Dennis Y.C. Leung & Gabriel C.K. Lam

Dept. of Mechanical Engineering

The University of Hong Kong

Water Scrubbing Technology for

reducing marine emissions

Background

• Hong Kong is one of the busiest ports in the world and suffering

from serious air pollution due to the emissions from ships

including ferries, barges, and ocean going vessels.

• Most of the ships (particularly those cruise ships and ocean going

vessels) still use poor quality high sulphur fuels producing

enormous amount of SO2 and dark smoke.

Background (cont’d)

• Marine vessels emissions have been increasing over the

past decades and become the top emitter of SO2, RSP and

NOx in HK.

4Source: EPD website http://www.epd.gov.hk/epd/english/environmentinhk/air/data/emission_inve.html

Background (cont’d)

• In recent years the government started to look into ways to

improve the emissions from marine sources. e.g. H.K. has

just legislated on the use of low sulphur diesel (<0.05%) in

April 2014 under the APC (Marine Light Diesel) Regulation.

• A desk top study has been conducted by HKU in 2008 on the

effectiveness of various APC control methodologies for

marine applications.

• It was found that wet scrubbing technology is a low cost and

effective technology that is suitable for ferry fleet in Hong

Kong.

Clean air

• a basic chem. engg. unit operation

• usually carried out in a column or tower in

which the gas to be cleaned comes into

contact with fresh liquid introduced at the

top;

• Commonly used nowadays for absorption of

air pollutants (e.g. FGD in HEC and CLP)

Gas

Absorbing liquid

Principle of wet scrubbing

Background (cont’d)

• The pollutant removal effectiveness of the scrubbing

technology in marine application will be the same, if not

better than the common measures of reducing fuel S or

installing other exhaust after-treatment devices such as

diesel oxidation catalysts and particulate filters.

• Subsequently, a seawater scrubber has been designed,

installed and tested in a ferry (Star Ferry) with 6 x 60 kW

engines.

Characteristics of Star Ferry fleet

Using Crossley 2-stroke marine diesel engine

• Provide steady propulsion even at low rpm

Low rpm(from normal @190 rpm

to 340 rpm max)

Low fuel consumption: ~4 litre/km.

Low exhaust gas temperature (<170℃)

Have tried other fuels such as ULSD, emulsified fuel but the effects

are not satisfactory

Very congested engine room

Testing results on using other fuels

Low S* diesel

ULSD* Emulsified*

Fuel

SO2 (g/kW-hr) 1.80 0.33 1.90

Smoke

(HSU)

Av. 5.02 12.24 3.36

Cruise 4.01 11.93 3.17

Max. 77.85 65.90 32.16

Particulate (g/kW-hr) 1.61 1.93 1.60

* Results from previous studies of Star Ferry

Wet-scrubber installed

Wet scrubber in operation

Testing results on using other

fuels and scrubber alone

Low S* diesel

ULSD* Emulsified*

Fuel

Seawater Scrubber

SO2 (g/kW-hr) 1.80 0.33 1.90 0.17

Smoke

(HSU)

Av. 5.02 12.24 3.36

>70% for max

Cruise 4.01 11.93 3.17

Max. 77.85 65.90 32.16

Particulate (g/kW-hr) 1.61 1.93 1.60 -

NOx (g/kW-hr) - - - -

* Results from previous studies of Star Ferry

Comparison between flumes with and

without the wet scrubber

With scrubber

Withoutscrubber

Background (cont’d)

• The scrubber was found to be very efficient in removing air

pollutants from the exhaust gas, in particular, a reduction in dark

smoke of ~70%, SO2 ~90%, and unburnt HC ~40% can be

achieved.

• Despite the system is very effective in removing SO2 and dark

smoke, it has very little effect on NOx in the exhaust gas due to the

poor solubility of NO in seawater.

• NOx is a pollutant very difficult to be removed or reduced due to

its complicated formation mechanism (prompt, thermal and fuel

NOx).

