Experimental Studies on CO 2 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:******************
Abstract —Experimental studies on carbon dioxide capture in a spray scrubber are carried out.  Fine spray of sodium hydroxide (NaOH) solution is used as CO 2 absorbent.  Effects of different operating and design parameters, including concentration of NaOH solution, liquid flow rate, total gas flow rate, initial temperature and concentration of carbon dioxide on CO 2 removal efficiency are investigated. Keywords-carbon dioxide capture; NaOH solution; spray;
flue gas
I.  I NTRODUCTION  The problem of global warming due to increasing atmospheric CO 2 concentration is arguably the most important environmental issue that the world faces today. It is imperative to reduce carbon emission by using new energy, saving energy, improving energy efficiency and carbon capture and storage (CCS).  In the coming few decades, new
energy will not play a major role in the energy supply. The
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 emission reduction by CCS, which will cut down the
consumption of climate change mitigation [2]. Fossil-fired
discharged CO 2more in China, so CO 2plants is significant. Several technologies of CO 2capture CO 2(MEA) process for CO 2In wet scrubbing techniques, CO 2CO 2plate tower and bubble reactor. While, the spray scrubber presents many advantages as gas-liquid contactors such as high levels of gas treatment efficiency, low pressure drops, the possibility to work in a wide range of liquid to gas flow rate ratios, and low investment costs [8]. A strategy, suggested by Storaloff et al. [9], is to generate a fine spray of the absorbing solution for providing large surface to the
atmospheric air flow through an open tower. Storaloff et al.
[9] studied the feasibility of a NaOH spray-based contactor by estimating the cost and energy requirement per unit CO 2 captured. However, the flue gas that power plants generate is
not taken into account. And the operation and design
parameters’ study is lack. So, in this work, the performance of NaOH spray scrubber is evaluated experimentally under various conditions to study effects of operation parameters, including concentration of NaOH solution, liquid flow rate, total gas flow rate, initial temperature and concentration of
carbon dioxide. II.  E XPERIMENTAL S ETUP AND R EACTION M ECHANISM  A. Scrubber for CO 2 Capture
A novel scrubber for carbon dioxide capture is designed, which is combined with the NaOH solution fine spray and solution
2009  International Conference on Energy and Environment Technology
laboratory-scale reactor. The schematic diagram of experimental for studying the removal of CO 2 by NaOH solution fine spray is shown in Figure 1. The CO 2 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 CO 2 gas and N 2 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 CO 2 concentration is measured by the CO 2 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 CaCl 2, 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 CO 2 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 CO 2 with NaOH Solution
The absorption of CO 2 into hydroxide solutions has been widely studied. The stoichiometric equation of CO 2 reaction with hydroxide solutions may be written as [10]
CO 2(l)  + 2OH − → CO 32− + Η2Ο. (1)
The reaction rate is normally given by
r  = k 2[CO 2][OH −]
The second-order rate constant (k 2) for the CO 2-NaOH system has been correlated as a function of activation energy and ionic strength (I C ) [11]:
log(k 2) = 11.895 − 2382/Τ + 0.221ΙC  − 0.016Ι
C 2
. (3)
III. R ESULTS AND D ISCUSSIONS
The effects of several operating and design parameters such as concentration of NaOH solution, CO 2 inlet concentration, total gas flow rate, NaOH solution flow rate and initial temperature on the CO 2 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 CO 2 removal efficiency under different concentrations of NaOH solution. 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 CO 2 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 CO 2 removal efficiency.  When the value of NaOH solution concentration is 2% (w/w), the CO 2 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 CO 2 removal efficiency increases to the high level.  The maximum value of CO 2 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 CO 2 removal efficiency is reduced.  When the value of the concentration of NaOH solution is 15% (w/w), the CO 2 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.
E XPERIMENTAL P ARAMETERS  Concentration of NaOH Solution w/w, %
Flow Rate of NaOH Solution ml/min
Total Gas Flow Rate L/min
Temperature
C
Concentration
of CO 2 at the Inlet, v/v, %
2~15 180 7.6 28, 35 15 5 120~200 7.6 28 15
5 180
7.6~24.7
28 15
5 180 7.
6 28~54 15 5 180 7.6 32 7~15
5055606570758085
90
95
100
C O 2 r e m o v a l  e f f i c i e n c y , %
NaOH concentration, % (w/w)
Figure 2. Effect of concentration of NaOH solution on CO 2 removal
efficiency
In general, the CO 2 removal efficiencies at 350C are higher than that at 280C.  The difference of the CO 2 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 CO 2 removal efficiency is investigated.  Figure 3 shows the CO 2
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 CO 2 at the inlet is 15% (v/v), and the initial temperature of the reactor is 280C.  Experimental results show that the CO 2 removal efficiency increases from 81% to 91.7% when the NaOH solution flow rate increasing from 120 ml/min to 200 ml/min.
