Nox Osx
A process for removing nitrogen oxides (NOx) and sulfur oxides (SOx) from gaseous streams. The gaseous stream is contacted with an aqueous solution of chloric acid in an oxidation scrubber (14). Internal combustion engines can produce all three nitrogen oxides. Nitrous oxide (N 2 O), also known as 'laughing gas'. It is a serious greenhouse gas, and is defined as being 298 times as bad as CO 2 because of its radiative effect, and the time taken to break it down.
With the passage of the 1990 Clean Air Act Amendments, many chemical and metal industries and utility producers will now be required to further limit the amount of NOx (Nitrogen Oxides) produced. NOx is a precursor to ozone in the atmosphere, and is believed to be a major contributor to acidic deposition (acid rain).
Formation of ozone: NOx is produced in a variety of different processes, including combustion equipment, gas turbines, incinerators, kilns and power plants. NOx also is emitted as by product from many metal treatment processes where nitric acid is used as an oxidant. Plating or catalyst recovery involves the reaction of nitric acid and transition metals also forming NOx. Substantial amounts of NOx also can be generated in the specialty chemical industry when nitric acid is used as a reagent.
The denitration processes for removal of NOx are classified into two groups: in one, NOx is absorbed by means of solutions, and in the other NOx is reduced to N2 by means of a reducing gas under the presence of a catalyst.
Selective catalytic reduction (SCR) is a chemical process that changes the oxides of nitrogen into N2 and H2O. The reactions take place at a temperature of 600-750 F in the presence of a catalyst. Ammonia is injected into the exhaust gases prior to their passing into the SCR. NOx removal efficiencies with SCR range from 80 to 90% [2]. An NH3/NOx mole ratio of 1.0 to 1.5 is normally used although the theoretical ratio is about 0.67. Although a portion of the excess ammonia decomposes in the reactor, a considerable amount of it would remain in the treated gas and may cause problems. For example, ammonia may combine with SO3, which is present in a small amount even after the wet scrubbing, to form ammonium bisulfate which condenses in a heat exchanger. However, catalysts are affected by dust and most are poisoned by sulfur and chlorine compounds.
For treatment of contaminated gas, a wet removal process may be carried out first to remove dust and poisonous chemicals, but in this case, the gas temperature drops to 120-150 and must then be reheated to 600-750 F, a large heat exchanger and a considerable amount of fuel are needed. In addition, mists from the scrubber may cause corrosion of the heat exchanger and contamination of the catalyst. Over a period of time, the materials in the catalyst-ceramics and zeolites degrade and must be replaced depending upon the industrial sources of the gases. Because the catalysts are made up of heavy metals, disposal of spent catalysts can also be a problem.
In recent years, many wet processes for NOx removal have been developed with the aim of removing NOx and SOx simultaneously. Scrubbing process also used to remove NOx from NOx rich gases is produced in a relatively small amount at metal-dissolving, nitric acid and chemical plants, etc.
This article describes the NOx chemical reaction associated with wet removal processes. The aim, then, is to provide a reasonably clear and uncomplicated basis for evaluation of potential treatment methods to help determine which may be practically applicable in a given situation.
Several oxides of nitrogen are found in the atmosphere but only nitric oxide (NO) and nitrogen dioxide (NO2) are important as air pollutants. The symbol NOx is frequently used to represent the composite of the two. The other nitrogen oxides seldom occur in appreciable quantities and then only under special conditions.
Essentially, NOx contains nitric oxide and nitrogen dioxide in varying proportions. This fluctuating ratio, and the fact that these compounds exhibit quite different properties when contacted with water (as would occur in a wet scrubber) complicate the treatment of NOx.
NO2 gas has fairly high solubility and reactivity to water and in aqueous solutions or alkalis as compared with NO, and can be removed by wet scrubbing. On the other hand, gaseous NO is only slightly soluble in water and is not very reactive with typical aqueous solutions. Nitric oxide does react with oxygen as follows:
2NO+O2 = 2NO2
This equation implies the coexistence of NO and NO2. Calculated equilibria indicates that the stability of NO2 decreases with increasing temperature. Nevertheless, from an equilibrium standpoint, the absolute concentration of NO2 increases with temperature while the ratio of its concentration to that of NO decreases with increasing temperature. The equilibrium concentration of NO varies with temperature; it is negligible below 1000 F but quite significant above 2000 F. (Table 1)
The oxidation of NO is concentration dependent to a marked degree as illustrated in Table 2, which shows the time required for half the NO present in air at various concentrations to be oxidized to NO at ambient temperature. This reaction proceeds more rapidly at a lower temperature than at raised temperatures.
