Study Of Sox And Nox Environmental Sciences Essay

Shipping has been and will continue to play a fundamental role in the past, present and future world economy moving 80-90 of world trade by volume. The most fuel efficient means of moving cargo is by ships, and around 70% of ship emissions occur within 400km of land [1]. From wars to the everyday life, ships have become an integral part of modern commercial and military systems. Fishing boats are used by millions of fishermen throughout the world. Military forces operate vessels for combat and to transport and support forces ashore. Commercial vessels, nearly 35,000 in number, carry around 10 billion tons of cargo. The demand for sea transport is mainly generated through the import of raw material or the trade of manufactured products. These ships are an important link in the international system of movement of goods. Largely to the rapid economic development in countries in Asia, Economists are forecasting a doubling or tripling of trade volumes in the next few decades. This also makes the shipping industry an extremely competitive industry. This has driven many ship owners to operate under “flags of convenience” that are well- known for their leniency in implementing and enforcing international maritime legislation. For example two-thirds of the world’s ships are registered in developing countries such as Panama. These are just flags of convenience, to evade tougher rules on safety and pay for sailors.

Air pollution from ships is growing, while compared to on-shore emissions. This is mainly due to the fact that marine vessels have the same power generation and utilization capacities of land based power plants but are not subjected to the controls, regulations and monitoring. Marine vessel engines also use the cheapest, lowest quality, high sulphur fuel called ‘Bunker Fuel’ which results in the production of emissions that are extremely bad for air quality and public health. The emissions generated from these vessels threaten the quality of air and public health in the area around the ports, in coastal regions, along inland waterways and also inland areas through air transport of emissions.

Oceanic vessels mainly use diesel marine engines as the main power source for propulsion systems. These engines provide dependable service and usually have a service life on the order of decades. But these engines are also designed to utilize the cheaper, poorer quality residual fuel mainly in response to growing fuel costs and the huge quantities of oil needed to operate vessels. The combustion process releases into the atmosphere several pollutants particulate matter (PM), Carbon dioxide (CO2), sulfur oxide (SOx), nitrogen oxides (NOX), volatile organic compounds (VOCs), elemental and organic carbon. These pollutants pose a major health risk to living organisms they come in contact with and hence are the cause of worry.

Formation of NOx and SOx

The presence of NOx and SOx in the emissions are a direct result of the type of engine and the fuel used.

Engines

Most of the ships use diesel engines to generate power, the largest of which operate on the two-stroke principle. This type of engine is commonly found as the main engine in the slower speed (less than 150 rpm engine), larger marine vessels. As these mix lubrication oil with the fuel to lubricate the crankshaft bearings, therefore there are large amount of unburnt fuel and oil that exit the engine through the exhaust and so they generates more pollution than the four-stroke engines. The 2-stroke diesel engine operates by first compressing the intake air followed by injecting the fuel for combustion. Because this process compresses only air and not air and fuel, the engine has a higher compression ratio which helps it produce more power and operate more efficiently. As a result of the high compression ratios diesel engines need to be designed stronger, increasing the amount of raw materials needed and increasing the cost of the engines. The combustion process in two-stroke engines has larger combustion chambers and a different fuel injector orientation than the four-stroke engines. The combustion time is a lot longer than that of medium and high speed four-stroke engines due to the slower speed of the two-stroke engines. The more time that the fuel and gases have in the combustion chamber the more complete the combustion will be, making for higher in chamber temperatures and more efficient process when compared to the four-stroke process.

Diesel four-stroke engines are commonly found in the medium to large marine vessels which usually operate between 250 and 850 rpm. These types of engines are favored on ships that are limited in their headroom such as cruise ships, passenger ferries. In comparison to the two-stroke cycle the four strokes are found to be more efficient, require less maintenance, are cheaper and lower polluting but are not as powerful.

Fuels

Most ocean going vessels use bunker fuel, also known as Intermediate Fuel Oil (IFO), Heavy Fuel Oil (HFO) or Residual Oil (RO), in their main engines. Bunker fuel contains high levels of sulfur, ash, and also nitrogen compounds and generates higher emissions than distillate fuels. It is a liquid fuel which is fractionally distilled from crude oil and is formed at the bottom on the distillation column.

The usual composition of the Bunker Fuels used is given in the table below:

Name

Approximate (%)

Carbon

85 – 87

Hydrogen

10 – 12

Sulphur3

1.5 – 3.5

Nitrogen

0.1 – 0.8

Ash

0.01 – 0.08

A comparison of the sulfur content of different types of diesel fuels is given below:

Name

Category

Max Sulfur (%)

Grade No. 2-DS15

Distillate

0.0015%

Grade No. 2-DS500

Distillate

0.05%

Grade No. 2-DS5000

Distillate

0.5%

LS MGO (0.1%)

Distillate

0.1%

LS MGO (0.2%)

Distillate

0.2%

DMX

Distillate

1.0%

DMA

Distillate

1.5%

DMB and DMC

Distillate

2.0%

Intermediate Fuel Oil

Residual

1.5%

Intermediate Fuel Oil

Residual

4.5%

Intermediate Fuel Oil

Residual

5.0%

(Starcrest 2005: Evaluation of Low Sulfur Marine Fuel Availability)

Formation of NOx

In conventional diesel engines, fuel injection occurs at high pressures into compressed air. The compression of the air makes the temperature rise to a level where it causes the ignition of the fuel. Combustion occurs at 2000oC around the periphery of the fuel spray. The high temperatures cause oxides of nitrogen to be formed due to combination of nitrogen and oxygen during the combustion process. The NOx in flue gases from combustion are generally nor due to the fuel used as the fuel itself contains a negligible amount of Nitrogen content, instead it’s the combustion process that produces NOx. NOx pollutants are formed by 3 methods; thermally-generated, ¬‚ame-generated, or fuel-bound NOx [2].

