Biogas upgrading: membrane, PSA, water scrubber and amines — a comparison

The four most common technologies for upgrading biogas to biomethane are membranes, pressure swing adsorption (PSA), the water scrubber and the amine scrubber. All of them separate CO₂ from methane, but they differ in mechanism, type of energy consumed, methane losses and the kind of CO₂ to expect at the outlet. The comparison below is qualitative: specific parameters are given by offers and tests for a given system.

Working principles

Membranes. Modules with selective fibres or sheets, through which CO₂ (as well as water vapour and some other gases) permeates faster than CH₄. Separation takes place under pressure, usually in several stages with recirculation of the stream. Described in the text Membrane upgrading.

PSA (pressure swing adsorption). A bed of adsorbent (e.g. activated carbon or molecular sieves) selectively retains CO₂ under pressure, and regeneration takes place by lowering the pressure. Cyclic operation in several columns. Description: PSA in upgrading.

Water scrubber. Biogas under pressure contacts water in a column; CO₂ dissolves in water far better than CH₄. The water is regenerated by lowering the pressure and stripping with air. Description: Water scrubbing.

Amine scrubber. CO₂ binds chemically with an amine solution in an absorber. The solvent is regenerated by heating in a desorber, which requires heat, and the CO₂ comes out relatively pure. Description: Amine scrubbing.

Comparison

Methane losses are a key parameter: besides the loss of feedstock, they mean emission of a greenhouse gas or the need for afterburning. The scale of the phenomenon is described in the text Methane slip in upgrading.

Energy

Pressure-based technologies (membranes, PSA, water scrubber) mainly consume electricity for compression, and their consumption depends on the configuration. Amines require heat, so they pay off most when cheap low-temperature heat is available, e.g. waste heat. The energy consumption balance is discussed in the text Energy consumption in upgrading; electrical and thermal energy consumption should be compared together, taking into account the value of each.

Selection criteria

1. Heat availability. Waste heat favours amines; its absence increases the importance of electrical technologies.
2. Scale. Modular membranes scale down well; scrubbers and amine plants gain at larger flows.
3. Requirements for the CO₂. If the CO₂ is to be used (dry ice, food-grade, greenhouses), its purity and quantity count, not only the quality of the biomethane; see Biogenic CO₂ from biomethane plants.
4. Methane losses and afterburning. Environmental requirements and the economics of losses.
5. Local conditions. Access to water, space, noise, pressure in the target grid.
6. Requirements for the pre-cleaned gas. See Pre-cleaning of biogas.
7. Guarantees and servicing. Guaranteed losses, energy consumption, availability and access to service.

A comprehensive way of comparing is proposed in the text How to choose an upgrading technology: a decision matrix, and an overview of five methods can be found in the article Upgrading technologies: a comparison of five methods.

What to avoid

• Comparing offers without harmonised assumptions (gas composition, pressure, product quality, losses).
• Omitting the cost of heat or its alternative value.
• Assessing only on capital expenditure, without operating costs, material replacement and methane losses.
• Assuming that all technologies give the same CO₂ at the outlet.

How to read offers

Each bidder states methane losses, energy consumption and product quality under specific assumptions (composition and pressure of the raw gas, temperature, required product composition). For the comparison to make sense, offers must be set side by side under the same assumptions, and it must be checked which parameters are guaranteed and which only predicted. Control questions: what does the scope of supply include (pre-cleaning, compressors, cooling, off-gas afterburner)? What are the conditions of the methane loss guarantee? What happens when the load falls? What is the scope of servicing and the availability of parts? What are the costs of replacing consumable elements (membranes, adsorbent, solvent)?

Practical conclusions

• With available, inexpensive waste heat, amine technology can be advantageous, especially when low methane losses or pure CO₂ are important.
• At smaller scales and where simple operation is needed, membranes are often considered.
• The water scrubber and PSA have a long history of use and well-understood operating conditions.
• The final choice is best preceded by a total life-cycle cost analysis covering energy, servicing, methane losses and the value of the CO₂.