(90a) Cradle-to-Gate Life Cycle Assessment of Light Olefin Production Via Methanol-to-Olefins Route Using Waste-Derived Feedstock and Steel Mill Off-Gases

The discovery and development of efficient technologies that enable the use of CO₂ as a starting material for chemical synthesis (at scale) is arguably one of the most significant scientific challenges of our era. But a key question is if the cure will not be worse than the disease? Renewable energy, specifically renewable power, is generally assumed to be the energy source for CO₂ utilization. Various options are available for using renewable power to mitigate CO₂ emissions, including methanol-to-olefins (MTO) based conversion of CO₂ to olefins, electrolysis of water to produce hydrogen, and powering battery electric vehicles. Thermodynamic and practical arguments can be used to rank the effectiveness of renewable power for CO₂ mitigation for these options. For example, renewable power used to reduce water to hydrogen, which is then used to convert CO₂ into liquid fuels, is only one-fourth as effective at mitigating CO₂ emissions compared to use of the renewable power directly. The direct electroreduction of CO₂ to liquid fuels, even if achieved with 100% thermal efficiency, is only one-third as effective at mitigating CO₂ compared to using electricity directly.

Given the complexity of these processes and their potential environmental implications, a comprehensive study is essential to assess their sustainability. This study aims to compare the environmental performance of four innovative MTO-based processes – GreenH2-A, GreenH2-S, CH4PyrH2, and MPO-Gas – using life cycle assessment (LCA) with a cradle-to-gate system boundary and a functional unit of 1 kg of high-value chemicals (mostly ethylene and propylene).

The GreenH2-A process utilizes alkaline electrolysis powered by wind energy to produce hydrogen from desalinated seawater, while steel mill off-gas serves as the CO₂ and CO source. The off-gas treatment involves hydrogen recovery from coke oven gas (COG) using pressure swing adsorption (PSA), combustion of the remaining gas for process heat, and mixing of the resulting flue gas with blast furnace gas (BFG) and basic oxygen furnace gas (BOFG). The combined gas stream then undergoes CO and CO₂ recovery using COSORB and monoethanolamine-based carbon capture processes, respectively. The recovered CO₂ is mixed with hydrogen and is fed to a reverse water-gas shift (RWGS) reactor, where it is converted into syngas over Ni/Al₂O₃ catalyst at a temperature of 750°C and pressure of 2 bar. The GreenH2-S process is similar to GreenH2-A but employs solid oxide fuel cell (SOFC) electrolysis as the source of make-up hydrogen. In the CH4PyrH2 process, methane pyrolysis using a molten tin (Sn) bubble column reactor is employed as the hydrogen source, where methane undergoes thermal decomposition into hydrogen and solid carbon at 1175°C and atmospheric pressure. The MPO-Gas process relies on the gasification of plastic waste as the sole syngas source, making it grid-independent. The gasification process consists of three main steps: gasification of plastic waste using pure oxygen and steam at 800°C and 1.5 bar, a water-gas shift reaction to increase the H₂/CO ratio, and a separation train.

In all processes, the produced syngas then undergoes methanol synthesis over a Cu/ZnO/Al₂O₃ catalyst in a fixed-bed reactor, followed by methanol condensation and purification via distillation. The methanol then undergoes the MTO process, converting it to light olefins using a silicoaluminophosphate (SAPO-34) catalyst in a fluidized-bed reactor at atmospheric pressure. The product stream is then separated to obtain high-purity olefins.

LCI data for the MTO-based processes are obtained through process simulations using Aspen HYSYS. The LCA results are compared to those of conventional steam cracking and electrified propane cracking. The study adheres to the guidelines provided by the WBCSD for LCA of chemical products. By identifying the main contributors to environmental impacts, this study will provide insights for process improvements and policy decisions, addressing the critical question of whether these innovative routes are indeed a solution or a potential burden to the environment.