Semi-natural grassland biomass for combustion: influence of botanical composition, harvest date and site conditions on fuel composition

Bioenergy generation by combustion is an alternative utilization of biomass from semi-natural grasslands no longer needed for forage production. Variability of fuel composition affects the suitability of grassland herbage for combustion, in terms of ash-related problems and harmful emissions of N, S and Cl compounds. A field experiment was established at six semi-natural grassland sites in southwest Germany to investigate the importance of botanical composition, harvest date and site effects in influencing fuel composition. The first growth at each site was harvested at five dates from June to October. Concentrations of N, S, K, Ca, Mg and ash were lower in grasses than forbs. Between June and October, K concentrations decreased and Ca concentrations increased in grasses and forbs. Ash concentration increased only in grasses, while yields remained stable. Considerable site effects were also found, particularly for K and Cl. The findings are discussed in relation to knowledge of combustion technology. Fuel composition of the investigated grassland biomass was less suitable for combustion than that of perennial energy grasses, but comparable to that of cereal straw. Most critical parameters were high N concentrations, cause of NOx emissions, and high ash, K and Cl concentrations, responsible for particle emissions, fouling and corrosion. Although combustion of semi-natural grassland biomass for small-scale residential boilers cannot be recommended currently, results of this study suggest that utilization in medium-scale combustion units may offer realistic perspectives, creating synergies between bioenergy production and biodiversity conservation.