Thermal NOx

Thermal NOx is formed by oxidation of nitrogen in air and requires suf¬cient temperature and time to produce NOx.

O2 ƒ  O + O

The O atoms react with N2 through a relatively slow reaction:

O + N2 → NO + N, (4)

the N atoms formed in this reaction quickly react with O2

N + O2 → NO + O

Flame-generated NOx

As the name implies, ¬‚ame-generated NOx occurs in the ¬‚ame front, created on the short time scale associated with primary combustion reactions. There are a variety of chemical mechanisms involved, all linked to intermediate combustion species that exist only in the reaction zone of the ¬‚ame.

Fuel NO

Fuel NO originates from the organic nitrogen components present in the fuel reacting with air during combustion. Its formation is not strongly temperature dependent like in the case of thermal NO.

Formation of SOx

SOx occur primarily from the combustion of petroleum-derived fuels that contain sulfur. The sulphur content of exhaust gases is directly proportional to the amount of sulphur in the fuel burnt. This sulfur is oxidized to sulfur dioxide or trioxide during the combustion process. Since Sulfur is an important lubricant for the engine it is not convenient for ships to use fuel that has no sulfur in it. SOx in flue gases are the product of the combustion of the fuel containing Sulfur and the amount of sulfur oxides in the emitted gases are directly dependent on the sulfur concentrations present in the fuel. The prime constituent of SOx is SO2 and the usual process of formation is as shown below.

Pyrolysis:

Fuel-S (s) + heat ƒ  H2S + COS + Char-S (s)

H2S + 1½O2 ƒ  SO2 + H2O

SO2 + ½ O2 ƒ  SO3

The oxidation of SO2 to SO3 is an extremely slow reaction and requires temperatures above 1100oC if it is to occur as a normal gas phase reaction. It can progress at lower temperatures in presence of catalysts such as oxides of iron, vanadium, nickel.

Environmental Impact of NOx and SOx

NOx emissions contribute to the formation of photochemical smog. Photochemical smog leads to elevated levels of ozone and production of hazardous organic compounds. Ground level Ozone is formed when NOx and volatile organic compounds (VOCs) react in the presence of heat and sunlight[3].

NO2 → NO +O

O + O2 → O3

When inhaled, even at very low concentrations, ozone can cause acute respiratory problems. Children, people with lung diseases such as asthma, and people who work or exercise outside, are susceptible to adverse effects such as damage to lung tissue and reduction in lung function.

NOx and sulfur dioxide react with other substances in the air to form acids, which fall to earth as rain, fog, snow or dry particles. Some may be carried by wind for hundreds of miles. Acid rain damages, causes deterioration of cars, buildings and historical monuments, and causes lakes and streams to become acidic and unsuitable for many fish.

NOx reacts with ammonia, moisture and other compounds to form nitric acid and related particles. Human health concerns include effects on breathing and the respiratory system, damage to lung tissue, and premature death.

Increased nitrogen loading in water bodies, particularly coastal estuaries, upsets the chemical balance of nutrients used by aquatic plants and animals. Additional nitrogen also accelerates “eutrophication,” which leads to oxygen depletion and reduces fish and shellfish populations.

One member of the NOx, nitrous oxide, is a greenhouse gas. It accumulates in the atmosphere with other greenhouse gasses causing a gradual rise in the earth’s temperature.

In the air, NOx can react with organic chemicals, to form a wide variety of toxic products, some of which may cause biological mutations. Examples of these chemicals include the nitrate radical, nitroarenes, and nitrosamines. Moreover, ground level ozone can be involved in a series of reactions with hydrocarbons to form aldehydes, various free radicals and other intermediates, which can react further to produce undesired pollutants.

Peak levels of SO2 in the air can cause temporary breathing difficulty for people with asthma who are active outdoors. Longer-term exposures to high levels of SO2 gas and particles cause respiratory illness and aggravate existing heart disease. SO2 reacts with other chemicals in the air to form tiny sulfate particles which gather in the lungs and are associated with increased respiratory symptoms and disease, difficulty in breathing, and premature death.

Regulation and Emission control Initiatives

Due to the obvious dangers of the emissions from shipping industry a set of regulations and protocols needed to be setup. In 1948, at a conference in Geneva the United Nations (UN) adopted a convention establishing the Inter-governmental Maritime Consultative Organization (IMCO), later renamed the International Maritime Organization (IMO). The IMO convention went into force in 1958. The IMO is responsible for developing and adopting regulations, and delegates the power to implement and enforce the regulations to its member states. In 1973 the IMO adopted the International Convention for the Prevention of Pollution from ships (MARPOL), which was modified in 1978. Annex adopted by the MEPC for example sets limits on SOx and NOx emissions from ships and the sulfur and nitrogen content in fuels. The Regulations for the Prevention of Air Pollution from Ships (Annex VI) seek to minimize airborne emissions from ships (SOx, NOx, ODS, VOC) and their contribution to local and global air pollution and environmental problems.  Annex VI entered into force on 19 May 2005 and a revised Annex VI with significant tighten emissions limits was adopted in October 2008 which entered into force on 1 July 2010. MARPOL Annex has set the SOx emission in Year 2020 to Sulfur < 0.5% w/w globally[4]. It also makes work plans to identify and develop the mechanisms needed to achieve the reduction of harmful substances in ship emissions. The IMO also has made new regulations making the Energy Efficiency Design Index (EEDI) compulsory for new ships and the Ship Energy Efficiency Management Plan (SEEMP) compulsory for all older ships. The EEDI sets a minimum efficiency standard that new ships must meet, but it permits owners to choose which technologies they use to achieve the EEDI. The SEEMP aims to improve the energy efficiency of a ship’s operation through increased fuel efficiency and planning the voyage of the ships.

