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The Formation, Effects and Control of Oxides of Nitrogen in Diesel Engines

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The Formation, Effects and Control of Oxides of Nitrogen in Diesel Engines International Journal of Applied Engineering Research ISSN 0973-4562 Volume 13, Number 6 (2018) pp. 3200-3209 © Research India Publications. http://www.ripublication.com The Formation, Effects and Control of Oxides of Nitrogen in Diesel Engines Maroa Semakula1and Prof Freddie Inambao2 1,2University of Kwazulu-Natal Durban South-Africa. Abstract HPL High Pressure Loop EGR The transport service industry is a heavy user of diesel IDICI Indirect Injection Compression Ignition propelled engines as prime movers of goods and services. The diesel propelled engine is praised due to its high fuel efficiency, L Length or Piston Stroke reliability and durability. However, the nitrogen emissions as a result of diesel fuel combustion characteristics raise major LPL Low Pressure Loop EGR concerns for the manufacturing industry, environmentalists and health care researchers. The manner in which diesel engines M Organic Residue combust their fuel is the main cause of the nitrogen oxide NG Natural Gas emission proportion. Although there are other sources of nitrogen oxide emission, this work will cover the sources of NO Nitrogen Oxide nitrogen oxides and their formation within the diesel engine, their routes of formation, identify the mechanisms under which NOX Oxides of Nitrogen Excluding Nitrogen Trioxide the formations occur, identify their types and interactions, look Up at the various effects of the oxides of nitrogen on human health and the overall damage to the natural environment, and look OH Water or Hydroxide Radical critically at control systems. P Pressure Keywords: Nitrogen Oxides, Human Health, Formation Mechanisms, Durability. PCCI Pre-Mixed Charge Compression Ignition PM Particulate Matter NOMENCLATURE AND ABBREVIATIONS rc Compression Ratio A/F Air-Fuel Ratio RCCI Reactivity Charge Compression Ignition ATDC After Top Dead Centre SCR Selective Catalytic Reduction B Cylinder Bore SI Spark Ignition BTDC Before Bottom Dead Centre SO2 Sulphur Dioxide CA Crank Angle SOI Start of Injection CI Compression Ignition UV Ultra-Violet Rays CN Carbon of Nitrogen Vc Clearance Volume CO Carbon Oxide Vd Swept or Displaced Volume DICI Direct Injection Compression Ignition λ The Air Excess Factor Symbol Lambda DOC Diesel Oxidation Catalyst DPF Diesel Particulate Filter INTRODUCTION EGR Exhaust Gas Recirculation The modern-day diesel engine, also known as the compression ignition engine, offers high fuel efficiency, low engine noise, F/A Fuel-Air Ratio reliability and durability during its life and service. However, diesel engines produce more oxides of nitrogen emissions than HCCI Homogeneous Charge Compression Ignition their counterparts, spark ignition (SI) engines. This collection of oxides of nitrogen emissions are collectively referred to as HCN Hydrogen Cyanide NOX. Along with particulate matter emissions (PM), these 3200 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 13, Number 6 (2018) pp. 3200-3209 © Research India Publications. http://www.ripublication.com emissions have become major environmental and health Prompt NOx or The Fenimore Mechanism concerns and have also become important themes in global Prompt NO , also known as Fenimore mechanism (named after discussions and forums. X the person who discovered it [8], accounts for the smallest Due to the ever increasingly stringent regulations on emissions contribution to the quantity of NOX. It is usually formed at the by environmental protection agencies and governments, there flame front especially in rich fuel-air ratio areas due to has been development and improvement in design to unavailability of oxygen, from the radicals of CH through their accommodate and conform with the growing list of emission oxidation when they react with molecular nitrogen (N2), standards, especially developments in diesel fuel improvement leading to the formation of cyanhydric acid (HCN) and nitrogen [1] and [2]. Among the fuels for the 21st century being oxide (NO) at the termination of the reactions. The general proposed as an alternative to diesel fuel is natural gas (NG), scheme of prompt NOX causes hydrocarbon radicals to react which is highly promising and attractive according to Abdelaal with molecular nitrogen to form amines or cyano compounds and Hegab (2012) [3]. The advantages of NG include which are then transferred and converted to intermediate availability, lower price, and reduction in CO emissions due the compounds thus forming NO as can be shown in equation 1 and low carbon to hydrogen ratio. Further, because of its high equation 2 below and expressed as: octane number, NG has a high auto-ignition temperature 퐶퐻 + 푁 ⟷ 퐻퐶푁 + 푁 Equation 1 characteristic which eliminates compression ignition (CI) 2 engine knock such as it occurs during high compression ratios 퐶 + 푁2 ⟷ 퐶푁 + 푁 Equation 2 due to the low octane number of diesel fuel. Above all, it is eco- friendly with clean combustion compared to conventional diesel fuel engines. NOX emissions are approximately 50% to Where equation 2 forms