A study on the limestone sulfation behavior under oxy-fuel circulating fluidized bed combustion condition

Transcription

1 11th European Conference on Coal Research and its Applications A study on the limestone sulfation behavior under oxy-fuel circulating fluidized bed combustion condition Wei Li, Shiyuan Li Institute of Engineering Thermophysics Chinese Academy of Sciences 11 Beisihuanxi Road, Beijing, China

2 Content Background 50 kw Oxy-fuel CFBC System Results and Discussion Conclusion 2

3 Background Oxy-fuel combustion is one of the most promising CCS technology. It is believed that the oxy-fuel CFB combustion will be a better choice for CO 2 capture. fuel flexibility, low NOx emission and efficient sulphur removal, etc. The high SO 2 concentration would cause the corrosion of the tube and device due to the sulphate deposition. The limestone can behave in the way of either direct or indirect sulfation depending on the bed temperature and atmosphere The aim of the paper is to investigate the limestone sulfation behavior under oxy-fuel circulating fluidized bed combustion condition. 3

4 50 kw Oxy-fuel CFBC System Diameter: 50 mm Height: g 3250 mm Combustion mode air, O 2 /CO 2, O 2 /N 2 Three zones of electrical heaters and a solid cooler. The oxygen concentrations of inlet gas range from 21% to 50% 50 kw Oxy-fuel CFBC System 4

5 Results and Discussion Procedure A small cage made from stainless steel Limestone particles size :2 mm - 3 mm Limestone :weight of 5 g Reaction time:15 min,30 min,45 min,60 min Analysis: XRF, SEM, BET Condition Stainless steel cage Temprature:850 o C,900 o C, 950 o C Inlet oxygen concentration:40% Fluidization velocity:2.5m/s 5

6 Results and Discussion The calcination of limestone o C, Oxy-fuel 900 o C, Oxy-fuel 950 o C, Oxy-fuel 850 o CAir C, CaO (%) Time (min) It The calcination of limestone is normally determined by the CO 2 partial pressure is indicating and that temperature the sulfation in mechanism the system switches together. from The direct increasing to indirect of route asthe temperaturet rises from 850 temperature would promote the calcination o C to above 900 of limestone. o C with the inlet And the high oxygen concentration of 40%. concentration of CO 2 would inhibit calcination of limestone. 6

7 15 min 30 min 45 min 60 min Results and Discussion Scanning electron microscope (SEM) 850 o C Air 15 min 30 min 45 min 60 min During the indirect sulfation process, the product layer looks loose and porous before 45min. It is caused by the flow of CO 2 formed by the limestone calcination.the sintering of the CaO causes the dense product layer in 60 min. 850 o C 15 min 30 min 45 min 60 min During the direct sulfation process, the CaSO 4 product emerge in the surface of the limestone at the beginning, then grow and gradually cover the particle surface, and a continuous product layer is formed in the end, which is very dense and less porous. 900 o C 15 min 30 min 45 min 60 min 950 o C

8 Results and Discussion Nitrogen adsorption analysis BET Surface Area (m 2 /g g) o C, Oxy-fuel 900 o C, Oxy-fuel 950 o C, Oxy-fuel 850 o C, Air Time (min) It is suggested that the temperature and gas compositions all play important roles in determining the pore structures of the product layer during the limestone indirect sulfation process. 8

9 Results and Discussion m conversio on (%) Calciu Calcium conversion o C, Oxy-fuel 900 o C, Oxy-fuel 950 o C, Oxy-fuel 850 o C, Air Time (min) During the indirect sulfation During process, the rate directof sulfation calcium process, conversion it isfirst observed increase that and the The rate then decrease of calcium l i markedly. conversion i Before of indirect limestone 45 min, the sulfation sulfation declinesis controlled is gradually always with by chemical reaction reaction time. And and/or there gas is higher diffusioni than through that h theof porous direct no longer clearly observable product sulfation. layer. And And the the highest increasing final change in calcium conversion of calcium the conversion BET surface afterarea 60 min is after benefit 45 to min. enhance The was found at 900 o the reduction sulfation in the C under the reaction reaction rate. Once rate the should product be caused become O 2 /CO 2 combustion by dense, a decrease a relatively condition. of non- the BET porous surface product area, which exerts results a in moderate the decrease resistance of gas diffusion to gas rate penetration, through the and product reaction layer. rate becomes dramatically reduced at this stage. 9

10 Conclusion It was found that the increasing of temperature would promote the calcination of limestone and the high concentration of CO 2 would inhibit calcination of limestone. The formation process of the product layer is completely different betweeneen the direct and indirect sulfation and is almost the same during the indirect sulfation under the oxy-fuel and air combustion. The calcium conversion of indirect sulfation is always higher than that of direct sulfation. And the highest final calcium conversion after 60 min was found at 900 o C under the oxy-fuel combustion condition. 10

11 Corresponding author: address: (S.Y.Li) 11