2012|13 Annual Report Fraunhofer IGB

Lignin in anaerobic digestion Alongside liquid manure, agricultural biogas plants primar- ily use renewable raw materials or “energy crops”. Against this backdrop, recent cultivation attempts have been aiming at increasing the yields per hectare. Higher yields, which ap- pear to promise a higher biogas yield per area of cultivation, usually come at the cost of an increased proportion of lignin, the structural and support substance in plants. Lignin, how- ever, is not metabolized by the anaerobic mixed bacterial cul- ture in biogas plants, which means that the increased biomass can only partially be converted into biogas [1]. The remainder (lignin) causes an increase in digestion residues and gener- ally requires disposal. The overall efﬁciency and economic at- tractiveness of the anaerobic digestion process for generating methane can be considerably improved by pre-treating the substrates and through utilization of the product as a raw ma- terial. Aims of the project The aim of the project being presented is the complete utili- zation of the substrates typically used by renewable raw ma- terials plants by means of integrated material and energetic use. Carbohydrates, fats and proteins are digested to produce biogas and the non-digestable lignin is separated off and used as a raw material. In addition, the intention is to convert methane in the presence of carbon dioxide oxidatively to C1- oxygenates (formaldehyde and methanol) using membrane reactors. These compounds may either be utilized materially for the synthesis of chemical raw materials or for improving the transport and storage properties of the gaseous methane (project partner LIKAT). This lays important foundations for an integrative concept as envisaged by cascade utilization and improves the international competitiveness of German tech- nology providers. The latest technology In methanogenic mixed cultures, organic material is converted into methane in several reaction steps. Macromolecules like carbohydrates, fats and proteins are initially hydrolyzed before being converted into organic acids, alcohols (acidogenic bac- teria), acetic acid (acetogenic bacteria) and ultimately to meth- ane and CO2 (methanogenic archaea) [2]. Experience to date concerning the separation of lignin from plant material leads one to expect that through disintegration and separation of the lignin the lignocellulose structure will be broken up. This ought to produce better substrate availabil- ity of freely available carbohydrates in the form of cellulose, a reduction in the proportion of compounds that are difﬁcult to hydrolyze, and hence higher conversion rates in the diges- tion process. The substrate samples were initially tested on a laboratory scale and are currently being tested on an industrial scale in a methanogenic degradation process with high load- ing rate in ideally mixed bioreactors. Results from the anaerobic degradation The substrates forage rye silage, maize silage and sorghum silage were each used in unground form, in ground form and after lignin extraction (Fig. 2) for biogas production on a 1-liter laboratory scale (Fig. 1). In the three or four loading 118 TOTAL ENERGETIC AND MATERIAL UTILIZATION OF CROP PLANTS: MORE BIOGAS WITHOUT LIGNIN? Dr. rer. nat. Yasemin Sterr, Prof. Dr. rer. nat. Dieter Bryniok ENERGY 1 2 Unground substrates Ground substrates Substrates after lignin extraction SorghumsilageMaizesilageForagesilage

2012|13 Annual Report Fraunhofer IGB

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