Landfill gas compression: interfaces, analysis and design

Why landfill gas is a moving target

Landfill gas is mostly methane and carbon dioxide, with nitrogen, oxygen, water vapour and trace compounds. Unlike digester biogas from a controlled vessel, it is drawn from a large, variable waste mass through a network of wells. Its composition shifts with waste age and type, moisture, temperature, cover condition, rainfall and how the wellfield is balanced.

The gas reaching the compressor can change hour by hour and season by season. In the Indian monsoon, rainfall raises moisture and liquid in the collection system; in summer, heat changes gas temperature and condensation patterns. A single supplier duty point is not enough unless minimum, normal, maximum and upset cases are also described.

The interface chain from well to end use

A typical arrangement runs from extraction wells through collection laterals and the main header, then isolation and liquid knockout, conditioning, an inlet buffer with monitoring, the blower or compressor, aftercooling and condensate separation, further gas treatment, metering and isolation, and finally the flare, gas engine, boiler or upgrading plant.

Collection pipework usually runs below atmospheric pressure, while discharge equipment runs above it. That transition is the critical boundary, and responsibilities must be clear on each side.

Gas analysis that a supplier can actually use

Give measured mole fractions of methane, carbon dioxide, nitrogen and oxygen, plus the relevant trace species: hydrogen sulphide, water vapour, siloxanes, halogenated compounds and volatile organics. For each result, state the date, wellfield operating state, analysis method, detection limits and whether it represents a minimum, typical, maximum or upset case.

Say whether results are on a wet or dry basis and at what pressure and temperature the sample was taken. Check that the main components sum to about 100%. The supplier must size the machine on the measured mixture’s molecular weight and properties, not on pure methane or air.

Collection vacuum and suction pressure

Field operators often talk about suction as gauge vacuum, but calculations need absolute pressure: P(abs) = P(atm) + P(gauge). A suction of −0.10 bar(g) at a local atmospheric pressure of 1.00 bar(a) is 0.90 bar(a).

Too much suction can pull air in through cover cracks and leaks, raising oxygen and creating a safety and combustion problem. Too little leaves gas uncollected and can increase surface emissions. The collection engineer sets acceptable vacuum limits; the compressor supplier must show stable operation across that whole range. Trips must not defeat the site’s gas-management philosophy, for example by stopping extraction without a flare back-up.

Flow definition and the compression ratio

Keep actual suction flow (m³/h at measured inlet pressure and temperature) separate from normal or standard flow (Nm³/h at stated reference conditions). For a screening conversion at constant composition, Q(actual) = Q(normal) × (P(ref) ÷ P(suction)) × (T(suction) ÷ T(ref)), with absolute pressures and temperatures in kelvin.

Worked example

Take a hypothetical feed of 1,000 Nm³/h, dry basis, at reference conditions of 273.15 K and 1.01325 bar(a), with 52% methane, 43% carbon dioxide, 4% nitrogen and 1% oxygen. The methane share is 520 Nm³/h.

1. At 0.90 bar(a) and 30 °C (303.15 K) suction, the actual dry inlet flow is about 1,000 × (1.01325 ÷ 0.90) × (303.15 ÷ 273.15), roughly 1,250 m³/h. Water vapour in saturated gas adds to this.
2. With discharge at 2.50 bar(a), the pressure ratio is 2.50 ÷ 0.90, about 2.78.
3. If methane falls to 48% at the same total flow, the methane share drops to 480 Nm³/h, a reduction of 40 Nm³/h, or about 7.7%. The end-use owner must check fuel quality and engine response, and the supplier must re-check power and temperatures.

An ideal isentropic estimate of discharge temperature uses T2 ÷ T1 = r^((k−1)/k), where k is the mixture’s heat-capacity ratio. That is useful for screening only. Real discharge temperature depends on machine efficiency, cooling and the actual gas, and must come from the supplier.

Which machine suits landfill gas?

Whatever the type, the package needs gas-tight sealing, materials suited to wet acidic gas, protection against liquid, and electrical equipment to the site’s hazardous-area classification.

Liquids, contaminants and pretreatment

Landfill gas leaves the wellfield close to saturation, and cooling in pipework produces condensate that is often acidic and contaminated. Liquid slugs can damage compressors and blowers. Provide knockout vessels, drains with gas-tight seals, sloped pipework and level protection ahead of the machine.

• Cool and dry the gas where the end use or machine requires it.
• Remove hydrogen sulphide where corrosion or end-use limits demand it.
• Remove siloxanes ahead of engines and boilers that set limits for them.
• Route all condensate to approved collection and treatment, never to open drains.

