Evaluating intermediate crops for biogas production – Effects of nitrogen fertilization and harvest timing on biomass yield, methane output and economic viability

Intermediate crops (ICs) are grown on large areas in Sweden and elsewhere for their function as cover or catch crops and to increase soil fertility, and they are usually soil-incorporated. The aim of this study was to investigate if aboveground biomass from ICs can be a sustainable source of biofuel feedstock. For that, the biomass yield of fertilized and unfertilized ICs was studied in field experiments and their energy potential determined using methane potential assays. Furthermore, we estimated the economic viability of biogas vehicle fuel production using the IC biomass. Our results indicated that it is economically viable to produce biomethane gas for vehicle fuel quality, from several intermediate crops grown in Northwest Europe, when the intermediate crop biomass was harvested with a self-loading forage wagon, used fresh or as silage as biogas feedstock and processed to methane gas in a large scale biogas plant. Nitrogen fertilization of intermediate crops was useful only when the intermediate crop is established early enough for the plant to make use of the nitrogen and 3 of the 9 investigated IC species could be grown economically feasible even without nitrogen fertilization and covering the full feedstock production costs. Other factors important for economic viability were a high gross methane yield per hectare combined with a high dry matter content in the IC biomass, with high dry matter yields to be prioritized over a higher specific methane yield. Further research is needed on the impact of IC maturity on methane production and suitable harvest technology. • Biomass and methane yield of intermediate crops (ICs) grown in Northwest Europe. • Economic viability for production of transportation fuel was assessed. • It was economic viable to produce fuel from several unfertilized and fertilized ICs. • High gross methane yield per hectare and high dry matter content were important. • Nitrogen fertilization was useful only when the ICs were established early enough.