Sustainable N fertilizer production based on a loop: Straw - biogas – ‘Haber-Bosch’ process

CONTEXT: A concept was presented to produce N fertilizer by digesting straw in a biogas plant and transporting bio-methane to a fertilizer plant through a gas grid substituting natural gas.OBJECTIVE: The rational was to find out if straw, a residue not infringing on food production and not needing extra cropland, converted to methane in a biogas reactor, is a viable option for ammonia synthesis.METHODS: Scenarios calculations were based on straw from 1 ha wheat producing 7 Mg of grain using 150 kg of fertilizer N. Removal rates of straw were 60 or 100%. Digestion residues were returned to fields within a radius of 50 km. Three scenarios with varying straw removal and low to high efficiency of the biogas and fertilizer plant were assessed.RESULTS AND CONCLUSIONS: A sensitivity analysis showed the combination of a medium to high efficient biogas and fertilizer plant using between 3.5 and 7 Mg straw covered the energy need for crop management, transportation of straw and digestate and N fertilizer production and still delivered a surplus of 838?9192 kWha ha-1. The amount of 1 Mg of straw enabled the production of 94 to 173 kg fertilizer N.SIGNIFICANCE: Use of straw can be a useful alternative for N fertilizer production making agriculture more sustainable.