Gemi dizel motorlarından kaynaklanan emisyonların kontrolü ve SCR teknolojisi

Abstract

Bu çalışmada, gemilerden çıkan dizel motor egzost emisyonları, bunların miktarı, zararları ve çeşitleri incelenmiştir. Bu emisyonların global hava kirliliğine katkısı da ayrıca incelenmiştir. Daha sonra bu zararlı emisyonlar hakkında dünyada uygulanan sınırlamalar, yeni uluslararası düzenlemeler ve getirilen kurallar belirtilmiştir. Çalışmada deniz dizel motorlarının egzost emisyonlarının kontrolü ve azaltılması için uygulanan çeşitli metod ve teknolojiler anlatılmakta, özellikle SCR (Selective Cataltytic Reduction) ayrıntısıyla incelenerek en iyi çözüm yolu olarak sunulmaktadır. Bunun yanında, dünyanın önde gelen dizel motor üreticileri ve mühendislik firmalarının NOx' in azaltılması konusunda yaptığı çalışmalar verilmektedir. Son olarak, Lıoyd' s Register tarafından 10 yıl önce başlatılan ve birbirine bağlı bir dizi araştırmadan oluşan Deniz Emisyonları Araştırma Programı' nın çeşitli etapları, amaçlan ve uygulanışı anlatılmaktadır.

Enviromental isues are becoming more topical th an ever and the concern for four enviroment, both on local, reginoal level as well as on a global scale, is extending through all sections of society in the developed world. On a global scale the chief subject of concern is probably air pollution. This true also for certain regional and local areas such as southern Californiya, parts of Europe and of course all the major cities around the world. Air pollution may be defined as any atmosferic condition in which subtances are present at concentrations high enough above their normal ambient levels to produce a measurable (negative) effect on man, animals, vegetation ormaterials. The natural occurrence of airborne subtancesd varies over time. The global concentration of CO2, for instance, varies according to a pattern reflected by the seasonal growth and dieback of plants, which soak up prodigious amounts of CO2 during spring and summer. For every substance or combination of substances there is a level of concentration critical to the enviroment. If the critical level exceeded over a period of time the enviroment becomes overstressed or even irreversibly damaged. A small increase of man- made ' superimposed ' air pollution will eventually push the natural or background ' pollution ' over the critical limit. This, in a very condensed form, is the process leading to enviromental fatigue. The magnitude of the enviromental stress or load has been reduced in many local areas during the past 1 5 - 20 years. Southern California is but one example where stringent control of road traffic emissions has led to reduced levels of, for instance, hydrocarbons ( HC ) and carbon monoxide ( CO ). On the regional and global scale, however. The enviromental stress is still on the increase. The Baltic Sea is one region where, in spite of extensive control measures applied, the enviromental stress is inreasing. Air pollution from stationary, as well as mobile, sources in the region and ' imported ' air pollution are two contributing factors to this situation.. On a global scale the state of the enviroment will deteriorate for a long time yet until a balance is again reached. One effective way too restore the delicate ' eco balance ' in nature is to use emission control whenever and whereever possible. Contemporary DI medium speed marine diesel engines produced about 1.7 Nm3 exhaust gas/ s and MW, irrespective of make, type of fuel used and duty. Less than % 0.5 of thie gasflow is regarded as a pollution, not counting CO2 inthis category of pollution.. In fact the exhaiust from a diesel will not differ very much from the human ' exhaust ' especially if we compare exhaust from diesel with that of a heavy smoker. (A cigarette produces approximately 120 ppm of NOx and considerable amounts of CO. ). To capture and ' neutralise ' such small concenrations is a very difficult task indeed, especially if we consider, for instance, that the size distribution of Xlll diesel exhaust particulate ( PM ) is such that the bulk ( > % 70 ) of the PM is smaller than the size of bacteria or less than lum across. In spite of the relatively moderate concentrations of noxious substances present in diesel exhaust the sheer size of large marine diesels means that the emissions on a mass basis from a diesel powered ship can be very substantial. A ship without exhaust control and a total installed engine powered of 20 MW will produce vast amounts of NOx, PM, etc every year. The exhaust emissions from the engine wil in general comprise oxygen, nitrojen, carbon monoxide, carbon dioxide, water vapour, particulates, unburnt and partially combusted hydrocarbons and the oxides of sulphur and nitrogen together with a variety of minor components most of which are generally of little importance but in some instances give rise to considerable concern. Oxygen and nitrogen The free oxygen component of the exhaust gas derives from the excess air ratioi used by engine. As a consequence, a considerable proportion of oxygen can be present in the exhaust gas of marine engine. Furthermore, since nitrogen forms about % 78 of the atmosphere by volume the substantial component of the engine exhaust comprises this gas. In general nitrogen is an unreactive gas, however, a small but important amount does react chemically to form the various oxides of nitrogen ; for example NO, NO2, N2O. etc. Oxides of nitrogen The temperatures and pressures encountered in the modern marine engine combustion chamber will inevitably lead to chemical reactions taking places which form a series of nitrogen oxides. The major component of these oxides which forms during the initial part of the combustion process will be nitrogen oxide, NO. However, later in the cycle and also during the flow through the exhaust system a proportion of the NO will convert into higher order oxide components: NO2 and N2O. The relative quantities of these latter two components are generally