Reducing CO2 emissions of existing ethylene plants: Evaluation of different revamp strategies to reduce global CO2 emission by 100 million tonnes - ScienceDirect

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Journal of Cleaner Production

Volume 362, 15 August 2022, 132127

Journal of Cleaner Production

https://doi.org/10.1016/j.jclepro.2022.132127 Get rights and content

Highlights

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LCA of different revamp strategies of steam cracking technologies can lead to huge CO2 reductions.
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Oxyfuel combustion with CCS reduces the carbon footprint up to 24%.
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Impact of improvements in combustion efficiency alone are marginal.
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Biogas fuel allows to reduce the carbon footprint by 21.2% without major investments.

Abstract

Steam cracking is the leading technology to produce light olefins and the most energy-consuming process in the chemical industry, using approximately 10% of the sector's total primary energy globally. Six technological improvements, requiring limited hardware changes and implementable today, have been evaluated: high emissivity coatings, 3D reactor designs, advanced coil material, oxy-fuel combustion, carbon capture and storage (CCS), and biogas firing. These technologies have the potential to reduce both energy consumption and raw material demand. Life cycle inventory data are obtained from the first principles-based COILSIM1D model of the furnaces, validated with industrial data, combined with a Petro-SIM model of the overall mass and energy balance of the plant. Cradle-to-gate life cycle assessment (LCA) shows that a major contribution to the overall carbon footprint of light figureolefin production is related to the feedstock supply. Oxy-fuel combustion combined with CCS followed by biofuel combustion has the highest potential to reduce the carbon footprint, and reach a target of over 100 × 10⁶ metric tons/yr of CO₂ eq. worldwide. On the other hand a combination of radiant coatings and novel reactor coil design yields in a marginal 0.5% reduction or potentially 3 × 10⁶ metric tons/yr of CO₂ eq. worldwide. In order to decrease the carbon footprint more one would have to consider both substantial hardware changes - requiring a large investment - and the use of non-fossil derived feedstocks.

Introduction

Ethylene and propylene are the most important building blocks of the chemical industry with an annual production of 1.85 × 10⁸ t and 1.3 × 10⁸ t respectively (Zhao et al., 2021). Steam cracking (SC) of hydrocarbons is and will be (Amghizar et al., 2017) the leading production process for ethylene and propylene and globally uses 8% (Ren et al., 2008) of the sector's total primary energy input. The cracking process is highly endothermic and, depending on the feed, the energy consumption for the production of 1 kg of ethylene ranges from 15 to 27 MJ/kg (Zimmermann and Walzl, 2009). The corresponding carbon footprints are estimated as 1.56 metric tons (t) of CO₂ equivalent per ton of ethylene (Keller et al., 2020) or 2.4 tons of CO₂ equivalent per metric ton of propylene (Zhao et al., 2017). This results in a dashing 543 MT of CO2 equivalent just for producing olefins by steam cracking. A key question that is on many corporate tables is “how can we reduce these emissions as soon as possible, preferably with as limited number of changes to the existing process”. Fortunately, the bulk part of the direct (scope 1) emissions, more than 80%, can be attributed to fuel combustion in the steam cracking furnaces. This explains the focus of evaluating hardware changes to the cracking furnaces and see how greenhouse gas (GHG) emissions can be reduced.

A first question is if the efficiency of the steam cracking process can be further improved because state-of-the-art furnaces are already heavily optimized. The state of the art of radiatively heated steam cracking coils, i.e. of the vertically suspended cylindrical reactor tubes in which the cracking takes place, has recently been summarized by Symoens et al. (2018) and Amghizar et al. (2020). One way to increase the efficiency of the radiative heat transfer from the burners in the furnaces to the process gas in the coils is by increasing the emissivity of the refractory linings of the furnaces (Vangaever et al., 2021). Emissivity is defined as the ratio of total radiative output from a surface per unit of time per unit of surface area at a specific temperature and wavelength to that of a black surface under the same conditions. If the refractory surface emissivity is increased then the heat demand of the cracking process in the coils can be supplied at a lower flue gas temperature, thereby increasing the efficiency of radiant heat transfer and decreasing the heat loss via the flue gas leaving the radiant section of the furnace.

Another hardware change is the implementation of novel reactor geometries and materials to reduce energy consumption. Several 3D coil geometries have been designed to improve heat transfer between the inner coil wall and process gas flow. The resulting lower wall temperature and rate of coke deposition on the inner wall (Amghizar et al., 2020) increases the run-length, i.e. the time between two shutdowns required to remove the coke (commonly referred to as decokes). However, 3D coils lead to a higher pressure drop than coils with a regular circular cross-section which reduces the selectivity to ethylene. Besides geometry, novel coil materials such as aluminum-rich alloys, have been considered as well. The aluminum in the metal alloy substrate forms a thin, inert, stable alumina layer on the inner surface that prevents catalytic coke formation (Sarris et al., 2020), ultimately leading up to ∼25% reduction in coking rate. Less frequent decoking implies less fuel consumption and less CO₂ emissions for every unit of production.

