Post-consumer absorbent hygiene products (AHPs), such as infant diapers, feminine hygiene products and adult incontinence products, present unique challenges for recycling, because they contain compound materials like superabsorbent polymers and non-woven fabrics, as well as bio-hazardous waste, which are difficult to separate. An advanced gasification process now being piloted at TU Wien (Technical University of Vienna; www.tuwien.at) could offer a path to solve the challenge of recycling complex material mixtures and increase the circularity of this set of products.
The gasification pilot project was jointly designed and conducted by a team at TU Wien, along with Essity (Stockholm, Sweden; www.essity.com), Europe’s largest producer of AHPs, and BASF (Ludwigshafen, Germany; www.basf.com), to convert AHP waste to synthesis gas (syngas; CO + H2), along with methane.
Source: BASF
While most gasifiers use single reactors or sequential-staged reactors, the 100-kW pilot gasification reactor at TU Wien uses a dual-fluidized bed (DFB) approach. In a DFB system, two interconnected fluidized-bed reactors transfer bed particles between oxidizing and reducing environments. The specialized TU Wien DFB design prevents issues related to ash formation in gasifiers. “AHP waste streams contain high levels of biologically derived salts, which result in low ash fusion temperatures,” explains Stephan Deuerlein, technical service, superabsorbents Europe at BASF. “This can cause operational issues, such as instabilities or blockages due to agglomeration or fouling.” The selection and implementation of innovative bed materials with catalytic properties, in combination with tight temperature control in the DFB prevents ash-related issues, Deuerlein says.
And while most existing recycling concepts for AHPs require the AHPs to be removed from the general waste stream to be sanitized in a pre-treatment process and require the absorbent polymers to be separated from the other AHP components, gasification also omits the need for pre-treatment and allows highly efficient gasification of AHP waste, Deuerlein says.
“This pilot trial was a first proof-of-concept that AHPs can be recycled as co-feed with municipal solid waste (MSW) without pre-treatment and separation of AHPs from other waste or separation of diaper components,” adds Deuerlein.
The H2-to-CO ratio of the product syngas depends on the process conditions and the feed composition (AHP-to-MSW ratio). The trial yielded a ratio 3.0–3.6. This high H2-CO ratio is directly suitable for downstream synthesis, Deuerlein notes. ■