Saline Water Conversion Corporation

www.swcc.com

CARBOHYDRAZIDE vs HYDRAZINE:
A Comparative Study

Dr. Mohammed Mahmoodur Rahman

Co-Investigators
Mr. Saad A Al-Sulami
Engr. Fahd Al-Muali
Mr. Kither Mohammed
Mr. A.J. Shahrani (Jubail Plants)

Saline Water Desalination

Research Institute 1
www.swdri.com
 CONTENTS

1 Background

2 Introduction

3O Objectives

4 Experimental

5 Results and Discussion

6 Conclusions

2
2
 www.swdri.com

BACKGROUND

Saline Water Conversion Corporation (SWCC) produces

electricity and drinking water through its dual-purpose
plants. The total power generated from all these plants
exceeds 5000 MW. This enormous power is being
generated from 55 high pressure boilers.

Hydrazine has been extensively used in SWCC high

pressure boilers as an effective oxygen scavenger for the
last several decades. However, recent studies with
hydrazine have indicated some difficulties both of
technological nature and those connected with its toxicity
and explosion hazards.

3
 www.swdri.com

Despite the fact that all SWCC power plants are taking
necessary safety measures while handling hydrazine, it
has become a serious desire of the top SWCC
management to search for a suitable alternative to
hydrazine for all SWCC boilers, that provides excellent
oxygen scavenging, non-toxic and safe while handling.

4
 www.swdri.com

PURPOSE OF OXYGEN CORROSION CONTROL

FAILURE PREVENTION
Corrosion Minimisation
EQUIPMENT RELIABILITY
Uninterrupted Production
Routine Maintenance
reduces crisis maintenance
allows planned preventive maintenance
ECONOMICS
Decrease Overall Maintenance Cost
Decrease Downtime Cost

5
www.swdri.com

OXYGEN CORROSION

CORROSION RATE DOUBLES WITH EVERY

10C INCREASE IN WATER TEMPERATURE
METAL LOSS
LOCALISED
RAPID FAILURE
PIT FORMATION

6
 DETAILED OXYGEN CORROSION OF IRON

O2
Fe(OH)3 O2
Fe2+ OH-
WATER
ELECTRON FLOW

ANODE CATHODE
ANODE REACTION CATHODE REACTION
Fe. = Fe++ 2e- 1/2 O2 + H2O + 2e- = 20H-

MECHANISM

IRON IS OXIDISED ON THE SURFACE (ANODE) - METAL LOSS

OXYGEN IS REDUCED (CATHODE)

7
7
www.swdri.com

DETAILED OXYGEN CORROSION OF IRON

RED OXIDE
PRECIPITATION (8)

CAP
H2
OXYGEN REDUCTION
(CATHODE) (2) Fe2O3 RED OXIDE
BLACK OXIDE
O2 OH- MAGNETITE

Fe3O4
Fe(OH)2+ + FeOH++ BLACK OXIDE
PRECIPITATION
(CATHODE) (7)
ELECTRON
FLOW H+ + FeOH+
OXIDATION (5) AND
HYDROLYSIS (6)
HYDROGEN
EVOLUTION PIT Fe++
(CATHODE) (4) HYDROLYSIS OF
e- DISSOLVED IRON
LOWERS pH (3)
ACID SOLUTION Fe
WITH REDUCED
OXYGEN CONTENT IRON DISSOLUTION
(ANODE) (1)

8
www.swdri.com

OXYGEN CONTROL PROGRAM

MECHANICAL CHEMICALS
Deaerators (Oxygen Scavengers)

9
www.swdri.com

TYPES OF OXYGEN SCAVENGERS

INORGANIC (non volatile ) SOLID

Contribute to the TDS of SODIUM BISULFITE
the Boiler Water SODIUM SULFITE

ORGANIC (volatile ) NON-SOLIDS

Do NOT contribute to the HYDRAZINE
TDS of the Boiler Water HYDROQUINONE
DEHA
CARBOHYDRAZIDE
ASCORBIC ACID
ISO-ASCORBIC ACID

