Energy consumption in biogas upgrading: the kWh per m³ balance and share of the gas energy

Biogas upgrading usually uses of the order of 0.2–0.35 kWh of electricity per m³ of raw biogas in pressure technologies (membranes, PSA, water scrubbing), and amine scrubbing additionally needs heat of the order of 0.5–0.8 kWh/m³. Since 1 m³ of biogas with 55% CH₄ carries approx. 5.5 kWh of energy, electricity consumption is a few percent of the gas energy, and the heat in amine scrubbing – over a dozen percent. The values are indicative; every supplier gives its own guarantees.

Two ways of counting

Energy consumption indicators can be related to:

• m³ of raw biogas – this is how technology suppliers most often give them;
• m³ of biomethane – conversion: we divide the consumption by the share of biomethane produced in the biogas stream (approx. 0.5–0.55 m³ of biomethane from 1 m³ of biogas at 55% CH₄, because biomethane contains approx. 98% CH₄).

Example: 0.25 kWh/m³ of biogas corresponds to approx. 0.46 kWh/m³ of biomethane, and so about 4.6% of the energy contained in the biomethane (approx. 10 kWh/m³). The assessment "consumption of 0.25 kWh/m³" without stating the reference is therefore sometimes misleading.

Components of the energy balance

1. Pretreatment – desulphurisation, drying, filters; often omitted from offers, yet it consumes electricity (coolers, blowers) and possibly heat (regeneration of carbon).
2. CO₂ separation – compressors, pumps, vacuum, regeneration heat.
3. Compression of the biomethane – for a medium-pressure grid a few to a dozen or so bar is usually needed; this requires an additional compressor if the upgrading system delivers gas at low pressure (amine scrubbing).
4. Off-gas oxidiser – supplementary fuel or electricity.
5. Auxiliary – cooling, space heating, automation, lighting, the injection station.

Example: a balance for 1000 m³/h of biogas

Assumptions: biogas 55% CH₄, energy 5.5 kWh/m³ (5500 kW), 8000 h/year, slip 0.5%.

The figures in the table are assumed for the exercise, not measurement results. They do, however, show the point of the comparison: the amine variant uses less electricity, but needs heat, whose cost and emissions must be assessed.

Energy cost

At an assumed electricity price of PLN 600/MWh the membrane variant uses 300 kW × 8000 h = 2400 MWh, which costs approx. PLN 1.44 million per year. The amine variant: 180 kW × 8000 = 1440 MWh of electricity, i.e. approx. PLN 0.86 million, plus heat 600 kW × 8000 = 4800 MWh. If the heat costs PLN 40/MWh (waste heat), the additional cost is approx. PLN 0.19 million; if PLN 250/MWh (from a gas boiler) – PLN 1.2 million, i.e. more than the saving on electricity. The result obviously depends on utility prices, which in every project must be adopted locally.

Impact on emissions

The consumption of electricity and heat increases the carbon footprint of the biomethane if the energy comes from a grid with a high share of fossil fuels. In the RED methodology, emissions from electricity consumption in the upgrading process enter the balance. Supply from own renewable sources (e.g. photovoltaics, biogas cogeneration) lowers this component.

What to measure

• Electricity consumption of the whole installation (a dedicated meter), not just the separation module.
• Consumption of heat and auxiliary fuels.
• Flow of biogas and biomethane and the composition (a chromatograph or analyser).
• Indicator: kWh of auxiliary energy / MWh of biomethane energy.

How to reduce energy consumption

The simplest measures are heat recovery from compression (for heating the digesters or for regeneration), variable-speed drives in compressors and pumps, which better match power draw to flow, and avoiding excessive recirculation. Supplying the installation with energy from own renewable sources is also important. It is worth monitoring the indicator of energy consumption per m³ of biomethane over time: its rise is sometimes the first signal of wear of the membranes or sorbent, before the quality parameters of the gas deteriorate.

Summary

Energy intensity alone does not decide the choice of technology, but it is one of three pillars alongside methane losses and CAPEX. When comparing offers, demand consistent balance boundaries and reference to a single unit, preferably m³ of biomethane or MWh of energy sold.