外文文献英文NAOH吸收CO2

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2009 International Conference on Energy and Environment Technology

Experimental Studies on CO2 Capture in a Spray Scrubber using NaOH Solution

Niu Zhenqi, Guo Yincheng*, Lin Wenyi

Department of Engineering Mechanics, School of Aerospace

Tsinghua University Beijing 100084, China E-mail: guoyc@

Abstract—Experimental studies on carbon dioxide capture in a plate tower and bubble reactor. While, the spray scrubber spray scrubber are carried out. Fine spray of sodium presents many advantages as gas-liquid contactors such as hydroxide (NaOH) solution is used as CO2 absorbent. Effects high levels of gas treatment efficiency, low pressure drops, of different operating and design parameters, including the possibility to work in a wide range of liquid to gas flow concentration of NaOH solution, liquid flow rate, total gas flow rate ratios, and low investment costs [8]. A strategy, rate, initial temperature and concentration of carbon dioxide suggested by Storaloff et al. [9], is to generate a ne spray of on CO2 removal efficiency are investigated. the absorbing solution for providing large surface to the

atmospheric air ow through an open tower. Storaloff et al.

Keywords-carbon dioxide capture; NaOH solution; spray;

[9] studied the feasibility of a NaOH spray-based contactor flue gas

by estimating the cost and energy requirement per unit CO2 captured. However, the flue gas that power plants generate is

I. INTRODUCTION not taken into account. And the operation and design

The problem of global warming due to increasing parameters’ study is lack. So, in this work, the performance atmospheric CO2 concentration is arguably the most of NaOH spray scrubber is evaluated experimentally under important environmental issue that the world faces today. It various conditions to study effects of operation parameters, is imperative to reduce carbon emission by using new energy, including concentration of NaOH solution, liquid flow rate, saving energy, improving energy efficiency and carbon total gas flow rate, initial temperature and concentration of

carbon dioxide. capture and storage (CCS). In the coming few decades, new

energy will not play a major role in the energy supply. The

II. EXPERIMENTAL SETUP AND REACTION MECHANISM contribution to the carbon emission reduction through saving

energy and improving energy efficiency is after all limited

[1]. It has huge potential for the contribution to carbon A. Scrubber for CO2 Capture

A novel scrubber for carbon dioxide capture is designed, emission reduction by CCS, which will cut down the

consumption of climate change mitigation [2]. Fossil-fired

which is combined with the NaOH solution fine spray and discharged CO2more in China, so CO2plants is significant. Several technologies of CO2capture CO2(MEA) process for CO2In wet scrubbing techniques, CO2CO2solution

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laboratory-scale reactor. The schematic diagram of experimental for studying the removal of CO2 by NaOH solution fine spray is shown in Figure 1. The CO2 and NaOH solution reactor is made of stainless steel with 120 mm inner diameter and 1300 mm height. The artificial flue gases are obtained from the mixture of pure CO2 gas and N2 gas from cylinders. The influent mixture gas and the NaOH solution are heated to the desired operating temperature by electric heaters before fed into the reactor. A thermocouple is placed at each inlet of the flue gas and the NaOH solution and the temperatures are continuously monitored. The CO2 concentration is measured by the CO2 analyzer. Before the sampling gas entering the analyzer, its water vapor due to the evaporation of the NaOH solution must be removed. At first, the sampling gas is induced into the small gas dryer filled with desiccant CaCl2, where the most of water vapor is absorbed. Then, the sampling gas enters into a filter where the left water vapor is absorbed.

B. Spray of NaOH Solution

In order to make CO2 and NaOH solution contact and react thoroughly, two atomizers are placed at the upper part of the reactor and the Sauter mean diameters (SMD) of the NaOH solution spray are 30 μm to 40 μm when the pressure of the pump are 0.69 MPa to 1.11 MPa, and flue gases are fed into the reactor from the bottom of the reactor, thus the fine spray of NaOH solution and flue gas stream are in counter flow pattern.

C. Reaction of CO2 with NaOH Solution

The absorption of CO2 into hydroxide solutions has been widely studied. The stoichiometric equation of CO2 reaction with hydroxide solutions may be written as [10]

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), experiments are carried out at two different initial temperature of 280C and 350C respectively. It can be found that the concentration of NaOH solution plays an important role on the CO2 removal efficiency. When the value of NaOH solution concentration is 2% (w/w), the CO2 removal efficiencies are only 58.9% and 63.2% at the two different initial temperature of 280C and 350C respectively. With the values of concentration of NaOH solution increasing, the CO2 removal efficiency increases to the high level. The maximum value of CO2 removal efficiency is achieved when the concentration of NaOH solution is 10% (w/w) at the temperature of 350C. Whereas, when the concentration of NaOH solution is higher than 10% (w/w), the CO2 removal efficiency is reduced. When the value of the concentration of NaOH solution is 15% (w/w), the CO2 removal efficiency is 90.7% which is lower than the maximum value of 95.4% when the concentration of NaOH solution is 10% (w/w).