10
www.swdri.com

Sulphite

REACTION:

2Na2SO3 + O2 2Na2SO4

SCAVENGER DECOMPOSITION:
Na2SO3 + H2O SO2 + 2 NaOH
4 Na2SO3 3 Na2SO4 + Na2S
Na2S + 2H2O NaOH + H2S

11
www.swdri.com

Sulphite

ADVANTAGES: DISADVANTAGES:
TRUE RESIDUAL TEST CONTRIBUTES TO TDS
VERY FAST REACTIVITY BREAKS DOWN AT 42
WITH OXYGEN Kg/cm DRUM PRESSURE
AVAILABLE IN LIQUID AND
DRY FORMS
INEXPENSIVE
SULFITE : FDA Approved

12
www.swdri.com

Hydrazine

REACTION:
N2H4 + O2 N2 + 2H2O

DECOMPOSITION REACTION:
2N2H4 + HEAT + 2H2O 4NH3 + O2

CONTROL LIMITS:
RESIDUAL N2H4 AT ECONOMIZER INLET

13
www.swdri.com

Hydrazine

ADVANTAGES:
DOESNT CONTRIBUTE TO TDS
TRUE RESIDUAL TEST

DISADVANTAGES:
POOR REACTIVITY WITH LOW TEMPERATURE
EXPENSIVE COMPARED TO SULFITE
SUSPECT CARCINOGEN
REQUIRES SPECIAL HANDLING / FEED EQUIPMENT
DECOMPOSES TO NH3 WHICH CAN LEAD TO
COPPER CORROSION

14
www.swdri.com

Hydroquinone
OH O

O + 1/2 O2
O

OH O
Hydroquinone Benzoquinone
REACTION:
C6H6O2 + 1/2O2 H2O + C6H4O2
CONTROL LIMITS:
DISSOLVED OXYGEN TEST
TYROSINE AND LEUCO CRYSTALS VIOLET
TEST
IRON REDUCTION TEST

15
www.swdri.com

Hydroquinone

ADVANTAGES: DISADVANTAGES:
DOESNT CONTRIBUTE TO TDS
REACTS FASTER THAN
MORE EXPENSIVE
HYDRAZINE AT LOWER Temp THAN HYDRAZINE
DOESNT REQUIRE SPECIAL CAN INCREASE
HANDLING CATION CONDUCTIVITY
GOOD FOR LAY-UP
NOT CARCINOGENIC

16
www.swdri.com

Ascorbic acid
CH2OH CH2OH

CHOH CHOH O
O
O O
CH C + 1/2 O2 CH C + H2O

C C C

OH OH O O
Ascorbic Acid Dehydroascorbate

REACTION:
C6H8O6 + 1/2O2 C6H6O6 + H2O

CONTROL LIMITS:
DISSOLVED O2 TEST
IRON REDUCTION TEST

17
www.swdri.com

Ascorbic acid

ADVANTAGES: DISADVANTAGES:
WORKS WELL IN pH COST PER Kg is HIGH
RANGE (7-11) IS NOT THERMALLY STABLE
CONTRIBUTES NO NON-VOLATILE PRODUCT
TDS (ACIDIC)
OXYGEN ATTEMPERATE WITH CAUTION
SCAVENGING DECOMPOSITION OF ACIDIC
PRODUCTS MAY END UP IN
CONDENSATE CIRCUIT

18
www.swdri.com

Diethyl hydroxylamine (DEHA)

O
CH3 CH2
4 N - OH + 9 O2 8 CH3 - C + 2N2 + 6H2O
CH 3 CH 2
O
Diethyl Hydroxylamine H
Acetic acid

