An investigation on NO removal by wet scrubbing using NaClO 2 seawater solution

The experiments of cyclic scrubbing the flue gas continuously by using different solutions
to remove NO had been conducted. Figure 2 shows the changes of NO concentrations in exit flue gas during the scrubbing process.
From the results, we can see that when seawater without NaClO
₂
addition is used to treat the flue gas, NO concentration in exit flue gas nearly does
not change, which demonstrates that seawater itself does not react with NO. For both
freshwater and seawater, when there is 20 mM NaClO
₂
added in the solution, NO concentration decreases sharply to zero within several minutes
at the beginning of the scrubbing process. It suggests that NO in flue gas has been
completely removed. With the proceeding of the cyclic scrubbing process, the complete
removal of NO can last for tens of minutes. Finally, NO concentration increases fast
to the initial level (about 1000 ppm), which indicates that NaClO
₂
in the solution has been depleted.

Fig. 2. NO absorption of 20 mM NaClO
₂
scrubbing solution

From Fig. 2, it can be seen that, with the same concentration of NaClO
₂
in the solution, the breakthrough times, which refers to the duration of complete
removal of NO from the flue gas, are obviously different from freshwater to seawater.
The breakthrough times for NO absorption by scrubbing solution containing various
concentrations (5–20 mM) of NaClO
₂
are illustrated in Fig. 3.

Fig. 3. The breakthrough times for NO absorption for cyclic scrubbing of NaClO
₂
solutions

When the concentration of NaClO
₂
in the scrubbing solution is 5 mM, a complete removal of NO from the flue gas can
not be achieved during the whole scrubbing process, so the breakthrough time is zero
for 5 mM NaClO
₂
solution. From Fig. 3, one can see that, the breakthrough times for NaClO
₂
seawater solution have been enhanced obviously compared with those for freshwater
solution of the same concentration of NaClO
₂
oxidant. When the concentration of NaClO
₂
is 15 mM, the breakthrough time for seawater can be improved by 34.3 % compared with
that for freshwater. Since the breakthrough time for NO absorption during the cyclic
scrubbing process represents the ability of NO absorption for the scrubbing solution,
NaClO
₂
seawater exhibits much higher NO absorption potential than NaClO
₂
freshwater.

Figure 4 shows the change of NO
₂
concentration in exit flue gas during the cyclic scrubbing process. As the simulated
flue gas treated in our experiments is the mixture of N
₂
and NO, the initial concentration of NO
₂
in exit flue gas is zero. With the start of the scrubbing process, the concentration
of NO
₂
in exit flue gas increases gradually due to the oxidation of NO into NO
₂
by NaClO
₂
(Brogren et al. 1998]).

(1)

The equilibrium reaction between NO and NO
₂
will occur simultaneously as in Eqs. (2) and (3).

(2)

(3)

It follows that some NO
_x
are absorbed via the hydrolysis of N
₂
O
₃
and N
₂
O
₄
as in Eqs. (4) and (5).

(4)

(5)

Fig. 4. The change in concentrations of NO
₂
in exit flue gas during the scrubbing process

Both N
₂
O
₃
and N
₂
O
₄
are very easy to react with H
₂
O, so most of them will be removed during the wet scrubbing process. To some extent,
N
₂
O
₃
and N
₂
O
₄
can be considered as intermediate products. The NO
_x
in exit flue gas are mainly NO and/or NO
₂
. The reaction between NO and NO
₂
, as well as the hydrolysis of N
₂
O
₃
and N
₂
O
₄
, will not affect the analysis result of NO
_x
absorption obviously.

Figure 4 also shows that, for 20 mM NaClO
₂
seawater solution, the concentration of NO
₂
in exit flue gas is more stable than that of freshwater solution, suggesting that
seawater is better for balancing the NO absorption during the cyclic scrubbing process.
Besides, for 20 mM NaClO
₂
seawater solution, the total removal efficiency of NO
_x
(here it is the sum of NO and NO
₂
) can be approximately calculated as 70 % during the cyclic scrubbing process.

Compared with freshwater, artificial seawater used in our experiments contains 419.74 mM
NaCl, 54.62 mM MgCl
₂
, 28.79 mM Na
₂
SO
₄
, 10.45 mM CaCl
₂
, 9.32 mM KCl and 2.39 mM NaHCO
₃
. Apart from these six kinds of salts, there is a little NaOH used to adjust the mixture
solution pH to 8.2. Here it can be seen that, although the components of seawater
solution seem to be a little complex, it can be considered as a buffering solution,
because it does not react with NO directly. In order to investigate the effect of
the buffer capacity of seawater on NO absorption by NaClO
₂
solution, the experiments of titrating 20 mM NaClO
₂
solutions with 0.2 M HCl are performed. The changes in solution pH value with the
addition of HCl are shown in Fig. 5.

