Mitsubishi Heavy Industries Technical Review Vol. 51 No. 3 (September 2014)11

Application of Electrolytic Denitrification System toTreatment of Waste Water from Power Plants

M i ts u b i s h i H e a v y I n d u s t r i e s
Environmental & Chemical Engineering
C o . , LT D .
Wa s t e R e s o u r c e R e c y c l i n g P l a n t
DivisionL i q u i d Wa s t e P r o c e s s & E l e c t r o l y s i sTe c h n o l o g y D e pa r t m e n tHydrazine has been used at thermal power plants as a water treatment agent. There is a trend,
however, toward banning the use of hydrazine mainly in Europe. As an alternative option, theadoption of “high-pH treatment (high-AVT)” is suggested.
The use of high-AVT results in an increase in ammoniacal nitrogen levels in waste waterfrom power plants. As the discharge of nitrogen is regulated in many countries, appropriate
measures should be taken. This report introduces an electrolytic denitrification system that is usefulfor abiding by the regulations on nitrogen discharge and is a proprietary technology of Mitsubishi
Heavy Industries Environmental & Chemical Engineering Co., Ltd.

|1. Background of emerging need for denitrification technology
When “high-pH treatment (high-AVT)” is adopted for water treatment at thermal power
plants instead of hydrazine, ammoniacal nitrogen levels in waste water increase, necessitating thatsuitable measures be undertaken.
(1) High-AVT-induced change in waste water qualities
The use of high-AVT results in an increase in ammoniacal nitrogen levels in boiler waste
water (Table 1).
Table 1 High-AVT-induced change in the quality of waste water
Adopted method
Ammoniacal nitrogen level inboiler water
Ammoniacal nitrogen level inthe storage tank(2)Hydrazine treatment

High-AVT

30 mg N/L

100 mg N/L

15 mg N/L or less

50 mg N/L

Trend of nitrogen regulations
As many countries have regulations on nitrogen in effluent, ammoniacal nitrogen
treatment is needed for waste water that is discharged from boilers with high-AVT (Table 2).
Table 2

Nitrogen regulations by country (Surveyed in 2012)

ItemTotal nitrogen
T-N

World BankJapanSaudi Arabia
Indonesia
VietnamChina
BrazilSouth Korea

Effluent criteria
Ammoniacal nitrogen
NH4-N
10 mg/L or less

120 mg/L(Daily average: 60 mg/L)
15 mg/L
60 mg/L

1 mg/L (30-day average)
1 mg/L (30-day average)
5 mg/L15 mg/L
5 mg/LMitsubishi Heavy Industries Technical Review Vol. 51 No. 3 (September 2014)12|2. Principle of electrolytic denitrification system
A chloride ion-containing solution such as seawater is electrolyzed to produce hypochlorousacid. Ammoniacal nitrogen is oxidized by the hypochlorous acid and is released as harmless N2 gas
into the air.
(1) Chlorine formation
Chlorine (Cl2) is produced at the anode. Chlorine is then instantaneously subjected todisproportionation decomposition near the electrode to form hypochlorous acid (HClO).
→ Cl2 + 2eAnode2ClCl2 + H2O→ HClO + HCl
Cathode 2H2O + 2e
→ 2OH- + H2
2NaOH
→ 2Na+ + 2OH(2) Nitrogen removal
Ammoniacal nitrogen in water reacts with the hypochlorous acid (HClO) produced as aresult of the chlorine formation reaction and is converted into harmless N2 gas.
2NH3 + 3HClO → N2 ↑ + 3HCl + 3H2O

|3. Ammonia removal from boiler waste water(1)

Verification of the effect on waste water from an actual power plant (boiler effluent)
The waste water from an actual power plant facility (boiler effluent) is mixed with anelectrolytic hypochlorous acid solution to evaluate the ammonia removal performance. It has
been confirmed that the ammoniacal nitrogen levels in the treated water meet the discharge
regulations (i.e., 10 mg/l or less), thus demonstrating that the treatment is sufficiently effective
for power plant waste water.
(2) How to control/detect the end point
Ammoniacal nitrogen-containing waste water from power plants is oxidized by mixing
with an electrolytic hypochlorous acid solution. In batch processing, as the ammoniacal
nitrogen contained in the mixture solution is removed, pH levels will change as shown in
Figure 1. Therefore, measured pH levels can be an indicator of the reaction end point in theoxidation treatment.
The necessary amount of hypochlorous acid is determined based on the ammoniacal
nitrogen level or the quantity of waste water. The supply of hypochlorous acid can be regulated
by controlling electrolytic currents, making it possible to keep up with changes in the quality ofwaste water, etc.

Figure 1 Confirmation of the reaction end point in theoxidation treatment|4. Application to the waste water process of power plants
Figure 2 shows the waste water treatment process of power plants, while the design
specifications for ammonia treatment are given in Table 3. Hypochlorous acid solution, which isformed as a result of the electrolysis of seawater or other solutions, is mixed in the mixing tankwith the power plant waste water (boiler effluent) collected in the storage tank, whereby
ammoniacal nitrogen in the solution decomposed through oxidation. Once the ammoniacal nitrogenMitsubishi Heavy Industries Technical Review Vol. 51 No. 3 (September 2014)13level in the treated water is confirmed to be at or below the regulation limit (i.e., 10 mg/l), it can bedischarged.
Table 3 An example of the design
specifications for ammonia
treatment
Item
Boiler capacity

Figure 2 Flow diagram of waste water treatment
process at power plants

Capacity
500,000 kW/unit
50 mg N/L
Nitrogen level in the(After being
waste water of storage
diluted at reducedtanktemperature)
1,000 m3/week(After being
Flow rate
diluted at reducedtemperature)
Treated amount of
50 kg N/weekammoniacal nitrogen
Capacity ofelectrolytic formation 405 kg Cl2/weekof chlorine|5. Comparison with other ammonia treatment methods
Our ammonia treatment method using the electrolytic denitrification system is superior toother treatment methods such as ion exchange and ammonia stripping (Table 4).
Table 4 Comparison of our electrolysis method with other ammonia treatment methods
Ion exchange
ShortAmmonia stripping
LongElectrolysis
ShortRatingGoodIntermediate

Large

Small

Good

Operability

No special operatingprocedures.
Unexpectedly increased
concentrations may makethe ion exchange capacity
insufficient.

Uses steam and NaOH
solution in large
quantities. Low treatment
efficiency at low
concentrations.

Change inconcentration can be
immediately handled
by modulating thecurrents.

Good

MaintenanceResin regeneration andreplacement

Acid cleaning of theelectrolysis cell
Electrode replacement

Good

Construction
periodShort

Large-scale maintenance,
because of many
auxiliary facilities
Long, because of many
auxiliary facilities.

ShortGood

Steam and pH adjusters
Ammonia gas treatment(as an extra)

Electricity
Electrolysis cellmaintenanceGood

Ammonia gas
Alkaline waste water

SeawaterAcid effluent

GoodMeasures against
ammonia gas diffusion
Recovery and treatment

Necessity of chloride
ion-containing
solutions such asseawater

Good

1040%

385%

100%

Good

187%

103%

100%

Good

Treatment time
Area forinstallation

Necessary
operational
costs
Waste
Issues to beaddressed
OPEX(operatingexpenditure )CAPEX (capitalexpenditure)

Resin regeneration andreplacement
Concentrated ammonia
solution treatment (as anextra)
Used ion exchange resins
Concentrated ammonia
solution (desorption)
Regeneration Treatmentof regeneration waste
water
Lack of ion selectivity