10 May,2017 UTC+8 Views:
Lime is produced by calcination of calcium carbonates in industrial kilns. The mineral calcite containing the calcium carbonates is the main component in naturally abundant limestone. The limestone is quarried or mined, mechanically pretreated and delivered to the lime plant. One of the most common kiln types is the rotary kiln. Calcination is an endothermic reaction requiring heat to evolve gaseous carbon dioxide from the calcite to form lime .The calcination starts between 800°C and 900°C and the operational solid temperature usually reaches 1000– 1200°C. The calcination temperature is dependent on the partial pressure of carbon dioxide in the kiln. A simplified illustration of a rotary kiln for lime production is presented in Figure 1. The upper setup shows the conventional air-fired process used as a reference case in this work. The lower setup shows the oxyfuel process with added air separation unit and flue gas recirculation are used for simulation cases in this work. The limestone is fed in the upper end. The feed is usually a 10–40 mm fraction of the limestone produced in the quarry. A small inclination of the kiln enables the material to move downward during rotation. The rotation is slow and the residence time is usually in the range of 5–10 h. It is a countercurrent process. When the limestone enters the kiln, it is dried and heated by the flue gases flowing in the opposite direction. At the lower end, the fuel is fed into the kiln through a burner. When exiting the kiln through the lower end, the limestone has released its content of carbon dioxide and the majority of the material is calcined into calcium oxide. Residual content of carbon dioxide in the product is usually in the range of 0.1–2.0 mass percent, depending on customer application. The mass balance shows that at full calcination, up to 44% of the feed is released as carbon dioxide gas during the process. After the kiln the lime product enters a cooler. The cooling is done with air to suitable product temperature. The cooling procedure is fast so that the product remains as lime and carbonation, which is uptake of carbon dioxide from the atmosphere, is minimal.
Oxyfuel combustion aims to concentrate carbon dioxide in the flue gases enough for utilization or storage. Replacing air (79% nitrogen and 21% oxygen) with pure oxygen can decrease the volume of flue gases from the process. This also increases the carbon dioxide concentration since no nitrogen is added to the system. Combustion in elevated oxygen concentrations increases the flame temperature. This increase in flame temperature changes the heat load of the kiln combustion area. Thus, flame temperature needs to be controlled. This is achieved by recirculating the flue gas back to the lower end of the kiln. This work studies the effect of recirculation levels 50%, 60%, 70%, and 80%. The recirculated flue gas has a high carbon dioxide concentration. The increased carbon dioxide concentration increases the calcination temperature of the raw material. This will change the heat balances of the kiln.
