Landfill-gas collection systems require equipment matched to extraction conditions, gas quality, treatment requirements and pressure limits. It typically contains methane and carbon dioxide as its main components, together with water vapour and smaller quantities of nitrogen, oxygen, hydrogen sulphide, siloxanes and non-methane organic compounds.
An active landfill-gas collection system uses mechanically induced vacuum to draw this gas through extraction wells and convey it to a central treatment, flaring or energy-recovery facility. A gas-tight roots blower can provide the required suction at the wellfield and positive pressure on the discharge side.
The objective is not to apply the highest possible vacuum. The blower must collect gas effectively without drawing excessive atmospheric air into the landfill. Wellfield balancing, condensate management and gas-quality monitoring are therefore as important as blower capacity.
How an Active Landfill-Gas System Operates
A landfill-gas collection and control system can include:
- Vertical extraction wells
- Horizontal collection trenches
- Wellhead valves and monitoring points
- Lateral collection pipes
- Main gas headers
- Condensate traps and sumps
- Knockout vessel
- Gas filters
- Roots Blower or another gas mover
- Flare
- Engine, boiler or other energy-recovery equipment
- Gas-treatment and upgrading systems
- Supervisory control and data acquisition—SCADA
Vertical wells are normally drilled into completed or capped sections of the landfill. Perforated pipe allows gas to enter the well below the cover, while the upper section is sealed to limit atmospheric air infiltration.
Horizontal collectors may be installed progressively as waste is placed. They can begin collecting gas from active areas before final closure and may operate alongside vertical wells.
Individual wells connect to lateral pipelines, which combine into a main header. The blower creates a pressure gradient that pulls gas from the waste mass toward the central processing skid.
On the discharge side, the blower provides enough pressure to move the gas through treatment equipment and into:
- An open or enclosed flare
- A reciprocating gas engine
- A boiler or kiln
- A gas turbine
- A direct-use pipeline
- A renewable natural gas upgrading plant
The required treatment depends on the final use. Gas sent directly to a flare may need primary moisture and particulate removal, while engines and upgrading systems can require additional H₂S, siloxane, VOC and moisture control.
Selecting a Roots Blower for Landfill-Gas Extraction
Blower selection should use measured field data and a professionally developed landfill-gas generation and recovery model. The design flow should reflect both current gas availability and expected changes over the project life.
A preliminary relationship is:
Expected Landfill-Gas Collection Flow
- Qcollected = expected recovered landfill-gas flow
- Qgenerated = estimated gas generation
- ηcollection = expected collection efficiency expressed as a decimal
Both terms contain uncertainty. Gas generation changes with waste composition, moisture, temperature, landfill age and operating practices. Collection efficiency changes with cover condition, well spacing, vacuum and maintenance.
The blower should therefore cover an operating range rather than one fixed design point.
The total blower differential pressure is:
Δpblower = pdischarge,g − psuction,g
Where gauge pressure is used consistently.
For example, if the blower inlet operates at −150 mbar(g) and its discharge operates at +250 mbar(g):
Δpblower = 250 − (−150)
Δpblower = 400 mbar
The corresponding pressure ratio must be calculated from absolute pressures:
Pressure ratio = pdischarge,abs / psuction,abs
Using gauge pressure directly in a pressure-ratio calculation produces an incorrect result.
The required differential pressure must include:
- Wellfield vacuum
- Suction-header friction
- Knockout-vessel loss
- Filter resistance
- Discharge-pipeline loss
- Gas-treatment pressure drop
- Flare or energy-user inlet pressure
- Control-valve losses
- End-of-service filter resistance
A roots blower is often suitable where relatively stable volumetric flow is required across changing resistance. Centrifugal and regenerative blowers are also used in landfill-gas systems. The appropriate technology depends on total gas flow, required vacuum, discharge pressure, turndown and gas condition.
A standard air blower should not be selected from an air-performance curve and then connected directly to landfill gas. Methane, carbon dioxide, contaminants and suction conditions change gas density, internal leakage, absorbed power and discharge temperature.
Actual flow should be related to its reference basis:
Actual Volumetric Flow Conversion
- Qactual = volumetric flow at actual inlet conditions
- Qreference = flow at stated reference conditions
- Tactual = actual absolute inlet temperature
- Treference = reference absolute temperature
- Pactual = actual absolute inlet pressure
- Preference = reference absolute pressure
- Zactual = compressibility factor at actual inlet conditions
- Zreference = compressibility factor at reference conditions
The effect of suction vacuum is important. As inlet absolute pressure falls, a given actual inlet volume contains less gas mass. Flow values should always identify whether they represent actual or standard conditions.
Variable Flow Over the Landfill Life
Landfill-gas production rises, reaches a peak and then declines over time. New cells may be added while older areas produce less gas. Seasonal moisture and temperature also affect generation and recovery.
A blower station should accommodate this changing duty through:
- Variable-speed control
- Multiple blowers operated in stages
- Different blower sizes
- Automatic duty rotation
- Controlled recycle where required
- Future connection capacity
Installing one large fixed-speed blower for projected peak gas flow can create inefficient operation during early and late project years. Staged units provide a wider control range and maintain standby capacity.
The lowest operating flow must also be checked. A positive-displacement blower should not run below its permitted speed or outside its allowable temperature range.
Wellfield Vacuum Control
The purpose of wellfield tuning is to collect methane while limiting air infiltration. Each extraction well has a zone of influence, and wellhead valves are adjusted to balance gas recovery across the landfill.
Excessive vacuum can draw atmospheric air through:
- Cracks in the landfill cover
- Poorly sealed well penetrations
- Damaged lateral connections
- Dry or failed bentonite seals
- Active filling areas
- Exposed waste
- Leaking condensate equipment
Air infiltration reduces methane concentration and increases oxygen and nitrogen in the collected gas. This can lower heating value, reduce energy-recovery output and increase the treatment required for RNG production.
More seriously, sustained oxygen entry into the waste mass can promote aerobic activity, heat generation and subsurface fire.
Increasing blower speed is therefore not an acceptable universal response to falling methane flow. The cause may be a cover leak, flooded well, broken lateral, condensate blockage or excessive extraction from one part of the landfill.
Glossary
Condensate drainage: Removal of liquid accumulated in gas wells, low points or collection headers.
Condensate trap: Low-point vessel that collects liquid from a gas line.
Flare diversion: Routing of gas to a flare when recovery equipment cannot accept it.
Landfill wellfield: Network of extraction wells that collects gas from waste layers.
Methane fraction: Share of methane by volume in landfill gas.
Vacuum header: Common pipe that gathers gas from multiple extraction wells.
Well balancing: Adjustment of extraction at individual wells to manage gas quality and vacuum.
Wellfield balancing: Adjustment of individual extraction flows to control landfill-gas collection.