Renewable Natural Gas (RNG) & Biogas Upgrading Screener

Biogas Upgrading Technology Principles

Raw biogas produced via anaerobic digestion is composed primarily of methane ($50\text{--}65\%$) and carbon dioxide ($35\text{--}50\%$), saturated with moisture and contaminated with $H_2S$ and siloxanes. To achieve pipeline biomethane quality ($>97\%\ \text{CH}_4$), three major separation mechanisms are industrially deployed:

1. Polymeric Membrane Permeation (3-Stage Cascade)

Hollow-fiber polyimide membranes exploit the differing kinetic diameters and solubility of gas molecules. $CO_2$ ($3.30\ \text{Å}$) and $H_2O$ permeate through the polymer matrix up to $40\text{--}60\times$ faster than non-polar $CH_4$ ($3.80\ \text{Å}$). A modern 3-stage membrane cascade recycles the second-stage permeate back to the compressor suction, yielding recovery rates exceeding $99.2\%$ with methane slip below $0.8\%$.

2. Pressure Swing Adsorption (PSA/VPSA)

PSA utilizes Carbon Molecular Sieves (CMS) or Zeolite 13X in a multi-vessel cyclic sequence (adsorption at $6\text{--}8\ \text{barg}$, depressurization, vacuum desorption at $0.15\ \text{bara}$, and repressurization). Methane recovery is typically $96\text{--}98\%$. While robust, high valve-cycling frequency and desiccant attrition can incur higher maintenance OPEX.

3. Pressurized Water Scrubbing (PWS)

PWS relies on the physical solubility differential described by Henry's law: at $15^\circ\text{C}$, $CO_2$ is approximately $26\times$ more soluble in water than $CH_4$. Raw biogas is compressed to $8\text{--}10\ \text{barg}$ and contacted with counter-current water in a packed tower. The rich water is depressurized in a flash tank to recover dissolved methane, then regenerated in an air stripper. PWS avoids chemical solvents, but requires large civil footprints and substantial pumping power.