Process optimization for a flexibly operated plastic waste-to-methanol plant

In the future, power grid operators must mitigate fluctuations in the large amounts of energy produced from renewable energy sources. Power-to-X plants can support the operators in this regard by 1) enabling intermittent energy storage in green fuels and chemicals and 2) operating flexibly in order to provide ancillary services. Therefore, in this work, we propose a multi-stage optimization approach that determines optimal operating points for a plastic waste-to-methanol (PWtM) plant for given time-varying electricity prices. The plastic is gasified in a fluidized bed reactor (FBR), and an electrolysis unit supplies both oxygen for the gasification and the hydrogen needed to upgrade the produced synthetic gas that is converted to methanol. PWtM plants can also support a circular plastics economy where the methanol is used to produce new plastic. We assume that all chemical reactions in the FBR reach equilibrium instantaneously, and the main operational constraint is that only trace amounts of solid carbon are allowed to form. Finally, we present numerical examples of the solutions to both single- and multi-stage optimization problems.