2(C2H5)2NOH + O2 2(C2H5)CH3 CHN=O + 2H2O

DEHA Oxygen Nitrone Water

2(C2H5)CH3CHN=O + 2H2O 2C2H5HNOH + 2CH3CHO

Nitrone Water Ethylhydroxylamine Acetaldehyde

2C2H5HNOH + O2 2CH3CH=NOH + 2H2O

Ethylhydroxylamine Oxygen Acelaloxime Water

CH3CH=NOH + H2O CH3CHO + NH2OH

Acetaloxime Water Acetaldehyde

4NH2OH + O2 2N2 + 6H2O

Oxygen Nitrogen Water

2CH3CHO + O2 2CH3COOH
Acetaldehyde Oxygen Acetic acid

19
 www.swdri.com

Diethyl hydroxylamine (DEHA)

ADVANTAGES: DISADVANTAGES:
No solids contribution to the Cost per kg is high
boiler Is not thermally stable
Steam volatile magnetite Non-volatile product (acidic)
promoter Decomposition of acidic products
Simple to dose and control may end up in condensate circuit
Effective oxygen scavenger

20
www.swdri.com

ORGANIC OXYGEN SCAVENGERS

ADVANTAGES DISADVANTAGES

No solids contribution to the all organic oxygen

boiler scavengers contribute
to cation conductivity
Steam volatile magnetite
promoter all organic oxygen
scavengers potentially
Simple to dose and control
decompose into acid
Effective oxygen scavenger species (organic acids)

21
Performance Criteria for Oxygen Scavenger

The scavenger itself does not react corrosively with materials of

construction and does not lower the pH to corrosive levels (pH < 8).

Its reaction with oxygen is as rapid as possible, particularly in

systems with high flow rates.

The scavenger promotes the formation of passivating metal oxide

films.

Reaction products with oxygen are not corrosive.

Dissolved solids contribution avoided.

The scavenger should not interfere with the action of other treatment
chemicals.

It should not be toxic and much safer and easy to handle.

It should be economical.

22
22
 Carbohydrazide
O

NH2 - NH - C - NH - NH2 + 2 O2 CO2 + 2N2 + 3H2O

Indirect reaction
> 135C
l (N2H3)2CO + H2O 2 N2H4 + CO2
l 2 N2H4 + 2 O2 4 H2O + 2 N2

Decomposition
> 200C
l(N2H3)2CO + H2O 2 NH3 + N2 + H2 + CO2
l 1 ppm Carbohydrazide liberates
l 15 ppb NH3
l 14 ppb CO2

23
23
 Carbohydrazide

An oxygen scavenger that contributes no

inorganic solids to the feed water or boiler
water
An oxygen scavenger that DOES NOT
decomposes in organic acid species
Contributes to passivation
Controls oxygen corrosion

24
24
 Carbohydrazide
%C
Chemical/Formula Reaction and/ Breakdown Products
(wt.)
The byproducts and the percentage Nitrogen,
Hydrazine
carbon content of oxygen scavengers N2H4
0 Water,
Ammonia
in the table clearly illustrates that the Cabohydrazide Hydrazine,
Nitrogen,
byproducts of carbohydrazide contain (N2H3)2CO
13.3
Water,
Ammonia
no harmful organic compounds or Carbon Dioxide
Erythrobic acid Dihydroascorbic acid
acids [CEGB Report V 14, 1991]. Salts of Lactic and Glycolic
C6H8O6 40.9 Carbon Dioxide
Diethylhydroxylamine Acetaldehyde
Acetic acid
On the basis of above data and its (CH3CH2)2NOH Acetate ion
Dialkylamines
wide application in different power 53.9 Ammonia
Nitrate
houses internationally as well as Nitrite
locally Carbohydrazide was selected Methylethylketoxime Methylethylketone
Hydroxylamine
(CH3)(CH3CH2)C=NOH
for the evaluation. Nitrogen
Nitrous Oxide
55.2
Ammonia
Carbon Dioxide
Hydroquinone Benzoquinone
Light Alcohols
C6H4(OH)2 Ketones
65.5 Low Molecular Weight Species
Carbon Dioxide

25
25
 OBJECTIVES

To evaluate the suitability and efficiency of carbohydrazide

as an alternative oxygen scavenger to hydrazine in the
high-pressure boiler.

