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.

Expla­na­tion models

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.
Orig­i­nal article (German): Studie: Steigerung des Bio­gaser­trages durch die Zugabe von Pflanzenkohle (Study: Increase of the biogas yield through the addi­tion of biochar)
Author: Jan-Markus Rödger, Walde­mar Ganagin, Andreas Krieg, Chris­t­ian Roth, Achim Loewen
Pub­lished in: Müll & Abfall 9/13, p. 476–481