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

Abstract

CONTEXTA 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. OBJECTIVEThe 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. METHODSScenarios 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 CONCLUSIONSA 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. SIGNIFICANCEUse of straw can be a useful alternative for N fertilizer production making agriculture more sustainable.