To determine the consequences of degradation by-

products on boiler system.

To evaluate the ability of the alternative oxygen scavenger

in forming and maintaining an oxide film in the boiler.

To evaluate whether the alternative oxygen scavenger is

generating any negative effects on the efficiency of the
boiler.

26
26
 EXPERIMENTAL

Two boilers # 81 and # 82 of PhaseII at Al-Jubail

Plants, each generating 130 MW/h, were selected
for the trial tests in consultation with chemical
manufacturing company.

Boiler # 81 was run with carbohydrazide and

boiler # 82 with hydrazine. Both these boilers are
pressurized box type water tube boilers. The
maximum continuous rating (MCR) of steam is
710 tons/h.

27
27
 Test Conditions (Boiler chemistry)

Feed Water/
Parameters/ Daerator Boiler Blow Saturated
Condensate Economizer
Samples out Down (Drum) Steam
inlet
pH 8.5 9.2 ------- 8.7 9.2 9 9.8 8.7 9.2
Sp. Conductivity (S/cm) <3 ------- <3 <50 <3
Cat. Conductivity (S/cm) <0.5 ------- ------- ------- <0.5
Copper (ppb) <5 -------- <5 <20 ----
Ammonia (ppm) <0.3 ------- <0.3 ------- <0.3
Iron (ppb) <10 ------ <10 <50 ------
Dissolved O2 (ppb) <20 <10 Nil or <7 ------ ------
Hydrazine (ppb) ------ ------- 10 - 20 ------ ------
Silica (ppb) <20 ------- <20 1000 <20
Sodium (ppb) <10 ------- <10 ------ <10
Chloride (ppm) <0.01 ------- <0.05 <0.5 <0.05
Phosphate (ppm) ------ ------- ------- 5 - 10 ------
P-Alkalinity(ppm) ------ ------- ------- <5 ------
M-Alkalinity(ppm) ------ ------- ------- <15 ------

28
28
 Test Conditions (Operations)

Water/Steam
S. No. Equipment Capacity Pres. Temp. Flow
(Bar) 0C M3/H
1 Condensate ---------- 9.5-9.8 99-121 ----
2 Deaerator 86 M3 5 156 700
3 25 M3 ---- --- ----
4 Boiler feed Pump 900 M3 103 ---- 697
5 HP Heater 1 ---- 103 160-195 697
6 HP Heater 2 ----- 103 195-233 697
7 Economizer ----- 102 230-295 700
8 Boiler 12 1M3 104 310 700
9 Super heater 39 M3 95 515 700
Hydrazine tank
10 560 L 150 ---- 50L/H
pump
Phosphate tank
11 560 L 150 -------- 50L/H
pump

29
29
TYPICAL BOILER SHOWING WATER AND STEAM FLOW

30
30
BOILER WATER CYCLE AND SAMPLING POINTS
Saturated steam

Boiler water
(boiler drum)

Feed water

Make-up water

Brine heater
condensate

31
31
 RESULTS AND DISCUSSIONS
Optimization of Carbohydrazide
Chemical feed rates for hydrazine were established to maintain hydrazine residual
around 20 ppb in feed water.
At the start with the recommended dosage of 1.5 ppm of carbohydrazide, determination
of carbohydrazide in feed water was observed to give inconsistent values. The
inconsistency observed was attributed to the high temperature of the feed water (235oC)
because at temperatures above 150oC carbohydrazide hydrolyzes to hydrazine.
250 30

25
200

FW Hydrazine (ppb)
20
FW CHDZ (ppb)

150
15
100
10

50
5

0 0
5/17/05 5/31/05 6/14/05 6/28/05 7/12/05 7/26/05 8/9/05 8/23/05 9/6/05 9/20/05 10/4/05
Date
CHDZ Limit CHDZ Hydrazine Limit Hydrazine

