Anaerobic digester gas recirculation can support mixing by returning biogas through a distributor, subject to digester design and process requirements. It helps distribute incoming organic material, maintain a more uniform temperature, improve contact between microorganisms and biodegradable substrates, and limit the accumulation of settled solids. Depending on the digester design, mixing may be provided mechanically, hydraulically or by recirculating compressed biogas.
In a gas-mixing system, biogas is withdrawn from the digester headspace, conditioned as required and compressed by a suitable gas blower. The pressurized gas is then returned through distribution pipes, lances, draft tubes or injection nozzles installed below the liquid surface. As the gas bubbles rise, they transfer momentum to the surrounding slurry and generate vertical circulation inside the digester.
A positive-displacement roots blower can be suitable for this duty when the process requires a controlled gas flow at a relatively low or moderate differential pressure. However, the machine must be engineered for direct contact with wet, combustible and potentially corrosive biogas. A standard air blower should not automatically be specified for anaerobic digester gas recirculation.
How Biogas Recirculation Mixes the Digester
The blower does not introduce atmospheric air into the anaerobic process. It recirculates gas that has already been produced and collected in the digester headspace. This distinction is critical because introducing uncontrolled oxygen could interfere with anaerobic conditions and create a flammable mixture within the gas system.
A typical gas-recirculation loop includes:
- A gas take-off connection from the digester headspace
- A condensate separator or knockout vessel
- Gas piping designed to drain accumulated moisture
- A gas-tight positive-displacement blower
- Non-return and isolation valves
- Gas-flow and pressure instrumentation
- A submerged gas-distribution system
- Safety interlocks connected to the plant control system
The compressed biogas is commonly injected near the lower part of the digester. The required injection arrangement depends on the tank geometry, slurry depth, solids concentration, viscosity and the desired circulation pattern. Multiple injection levels or distribution points may be necessary to reduce stagnant zones in large digesters.
As the bubbles move upwards, they entrain liquid and create bulk circulation. Properly designed circulation can assist with temperature equalization, substrate dispersion and suspension of settleable material. It may also help limit floating layers and localized accumulation, although separate surface-mixing equipment may still be required where heavy crust formation is expected.
Gas mixing should not be presented as a universal solution for every digester. Plug-flow digesters, covered lagoons, high-solids reactors and complete-mix digesters have different operating requirements. Mechanical or hydraulic mixing may be more appropriate where the slurry is highly viscous, contains long fibres or requires strong localized shear. The mixing technology should therefore be selected by the digester process designer rather than solely by the blower supplier.
Why Roots Blowers Are Used for Gas Mixing
Roots blowers are positive-displacement machines. Their two synchronized lobed rotors transfer discrete volumes of gas from the inlet to the discharge without internal compression. The required pressure develops in response to the resistance of the downstream system.
This operating principle provides several advantages for anaerobic digester gas mixing:
- Relatively stable volumetric flow over a practical range of discharge pressures
- Predictable response to speed control
- Suitability for continuous industrial operation
- Oil-free gas transport within the compression chamber
- Capability to overcome hydrostatic pressure and distribution-system losses
- Straightforward capacity control using a variable-frequency drive
- Availability in duty-and-standby package arrangements
“Oil-free gas transport” refers to the compression chamber. Bearings and timing gears still require lubrication, but the lubricating oil is separated from the process gas by the machine’s sealing arrangement.
Unlike an aeration blower, a digester gas blower handles a wet fuel gas containing methane, carbon dioxide, hydrogen sulphide and other trace compounds. The gas is normally saturated with water vapour when it leaves the digester. Cooling in the suction pipe can therefore generate condensate before the gas reaches the blower.
Liquid carryover can cause corrosion, deposits, rotor damage and severe operating disturbances. The suction system should include effective condensate removal, suitable drainage and high-level protection. Pipework must be arranged so that condensed liquid cannot collect unnoticed or drain back into the blower.
Differential Pressure and Gas-Flow Evaluation
The blower must generate sufficient discharge pressure to overcome the hydrostatic pressure at the injection depth together with the pressure losses in the gas-distribution system.
A simplified discharge-pressure relationship is:
Digester Gas-Recirculation Pressure Requirements
- pdischarge,abs = required absolute pressure at the blower discharge
- pheadspace,abs = absolute pressure in the digester headspace
- ρslurry = density of the digester slurry
- g = gravitational acceleration
- h = vertical distance between the slurry surface and the gas injection point
- Δppipeline = pressure loss through the discharge piping
- Δpvalves = pressure loss through valves and control components
- Δpinjectors = pressure required across the injection or distribution devices
- Δpblower = blower differential pressure
- psuction,abs = absolute pressure at the blower suction
The suction pressure may be slightly lower than the digester headspace pressure because of losses through the suction pipe, moisture separator and other upstream equipment. Both suction and discharge pressures must therefore be evaluated on an absolute-pressure basis during final selection.
The static liquid head can form a substantial part of the total pressure requirement. Deeper injection points require more discharge pressure, even when the gas pipeline is short. Deposits, partially obstructed injection holes or poorly balanced distribution branches can increase the required pressure further.
Blower capacity must be specified at actual inlet conditions. Normal cubic metres per hour and actual cubic metres per hour are not interchangeable. For the same mass or molar gas flow, an approximate conversion is:
Actual Blower Inlet Flow Conversion
- Qactual = volumetric flow at the blower inlet
- Qreference = volumetric flow stated at reference conditions
- Tactual = actual absolute gas temperature
- Treference = reference absolute gas temperature
- Pactual = actual absolute pressure
- Preference = reference absolute pressure
- Zactual = gas compressibility factor at actual inlet conditions
- Zreference = gas compressibility factor at reference conditions
Gas composition affects density, temperature rise, absorbed power and blower performance. The calculation should use the expected operating range rather than a single nominal methane percentage.
The required gas flow should not be selected from digester volume alone. Tank dimensions, injection depth, slurry rheology, total-solids content, feedstock characteristics and the performance of the selected gas-distribution system all influence the mixing demand. Pilot data, process-licensor recommendations or validated computational modelling may be necessary for unusual digesters.
Glossary
Digester mixing: Movement that distributes feed, heat and microorganisms in a digester.
Gas lift: Liquid circulation created by gas injection and buoyancy.
Gas recirculation: Return of process gas to a vessel to promote mixing or circulation.
Gas residence time: Average time gas remains in a digester headspace or recirculation path.
Headspace: Gas-filled space above the liquid or slurry in a vessel.
Headspace pressure: Gas pressure in the space above the digester liquid or slurry.
Recirculation flow: Gas flow returned through the mixing loop per unit time.
Sparger: Pipe or device that releases gas into liquid or slurry.