Biogas Upgrading Unit

Overview

A biogas upgrading unit converts raw biogas, roughly 55% methane and 45% carbon dioxide, saturated with water and laced with hydrogen sulfide, into biomethane that is chemically interchangeable with natural gas. Upgrading is what lets a digester feed the gas grid or fuel CNG vehicles instead of running a local engine: the product leaves at ≥97% CH4, dried, sulfur-free, and odorized. Several separation technologies compete (water scrubbing, amine scrubbing, PSA), but membrane plants have taken most of the small and mid-scale market because they have no process water, no chemicals, fast start-up, and nearly everything fits in one 20 ft container on the Skid Frame.

Pretreatment

Membranes and compressor both demand clean, dry gas. Raw biogas arrives warm and saturated; the Gas Dehumidifier chills it to a 5 °C dew point in the Gas Cooler, with a Demister Pad and Condensate Traps removing the condensed water. The Gas Reheater then lifts the gas ~10 °C above dew point so nothing condenses downstream.

Sulfur and siloxanes are the membrane killers. Two Activated Carbon Filter vessels in lead/lag take H2S below 1 ppm and adsorb the siloxanes that would otherwise deposit as silica inside the fibers; running two beds means the lag bed catches breakthrough while the exhausted lead bed is swapped without stopping the plant. A Coalescing Filter catches carbon dust before the compressor.

Compression

Membrane separation is driven by partial-pressure difference across the fiber wall, so the feed must be compressed. The Screw Compressor is an oil-injected twin screw, the Screw Block's injected oil seals the rotor mesh, absorbs the heat of compression, and allows 1 to 16 bar in a single stage. The Oil Separator keeps carry-over below 5 mg/m³, the Aftercooler drops the ~85 °C discharge to near ambient, and a Recycle Valve matches throughput to whatever the digester is producing hour by hour. Compression is the plant's energy budget: 0.20–0.30 kWh per Nm³ of raw gas, essentially all of it here.

Membrane separation

Each Membrane Module packs a Hollow-Fiber Bundle of ~50,000 polyimide hollow fibers into a 20-bar shell. Gas flows along the fiber bores; CO2, a smaller and more soluble molecule in the polymer, permeates the wall 30–60 times faster than CH4. CO2 (with most of the water vapor and O2) emerges on the low-pressure shell side as permeate, while methane concentrates in the high-pressure retentate. The Potting Heads, epoxy tube sheets anchoring the fiber ends, are the only barrier between the two sides, which is why modules are pressure-tested individually.

A single pass cannot give both high purity and high recovery, so the plant runs three stages. Stage 1 makes the rough cut; its retentate passes to stage 2, which polishes the product to ≥97% CH4. Stage 1 permeate still carries worthwhile methane, so stage 3 reprocesses it; the Permeate Recycle Line returns stage 2 and 3 permeates to compressor suction. The arrangement pushes overall methane recovery above 99% at the cost of recompressing the recycle. The Interstage Control Valves set each stage cut, and the Feed Trim Heater holds the feed at 25–40 °C where polyimide's selectivity-permeance balance is best.

Analysis, product, and offgas

Quality gating is continuous. The Sample Conditioning System switches feed, product, and offgas streams through the NDIR Analyzer (CH4/CO2), Oxygen Cell, H2S Sensor, and Dew-Point Sensor, with automatic weekly zero and span checks against certified calibration gas. If the product drifts off-spec, the Quality Reject Valve loops it back to feed within seconds, the grid never sees a bad molecule. On-spec gas is measured by the custody-transfer Product Gas Meter, odorized with THT by the Odorant Doser, and enters the grid through the Grid Check Valve.

The permeate is not vented. Its ~0.5% residual methane is 28 times worse than CO2 as a greenhouse gas, so the Offgas Blower feeds it to a Regenerative Thermal Oxidizer, whose ceramic beds oxidize the slip autothermally behind a Flame Arrester.

Operation

The plant runs unattended. The PLC in Control System sequences start-up, tracks digester output through the recycle valve, executes the quality logic, and reports through its Compute SoC Module telemetry link. Safety is layered in the skid itself: Safety Relief Valves on every pressure section, Gas Detectors tripping the Ventilation Fan at 10% LEL and a full shutdown at 20%, and all relief routed to the Emergency Vent Stack. Membrane life is 8–10 years if pretreatment holds; a poisoned carbon bed shows up within weeks as rising stage-1 permeate methane, which is the number operators watch first.