Utilization of semi‐natural grassland through integrated generation of solid fuel and biogas from biomass. III. Effects of hydrothermal conditioning and mechanical dehydration on solid fuel properties and on energy and greenhouse gas balances

The integrated generation of solid fuel and biogas from biomass (IFBB) procedure separates biomass into a readily digestible press fluid, from which biogas is produced, and a fibrous press cake that is used as solid fuel. The effects of mechanical dehydration and prior hydrothermal conditioning (5, 60 and 80°C) on biomass from five species‐rich, semi‐natural grasslands, typical of mountain areas of Germany were investigated. Proportional reduction of ash constituents in the press cake compared with the parent material was up to 0·80, 0·61 and 0·81 for potassium, magnesium and chloride, respectively, at 60°C, resulting in potassium, magnesium and chloride concentrations in the press cake of 2·43, 1·22 and 0·93 g kg^?1 dry matter (DM). Emission‐relevant constituents were reduced by up to 0·19 (nitrogen) and 0·39 (sulphur), yielding nitrogen and sulphur concentrations of 11·13 and 0·97 g kg^?1 DM respectively. Ash softening temperatures were significantly increased up to 1250°C, falling within the range of wood fuels. Thus, quality of IFBB fuels is superior compared with conventional hay and is comparable to hay of delayed harvest in winter or the next spring. Calculated energy conversion efficiency for IFBB was up to 0·51, compared with a maximum of 0·22 for anaerobic whole crop digestion (WCD) and 0·74 for combustion of hay (CH). High energy demands in IFBB resulted in a greenhouse gas mitigation potential of up to ?4·40 t CO₂eq ha^?1 which is lower than for CH (up to ?6·17 t CO₂eq ha^?1), but higher than for WCD, which mitigated up to ?2·24 t CO₂eq ha^?1.