Aug-2022

Techno-economic metrics of carbon utilisation: Part 1

Technological and economical parameters of carbon utilisation and how these parameters vary widely depending on external and technology-specific variables.

Joris Mertens, Mark Krawec and Ritik Attwal
KBC (a Yokogawa company)

Article Summary

Carbon capture, utilisation, and storage (CCUS) is often confused with carbon storage (CS) rather than carbon utilisation (CU). This misunderstanding is logical since, ultimately, CS is a form of waste disposal while CU refers to the new circular world that emphasises more efficient use of resources. With CU being in general more expensive than CS, some CU technologies need further development, which explains the current focus on storage.

Currently, Yokogawa, a leading provider of industrial automation and test and measurement solutions, is performing a strategic decarbonisation study of the Goi industrial area in the Chiba Prefecture at Tokyo Bay (Yokogawa, 2021). The purpose of this research is to make the industrial area net carbon neutral by 2050, preferably using CU rather than CS.

Figure 1 shows the technological and economic parameters in play for CU. Economically, capital costs and different operational costs will affect project viability. In addition, product market demand and the technical readiness level (TRL) for a given CU technology should be considered.

Carbon utilisation technologies
KBC performed a techno-economic evaluation of the nine CU technologies listed in Table 1. Table 1 also includes the feeds, other than CO2, and the operating temperature of the CU paths. For most of the feed and product pricing, KBC relied on third-party market intelligence from Argus Media.

The Fischer-Tropsch (FT) and Oxo synthesis configurations considered in the referenced articles consume CO rather than CO2. Therefore, a reverse water gas shift (RWGS) step is included upstream to convert the CO2 into CO. The RWGS step includes CO2 capture to recycle the unconverted CO2.

The methanation process considered uses CO2, not syngas (CO). However, due to low CO2 conversion per reactor pass, a CO2 capture/recycle step is required, too.

Four of the nine CU technologies were simulated partially or entirely using KBC’s Petro-SIM software. Figure 2 shows the Petro-SIM simulation model of the Oxo process. The technologies were simulated when a process flow diagram was missing, or the assumed process heat integration was incomplete or unrealistic.  

Operating cost
Many CU technologies require significant amounts of hydrogen. In the upcoming Part 2 article, we will demonstrate that the hydrogen used for this study should have a very low carbon intensity. The cost of the green hydrogen used and the revenue generated from utilising CO2 will significantly impact CU technology economics.

Two price scenarios were considered (see Table 2). The 2030 scenario employs a high price of green hydrogen and a low price of CO2. The 2050 scenario adopts a much lower price of green hydrogen and a much higher price of carbon emissions. The price sets correspond with possible carbon and hydrogen pricing in 2030 and 2050. These are semi-arbitrary and based on price scenario trends, not on an in-depth analysis of current and upcoming legislation, carbon markets, and green hydrogen project pipeline. The primary purpose is to demonstrate the sensitivity of the CU economics with carbon and hydrogen pricing.

The 2030 and 2050 price estimates have been established with a more rigorous market analysis by Argus Media for the other feeds (propylene, PO) and the CU products. Yokogawa and KBC established price estimates for the carbonation feeds and PPC products. The estimates are based on price data before third-quarter 2021 inflation rates hit, when natural gas prices wavered around USD 40/MWh rather than surpassing USD 100/MWh.

Figures 3 and 4 show the operating cost/revenue breakdown for the different CU technologies under the two Hâ‚‚/CO2 pricing scenarios. Hydrogen, other utilities (electricity, fuel, steam), and fixed operating cost are shown on the debit side of the graph, below the zero axis. Revenue streams generated by the product/feed differential and CO2 utilisation are shown as positive bars in the chart. The resulting operating cost/revenue balance (EBITDA) is plotted in Figures 5 and 6.

The charts demonstrate that hydrogen is the key driver of operational costs for many of these technologies. These technologies will only become economically profitable if green hydrogen costs drop significantly, although product pricing can play a decisive role, too. The following sections discuss the operating cost elements in more detail, as well as the capital cost and technology readiness.

Hydrogen consumed
The hydrogen utilisation intensity (HUI) heavily depends on the CU technology considered and largely correlates with the destination of the oxygen atoms in the utilised CO2 molecule. The process of producing oxygen-free products involves separating the oxygen atoms in the CO2 molecule from the carbon atom, which is done by binding the oxygen with hydrogen and generating water. Hence, hydrogen is not only required to generate hydrocarbon but also to capture the oxygen atoms of the CO2 into water molecules.

The HUI and carbon utilisation intensities (CUI) of the process can be defined as the tonnes of hydrogen and carbon consumed to produce one tonne of product, respectively. Figure 7 is a theoretical hydrogen intensity chart. The x and y axes are the ratio of the number of oxygen and hydrogen-to-carbon atoms in the product. Acetic acid (CH₃COOH), for example, has an #H/C and #O/C ratio of 2 and 1, respectively. The lines on the graph show lines of equal hydrogen intensity, i.e. lines of equal hydrogen intake for products produced from CO2, as a function of the oxygen and hydrogen content of the products. Acetic acid is located close to the line of 0.13 tH2/tproduct. Therefore, a green acetic acid facility with a production capacity of 100 t/h of acid out of CO2 will require just over 13 t/h (~150 kNm3/h) of hydrogen.

The graph shows that even the production of hydrogen-free carbon from CO2 requires more than 0.3/t of hydrogen per tonne of product, only to remove the oxygen atoms from CO2. At the same time, the hydrogen requirement drops below 0.2 t/t in most cases if the product contains oxygen.

Producing oxygen-free products from CO2 requires more hydrogen to remove both oxygen atoms. In addition, removing oxygen from the CO2 results in products with lower molecular weight, which further increases the Hâ‚‚ requirement, at least if expressed per tonne of product. 

Figure 8 shows the HUI for each of the nine technologies considered. The carbonation, urea, polyol, and PPC technologies require no hydrogen because the CO2 molecule is bound to another molecule without prior oxygen removal.

Note that the CUI of the FT process tested is close to 0.52 tH2/tProduct, which is considerably higher than the theoretical intensity of around 0.44 t/t (see Figure 7). This is because a significant purge of a syngas stream is applied in the specific process set-up considered in our study (Zang et al., 2021).