Global scale life cycle environmental impacts of single- and multi-walled carbon nanotube synthesis processes

Abstract

Carbon nanotubes (CNTs) are well known for their mechanical resistance, durability and flexibility, which make them preferable for a wide variety of applications. The global production volume of CNTs is expected to reach 7,000 tons by 2025. This work performs cradle-to-gate life cycle assessments (LCAs) of industrially preferred single- and multi-walled CNTs synthesis processes. The aim is to evaluate global environmental impacts associated with raw materials acquisition and manufacturing and identify hotspots in CNTs production. Eight single-walled and seven multi-walled CNTs synthesis processes are evaluated using LCA. A mass based functional unit is selected as 1 kilogram of CNTs produced, and LCAs are conducted using SimaPro 8.5.2 Software with Tool for Reduction and Assessment of Chemicals and Other Environmental Impacts (TRACI 2.1) and Cumulative Energy Demand (CED) impact categories. It is expected that industrial scale production provides significant material and energy savings as well as reduces environmental impacts per unit mass of the product, due to the use of efficient equipment and recycling of reagents. Therefore, hypothetical scaling up scenarios are applied in order to estimate associated impacts. Lastly, industry-based impact projections are developed for industries where the majority of CNTs are used using the Laplace criterion. The results showed that chemical vapor deposition is the most impactful route for manufacturing single- and multi-walled CNTs. Whereas, high pressure carbon monoxide route for producing single-walled CNTs, and arc discharge route for manufacturing multi-walled CNTs are found to be the least environmentally impactful techniques among different processes considered. Results indicate that the preference of synthesis process dominates the overall environmental cost of the CNTs as well as CNTs-enabled products. Additionally, using different scaling up scenarios, it is projected that the environmental emissions associated with producing CNTs may be reduced up to 88% globally. As industries use particular routes to synthesize the CNTs to be embedded in their products, it is found that the sectoral environmental impacts are not proportional with the industrial shares. CNTs offer technological advances to conventional products (e.g. heated jacket). However, thinking from a global scale, manufacturing CNTs has significant environmental impacts. This study provides segmented impact projections for industries, which then may be used to inform sectoral cradle-to-grave environmental impacts as a function of manufacturing processes. Based on the desired characteristics of produced CNTs (e.g. diameter, surface area), manufacturing CNTs with environmentally responsible production routes may help decreasing global environmental impacts significantly.