It may then be concluded that it is impossible to reduce effluent concentration below a few hundred parts per million NOx in absorption equipment of practical dimensions when the entering concentration is in the low percent range. The slow oxidation rate for NO in air can be greatly improved by adding an oxidant such as ozone (O3) or chlorine dioxide (ClO2). The oxidation of NO in the gas phase by ozone or chlorine dioxide occurs much more rapidly than oxidation in the liquid phase because the rate of absorption of NO in the aqueous solution is slow. Ozone is capable of oxidizing NO not only to NO2 but also to N2O5 which rapidly reacts with water or alkaline solutions to form nitric acid or nitrates. Ozone, however, is fairly costly making it usually uneconomical.
It has been shown that the use of stoichiometric amounts of ClO2 eliminates approximately 95 percent of the NO in the gas in concentrations of up to at least 24 ppm in less than 2 seconds [4]. Chlorine dioxide is less costly than ozone, but there are inherent difficulties involved in its storage or recovery in terms of equipment maintenance. This is due to its reactive and hazardous nature.
For all practical purposes it is impossible to remove NO gas by wet scrubbing in the situation in which the gas does not contain NO2. It is also known that gaseous NO present in amounts approximately equal to or less than that of gaseous NO2 in a waste gas, when brought into contact with an alkali solution, forms a nitrite and is thereby absorbed as indicated by the formula (2)
If in this case NO2 is present in excess to NO, it reacts with an alkali solution to form nitrate and nitrite and is thereby absorbed as indicated in (3):
where NO and NO2 are present in equal volumes (NO2:NO mole ratio is 1) reaction (2) will principally proceed, while reaction (3) will become secondary. If the ratio by volume of NO to NO2 is greater than 1 the NO equal in volume to NO2 will react to the nitrite, but the excess NO will essentially remain unchanged. Therefore, the reaction between an alkaline solution and NO/NO2 is optimal at a 1:1 molal ratio of the oxides.
It has been determined that the controlling mechanism of NOx absorption is different according to the relative concentration of NO and NO2. But when the NOx concentration is low, N2O3 (reaction 1) does not form in significant amounts even when the NO:NO2 mole ratio is 1 and the absorption rate is low. The underlying reason is that a high level of NO/NO2 concentration is needed to get the high NOx removal efficiency using an alkaline solution. This means that the complete oxidation of NO to NO2 should be done at low concentrations of NO (less 500 ppm) even when the NO:NO2 ratio of the inlet mixture is 1.
However, when the liquid phase concentration of nitrite and nitrate is relatively high, (Reaction 3) an increase in its concentration causes a decrease in the percent of NO+NO2 removal from the gas, due to the secondary generation of NO that takes place:
The reaction (4) limits the degree of absorption that takes place using aqueous sodium hydroxide as the scrubbing liquid. Consequently even after the gas phase oxidation of NO to NO2, it is possible to get the new evolution of NO in the liquid phase.
If the gas stream contains NOx and SO2 simultaneously, better results of NOx absorption using sodium hydroxide may be achieved. When this mixture is contacted with aqueous NaOH solution, the SO2 reacts very quickly and forms the sodium sulfite or bisulfite, which can react with NOx. The sodium sulfite exhibits a higher reaction rate with NO2 when compared with sodium hydroxide [3]. Although the reaction mechanisms are not clear, the main reaction that likely occurs when a large excess of sulfite ion is present may be described simply as follows:
NO, however reacts very poorly even with sulfite solutions. The reaction of NO can be promoted by use of a liquid catalyst. Better results are obtained for the absorption of SO2 and NOx by ammonia solutions containing a soluble catalyst in comparison with aqueous NaOH solution. The overall NO reaction, where the intermediate compounds are ammonium sulfite and bisulfate, may be expressed as:
It was found that for recovery of 80% of 200ppm NOx the gaseous concentration of SO2 must be more than 1200 ppm [3]. The use of sulfite solution may not be suitable for highly effective removal for large quantities of waste gas particularly when it is oxygen rich resulting in the oxidizing of sulfite to unreactive sulfate. The fact is that a single stage of wet scrubbing cannot provide highly efficient NOx removal generally and especially for NO. For this application, it is preferable to use a first stage for oxidation and second stage for absorption.
