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Investigation of SO2 removal characteristics with limestone under oxycombustion conditions

Avşaroğlu, Sevil
One of the technologies to increase combustion efficiency and decrease CO2 and other emissions is Oxy-Fuel Combustion. Combustion efficiency is higher and emissions are lower when the oxygen concentration of air is higher. In this thesis different characteristics of two indigenous Turkish lignites are investigated by Thermal Gravimetric Analysis (TGA). Pyrolysis is carried out under both N2 and CO2 atmospheres and combustion characteristics is also examined. CO2 acts as an inert gas at lower temperatures. Three temperature regions in pyrolysis are due to moisture release, volatile matter release and calcite decomposition in N2 and char gasification in CO2 atmosphere. In combustion study, the third temperature region shows the oxidation of char. Due to Turkish lignites having high sulphur content, capturing of SO2 emissions with limestone addition during oxy-combustion is studied. The studies showed the main effect of CO2 concentration is to determine whether the limestone will undergo calcination (indirect sulfation) or not (direct sulfation) at the same temperature. Generating oxycombustion conditions that allow indirect sulfation, results in a more effective use of limestone to capture SO2. At the end of eight-hour period, the sulfur conversion of direct sulfation at 800 °C was 30%. However, for the indirect sulfation sulfur conversion was 58%. For 15% CO2, sulfur conversion doubles at higher temperatures due to calcination and indirect sulfation of limestone. In sulfation studies, the other parameters that are examined were temperature, SO2 concentration in the gas mixture, particle size and limestone type. When Çan and Çumra limestone are compared, calcination of these limestones occurred at the same condition. However, Çan limestone resulted in higher sulfur conversion values (about 60%) due to higher surface area. Dolomite results showed lower sulfur conversions (about 12% at 800 °C, and 28-37% at 900 °C) as compared to limestone samples.