Thermochemical Conversion of Wastewater Sludge from Ribbed Smoked Sheet (RSS) Rubber Production: Pyrolysis Product Distribution, Physicochemical Characteristics, and Energy Potential

The rapid depletion of fossil fuels and increasing environmental concerns has intensified the search for sustainable energy recovery from biomass residues. Wastewater sludge generated during ribbed smoked sheet (RSS) rubber production is an underutilized by-product rich in organic matter and therefore represents a potential feedstock for thermochemical conversion. This study investigated the slow pyrolysis of RSS wastewater sludge and evaluated the effects of pyrolysis temperature (350–500 °C) on product distribution, fuel properties, and thermochemical characteristics. Wastewater sludge collected from an RSS cooperative in Chumphon Province, Thailand, was pyrolyzed in a laboratory-scale reactor under a nitrogen atmosphere, and the feedstock and pyrolysis products were characterized using proximate and ultimate analyses, thermogravimetric analysis (TGA), gas chromatography, and physicochemical measurements. The results showed that pyrolysis temperature significantly influenced product yields and fuel quality. The yields of pyrolysis gas (PG), biochar, and liquid products (LPs) ranged from 41.87 to 45.78% wt, 23.41 to 28.75% wt, and 29.19 to 30.81% wt, respectively. The LPs contained high oxygen contents (72–81% wt) and exhibited relatively low higher heating values (HHVs) of 16.87–21.75 MJ kg−1, indicating limited suitability for direct fuel applications without upgrading. In contrast, biochar produced at higher temperatures exhibited increased carbon content (60.18–77.15% wt) and HHVs of up to 28.45 MJ kg−1, demonstrating promising potential as a renewable solid fuel, although its relatively low bulk density (97.28–135.29 kg m−3) suggests that densification would improve practical utilization. The PG fraction exhibited low HHVs (0.21–2.95 MJ kg−1) owing to the predominance of CO2 and other non-combustible gases under slow pyrolysis conditions, making it more suitable for localized process heat recovery than as a primary gaseous fuel. Overall, this study demonstrates that RSS wastewater sludge can be effectively valorized through slow pyrolysis into biochar, liquid products, and pyrolysis gas. The findings provide quantitative information for optimizing pyrolysis operating conditions and offer engineering guidance for the sustainable utilization of rubber-processing sludge as a renewable energy resource within a circular bio-economy framework.