32
32
 Optimization of Carbohydrazide
1.2
At the start with the

FW Ammonia (ppm)
1
0.8
recommended dosage 0.6
of 1.5 ppm of 0.4
0.2
carbohydrazide, high 0
5/17/05 5/31/05 6/14/05 6/28/05 7/12/05 7/26/05 8/9/05 8/23/05 9/6/05 9/20/05 10/4/05
values for ammonia Date
CHDZ Hydrazine Limit
and copper were SS Ammonia (ppm)
1
recorded. 0.8
0.6
0.4
The dose rate was 0.2
0
later reduced from 1.5 5/17/05 5/31/05 6/14/05 6/28/05 7/12/05 7/26/05 8/9/05 8/23/05 9/6/05 9/20/05 10/4/05

to 0.7 ppm till the CHDZ Hydrazine Limit

Date

parameters (copper & 0.8

BHC Ammonia (ppm)

ammonia) were 0.6

maintained within the 0.4

0.2
normal range and
0
stabilized. 5/17/05 5/31/05 6/14/05 6/28/05 7/12/05 7/26/05 8/9/05 8/23/05 9/6/05 9/20/05 10/4/05
Date
CHDZ Hydrazine Limit

33
33
 Boiler Chemistry
9.7
pH Control 9.2

FW pH
8.7
pH values for both 8.2
hydrazine and 5/17/05 5/31/05
Carbohydrazide
6/14/05 6/28/05
Hydrazine
7/12/05
Limit
7/26/05 Date8/9/05 8/23/05 9/6/05 9/20/05 10/4/05

Carbohydrazide 10.2
were found to be BBD pH

maintained in the 8.2

5/17/05 5/31/05 6/14/05 6/28/05 7/12/05 7/26/05Date 8/9/05 8/23/05 9/6/05 9/20/05 10/4/05
range of 8.6 and 9.0 9.4
Carbohydrazide Hydrazine Limit
9.2
BHC pH

for Brine heater 9

8.8
8.6
condensate (BHC), 8.4
8.2
Feed water (FW), 5/17/05 5/31/05
Carbohydrazide
6/14/05 6/28/05
Hydrazine
7/12/05 7/26/05 Date
Limit
8/9/05 8/23/05 9/6/05 9/20/05 10/4/05
9.4
Saturated steam 9.2
9
8.8
SSpH

(SS) and Deaerator 8.6

8.4
8.2
outlet (DAO) 5/17/05 5/31/05 6/14/05 6/28/05 7/12/05 7/26/05Date 8/9/05 8/23/05 9/6/05 9/20/05 10/4/05
Carbohydrazide Hydrazine Limit
whereas for Boiler 9.4
9.2
9
blow down (BBD) it
DAO pH

8.8
8.6
was in the range of 8.4
8.2

9.0 and 10. 5/17/05 5/31/05

Carbohydrazide
6/14/05 6/28/05
Hydrazine
7/12/05
Limit
7/26/05
Date
8/9/05 8/23/05 9/6/05 9/20/05 10/4/05

34
34
 Boiler Chemistry
14

12
Dissolved Oxygen 10

DAO Disol. O2
8
Average dissolved oxygen 6
with both hydrazine and 4
Carbohydrazide in the DAO 2
was found to be 7 ppb, 0
whereas in the BHC it was 5/17/05 5/31/05 6/14/05 6/28/05 7/12/05 7/26/05 8/9/05 8/23/05 9/6/05 9/20/05 10/4/05
Date
found to be high at an CHDZ Hydrazine

average of 87 ppb. This 250

was attributed to the air 200

BHC Disol. O2

leakage. 150

100

50

0
5/17/05 5/31/05 6/14/05 6/28/05 7/12/05 7/26/05 8/9/05 8/23/05 9/6/05 9/20/05 10/4/05
Date
CHDZ Hydrazine

35
35
 Boiler Chemistry
Control of Copper levels 40
35
With hydrazine, the average