MARPOL defines certain sea areas as “special areas” or Emission Control Areas (ECA) in which, due to their ecological and oceanographic condition and due to their marine traffic, the adoption of special mandatory methods for the prevention of sea pollution is required. Under the Convention, these special areas are provided with a higher level of protection than other areas of the sea. There are 3 designated ECAs in effect in the world, sulphur oxide ECAs only in the Baltic Sea area and the North Sea area. A fourth area, the United States Caribbean Sea EC Area, covering waters adjacent to the coasts of Puerto Rico and the United States Virgin Islands, was labeled under MARPOL amendments adopted in July 2011, and will be bought into force on 1 January 2013, with the new ECA coming into force 12 months later on 1 January 2014.

The North American ECA under the International Convention for the Prevention of Pollution from Ships (MARPOL), comes into effect from 1 August 2012, applying strict standards to be followed on emissions of nitrogen oxide (NOx), sulphur oxide (SOx), and particulate matter for ships trading in and around the coasts of Canada, the United States and the French overseas. 

With respect to NOx emissions, Ship diesel engines installed on a ship constructed on or after 1 January 2011 will have to comply with the “Tier II” standard set out in regulation 13 of MARPOL Annex VI. AN engines fixed on a ship constructed on or after 1 January 2016 will have to comply with the more stringent Tier III NOx standard, when operating in a particular NOx EC Area. 

Tier

Ship construction date on or after

 Total weighted cycle emission limit (g/kWh)

n = engine’s rated speed (rpm)

n<130 n = 130 – 1999 n>=2000

I

1 January 2000

17.0

45.n-0.2

e.g., 720 rpm – 12.1

9.8

II

1 January 2011

14.4

44.n-0.23

e.g., 720 rpm – 9.7

7.7

III

1 January 2016*

3.4

9.n-0.2

e.g., 720 rpm – 2.4

2.0

Sources: IMO.org Nitrogen Oxides (NOx) – Regulation 13

In brief, the provisions of MARPOL Annex VI and the year of passing the regulation are:

2005 – NOx regulation Tier1 for new engines post 2000

2010 – Emission Control Area fuel sulfur 1% (currently 1.5%)

2011- Global Tier 2 NOx for new engines (IMO Tier 1 less 15 to 20%) (engine tuning)

2012 – Global fuel sulfur 3.5% (currently 4.5%)

2015 – Emission Control Area fuel sulfur set to 0.1%

2016 – Emission Control Area Tier 3 NOx for new engines (IMO Tier 1 less 80%) (exhaust gas after treatment)

2020 – Global fuel sulfur 0.5% – if refineries can produce it, to be reviewed again in 2018

The European Union’s (EU) commission on transportation and the environment adopted a strategy to reduce air pollution generated by ocean-going vessels. The directive calls for passenger vessels operating in European territorial seas to use marine fuel with a sulfur content of less than 1.5%, and all ships in EU ports to use marine fuel with sulfur content less than 0.1%

The California Air Resources Board (CARB) has been involved in the development and evaluation of measures to reduce emissions created by international trade. With several large ports participating in International Trade and growing trade increases CARB has been one of the lead agencies responsible for attempting to control emissions from marine vessels and port related activities.

Table 1: Fuel Requirements for Ocean-Going Vessels

Fuel

Requirement

Effective

Date

ARB’s California OGV Fuel Requirement

Percent Sulfur Content Limit

Phase I

July 1, 2009

Marine gas oil (DMA) at or below 1.5% sulfur; or

Marine diesel oil (DMB) at or below 0.5% sulfur

August 1, 2012

Marine gas oil (DMA) at or below 1.0% sulfur; or

Marine diesel oil (DMB) at or below 0.5% sulfur

Phase II

January 1, 2014

Marine gas oil (DMA) at or below 0.1% sulfur; or marine diesel oil (DMB) at or below 0.1% sulfur

Source: California Air Resources Board Marine Notice 2011-12

There are also incentive based approaches like the one used by Sweden where on reducing their air emissions they receive incentive of paying reduced fairway dues.

Some Important taxes and incentives in place to encourage reduced emissions include:

• Norwegian NOx tax on all industries including domestic shipping to meet Gothenburg Protocol obligations 7 – continuous measurement or calculation based on default indices (tax in NOK = 15 x kg NOx emitted in Norwegian territory).

• Sweden differentiated harbour dues – NOx emissions, fuel sulphur content.

• Vancouver differentiated harbour dues – fuel sulphur content.

Emission Control

Clean emission is obtained by Removal of NOx, SOx and PM from the gases produced by the burning of the fuels. Emission control technologies can be implemented either as a feature of the original design of the vessel or as a retrofit control device added onto an existing vessel. The three main ways of Emission control are Engine Modifications, Fuel Switching and Treatment of the flue gases

NOx Removal

New engines are made to meet the IMO Tier 2 NOx limits, but this is accompanied with an increase in fuel used. This is usually achieved by optimization of combustion process, delayed injection timing and increased fuel injection pressure, amplified compression ratio, reduced initial air temperature and enhanced injection patterns.

