Karim Allana Discusses Decarbonizing Steel and Cement: Possible Solutions for a More Sustainable Built Environment

The world we take for granted would quite literally not exist without steel and cement. Each year, thousands of furnaces around the world churn out billions of tons of these materials for use in buildings, roads, bridges, and other pieces of infrastructure.

Unfortunately, the production of these absolutely critical materials comes at a great environmental cost. The steel and cement industries collectively account for more than 15% of global greenhouse gas emissions, contributing significantly to the worsening problem of climate change.

Reducing the planet-warming potential of steel and cement is a top priority for governments around the world, but the path to net-zero won’t be easy for either industry.

Why Are Steel and Cement So Carbon-Intensive?

Steel and cement are known as “hard to decarbonize” industries. This is because their production — for now, at least — releases greenhouse gasses not only through the combustion of coal or natural gas, but through the more fundamental chemical processes that occur during the transformation of limestone into cement and iron ore into pig iron, which is then further processed into steel.

Both processes, known respectively as “clinkering” and “reduction,” release or generate carbon dioxide separately from the combustion process itself. This is important because even if we were able to eliminate or capture the carbon emissions from combustion, we’d be left with unacceptably high “process emissions.”

Fortunately, dozens of teams — including some promising startup companies that have attracted substantial amounts of outside funding — are working on one challenge or the other.

Pathways to Lower-Carbon Cement

Pathways to fully-decarbonized cement require both a lower-emissions combustion process or the elimination of the combustion step altogether, as well as a zero-emissions alternative to current clinkering processes.

• Fuel substitution. Currently, the intense heat required for clinkering is overwhelmingly produced by burning fossil fuels, mainly natural gas. Substituting clean-burning hydrogen for natural gas can reduce combustion-related emissions by up to 35%, according to a recent study.
• Carbon capture. Carbon capture and storage, or CCS, can further reduce emissions from cement production. From a technical standpoint, well-designed systems may be capable of nearly eliminating emissions from both combustion and clinkering. However, the cement industry is highly competitive, and it’s not yet clear that CCS will ever be cost-competitive with “unabated” production. Additionally, CCS has yet to be demonstrated in commercial cement production facilities.
• Alternative chemical processes. The “holy grail” of cement decarbonization is alternative chemistry — that is, clinkering (or something like it) without the carbon emissions. Though no option has yet to be proven at commercial scale, companies like Sublime Systems have demonstrated electrochemical cement production with smaller amounts of raw material.

Pathways to Lower-Carbon Steel

Today, converting iron ore to high-grade “reduced iron” occurs in coal-fired blast furnaces and utilizes high-quality metallurgical coal as a chemical input, further increasing carbon emissions. But these alternatives have the potential to reduce or completely eliminate the industry’s global warming impact.

• More recycling. Absent any concerted action, steel production’s absolute carbon emissions will probably decline in the long term as less primary steel is needed for urban development. Eventually, most raw steel will be recycled in electric arc furnaces, which can be powered with renewable energy rather than fossil fuels.
• Molten oxide electrolysis. This process, which is being pursued by Boston Metals and other startups, uses powerful electric currents to purify iron ore without releasing significant amounts of carbon dioxide. The resulting pig iron is then transformed to steel in electric arc furnaces.
• Hydrogen direct reduction. This process uses hydrogen rather than coal to reduce iron. If the hydrogen is produced using green electricity, the resulting carbon emissions are minimal. Like pig iron produced through molten oxide electrolysis, the result can be “finished” in a zero-emissions electric arc furnace.

Let’s be clear that decarbonizing steel and cement won’t be fast or easy. However, we have the technology to do it; now, we only need the will.