Carbon footprint comparison – duplex stainless steels vs competing materials

By Claes Tigerstrand, Outokumpu

Renewable energy production, from the refining of battery metals to biofuel production, hydrogen production and storage, carbon capture, plastic recycling, etc, calls for new material solutions. This transition brings demanding process environments which often operate at high (or low) temperatures and pressures, making the material selection more complicated. Hence, stronger, more corrosion resistant materials with a lower carbon footprint are now in demand.

The challenge is how an engineering company or a materials engineer working on a large investment project can compare the carbon footprint of various material solutions and draw unbiased conclusions about their performance.

The author of this article has extensive experience in stainless steel and represents a stainless steel manufacturer with a strong green profile. Lots of improvements are being made to reduce emissions in stainless manufacturing, but how do these compare to other materials? I have less knowledge of the competing materials, but can I identify similar data for them to enable a comparison? Let’s find out.

Let’s start with the toolbox. A life cycle assessment (LCA) study can be used for this task. This method quantifies various types of emissions, where the global warming potential (GWP) – carbon dioxide equivalents (CO2e) is the most sought. See Figure 1 for a complete assessment spanning the entire product life cycle (cradle to grave) from the extraction of resources through production, use, disposal, and potential recycling. The LCA methodology is standardised, mainly through ISO 14040 and ISO 14044.

Life cycle assessment

There are four main steps to follow in an LCA:

1. Define scope and boundary conditions,
2. Emission data collection (Life cycle inventory),
3. Emission calculation for the different stages of the product life cycle,
4. Validation of results.

Collecting emission data is not easy. Where can you find carbon emission data for materials? How reliable is the data? Can data be compared? Are they derived using the same systematics? How do we ensure that we are comparing ‘apples to apples’ and not ‘apples to oranges’? Extensive knowledge and experience are required in both production processes and material properties, as well as environmental assessments.

‘Trustworthy’ carbon footprint data can be found from three primary sources:

1. Environmental Product Declarations (EPD),
2. Product Carbon Footprints (PCF), and
3. Verified databases.

Third-party verified databases are trusted sources used for all types of LCA calculations, including EPDs and PCFs.

Environmental Product Declarations (EPD)

The building and construction sector is at the forefront of developing and putting EPDs into practice. An EPD is a third-party verified report according to ISO 14025, which declares various emission data. The methodology for the calculations is based on predefined rules for a certain product type (PCR) using LCA systematic according to either EN 15804 or ISO 21930. Typically, the boundary condition for assessing a material is limited to a so-called cradle-to-gate scenario, meaning emissions from raw material supply, transport and manufacturing, and potentially information about waste processing and recyclability are included (Figure 1). However, the construction and use phases are normally excluded from the LCA scope.

Figure 1. The main stages of a product life cycle.

Product Carbon Footprints (PCF, ISO 14067)

Product Carbon Footprint (PCF), CO2e, is estimated based on partial LCA similar to EPD but using slightly different rules, covered by ISO 14067. A third party can also verify a PCF according to ISO 14064. The methodology behind a PCF encourages the use of primary emission data directly provided by the raw material suppliers. While EPD only allows industry average emission data for raw materials taken from verified databases (so-called secondary data). The objective of the stricter rules behind an EPD is to ensure comparability between different EPDs.

On the other hand, when a company is at the forefront of implementing decarbonisation measures, e.g., using selected suppliers, a PCF calculated according to ISO 14067 is often less conservative than an EPD. Moreover, under such changing conditions, an EPD report is soon out of date unless it is verified and republished.

Can a PCF value be compared with a CO2e value from an EPD? Basically not. They are calculated based on different methodologies and rules. Nevertheless, I argue differently. For stainless steels, we prefer to use PCF as they more accurately describe our products.

The PCF represents a specific product rather than a group of products, which often is the case of the EPD for practical reasons. The calculation is based on a 12-month rolling average. The latest third-party validated PCF can, in this case, be obtained from the supplier. The PDF is communicated on the material test report (certificate) along with all other material-specific test data.

The EPD is a published report valid for up to 5 years and is either obtained directly from the supplier or databases at official EPD program operators.

Figure 2. Carbon footprint (CO2e) comparison of various materials based on both PCF (ISO 14067) and EPD.

Carbon footprint comparisons

Stainless steels (from PCFs) – In Figure 2, PCF values are shown for type 316L, duplexes, high-alloy austenitics and nickel alloy 825. All grades are produced by electric arc furnace (EAF) and delivered in hot rolled condition from the same mill¹. Typically, PCF increases with increasing alloying content. The PCFs for duplex grades are generally slightly higher than type 316L but lower than high-alloy austenitics and nickel alloy 825.

Thus, the more advanced grades require a higher share of virgin raw materials, e.g., chromium, nickel and molybdenum. The emissions generated by recycled raw materials are limited to scrap handling and transport, while virgin raw materials have the greatest impact on the carbon footprint. Therefore, it is crucial to choose the right raw material suppliers to minimise the PCF.

The target for the ‘low-CO₂’ versions of both 316L and LDX 2101 depicted in green in Figure 2 is to reduce the emission level by 50% compared to the regular versions, to below 1 kg CO₂e per kg. This reduction is obtained by maximising the recycled content, using biofuel, focusing on the best possible heat and production efficiency through the mill, and minimising all transports needed for the manufacturing.