An exception to this is the particular NOx Scrubber [6] [7]. This technology utilizes a so-called “surface active media” in a counter current packed tower design generally scrubbing with alkali media. For total NOx concentrations typically >2000 ppmV, NO2:NO mole ratio 2:1 or greater, and O2:NOx mole ratios >5:1 exceptionally high NOx removal (90-99%) can practically be achieved.
Several liquid phase oxidants can be used such as in general hydrogen peroxide potassium permanganate, sodium hypochlorite. The hydrogen peroxide needs care in handling. The potassium permanganate requires the added maintenance to remove the manganese dioxide, a precipitate that forms on the packing. Practically speaking, the most economical of the oxidizing agents is sodium hypochlorite. This usually comes in the form of an alkaline solution to prevent decomposition of sodium hypochlorite to Cl2 and Cl2O and to result in the optimum oxidizing properties. The optimum pH of that scrubbing solution is about 9, where the oxidizing properties of NaOCl are the best. This pH value is where reaction NaOCl NaClO is close to equilibrium and the concentration of NaClO (sodium isohypochlorite) which has the tendency to release the active oxygen is maximum. The optimal pH increases with increasing gas contact time [9]. The oxidizing reaction of NO by sodium isohypochlorite is as follows:
The liquid phase utilized in absorption towers can consist of various chemicals. In this case, alkaline solutions, sodium bisulfite sodium hydrosulfide are used in the scrubbing solutions [10]. For example, when sodium hydrosulfide is used, the NOx reaction may be as follows:
For oxidizing towers the normal engineering design approach for absorption, based on specific mass-transfer and reaction rate data, is not valid. Accordingly, a large mass-transfer surface is usually required.
For the absorption tower it is suggested to determine the relative effects of mass-transfer and chemical reaction for the absorption of NO-NO2 mixture.
Where Kga is the overall absorption coefficient kga is the gas-phase mass transfer coefficient and kla is the liquid-phase mass transfer coefficient. The factor is the coefficient which represents the effect of chemical reaction and H is Henry’s law coefficient. The mass transfer coefficient for NO + NO2 were calculated by determining the coefficients for CO2 in water (sparingly soluble gas, liquid phase resistance rate limiting) and SO2 in water (highly soluble gas, resistance of gas and liquid phases comparable) and correcting for the differences in diffusion rates, viscosity’s and densities of CO2, SO2 and NO + NO2. It was concluded that the boundary of chemical interaction between reacting components move toward the liquid surface with increasing liquid flow rate and that the rate is influenced by both diffusion of the active component in the gas and diffusion of the active component as well as the reaction product in the liquid.
A practical and economical design for the wet scrubbing of NOx can therefore be arrived at. However, given the somewhat unusual design factors involved relative to more straight forward absorptive mass transfer chemical systems specific knowledge of the principles involved along with availability of empirical data is critical to determining an effective design.
Emission Control Areas (ECAs), or Sulfur Emission Control Areas (SECAs), are sea areas in which stricter controls were established to minimize airborne emissions from ships as defined by Annex VI[1] of the 1997 MARPOL Protocol.
Sox Air Pollution
The emissions specifically include SOx, NOx, ODSs and VOCs[2] and the regulations came into effect in May 2005.[3][4] Annex VI contains provisions for two sets of emission and fuel quality requirements regarding SOx and PM, or NOx, a global requirement and more stringent controls in special Emission Control Areas (ECA).[5] The regulations stems from concerns about 'local and global air pollution and environmental problems' in regard to the shipping industry's contribution. In July 2010, a revised more stringent Annex VI was enforced in the Emission Control Areas with significantly lowered emission limits.[2]
As of 2011 there were four existing ECAs: the Baltic Sea, the North Sea,[4] the North American ECA, including most of US[6] and Canadian coast[5] and the US Caribbean ECA.[5] Also other areas may be added via protocol defined in Annex VI. ECAs with nitrogen oxides thresholds are denoted as Nitrogen Oxide Emission Control Areas (NECAs).