Below approximately 1700K, the residence time in conventional combustors is not long enough to produce large amounts of thermal NOx. Where temperatures higher than 1700K can’t be avoided, it is necessary to reduce the residence time to limit NOx formation, which favors shorter combustor designs. Ideally if the fuel contains Nitrogen or nitrogenous compounds, they should be removed from the fuel before it is used for combustion, or it will be lead to formation of NOx during fuel combustion. Where this is not feasible, rich-lean strategies have the most potential to reduce NOx pollutants. In this approach, combustion is ¬rst carried out under fuel-rich conditions, followed by completing combustion under fuel lean conditions.

The use of fuel water mixture and direct water injection can result in 25% to 50% reduction of NOx. Addition of water to the combustion zone causes reduction in the peak combustion temperature and leads to reduced NOx in the flue gas stream.

The common technique for NOx removal from the flue gases is the adding ammonia to flue gas which passes through catalyst layers, which leads to decomposition of the NOx into harmless nitrogen and steam. SOx are usually removed by wet scrubbing methods which utilize an alkaline solution such as seawater or a solution of freshwater and lime to react with the sulfur present and neutralizing it to form sulphites or bisulphites. Particulate matter is removed using filters or by Electrostatic precipitators which removes particles from the emission using electrostatic charge.

Selective Catalytic and Selective Non-Catalytic Reduction

Selective catalytic reduction (SCR) is a process of converting NOx into nitrogen and water using a catalyst[5]. Selective Catalytic Reduction (SCR) and Selective Non Catalytic Reduction (SNCR) are end-of-pipe removal techniques that use ammonia or urea to convert NOx to nitrogen and water. The difference between SCR and SNCR is that SCR utilizes a catalyst which allows the NOx reduction reaction to occur at a lower temperature while SNCR requires temperatures in the range of 800 to 1100 °C.

The flue gas is mixed with ammonia (or urea) and then passed through a catalyst chamber where the NOx reduction reaction occurs. SCR catalysts are made of a ceramic material that is a mixture of carrier (titanium oxide) and active catalytic components are usually either oxides of base metals (oxides of vanadium, tungsten). The two commonly used shapes of SCR catalyst are honeycomb and plate.

The Chemical Reaction occurring is as shown below.

4NO + 4NH3 + O2 –> 4N2 + 6H2O

2NO2 + 4NH3 + O2 –> 3N2 + 6H2O

The common problem with SCR appears to be their narrow temperature operation bands, the small residence time of the exhaust gas, catalyst poisoning, ammonia leakage and the discharge of catalyst mass under the high temperature conditions. SNCR on the other hand practical has the requirements of high temperature, long time and mixing to effectively remove the NOx.

NOx removals by dielectric barrier discharges

Non-thermal plasma (NTP) processing are currently being investigated intensively as a promising technology for treatment of exhaust gases from diesel engine. NTP technologies mainly include surface discharge, electron beam irradiation, pulsed corona discharge (PCD) and dielectric barrier discharge (DBD)[6].

NO2 could be removed completely in the specific power range, but were produced again for too much power input. Furthermore, subsequent researches are being carried out for an attempt to NOx removal in combination with catalysts

In the above figure you can see the PM, HC and NOx removal efficiencies as a function of peak voltage (applied frequency = 15.5 kHz; engine load = 50%; initial PM concentration = 73.67 Î¼g/L; initial HC concentration = 107.6 ppm; initial NOx concentration = 476.3 ppm). In the range of 4-7.5 kV (34-178 J/L) in the peak voltage, a significant increase of NOx removal efficiency from 0.5% to 67.3% is observed. However, on further enhancing the peak voltage from 7.5 kV to 10 kV, lower NOx removal efficiency was attained[6].

Further studies in this technology is undergoing.

NO removal using micro-organisms

Recently bio-filtration techniques, have been tested to remove NOx from flue gas. Upto 90% of NOx removal have been shown by Lab scale pilot biofilters, filled with wood and compost, at a pH between six and seven at room temperature. Studies using lab scale trickle bed biofilters under aerobic conditions have shown that the removal of NOx by the action of microbes. The identification of the biologically active species and testing of larger reactors are under investigation.

NO removal using wet absorption with metal chelates

The wet absorption of NOx concept consists of absorption of the gas phase element in a liquid containing a suitable reactant. This method is very convenient for gas-phase components which are poorly soluble in the liquid phase. The reactant should react with the gas-phase component to enhance the absorption rate. NO is poorly soluble in water. Increase of the absorption rates of NO in an aqueous solution can be achieved by adding metal salts capable of reacting with NO to the liquid phase. Particularly, divalent iron chelate complexes are very suitable for reactive NO absorption. Well known chelates able to form stable metal complexes are: EDTA (ethylenediaminetetraacetic acid), NTA (nitrilotriacetic acid), MIDA (methyliminodiacetic acid), HEDTA (hydroxy-ethylenediaminetriacetic acid) and DMPS (dimercaptopropanesulfonic acid).

Within the family of iron chelates, Fe II(EDTA) shows a high reactivity towards NO and forms a highly stable complex which is sensitive to oxygen, and could lead to the formation of ferric chelate, Fe III(EDTA). The latter is not capable of binding NO.