Background (cont’d)

There are 3 sources of NOx during a combusting process:-

1. Thermal NOx:

oxidation of atmo. molecular N2 at high temp.

2. Prompt NOx:

rapid oxidation of atmo. nitrogen in the presence of HC radicals (generally insignificant)

3. Fuel NOx:

Direct conversion of fuel bound nitrogen

Background (cont’d)

• Conventional ways of removing NOx from exhaust include

post-combustion control and exhaust gas treatment.

• Post-combustion control mainly modifies the combustion

chamber/burners to reduce the peak combustion temperature-

mainly incorporated in modern boilers and engines

• Exhaust gas treatment is more flexible and mostly used to

further reduced the NOx from boilers and engines.

• Exhaust gas treatment technologies for NOx reduction

include: Selective catalytic reduction (SCR) and absorption

(NO2 only).

SCR technology for NOx reduction

• Commonly used for stationary sources (e.g. Town Gas & CLP) and

new vehicles (Euro 5 & beyond)

• Normal efficiency 50-90%

• Need a higher exhaust gas temperature (>200℃)

• Need to store urea on board

• High in capital and running cost

Wet scrubbing technology

• As mentioned before, the wet scrubbing technology was found to

be very efficient in removing dark smoke, SO2 and HC from the

exhaust gas but has very little effect on NOx

• According to the following equation, NO can be removed by first

oxidized to NO2 and then removed by water:

NO + O3 NO2 +O2

2NO2 + H2O + 0.5O2 2HNO3

• Feasibility of further NOx removal in the scrubber previously

developed has been pursued by additional of O3 as the oxidation

agent.

Laboratory setup for testing the De-NOx

system –vertical and single nozzle

Laboratory setup for testing the

de-NOx system- horizontal and multi-nozzle

Main scrubber design

22

L/G ~16.6 (mass)

23

Ozone generator(380V, 3ph, 50Hz)

Monitoring/measurement equipment

24

Measured Fuel Flow to Engine

Data set Measured

(L/min.)

Average for

6 × 60kW

Average for

3 × 60kW

(L/min.) (kg/h)* (kg/h)

1 0.895 0.895 45.96 22.882 0.905

3 0.885

4 0.965 0.966 49.60 24.805 0.980

6 0.985

7 0.950

8 0.960

9 0.955

10 0.245 0.271 13.92 6.9611 0.270

12 0.275

13 0.295

14 0.300 0.298 15.30 7.6515 0.295

Measured O2 Content and Calculated Gas

Flows at Scrubber Inlet

Engine

Speed

(rpm)

Data set Measured

O2 (%)

Average O2

(%)

Dry Basis Gas

Flow

(Nm3/kg fuel)

@ 0oC, 1 atm.

Wet Basis Gas

Flow (Nm3/kg

fuel) @ 0oC, 1

atm.

Wet Basis

Gas Flow

(Nm3/h)

300 1 16.8 16.8 53.55 55.69 12792 16.9

3 16.9

4 16.7

5 16.7 16.8 53.55 56.76 14076 16.7

7 16.7

8 17.0

9 16.9

10 16.9

11 16.8

12 16.7

190 13 19.1 19.0 115.57 120.19 83714 19.0

15 19.0

16 19.2 19.1 121.99 129.31 989

Ozone Generator Output

Data set Engine

speed

(rpm)

Ozone

Generator

Manual

Output %

Ozone

Generator

flow rate from

rotameter

(m3/h) @ 20oC

Ozone

Generator

rotameter

pressure

(MPa)

Ozone

Generator

gas flow rate

(Nm3/h)

Ozone Output

Conc. by

Teledyne 465H

(g/Nm3)

Calculated

Ozone

Output

(g/h)

