Biogas potential of green biomass after protein extraction in an organic biorefinery concept for feed, fuel and fertilizer production

The biogas potential of the residual fractions of four organically grown green crops after protein extraction was studied. The protein extraction method involved screw pressing of freshly harvested biomass to obtain a plant juice, followed by precipitation of the proteins. After protein extraction, 95% of organic matter was still present in the residual press cake and juice. Methane yields in the range of 219-375 and 429-539 ml-CH₄ g-VS^-1 were obtained for the mono-digestion of press cake and the residual juice, respectively, and up to 81% of the methane potential of the fresh crops was recovered in the two residual fractions when evaluated separately. Co-digestion of the press cake and the residual juice at the organic matter ratio at which those fractions leave the biorefinery, resulted in a methane yield of 400 ml-CH₄ g-VS^-1 according to the regression model equation developed for red clover. Consequently, 65% of the methane potential from fresh red clover could be recovered by co-digestion of the residual fractions from the green biorefinery after extraction of proteins.