Fig. 5. The change of pH of NaClO
₂
(20 mM) scrubbing solution with addition of HCl (0.2 M)

It is known that water itself is a buffering medium, even in the absence of an added
buffering reagent. Unlike water, seawater is of natural alkalinity, defined as the
sum of the concentrations of the alkaline species contained in seawater. The value
of alkalinity is not fixed, but changes according to the geographical area. In a general
way, seawater alkalinity can be expressed as follows (Giuseppe et al. 2012]):

(6)

where the first species HCO₃^?
represents the dominating contribution to seawater alkalinity. It is related with
the seawater buffer capacity directly.

As shown in Fig. 5, the initial pH value of NaClO
₂
freshwater is about 10.5 and it drops sharply with the addition of HCl. An addition
of 6.6×10
^?2
mmol HCl can make the pH value of 0.2 L NaClO
₂
freshwater drop below 7. Whereas the initial pH value of NaClO
₂
seawater is 8.6, which is just a little higher than that of seawater alone. With the
addition of HCl, the pH value of NaClO
₂
seawater drops slowly. It requires 1.14×10
^?1
mmol HCl to make the solution pH drop to 7. The result shows that, for NaClO
₂
solution, seawater has a much better buffering ability in maintaining the solution
pH than freshwater. It is well known that the buffering ability of seawater is mainly
related with the bicarbonate salt which is only 0.48 mmol in 0.2 L seawater. Though
the amount of HCl addition, that can make pH value of NaClO
₂
seawater drop to below 7, seems to be much less compared with the molar quantities
of bicarbonate salt and NaClO
₂
(5 mmol) in the solution, the buffering ability of seawater might be high enough to
enhance the breakthrough time for NaClO
₂
seawater during the cyclic scrubbing experiments.

The changes of the solution pH during the cyclic scrubbing process are presented in
Fig. 6. For 20 mM NaClO
₂
freshwater solution, with the cyclic absorption of NO, the solution pH drops quickly
to below 4, resulting that the majority of the NO removal process is in acidic condition.
After scrubbing for several minutes, a greenish yellow color is observed in NaClO
₂
freshwater solution, which is confirmed to be ClO
₂
. ClO
₂
is formed by the acidic decomposition of NaClO
₂
in Eq. (7) (Yang and Shaw 1998])

(7)

ClO
₂
is found to be the active intermediate for NO oxidation. It does not react with water
or ionize in solution, so it could remain as a dissolved gas or be stripped out from
the liquid. ClO
₂
escaped from the liquid contributes to the oxidation of NO to NO
₂
in the gas phase

(8)

when the amount of ClO
₂
is slightly in excess of that needed for the NO oxidation, NO will be absorbed completely
during the breakthrough time. However, if the amount of ClO
₂
is largely in excess of that needed for the oxidation of NO, the redundant ClO
₂
will escape from the scrubber reactor without making contribution to the oxidation
of NO. One can deduce from Figs. 2, 6 that, for 20 mM NaClO
₂
freshwater solution, a certain amount of ClO
₂
has possibly escaped into the atmosphere due to the low pH value during the majority
process of NO absorption. In addition, it is reported that, in acidic medium (pH range
of 3.5–4.0), a reaction will occur in Eq. (9) (Brogren et al. 1998]).

(9)

The formation of ClO₃^?
consumes a small quantity of ClO₂^?
, which makes no contribution to the oxidation of NO, too.

Fig. 6. The change in solution pH value during the scrubbing process

It can be seen from Fig. 6 that, for NaClO
₂
seawater, the solution pH remains above 7 during the major part of the cyclic scrubbing
process owning to the buffering ability of seawater. The neutral or slightly alkaline
solution is beneficial to suppress the formation of ClO
₂
effectively. In alkaline medium, NO is directly oxidized by ClO₂^?
in Eq. (1) and NO
₂
is absorbed in Eq. (10).

(10)

Thus, the reason for the extension of the breakthrough time for NaClO
₂
seawater might be mainly ascribed to the improved utilization of NaClO
₂
. The acidic decomposition of NaClO
₂
for NaClO
₂
freshwater and further the escape of ClO
₂
into atmosphere have lowered the utilization of NaClO
₂
in freshwater scrubbing solution to some extent. The other factors (such as the transition
metal ions, ion strength and solution pH etc.) may also influence the NO absorption
by NaClO
₂
seawater, which will be explored in the future in detail and the results will be reported
in other forms.