Stainless Steels (from EPDs) – Figure 2 shows the total CO₂e values declared in recently published EPDs from us and other stainless steel suppliers.

The complete list of references is at the end of the article, listed in the order of appearance left to right from ‘lowest’ green, ‘low’ blue to ‘high’ grey emission levels in Figure 2. Note that the CO₂e value from the EPD for type 316 conforms well with the PCF, 1.5 vs 1.3 kg CO₂e kg, mainly due to low additions of virgin metals², and the other manufacturers are slightly higher^{3, 4}. The duplexes from the EPD (average of many grades) have significantly higher CO₂e values than the PCFs, as indicated in green ‘lowest’ value⁵. In addition, one manufacturer from East Asia representing an undefined mix of austenitic and duplex grades is depicted in blue⁶, and one European manufacturer in grey⁷. Note, none of the EPDs represent the same duplex grade mix. How do stainless steel CO₂ values compare to other materials? Let’s start with carbon steels.
Carbon steels (from EPDs) – There is a clear market trend towards more high scrap (near 100%) electric arc furnace (EAF) produced carbon steel. A recently published EPD was declared to be as low as 0.5 kg CO₂e per kg of steel⁸ (Figure 2). Otherwise, the best-in-class currently hovers around 0.8 kg CO₂e per kg from steel production in either North America or Europe^{9, 10}. An average value of 1.7 kg CO₂e kg representing several steel mills in the USA is provided by AISC¹¹. For blast furnace steel production (BOF) out of iron ore, the declared CO₂e emission in identified EPDs are about 2.7 – 3.0 kg CO₂ per kg for hot rolled plates^{12, 13}.

Aluminum, nickel and titanium alloys (from EPDs) – Many EPDs are available for aluminum alloys, mainly from Europe, North America, and a few from Asia. A selection is presented in Figure 2^{14, 15, 16} . The declared CO₂e values vary greatly depending on how much scrap is used for their production (near 100%, 80% and unknown, respectively).

Nickel alloys require more virgin alloying elements to obtain the right composition and quality so the carbon footprint soars with increasing nickel content. Consequently nickel suppliers with the strongest focus on reducing their emissions should be awarded. Only a couple of EPDs have so far been identified for nickel alloys¹⁷. It is the same situation for titanium alloys, with few EPDs found. Titanium production relies largely on the energy-intensive sponge process . The available EPD is from Japan and the declared CO₂e value is significantly lower for the ‘green’ product where more than 50% scrap is used¹⁸ compared to an unknown scrap ratio in the regular version from the same supplier¹⁹.

Fiber reinforced plastics (CO₂ from partial LCA) – Finally, (glass) fiber reinforced plastics (FRPs) are growing as an alternative material for chemical industry process equipment. EPDs for FRPs as such has not been found. However, by using a trusted database, the carbon footprint of its constituents (60% E-glass, vinyl-ester resin) can be found and based on that, a ballpark estimate can be calculated20. However, in Figure 2, a calculated value published by a tank manufacturer is given which roughly coincides with the database value of its constituents21. Nevertheless, the most obvious difference between the metals and the FRP is that the end-of-life processing of the wastes is not considered. It is well-known fact that waste generation and lack of recycling possibilities is a burden for FRP products.

Concluding remarks

An interesting observation from this study is that stainless steels, in general, perform very well. The PCFs of regular type 316, lean duplex 2101 and duplex 2205 are on a similar level to carbon steel, and the low-CO₂ versions are also in the same range.

Another important discussion point related to the emission levels presented in Figure 2 is that the value proposition for each material varies greatly. High-alloy austenitic stainless steels, nickel and titanium alloys are only considered for critical applications when the environment demands them, while the other materials are more commonly used engineering materials.

I would also argue that comparing carbon steel to stainless steel is very much like comparing apples to oranges. It would make more sense to compare similar functional properties. Carbon steel most often requires some form of corrosion protection, such as coatings or linings. A complete LCA is required to consider the impact of the surface protection.

The same applies to structural efficiency, which is strongly influenced by which material is chosen. Design strength, modulus of elasticity, and density are important parameters to consider.

Super duplex 2507 is a good example, benefitting from both high strength and high corrosion resistance along with a low CO₂e value compared to the other high performance materials.

What are the key insights from my study:

• Carbon footprint emission factors for different materials are obtainable, but are they directly comparable? An EPD should be comparable to another EPD if they are based on the same PCR. In fact, this is not always true as they mostly cover several products, and the declared emissions are based on industry averages rather than specific emissions. PCFs based on ISO 14067 are more specific but not yet generally available for engineering materials. Nevertheless, some representatives of the chemical industry have selected PCF for their products²².
• Extensive knowledge about raw material extraction, processing, and the properties of all material candidates is essential to carrying out a reliable LCA.
• It is important that the same functional properties are considered when comparing different materials.
• More guiding principles to support current standards and methodologies are needed to know better how to compare different materials and alloys for a specific application.
• With the above considerations in mind, the PCF emission levels for duplex stainless steels appear to be very attractive compared to other common materials used in the process industry sector. There is a clear positive trend towards net-zero emissions for all types of material solutions strongly driven by the increased use of recycled raw materials.

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