What is the impact of adding limestone desulfurizer to a circulating fluidized bed boiler on its efficiency?

This post was last edited by zgj2405 on 2011-6-11 at 15:58. Currently, research on the environmental issues related to circulating fluidized bed boilers has progressed quite deeply; not only are there in-depth understandings of major issues such as low NOX emissions and limestone-based desulfurization within the boiler, but attention has also been paid to issues like N2O, CO, and CXHY, which were previously given less consideration. However, at present, there is only a qualitative understanding of the impact of limestone-based desulfurization on the efficiency of circulating fluidized bed boilers, with no quantitative analyses yet conducted. This paper focuses on analyzing, through practical examples, the impact of adding limestone desulfurizer into the furnace on boiler efficiency. 1. Calculation of the amount of limestone required for limestone-based desulfurization The chemical reaction equations for limestone-based desulfurization are as follows: S + O2 → SO2 CaCO3 → CaO + CO2 CaO + SO2 → CaSO3 CaSO3 + 1/2 O2 → CaSO4 CaSO4 + CO → CaO + O2 + CO2 From these reaction equations, it can be seen that one mole of sulfur (32 kg) requires one mole of calcium carbonate (100 kg) to carry out the reaction. During combustion, approximately 28.5% of the sulfur in coal remains in the ash, while 71.5% is released in gaseous form. According to the operating data of industrial circulating fluidized bed boilers, as the Ca/S ratio increases, the desulfurization efficiency rises rapidly when the Ca/S ratio is below 2.5; however, further increases in the Ca/S ratio result in only a slight increase in desulfurization efficiency. The normally recommended Ca/S ratio is 2–2.5. The direction of the desulfurization reaction varies in different atmospheres. In an oxidizing atmosphere, the combination of CaO and SO2 is dominant, whereas in a reducing atmosphere, the decomposition of CaSO4 is dominant. The designed coal consumption for the boiler at Jidong Xincun Power Plant is 8726.4 kg/h, with an average sulfur content in the coal of 0.5%. Using a Ca/S ratio of 2.5, it is calculated that 366.5 kg of limestone needs to be added per hour. 2. Impact of limestone desulfurization addition on the heat input to the furnace The thermochemical reactions involved in limestone desulfurization include an endothermic reaction of CaCO3 calcination and an exothermic sulfate formation reaction. The corresponding thermochemical equations are as follows: CaCO3 ——→ CaO + CO2 – 1830 KJ/kg CaCO3 CaO + SO2 + 1/2 O2 ——→ CaSO4 + 15141 KJ/kg S Once limestone is added to the furnace via the coal feeder screw, the bed temperature drops significantly. This is because, upon entering the furnace at high temperatures, limestone undergoes calcination and decomposition first; this is an endothermic process. Additionally, since the limestone mixes with the coal before entering the furnace, and when the speed of the screw coal feeder remains constant, the amount of coal entering the furnace decreases as more limestone is added, resulting in a reduced heat release. Both of these factors lead to a decrease in bed temperature. Moreover, the greater the amount of limestone added, i.e., the higher the Ca/S ratio, the greater and faster the drop in bed temperature. When the Ca/S ratio is 2.5, the bed temperature drops from 1000°C to 980°C within 10 minutes, a reduction of 20°C. Then the temperature rises to 990°C. Since the sulfation reaction between calcium oxide, which is formed from the decomposition of limestone, and sulfur dioxide is an exothermic process, and since some of the calcium oxide particles are separated by the cyclone separator and return to the furnace bed to continue the exothermic sulfation reaction, this leads to an increase in the bed temperature. The heat required to convert the amount of limestone added per hour into CaO is 670,695 kJ. Assuming a desulfurization efficiency of 80%, the heat released during the sulfation reaction is 528,505 kJ, with a loss of heat of 142,190 kJ. 3. Effect on the physical and thermal losses of ash The limestone added to the furnace consists mainly of CaCO3 (100 g/mol); after decomposition and sulfation reactions, its main components become CaO (56 g/mol) and CaSO4 (136 g/mol), which are then discharged from the boiler along with the ash and fly ash. According to actual measurements, the content of unutilized CaO in the ash and slag reaches 12.3%. Additionally, complex chemical reactions may occur in the furnace involving substances other than CaCO3 in the limestone; however, since the weight of these substances changes either increase or decrease as a result of various reactions, the overall change is minimal, and their content is relatively low. Therefore, they can be considered constant in weight calculations. After cooling, the ash absorbs some of the heat and is discharged at a temperature of 150°C. The weight of 366.5 kg of limestone after the reaction is as follows: the weight of unreacted CaO is 25.3 kg, and the weight of CaSO4 is 437 kg. The specific heat of the slag is 0.96 kj/kg, and the heat loss is 66571.2 kj. 4. Impact on flue gas heat loss As can be seen from the desulfurization formula, the amount of CO2 produced as a result of the reaction of CaCO3 is 82 m3. During the reaction of SO2 with CaO, an additional 15.26 m3 of oxygen is consumed, and 73 m3 more air is required. In total, the volume of flue gas increases by 140 m3, resulting in an increase in flue gas heat loss of 30,458 kJ. The density of the flue gas ρg = 1.295 kg/Nm3, and its specific heat Cpg = 1.12 kJ/(kg·℃). 5. Conclusion: The endothermic process of limestone calcination and the exothermic sulfate reaction are the main factors affecting the efficiency of circulating fluidized bed boilers. The additional physical heat losses due to the addition of limestone are also important factors that affect such boilers’ efficiency. The increase in flue gas volume resulting from the addition of limestone leads to increased heat losses in the exhaust gases, but this has a relatively smaller impact on the efficiency of circulating fluidized bed boilers. The desulfurization process using limestone addition inside the CFB boiler results in only a limited reduction in the boiler’s thermal efficiency; according to the calculations based on the conditions of this experiment, the boiler’s thermal efficiency decreases by approximately 0.22%.

Thanks for the great post; it at least provides a clear approach.