Environmental life cycle assessment of olefins and by-product hydrogen from steam cracking of natural gas liquids, naphtha, and gas oil

Steam cracking is an energy-intensive process used to convert natural gas liquids, naphtha, and gas oil into ethylene and propylene, as well as other chemicals. It is the primary source of ethylene, one of the most important building blocks for the chemical and plastics industry. Steam cracking also co-produces hydrogen which is typically combusted with the tail gas onsite for process heat, but alternatively could be separated and sold as a by-product. This study provides a detailed life cycle inventory for the United States steam cracking industry based on publicly-available, facility-specific information; provides industry average results; and assesses variability across facilities, feedstocks, and technologies. This life cycle inventory provides the baseline needed for comparison of plastic alternatives designed to improve recyclability and reduce the environmental effects of plastics. Likewise, the environmental profile of by-product hydrogen from steam crackers is important for assessing its potential benefit in decarbonizing transportation and/or industry, considering the energy use for separation and compression, as well as the make-up fuel requirements. We present the cradle-to-gate results for all steam cracking products and find the life cycle GHG emissions for average U.S. ethylene and propylene are 1.13 kg CO2e per kilogram using a mass allocation, 1.05 kg CO2e for facilities that combust their hydrogen, and 1.30 kg CO2e for facilities that separate by-product hydrogen for use. With natural gas production and ethylene demand continuing at high levels in the United States, decarbonizing steam cracking would be an important step toward mitigating emissions from the chemical industry. Similarly, the benefit of using by-product hydrogen to decarbonize other processes is dependent on the relative benefit of the hydrogen application compared with the alternative energy source used for steam cracking process heat. Results are also reported for criteria air pollutant emissions, energy use, water use, and a series of life cycle impact potentials.