Attrition of limestone during calcium looping cycles for CO 2 capture in fluidized bed

Transcription

1 Attrition of limestone during calcium looping cycles for CO 2 capture in fluidized bed Antonio Coppola, Fabio Montagnaro, Piero Salatino and Fabrizio Scala This work has been carried out in the framework of the European Commission Research Fund for Coal and Steel Contract no. RFCR-CT (CAL-MOD)

2 Scope of this work Attrition of an Italian limestone during calcium looping cycles for CO 2 capture was studied in a lab-scale FB apparatus. Batch experiments under alternating calcination-carbonation conditions were carried out to investigate the effect of chemical reactions and temperature changes on the attrition propensity of the sorbent particles. Attrition processes were characterized by following the modifications of bed sorbent PSD and the elutriation rates of fines throughout conversion over repeated cycles. Different bed temperatures and CO 2 and SO 2 inlet concentrations were tested in the experiments.

3 Experimental apparatus 40 mm ID (1) gas preheating/premixing section; (2) electrical furnaces; (3) ceramic insulator; (4) gas distributor; (5) thermocouple; (6) fluidization column; (7) two-exit head; (8) sintered brass filters; (9) hopper; (10) SO2 scrubber; (11) stack; (12) cellulose filter; (13) membrane pump; (14) gas analyzers; (15) personal computer; (16) manometer; (17) mass flow meter/controller.

4 Experimental conditions Tests without SO 2 Calcination/Carbonation Condition 1 Condition 2 Condition 3 Duration [min] 15/15 35/15 20/15 Temperature [ C] 850/ / /700 Inlet CO 2 [%v/v] (balance air) 0/16 20/16 44/16 Tests with SO 2 Calcination/Carbonation Condition 4 Condition 5 Duration [min] 35/15 35/15 Temperature [ C] 850/ /700 Inlet CO 2 [%v/v] 20/16 20/16 Inlet SO 2 [ppmv] 0/1800 0/110

5 Experimental procedures Five calcination/carbonation cycles were carried out under all experimental conditions; An initial amount of 20 g of fresh limestone, sieved in the range mm was fed to a bed consisting of 150 g of silica sand in the range mm; The fluidizing velocities were 0.7 and 0.6 m/s in the calcination and in the carbonation stages, respectively; The progress of calcination/carbonation (and sulphation) was followed by measuring the CO 2 (and SO 2 ) concentration at the exhaust; The duration of each stage was such that calcination/carbonation was complete; After each step the bed was discharged, the limestone was separated from the sand by sieving, and its PSD was measured; During the runs the rate of fines generation was determined by measuring the amount of fines elutriated from the reactor by means of the brass filters.

6 Results: CO 2 capture capacity (no SO 2 ) Capture capacity [gco 2 /initial gcaco 3 ] 0,30 0,25 0,20 0,15 0,10 0,05 0,00 Condition 1 Condition 2 Condition 3 Cycle # Highest capacity: Condition 1 (calcination in air); Slightly lower capacity: Condition 2 (calcination in CO 2 ); Lowest capacity: Condition 3 (calcination in CO 2 at higher temperature 900 C).

7 Results: PSD changes (no SO 2 ) in Condition 1 1,0 0,8 1 st calcination 3 rd carbonation 5 th carbonation 0,05 0,04 1 st calcination 3 rd carbonation 5 th carbonation PSD [µm -1 ] 0,6 0,4 PSD [µm -1 ] 0,03 0,02 0,2 0,0 26,5 82,5 146,0 196,0 231,0 275,0 327,5 d [µm] 377,5 500,0 0,01 0, d [µm] The PSD does not change appreciably during the cycles, indicating a limited occurrence of particle fragmentation; The only significant difference is the amount of fines (<100 µm) appearing after the first calcination that was not found in the following cycles.

8 Results: PSD changes (no SO 2 ) after 5 th cycle Condition 1 Condition 2 Condition Condition 1 Condition 2 Condition 3 PSD [µm -1 ] PSD [µm -1 ] d [µm] d [µm] Tests carried out under the three experimental conditions showed the same qualitative trend.

9 Results: Elutriation rate (no SO 2 ) in Condition 1 0,10 1 st cycle 2 nd cycle 3 rd cycle 4 th cycle 5 th cycle E [g/min] 0,08 0,06 0,04 Calcination Carbonation 0,02 0, Time [min] Fines elutriation rate decreases with the number of cycles; In each cycle the elutriation rate shows an initial peak, caused by a combination of rounding off, thermal shock and rapid release of CO 2 (only during calcination).

10 Results: Elutriation rate (no SO 2 ) in Condition 1 0,05 0, E [g/min] 0,03 0,02 4 %CO 2 [v/v] 3 0, , Time [min] 0 In Condition 1, during 1 st calcination, E shows a peak shifted to about 2 min after the beginning, approximately at the same time as the CO 2 concentration peak; The peak might be related to the large rate of CO 2 release during calcination.

11 A Condition 1 Calcination Carbonation Results: Total amount of elutriated fines (no SO 2 ) B C Condition 2 Condition 3 Cycle # In all conditions the amount of fines decreases with the cycle number; When considering that the calcination stages have a different duration under the different conditions, the average fines elutriation rate is approximately the same during calcination and carbonation stages; The largest amount of fines was obtained in the tests carried out in Condition 1; Fines generation was much smaller under Condition 2 (20% CO 2 during calcination); When the calcination temperature was increased at 900 C (Condition 3), a larger amount of fines was collected.

12 Results: CO 2 capture capacity (with SO 2 ) Capture Capacity [gco 2 /initial gcaco 3 ] 0,30 0,25 0,20 0,15 0,10 0,05 Capture Capacity 1800ppm 110ppm 0ppm 0,00 Cycle # The presence of SO 2 in the gas decreases the CO 2 capture capacity of the sorbent; The effect of the SO 2 concentration level is not as large as expected.

13 Results: Ca conversion to CaSO 4 SO ppm SO 2 110ppm %X Ca 8 %X Ca Cycle # Cycle # The effect of the SO 2 concentration level on Ca conversion to CaSO 4 is large.

14 Etot [g] 1 0,1 0,01 SO 2 0ppm Calcination Carbonation Results: Total amount of elutriated fines (with SO 2 ) Etot [g] Etot [g] 0, ,1 0,01 0, ,1 0,01 SOX Data 2 110ppm X Data SO ppm Calcination Carbonation Calcination Carbonation In all conditions the amount of fines decreases with the cycle number; Fines elutriation increases when SO 2 is present in the gas during carbonation; This effect is enhanced at high SO 2 concentration, especially during the carbonation stage; No significant difference was found in the PSD analysis when SO 2 was present in the gas during carbonation. 0,001 Cycle #

15 Conclusions The limestone exhibited the typical decrease of the uptake capacity with increasing number of cycles. The presence of CO 2 during calcination and especially a higher calcination temperature determined a decrease of sorbent capacity for all the cycles, most likely due to the enhancement of particle sintering. The analysis of PSD of bed material over repeated calcination/carbonation cycles indicated that particle fragmentation was always very limited. The fines elutriation rate was relatively large during the first cycle and decreased with the number of cycles. The presence of CO 2 during calcination led to a significant decrease of fines generation, while a higher calcination temperature produced an increase of the particle attrition rate. The presence of SO 2 in the gas decreased the CO 2 capture capacity of the sorbent, but did not affect significantly the attrition rate.

16 THANK YOU and see you in Naples! (... a few days left for abstract submission...)