Addendum: Carbon-negative production of acetone and isopropanol by gas fermentation at industrial pilot scale

Proper LCA methodology requires a system boundary that accurately compares the new pathways to the conventional routes. LCA can be calculated on either a cradle-to-gate or a cradle-to-grave basis. Cradle-to-grave analyses make modeling assumptions on the end-of-life treatment and use of products. In cases where end-of-life is not specified, cradle-to-gate assessments may be more appropriate than cradle-to-grave if the product has a variety of use cases. Isopropanol and acetone have various end-of-life scenarios^1,2, including use as a solvent, where the carbon in the molecule degrades into the environment, or as an intermediate for polymer production, where the carbon could be stored long-term or recycled into other products, achieving circularity. Given the diversity of potential final uses, we presented the LCA on a cradle-to-gate basis, allowing readers the ability to calculate end-of-life scenarios for their specific products.

Our choice of a cradle-to-gate system boundary is supported by multiple sources. ISO 14040:2006 and ISO 14044:2006 outline the guidelines and framework for conducting LCAs^3,4. ISO 14044:2006 notes that “Systems shall be compared using the same functional unit and equivalent methodological considerations, such as performance, system boundary, data quality, allocation procedures, decision rules on evaluating inputs, and outputs and impact assessment.” All LCA comparisons between our synthetic biology processes and conventional fossil production routes use the same system boundary and cradle-to-gate assumptions. Importantly, the greenhouse gas savings for our processes relative to conventionally derived chemicals (on a per kilogram of acetone or kilogram of isopropanol basis) would not differ between a cradle-to-gate and a cradle-to-grave calculation as end-of-life fates would be applied equally to both. Therefore, use of a cradle-to-gate system boundary provides all necessary calculations to determine the environmental performance of isopropanol and acetone produced from steel mill off-gas.

Several methodologies for performing LCAs of carbon capture and utilization (CCU) technologies, such as LanzaTech’s gas fermentation process, have been developed and are internationally recognized. International Sustainability & Carbon Certification (ISCC) has put forth a Carbon Footprint Certification⁵ methodology that provides a detailed approach to calculating product carbon footprints of CCU products. The ISCC Carbon Footprint Certification provides clarity on eligible CCU feedstocks, including biogenic CO₂, direct air capture and postindustrial gases. ISCC also provides a detailed formula used to calculate product carbon footprints of CCU products at the gate, which is consistent with the cradle-to-gate methodology presented in the paper. Additionally, the Roundtable for Sustainable Biomaterials (RSB)⁶ supports calculating greenhouse gas emissions at the gate for intermediate products, which are products intended to be used in the creation of other materials (e.g., ethylene glycol use in polymer productions), stating that “Operators shall calculate lifecycle cradle-to-gate GHG emissions of certified biomass and/or intermediate products.” These certifications represent two of the most common and recognized certification schemes for sustainable chemicals and products.

In the fuels market, the Renewable Energy Directive (EU)⁷, Renewable Transport Fuels Organization (UK)⁸ and Carbon Offsetting Reduction Scheme for International Aviation (global)⁹ all treat CCU products made from industrial waste off-gases the same way. The CCU product can take credit for the avoided release of the off-gas to the atmosphere so long as the original emitter is still penalized with the release of the gas. These standards also require that the waste gas cannot be purposely generated, meaning that it must be the unintended result of the main production process (i.e., steelmaking in this case). However, it can be difficult for the LCA community to verify this requirement^10,11, especially when utilization of that waste feedstock can result in a new valuable product.

Unlike for fuels, there is not yet one common approach for the chemicals sector. However, the LCA methodology we used is supported by several guidelines and frameworks that have been developed and recognized by third parties, including government policymakers, research institutions and academia. The Global CO₂ Initiative’s Techno-Economic Assessment & Life Cycle Assessment Guidelines for CO₂ Utilization¹² state that ‘carbon-negative’ results are possible at the gate and that cradle-to-gate analyses can be used for a comparative analysis: “…cradle-to-gate analysis, negative LCA results can be computed. However, such negative LCA results only reflect a comparison. In particular, negative LCA results do not necessarily imply that the CCU product is carbon-neutral or even has negative emissions over its entire life cycle.” Similarly, the Together for Sustainability (TfS)¹³ methodology for calculating product scope emissions outlines the same carbon accounting methodology for CCU methodologies: “…the cradle-to-gate PCF is provided considering a credit for the avoided waste treatment from the first life cycle.”

Because there is no consensus on whether to take avoided emissions credits in LCA for non-fuel chemicals, and end of life was excluded from the analysis, we performed the LCA with avoided emissions credits. It is the use of avoided emissions credits and a cradle-to-gate boundary that leads to negative carbon intensities of the products. The calculations could change depending on the feedstock and the end-of-life scenario.

