Utilization of semi-natural grassland through integrated generation of solid fuel and biogas from biomass. I. Effects of hydrothermal conditioning and mechanical dehydration on mass flows of organic and mineral plant compounds, and nutrient balances M. Wachendorf*, F. Richter*, T. Fricke*, R. Graß* and R. Neff† *Department of Grassland Science and Renewable Plant Resources, University of Kassel, Witzenhausen, Germany and †Landesbetrieb Landwirtschaft Hessen (LLH) Landwirtschaftszentrum Eichhof, Schloss Eichhof, Bad Hersfeld, Germany Abstract The use of semi-natural grasslands for the production of renewable energy through conventional conversion techniques faces major limitations because of chemical and physical properties of the biomass. A new conver- sion procedure was developed which separates the biomass, as silage, into a liquid phase for biogas production and into a solid fraction to be used as fuel. Separation (mechanical dehydration) is carried out with a screw press after mashing with water (hydro- thermal conditioning). The effect of hydrothermal conditioning at different temperatures (5, 60 and 80°C) and mechanical dehydration on mass flows of plant compounds into the press fluid was investigated for five grassland pastures typical of mountain areas of Germany. Results show that 0Æ18 of the crude fibre was transferred into the fluid, whereas more digestible organic compounds, such as crude protein and nitro- gen-free extract, showed mass flows of 0Æ40 and 0Æ31 respectively. While 0Æ52–0Æ89 of potassium (K), mag- nesium (Mg) and chloride (Cl), which are detrimental for the combustion of the press cake, were transferred into the press fluid, more than 0Æ50 of calcium, which has positive combustion properties, remained in the press cake. Significantly (P <0Æ05) higher mass flows were detected at conditioning temperatures of 60°C (K and Mg) and 80°C (crude fibre and nitrogen-free extract) compared with the 5°C treatment. Because of the separation of solids and liquids, high proportions of P (0Æ61–0Æ74) and K (0Æ64–0Æ85) but only 0Æ32–0Æ45 of nitrogen exported from the grassland would be recycled with an application of the digestates from the anaerobic digestion of the press liquid. Keywords: semi-natural grassland, nature conserva- tion, biogas, solid fuel, bioenergy Introduction The rising consumption of energy globally, the growing shortage of fossil fuels and a world-wide increase in CO 2 emissions means that there is an urgent need to develop sustainable, eco-friendly energy supplies. The Commis- sion of the European Union (EU) has set the goal of raising the proportion of electricity made from renew- able energy in the EU to 0Æ21 by 2010 and the proportion of energy consumption from renewable energy to 0Æ20 by 2020 (Anonymous, 2006). Energy from biomass can make a significant contribution, yet its potential is far from being fully exploited. However, the cultivation of energy crops often competes with food production for limited agricultural land. In practice, there are only few crops, such as maize, rapeseed and cereals, which are suitable for bioenergy production. Their cultivation is restricted to arable soils and their production requires the intensive consump- tion of resources which may result in reduced efficien- cies and in increased greenhouse gas emissions (Crutzen et al., 2007), soil erosion, nutrient leaching and reduced soil fertility (Graß and Scheffer, 2005). To achieve the goals established by the EU, the potential of all possible sources of biomass, such as municipal Correspondence to: M. Wachendorf, Department of Grassland Science and Renewable Plant Resources, University of Kassel, Steinstrasse 19, 37213 Witzenhausen, Germany. E-mail: mwach@uni-kassel.de Received 27 June 2008; revised 16 December 2008 Ó 2009 Blackwell Publishing Ltd. Grass and Forage Science, 64, 132–143 doi: 10.1111/j.1365-2494.2009.00677.x 132