To confirm the composition of reaction products, the anions in the spent liquor are
determined using ion chromatography. Ion chromatographic analysis of spent scrubbing
liquor and reagent blank is depicted in Fig. 7. Since the concentration of Cl^?
in seawater is relatively too high, it results in that ion chromatography can not
be used to analyze the spent liquor of NaClO
₂
seawater scrubbing solution directly. Therefore, the spent liquor of NaClO
₂
seawater scrubbing solution should be treated before doing ion chromatography analysis.
Firstly, the spent liquor is diluted 100 times in order to reduce the consumption
of Ag cartridge. Then, the diluted solution is filtered by Ag cartridge to remove
excessive Cl
^?
ions from the seawater solution. Actually, the spent liquor of NaClO
₂
freshwater scrubbing solution can be analyzed by ion chromatography directly. For
comparison, the spent liquor of NaClO
₂
freshwater is also treated by diluting 100 times, but it is not filtered by any Ag
cartridge.

Fig. 7. Ion chromatograms of absorption solution

Figure 7 indicates that Cl^?
and NO₃^?
are the major anion products in the spent liquor. There is no ClO₂^?
in the spent liquor because NaClO
₂
in the solution has been consumed completely during the cyclic scrubbing process.
For 20 mM NaClO
₂
freshwater, there is certain amount of ClO₃^?
in the spent solution, which may be formed by the acidic decomposition of ClO₂^?
in Eq. (9).

As shown in Fig. 7, the concentration of Cl^?
in freshwater represents the actual value of Cl^?
which exist in the spent liquor and comes from the NaClO
₂
oxidant. While the concentration of Cl^?
in seawater represents the value of Cl^?
left in the sample solution after being filtered by Ag cartridge.

Note that the treatment of the spent liquor does not affect the concentrations of
other ions, such as NO₃^?
and ClO₃^?
. As shown in Fig. 7, the concentration of NO₃^?
in spent liquor of NaClO
₂
seawater is a little higher than that in freshwater. It means that, with the same
concentration of NaClO
₂
, the seawater solution has absorbed more NO during the cyclic scrubbing process than
freshwater solution. It also demonstrates that, seawater is helpful to improve the
utilization of NaClO
₂
compared with freshwater.

According to the measurement results of ion chromatography, the NaClO
₂
utilization for the scrubbing solution may be calculated approximately as follows.
For 20 mM NaClO
₂
seawater, NO₃^?
ion concentration in the spent solution is about 2.72 mmol that can be calculated
from the measurement result of ion chromatograms. As NO₃^?
derives from the absorption of NO by NaClO
₂
through the Eq. (11), so one can deduce that, the molar quantity of NO₃^?
ions correspond to equal molar of NO, which has consumed 2.04 mmol NaClO
₂
during this process.

(11)

Because NO is absorbed completely during the breakthrough time, the amount of NO can
be calculated approximately as the product of the flow rate of flue gas, NO concentration
and the breakthrough time, which is about 4.02 mmol. From Eq. (1), we can see that, NO
₂
in the exit flue gas derives from NO oxidation by NaClO
₂
, which can be calculated by substracting the molar quantity of 2.72 mmol from the
molar quantity of 4.02 mmol. From Eq. (1), it can be seen that 1.3 mmol NO
₂
has consumed 0.65 mmol NaClO
₂
. Therefore, the sum of NaClO
₂
that has made contribution to the absorption of NO
_x
during the scrubbing process is 2.69 mmol. Since the addition of NaClO
₂
in the initial seawater solution is 4 mmol, the utilization of NaClO
₂
in seawater solution can be calculated as 67.25 %.

Similarly, for 20 mM NaClO
₂
freshwater, NO₃^?
ion concentration in the spent liquor is about 2.34 mmol, which has consumed 1.76 mmol
NaClO
₂
according to Eq. (12). NO that has been absorbed by NaClO
₂
freshwater during the whole breakthrough time is about 3.29 mmol. NO
₂
in the exit gas is 0.95 mmol, which has consumed 0.48 mmol NaClO
₂
accordingly. Thus the sum of NaClO
₂
that has made contribution to the absorption of NO
_x
during the whole scrubbing process is 2.24 mmol. Since the addition of NaClO
₂
in the initial freshwater solution is also 4 mmol, the utilization of NaClO
₂
in freshwater solution can be calculated as 56 %.

(12)

After in all, the utilization of NaClO
₂
in seawater is enhanced by about 20.1 % compared with that of NaClO
₂
in freshwater solution, which agrees well with the comparison result of the breakthrough
times in Fig. 3.