Amine scrubbing: the lowest methane losses, but heat is needed for regeneration

Amine scrubbing is chemical absorption: CO₂ from the biogas reacts in a column with an aqueous amine solution, and the solvent saturated with CO₂ is heated in a regeneration column, where it releases carbon dioxide. Because the reaction is highly selective towards CO₂, methane is practically not absorbed – losses are usually below approx. 0.1–0.2%. The price for this is paid in heat: regeneration requires a temperature of the order of 110–140°C.

The course of the process

1. Pretreatment – especially desulphurisation. Excess H₂S degrades the solvent or contaminates the CO₂ stream.
2. Absorption – biogas at a pressure close to atmospheric (or a slight overpressure) enters at the bottom of the column, and the amine solution flows down from the top. The CO₂ reacts with the amine, forming compounds (e.g. carbamates and bicarbonates).
3. Heat exchange – the rich solvent is heated in an exchanger by the heat of the solvent returning from the regenerator, which reduces the heat demand.
4. Regeneration – in the stripping column the solvent is heated with steam or hot water; the released CO₂ leaves the installation in a stream of high purity (>99% after drying).
5. Circulation – the cleaned solvent is cooled and returns to absorption.

The biomethane comes out at low pressure, so before injection into the grid it is additionally compressed and dried.

Energy: heat dominates

The electricity demand is small (fans, pumps, compression of the gas after the scrubber), whereas the heat demand is significant. The literature gives orders of magnitude of about 0.5–0.8 kWh of heat per m³ of biogas, depending on the solvent, the CO₂ concentration and the degree of heat recovery. The economic value depends on the local source: waste heat from a CHP engine, a biomass boiler or an industrial plant may be cheap, whereas heating with gas can be costly and worsens the emissions balance.

Pure CO₂

The off-gas from the regenerator is almost pure biogenic CO₂. After additional treatment (drying, removal of odours) it can be sold to industry (beverages, food, greenhouses, production of e-fuels) or liquefied. This is the particular advantage of amines over technologies in which the off-gas contains impurities and methane.

Disadvantages and risks

• Solvent degradation under the influence of oxygen, high temperature and contaminants; top-up and disposal of spent amines are necessary.
• Health and safety – amines can be irritating and require appropriate storage and handling procedures.
• Foam and corrosion – contaminants (oils, particles) cause foaming in the columns.
• Lower flexibility under variable load than in membranes; longer start-up time.
• Heat infrastructure – a regeneration column, exchangers, a heat source.

A calculation example

Assumptions: 1000 m³/h of biogas, 52% CH₄, heat demand of 0.6 kWh/m³ of biogas, 8000 h/year, waste heat at PLN 40/MWh, heat from a gas boiler at PLN 250/MWh (both prices assumed). Heat: 1000 × 0.6 × 8000 = 4800 MWh/year. Cost of waste heat: approx. PLN 192 thousand; from the gas boiler: approx. PLN 1.2 million. The gain from the lower methane loss (0.1% instead of 1%) at 520 m³/h of CH₄ is 520 × 0.009 × 8000 ≈ 37 thousand m³ CH₄ ≈ 374 MWh, i.e. at PLN 200/MWh approx. PLN 75 thousand per year. Conclusion: the methane saving rarely covers the cost of heat bought from fuel, but it starts to count with waste heat and valuable CO₂ sales.

When to consider amine scrubbing

• When cheap, low-emission heat is available.
• When the CO₂ is to be sold or utilised (CCU), so purity counts.
• When a very low methane loss is required, e.g. because of the GHG balance or environmental requirements.

Heat integration with the biogas plant

In a typical biogas plant with cogeneration, part of the heat is used to heat the digesters, and the surplus is often unused. After switching to biomethane the CHP engine usually stops running, so the existing heat source disappears. Therefore in the design the heat balance of the whole must be calculated: heat for digestion, heat for amine regeneration and possibly heat from a biogas or biomass boiler. If the heat for the scrubber comes from a boiler fired with raw biogas, methane that could have been sold is burned, which lowers the sales volume and affects the emissions balance. For this reason one of the key decisions is the choice of heat source: a biomass boiler, a heat pump or waste heat from a neighbouring plant.

Summary

Amine scrubbing is a technology for projects in which pure CO₂ and minimal methane loss provide value, and heat is available at a sensible price. In the cost balance it is thermal energy that counts, not just electricity.