Transition pathways to electrified chemical production within sector-coupled national energy systems

TL;DR

This study analyzes how a sector-coupled national energy system can accommodate atransition of the chemical industry to electrified production via CCU and electrolysis-based hydrogen. Using the SecMOD optimization framework, it models seven base/high-value chemicals within the German energy system and derives a Cost-Avoided merit order to determine when electrified production becomes favorable relative to fossil routes. The results show that decarbonizing electricity, heat, and mobility precedes chemical electrification, which starts in 2040 and culminates in 2045 with a largely electrified industry that relies on substantial green-energy imports (about 41% of electricity) and residual emissions offset by CO_{2} removal. The work further reveals that partial electrification, supported by dispatchable processes, can provide essential flexibility to a renewables-dominated grid and reduce import dependence, offering a pragmatic pathway toward net-zero with policy mechanisms to encourage diversified, oversized chemical capacities.

Abstract

The chemical industry's transition to net-zero greenhouse gas (GHG) emissions is particularly challenging due to the carbon inherently contained in chemical products, eventually released to the environment. Fossil feedstock-based production can be replaced by electrified chemical production, combining carbon capture and utilization (CCU) with electrolysis-based hydrogen. However, electrified chemical production requires vast amounts of clean electricity, leading to competition in our sector-coupled energy systems. In this work, we investigate the pathway of the chemical industry towards electrified production within the context of a sector-coupled national energy system's transition to net-zero emissions. Our results show that the sectors for electricity, low-temperature heat, and mobility transition before the chemical industry due to the required build-up of renewables, and to the higher emissions abatement of heat pumps and battery electric vehicles. To achieve the net-zero target, the energy system relies on clean energy imports to cover 41\% of its electricity needs, largely driven by the high energy requirements of a fully electrified chemical industry. Nonetheless, a partially electrified industry combined with dispatchable production alternatives provides flexibility to the energy system by enabling electrified production when renewable electricity is available. Hence, a partially electrified, diversified chemical industry can support the integration of intermittent renewables, serving as a valuable component in net-zero energy systems.

Figures (5)

• Figure Figure 1: Schematic of processes included in the chemical industry model for producing base and high-value chemicals (blue boxes on the right). Olefins comprise ethylene and propylene, BTX comprise the aromatics benzene, toluene, xylene. Processes are in boxes, whereas products are in circles. Processes are grouped by color based on the main product. SMR: steam methane reforming, HB: Haber Bosch, DAC: direct air capture, syngas: synthesis gas, pyr. gas: pyrolysis gas, NG: natural gas, MTO: methanol to olefins, MTA: methanol to aromatics, $\mathrm{NH_{3}}$: ammonia, $\mathrm{H_{2}}$: hydrogen, $\mathrm{CO_{2}}$: carbon dioxide, MeOH: methanol, CCU: carbon capture and utilization. *Three SMR processes are considered: one for $\mathrm{H_{2}}$ production, and two for syngas production. For syngas, SMR with $\mathrm{H_{2}}$ skimming and SMR with $\mathrm{CO_{2}}$ import are considered bachmann2023syngas.
• Figure Figure 2: Schematic representation of the time-dependent merit order curve arising from the Cost-Avoided of each product, $\Delta C_{i,t}^{elec}$, and their electricity demands, $E_{i,t}^{P}$. Here, for explanatory purposes, we show the intersection of two possible renewable electricity (RE) availabilities for a single merit order curve. However, a distinct merit order curve exists in each time step.
• Figure Figure 3: GHG emissions, in million tonnes $\mathrm{CO_{2}}$-eq, of the integrated energy system and chemical industry model by sector. The chemical industry transitions last along with medium temperature (MT) and high temperature (HT) heat starting in 2040. LT: low temperature.
• Figure Figure 4: Transition of chemical production from 2035 to 2045. Years prior to 2035 have the same production mix as 2035 and are therefore excluded from the figure. Methanol becomes an important intermediate for the production of electrified olefins and aromatics, leading to a 25-fold increase in methanol production between 2035 and 2045. The specific electrified, fossil, and methanol-based processes are found in \ref{['tab:chemical processes']}. elec (via methanol) refers to the methanol-to-olefins and methanol-to-aromatics processes. elec ($\mathrm{H_{2}}$ by-product) refers to methanol produced via CCU using by-product $\mathrm{H_{2}}$ from synthesis gas production and $\mathrm{CO_{2}}$ from chemical industry point sources. SMR + HB ($\mathrm{CH_{4}}$ by-product) refers to ammonia produced via steam methane reforming + Haber-Bosch using by-product $\mathrm{CH_{4}}$ from other electrified processes.
• Figure Figure 5: TOP: electricity supply and demand for each hour in the year 2040. The hours are ordered from highest to lowest excess renewables after full electrification of the electricity, residential and low-temperature (LT) heat, and mobility sectors. Due to the time series aggregation, the hours repeat themselves, causing the steps in the figure. Electricity storage is excluded from the figure. MIDDLE: load-duration curve for the year 2040, with the hourly breakdown of electrified vs. fossil-based production for medium and high-temperature (MT + HT) heat and for each chemical product. The hours are in the same order as in the electricity balance plot (top figure). In hours with high excess renewables (dashed green line), all chemicals and heat are produced via their electrified process up to the installed capacities. In hours with low excess renewables (dashed red line), only a subset of chemicals are produced via their electrified processes. This behavior is explained by the merit order curves created by the Cost-Avoided and the electricity demand of each product (bottom figures). BOTTOM: merit order curves of electrified products in the sector-coupled energy system. Each curve corresponds to a separate hour, identified by the red and green dashed lines crossing the top and middle figures. The red and green dashed lines show the renewable electricity supplied for the electrified energy sectors. Everything to the left of the intersection between the renewable electricity supply and the merit order curve is produced electrically for that hour. Mobility, residential heat, and LT heat are fully electrified in every hour (dashed gray lines). The excess renewables (red and green brackets) are then used for electrification of chemicals and MT + HT heat. There is a tranche of electrified ammonia that is prioritized over electrified MT + HT heat in the middle figure while having a lower Cost-Avoided. Although not shown in this figure, the Cost-Avoided of ammonia can be split into two parts depending on whether the point-source $\mathrm{CO_{2}}$ emissions from fossil-based ammonia can be used downstream to produce CCU-based methanol (Supplementary Section 3) Thus, the prioritized tranche of electrified ammonia corresponds to the portion with a higher Cost-Avoided. *Heat production is shown in tonne natural gas equivalents using a heating value of $\mathrm{15.4~\frac{MWh}{tonne}}$.