How Green Can Steel Get?

March 25, 2023 § 1 Comment

Steel is considered a “hard to abate” commodity because the production process uses a lot of fossil fuel and alternative processing methods are not readily available. The first step in the production is reducing iron ore (an oxide) to metallic iron. This is a continuous process performed in a vertical shaft furnace known as a blast furnace and the reducing agent is a form of processed coal known as coke. This is the primary culprit responsible for the high carbon footprint of steel, which is estimated to be about 2.2 tonne CO2e per tonne steel. By comparison, that other hard to abate structural commodity, cement, has a footprint of about 1 tonne CO2e per tonne cement.

The molten iron containing a few percent carbon is transferred from the blast furnace directly to a Basic Oxygen Furnace, where much of the carbon is oxidized to produce steel, which requires the carbon to be a fraction of a percent. This is known as primary steel. Steel produced from remelting scrap iron and steel is known as secondary steel, and has a very small carbon footprint, but is in relatively short supply.

A recent report from the Rocky Mountain Institute (RMI) provides a review of alternate ironmaking with lower carbon footprint. Their figure is reproduced below.

Courtesy RMI

They highlight the Direct Reduction Iron (DRI) process as the primary means to greener steel. This process has a vertical shaft variant which uses synthesis gas (syngas), a mixture of CO and H2, as the reducing agent, instead of coke. The operating temperatures are also less than half that in blast furnaces. The result is a reduction of associated carbon to 0.8 tonne CO2e per tonne steel, after the iron is converted to steel in an electric arc furnace (labeled EAF in the figure).

The report advocates a recent variant comprising substituting H2 for the syngas, piloted by a Swedish entity Hybrit. This is straightforward because syngas can be reacted with water in what is known as the Water Gas Shift reaction to produce H2 and CO2, which, if sequestered, makes the hydrogen carbon free (although saddled with the color blue, rather than green). Alternatively, green hydrogen could be produced from electrolyzing water with carbon-free electricity. The report advocates this approach, and further estimates that if green electricity is used in the EAF as well, the carbon emissions associated with a tonne of steel drop to 0.1 tonne (see figure).

So, there you appear to have it. Switch to the DRI/EAF process and use green electricity for the EAF and to produce hydrogen as the reducing agent. The RMI report notes that the steel industry has vertically integrated to ensure supply of relatively scarce coking coal. It advocates that the new process do the same with respect to green electricity supply. This may well be necessary because grids will not be carbon-free for a long time (see  Captive supply will also have a lot of competition. But it could be done, certainly over time.

But there is a fly in that ointment. This is the fact that the DRI process can only use high grade iron ore, with over 64% iron, preferably over 67%. Those who don’t care why should skip the rest of this paragraph. In a blast furnace the mineral impurities such as silica and alumina are removed by combining with oxides of Ca and Mg to form a molten phase known as slag. This floats on top of the molten iron and both are removed continuously. In the DRI process the temperatures are too low for slag formation. Consequently, very small proportions of mineral impurities are tolerated. These small amounts are slagged in the EAF. Low impurities equate to high grade iron ore. Hence the requirement for the ore to be high grade.

Such high-grade ore is in very short supply. Most of the known reserves are in Brazil and Australia. The DRI process has been commercial for decades, but only about 7% of the steel supply comes from this source. The shortage of supply (and of world reserves, for that matter) and the higher cost of the high-grade ore are contributory factors.

Before we get into my opinion on the way forward, two other avenues to green(er) steel bear mention. A story in The Economist describes a way to clean up the blast furnace process. The CO2 emitted is broken down into CO and oxygen using perovskites (essentially known science). The CO is used as the reducing agent in the furnace (instead of coke) and the oxygen is used in the steelmaking. There are practical issues in replacing the structural aspects of the coke. But the allure is that it modifies existing capital equipment. A complete departure from the blast furnace is electrolytic steel. The clever bit in a recent embodiment is their inert anodes. But the electricity must be carbon-free for the steel to qualify as green. And the process uses a lot of it: 4 MWh per tonne of steel. Scaling to anywhere close to the world usage of 2 billion tonnes per year means the need for a high fraction of all the power produced, leave alone the clean power. And as we noted earlier, carbon-free grids are not in the immediate future.

Where does that leave us? My favorite is the DRI/EAF with hydrogen, especially if we are not too choosy on the color of hydrogen; blue will do till green is feasible at scale. It is a tweak to an accepted process, essentially the same work force, so more easily acceptable. And that can be important for a staid industry such as iron and steel. The high-grade ore is the main hurdle to scale. Magnetite is the highest-grade variety, and it could be actively prospected. There will not be enough. We need another source.

One such is ultramafic rocks such as olivine, which are some of the most abundant minerals on earth and close to the surface. These are mixed silicates of iron and magnesium (in the main). Early-stage research offers the promise of extracting the Fe portion, and as luck (and thermodynamics) would have it, the Fe will be in a valence state making the oxide magnetite.

The CO2 in blast furnace emissions can be captured and stored for under USD 50 per tonne of CO2 with technology available today. This is well below the carbon penalty in Europe today. Partial use of hydrogen as a coke substitute would be minimally intrusive.

The two approaches above could handle the bulk of the decarbonization. They could be supplemented by electrolytic steel where captive carbon-free electricity could be arranged.

And don’t forget that Kermit the Frog said*, “It’s not easy being green”.

Vikram Rao

March 25, 2023

*Bein’ Green, Song by Kermit the Frog (Jim Henson), 1970, written by Joe Raposo


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