Johnson County, Kansas, USA
Nitrogen Rejection for Landfill Gas to RNG
Project details
Technology: Nitrogen rejection PSA
Commissioned: 2019
Biogas Source: Landfill
End Use: Pipeline injection RNG
Specifications
Capacity: 4,823 Nm³/h / 3,000 SCFM
Inlet Gas Composition: 87.7% CH₄, 5% CO₂,
7% N₂, 0.3% O₂, 100%RH
Design Specifications RNG / Biomethane: 97% CH₄, 0.06% CO₂,
2.9% N₂, 0.04% O₂
Power Consumption of Biogas Upgrading Plant:
0.42–0.48 kW/scfm (0.26–0.30 kWh/Nm³) raw biogas
At the Johnson County Landfill near Kansas City, an IVYS nitrogen-rejection PSA was retrofitted to an existing landfill-gas-to-RNG plant. It reduced N2 from 7% to 2.9% and increased product gas CH4 from 87.7% to 97%, making it suitable for pipeline injection.
The Challenge: Too Much Nitrogen for the Pipeline
Since 2008, Aria Energy’s Johnson County plant had converted landfill gas into RNG, producing about 3,480 MMBtu/day. Its existing process removed CO2, but air entering the landfill’s collection wells left about 7% N2 in the treated gas. At 87.7% CH4, the gas fell short of pipeline heating-value and inert-gas limits. Aria needed a polishing step that would:
- Remove nitrogen, not just CO2: N2 and CH4 molecules are similar in size, so standard CO2-removal PSA cannot separate them.
- Fit an operating plant: connect downstream of the existing CO2 removal system without rebuilding the front end.
- Keep methane losses low: at 3,000 SCFM, every percentage point of recovery represents significant revenue.
The Solution: A Nitrogen Rejection Unit Built Around Rotary Valves
IVYS (formerly Xebec Adsorption) designed a nitrogen rejection unit (NRU) for installation downstream of the existing CO2 removal step:
- Feed: Treated gas exits the existing system.
- PSA: Three synchronized G4 rotary valves switch between nine identical adsorbent beds to separate N2 and O2 from CH4.
- Product: RNG, ready for pipeline delivery.
- Exhaust: Vacuum pumps draw nitrogen-rich tail gas through a surge tank.
Performance: From 7% to 2.9% Nitrogen & Why It Matters for Landfill Operators
As landfills age, air intrusion increases nitrogen levels, while many older RNG plants were designed only to remove CO2. Johnson County shows how adding a PSA can restore pipeline specs without replacing the plant.
- Retrofit, not rebuild: connects downstream of existing CO2 removal, with recycle routed back to the existing blower.
- One unit, three inerts: removes N2, O2, and residual CO2 in a single PSA.
- High recovery for an NRU: 93% expected.
- Low drive power: rotary valves replace dozens of switching valves.
- Built for long cycling life: vessels are designed and fatigue-analyzed for 4 million pressure cycles.
