Products > Biogas plants > Plant Design

Waste to Energy

Delivery and removal of material / Filling of fermentation boxes

Sample construction of a dry fermentation plant
Sample construction of a dry fermentation plant

The box-shaped fermentation containers (boxes) are constructed using gas-proof reinforced concrete and are operated in batch mode. The fermentation substrate remains in the digester for a period of approx. 4-5 weeks in each case. The removal/filling process takes 1front-end loader about 1 working day per fermentation box (including cleaning work).

The biomass intended for fermentation is brought to the mixing area in front of the fermentation box in question on the filling day. Approx. 50% of the removed, fermented charge is separated and can then be further processed. The other approx. 50% of the charge is used as inoculation material. The fresh material (biomass) is mixed with the inoculation material and fed into the fermentation box. This produces an average statistical holding time of approx. 8-10 weeks for the fermentation substrate in the digester. The fermentation substrate is mixed with wheeled loaders inside the planned mixing and manoeuvring area and inside the fermentation boxes. The emptying/filling of the fermentation boxes is also handled via the wheeled loaders.

Work safety

The patented BEKON purging method allows the fermentation boxes to be rendered inert in a controlled way before opening and after closing them to exclude the possibility of atmospheres that could lead to explosions. The resulting exhaust mixture is safely disposed via the integrated chimney and torch system.

Furthermore, the respective fermentation box is always rinsed with fresh air during the emptying/filling process for safety reasons.

The fermentation boxes are closed with a hydraulically operated, gas-proof gate. A permanent sealing of the fermentation gates is guaranteed by an inflatable sealing lip placed directly on the gate. Electronic pressure controllers ensure a constant monitoring of the sealing lip’s interior pressure – and adjust it automatically if need be. Large decreases in pressure are recorded in the plant’s control system and trigger specific reminders and warnings.

Fermentation process / Operation of fermentation boxes

Dry fermentation plant
Dry fermentation plant, view of digester retention gate

The substrate is anaerobically fermented in the fermentation boxes in batch operation for the entire holding time – without further mixing or stirring. Since no pumps, stirring tools or other mixing and conveying equipment are necessary to move the biomass, BEKON technology possesses an exceptionally high tolerance for mechanical impurities in the fermentation substrate. For this reason, the tendency to develop problems and the required maintenance and repair costs during the fermentation process and for the operation of the entire plant is accordingly lower.

To accelerate the anaerobic fermentation process after closing the fermentation gates and to stimulate biogas production, each newly filled fermentation box is systematically rendered inert again. This guarantees high gas yields of good quality early on.

The fermentation boxes possess percolate sprinkler systems that are equipped with outlet nozzles specially developed by BEKON. This ensures an especially even and effective distribution of the percolate over the entire biomass. The sprayed percolate provides ideal, continuous moisture for the fermentation substrate to guarantee the best possible gas yields. The sprayed percolate is systematically extracted via a drainage trough on the fermentation floor and via lateral drains, cleaned in a separate filter system, tempered in the percolate tank, temporarily stored and, if need be, sprayed on the fermentation substrate again to moisturise it.

The fermentation process in the fermentation boxes takes place in the mesophilic temperature range at approx. 38°C; the tempering occurs with floor and wall heating. This allows the entire contact surface of the fermentation substrate / digester to be used for the transfer of heat. The heating loops are integrated into the concrete walls during the building of the fermentation boxes so that no undesirable installations are required inside the containers. Overall, this facilitates an ideal control over the temperature in the fermentation boxes.

Agro-Energy

View of a dry cycle fermentation plant
View of a dry cycle fermentation plant

The patent-protected BEKON DCF technology (dry cycle digester) consists of a round digester (plug-flow digester) and a storage tank for the liquid fermented substrate, which is usually installed in a central location.

The BEKON dry cycle digester represents a systematic further development of the proven BEKON technology for continuous flow-through systems. The wet fermentation methods with conventional stirring technology as usually used in renewable resource plants require both significantly larger digesters and corresponding end storage capacities for the liquid fermented substrate due to the lower dry substance content of the input.

In contrast to such systems, the BEKON DCF plants developed specially for renewable resource plants offer the operator an extremely attractive economic alternative to wet fermentation plants.

The BEKON DCF plant significantly reduces construction costs thanks to its compact design. Furthermore, the efficient digester structure and the robust conveyor technology offer ideal operating and maintenance costs. The innovative process control engineering for the plant guarantees minimal personnel costs.

Preparing and feeding the material

Approx. 1 to 2 times per day, the charging device for the dry cycle digester is filled. The supplied material is dumped directly into the feed silo with a telescope loader. The charging device (feed silo) is located right next to the dry cycle digester and consists of a rectangular container that opens on top. A floor conveyor is installed on the floor of this plastic silo and operated fully automatically. The floor conveyor sends the filled material to the dosing station. In the next step, a corresponding acceptance conveyor feeds the material into a closed mixing pump that consists of a screw conveyor and an eccentric pump. This pump facilitates the best-possible mixture of fresh substrate and inoculated fermented substrate before the mixture is conveyed to the digester.

Fermentation process

The BEKON DCF technology consists of one or more dry cycle digesters: A dry cycle digester serves as the main digester, another dry cycle digester as the secondary digester and the storage tank is for the fermented substrate. The fed fermentation substrate circles through the main digester in plug flow. 

The continuous separating and feeding of renewable resources and the systematic percolation in the main digester create a natural movement that ensures the ideal transport of the fermentation substrate through the container. The conventional energy- and maintenance-intensive stirring tools are no longer necessary in the digester. The fermentation process in the BEKON dry cycle digesters takes place in the mesophilic temperature range at approx. 38°C; the tempering is handled with floor and wall heating. This allows the entire contact surface of the fermentation substrate / digester to be used ideally for the transfer of heat. The heating loops are integrated into the concrete walls during the building of the containers so that no undesirable installations are required inside the containers. This facilitates an ideal control of the temperature.

The percolate extracted from the main digester is stored in the inner ring.  This is partially used for the inoculation of fresh material and the constant substrate supply of solid material, and is systematically returned for this purpose to the main digester.

After fermentation, a portion of the fermented renewable resources is sent to the digester or the feed system as inoculation material. For this purpose, there is a connection between the separator pump and feed pump.

Depending on the requirements, percolate can be systematically added to or withdrawn from different places of the plug flow. The distribution of the percolate is controlled by computer in accordance with a set rhythm.

Separating material

Separating material
Separator system for fermentation substrate

After completing the fermentation phase, the fermentation substrate is conveyed out of the digester with pumps. In the separator, the fermented substrate is divided into its fluid and solid components. The solid fermented substrate can then be used directly as organic fertilizer and brought e.g. to the agricultural land of the substrate supplier.