Material-energetic full utilization of cultivated plants: without lignin to more biogas?

In addition to liquid manure, agricultural biogas plants are mainly operated with renewable resources (NaWaRo), so-called energy crops. Against this background, breeding experiments have recently been aimed at increasing yields per hectare. However, higher yields, which seem to promise a higher biogas yield per cultivated area, are usually paid for with a higher proportion of lignin, the structural and supporting substance of the plants. However, lignin is not metabolised in biogas plants by the anaerobic mixed bacteria culture, so that only part of the increase in biomass can be converted into biogas [1]. The rest (lignin) leads to an increase in fermentation residues and usually has to be disposed of. The overall efficiency and economic attractiveness of the anaerobic digestion process for the production of methane can be considerably improved by pre-treating the substrates and recycling the product.

The aim of the presented project is a complete utilization of the substrates typical for NaWaRo plants by an integrated energetic-material utilization. Carbohydrates, fats and proteins are fermented to biogas and the non-fermentable lignin is separated and recycled. In addition, methane is to be oxidatively converted to C1 oxygenates (formaldehyde and methanol) by means of membrane reactors in the presence of carbon dioxide. These compounds can either be used for the synthesis of chemical raw materials or for improving the transport and storage properties of the gaseous methane (project partner LIKAT). This will create important foundations for an integrative concept in the sense of cascade use and strengthen the international competitiveness of German technology providers.

In methanogenic mixed cultures, organic matter is converted to methane in several reaction steps. Macromolecules such as carbohydrates, fats and proteins are first hydrolyzed before they are converted to organic acids, alcohols (acidogenic bacteria), acetic acid (acetogenic bacteria) and finally to methane and CO₂ (methanogenic bacteria) [2].

According to previous experience with lignin separation from plant material, it can be expected that the lignocellulose structure will be expanded by the digestion and lignin separation. This should lead to a better substrate availability of freely accessible carbohydrates in the form of cellulose, to a reduction of the proportion of poorly hydrolysable compounds and thus to higher conversion rates in the fermentation process. The substrate samples were therefore initially investigated on a laboratory scale and currently on a pilot plant scale in a methanogenic degradation process with high space loading in ideally mixed reactors.

The substrates green rye, maize and sorghum silage were each uncrushed, crushed and, after lignin extraction, used for biogas production on a 1 litre laboratory scale. In the three or four feeding cycles, all substrates were converted to biogas at stable pH values and fatty acid concentrations within 7 days. In addition, the chemical oxygen demand content (COD content) was reduced. Nutrients important for the fermentation, such as NH₄⁺-N and PO₄^3--P were present in the reactor in sufficient concentration over the entire fermentation period.

Compared to lignin-extracted maize and green rye silage, a higher biogas yield of about 800-850 NmL/g oTR was achieved with lignin-extracted sorghum silage. Basically, the substrates without lignin showed a higher biogas yield due to the expanded lignocellulose structure. The mechanically shredded substrates generally showed a higher biogas production compared to unshredded substrates. Only with sorghum silage this was not observed. Here we suspect that components were released which impair the biogas process.

Lignin extraction upstream of biogas production represents a promising increase in biogas yield with simultaneous value creation of the raw material lignin as a starting material for the chemical industry. For example, lignin is an adequate substitute for phenol in the production of resins. Subsequent to the project, the technical results are to be transferred to industrial scale in order to enable series production of plants for lignin preparation, biogas production and utilisation in the medium term.

We would like to thank the Federal Ministry of Education and Research (BMBF) for funding the project "Energetic-material-energetic full utilization of cultivated plants; lignin separation, fermentation and partial oxidation; sub-project: Investigations on biogas production from NaWaRo", funding reference number 03SF0362A.

Fraunhofer ICT, Pfinztal | Leibniz Institute for Catalysis e.V. at the University | Rostock (LIKAT), Rostock | Hüttenes-Albertus Chemische Werke GmbH, Düsseldorf | Hölle & Hüttner AG, Tübingen | Johann Dudek Maschinenbau, Berlin | SLP GmbH, Wurmannsquick | BL1 GmbH Lichtenau, Lichtenau

[1] Fachagentur Nachwachsende Rohstoffe e. V., FNR (2009) Handreichung Biogasgewinnung und -nutzung

[2] De Lemos Chernicharo, C. A. (2007) Anaerobic Reactors, IWA Publisher