We recognize that other methodologies exist. For example, the Environmental Footprint (EF)¹⁴ methodology does not account for carbon uptake, such as atmospheric or avoided industrial carbon release, in carbon footprint calculations. In carbon accounting, this is designated as 0/0, with the first and second numbers representing carbon release and uptake, respectively. The 0/0 approach does not consider avoided emissions (represented as 0 in LCA calculations) but also considers no penalty for its end-of-life phase, meaning negative carbon intensities for waste and biogenic feedstocks are not possible at the gate although the relative greenhouse gas emissions savings compared to fossil sources would remain the same. It is therefore recommended to perform a cradle-to-grave analysis when using 0/0 methodologies to compare the environmental benefit of chemicals from CCU or biogenic sources to conventional fossil sources. On the other hand, using a +1/–1 approach (the methodology used in this paper) means that a credit is taken for carbon uptake (represented as –1) and a penalty is given for carbon release (indicated by +1). Therefore, a cradle-to-gate analysis is sufficient to capture the environmental benefits compared to the conventional fossil source.

It should be noted that LCA methodology is different than CDR methodology¹⁵. The main difference is that LCA allows calculation of avoided emissions whereas CDR does not. Thus, LCA allows ‘carbon-negative’ products that do not necessarily mean net atmospheric carbon removal. The LCA in our study includes an avoided emissions credit for steel mill off-gas, which results in the carbon-negative result at the gate. The credit arises from diverting the off-gas from the prior use in which the CO₂ emissions would be released to the atmosphere. This methodological choice means that the emissions of the steel production process do not change and that the steel production process is burdened with the carbon release to the atmosphere.

Supplementary Table 11 uses steel mill off-gas as an example to highlight carbon accounting and carbon claims using LCA methodology. The example includes only the emissions due to the off-gas feedstock and excludes chemicals or utilities used during processing. The steel mill product will always be considered carbon-positive for fossil carbon sources and carbon-neutral for biogenic sources. The CCU product can be carbon-neutral or carbon-negative depending on the end-of-life scenario. In fact, the CCU product is agnostic to the carbon source because the steel mill off-gas is considered a waste in many methodologies.

Supplementary Table 12 provides carbon accounting and carbon claims using the CDR methodology, which considers the steel mill and CCU products together. In this case, all carbon claims are dependent on the carbon source and end-of-life scenario. Carbon-negative claims can be applied only to biogenic carbon feedstocks with sequestration end-of-life scenarios.

We would like to highlight that the gas fermentation process described in the study is agnostic to feedstock as it requires only syngas. Steel mill off-gas was chosen in the LCA as it is the most prevalent commercial application of the gas fermentation technology today¹⁶. The same LCA methodology would be applied to any accessible waste feedstock with the same gas fermentation process, including solid waste, atmospheric or waste CO₂, and any waste feedstock of biogenic origin. Regardless of the type of waste feedstock chosen, the engineered autotrophic acetogen Clostridium autoethanogenum can convert the syngas to isopropanol or acetone using the same gas fermentation technology with lower greenhouse gas emissions than conventional fossil routes when using the system design described in the paper.

Supplementary Information accompanies the online version of this amendment.

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Author notes

1. These authors contributed equally: Fungmin Eric Liew, Robert Nogle, Tanus Abdalla.

Authors and Affiliations

1. LanzaTech Inc., Skokie, IL, USA

   Fungmin Eric Liew, Robert Nogle, Tanus Abdalla, Christina Canter, Rasmus O. Jensen, Lan Wang, Jonathan Strutz, Sashini De Tissera, Alexander P. Mueller, Zhenhua Ruan, Allan Gao, Loan Tran, Jason C. Bromley, James Daniell, Robert Conrado, Séan D. Simpson, Steven D. Brown, Ching Leang & Michael Köpke

2. Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL, USA

   Blake J. Rasor, Ashty S. Karim & Michael C. Jewett

3. Chemistry of Life Processes Institute, Northwestern University, Evanston, IL, USA

   Blake J. Rasor, Ashty S. Karim & Michael C. Jewett

4. Center for Synthetic Biology, Northwestern University, Evanston, IL, USA

   Blake J. Rasor, Ashty S. Karim & Michael C. Jewett

5. Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

   Payal Chirania, Nancy L. Engle, Timothy J. Tschaplinski, Richard J. Giannone & Robert L. Hettich

6. Graduate School of Genome Science and Technology, University of Tennessee, Knoxville, TN, USA

   Payal Chirania

7. Robert H. Lurie Comprehensive Cancer Center, Northwestern University, Chicago, IL, USA

   Michael C. Jewett

8. Simpson Querrey Institute, Northwestern University, Chicago, IL, USA

   Michael C. Jewett

Corresponding authors

Correspondence to Ching Leang, Michael C. Jewett or Michael Köpke.