Biogas production from energy crops – effect of various enzymes on biogas yield and fermentation time

The start up research aimed at investigating if there is a possibility to enhance biogas yield and decrease fermentation time from anaerobic digestion of energy crops by adding various enzymes. In a first step, the aim was to answer the question: "Do enzymes affect biogas yield?". If so, then additional experiments are planned that examine the effects in more detail. The Division of Agricultural Engineering (ILT) of the Department of Sustainable Agricultural Systems carried out measurements on substrate and energy turnover during anaerobic digestion of various energy crops in 1 litre batch digesters according to the international standard DIN 38414. Novozymes A/S supplied the following enzymes: Alcalase (protease), Novozym 51008 L (cellulase), Novozym 342 (mix of cellulase and hemicellulase), and Resinase A2X (lipase). The effect of a mix of 8 enzymes was tested, as well. The start up research concentrated on two types of energy crops: - starch rich energy crops: maize silage - protein rich energy crops: clover grass silage. The enzymes were added to the energy crops (directly into the batch digester). The default treatments were maize silage and clover grass silage without enzyme addition. Each treatment was replicated three times in order to allow a statistical evaluation of the results. Fermentation time, specific biogas yield, methane content in the biogas, and the specific methane yield were measured. Composition of maize silage and clover grass silage was analysed, as well. Biogas consisted of methane (50 – 80 %), carbon dioxide (20 – 50 %), and trace levels of other gases. Biogas quality (CH4 and CO2) was analysed 11 times in course of the 6 weeks digestion. Biogas production is given in norm litres per kg of volatile solids: Nl (kg VS-1). Results have proved positive effects from adding different enzymes to biomass degradation through an increase in biogas yield. Enzyme effects were different with digestion of maize, and clover rich grass. Differences in biogas quality were measured with the tested enzymes, but these were not statistically significant. The highest specific methane yields during anaerobic digestion of maize were monitored in the case of the enzymes Resinase A2X with 18 % and Novozym 342 with 12 % increase. Alcalase showed a decrease of 3% compared with untreated maize. The differences in methane yield obtained by the mentioned enzymes were statistically significant at p ≤ 0.05. In the case of clover rich grass, the highest increase was monitored after addition of Novozym 342 (67 %), Resinase (40 %), and Novozym 51008 L (38 %) compared with untreated grass. Maize, and clover grass differ in their composition. Clover rich grass has a much higher content of cellulose (27.3 %), and protein (20.1 %) than maize (19.4 % cellulose, 6. 7% protein). Maize is rich in starch (44.6 5). Additional profits from electricity and heat production were calculated from the additional methane yield that was produced when enzymes were added to clover grass silage. From untreated clover grass silage, 910 kWh electricity, and 1380 kWh heat can be produced in a combined heat and power unit. This gives a profit of 128 € (electricity), and 28 € (heat). Addition of enzymes results in an increase in methane production and thus an increase in profits from electricity and heat. The biggest profit increase of 103 € per t of volatile solids can be achieved from Novozym 342 addition. The start up research has proven a promising potential of enzymes to enhance biogas yield. The results propose future investigations focussing on additional enzymes, and on the effect of optimised enzyme mixtures. Finally, it is essential to validate results on commercial biogas plants.