Dry fermentation

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The dry fermentation or dry fermentation is a special process for the production of biogas . Comparatively dry, fibrous and contaminated biomass such as biowaste from separate collection, organic fractions from residual waste, manure and green waste, which can be problematic in wet-fermenting biogas plants , can be tapped. With regard to the tank-or-plate debate, dry fermentation offers the possibility of using substrates that do not compete with food production. The ambiguous designation of the process as "dry" serves to distinguish it from wet fermentation. The substrates used usually still contain a very high proportion of water (up to 70%). In addition, depending on the process variant , the material is mashed with process liquid before fermentation or sprayed with aqueous liquid during fermentation in order to enable the necessary microbial processes ( anaerobic degradation ). In contrast to wet fermentation, dry fermentation offers the possibility of using organic waste for energy.

Biogas plants built from 2004 to 2008 could, under certain conditions, take advantage of the technology bonus for dry fermentation guaranteed by the Renewable Energy Sources Act 2004 , which enables a fee increased by 2 cents per kWh of electricity fed in for 20 years.

Procedure

There are different process variants for dry fermentation. The basic distinction is made through the division into continuous processes ( plug flow fermenter ) and discontinuous or batch processes ( garage fermenter ).

So-called plug flow fermenters are used for the continuous processing of substrates in dry fermentation. The substrate is conveyed through the horizontally positioned fermenter as a “plug” by means of large hydraulic piston pumps . Due to the high dry matter content , it is possible to mix the fermenter contents only locally with an agitator and not between loading and output, so the first in - first out principle applies . As a result, a minimum residence time of the substrate in the fermenter can be ensured and thus the substrate can be sanitized.

In batch operation with regular emptying and refilling, a stackable substrate is used, which is not previously moistened. The staggered operation of several fermenters on one system, however, allows a fairly even gas production to be achieved, which allows high utilization of the downstream components, such as a block-type thermal power station . In dry fermentation, the inoculation of the newly filled fermenter with anaerobic microorganisms is crucial in order to start degradation quickly in the absence of oxygen. Inoculation takes place either by remixing digestate from the previous batch or by moistening with percolates. Liquid ( percolate ) that escapes during fermentation is collected and returned to the fermentation material ( fermentation substrate ) from above.

The temperatures required for anaerobic degradation are achieved by wall and / or floor heating or indirectly by heating the percolate. Like any other biogas plant , dry fermentation plants can be operated both mesophilically (approx. 40 ° C) and thermophilically (approx. 55 ° C).

The digestion period in dry fermentation is between two and four weeks , depending on the process conditions and the quality and nature of the substrate . The gas yield can equal or exceed that of conventional wet fermentation, but is often significantly lower.

In the dry fermentation of organic waste is followed by the production of biogas necessarily a composting of the digestate, before they can be recycled as a compost in the nutrient cycle.

Advantages and disadvantages

Table of advantages and disadvantages of dry fermentation

According to the state of the art and taking into account the retention time of the material to be fermented in the corresponding fermentation concept, batch process or plug stream, the gas yields from 100% input material are the same. Both types of plant have a high degree of automation, demonstrably meet the hygienic standard and, depending on the mass balance and design based on the total mass to be fermented, have high processing capacities. Due to the relatively high technical effort involved in plug flow systems, higher maintenance and repair costs are inevitable. The trend in the German-speaking area is slightly in the direction of continuous processes.

Compared to the plug-flow fermenter and most wet fermenters, the operating mode in batch mode requires less maintenance and is less complex. Susceptible components of wet fermentation, such as pumps and agitators, are omitted, so that maintenance and operating costs can be lower. If necessary, discontinuous dry fermentation can also be carried out in mobile plants the size of a container. In addition, due to the composition of the substrate used, the resulting biogas has a lower hydrogen sulphide content (H 2 S). With appropriate gas storage facilities, which should be installed in all batch systems under the condition of peak load operation depending on a stock exchange electricity price according to the state of the art, a constant gas production, even in min. four fermenters, possible without loss of performance.

For certain substrates, such as biowaste , landscape maintenance material, grass silage and others, dry fermentation has proven to be well suited due to its insensitivity to contaminants. Especially in the future, dry fermentation will be used more and more due to the waste issue.

