Why heavy industry is different

Heavy industry—cement, steel and refining—produces roughly a fifth of global CO2 emissions. Unlike a power station, much of that CO2 is not just from burning fuel. In cement, about 60% comes from the chemical reaction that turns limestone into clinker. In steel, coke acts as both fuel and reducing agent. In refining, hydrogen production and process heat dominate. You cannot simply switch fuel and call it done; you must capture the process CO2 too. Flue gases are often dilute—10–15% CO2 in cement kilns, 20–30% in steel off-gas—which makes capture harder than from a modern gas plant. Plants run for decades, so capture kit must be robust, modular and easy to retrofit.

Solvent systems: the workhorse of post-combustion capture

Chemical absorption with amine solvents is the most mature option. Flue gas passes through an absorber, where a solvent such as MEA or a newer hindered amine captures CO2. The rich solvent is heated in a stripper, releasing a pure CO2 stream for compression. Regeneration typically needs 2.5–4 gigajoules per tonne of CO2, usually as low-pressure steam, often from a dedicated boiler or the plant’s steam cycle. Pre-treatment is essential: sulphur oxides, nitrogen oxides and particulates degrade the solvent and cause corrosion. Advanced solvents, including water-lean and phase-change types, can cut regeneration energy to 2–2.5 GJ per tonne. But solvent make-up, degradation and waste handling remain practical headaches. For cement, kiln waste heat may not be enough, so you may need extra fuel—unless you electrify the reboiler with clean power.

Membrane separation: leaner, but stream-specific

Membranes use a pressure difference to separate CO2. Polymeric membranes work well when CO2 is already concentrated—for example, refinery hydrogen purification tail gas or some steel off-gas. They need no steam, no solvent make-up and have a small footprint, making them attractive for retrofit. The catch: for dilute streams like cement kiln flue, you need multiple stages and pre-compression. That can push electricity demand up to levels rivalling solvent systems. Facilitated transport membranes, using a chemical carrier for better selectivity, show promise but are still developing. In refining, membranes already recover hydrogen commercially, so adding CO2 capture is a natural extension. For steel, they can handle hydrogen-rich top gas after a shift reaction. Rule of thumb: the higher the CO2 concentration, the better membranes look.

What capture looks like in cement, steel and refining

• Cement: Post-combustion solvent capture on the kiln flue is the front-runner for retrofits. Oxyfuel firing raises CO2 to 80% or more, easing capture, but needs an air separation unit—a large energy load. Calcium looping uses lime-based sorbents at high temperature and integrates with the kiln’s raw meal, though it is less proven commercially.
• Steel: Blast furnace gas contains 20–30% CO2. After a water-gas shift to boost hydrogen, CO2 can be captured by solvents or membranes, and the hydrogen-rich gas becomes a cleaner fuel. Direct reduced iron (DRI) with natural gas gives a concentrated CO2 stream, easy to capture. Top gas recycling also yields a capture-ready stream while cutting coke demand.
• Refining: Steam methane reforming for hydrogen gives high-purity CO2—often above 90%—capturable with pressure swing adsorption or membranes at low cost. Fluid catalytic cracking flue gas is much more dilute and needs solvent capture. Many refineries already separate CO2 for food-grade use; the shift is to scale that up for geological storage.

The energy penalty—and how to shrink it

Every capture system consumes energy. For solvent capture, the parasitic load can be 15–30% of a plant’s output. On a one-million-tonne-per-year cement kiln, that might mean an extra 20–30 megawatts for regeneration, compression and pumps. If that energy comes from fossil fuels, you can end up capturing CO2 while generating more upstream. The fix is to integrate capture with clean heat and power: waste heat recovery, heat pumps, mechanical vapour recompression, and renewable electricity for compressors. Membranes avoid steam but shift the burden to electricity, so their climate benefit depends on grid carbon intensity. Advanced solvents can bring the penalty down, but there is no free lunch. Successful projects treat capture as part of a site-wide energy strategy, not a bolt-on.

Practical realities: purity, transport and engineering

Captured CO2 is typically 95–99% pure. It must be dehydrated and compressed to around 100–150 bar for transport by pipeline or ship. Impurities such as oxygen, nitrogen, sulphur oxides and water can corrode pipelines or affect storage, so design the capture unit with the whole chain in mind. UK industrial clusters are developing shared pipelines and storage in saline aquifers or depleted gas fields. Costs are around £50–100 per tonne of CO2 captured, plus £10–20 for transport and storage. The energy penalty remains the biggest technical lever on cost. When planning a project, start with a detailed flue gas characterisation. Match technology to stream: solvents for dilute gases, membranes for concentrated ones. Plan for solvent management or membrane replacement. And engage your heat integration engineer early. Carbon capture is not one silver bullet—it is a toolbox, and heavy industry can use it well with careful, site-specific design.