Steam Cracking in Dual Fluidized Beds - One Step Towards Complete Recyclability of Plastic Waste Using Thermochemical Conversion

Doctoral thesis, 2024

Circular use of plastic materials is crucial for reducing dependence on fossil resources and mitigating the environmental pollution from plastic waste. Thermochemical recycling offers a way to produce new plastics from plastic waste. This approach faces numerous challenges that impede its development to an industrial scale. The main challenges include reactor robustness, feedstock sensitivity, and the selective production of monomers. This thesis presents steam cracking in a dual fluidized bed (DFB) reactor as a solution to the challenges in thermochemical recycling. An ideal operating window for the selective production of light olefins and monoaromatics is determined, and these operational parameters are applied to various plastic waste streams. The bed materials tested include silica sand, olivine, bauxite, and feldspar. The development of catalytic activity in the bed materials within DFB steam crackers is also experimentally demonstrated.

The results show that the cracking severity achieved in a DFB steam cracker at temperatures between 700 and 825°C is suitable for the selective production of light olefins and monoaromatics.Within this operating window, pure polyolefin feedstocks yield up to 50% C2– C4 olefins and 20% BTXS. The yield of light olefins remains proportional to the polyolefin content of unsorted plastic wastes, regardless of the presence of other polymers. The nonpolyolefin materials in plastic waste, such as PET and cellulose, are selectively converted into aromatics and syngas. Although PET and cellulose contents also lead to significant coke formation, an uninterrupted steam cracking operation without intermittent decoking procedures is demonstrated. Silica sand, olivine, feldspar, and bauxite exhibit no significant catalytic activity in their natural state. In certain situations, olivine and bauxite develop catalytic activity. The accumulation of biomass ash in olivine enhances syngas production through the steam reforming of aromatic precursors. The accumulation of transition metal oxides within the bed material negatively impacts light olefin production. Using bauxite in a reduced oxidation state promotes hydrogenation reaction, thereby enhancing the production of light olefins.

The outcomes of this thesis demonstrate that a DFB steam cracker enables direct production of light olefins from plastic waste without the need for presorting procedures or catalysts. Most of the data presented in this work are obtained from experiments conducted at a scale relevant to the petrochemical industry, showcasing the scalability and technology readiness of the DFB steam cracking process.