1 300 80 23 0.0 22.59 43.3 978.1

2 300 53 26 0.0 25.54 22.7 579.7

3 300 25 26 0.0 25.54 10.6 270.7

4 300 80 26 0.0 25.54 32.7 835.0

5 190 80 32 0.0 31.43 31.5 990.0

6 190 71 30 0.0 29.46 22.7 668.8

7 190 80 24 0.0 23.57 42.8 1008.9

8 300 79 19 0.0 18.66 53.5 998.3

9 190 79 20 0.0 19.64 50.5 992.0

10 300 79 20 0.0 19.64 51.2 1005.7

11 300 79 20 0.0 19.64 53.0 1041.1

Dilution and Mixed Ozone Concentration at

Scrubber Inlet due to the Main Ozone Generator Output

Data

set

Engine

speed

(rpm)

Main Ozone

Generator

ozone output

(g/h)

Small Ozone

Generator

ozone output

(g/h)

Main Ozone

Generator

gas flow rate

(Nm3/h)

Engine

exhaust gas

flow rate

(Nm3/h)

Deduced

Dilution to

Engine

Exhaust Gas

Calculated

Ozone Conc.

at scrubber

inlet (ppm)

1 300 978.1 14.9 22.59 1279 1.77% 356.0

2 300 579.7 14.9 25.54 1279 2.00% 212.7

3 300 270.7 14.9 25.54 1279 2.00% 102.2

4 300 835.0 14.9 25.54 1279 2.00% 304.0

5 190 990.0 14.9 31.43 837 3.76% 540.0

6 190 668.8 14.9 29.46 837 3.52% 368.2

7 190 1008.9 14.9 23.57 837 2.82% 555.2

8 300 998.3 14.9 18.66 1407 1.33% 331.7

9 190 992.0 14.9 19.64 989 1.99% 465.9

NO and NOx reduction (dry basis)

Data set Engine

Speed

(rpm)

Scrubber Inlet (ppm) Scrubber Outlet (ppm) Removal efficiency

(%)

NO NOx NO NO2* NOx NO NOx

1 300 602 620 490 34 524 17.0% 13.8%

2 300 640 659 520 81 601 17.1% 7.0%

3 300 590 606 384 67 451 33.6% 24.1%

4 190 330 340 152 34 186 52.2% 43.2%

5 190 309 315 132 38 170 55.8% 44.1%

6 190 309 319 75 40 115 75.0% 62.9%

7 300 646 666 398 26 424 37.6% 35.5%

8 190 332 342 99 18 117 69.6% 65.1%

Other issues of concern

30

Effluent discharge

Flow rate(m3/day)

Determinand

≦10 ＞10 and≦200

＞200 and≦400

＞400 and≦600

＞600 and≦800

＞800 and≦1000

＞1000

and≦1500

＞1500

and≦2000

＞2000

and≦3000

＞3000

and≦4000

＞4000

and≦5000

＞5000

and≦6000

pH (pH units) Temperature (℃)

Colour (lovibond units) (25mm

cell length)

Suspended solids

BOD

COD

Oil & Grease

Iron

Boron

Barium

Mercury

CadmiumOther toxic metals individually

Total toxic metals

Cyanide

Phenols

Sulphide

Total residual chlorine

Total nitrogen

Total phosphorus

Surfactants (total)

