Carbon dioxide reduction measures in steam cracker plants: Process integration and opportunities for autothermal methane reforming for fuel gas valorisation

Abstract

As Sweden moves towards a future with stricter CO2 emission goals, significant changes will be required for the Swedish petrochemical industry. Changes need to be made to existing infrastructure or build new infrastructure in order to lower emissions. This work focuses on ethane steam cracking, an energy-intensive process. The energy intensity of the process stems from cracking, that is, the decomposition of hydrocarbons into lighter hydrocarbons with the main product being ethylene. Ethylene is a key intermediate in the petrochemical industry, mainly used for producing polyethylene. By-products of cracking ethane and other hydrocarbons such as naphtha include H2 and CH4. These are often used internally as fuel gas for the steam cracker furnace. In order to valorise the fuel gases further, an autothermal reformer (ATR) could produce synthesis gas (i.e. a mix of H2, CO2, and CO) from the cracked CH4. This work investigates reducing emissions for an ethane steam cracker by introducing hydrogen firing with H2 produced through ATR and oxyfuel combustion for the cracker, with the option of recirculating depleted synthesis gas produced through ATR to the ethane cracker furnace. Utilisation options for the synthesis gas produced in the ATR were considered by investigating methanol synthesis along with methanol-to-olefins in order to increase the yield of light olefins, mainly ethylene. The results indicate that hydrogen firing and oxyfuel combustion both have the potential to reduce CO2 emissions for an ethane steam cracker. Hydrogen firing does not require carbon capture since no carbon is emitted, and oxyfuel combustion does not require any adsorption capture to increase the concentration due to an already high concentration of CO2. Furthermore, the hydrogen firing case requires the least amount of additional specific energy in order to reach lower emissions. However, since the reformation of CH4 produces its fuel, the hydrogen firing case depends more on the ATR to produce H2 compared to the oxyfuel combustion cases. In the case that the depleted synthesis gas from hydrogen firing can be utilised for methanol synthesis a large amount of electricity is required to produce additional H2 in an electrolyser, as the majority of the hydrogen produced from the ATR is combusted inside the ethane steam cracker furnace. For oxyfuel combustion, a higher amount of energy is required to achieve a reduction of CO2 emissions than in the hydrogen firing case. On the other hand, oxyfuel combustion has the potential to utilise the synthesis gas produced in the ATR to synthesise methanol.