Dry fermentation still has room for development
Very few mechanical-biological waste treatment plants have a fermentation stage.
The only way to ensure a preferably extensive supply of renewable energy is by using a combination of various processes and by exploiting the existing potential. The process of dry fermentation offers a wide range of options to achieve this aim through the use of biomass. The method is, however, still hardly being used in mechanical-biological treatment plants.
By Rolf Liebeneiner, Thomas Luthardt-Behle and Ulf Theilen from the Institute for Waste Water Management and Anaerobic Process Technology at the Gießen-Friedberg University of Applied Sciences
The proportion of electricity being produced from renewable energy sources is constantly growing and European countries have set themselves some partly very ambitious targets in their efforts to increase its use. These objectives can only be reached by employing a variety of renewable energy sources and skilfully combining each of their specific advantages. The generation of biogas through dry fermentation processes plays a major role in this endeavour. In contrast to wet fermentation systems, dry fermentation requires no fluid to be added to the substrate. This firstly saves on container volume and secondly it reduces the cost of separating the solid matter from the liquid at a later point in the process. Substrates that lead to operating problems during wet fermentation due to their structure or the proportion of impurities they contain can still be processed using dry fermentation methods.
The dry fermentation of wastes is a process that has already been implemented successfully for a number of years. Particularly since the amendment of the Renewable Energy Sources Act (EEG) in Germany in August 2004 and the resulting granting of the technology bonus for dry fermentation plants, the established techniques used by the waste disposal industry have been adapted to suit the numerous applications connected with the fermentation of energy crops. Although the technology bonus for dry fermentation plants is being dropped due to the latest amendment (2009), the advantages with regard to reactor volume, the choice of substrate and susceptibility to impurities still remain.
Dry fermentation in MBWTs
The dry fermentation method is established in Germany for the fermentation of residual and organic wastes. In 2007 there were 50 mechanical-biological waste treatment plants (MBWTs) with an annual processing capacity of 5.6 million Mg operating in Germany. Eleven of these plants have a fermentation stage integrated (22 per cent of the treatment capacity). Almost half of the fermentation plants work according to the dry fermentation principle. In the field of organic waste fermentation, around 47 per cent of the organic wastes treated by fermentation were processed using dry fermentation methods (status 2004). The capacity is likely to be expanded. Dry fermentation is going to increase in significance as a result of the latest amendment to the Renewable Energy Sources Act (EEG).
Discontinuous processes usually consist of boxes into which the substrate is emptied by means of wheel loaders. After the filling is completed the boxes are sealed airtight and the anaerobic degradation begins after inoculation either by means of percolation or packing. The process does not require moveable equipment to be installed in the fermenter. The fermented substrate is then removed by wheel loader after approximately three to four weeks and the boxes are then filled anew. The fresh substrate is partly mixed with residual fermented material before filling in order to accelerate the process. Several boxes have to be operated in chronologically staggered order to ensure the continual production of gas.
The idea of continuous processes has been borrowed from the waste disposal industry and the technical processes have been adapted to suit structurally rich, dry substrates. The circulation is done either by means of slowly turning mixers or solid matter pumps.
A wide variety of substrates can be used in dry fermentation processes. The main advantages of dry fermentation systems are the compact construction due to smaller material streams and reactor volumes and therefore lower energy consumption as well as reduced output of residues. These advantages are, however, to be seen in the light of relatively little practical experience.
Current research project
A research project currently being carried out on behalf of the Agency for Renewable Resources (FNR) by the Institute for Waste Water Management and Anaerobic Process Technology at the Gießen-Friedberg University of Applied Sciences together with the company Pöyry Environment GmbH is investigating the use and optimisation of continuously running dry fermentation processes for energy crops by means of laboratory and semi-technical fermentation tests on various types of biomass. The project is studying the effects of upstream treatment on the gas yield as well as the analysis of the fermentation residue regarding its effective use as fertiliser.
The dry fermentation process is being analysed both on a laboratory scale (7 litres thoroughly mixed, 40 litres plug flow) as well as on a semi-technical scale (800 litres plug flow). The results are to provide information on the efficiency of the dry fermentation process and determine the suitability of various substrates. The tests will be carried out under both mesophilic and thermophilic conditions in order to determine the influence of the process temperature on the various substrates.
The research work is also focussing on steam treating as well as finely grinding the biomass prior to fermentation and analysing the effects of these processes on the biogas yield. An additional part of the project is dealing with the agricultural recycling of the residues resulting from the dry fermentation process. The fermentation residues resulting from the thermophilic dry fermentation of maize silage from the semi-technical testing plant are being analysed in cooperation with the Landesbetrieb Landwirtschaft Hessen (the agricultural department of the German federal state of Hesse).
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