03 Process and Systems Engineering

• Dry Matter (DM) content substrate (solid/dry fermentation)

• Feed (continuous, discontinuous)

Most important process characteristics

• Mixture

• Number of process steps (single-stage, multi-stage)

• Process temperature (unheated, psychophil, mesophil, thermophil, hyper-thermophil) —> mostly thermophil

• Wet fermentation - which is pumpable and stored in large round containers

• Solid fermentation - dry fermentation and stackable when storing

• Limit 20 % DM content

• Mostly 30 % DM content

• < 15 % DM —> pumpable

• Process of putting substrate to plant

• Wet fermentation - mostly (quasi) continuous which flow through process (most plants use this type)

• Solid fermentation - mostly discontinuous using batch process (not many plants use this type)

• Same amount of digestate fed is removed from digester

Advantages

• Countinuous gas production

• Automatic feed

• Good use of digester capacity

Disadvantages

• Part of fresh substrate is removed

• Digester is filled completely with substrate and sealed airtight

• No adding or removing substrate till end when all is emptied.

Advantages

• Easy process for substrate with high DM content

Disadvantages

• Injection of bacteria at beginning

• Discontinuous production (solved by time-shifted filling of containers)

  
  

—> Depends on the fermentation process

• Fully mixed digester (wet)

• Plug flow process (wet and dry)

• Batch method

Fermenter can be built from armored concrete or steel/ high-grade steel

• It is inflatable with double foil with air in between

• The external foil is for weather production and the internal is gas membrane for storage.

Advantages

• Cost effective

• Additional gas storage

• Can be opened over large area by removing foil

  
  

Disadvantages

• No access from above

• Heat losses

• More sensitive to weather conditions

• Gas tightness (simple foils)

• It is important to keep specified temperatures for fermenter

• Mesophilic (30 – 45 °C)

• Thermophilic (45-55 °C).

• Internal (wall) heating allows for good heat transfer for the substrate —> Heating pipes on walls could be damaged because of exposure and may cause deposit buildups

Why mixing?

• Intensive contact bacteria/substrate

• Distribution of heat

• Improvements of degassing

4 types of stirers

1. Mechanical - in continuous or intermittent operation

2. Submersible stirrers - rod type and paddle type

3. Hydraulic - circulation with hydraulic pump

4. Pneumatic - injection of biogas

—> Has no stirrers, composition is heat up with air, 20 days retention time and no constant methane production

Advantages

• Simple Construction

• Easy feeding/remove

• Low energy demand because of no stirrer and heater

• High DM content processable

Disadvantages

• High Investment cost

• Increased emissions when opening the door

• Liquid substrate(slurry)

  • Stored in storage pool.

  • Storage pool acts as a buffer.

  • Mostly not gastight(open or closed with a cover)

• Solid substrate(stackable)

  • Stored in bunker silos

—> Liquid/pumpable substrate

• Pumps mostly driven by electric motors

• Shredders in front of the pump or integrated

Thin fluid < 8 % DM —> Rotodynamic pump

Thick fluid —> Displacement pump (eccentric screw or eccentric rotor pump)

—> Solid/stackable substrate

Transport by

• Wheel loader

• Push floors

• Charging screw

Feeding

• Indirect

  • Via storage/mixing tank (stirrer/shredder needed)

  • Via fluid flow (in slurry stream, via branch pipe)

• Direct

  • Charging screws

  • Charging screws with feed hopper and supply tank