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removal efficiency profile at different NaOH solution flow rates. In these cases, the concentration of NaOH solution is 5%, the total gas flow rate is 7.6 l/min, the concentration of CO2 at the inlet is 15% (v/v), and the initial temperature of the reactor is 280C. Experimental results show that the CO2 removal efficiency increases from 81% to 91.7% when the NaOH solution flow rate increasing from 120 ml/min to 200 ml/min.

95

CO2 removal efficiency, %

90

D. Effect of Initial Temperature in the Tower

Figure 5 shows the CO2 removal efficiency under

different initial temperature of the reactor. In these cases, the flow rate of NaOH solution is 180 ml/min. the total gas flow rate is 7.6 l/min, the concentration of CO2 at the inlet is 15% (v/v), and the concentration of NaOH solution is 5% (w/w). In the experiments, the initial temperatures of the tower are 280C, 320C, 350C, 380C, 450C and 540C. It can be found that an increase in temperature results in higher absorption performance, which is primarily caused by the increasing absorption rate constant as described in (1).

100

85

80

CO2 removal efficiency, %

95

75

85

NaOH volume flow rate, ml/min

80o

Figure 3. Effect of NaOH flow rate on CO2 removal efficiency

The initial temperature in the tower, C

90

C. Effect of Total Gas Flow Rate

Figure 4 shows the CO2 removal efficiency when total gas flow rate of CO2 and N2 changing from 7.6 l/min to 24.7 l/min. In these cases, the concentration of NaOH solution is 5%, the flow rate of NaOH solution is 180 ml/min, the concentration of CO2 at the inlet is 15% (v/v), and the initial temperature of the reactor is 280C. Experimental results show that the total gas flow rate has remarkable effect on the CO2 removal efficiency. It is found that the CO2 removal efficiency declines from 90.2% to 41% when the total gas flow rate changing from 7.6 l/min to 24.7 l/min under the above experimental conditions. The main reason for low CO2 removal efficiency at high total gas flow rate is that the reaction between CO2 and NaOH solution is insufficient. With total gas of flow rate increasing, the velocity of the mixture of carbon dioxide and nitrogen increases. Thus, the contract time between CO2 and NaOH solution spray is reduced which gives rise to low CO2 removal efficiency.

Figure 5. Effect of initial temperature in the tower on CO2 removal

efficiency

CO2 removal efficiency, %

gas total volume flow rate, l/min

Figure 4. Effect of total gas flow rate on CO2 removal efficiency

E. Effect of Inlet Concentration of Carbon Dioxide

The influence of CO2 inlet concentration on the CO2 removal efficiency is also investigated. Figure 6 shows the CO2 removal efficiency when the inlet concentration of CO2 changing from 7% to 15% (v/v). In these cases, the concentration of NaOH solution is 5%, the flow rate of NaOH solution is 180 ml/min, the total gas flow rate is 7.6 l/min, and the initial temperature of the reactor is 280C. Under the above experimental conditions, experimental results show that the CO2 removal efficiency is larger than 90% at different CO2 inlet concentrations. The CO2 removal efficiency declines a little with the inlet concentrations of CO2 increasing.

From Figure 2, Figure 3 and Figure 4, we can find that the higher concentration of NaOH solution, the larger flow rate of NaOH solution and the lower flow rate of total gas mixture of nitrogen and CO2 are beneficial to promote CO2 removal efficiency. According to the experimental parameters given in Table I, equivalence ratios are calculated and given in Table II together with CO2 removal efficiencies, the ratio of NaOH flow rate to total gas flow rate has the same value of 0.0237 l/l in the experiments given in Table II. It is found that the equivalence ratio of NaOH to CO2 is a key parameter when comparing CO2 removal efficiency at different experimental conditions. In general, with the equivilence ratio of NaOH to CO2 increasing, the CO2 removal efficiency increases. It seems that there exists a critical value of the equivilence ratio of NaOH to CO2. Thus, in order to achieve a higher CO2 removal efficiency, the equivilence ratio of NaOH to CO2 should be larger than 4.43 as shown in Table II. Whereas, when the equivilence ratio of