The BiodeNOx process

The BiodeNOx process consists of two steps: absorption of NO in an aqueous Fe II (EDTA) solution and biological regeneration of the iron chelate solution by denitrifying bacteria[7].

In the first step, the flue gas with NOx is brought in contact with an aqueous Fe II(EDTA) solution. The iron chelate then binds to the NO and forming a stable nitrosyl complex. The reaction that occurs is:

Fe II ( EDTA) + NO ƒ  Fe II ( EDTA)( NO)

Flue gases also have oxygen and this causes oxidation of part of the FeII(EDTA) complex:

4 Fe II ( EDTA) + O2 + 2 H 2 O ƒ  4 Fe III ( EDTA) + 4OH −

Since Fe III (EDTA) cannot bind NO this reaction is highly undesired because it casues the efficiency of the process to go down. Oxygen is also thought to be responsible for the chemical degradation of chelate complex.

The regeneration occurs in the presence of denitrifying microbes. The regeneration reaction is as follows:

6 Fe II ( EDTA)( NO ) + C2H5OH ƒ  6 Fe II ( EDTA) + 3 N2 + 2CO2 + 3H2O (10)

12 Fe III ( EDTA) + C2H5OH + 3H2O ƒ  12 Fe II ( EDTA) + 2CO2 + 12 H

Both the complexes, Fe II (EDTA)(NO) and Fe III (EDTA) are regenerated to form Fe II (EDTA). Ethanol can be used as an electron donor, while electron donors like methanol also can be used.

SOx Removal

The technologies and techniques used in the control of SOx in flue gases can be separated into three classes; pre-combustion control, combustion control and post-combustion control.

Pre-treatment control deals with reduction in sulfur content of fuel before it is combusted by use of low sulfur fuel. Reduction in SOx emission during the Combustion process is achieved by use of a fluidized bed made up of limestone for combustion of the fuel. But the pretreatment and combustion control of SOx are not feasible solutions as they are expensive and complex. Due to this the focus of SO2 control is usually on post treatment techniques also called Flue Gas Desulfurization (FGD).

FGD can be generally categorized into 2 types; Wet and Dry, depending on the reacted products obtained after the flue gas is desulfurized.

The Tower setup or system of FGD can be broadly classified into Packed/Fluidized Bed, Venturi and Spray Towers. The flow of the Flue gas and Sorbents are further differentiated into Cross Flow, Counter Flow and Co-flow.

Wet Scrubbing

Wet absorption scrubbing processes use the solubility of sulfur dioxide in aqueous solutions to remove it by bringing the gas in contact with alkaline scrubbing liquid in an absorber, the scrubbing liquid contains an alkali dissolved in it for the absorption of Sulfur Oxide[8].

Wet FGD scrubbers can either be non-regenerable or regenerable. In non-regenerable processes the SO2 is irreversibly bound to the sorbent used and hence produces a wet waste that needs to be disposed or a product that has a saleable value. Regenerable processes produce a product where the SO2 can be extracted as liquid or a gas and the sorbent can be reused as a scrubber.

Sea Water Scrubbing

For ships seawater scrubbing is an attractive method of scrubbing as the scrubbing agent does not have to be stored on the ship and the waste slurry produced can be dumped back into the sea.

When SOx comes into contact with seawater there is a fast and efficient reaction between the SOx, water and also the Calcium Carbonate (CaCO3) in the seawater, to form Calcium Sulphate and CO2[9]. The reaction neutralizes the acidity of SOx, and consumes some of the buffering capacity of the seawater. During seawater scrubbing the SO2 is finally converted to sulphuric acid. 95 % of the SO2 is eliminated by this technique.

Reaction with only sea water:

S0 2 (gas) –> S0 2 (aq) + 2H 2 0 –> HS0 3 ” + H 3 0 +

HS0 3 + H 2 0 –> S03-2 + H 3 0 +

HC0 3 – + H 3 0 + –> C0 2 + 2H 2 0

As the conversion of SO2 to SO42- consumes oxygen, aeration of the effluent is necessary. The system incorporates a high degree of recirculation, thus ensuring that sulphur oxides are given adequate time and oxygen contact to be converted to SO4.

There are arguments for and against the use of sea water scrubbers. According to some laboratory experiments and field evidence, acidic waste streams from sea water scrubbing systems introduced in full strength seawater leads to observable effects on ambient pH only for extremely short periods of time. While some others say that the acidic waste stream can potentially cause serious harm to the sea creatures in the area where the stream is let out by causing a change in the pH of the sea water in that area. Another major hindrance in use of sea water scrubbers is that some areas have laws that specifically do not allow the dumping of the acidic waste stream into the sea.

Lime or Limestone Scrubbing

The preferred sorbent in operating wet scrubbers is limestone followed by lime because of their availability and relative low cost. Both lime and limestone follow almost the exact process chemistry and have similar FGD equipment systems. The alkaline solids are powdered Wet Scrubbers usually produce a wet mixture of calcium sulphate and calcium sulphite (sludge) as a final reaction product. The layout and flowchart of a generalized wet scrubber is shown in figure 5-1.

The solid scrubber is first crushed to obtain a fine power. It is then dissolved in water to form an alkaline sorbent slurry which is sent to the absorber where the desulfurization of the flue gas occurs.

The chemistry involved in the scrubbing process is shown below.

Scrubbing using lime(CaO) or limestone(Ca(OH2)) involves spraying of an alkaline slurry with the SO2 in the flue gas. The Insoluble Calcium sulfite (CaSO3) and calcium sulfate (CaSO4) salts are formed which are easily filtered out.