small: of the order of % 5 - 10 of the total quantity of NOx with N2O being smaller of the two. In general the oxides of nitrogen have implications for acid rain since the conversion of NO, NO2 will continue to take place in the atmosphere after expulsion from the engine; in this latter state it is soluble in water and, therefore, will increase the acidity of the soil by falling as rain. In addition these oxides also contribute to photochemical smog production. Furthermore, NîOfrom all sources, both marine and industrial, is a contributor, estimated to be of order of some % 5, to the global warming effect. The control of NO emissions can be achieved by catalytic conversion in which the NOx content is reduced by NH3 to N2 and H2 O: techniques of this type are able to reduce the NOx content considerably. One possibilty that has been sugggested for commercial NOx control is the water emulsification of the fuel oil in order to inhibit the NO formation and the follow this with an exhaust catalytic conversion system. The alternative to control processes of this type is to attempt to introduce major XIV changes to the underlying design philosophy of diesel engines that has been prevalant over the last 10 - 20 years and which has centred on the pursuit of engine efficieny. This philosophy, based as it is on improving fuel economy, has lead directly to the use of high temperatures and pressures which are conductive to increased NOx formation. Hence, it is difficult to reconcile a departure from this philosophy on an overall consideration of enviromental matters since it would conflict with the energy management concepts which limit the other component exhaust species, chiefly carbon dioxide. Carbon dioxide The basic equilibrium combustion equations relating to hydrocarbons dictate that carbon dioxide and water vapour will be formed proportions determined primarily by the structure of the hydrocarbon in the fuel. As such, the production of CO2 is a function of the quantity of fuel burnt, which to large extent is determined by the engine power required, plant efficiency and the chemical structure of the fuel being burnt. The importance of CO2 as an exhaust component lies in its contribution to the global warming effect. It is estimated that carbon dioxide accounts for some % 55 of the greenhouse gases of which some % 40 comes from energy conversion of all kinds: power generation, transport, industry and domestic usage. Its control, assuming that conventional fuels remain in use, wil lie in energy mangements techniques since to reduce CO2 it is necessary to reduce the amount of fuel burnt to achieve the required power output or, alternatively, reduce the overall energy requirement by more efficient operation and energy conservations strategies. Carbon monoxide Carbon monoxide formation is principally a function of the excess air ratio and the temperature of the combustion within cylinder. In general this component should be found in relatively low concentrations in the higher power ranges, but at low power or in badly maintained engines the proportion of CO is found to increase considerably in its relative concentration. Oxides of sulphur The oxides of sulphur derive directly from the sulphur content of the fuels used. The sulphur in the combustion chamber reacts and principally forms SO2 but also a lesser quantity of SO3. The use of alkaline lubricants only protect the engine surfaces from corrosion and convert a relatively insignificant proportion of the SOx produced by the combustion process to calcium sulphate. The only methods for controlling the SOx componentsin the engine exhaust are either to remove or limit the sulphur content from the fuel prior to the bunkering operation or, alternatively, to ' scrub ' the exhaust gases by means of a washing system. The latter concept of ' scrubbing ' the exhaust gases clearly leads to the requirement for a disposal system to be introduced into the vessels machinery system whilst, in the former case of limitng the amount of sulphur in the fuel, the price of the XV fuel would be considerably influenced by the amount of sulphur removal required which, of course, is a variable depending upon the origin of the crude stocks. This would then place a differential cost penalty on various production sources and wil also introduce a land based sulphur disposal problem. As a concequence, low sulphur fuel costs versus the cost of providing scrubbing systems will require careful evaluation by the shipowning and refining communities; indeed it is quite likely that the optimal economic solution will vary between operators. The importance of sulphur emissions, as is generally known, lies in the contribution they make to the increased acidity of precipitation and the consequent effect this has on the land and water chemistry. Hydrocarbons During combustion small quantities of hydrocarbons remain unburnt whilst others will be formed. In general the concetration of hydrocarbnsin the exhaust gases varies with engine load and can be considered to follow a similiar trend to the carbon monoxide component. Again, the hydrocarbon content assumes an importance in the context of global warming where the sum of all hydrocarbons emitted from all industrial sources of energy conversion are estimated to contribute some % 20 to this effect. Particulates The particulate component of exhaust emissions comprises, in the main, unburnt carbon from the fuel, absorbed hydrocarbons, sulphur components and a selection of trace metals from the fuel oil together with calcium particles from the lubricating oil. The particulate cointent of the emission is difficult to quantify with precision. However, their relative content is a function of the fuel quality and combustion efficiency and, therefore, related to engine load and condition. Marine diesel engines builders all over the world are working hard to meet the emission regulations proposed by the International Maritime Organization ( IMO ). These