Prepare the biomass
Fresh feedstocks are mixed with microorganism-containing digestate or biofiltrate to form a pumpable biosuspension.
Technical solution for pumpable biomass
Plug-flow anaerobic digestion process
The process transfers the advantages of defined plug flow to the wet anaerobic digestion and methane fermentation of pumpable biomass. A hydraulically controlled outer and inner digester provides defined minimum treatment times, retention of adapted microbial cultures and largely continuous operation.
Technical basis: DE 10 2016 009 223 A1Initial challenge
Fully mixed continuous digesters do not ensure a defined minimum treatment time
In a continuously fed stirred digester, part of the freshly added biomass may leave the vessel with the next discharge, while other fractions remain substantially longer than required for biological degradation. The mean hydraulic retention time therefore does not represent the actual minimum treatment time of the entire material stream.
Known plug-flow reactors are frequently designed for bulk solids or depend on mechanically intensive conveying and mixing systems. This process solution instead creates a continuous plug flow for pumpable biosuspensions with high dry-matter contents.
Pumpable biomass and biosuspensions
12–20% dry matter, preferably at least 15%
Minimum treatment time of at least 15 days
Hydraulically connected outer and inner digester
Process sequence
Continuous material flow along a defined treatment path
The supplied biomass is not fully dispersed throughout the entire digester, but passes consecutively through spatially distinguishable process zones.
Feed the outer digester
Feeding takes place in the upper zone. The batch is mixed only into the upper substrate layer of the outer digester.
Guide the plug flow
Displacement moves the digesting substrate downwards in the outer digester, through the bottom connection and then upwards in the inner digester.
Use the pressure difference
Different gas pressures create a difference in liquid levels and provide hydraulic energy for fluidisation and material transport.
Discharge the digestate
Each feeding step displaces and discharges a corresponding quantity of digestate at the highest liquid level of the inner digester.
Plant principle
Hydraulic plug flow through the outer and inner digester
The material path runs from the upper zone of the outer digester through the bottom connection into the inner digester and to the digestate discharge. Separate pressure control of the two gas spaces creates the different liquid levels.
Technical design parameters
Key operating and design features
Dry-matter content
The process is intended for pumpable biosuspensions with 12 to 20% dry matter, preferably at least 15%. The higher solids content reduces separation tendencies.
Digester volume
According to the published application, the usable digestion volume is selected at no less than 25 times the daily biomass feed volume.
Organic loading rate
The fresh biomass feed is limited to a maximum of 6 kg organic dry matter per m³ of usable digestion volume and day.
Separate gas-pressure control
In the described plug-flow operation, the absolute outlet pressure is limited to a maximum of 1.45 bar in the outer digester and 1.05 bar in the inner digester.
Floating-layer control
If required, gas overflow channels can be activated temporarily or additional fluidisation devices can be used to break up disruptive floating layers in the inner digester.
Sediment discharge
Temporary pressure equalisation between the gas spaces generates a near-bottom circular flow. This transports sediment-rich material towards the designated discharge nozzles.
Client benefits
Defined treatment with reduced mechanical effort
Defined minimum treatment time
The defined material path reduces hydraulic short-circuiting and provides a minimum treatment time much closer to the mean retention time than in a fully mixed continuous digester.
Improved substrate utilisation
Premature discharge of insufficiently converted biomass is reduced. This can improve utilisation of the energy potential contained in the organic matter.
Adapted microbial cultures
Along the plug flow, microbial cultures can adapt to the gradually changing substrate quality. Recycled biofiltrate supports the return of active microorganisms and spores.
Less mixing energy and wear
Gas release provides a substantial part of the micro-circulation. Mechanical agitators are used mainly for feed incorporation or as on-demand fluidisation devices.
Hygienisation potential
The defined treatment time can support whole-stream hygienisation when suitable temperatures are maintained. The effects described in the application must be verified for the specific substrate and operating concept.
Compact configuration
The coaxial outer and inner digester arrangement enables high filling levels, comparatively low land requirements and reduced heat-transferring external surfaces.
Example from the published application
Sizing example for a biosuspension made from grain-cleaning residues
In the described example, 8 t/day of grain-cleaning residues are mixed with 42 m³/day of biofiltrate to produce 50 t/day of biosuspension with approximately 18% dry matter. A plug-flow digester with 3,000 m³ usable volume and a daily feed of 50 m³ provides a mean retention time of 60 days and a stated minimum treatment time of at least 18 days.
The figures do not constitute a general performance guarantee. Achievable methane yields, treatment times and hygienisation effects must be calculated and verified for the specific feedstock and project.
3,000 m³ usable digestion volume50 m³ biosuspension per day60-day mean retention timeat least 18 days minimum treatment time in the example
Technical background
The process solution presented is based on German published patent application DE 10 2016 009 223 A1, “Process for plug-flow methane fermentation and apparatus for carrying out the process”. Digester volume, pressure levels, loading, temperature control, safety systems, materials and integration into the overall plant must be designed for the specific project, site and feedstock.
Technical project assessment