Nitrogen oxides reduction by carbonaceous materials and carbon dioxide separation using regenerative metal oxides from fossil fuel based flue gas .

摘要

The ever-growing energy demands due to rising global population and continuing lifestyle improvements has placed indispensable emphasis on fossil fuels. Combustion of fossil fuels leads to the emission of harmful gaseous pollutants such as oxides of sulfur (SOx) and nitrogen (NOx), carbon dioxide (CO₂), mercury, particulate matter, etc. Documented evidence has proved that this air pollution leads to adverse environmental health. This dissertation focuses on the development of technologies for the control of NOx and CO₂ emissions.; The first part of the thesis (Chapters 2–6) deals with the development of carbon based post combustion NOx reduction technology called CARBONOX process. High temperature combustion oxidizes both atmospheric nitrogen and organic nitrogen in coal to nitric oxide (NO). The reaction rate between graphite and NO is slow and requires high temperature (>900°C). The presence of metallic species in coal char catalyzes the reaction. The reaction temperature is lowered in the presence of oxygen to about 600–850°C. Chemical impregnation, specifically sodium compounds, further lowers the reaction temperature to 350–600°C. Activated high sodium lignite char (HSLC) provided the best performance for NO reduction. The requirement of char for NOx reduction is about 8–12 g carbon/g NO reduced in the presence of 2% oxygen in the inlet gas.; The second part of this dissertation (chapter 7–8) focuses on the development of a reaction-based process for the separation of CO₂ from combustion flue gas. Certain metal oxides react with CO₂ forming metal carbonates under flue gas conditions. They can be calcined separately to yield CO₂. Calcium oxide (CaO) has been identified as a viable metal oxide for the carbonation-calcination reaction (CCR) scheme. CaO synthesized from naturally occurring precursors (limestone and dolomite) attained 45–55% of their stoichiometric conversion due to the susceptibility of their microporous structure. High surface area precipitated calcium carbonate (PCC) was synthesized that provided a mesoporous CaO structure upon calcination. This CaO structure attained more than 90% conversion towards the carbonation reaction at 650°C. The reactivity of the novel CaO structure was maintained close to 95% over two reaction-regeneration cycles at 700°C. Vacuum calcination proved beneficial in maintaining the structural integrity of the sorbent.