Development of Iron-based Oxygen Carriers in Recyclability, Physical Strength and Toxicity-tolerance for Coal-direct Chemical Looping Combustion Systems PDF Download

Author: Cheng Lung Chung
Publisher:
ISBN:
Category : Chemical engineering
Languages : en
Pages :

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Book Description
This dissertation presents investigations of chemical looping technology as a transformative process for combustion of fossil fuels for power generation with CO2 capture. Specifically, the dissertation seeks to synthesize and characterize a low-cost iron-based oxygen carrier that can be employed in a commercial chemical looping combustion system with realistic material lifetime and adequate resistance to toxicity from pollutants from fossil fuels such as coal. Two secondary metal oxides (Al2O3 and TiO2) as support materials for Fe2O3 and their respective reaction-induced morphological changes are presented. A novel iron-based oxygen carrier was consequently identified to be sustainable over 3000 redox cycles in high temperatures (1000 °C) at the lab scale without chemical and physical degradation. Oxygen carrier of the same design also exhibited high resistance toward attrition from circulation and fluidization in two pilot-scale demonstration units under representative conditions. Tolerance of the active ingredients of the iron-based oxygen carriers against common toxic elements in the fossil fuel feedstock, such as alkaline and sulfur compounds from conversion of coal, through multiple fixed bed experiments under conditions representative of the counter-current moving bed reducer and thermogravimetric experiments up to 9000 ppm of H2S. The likelihood of agglomeration and interaction of alkaline metals (Na, K) with the iron-based oxygen carriers were found to be extremely low under normal operating conditions. Instead, proper distribution of coal was more crucial to avoid agglomeration caused by melting of SiO2. Sulfur deposition on iron-based oxygen carriers, although observed, was reversible through regeneration with air and did not result in degradation in the recyclability of the oxygen carriers. A potential pathway for sulfur emission via the combustor spent air was also identified. The sulfur emission and distribution of the Coal-Direct Chemical Looping (CDCL) 25 kWth sub-pilot unit which utilized the iron-based oxygen carriers was determined with a custom heat-traced gas sampling system. More than 69% of the total amount of atomic sulfur from high sulfur coal was converted to SO2 and H2S in the reducer flue gas stream while less than 5% was released as SO2 in the combustor spent air. The missing atomic sulfur in the balance was attributed to sulfur retained in coal ash as inorganic sulfur compounds. A flue gas clean-up system targeting both H2S and SO2 is therefore recommended to meet the quality of CO2-rich stream for transportation and sequestration in a commercial CDCL system. The projected sulfur emission in the combustor spent air was under the US EPA sulfur emission regulation safe to be released to the atmosphere without a costly acid removal system. The findings demonstrate the robustness of the CDCL system, together with the iron-based oxygen carriers, to handle high sulfur coal without severe performance and economic penalties.