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Potential of Co-Feeding Pyrolysis in Arid Regions for Energy Production, Carbon Sequestration, and Soil Amendment
Ahmad, Waqas
Ahmad, Waqas
Date
2026-11
Author
Advisor
Type
Dissertation
Degree
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29.330-2025.01a Waqas Ahmad_COMPRESSED.pdf
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Description
A Doctor of Philosophy Dissertation in Materials Science and Engineering by Waqas Ahmad entitled, “Potential of Co-Feeding Pyrolysis in Arid Regions for Energy Production, Carbon Sequestration, and Soil Amendment”, submitted in November 2025. Dissertation advisor is Dr. Fatin Samara and dissertation co-advisor is Dr. Yassir Makkawi. Soft copy is available (Dissertation, Completion Certificate, Approval Signatures, and AUS Archives Consent Form).
Abstract
Co-pyrolysis can convert mixed organic residues into fuels and soil-conditioning carbon, yet practical guidance on temperatures and blend ratios that satisfy energy, agronomic, and safety requirements remains limited. This thesis established temperature-resolved mono-feed baselines for food waste (FW), sewage sludge (SLG), Salicornia (SA), and date-palm residues (DP) at 400, 500, and 600 °C, and then examined selected co-feeds (50% SLG+FW, 25% SLG + FW and 50% SA+DP). For each feed and temperature, product yields; higher heating values (HHV) of oils, gases, and chars; biochar stability and carbon-sequestration efficiencies (CS%); polycyclic aromatic hydrocarbons (PAHs); heavy metals; and soil-relevant properties were quantified. The most promising FW-SLG char was further evaluated in a pot trial on alkaline sandy soil. In the FW-SLG system, the FW-rich 25% SLG+FW blend at 400 °C produced the most energy-dense heavy oil (32.31 MJ kg⁻¹), exceeding both parent feeds. At 500 °C, FW-rich blends maximized total liquid yields, while at 600 °C, both FW and its co-feeds shifted toward gas-dominated energy. In the SA-DP system, the 50% SA+DP blend at 500 °C delivered the highest oil HHV (≈36.3 MJ kg⁻¹) and remained above 33 MJ kg⁻¹ at 600 °C, whereas SA alone produced alkaline, saline chars more suited to liming than to application in alkaline soils. Char stability was driven primarily by temperature: all chars produced at ≥500 °C met H/C < 0.7 and O/C < 0.4 criteria, while CS% peaked at 400 °C and declined with severity, reaching 44.1% for the 25% SLG+FW blend. Across recommended operating windows, Σ16-PAHs and toxic-equivalency (TEQ) values remained within certification thresholds. Metal-risk indices classified FW, SA, DP and the 50% SA+DP char as low-risk; the 25% SLG+FW char at 400 °C met soil-use standards, whereas the 50% SLG+FW blend shifted into Cd-dominated higher-risk categories. In alkaline sandy soil, the 25% SLG+FW biochar (400 °C, 0.6–2%) was essential for plant establishment: the control failed, while amended pots supported Ruellia ciliosa growth, with 2% producing the highest biomass. Overall, the study identifies temperature-blend windows in which co-pyrolysis of FW-SLG and SA-DP can be tuned to produce fuel fractions, stable carbon, and agronomically viable biochars within PAH and metal-safety constraints.