Title : "Experimental investigations and modeling on the sorptive removal of trace contaminants from coal-derived syngas".
Date and Time: December 26th, 2024 (Thursday) at 11.00 am
Through video conferencing (Teams Link).
https://teams.live.com/meet/9371612503492?p=bWd1sN7GCStR8juBFJ
Abstract:
Toxic trace contaminants such as mercury (Hg0), arsenic (As), selenium (Se), phosphorous (P), cadmium (Cd), lead (Pb), and their compounds are emitted from coal, biomass, and municipal
solid wastes during their gasification and combustion. The removal of trace contaminants from carbonaceous feedstock-derived syngas at high temperatures is desired to improve the thermal
efficiency of the integrated chemical processes. The present study demonstrates the use of theoretical calculations and experimental methods in designing novel sorbents for the removal
of trace contaminants from gas streams.
Density functional theory (DFT) calculations are performed to evaluate the binding of gas-phase trace contaminants viz., Hg0, AsH3, PH3, H2Se, Cd, and Pb on various metals. Gibbs free energy (ΔG) calculations demonstrate Pd and Pt to be suitable for Hg0 adsorption at high temperatures (200 ºC). Further, DFT calculations predict the tendency of Hg to segregate towards the surface of amalgams. Analysis of the electronic characteristics of the amalgam surface explains the poisoning effect of Hg0 on metallic catalysts. Pt-based sorbents are synthesized, and their efficacy in removing Hg0 at high temperatures (≥ 200 ºC) is shown. Pt/SBA-15 exhibits the highest efficiency, exceeding 95 % at 270 ºC, followed by Pt/S-40, Pt/ZrO2, Pt/CeO2, and Pt/TiO2. Hg0 removal is proposed to occur via amalgam formation over these sorbents. Moreover, Hg0 removal on Pt/ZrO2 and Pt/CeO2 is inferred to occur by Hg0 oxidation. Syngas components CO, H2, and H2O impede Hg0 adsorption, while CO2 does not influence it significantly. A fixed bed reactor model is developed to predict the temporal evolution of Hg0 over Pt/SBA-15. DFT calculations performed to evaluate the binding of other trace contaminants reveal Ir, Pd, Pt, Rh, and Ru to possess a strong affinity for all trace contaminants, making them suitable for their simultaneous elimination. The mechanism for the catalytic decomposition of AsH3, H2Se, and PH3 on various metals is proposed, and transition state analysis reveals their dissociation to be more feasible on Ir, Pd, Pt, Rh, and Ru surfaces, as compared to Ag and Au. Bimetallic sorbents comprising of Ag or Au along with Rh, Ru, Pd, or Pt are identified for Hg0 removal at high temperatures (200 ºC). Similarly, bimetallic compositions of Ir, Pd, Pt, Rh, or Ru on Ag, Au, or Cu are identified for other trace contaminants. Linear and tree-ensemble machine learning models are developed to predict the adsorption energy of trace contaminants on bimetallic surfaces, utilizing non-collinear descriptors that encompass the physical, electronic, and geometric attributes of the adsorbent and its constituent metals. The methodology and results of this work demonstrate a process for rapidly discovering and designing novel materials for the removal of trace contaminants and could be applied for the removal of other contaminants in coal, biomass, and waste plastic- derived gas.