TABLE I.

Concentration of NaOH Solution w/w, %

Rate of NaOH Solution ml/min

EXPERIMENTAL PARAMETERS Gas Flow Rate L/min

Concentration

Temperature

of CO2 at the 0

C

Inlet, v/v, %

7

CO2(l) + 2OH → CO32 + Η2Ο. (1)

10095

The reaction rate is normally given by

CO2 removal efficiency, %

90

85

r = k2[CO2][OH ]

80757065605550The second-order rate constant (k2) for the CO2-NaOH system has been correlated as a function of activation energy and ionic strength (IC) [11]:

log(k2) = 11.895 2382/Τ + 0.221ΙC 0.016Ι

III. RESULTS AND DISCUSSIONS

The effects of several operating and design parameters such as concentration of NaOH solution, CO2 inlet concentration, total gas flow rate, NaOH solution flow rate and initial temperature on the CO2 removal efficiency have been studied. Detailed parameters in experiments are given in Table I.

A. Effect of Concentration of NaOH Solution

Figure 2 shows the CO2 removal efficiency under different concentrations of NaOH solution. In these cases,

2

C. (3)

NaOH concentration, % (w/w)

Figure 2. Effect of concentration of NaOH solution on CO2 removal

efficiency

In general, the CO2 removal efficiencies at 350C are higher than that at 280C. The difference of the CO2 removal efficiency between two temperatures is from 1.5% to 4.3% at different concentrations of NaOH solution.

B. Effect of NaOH Solution Flow Rate

The influence of NaOH solution flow rate on the CO2 removal efficiency is investigated. Figure 3 shows the CO2

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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

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NaOH to CO2 is larger than 4.43, the difference between CO2 removal efficiencies at different experimental conditions is small. So, the equivilence ratio of NaOH to CO2 is suggested to have the value of 4.43 in order to save the reaction material of NaOH.

100

CO2 removal efficiency, %

95

90

The CO2 removal efficiencies are measured at different concentrations of NaOH solution, liquid flow rates, total gas flow rates, initial temperatures and CO2 inlet concentrations. Experimental results show 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. Besides, an increase in temperature results in higher absorption

performance and higher CO2 removal efficiency.

Experimental results show that the equivalence ratio of NaOH solution to CO2 flow rate is a key parameter and plays an important role in CO2 removal efficiency.

ACKNOWLEDGMENT

This research was supported by Beijing Municipal Commission for Science & Technology under Grant No.Z08040902950803.

85

80

CO2 inlet concentration, % (v/v)

Figure 6. Effect of inlet concentration of carbon dioxide on CO2 removal

efficiency

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Furthermore, when the equivilence ratio of NaOH to CO2 and the ratio of NaOH flow rate to total gas flow rate have nearly the same values at different conditions, the CO2 removal efficiencies have the nearly same values. In Figure 3, the concentration of NaOH solution is 5%, the flow rate of NaOH solution is 200 ml/min, the total gas flow rate is 7.6 l/min, and the initial temperature of the reactor is 280C, the inlet concentration of CO2 is 15% (v/v). The measured CO2 removal efficiency is 91.7%. In Figure 6, 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 320C, the inlet concentration of CO2 is 13% (v/v). The measured CO2 removal efficiency is 91.8% which is as the nearly same value as above in Figure 3.

TABLE II.

Equivilence Ratio of NaOH to CO2 CO2 Removal Efficiency, %

EQUIVALENCE RATIOS IN EXPERIMENTS

In Figure 3, the calculed equivilence ratio of NaOH to CO2 is 4.92 at the above experimental conditions, and the ratio of NaOH flow rate to total gas flow rate is 0.0263 l/l. In Figure 6, the calculed equivilence ratio of NaOH to CO2 is 4.88, and the ratio of NaOH flow rate to total gas flow rate is 0.0237 l/l. The difference of the equivilence ratio and the ratio of NaOH flow rate to total gas flow rate between these two cases is small, it results in small difference of CO2 removal efficiency between these two cases.

IV. CONCLUSIONS

Experimental studies on carbon dioxide capture are carried out in a scrubber using fine spray of NaOH solution.

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