are expected to take effect from 1998 and will govern primarily NOx and SOx emissions. For international waters, IMO is recomending NOx limits ranging from 17 to 9.8 g/ kWh depending on the engine speed, with a further target to reduce NOx % 30 by the year 2000. To meet the proposed regulation, the main thrust of engine builders has ben into primary solutions, to reduce NOx to acceptable levels without paying too much of a penalty interms of fuel consumption and efficieny. The possibility that primary measures such as retarded timing, variable injection rate etc, would provide a solution, if very sustantial NOx reductions are called for, for either two or four cycle engines in yhe furture looks bleak. Emission rated engines have been supplied by a reputable engine manufacturer and when checked for NOx emission factors they proved to be slightly worse than their economy rated equlavents. Most primary measures trade NOx against specific fuel consumption ( SFOC ), which means that the emission of CO2 increases. Since it is really not known for certain yet that NOx is worse than CO2 in yht long run, any primary measure that trades NOx for C02 should be avoided. Because diesel engines exploit the greatest possible temperature difference during the work cycle for high efficiency, and the fact that NOx XVI formation is strongly promoted by high temperatures, it is very difficult not impossible to combine very low NOx concentrations with high efficieny with any primary measures. This particularly true if poor quality heavy fuel oil is utilised, because this exludes the most promising primary measure - Exhaust Gas recirculation ( EGR ) EGR requires an exhaust gas almost free of abrasive particulates and exhaust gas cooling. Even with better fuels the particulate ( soot ) will be a major obstacle because as long as liquid fuel is injected into the combustion chamberthe formationof soot can never be avoided. This is another dilemma with the diesel engine that can be solved only by exhaust gas treatment such as particulate traps, or perhaps oxidation catalysts in combination with ultra low sulphur content in the fuel. It appears from current research and development work undertaken by engine builders that a % 50 NOx reduction is possible in some cases if no further savings in fuel economy are the objective. This is the real dilemma with the diesel engine in that HC, CO and PM are inversely propotional to NOx making it very difficult if not impossible to reduce all the various unwanted substances at the same time without some form of post - combustion treatment of the exhaust. Over the years numerous methods have been tried in order to alter the combustion by, for instance, introducing water into the combustion chamber where greater heat capacity is utilised to reduce high peak temperatures responsible for the NOx formation. None of this methods have proved very effective some of them produce severe'side effects'on the engine. Becauseof high oxygen content ( 10 - 15 % ) and low CO content ( 100 - 200 ppm ) in diesel exhaust the traditional threeway auotomobile catalyst will not provide a solution in respect of the control of diesel exhaust by post-combustion treatment for NOx, HC, CO and PM simultanously. Instead so-called SCR system can be used, where a reducing atmosphere is created over a catalyst by injecting small amounts of single atom nitrogen ( N ) based additive, such as ammonia ( NH3 ), into the exhaust, upstream. Marinesed urea based SCR systems which are compatible with low sulphur fiel oil ( approximately % 1 sulphur or less ) and exhaust temperatures of approximately 280 - 500 C, are commercially available and can be utilised for propulsion as well as auxiliary engines between 0.25 and 50 MW. For large bore slow - speed engines, where the exhaust temperature is below 280° C, it may be feasible to reheat the exhaust to 300° C to get well within the working range of the SCR. The extra energy required will almost be recovered by the exhaust boiler and the elevated gas temperature prevents condensation of sulphuric acid inside the boiler. If a supplementary oil firing is used, massive amounts of soot and unburned hydrocarbons from the fuel and lub oil will be burned off in the exhaust train, leading to a much cleaner exhaust gas and less boiler maintenance. Placing the SCR converter before the turbocharger is another, but less attractive, alternative that has been tried by several makers of large bore diesel engines. NOx can be reduced very effectively along with HC by commercially available SCR converters, without affecting the high efficiency of the diesel engine, at a moderate investment cost and a low operating cost. The operating cost ( urea and catalyst wear ) typically amounts to US 4$ 1 - 1.5 per MWh depending on operating conditions, NOx baseline and at % 90 NOx reduction. The SCR technology is the most cost effective method of all methods yet developed, where a high NOx reduction is required. This tecchnology can reduce NOx to neglibile levels, along with HC that XVU are converted to CO and CO2 at a rate of % 75 -90, without problematic emissions of other unwanted substances. Marine Research Programme is directed towards fulfillingthe identified need for an exhausted emissions data base and can be viewed as an extension of the continuing programme of study which Lloyd' s Register has been undertaking over the last decade or so. The progrmme is defined as a two phase initiative in which the phase are planned to overlap to a limited extent Phase I aims to quantify the exhaust emissions under the under the ' open sea ' or steady state operating conditions whilst Phase II addresses the' local ' or transient emission problem. Both phases of the programme will culminate in reports published by Lloyd' s Registerwith the Phase I report being generally available in the autumn of this year. Phase I is wholly funded by Lloyd' s Register, However, the second phase will be based on external.