Biomethane yield of energy crops and prediction of their biochemical methane potential (BMP) with near infrared spectroscopy (NIRS)

Anaerobic digestion process produces biomethane as a renewable energy source. To optimize the energy production, energy crops with a high biomethane yield per hectare should be identified. A large number of samples of maize, tall fescue, sorghum, spelt, miscanthus, immature rye, switchgrass, sunflower and hemp was cropped. The fresh biomass yield per hectare, the volatile solid (VS) content, the biochemical composition and the biochemical methane potential (BMP) were measured. Maize (annual plant) was shown to be the best methane yielding plant per hectare. Green miscanthus (perennial plant) appeared as a promising alternative to maize thanks to methane yields similar to maize. For the various energy crops tested, the biomethane yield per hectare was significantly more influenced by the biomass yield per hectare, especially on a VS basis, than by the BMP. The BMP on a crude matter basis was mainly influenced by the VS content. The BMP measurement is essential for the assessment of the biomethane yield per hectare of energy crops. However, it is a time-consuming assay and requires dedicated scientific instruments. Near infrared spectroscopy (NIRS) was assessed as a tool to quickly predict the BMP of energy crops. The near infrared (NIR) spectra of wet silages and of dried and ground silages were measured on the harvested energy crops. Both spectra types were used as predictors of the BMP of wet silages. For all energy crops, models based on NIR spectra of wet silages were able to predict the BMP on a crude matter basis. Specific prediction models were also developed for maize silages. The BMP on a volatile solids basis of crude wet silages was not well predicted. NIRS is a non-destructive and fast analytical technique to estimate the anaerobic digestibility of biomass. Methods based on NIRS could facilitate and fasten the selection of energy crops dedicated to anaerobic digestion.