Secondly, emissions next to greenhouse gases and CO₂ in particular like NO_x should also be considered (Djokic et al., 2017). In this context oxy-fuel combustion with flue gas recycle has been considered (Zhang et al., 2021). In these arrangements the flame is quenched with recycled flue gas instead of ambient air nitrogen for a more cost-effective design. Introduction of recycled flue gas allows one to reduce peak flame temperatures in order to avoid costly burner and coils redesign. Since oxy-fuel combustion does not use any air, virtually no NO_X is produced in the firebox. At the same time, the concentrated CO₂ stream leaving the stack allows for more cost-efficient carbon capture and storage (CCS). Captured carbon dioxide can be (a) used as feed for the chemical industry (Pappijn et al., 2020); (b) permanently stored in underground reservoirs; or (c) used for enhanced oil recovery (EOR). Although feasible, oxy-fuel combustion and CCS have not been carried out on an industrial scale until now because of the additional cost (Weikl and Schmidt, 2010).

Next to using pure oxygen, also alternative fuels should be considered. Those could result in lower emissions for their production and supply chain, i.e. lower SCOPE 3 emissions (WBCSD, 2004). For steam cracking the most logical choice is biogas. Biogas, which consists of primarily methane, originates from the anaerobic digestion of organic waste. Nowadays only a small fraction of the biogas potential is being used (Poeschl et al., 2010) making it a promising fuel of the future. Today, this biogas is typically burnt to generate electricity and heat, which is incentivized by regulatory support schemes. However, according to the literature, only one-third of the energy contained within the biogas can be transferred into electricity by direct combustion. The other part is lost as heat of which only one-third can be recuperated. At present, the most viable and sustainable pathway of biogas utilization is considered to be the purification and upgrading of biogas to biomethane (Haider et al., 2020) for further combustion in combined heat and power (CHP) unit or injection into the natural gas grid. However, it is expensive to upgrade biogas to biomethane (Verbeeck et al., 2018) so combustion of untreated biogas with high biogenic CO₂ content has potential. Despite the benefits of applying biogenic fuels, because of issues like fugitive methane and PM 2.5 emissions (Poeschl et al., 2012), a life cycle assessment is needed to assess the overall environmental impact when using it in a steam cracking furnace instead of fossil-derived fuel gas.

Various LCA studies for conventional and alternative olefin production systems are already available in the literature. Most publications simply compare fossil feeds to renewable feeds, e.g. (Keller et al., 2020) and (Van Uytvanck et al., 2014). Others such as (Delikonstantis et al., 2020) investigate alternative ethylene production pathways using a plasma-assisted process: such an approach will not be ready in the coming decade. Other publications (Holmgren et al., 2014) concentrate on process emissions and energy flows but do not account for the impact of feed supply.

A benchmarking of the best of the new steam cracking technologies under the same conditions and using the same feed is lacking in the literature. This work fills this gap and assesses the impact of nine combinations of technologies(further referred to as ‘cases’) on the overall environmental footprint of cracking products using a cradle-to-gate life cycle assessment. All these technologies can be implemented today, and hence can make a substantial impact in the coming decade. The additional objective is to identify multiplier effects: in other words, is the whole greater than the sum of the parts. To obtain reliable life cycle inventory, operating parameters of nine cases requiring limited hardware changes and implementable today, have been evaluated through process simulation, making this work a prospective study. The individual impact of the different process steps can be used to find ‘hotspots’ – stages that are responsible for the major share of certain harmful effects. The sustainability of the cases has been investigated among 16 impact categories using models suggested by the framework of the European Product Environmental Footprint (Fazio et al., 2018).

Section snippets

Process overview

The arrangement of a typical steam cracking furnace is shown in Fig. 1. The process flow diagram of the ethylene plant considered in this work is given in supplementary material SM1, including a detailed description of the utilities. Fig. 1 shows that first the hydrocarbon feed is heated by the flue gas in the feed preheater bank of the convection section, after which it is mixed with dilution steam (DS), and further heated to the desired coil inlet temperature (CIT) (550–750°C, depending on

Results and discussion

Life cycle assessment has been conducted for the midpoint indicators suggested by the industry standard guidelines (WBCSD, 2014) using models presented in Table 5 for all the concepts that are considered. The environmental burden of a functional unit obtained with each concept is shown in Table 6. Given the different scale of improvements, it is reasonable to divide all cases into two subcategories: conventional designs and novel concepts. Cases that represent traditional air fired furnaces

Conclusions

Nine scenarios of steam cracking innovations have been compared. For each of these cases a life cycle assessment has been carried out evaluating 19 impact categories based on the Environmental Footprint midpoint LCIA method. The biogas furnace reduces the carbon footprint by 21.2%, corresponding to 116 million metric tons if this innovative process solution would be applied in all steam cracking plants in the World. However, using biogas also has its downside because it increases terrestrial

CRediT authorship contribution statement

Oleksii Mynko: Conceptualization, Methodology, Software, Investigation, Writing – original draft, Data curation, Visualization. Ismaël Amghizar: Methodology, Software, Validation, Investigation, Writing – review & editing. David J. Brown: Conceptualization, Validation, Investigation, Supervision, Writing – review & editing, Project administration. Lin Chen: Software. Guy B. Marin: Conceptualization, Writing – review & editing, Supervision, Project administration. Rodrigo Freitas de Alvarenga:

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study is supported by the IMPROOF (Integrated Model Guided Process Optimization Of Steam Cracking Furnaces) project as a part of the Horizon 2020 framework program (H2020 Grant Agreement N° 723706) of the European Union. The research leading to these results has received funding from the European Research Council under the European Union's Horizon 2020 research and innovation program/ERC grant agreement n° 818607.

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