A novel strategy for syngas production through the oxy-gasification of pulp and paper mill sludge

Pulp and paper mill sludge (PPMS) is the main organic residual generated from the wastewater treatments of the pulp and paper industry. It is generally divided into primary sludge, characterised by cellulosic-rich fibre and ash, and secondary sludge, primarily composed of organic matter with a high microbial content. Due to the huge production of the paper-making industry, around 400 million wet tons of PPMS are produced annually. Nowadays, the management and disposal of PPMS are landfilling and/or incineration, incurring economic, environmental and social costs. The present work concerns the use of the gasification process as a promising valorisation method of primary and secondary PPMS, with the goal of the production of syngas. The syngas obtained by gasification is generally composed by CO, CO2, H2 and CH4, and it is commonly used as feedstock for the synthesis of key molecules within the chemical industry, such as methanol and hydrocarbons, or for plant energy recovery (heat or electricity generation). The composition of PPMS depends on several factors, however, due to the biological activity of secondary PPMS, only primary PPMS has been characterised, highlighting a total amount of 70 wt% of CaCO3 and 25 wt% of cellulosic fibre. The experimental activity has mainly focused on an oxy-gasification strategy. Initially, fir pellets were used as a benchmark, followed by experimentation with a blend of PPMS and wood pellets in a 1:4 wt/wt ratio. The gasification attempts have been conducted in a downdraft fixed bed reactor (≈100 KwTH) by adopting a mixture of O2/H2O as a gasifying agent. Notably, the co-gasification with both primary and secondary PPMS resulted in obtaining nitrogen-free syngas characterized by a high H2 content (40 mol%) and CO (21 mol%). However, only the syngas composition derived from primary PPMS has shown to be stable over time, displaying high values (≈80%) of Cold Gas Efficiency (CGE), which is an indicator of gasification effectiveness. Finally, the characterization of the side products, such as ash and tar (condensable hydrocarbon and aromatic molecules) has been also performed. In particular, FT-IR analysis revealed that the primary PPMS ash predominantly consisted of CaO. In conclusion, this work proposes the integrated valorization of PPMS through gasification, leading to a promising syngas composition for energy recovery and a high CaO content in gasification ash primary PPMS, which opened up possible strategies for its recovery and recycling.