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