Effect of Steam on Carbonation of CaO in Ca-Looping

Abstract

Ca-looping is an effective way to capture CO₂ from coal-fired power plants. However, there are still issues that require further study. One of these issues is the effect of steam on the Ca-looping process. In this paper, a self-madethermogravimetric analyzer that can achieve rapid heating and cooling is used to measure the change of sample weight under constant temperature conditions. The parameters of the Ca-looping are studied in detail, including the addition of water vapor alone in the calcination or carbonation stage and the calcination/carbonation reaction temperatures for both calcination and carbonation stages with water vapor. Steam has a positive overall effect on CO₂ capture in the Ca-looping process. When steam is present in both calcination and carbonation processes, it increases the decomposition rate of CaCO₃ and enhances the subsequent carbonation conversion of CaO. However, when steam was present only in the calcination process, there was lower CaO carbonation conversion in the following carbonation process. In contrast, when steam was present in the carbonation stage, CO₂ capture was improved. Sample characterizations after the reaction showed that although water vapor had a negative effect on the pore structure, adding water vapor increased the diffusion coefficient of CO₂ and the carbonation conversion rate of CaO.

Keywords: Ca-looping; carbonation conversion; catalysis; steam.

Figure 1

Sample weight vs. time for eight cycles of calcination/carbonation. (a) All calcination and carbonation periods lasted 300 s except the first calcination period, which was 360 s. (b) Calcination was completed and sent immediately to the carbonation furnace for carbonation, and all the carbonation periods were 300 s.

Figure 2

Influence of steam when it was present during both the calcination and carbonation stages on sorbent reactivity of BD limestone: calcination (80% CO₂ + 0%/10%/20% H₂O + O₂ balance, 900 °C); carbonation (15% CO₂+ 0%/10%/20% H₂O + N₂ balance, 650 °C).

Figure 3

Influences of steam on conversion of CaO calcined from BD limestone: calcination (80% CO₂ + 0%/10%/20% H₂O + O₂ balance, 900 °C); carbonation (15% CO₂ + N₂ balance, 650 °C).

Figure 4

Conversion vs. time curves of BD limestone: calcination (80% CO₂ + 0%/20% H₂O + O₂ balance, 900 °C); carbonation (15% CO₂ balance N₂, 650 °C).

Figure 5

Influences that adding steam only during the carbonation stage has on sorbent reactivity of BD limestone: calcination (80% CO₂, O₂ balance, 900 °C); carbonation (15% CO₂ + 0%/10%/20% H₂O + N₂ balance, 650 °C).

Figure 6

Conversion vs. time curves of different cycle numbers of BD limestone: calcination without water vapor (80% CO₂ + O₂ balance, 900 °C); only carbonation with vapor (15% CO₂ + 0%/20% H₂O + N₂ balance, 650 °C).

Figure 7

Influences of calcination temperature on sorbent reactivity of BD limestone: calcination (80% CO₂ + 0%/20% H₂O + O₂ balance) at different temperatures; carbonation (15% CO₂ + 0%/20% H₂O + N₂ balance) at 650 °C.

Figure 8

Decomposition curves of BD limestone calcination at 900 °C and 950 °C in 80% CO₂ + 0%/20% H₂O + O₂ balance.

Figure 9

Influences of carbonation temperature on sorbent reactivity of BD limestone: calcination (900 °C, 80% CO₂ + 0%/20% H₂O + O₂ balance); carbonation (15% CO₂ + 0%/20% H₂O + N₂ balance).

Figure 10

CaO carbonation conversion vs. time at different temperatures. (CaO was derived from BD limestone, calcination was at 900 °C in 80% CO₂ + O₂ balance, and carbonation was in 15% CO₂ + 0%/20% H₂O +N₂ balance.)

Figure 11

SEM images of samples under different conditions: (a) CaCO₃, (b) CaO, (c) CaO (calcined with water), and (d) CaCO₃ (calcined in water).

Figure 12

De of CO₂ vs. temperature in the presence of steam.

Figure 13

Tube furnace experimental system.