SO2 dissociation:

SO2 (gaseous) –> SO2 (aqueous)

SO2 + H2O –> H2SO3

H 2SO3 –> H+ + HSO3- –> 2H+ + SO3-

Lime (CaO) dissolution:

CaO(solid) + H2O –> Ca(OH)2 (aqueous)

Ca(OH2) –> Ca++ + 2OH-

Limestone(CaCO3) dissolution:

CaCO3 (solid) + H 2 O → Ca++ + HCO- + OH-

The ions of SO3– and Ca++ formed above then react to form CaSO3:

Ca++ + SO3– + 2H+ + 2OH- –> CaSO3 (solid) + 2H2O

Usually air is used to forcefully oxidize CaSO3 to form a Calcium Sulfate which being bigger crystals allows for easier and better filtering.

CaSO3 + H2O + 0.5O2 –> CaSO4 + H2O

In presence of excess oxygen CaSO4 is directly formed:

SO3– + 0.5O2 –> SO4–

Ca++ + SO4– + 2H+ + 2OH- –> CaSO4 (solid) + 2H2O

Ammonia Scrubber

Ammonia scrubbing FGD is a relatively new technology compared to the conventional methods with a removal efficiency of up to 95%[10]. The FGD configuration is similar to other WET FGD processes with a counter flow spray tower or tray design [11]. The ammonia is first vaporized using steam and then along with air is pumped into the absorber tank. Ammonium bisulfite solution is removed from the reaction tank.

Process Chemistry:

SO2 + H2O ƒ  H2SO3

H2SO3 + 2NH3 ƒ  (NH4)2SO3

H2SO3 + (NH4)2SO3 ƒ  2NH4HSO3

(NH4)2SO3 and NH4HSO3 on oxidation form (NH4)2SO4 and NH4HSO4 in the absorber.

(NH4)2SO3 + ½ O2 ƒ  (NH4)2SO4

NH4HSO3 + ½ O2 ƒ  NH4HSO4

The NH4HSO4 in the presence of water and ammonia is neutralized to give (NH4)2SO4.

NH4HSO4 + NH3 + H2O ƒ  (NH4)2SO4 + H2O

Ammonia, air and water are continuously replenished to maintain the desired pH and water level inside the reaction tank.

The main disadvantage in this method was the formation of aerosol in the stack flue gas formed due to presence of SOx, ammonia and water vapor. To combat this problem wet electrostatic precipitator is placed after the absorber to remove any aerosol particles if formed.

Ammonia scrubbing gives ammonia bisulphite as a by-product which has a high value as a fertilizer and therefore is a commercially viable method of FGD.

Sodium Scrubber

Sodium Hydroxide

Sodium hydroxide has an advantage as it is more soluble than the Calcium salts but it is more expensive. The use of Calcium Compound along with NaOH is called dual alkali system and it results in regeneration of NaOH and formation of insoluble calcium+sulfur salts which can be filtered out.

The Basic Chemistry involved in Sodium Hydroxide scrubbing is as shown below.

2NaOH + SO2 –> Na2SO3 + H2O

2NaOH + SO3 –> Na2SO4 + H2O

Na2SO3 + SO2 + H2O –> 2NaHSO3

The Sodium Sulphate and sulphite are then reacted with aqueous Lime.

2NaHSO3 + Ca(OH) 2 –> Na2SO3 + CaSO3.2H2O

Na 2SO3 + Ca(OH) 2 + 1/2H2O –> 2NaOH + CaSO3 . 1/2H2O(solid)

Na 2SO4 + Ca(OH) 2 –> 2NaOH + CaSO4

Sodium Sulpfite

The SO2 in the flue gas can be absorbed in a scrubber system using a concentrated aqueous solution of Na2SO3. The sodium Sulfite absorbs SO2 present in the flue gas when brought in contact in a absorber to produce aqueous sodium bisulfite.

Na2S03(aq) + S02(g) + H20(1) ƒ  2NaHS03(aq)

This aqueous Sodium Bisulfite obtained can be converted to produce elemental sulfur by a series of reactions[12].

First the NaHSO3 is reacted with NaHCO3 slurry.

NaHCO3(aq) + NaHSO3(aq) ƒ Na2S03(aq) + H20(1) + C02(g)

The sodium sulfite obtained can now be kept in solution and reused to scrub more SO2 or precipitated and reductively burnt to form Sodium Sulfide.

2Na2S03(1) + 3C(s) – 2Na2S(1) + 3CO2(g)

The Na2S is then reacted with water and CO2 to form Sodium Bicarbonate.

Na2S + CO2 + H2O ƒ  NaHCO3(aq) + H2S(g)

The resulting H2S gas can be reacted with SO2 to produce elemental Sulfur which can be sold making this process very economically favorable.

H2S + SO2 ƒ  2S(solid) + H2O(g)

Alternatively as done in the Wellman Lord Process the Sodium Bisulfite can directly be evaporated to produce Na2SO3 (regenerated and sent back for scrubbing) and SO2 gas, followed by reducing SO2 to produce Elemental Sulfur.

2NaHSO3 ƒ  SO2 + H2O + Na2SO3

Sodium Carbonate

In this particular process, SO2 is reacted with aqueous sodium carbonate solution to form sodium sulfite and sodium sulfate, which can be reductively burnt to produce sodium sulfide. Sodium sulfide is reacted with CO2 and water to regenerate sodium carbonate and produce hydrogen sulfide which can be reduced to form sulfur[13]. The chemistry is almost similar to the Sodium Sulfite process.

