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.

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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.

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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

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Approximate (%)

Carbon

85 – 87

Hydrogen

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10 – 12

Sulphur3

1.5 – 3.5

Nitrogen

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0.1 – 0.8

Ash

0.01 – 0.08

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

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Name

Category

Max Sulfur (%)

Grade No. 2-DS15

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Distillate

0.0015%

Grade No. 2-DS500

Distillate

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0.05%

Grade No. 2-DS5000

Distillate

0.5%

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LS MGO (0.1%)

Distillate

0.1%

LS MGO (0.2%)

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Distillate

0.2%

DMX

Distillate

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1.0%

DMA

Distillate

1.5%

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DMB and DMC

Distillate

2.0%

Intermediate Fuel Oil

Residual

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1.5%

Intermediate Fuel Oil

Residual

4.5%

Intermediate Fuel Oil

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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].

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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)

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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.

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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].

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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.

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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

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I

1 January 2000

17.0

45.n-0.2

e.g., 720 rpm – 12.1

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9.8

II

1 January 2011

14.4

44.n-0.23

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e.g., 720 rpm – 9.7

7.7

III

1 January 2016*

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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

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