Green C₂-C₄ hydrocarbon production through direct CO₂ hydrogenation with renewable hydrogen: Process development and techno-economic analysis

Highlights

• A new green C₂-C₄ hydrocarbon production from captured CO₂ and renewable hydrogen.

• The process includes CO₂ hydrogenation and energy-efficient CO₂ separation schemes.

• The process shows high carbon and energy efficiencies, and negative CO₂ emission.

• The unit production cost of C₂-C₄ hydrocarbon of the proposed process is 3.58 $/kg.

Abstract

This study proposes a novel process for the light hydrocarbons production from captured CO₂ and renewable hydrogen (H₂). The process consists of two main stages: i) A reaction stage in which CO₂ is converted into alkene-range hydrocarbons over a K-promoted Fe catalyst, and ii) a separation stage in which multiple technologies are integrated for the recycling of CO₂, carbon monoxide (CO), and H₂ and the purification of the main C₂-C₄ hydrocarbon products (e.g., light olefins) and byproduct (C₅₊). The methane (CH₄) and CO rejected in the separation stage are fed into a combined heat and power (CHP) process to generate utilities on-site while C₅₊ considered as by-product. In the rigorous process models developed in this work, the required sub-processes are integrated at a plant-wide level and the optimal operation conditions are determined to ensure high energy efficiency and low environmental impact, as well as compliance with the product specifications. As a result, the proposed process could achieve 99.2% and 42.0% carbon and energy efficiencies, respectively. The unit production cost for the C₂-C₄ hydrocarbons was estimated to be 3.58 USD/kg, and the CO₂ emission was estimated to be negative (−1.85 kg CO₂ per kg C₂-C₄ hydrocarbon). In addition, an economic sensitivity analysis was performed to identify the major cost-drivers, such as low CO₂ conversion or high prices for renewable H₂, and to propose solutions to these bottlenecks to allow the development of economically viable applications.

Introduction

C₂-C₄ hydrocarbons, such as light olefins, are important chemical building blocks for a variety of useful derivatives in the petrochemical industries [1], [2]. In particular, ethylene and propylene production in the petrochemical industries has shown significant growth in the global market during the past decades [3]. The global market demand for ethylene and propylene is predicted to reach 184 and 127 MMton, respectively, by 2022, corresponding to a 20% and 25% increase over 2017 [3], [4]. Conventionally, C₂-C₄ hydrocarbons have primarily been produced using oil fractions from petroleum refineries and natural gas processing plants [3], [5], [6], [7]. With the rapid depletion of petroleum sources [8], many alternative feedstock (e.g. biomass-based [9]) and technological pathways (e.g. methanol-to-olefin [10] or coal-to-olefin routes [11], [12]) have been investigated to satisfy the demand growth of C₂-C₄ hydrocarbons.

A concern about climate change and global warming is increasing as CO₂ emission has been predicted to continually and rapidly increased [13]. Since a recent report of the Intergovernmental Panel on Climate Change (IPCC) [14] suggested on the limiting global warming less than 1.5 °C by reducing global CO₂ emissions, more attempts have been made to mitigate climate change [15]. In this context, the environmental friendliness is one of the prioritized criteria in the light hydrocarbons production such as developing clean technologies, using renewable energy sources, or proactively utilizing CO₂ as a source for high-value chemicals [16]. Especially, the direct utilization of CO₂ has been considered as one of the most effective initiatives to mitigate climate change. A number of studies in the literature have examined new processes which utilizes CO₂ as a source (e.g., the direct CO₂ hydrogenation) as a promising route to value-added chemicals from a captured CO₂, especially coupled with renewable H₂ [17]. Do et al. developed the eco-friendly process for methanol production through the direct hydrogenation of CO₂ with a renewable H₂ [18]. Similarly, the techno-economic analysis of the power-to-dimethyl ether conversion using the CO₂ hydrogenation was examined by Michailos et al. [19]. Meiri and the colleagues studied the catalyst performance of the direct CO₂ hydrogenation for the hydrocarbon production and developed simulation models for techno-economic analysis [20], [21]. Najari et al. proposed and analyzed an advanced membrane reactor for the CO₂ hydrogenation to the hydrocarbon production [22]. While various studies on the CO₂ direct hydrogenation for producing value-added hydrocarbons are found in the literature, most studies focused on the long-chains hydrocarbon, or liquid fuels production [23]. Particularly, Smejkal et al. [24] performed a techno-economic evaluation of direct CO₂ hydrogenation technology to produce petrochemical and liquid fuel hydrocarbons. Zhang et al. [25] analyzed the technological and economic performance of a process for co-production of synthetic natural gas and liquid fuel hydrocarbons via direct CO₂ hydrogenation. Recently, a few studies have focused on light C₂-C₄ hydrocarbons via direct CO₂ hydrogenation. For instance, Najari et al. [26] estimated the kinetic parameters for the modeling and optimization of the hydrogenation reaction of CO₂ to C₁-C₄ hydrocarbons over a K-promoted Fe catalyst using an algorithm-based method. To the best of our knowledge, there is little study in the open literature to propose a complete process producing light C₂-C₄ hydrocarbons via direct CO₂ hydrogenation, and to analyze the technical, economic and environmental capability. Thus, it is necessary to develop rigorous models and examine the techno-economic and environmental performance of the light C₂-C₄ hydrocarbons via direct CO₂ hydrogenation to obtain further perspectives for industrial applications.

This work aims to develop and evaluate a complete process for the C₂-C₄ hydrocarbons production via direct CO₂ hydrogenation. One of the distinct features of the proposed process is a green route to C₂-C₄ hydrocarbons, which is obtained from the synergy of using a captured CO₂ and renewable H₂. In this work, the catalyst of Fe-K/γ-Al₂O₃ was introduced in the catalytic conversion unit to produce light hydrocarbons [26]. The diverse mixture of products with different physicochemical characteristics (e.g., a wide distribution of hydrocarbons, undesired by-products such as CO, and unreacted CO₂ and H₂) results in a complex separation scheme which is also another critical featured of the study. To achieve the goal, a conceptual but rigorous process model for the C₂-C₄ hydrocarbon production, including the reaction and separation sections, has been developed using the software Aspen Plus V10.0 [27]. Then, a techno-economic analysis study was performed to evaluate the technical and economic feasibility of the proposed process using different criteria: technical performance (carbon element efficiency and energy efficiency), environmental performance (emission CO_2-eq), and economic performance (unit production cost). In addition, major cost parameters, in particular the price of renewable H₂ as the most critical factor, were evaluated in a sensitive analysis study that identifies the key strategies to make the proposed process more economically.