| before 1 July 2010 | 1.50% m/m |
| between 1 July 2010 and 1 January 2015 | 1.00% m/m |
| after 1 January 2015 | 0.10% m/m |
| before 1 January 2012 | 4.50% m/m |
| between 1 January 2012 and 1 January 2020 | 3.50% m/m |
| after 1 January 2020[note 1] | 0.50% m/m |
Context[edit]
In 1972 with the United Nations Conference on the Human Environment, widespread concerns about air pollution led to international cooperation. Air pollution from 'noxious gases from ships' exhausts' was already being discussed internationally. On 2 November 1973 the International Convention for the Prevention of Pollution from Ships was adopted and later modified by the 1978 Protocol (MARPOL 73/78). MARPOL is short for Marine Pollution. In 1979, the Convention on Long-Range Transboundary Air Pollution, the 'first international legally binding instrument to deal with problems of air pollution' was signed.[3] In 1997 the regulations regarding air pollution from ships as described in Annex VI of the MARPOL Convention were adopted. These 'regulations set limits on sulfur oxide (SOx) and nitrogen oxide (NOx) emissions from ship exhausts and prohibit deliberate emissions of ozone-depleting substances.'[1] The current convention is a combination of 1973 Convention and the 1978 Protocol. It entered into force on 2 October 1983. According to the IMO, a United Nations agency responsible for the 'safety and security of shipping and the prevention of marine pollution by ships', as of May 2013, 152 states, representing 99.2 per cent of the world's shipping tonnage, are parties to the convention.[8]
SECAs or ECAs[edit]
As of 2011 existing ECAs include the Baltic Sea (SOx, adopted 1997; enforced 2005) and the North Sea (SOx, 2005/2006 adopted July 2005; enforced 2006),[4] the North American ECA, including most of US[6] and Canadian coast (NOx & SOx, 2010/2012)[5] and the US Caribbean ECA, including Puerto Rico and the US Virgin Islands (NOx & SOx, 2011/2014).[5][9]
The Protocol of 1997 ( MARPOL Annex VI ) included the new Annex VI of MARPOL 73/78, which went in effect on the 19th of May 2005.
SOx emissions control[edit]
The purpose of the protocol was to reduce and to control the emissions coming from the marine vessels’ exhausts that pollute the environment. MARPOL convinced IMO to control the average worldwide sulfur content fuels. The Annex states that a global cap is 4.5 %m/m on the sulfur content in fuel. However, MARPOL insist on it being 1.5 %m/m in some regions classified as “SOx Emission Control Areas” (SECAs).[10]
On the other hand, MARPOL came up with a way to avoid using an exhaust gas cleaning systems or anything else that would limit SOx emissions. In fact, the exhaust gas cleaning systems must be approved by the State Administration before put into use. The regulations on the exhaust gas cleaning systems are to set by IMO.[10]
The monitoring of sulfur content of residual fuel supplied for use on board ships is being performed by IMO since 1999. The IMO monitors it by the bunker reports around the world. According to The Marine Environment Protection Committee (MEPC) the worldwide average sulfur content in fuel oils for 2004 was 2.67 %m/m.[10]
Nitrogen Oxide (NOx) emissions – Regulation 13[edit]
NOx control requirements apply worldwide to any installed marine diesel engine over 130 kW of output power other than the engines used solely for emergency purposes not in respect of the marine vessel’s tonnage where the engine is installed. However, there are different levels of regulations that are based on the ship’s date of construction. Those levels are broken down into 3 Tiers. Tier I applies to the ships built after January 1st of 2000. It states that for engines below 130 rpm must have the total weighted cycle emission limit (g/kWh) of 17, engines that are between 130 and 1999 rpm must have no more than 12.1 (g/kWh), engines above 2000 rpm must have the limit of 9.8 (g/kWh). Tier II has the following requirements: 14.4 (g/kWh) for engines less than 130 rpm, 9.7 (g/kWh) for engines 130 – 1999 rpm, and for engines over 2000 rpm 7.7 (g/kWh) is the limit. Tier II limits apply to the ships constructed after January 1st of 2011.Tier III controls only apply in the specific areas where the NOx emission are more seriously controlled (NECAs) and apply to the ships constructed after January 1st of 2016. For engines under 130 rpm the limit is 3.4 (g/Kwh), engines between 130-1999 rpm the limit us 2.4 (g/kWh), engines above 2000 rpm must have the total weighted cycle emission limit of 2.0 (g/kWh).[11][12]
Incineration[edit]
Annex VI prohibits burning certain products aboard the ship. Those products include: contaminated packaging materials and polychlorinated biphenyls, garbage, as defined by Annex V, containing more than traces of heavy metals, refined petroleum products containing halogen compounds, sewage sludge, and sludge oil.