FW Copper (ppb)
30
concentrations for copper in feed 25

water was found to be 3 ppb and 20

15
in boiler water it was found to be 10
6 ppb, whereas with 5

carbohydrazide dosing, the 0

5/17/055/31/056/14/056/28/057/12/057/26/05 8/9/05 8/23/05 9/6/05 9/20/0510/4/05
average concentrations for Date
CHDZ
copper in feed water was found
50
to be 3 ppb and in boiler water it 45
40
was found to be 4 ppb.
BBD Copper (ppb)

35
30
25
This showed that copper levels in 20
15
feed water were maintained at 10
5
the baseline value whereas a 0
5/17/055/31/056/14/056/28/057/12/057/26/05 8/9/05 8/23/05 9/6/05 9/20/0510/4/05
33% reduction was found in the Date
CHDZ
boiler water (drum).

36
36
 Boiler Chemistry
Control of Iron levels 25

20
With hydrazine, the average

FW Iron (ppb)
15
concentrations for iron in feed
10
water was found to be 8 ppb
and in boiler water it was found 5

to be 19 ppb whereas with 0

5/17/05 5/31/05 6/14/05 6/28/05 7/12/05 7/26/05 8/9/05 8/23/05 9/6/05 9/20/05 10/4/05
carbohydrazide dosing, the Date
CHDZ
average concentrations for iron
60
in feed water was found to be 2
50
ppb and in boiler water it was
40
BBD Iron (ppb)

found to be 17 ppb.
30

20
This showed a reduction of iron 10
levels in the feed water by 75% 0
and in the boiler water (drum) a 5/17/05 5/31/05 6/14/05 6/28/05 7/12/05 7/26/05 8/9/05 8/23/05 9/6/05 9/20/05 10/4/05
Date
reduction of 10%. CHDZ

37
37
 Corrosion studies
Coupons
in Boiler
The exposed coupons were Drum
Before
test
visually examined. A uniform
and non-porous oxide film
was found to be adhered on
the coupons indicating the Coupons
in Boiler
protective nature of the Drum
after test
films. The corrosion rate of
0.044 mpy determined for Coupons
in HP
heaters
carbon steel coupons fixed after test

in drum at water side with

hydrazine dosing indicates
extremely low corrosion.
Coupons in
HP heaters
Before test

38
38
 Corrosion studies
The corrosion rates

Water side
determined with
carbohydrazide dosing for
carbon steel coupons fixed Front Back

in drum at water side and

steam side were 0.384 mpy

Steam side
and 0.444 mpy respectively,
whereas that determined for
carbon steel and 70/30 Front Back

cupronickel in HP heaters Carbon steel coupons after exposure in water drum under Carbohydrazide dosing boiler
#81
were 0.251 mpy and 0.128
mpy respectively. The
corrosion rates of the
material appear to be low
and effect of corrosion
Front Back
appears to be insignificant.
Cupronickel coupons after exposure in HP heater under Carbohydrazide dosing boiler
#81

39
39
 CONCLUSIONS
The results indicated the suitability and efficiency of
Carbohydrazide oxygen scavenger as an alternative to
hydrazine in SWCC high pressure boiler provided the
concentration of residual hydrazine (decomposition by-
product of Carbohydrazide) is maintained at levels
between 30-40 ppb in the feed water.
No harmful degradation by-products were found.

Carbohydrazide was found to be a good oxygen scavenger

at a concentration dose rate of 0.7ppm.

Optimized dose rate resulted in maintaining the boiler

chemistry within design limits.

40
40
 CONCLUSIONS
Iron levels measured at economizer inlet (boiler
feed water) were reduced by 75% whereas for
copper it was found to be maintained at the
baseline value. In the boiler water (drum) the
reduction in Fe and Cu were 10% and 33%,
respectively.

The corrosion rates indicated very little or

negligible corrosion due to either hydrazine or
Carbohydrazide. However hydrazine showed much
lower rates compared to Carbohydrazide.

41
41
 Acknowledgment

The authors gratefully acknowledge:

The cooperation of Jubail Plant personnel.

42
42
Saline Water Conversion Corporation
www.swcc.com

Saline Water Desalination

Research Institute 43
www.swdri.com