At selected process stages, roots blower applications for biogas involve gas transfer or boosting, subject to composition, pressure, flow and package compatibility. Raw biogas may need to be extracted under vacuum, recirculated through a digester, transferred from a gas holder, boosted to a CHP engine or delivered to an upgrading system. Other processes require controlled air rather than direct biogas handling.
Roots-type positive-displacement blowers are suitable for many low-pressure gas duties because they provide relatively stable volumetric flow against changing system resistance. Their performance can be valuable where biogas production fluctuates and the downstream equipment requires consistent inlet pressure.
Direct biogas service is fundamentally different from conventional air service. Raw biogas may be wet, corrosive, toxic and combustible. A blower used for this duty must be engineered as a process-gas machine with suitable sealing, materials, instrumentation and explosion protection. A standard aeration or pneumatic-conveying blower should not automatically be connected to a biogas pipeline.
Where Roots Blowers Are Used
Biogas is generated when microorganisms decompose organic materials under anaerobic conditions. Feedstocks can include animal manure, sewage sludge, food-processing residues, agricultural materials, municipal organic waste and landfill waste.
Raw biogas normally contains methane and carbon dioxide as its principal components. Methane concentration can vary considerably, commonly falling within a broad range of approximately 50–75%. Water vapour, hydrogen sulphide and smaller quantities of nitrogen, oxygen, ammonia, hydrogen, siloxanes and volatile organic compounds may also be present, depending on the feedstock and process.
Biomethane is produced by upgrading biogas to increase its methane content and reduce components that are unsuitable for the intended end use. The required treatment may include removal of:
- Carbon dioxide
- Hydrogen sulphide
- Water
- Particulates
- Siloxanes
- Volatile organic compounds
- Nitrogen and oxygen where applicable
The final specification depends on whether the biomethane will be used as vehicle fuel, compressed or liquefied, supplied to an industrial consumer or injected into a natural-gas network.
Roots blowers may support the following plant operations.
Biogas Boosting
A gas-tight roots blower can increase the pressure of conditioned biogas for delivery to CHP engines, boilers, burners, flares or upgrading equipment. The blower helps overcome losses through pipes, filters, heat exchangers, gas-treatment vessels and control valves.
CHP engines commonly require a stable fuel-gas inlet pressure. Raw biogas production, however, changes with feedstock loading, temperature and biological activity. A speed-controlled positive-displacement blower can help isolate the engine fuel supply from these upstream fluctuations.
The blower should be selected only after the downstream pressure requirement has been confirmed. If biomethane must be delivered at several bar or at transmission-pipeline pressure, a higher-pressure compressor may be required. A roots blower may serve as a low-pressure feed booster but should not be described as suitable for every grid-injection duty.
Landfill-Gas Extraction
Active landfill-gas collection systems use mechanically induced vacuum to draw gas through vertical wells or horizontal collection trenches. The gas passes through lateral pipes and a main header before reaching condensate separation, filtration, treatment, flaring or energy-recovery equipment.
A roots blower or exhauster can provide both the suction needed at the collection field and the positive discharge pressure required downstream.
Vacuum must be controlled carefully. Excessive suction can draw atmospheric air through the landfill cover or damaged well seals. This dilutes the methane, increases oxygen concentration and can contribute to subsurface heating or fire.
The blower is therefore part of a controlled wellfield extraction system. Maximum vacuum is not the design objective; stable collection with limited air infiltration is.
Anaerobic Digester Gas Recirculation
Some anaerobic digesters use recirculated biogas to improve mixing. Gas is withdrawn, compressed and injected into the sludge through suitable distribution equipment. The rising bubbles promote circulation, distribute heat and reduce solids settling.
A process-gas roots blower may be suitable where the required circulation flow and injection pressure remain within its operating range. The gas loop must include effective condensate management because cooling in the suction line can produce liquid accumulation.
Gas mixing is not used in every digester. Hydraulic and mechanical mixing systems are also common, so the blower should be connected with this application only where gas recirculation is part of the digester design.
Biogas Upgrading and Vacuum Regeneration
Pressure Swing Adsorption separates carbon dioxide and other components by adsorbing them onto materials such as activated carbon or molecular sieves. The adsorbent is regenerated by reducing pressure, and some systems apply vacuum during the regeneration cycle.
Roots vacuum blowers or boosters may be integrated into certain VSA or VPSA systems. Their suitability depends on cycle pressure, gas composition, required evacuation time and the backing-pump arrangement.
The term VPSA does not identify one universal machine configuration. Liquid-ring, dry-screw, claw and other vacuum technologies may also be used. Equipment selection is controlled by the upgrading technology supplier’s process design.
Biological Hydrogen-Sulphide Removal
Sulphur-oxidizing microorganisms can convert hydrogen sulphide into elemental sulphur or sulphate when a controlled oxygen source is available. This can occur in a digester headspace, biological scrubber or biotrickling filter.
A blower may supply the required oxidation air. In this service, the machine normally handles atmospheric air rather than biogas. Airflow must be measured and limited because excessive oxygen can create a flammable gas composition and prevent the treated gas from meeting its end-use specification.
Whether a roots blower is appropriate depends on the required air volume and pressure. Small systems may use another low-capacity air technology.
Double-Membrane Gas-Holder Support Air
A double-membrane gas holder uses an inner membrane to contain biogas and an outer membrane to provide weather protection and structural form. A support-air blower supplies atmospheric air to the space between the membranes.
This blower does not normally contact biogas. It maintains the required support pressure while relief and control equipment prevent excessive pressure.
The support-air requirement is genuine, but the air mover is not necessarily a roots blower. Side-channel or centrifugal blowers may be more appropriate for some membrane sizes. Selection should follow the gas-holder manufacturer’s airflow and pressure requirements.
Direct Gas Handling Requires a Dedicated Design
Oil-free compression describes the absence of lubricating oil inside the gas-conveying chamber. It does not automatically mean that a blower is gas-tight, corrosion-resistant or approved for combustible gas.
A roots blower intended for direct biogas service should be evaluated for:
Equipment Scope and Selection Limits
Separate air-side aeration and mixing from raw-biogas duties; gas service needs its own composition, moisture, pressure, materials, sealing and hazardous-area review.
Gas-Service Review
Separate air-side aeration and mixing from raw-biogas duties; gas service needs its own composition, moisture, pressure, materials and containment review.
Glossary
Biogas upgrading: Separation process that removes CO2 and contaminants from raw biogas.
Biomethane: Upgraded gas with increased methane concentration for fuel or grid use.
Biomethane specification: Required product-gas composition and contaminant limits for the intended use.
Digestate: Residual liquid or solid material remaining after anaerobic digestion.
Gas holder: Flexible or rigid storage vessel that buffers biogas production and demand.
H2S: Hydrogen sulfide, a toxic and corrosive contaminant in raw biogas.
Methane concentration: Volume fraction of methane in the gas at stated pressure and temperature.
Raw biogas: Unconditioned gas from anaerobic digestion, typically containing methane and CO2.