Controls, protection and maintenance

The control system must balance gas production, collection vacuum and end-use demand. Typical measurements include suction pressure, discharge pressure and temperature, flow, oxygen and methane content, knockout level and vibration. Oxygen rise is a key alarm because it signals air ingress.

Maintenance focuses on corrosion, fouling and liquid damage: inspecting impellers, rotors and valves, checking seals, monitoring oil condition on oil-flooded machines, cleaning coolers and verifying drains. Keep a log of gas analyses alongside maintenance findings so trends can be linked to causes.

Plan for the flare as part of the compression design. When the gas engine trips or the treatment plant is offline, gas must still be extracted and burned safely, so the blower, controls and flare must work together during every changeover. Check what happens on power failure and restart, including how the collection vacuum is restored without drawing air into the wellfield. Where grid supply is unreliable, standby power for the extraction blower is often as important as for the engine itself.

How Reddy Compressors helps

We sell new and used or refurbished gas compressors and blowers, rent them, service them and supply their spares, including process-gas and booster compressors, Roots, screw, centrifugal and side-channel blowers, as an independent multi-brand specialist.

• We help you assemble a complete enquiry: composition ranges, vacuum limits, flow cases and end-use requirements.
• We compare blower and compressor options for your pressure and flow range.
• We service existing machines, inspect for corrosion and fouling, and supply spares identified from the nameplate and the old part.
• We supply compressed air for instruments and maintenance at landfill and waste-to-energy sites.

Our team of more than 20 service engineers and technicians, based at Tarnaka, Secunderabad, covers Telangana and Andhra Pradesh, and we supply equipment and spares across India. Gas-treatment design, hazardous-area classification and emissions approvals remain with the project’s specialists.

What to send us

• Recent gas analyses with dates, basis and wellfield state.
• Collection vacuum range and required discharge pressure.
• Flow range in Nm³/h, with reference conditions.
• End use: flare, engine, boiler or upgrading.
• Nameplate photos of existing blowers or compressors.
• Current problems: corrosion, trips, liquid carry-over or unstable vacuum.

Frequently asked questions

What type of compressor is used for landfill gas?

For extraction and low-pressure delivery to flares and gas engines, centrifugal, side-channel or rotary lobe blowers are common. Where higher pressure is needed, such as for upgrading or pipeline injection, oil-flooded screw or reciprocating compressors are used. The choice depends on flow, pressure ratio, gas composition, liquid load and the end-use requirements, and the package must be gas-tight and corrosion-resistant.

Why does landfill gas composition keep changing?

Gas generation depends on waste age and type, moisture, temperature, cover condition, rainfall and how the wellfield is balanced. Excess suction can also draw air in, raising oxygen and nitrogen. Because of this, a compressor should be specified for minimum, normal, maximum and upset compositions, not one average analysis, and trends should be logged continuously where possible.

How do I convert collection vacuum to absolute pressure?

Add the gauge reading to local atmospheric pressure. For example, −0.10 bar(g) at 1.00 bar(a) local atmosphere is 0.90 bar(a). Always use absolute pressure for compression ratio, flow conversion and temperature calculations. Note that local atmospheric pressure varies with altitude and weather, so record it with the reading where accuracy matters.

What damages landfill gas compressors most often?

Liquid carry-over, corrosion from hydrogen sulphide and acidic condensate, and fouling from siloxanes and particulates are the usual causes. Oil-flooded machines can also suffer oil degradation. Good knockout and drainage, suitable materials, appropriate pretreatment and regular inspection of rotors, valves, seals and coolers prevent most failures.

Why is oxygen in landfill gas a concern?

Rising oxygen usually means air is being drawn in through cover cracks, leaks or excessive suction. It changes combustion behaviour, can create a flammable mixture in the system and may affect treatment equipment. Oxygen should be monitored continuously, with alarms and responses agreed by the collection engineer and safety authority.

Can Reddy Compressors service my landfill gas blower?

Yes, within the maker’s limits and the site’s hazardous-area rules. We service and supply spares for blowers and gas compressors of many brands, inspect for corrosion and fouling, and identify parts from the nameplate and the old part before quoting. Send nameplate photos and a description of the problem to start.

Related pages

• Biogas compression and pretreatment
• Process gas compressor selection requirements
• Gas compressor interstage cooling and separation
• Hazardous-area compressor package enquiries
• Gas properties and compression behaviour
• Specialist compression and vacuum engineering centre