Dry/Semi Dry Scrubbing

Dry or semi-dry Scrubbing processes involve injection of an alkaline sorbent (either or slurry or a solid form) into the direct path of the hot flue gas. The differentiating factor from wet FGD is that the waste or by-product of Dry Scrubbing is dry and scrubbed gas leaving the absorber isn’t saturated with water. Dry/Semi Dry FGD also consists of regenerable and non-regenerable methods.

Spray Dryers

A type of dry absorption system is called a spray dry scrubber. In this method of FGD, an alkaline slurry is sprayed into the hot gas stream at an upstream point from the particulate control device. As the slurry droplets are evaporating, sulfur dioxide absorbs into the alkaline sorbent droplet and reacts with the dissolved and suspended alkaline sorbent. The droplets containing the reacted pollutants in the desulfurized gas stream are then dried and collected usually by passing the gas stream in an ESP or fabric filter, and sometimes at the bottom of the scrubbing vessel. The semi-dry droplets that fall to the bottom of the vessel are either recycled or disposed. The generalized diagram of a spray dryer is as shown in figure 5-3

During the spray drying process mass transfer occurs in two distinct phases: moist and dry [14]. In the moist phase, the SO2 diffuses into the moisture layer of the surface of lime particles from the bulk gas and reacts with dissolved lime. The product of the reaction precipitates on the surface of the same lime particle. In the dry phase, SO2 diffuses into the unreacted core of lime particle through the products layer and a gas solid reaction occurs between the SO2 gas and solid lime.

The main process chemistry of a lime spray dry scrubber is as follows:

Ca(OH)2 + SO2 ƒ  CaSO3.1/2H2O + 1/2H2O

Ca(OH)2 + SO3 + H2O ƒ  CaSO4 .2H2O

CaSO3 + 1/2O ƒ  CaSO4

Slurry droplets must evaporate before they reach the side walls and before they leave the absorber.

Drying too rapidly can reduce pollutant collection efficiency as the primary removal mechanism is pollutant absorption into the slurry droplets, the slurry droplets and the gas stream must remain in Contact long enough to allow absorption.

Spray dry absorption systems have efficiencies that are similar to those for wet-scrubber-type absorption systems. These generate a waste stream that is dry and, therefore, easier to handle than the sludge generated in a wet scrubber. However, the equipment used to atomize the alkaline slurry is complicated and can require considerably more maintenance than the wet scrubber systems. Spray dryer type absorption systems operate at higher gas temperatures than wet scrubbers do and are less effective for the removal of other pollutants in the gas stream such as condensable particulate matter.

Dry-injection-type scrubber

Sulfur dioxide can be collected by adsorption systems. In this type of control system, Dry injection adsorption systems use finely divided (powdered) alkaline material to adsorb acidic gaseous pollutants. The injection system can either be Duct based or Furnace based depending on the location at which the sorbent is injected and made to contact the flue gas. In Duct based Sorbent Injection the sorbent is injected in the duct leading to the particulate filter or baghouse. In furnace type the sorbent is inject directly into the furnace where the high temperatures makes the sorbent porous increasing its surface area.

The usual sorbent used is Calcium Hydroxide, the process chemistry for the injection type using Ca(OH)2 is as follows:

Ca(OH)2 ƒ  CaO + H2O

SO2 present in the gas reacts with CaO in presence of air (O2) to form calcium sulphate.

CaO + SO2 + 1/2O2 ƒ  CaSO4

Sulfur dioxide adsorbs to the surface of the alkaline particles and reacts to form compounds that cannot be re-emitted to the gas stream. The Ca(OH)2 particles and any uncollected particles entrained in the gas stream are then collected in a fabric filter.

Sodium Bicarbonate is another excellent sorbent used in dry injection type scrubber due to its higher reactivity and ability to capture SO2 [15].

The main process chemistry is as follows:

NaHCO3 + SO2 ƒ  Na2SO3 +CO2 + H2O

Na2SO3 + 1/2O2 ƒ  Na2SO4

The efficiency with which pollutants are removed depends on the size of the reagent particles, the adequacy of dust cake formation, and the quantity of reagent injected. The amount of sorbent used in dry injection systems is almost four times the amount than that is stoiciometrically needed to adsorb the pollutants to ensure adequate adsorption. The feed rate is much higher than the feed rate for spray dryer absorber systems. Therefore, a dry-injection-type dry scrubber can be used on smaller systems as opposed to using the larger, more complicated spray-dryer-type dry scrubber. However, the dry injection system is slightly less efficient, and requires more alkali per unit of sulfur dioxide (or other acid gas) collected. Accordingly, the waste disposal requirements and costs are higher for adsorption systems than absorption systems.

Circulating Fluidized Bed scrubber

In Circulating Fluidized Bed scrubber process, a dry sorbent is reacted with flue gas which is previously humidified in a circulating fluidized bed. This circulation of the sorbent provides a high retention time and provides high removal efficiency of the SOx gas from the flu gas stream. The constant movement in the fluidized bed also causes scraping of sorbent, causing the unreacted alkali underneath the reacted sorbent layer to be exposed [16]. The usual sorbent used in Ca(OH)2, the hydrated lime reacts with the SO2 to produce CaSO3 or CaSO4. The by-products along with excess Ca(OH)2 and cleaned flue gas is then passed through a baghouse for removal of the particles.

A general flow of the CFB process is shown in figure 5-2.