75
80
85
90
95reactor technology
C O 2 r e m o v a l  e f f i c i e n c y , %
NaOH volume flow rate, ml/min
Figure 3. Effect of NaOH flow rate on CO 2 removal efficiency
C. Effect of Total Gas Flow Rate
Figure 4 shows the CO 2 removal efficiency when total gas flow rate of CO 2 and N 2 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 CO 2 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 remarka
ble effect on the CO 2 removal efficiency.  It is found that the CO 2 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 CO 2 removal efficiency at high total gas flow rate is that the reaction between CO 2 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 CO 2 and NaOH solution spray is reduced which gives rise to low CO 2 removal efficiency.
C O 2 r e m o v a l  e f f i c i e n c y , %
gas total volume flow rate, l/min
Figure 4. Effect of total gas flow rate on CO 2 removal efficiency
D. Effect of Initial Temperature in the Tower
Figure 5 shows the CO 2
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 CO 2 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).
8085
90
95
100
C O 2 r e m o v a l  e f f i c i e n c y , %
The initial temperature in the tower, o
C
Figure 5. Effect of initial temperature in the tower on CO 2 removal
efficiency
E. Effect of Inlet Concentration of Carbon Dioxide
The influence of CO 2 inlet concentration on the CO 2 removal efficiency is also investigated.  Figure 6 shows the CO 2 removal efficiency when the inlet concentration of CO 2 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 CO 2 removal efficiency is larger than 90% at different CO 2 inlet concentrations.  The CO 2 removal efficiency declines a little with the inlet concentrations of CO 2 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 CO 2 are beneficial to promote CO 2 removal efficiency. According to the experimental parameters given in Table I, equivalence ratios are calculated and given in Table II together with CO 2 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 CO 2 is a key parameter when comparing CO 2 removal efficiency at different experimental conditions.  In general, with the equivilence ratio of NaOH to CO 2 increasing, the CO 2 removal efficiency increases.  It seems that there exists a critical value of the equivilence ratio of NaOH to CO 2.  Thus, in order to achieve a higher CO 2 removal efficiency, the equivilence ratio of NaOH to CO 2 should be larger than 4.43 as shown in Table II.  Whereas, when the equivilence ratio of
NaOH to CO 2 is larger than 4.43, the difference between CO 2 removal efficiencies at different experi
mental conditions is small.  So, the equivilence ratio of NaOH to CO 2 is suggested to have the value of 4.43 in order to save the reaction material of NaOH.
80
85
90
95
100
C O 2 r e m o v a l  e f f i c i e n c y , %
CO 2 inlet concentration, % (v/v)
Figure 6. Effect of inlet concentration of carbon dioxide on CO 2 removal
efficiency
Furthermore, when the equivilence ratio of NaOH to CO 2 and the ratio of NaOH flow rate to total gas flow rate have nearly the same values at different conditions, the CO 2 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 CO 2 is 15% (v/v).  The measured CO 2 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 CO 2 is 13% (v/v).  The measured CO 2 removal efficiency is 91.8% which is as the nearly same value as above in Figure 3.
TABLE II.
E QUIVALENCE R ATIOS IN E XPERIMENTS
Equivilence Ratio of NaOH to CO 2    1.72 4.43 4.88 5.72 6.91 7.31 8.72 CO 2 Removal Efficiency, %
58.9 90.2 91.8 93.2 93.7 92.2 93.5
In Figure 3, the calculed equivilence ratio of NaOH to CO 2 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 CO 2 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 CO 2 removal efficiency between these two cases.
IV. C ONCLUSIONS
Experimental studies on carbon dioxide capture are carried out in a scrubber using fine spray of NaOH solution.
The CO 2 removal efficiencies are measured at different concentrations of NaOH solution, liquid flow rates, total gas flow rates, initial temperatures and CO 2 inlet concentrations.  Experimental results show that the higher concentration of NaOH solution, the larger flow rate of NaOH solution and the low
er flow rate of total gas mixture of nitrogen and CO 2 are beneficial to promote CO 2 removal efficiency.  Besides, an increase in temperature results in higher absorption
performance and higher CO 2 removal efficiency.
Experimental results show that the equivalence ratio of NaOH solution to CO 2 flow rate is a key parameter and plays an important role in CO 2 removal efficiency.
A CKNOWLEDGMENT
This research was supported by Beijing Municipal Commission for Science & Technology under Grant No.Z08040902950803.
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