the primary path and becomes the rate 80% less, with zero smoke and PM emissions – something that limiting step in the reaction chain of the entire mechanism. is very hard to achieve in basic diesel propelled engines [3]. However, it is vital to mention here that in the diesel engine Besides the use of alternative fuels as a method of reducing combustion process, NOX is generated through the Fenimore mechanism or prompt NOX and the thermal mechanism or NOX emissions, the use of other control strategies and measures besides fuels has become equally important in mitigating the Zeldovich mechanism only. The conversion of hydrogen cyanide (HCN) to form NO takes the following path as effects and impact of NOX emissions. One of the techniques that has gained widespread use, although not as a standalone expressed in the following equations as: technology, is exhaust gas recirculation (EGR). This technique 퐻퐶푁 + 푂 ⟷ 푁퐶푂 + 퐻 Equation 3 has been found to be an effective tool in reducing the emissions 푁퐶푂 + 퐻 ⟷ 푁퐻 + 퐶푂 Equation 4 of NOX [4, 5]. Although the combustion of biodiesels and its blends is 푁퐻 + 퐻 ⟷ 푁 + 퐻2 Equation 5 different from conventional diesel and fossil fuel in 푁 + 푂퐻 ⟷ 푁푂 + 퐻 Equation 6 compression ignition engines due to their physio-chemical fuel properties, they do also cause emission of NOX, PM, CO and HC [6]. There has been a tremendous development in 2.2 The Thermal NOx (Zeldovich Mechanism) combustion technologies for biodiesel fuel combustion in diesel compression ignition engines, to accommodate the The Zeldovich mechanism which takes the intermediate route developments witnessed in the growth and expansion in is an important mechanism in lean fuels Փ < 0.8 operating alternative fuels, especially the biodiesels. Among the under low temperature conditions as can be seen in the three strategies that have been developed are: homogeneous charge equations below with M as the organic residue [9] as: controlled ignition (HCCI), pre-mixed charge controlled 푂 + 푁 + 푀 → 푁 푂 + 푀 Equation 7 ignition (PCCI) and reactive charge controlled ignition (RCCI). 2 2 These strategies have been extensively studied and found to 퐻 + 푁2푂 ⟷ 푁푂 + 푁퐻 Equation 8 significantly reduce NOX and other emissions [7]. 푂 + 푁2 ⟷ 푁푂 + 푁푂 Equation 9 The Zeldovich scheme mechanism consists of three main SOURCES OF OXIDES OF NITROGEN AND reaction chains as shown in the following equations and are FORMATION coupled to fuel combustion chemistry through O2, O and OH species [10] as follows: There are five mechanisms of NOX formation in a diesel engine combustion process: 푂 + 푁2 ⟷ 푁푂 + 푁 Equation 10 Fenimore CN and HCN pathways. 푁 + 푂2 ⟷ 푁푂 + 푂 Equation 11 N O intermediate route or the Zeldovich mechanism. 2 푁 + 푂퐻 → 푁푂 + 퐻 Equation 12 Due to super equilibrium concentrations of O and OH. Radicals in the extended Zeldovich mechanism The thermal NOX formation of oxides of nitrogen depend on reactions. the following factors for their formation and propagation: The NNH route. Temperature especially in the reaction zone. Other than the disassociation process in the equation 2 of 3201 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 13, Number 6 (2018) pp. 3200-3209 © Research India Publications. http://www.ripublication.com oxygen, the reaction chain is inherently a temperature Due to the fact that hydrocarbon fuel contains organic bonded function as concluded by the studies conducted by nitrogen (organic nitrogenous compounds) some of the in- Dangar and Rathod (2013) [11]. oxidized nitrogen will eventually be oxidized to be NOX. The The equivalence ratio or the air-fuel ratio in the percentage amount of nitrogen going through this process reaction zone as it influences the atomic oxygen depends on the nature of the combustion process. This is concentration within a combustion mixture. It has compounded by the fact that light distillate fuels contain 0.06 been observed that the NOX emissions decrease with a % by volume of organic hydrogen, while the heavy distillates decrease in the air-fuel ratio [12]. contain 1.5 % or above. Thus, it can clearly be seen that The amount of time or duration of the reacting gases, depending on the amount of or percentage of nitrogen also called retention time, spent in the reaction zone at converted into fuel, NOX can contain large portions of and maximum temperature determine the amount of NOX percentages of total NOX emissions [20]. formed. The shorter the time the smaller the quantity The oxidation of the nitrogenous compounds is known to be of NOX and vice versa. less temperature dependent; thus, oxidation can even occur at low temperatures as the air excess ratio increases. However, during combustion a portion of the nitrogen fuel is changed to The NNH Mechanism NOX especially in fuel rich areas of combustion where This is a recently discovered reaction pathway [13], with two thermally formed NOX can be reduced to molecular nitrogen major reaction mechanisms which has been shown to be [21] as demonstrated by Figure 1 with the reactions taking the important especially in the combustion of hydrogen [14, 15] following forms: and hydrocarbon fuels which have large carbon to hydrogen 1 푁퐻 + 푂 → 푁푂 + 퐻 Equation 15 ratios [16].
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