Funding by the EEG

In the amendment to the Renewable Energy Sources Act (EEG 2004) that came into effect in 2004, a technology bonus of 2 cents / kWh was set for dry fermentation . The right to the bonus lasts for 20 years. In particular, farms without livestock farming should be able to get started with manure-free biogas production. In addition, dry fermentation should be promoted as a process that requires little energy in order to increase the efficiency of biogas production.

Since 2004, the EEG has defined processes with an average of 30% dry matter in the substrate as dry fermentation. Plants with wet fermentation can also meet this requirement, as the percentage of water increases due to the breakdown of dry matter during fermentation and ensures a viscous to liquid fermentation substrate. For example, by mashing the fresh substrate with fermentation substrate or fermentation residue , it can be introduced into the fermenter. Further requirements for dry fermentation according to EEG 2004 relate to the efficiency of the process. Since the water content increases during the degradation process, a process is only referred to as dry fermentation when there is a volume load of at least 3.5 kg of organic dry matter per cubic meter of effective usable volume and day. In addition, the content of free volatile acids ( acetic acid equivalent ) in the digestate is limited to 2000 mg / l.

With the entry into force of the EEG 2009, the possibility of this bonus for newly constructed plants no longer applies. The reason is that the technology is no longer considered new due to its widespread use. In addition, in plants with dry fermentation, the use of locally available liquid manure was mostly dispensed with, which, if unused, causes high methane emissions.

The EEG 2012 promoted the generation of biogas from biowaste with 14-16 cents / kWh ( § 27a EEG 2012).

The EEG 2014 provides for so-called direct marketing of renewable energies for biogas plants from 100 kW . In the future, only half of the installed electrical output will be funded. At peak times where consumers lose weight much power, morning, noon, evening, a targeted BHWK-control mode is to be used. In addition to the base load CHP that is calculated on the basis of the mass balance of the feedstock to be fermented, a corresponding CHP additional output will have to be installed in the future. This CHP is always switched on to base load operation when there is a demand for peak load. This requires a large volume of gas storage in almost all systems. Based on the knowledge of the current electricity price and its demographic development, it can be assumed that operators of a biogas plant can expect up to 2 cents / kWh additional yield at peak load times according to the current electricity feed- in law . The additional CHP to cover these peak loads is funded for 20 years with 40 € / kW on the total system output.

Application examples

In 2003, the Munich waste management company built a dry fermentation pilot plant. This plant was expanded and started in December 2007 with a capacity of 25,000 t / a

A dry fermentation plant has been in operation in the city of Augsburg since 2013. 55,000 t / a of separately collected organic waste from around 1 million residents from the surrounding districts and the city of Augsburg are processed into biogas. In a further step, the biogas produced is processed into biomethane using the so-called membrane process. Waste generates around 5,400,000 m³ / a of biogas, which corresponds to 28,000,000 kWh / a of biomethane fed into the natural gas network. Furthermore, the amount generated corresponds to the annual heating requirement of 3,000 households, or the annual electricity requirement of 3,700 households, or the CNG requirement of 3,000 cars (15,000 km / a). The plant is currently being expanded with a third fermenter to a capacity of approx. 80,000 t / a.

Further plants are being planned in Germany, driven primarily by the newly regulated EEG 2014 and the Recycling Management Act , which provide for the separate collection of biowaste in combination with energetic recovery.

Web links

literature

Individual evidence

  1. ↑ Fachagentur Nachwachsende Rohstoffe eV: Dry fermentation - state of development and further R&D needs. (PDF; 1.2 MB), Gülzow 2006.
  2. ↑ Design aid: Dry fermentation for continuous biogas processes. (PDF; 66 kB) Federal Ministry for the Environment, Nature Conservation and Nuclear Safety , January 2007, accessed on January 6, 2018 .
  3. ↑ Simon Thomas Groneberg: EEG and KWKG reloaded - motifs, results and open questions: Die… GRIN Verlag, 2010, ISBN 3-640-57634-9 , p. 16 f . ( limited preview in Google Book search).
  4. ↑ Regulations of the EEG 2014 for biowaste fermentation plants. (PDF; 343 kB) Discussion paper. Witzenhausen Institute for Waste, Environment and Energy GmbH, August 1, 2014, accessed on January 6, 2018 .