E. coli (count/100ml)

6-10

45

4

700

700

1500

50

20

6

6

0.1

0.1

2

4

1

0.5

5

1

100

10

30

5000

6-10

45

1

600

600

1200

50

15

5

5

0.1

0.1

1.5

3

0.5

0.5

5

1

100

10

20

5000

6-10

45

1

600

600

1200

50

13

4

4

0.05

0.05

1

2

0.5

0.5

5

1

100

10

20

5000

6-10

45

1

500

500

1000

30

10

3.5

3.5

0.001

0.001

0.8

1.6

0.5

0.3

5

1

100

10

20

5000

6-10

45

1

375

375

700

25

7.5

2.5

2.5

0.001

0.001

0.6

1.2

0.4

0.3

5

1

100

10

15

5000

6-10

45

1

300

300

600

20

6

2

2

0.001

0.001

0.5

1

0.3

0.2

5

1

100

10

15

5000

6-10

45

1

200

200

400

20

4

1.5

1.5

0.001

0.001

0.32

0.64

0.2

0.1

2.5

1

100

10

15

5000

6-10

45

1

150

150

300

20

3

1

1

0.001

0.001

0.24

0.48

0.1

0.1

2.5

1

100

10

15

5000

6-10

45

1

100

100

200

20

2

0.7

0.7

0.001

0.001

0.16

0.32

0.1

0.1

1.5

1

100

10

15

5000

6-10

45

1

75

75

100

20

1.5

0.5

0.5

0.001

0.001

0.12

0.24

0.08

0.1

1

1

100

10

15

5000

6-10

45

1

60

60

100

20

1.2

0.4

0.4

0.001

0.001

0.1

0.2

0.06

0.1

1

1

100

10

15

5000

6-10

45

1

40

40

85

20

1

0.3

0.3

0.001

0.001

0.1

0.14

0.04

0.1

0.5

1

50

5

15

5000

Table 9b Standards for effluents discharged into the marine waters of Victoria Harbour Water Control Zone(All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated)

(Enacted 1990)

Effluent sampling

32

Background sampling From scrubber outlet

Water Quality Sampling and Analysis Results

33

Conditions Measured Standard

Baseline (Ambient seawater)

Temperature oC 19.5 45

pH 8.0 6 – 10

Suspended Solids mg/L <2 200

COD mg/L <200 400

O&G mg/L <5 20

Total Nitrogen 0.6 100

300rpm cruising

Temperature oC 24.3 45

pH 6.5 6 – 10

Suspended Solids mg/L 13 200

COD mg/L <200 400

O&G mg/L 16 20

Total Nitrogen 1.6 100

190rpm cruising

Temperature oC 20.8 45

pH 7.0 6 – 10

Suspended Solids mg/L 3 200

COD mg/L <200 400

O&G mg/L <5 20

Total Nitrogen 1.7 100

DateS Limit in Fuel (% m/m)

SOx ECA Global

2000 1.5%4.5%

2010.071.0%

20123.5%

20150.1%

2020a 0.5%

a - alternative date is 2025, to be decided by a review in 2018

IMO Sulphur Limit

Tier DateNOx Limit, g/kWh

n < 130 130 ≤ n < 2000 n ≥ 2000

Tier I 2000 17.0 45 · n-0.2 9.8

Tier II 2011 14.4 44 · n-0.23 7.7

Tier III 2016† 3.4 9 · n-0.2 1.96

† In NOx Emission Control Areas (Tier II standards apply outside ECAs).

IMO NOx Emission Limits

Concluding remarks

1.A water scrubber has been installed and tested in one of the

ferries of Star Ferry Co., Ltd. for reducing the air pollutant

emissions from its exhaust.

2. The original scrubber is found to be effective in reducing

several important pollutants such as dark smoke, SO2 and

hydrocarbon but there is very little effect on NOx.

3. With the installation of an ozone dosing system, the NOx in

the exhaust gas can also be reduced with a reduction of ~75%

and 65% for NO and NOx respectively at low speed operation

mode and ~38% and 35% at high speed operation mode.

Concluding remarks (cont’d)

4. The performance of the scrubber can be further improved by

better design of the mixing chamber and reheater of the

scrubber.

5. Higher O3 injection will also increase the reduction of NOx.

6. The effluent discharge due to the scrubbing was also found to

be acceptable to local discharge criteria.

7. Wet scrubbing may provide an alternative solution for

reducing the pollutant emissions from marine sources.

38

Acknowledgement

This project is funded and supported by various bodies at 2

stages:

• The 1st stage: scrubber without de-NOx is funded by Star

Ferry Co. Ltd.

• The 2nd part: scrubber with de-NOx system is funded by

the Environment and Conservation Fund (ECF)

• Star Ferry Co. Ltd.

• Jockey Club Heavy Duty Vehicle Emission testing Centre

• Hongkong Electric Co., Ltd.

• Leung Wan Kee Shipyard Ltd.

Project team

40

Thank you!

41