Greenhouse gas policy[edit]
The Marine Environment Protection Committee (MEPC) has strongly encouraged members to use the scheme to report the greenhouse gas emissions. Those gases include carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. The purpose of making the guidelines on CO2 emissions is to develop a system that would be used by ships during a trial period.[13]
Regulations of 2013[edit]
In 2013 new regulations described in a chapter added to the MARPOL Annex VI came into effect in order to improve 'energy efficiency of international shipping'. The regulations apply to all marine vessels 400 gross tonnage or above. MARPOL requires the ship industry to use the EEDI mechanism that would ensure that all the required energy-efficiency levels are met. Also, all the ships are required to have a Ship Energy Efficiency Management Plan (SEEMP) on board, therefore, the seafarers always have a plan to refer to in order to maintain energy-efficiency levels required by the area the ship is at or sailing to at all times.
As for the additions to the Annex VI, there were corrections towards emissions, sewage, and garbage.Prior to the regulations adjusted in 2013 the sulfur emission control areas included: the Baltic Sea, the North Sea and the North American Area (coastal areas of the United States and Canada). However, the updated in 2013 version of Annex VI included the United States Caribbean Sea (specifically areas around Puerto Rico and the United States Virgin Islands) to the list.
As for the other regulations updates, there is possibility of establishing “special areas” where sewage discharge laws would be outstandingly stricter than in other areas, as well as, the few minor additions to the garbage disposal laws.[13]
Nox Oxygen
Notes[edit]
References[edit]
- ^ abNew rules to reduce emissions from ships enter into force, Briefing 23, International Maritime Organization, 18 May 2005, retrieved 4 May 2014
- ^ abAir Pollution and Greenhouse Gas Emissions, International Maritime Organization (IMO), 2014a, retrieved 4 May 2014
- ^ abPrevention of Air Pollution from Ships, International Maritime Organization (IMO), 2008, retrieved 4 May 2014
- ^ abcSulphur Oxides, International Maritime Organization, 2014, archived from the original on 23 December 2014, retrieved 4 May 2014
- ^ abcdeEmission StandardsInternational: IMO Marine Engine Regulations: Background, Diesel Net, September 2011, retrieved 4 May 2014
- ^ abCalifornia SECA Regulations Upheld by Supreme Court, Marine Link, 20 July 2012, retrieved 4 May 2014
- ^Sulphur oxides (SOx) – Regulation 14, International Maritime Organization (IMO), archived from the original on 2014-12-23
- ^Status of Convention, International Maritime Organization, 2013, archived from the original on 6 May 2014, retrieved 4 May 2014
- ^Special Areas under MARPOL, International Maritime Organization, 2014b, archived from the original on 1 May 2014, retrieved 4 May 2014
- ^ abc'Air pollution'. www.imo.org. Retrieved 2017-04-08.
- ^'Nitrogen oxides (NOx) – Regulation 13'. www.imo.org. Retrieved 2017-04-08.
- ^EPA,OECA,OAP,ITD, US. 'MARPOL Annex VI'. www.epa.gov. Retrieved 2017-04-08.CS1 maint: multiple names: authors list (link)
- ^ ab'Nitrogen oxides (NOx) – Regulation 13'. www.imo.org. Retrieved 2017-04-08.
Further reading[edit]
- 'A new ECA and speed reduction limits in South Korean ports'. DNV. 27 April 2020. Retrieved 11 June 2021.