PhD Dissertation Final Oral Defense (November 2025)

Title of dissertation: Potential of Co-Feeding Pyrolysis in Arid Regions for Energy Production, Carbon Sequestration and Soil Amendment
Name of candidate: Waqas Ahmad
Program: PhD in Materials Science and Engineering
Name of supervisor: Dr. Fatin Samara

Abstract:

Co-pyrolysis can convert mixed organic residues into fuels and soil functional carbon, yet practical rules for temperature and blend that satisfy energy, agronomy and safety at the same time remain unclear. This study establishes temperature-resolved mono-feed baselines for food waste (FW), sewage sludge (SLG), Salicornia (SA) and date-palm residues (DP) at 400-600 °C, then quantifies how co-feeds (FW-SLG, SA-DP) shift product yields, oil and gas energy, biochar performance (summarized as energy and acidity, stability, and nutrient-thermal profile), and safety (PAHs/TEQ and metal-risk indices), with a plant-growth screen in alkaline sandy soil. In FW-SLG, liquid quality and quantity diverge with severity: FW rich blends produce the most energy-dense oils at lower temperature, mid temperature maximizes total liquids and high temperature favors permanent-gas energy. In the SA family, the 50% SA+DP co-feed yields oils that are more energy-dense than either parent, and at lower temperature its oil yield also exceeds the single feeds. GC-MS confirms a compositional shift from oxygenate-rich spectra (fatty acids/esters/phenolics) at low severity to less-oxygenated hydrocarbons (olefins/alkyl aromatics) at high severity. Biochar functions map to soil context: low-temperature FW-SLG chars maintain mildly acidic to near-neutral reactions with higher exchange capacity, aligning with nutrient-poor alkaline sands; SA-DP chars are strongly alkaline and saline, suited to acid-soil correction and best blended or rinsed before use in neutral to alkaline soils. Stability increases with temperature (lower H/C and O/C; higher thermal recalcitrance), while mass-based carbon-sequestration efficiency peaks near 400 °C and declines as char yield falls, defining a stability yield trade-off. Safety is met for organics across identified windows; metal risk depends on feed and blend. The 25% FW+SLG char keeps all regulated metals within soil-use standards and sits one ecological-risk class lower than the 50% blend, where exceedances (e.g., Ba, Se, Ni) and Cd/Cu driven risk appear. Overall, the work delivers an operating map: select ~400 °C for sequestration, ~500 °C for liquids (FW-SLG), ≥600 °C for gas energy; choose 25% FW+SLG for alkaline sandy soils and SA-DP for acid-soil correction linking temperature and blend to energy, agronomy, and safety in one framework.

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