Some of the particles captured by the particle controller is sent back into the Fluidized bed as the exit stream contains a considerable amount of unused hydrated lime.

Dielectric Barrier Discharge and Combined plasma photolysis

Plasma-based and Dielectric Barrier Based removal of SO, from flue gases generated by the combustion of fuels containing sulphur relies altering SOx to a species that is more easily removed from the gas stream[10]. In moist gas streams, chemical removal is based on the generation of OH radicals which successively oxidize SOx to sulfuric acid, H2S04.

The basic setup is as shown below:

The wet scrubber present removes the gases and converts them into a form that can be easily captured.

The source of OH radicals in moist gas is direct electron dissociation impact of H20, and dissociative

excitation transfer reactions. A particularly efficient source of OH is the hydrogen abstraction reaction from H20 by O atoms.

e + O2 ƒ  O+O

O + H2O ƒ  OH+OH.

The reactions that occur in the DBD and scrubber are as follows:

SO2 + O ƒ  SO3

SO3 + H2O ƒ  H2SO4 (g) ƒ  H2SO4 (aq)

SO2 + OH- ƒ  HSO3……………..

HSO3 + OH- ƒ  H2SO4 (aq)

Due to the low equilibrium vapor pressure of H2S04, the H2S04 molecules will condense from the gas stream resulting in the formation of droplets. These droplets can then be removed by particle separation and removal devices (e.g., fabric filters or electrostatic precipitators).

Wet Sulphuric Acid (WSA) process

The Wet Sulphuric Acid process was developed by Haldor Topsoe. The process can be used to treat a variety of gas streams containing sulphur compounds and is an efficient process for recovering sulfur from various process gasses in the form of commercial quality sulfuric acid (H2SO4)[11].

The converter contains Haldor Topsoe VK-WSA catalyst which has been specially developed for this application. The SO2 undergoes conversion to SO3 in the presence of the catalyst.

SO2 + ½O2 = SO3 + 99 kJ/mole (in the presence of a vanadium (V) oxide catalyst)

At the exit of the converter the gas is cooled which allows the SO3 formed to react with the water vapour to form sulphuric acid in the gas phase.

SO3 + H2O (g) -> H2SO4 (g) + 101 kJ/Mol

The sulphuric acid gas is then mixed with water to form sulphuric acid in aqueous phase which is collected.

Activated Metal Oxides

Studies done by Johswich showed that activated iron Oxide (75% iron and 25% soda) is a suitable adsorbent for SO2 and S03 at temperatures between 100°C and 300°C. This substance is able to adsorb sulfur gases to almost 100% until it is exhausted which usually happens when it has adsorbed around 21% of its weight of SOx.

Another method is the use of alkalized alumina gel where the sulfur oxides are catalytically oxidized to sodium sulfates. This is prepared by precipitating aluminum sulfate with a soda solution to give a sulfate-free gel after washing and drying, consisting of 60% Aluminum Oxide and 30% Na2O. It is capable of adsorbing 90 % of the SO2 gas at temperatures in the range of 130°C to 330°C. It can adsorb 19 % to 24 % of its own weight and gets exhausted. Regeneration of the adsorbent material can be done by using reducing gases such as H2, CO, water­gas or producer-gas, in combination with CO2 at a temperature of about 600°C.This process causes the sodium sulfate to form soda, the sulfate being reduced to a mixture of H2S, COS and elementary sulphur[12].

Conclusion

Marine vessels play an important irreplaceable role in the international transportation of goods. Marine vessels represent one of the largest under-regulated threats to air quality. If these vessels were treated as onshore sources they would have been subject to regulatory emission controls years ago. The emissions from these vessels threaten the air quality and public health in port communities while berthing or maneuvering and in coastal communities while transiting along the coast.

There is a huge amount of complexity in the shipping industry too. The shipping industry is extremely difficult to regulate, as up to a dozen different countries could have a financial interest in the movement of goods from one country to another. The industry allows ship owners to register their vessels with foreign nations that least threaten their profits while the actual ship owners can be almost impossible to identify. As an example of the complex international nature of the industry a seemingly simple transaction between two companies in different nations could involve people and property from a dozen different nations. The vessel could have been built in Korea, registered in Panama, owned by a Greek businessman, chartered to a Danish operator, who employs a Filipino crew through a crewing agent in Cyprus, insured in the UK, transports German cargo in the name of a Swiss freight forwarder from a Dutch port to Argentina, through terminals that are leased to port operators from Hong Kong and Australia [13]. The complexity of the shipping industry and the numerous international stakeholders involved in every shipment shows that the most effective policy would be developed with a good understanding of the industry, and international relationships involved.

The technology to reduce emission from marine vessels does exist. There needs to be some form of motivation to implementing these technologies on their vessels. But due to the competitiveness of this industry this is proving to be very tough. If Shipping companies face take actions to reduce their emissions and their competitors don’t then they would be at an economic disadvantage because of the high capital and potential operating costs involved with installing emission control technologies.

The choice of technology used to reduce the pollutants in the emissions has to be done in a manner that is beneficial to the environment and also doesn’t put too much of a financial strain on the total cost of the vessel. The choice of technology also depends on the regulations and laws that the ship comes under. If a particular vessel is operating in an area that doesn’t allow scrubbing due to the need to dump the wastes stream into the sea then the only option available will be use of high quality fuel or in-engine modifications.

In the end the bodies making the regulations have to make sure that the standards set by them are balanced. Because even though the environment does need protection it shouldn’t be forgotten that shipping industry is the main vessel of economic development and global trade.