Influence of Torrefaction Temperature and Residence Time on the Physicochemical Properties and Structural Transformation of Market Waste-Derived Torrefied Solid Fuel

Global energy-related carbon dioxide (CO₂) emissions reached approximately 37.4 Gt in 2023, reflecting continued dependence on fossil fuels. In the Philippines, coal accounted for about 62% of total electricity generation in 2023, highlighting the need for renewable and lower-carbon fuel alternatives. At the same time, increasing urbanization has intensified municipal solid waste generation, which now exceeds 35,000 tons per day, with fruit and vegetable residues (FVR) comprising a substantial biodegradable fraction. This study investigated the effects of torrefaction temperature and residence time on the physicochemical properties, fuel quality, and structural transformation of FVR to assess its suitability as a torrefied solid fuel.

Torrefaction experiments were conducted using a full factorial 2² design under an inert nitrogen atmosphere at two temperatures (200°C and 300°C) and two residence times (30 and 90 min), with three replicates per condition. Prior to treatment, the feedstock was air-dried, oven-dried, and milled to particle sizes ranging from 0.25 to 2.00 mm. Product performance was evaluated through mass yield determination, proximate analysis, higher heating value (HHV), fuel ratio, visual colorimetric analysis, Fourier Transform Infrared (FTIR) spectroscopy, and Analysis of Variance (ANOVA).

ANOVA identified temperature as the dominant factor affecting torrefaction behavior and product quality. Samples treated at 200°C exhibited high mass yields of 94.92%–96.83%, indicating limited devolatilization, whereas torrefaction at 300°C reduced mass yield to 46.40%–52.74%. Despite lower product recovery, higher torrefaction severity improved fuel characteristics. HHV increased from 9060–9246 Btu/lb at 200°C and 30 min to 10,918–11,560 Btu/lb at 300°C and 90 min. Volatile matter decreased from 73.90%–76.50% to 39.70%–44.30%, while fixed carbon increased from 17.10%–20.40% to 41.60%–48.00%. Ash content increased from 5.70%–6.32% to 12.30%–14.20%, and fuel ratio rose from 0.2235–0.2760 to 0.9391–1.2091, indicating enhanced carbonization and improved solid fuel behavior. FTIR analysis further showed a reduction in hydroxyl and aliphatic C–H functional groups and a stronger aromatic C=C band near 1600 cm⁻¹, confirming the formation of a more aromatic and chemically stable carbonaceous structure. These changes were accompanied by progressive darkening from light brown feedstock to nearly black torrefied solids.

The results show that lower torrefaction severity favors mass retention, whereas higher severity improves fuel quality and structural stability, supporting the potential use of market waste-derived torrefied solids for renewable energy applications.