Which Upgrading Method Gives 98% Methane Purity? Technology Options Compared
Water scrubbing, pressure swing adsorption (PSA), amine scrubbing, and cryogenic separation all reach 98% methane purity routinely. Single-stage membrane systems usually land at 95–98% and need a second stage or a polishing step to hold 98% with margin. The real decision is not whether 98% is achievable—it is how much methane slip and how much cost per Nm³ that 98% carries.
Which Technologies Reach 98% and How
Four commercial routes reach 98% CH₄ reliably. Water scrubbing dissolves CO₂ in water at 8–12 bar. PSA adsorbs CO₂ on a molecular sieve at 4–10 bar. Amine scrubbing binds CO₂ chemically and regenerates with heat. Cryogenic separation liquefies CO₂ at about −160 °C. Single-stage membranes sit lower because CH₄ and CO₂ permeability overlap; a two-stage membrane or membrane-plus-PSA polish reaches 98–99%.
Comparative Data Table: Purity, Slip, and Scale by Technology
| Technology | Typical CH₄ purity | Methane slip | Best scale (Nm³/h raw) |
| Water scrubber | 96–99% | 0.5–2% | 500–5,000 |
| PSA | 96–99% | 1–3% | 200–2,000 |
| Membrane, single stage | 95–98% | 0.5–3% | 50–1,000 |
| Membrane, two stage | 97–99% | 0.3–1% | 200–3,000 |
| Amine scrubber | 97–99% | <0.5% | 1,000–10,000 |
| Cryogenic | 97–99.9% | <1% | 1,000–10,000 |
What 98% Costs You in Slip and Energy
Purity and recovery trade off directly. Pushing a single-stage membrane from 96% to 98% typically lifts methane slip from under 1% to 2–4%, losing saleable gas to the off-gas stream. Energy use also diverges: water scrubbing needs 0.20–0.40 kWh per Nm³ of raw biogas plus water treatment, PSA needs 0.15–0.30 kWh/Nm³, and cryogenic needs 0.40–0.70 kWh/Nm³ but yields liquid CO₂ as a marketable by-product.
Frequently Asked Questions (FAQ)
Q1: Which upgrading method gives 98% methane purity?
A: Water scrubbing, PSA, amine scrubbing, and cryogenic separation all deliver 98% CH₄ in normal operation. Membrane systems reach 98% only with a second stage or a polishing unit; a single stage typically delivers 95–98% with higher slip.
Q2: Can membrane upgrading reach 98% methane?
A: Yes, but not economically in one pass. A two-stage membrane configuration reaches 97–99% with slip held to 0.3–1%. A single stage can hit 98% at the top of its range only by accepting 2–4% methane loss.
Q3: What purity do I need for grid injection?
A: Most European and North American grid specs require 96–98% CH₄ plus limits on CO₂ (2–6%), O₂ (below 0.5–1%), H₂S (below 5 mg/Nm³), and a defined Wobbe index window. Targeting 98% gives margin against feed variation.
Q4: Does higher purity mean more methane loss?
A: Usually yes. Every technology sacrifices some CH₄ to the off-gas to strip CO₂ deeper. Best-in-class plants hold slip at 0.1–0.5% while still delivering 97–99% purity; poorly tuned units selling 98% purity can lose 3% or more.