Today’s energy system relies on sources that contain carbon (C), predominantly of fossil origin. Even in the post-fossil age there will be markets for indispensable C-based fuels and raw materials such as kerosene for aircraft or methanol and ethylene for the chemical industry. These can be synthesized using three resources – water, renewable electricity and sustainable carbon. Soon we will have to draw on the CO2 in the air, biomass, biogenic waste and as yet unavoidable plastic waste as sustainable C resources to meet future demand for synthesized C products. Thermochemical conversion technologies such as oxyfuel combustion and fluidized bed gasification can provide nearly all the carbon needed for P2X processes.
Oxyfuel combustion uses a mixture of O2, CO2 and water vapor rather than air as the oxidant, thereby producing an N₂-free oxyfuel flue gas consisting mainly of CO2 and water vapor. ZSW is developing a near-stoichiometric method of oxyfuel combustion using solid (biogenic) residues to provide process heat and oxyfuel flue gases with nearly 99% CO2 content by volume and residual O2 content greater than 0.5% by volume in dry conditions. This efficient integrated process serves to generate CO2 independently of CO2 sources for the production of C-based P2X products. The O2 required for oxyfuel combustion can be obtained directly from electrolysis when this process is combined with a P2X process, which again increases efficiency. ZSW has a comprehensive application-oriented test environment consisting of a fluidized bed reactor and a FLOX® reactor (flameless oxidation, FLOX®) at its disposal for development projects. The fluidized bed is designed for combustion temperatures up to 950°C. The FLOX® reactor can handle temperatures up to 1,100°C. The peak fuel thermal heat input is 15 kWth in each case.
ZSW takes an innovative approach to recycling mixed plastic waste, including residuals containing PVC. Plastic waste is hydrogenated with renewable hydrogen produced by electrolysis. This way, the raw materials may be reclaimed and reused rather than incinerated to produce thermal energy. ZSW is focusing on a highly efficient, direct method of producing methane in a fluidized bed reactor. The infrastructure in place today for natural gas can serve to store, distribute and use the methane – CH4, a natural gas substitute – produced via this method. As an alternative, hydrogenating gasification can produce a syngas suitable for use in other applications, for example, to yet again produce plastics in keeping with the principle of closed-loop, low-CO2 carbon recycling.
Autothermal fluidized bed gasification in a mixture of H2O and O2 is a suitable thermochemical means of converting solid (biogenic) residues into a syngas. ZSW’s development efforts in this area focus on coupling this gasification with P2X processes. This combination of processes puts the two products of electrolysis to productive use – the O2 goes to the gasifier, while the H2 can upgrade syngas. The hydrogen content in both plastic waste and biogenic residues is too low to ensure near-complete carbon transfer into the synthesized P2X product.
