be optimal (in the 1% mixture almost all particles settled on the soil, in the 5% mixture a large part of the carbonate floated on the surface).
Use in the main fermenter
In order to make a statement about the effect of biochar in the main fermenter, two variants were investigated here. One variant with fermentation residue and fresh corn silage (2) and another one with fermentation residue, fresh corn silage and biochar (3). A comparison of both variants after the entire test period (91 days) yielded an additional yield of almost 9% for the plant-charcoal variant. The following values were achieved over time by the addition of biochar:
- After 18 days an additional yield of just under 18.9
- After 40 days an additional yield of 21.7
- After 91 days an additional yield of 8.9
Use in secondary fermentersr
Two variants were also investigated here. On the one hand, a variant with fermentation residue and biochar (4); in the second variant (5), the biochar was screened and only large charcoal particles (>13mm) were introduced into the fermenter.
- Until the 40th day of the test, an additional yield of 17.3 % was measured for variant (4), but not quite as much for variant (5), approx. 3% less.
- From the 40th day up to the end of the test, variant (4) achieved an additional yield of 14 %, variant (5) even of 24 %.
Compared to variant 4, variant 5 produced approximately 3% less methane up to the 40th day of the test.
The addition of biochar therefore leads to better substrate utilisation in the main fermenter as well as to a significantly higher exploitation of the fermentation residue potential. Possible reasons for this are:
- Due to its large surface area, the biochar can bind interfering and inhibiting substances to itself and thus stabilises the biogas process.
- The relatively high proportion of mineral components of biochar ensures the supply of essential micronutrients to microorganisms through use in the fermenter.
- The porous structure of the biochar leads to a better exchange as well as more effective and faster degradation of the substrates and metabolic products, since the microorganisms in the biochar matrix are better protected.