10-09-2016, 04:08 PM
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Abstract - Extensive usage of compression ignition engines are big challenges in terms of pollution and depleting fossil fuel reserves. Reasonable improvements in emission levels were achieved with the efforts constituted by various researchers and automobile industries worldwide to control emission associated issues both on road and off road diesel vehicles with developed technology for cleaner diesel fuels, quick responding electronic control units, advanced engine design and effective exhaust after treatment. Nitrogen oxides (NOX) are a very important and interesting family of air polluting chemical compounds. This paper explains why NOXare important air pollutants and how NOX are formed and react in the atmosphere. This paper also discusses the principles on which all NOX control and pollution prevention technologies are based; available NOX technologies for various combustion sources; and performance and cost of NOX technologies.
INTRODUCTION
Theoretical combustion in engines should result CO2 and H2O as exhaust emissions. However, actual combustion will never be theoretical due to several operating and design conditions. Fuel comprises not only hydrocarbons but also traces of sulfur, nitrogen and other chemicals and air comprises several constituents other than O2 and N2. A number of compounds, both gaseous and solid, appear in the engine exhaust when combustion takes place between fuel and air. Heterogeneous nature of air fuel mixture in diesel engines, high combustion pressure and temperatures further complicates the combustion process. Regions of rich and poor mixture ratios along with local stoichiometric composition pockets exist within the combustion chamber. Fuel rich composition produce CO, soot, dark smoke and unburnt HC, on the other hand fuel oxygen rich regions generate NOX at high temperature conditions
. NEED TO CONTROL OXIDES OF NITROGEN
Atmosphere consists of about 79% nitrogen (N2) which dissociates into its atomic state (N) at high temperatures (around 1800 K and above). The single nitrogen atom can form different oxides like N2O, NO, N2O2, NO2, N2O3, N2O4and N2O5. EPA regulates NO2 only because it is the most prevalent form of NOX in the atmosphere (out of seven compounds of NOX family). Combustion primarily results emissions of NOX in the form of NO (about 85%) and are oxidized in the atmosphere to NO2 within short time and hence it is of primary concern. About 15% of nitrogen oxides from diesel engines are emitted as NO2, which is five times more toxic than NO. The other oxides of nitrogen emitted are negligibly small. Apart from an important pollutant, NO2 also reacts in the atmosphere to form ozone (O3) and acid rain. The family of NOX compounds and their properties are listed in Table 1.
. MAJOR SOURCES AND MECHANISM OF FORMATION OF NOX
About 50% of NOx emitted is contributed by automobiles and other mobile sources. The major sources of NOx formation during combustion processes can be explained by three different mechanisms, namely: 1. Thermal NOx, 2. Fuel NOx and 3. Prompt NOx (EPA Technical Bulletin, 1999).
3.1. Themal NOx
The nitrogen and oxygen molar concentrations and the combustion temperature control the concentration of thermal NOx. Combustion at temperatures well below 1600 K forms much smaller concentrations of thermal NOx. The strong triple bond in the nitrogen molecule breaks to its radicals at high temperature (greater than 1800 K) and nitrogen then reacts with excess oxygen to form NOx. Most of NO2 formed during combustion is decomposed quickly. NO can also disintegrate to form N2 and O2, but this rate is very slow. Thus, almost all of the NOx emitted is NO. It is generally accepted that in combustion of near stoichiometric fuel-air mixtures, the principal reactions governing the formation of NO from molecular nitrogen are given below. The first two reactions suggested by Zeldovich and third equation given by Lavoie et al. (1970)explain the formation of thermal NOX (John B. Heywood, 1988).
Abstract - Extensive usage of compression ignition engines are big challenges in terms of pollution and depleting fossil fuel reserves. Reasonable improvements in emission levels were achieved with the efforts constituted by various researchers and automobile industries worldwide to control emission associated issues both on road and off road diesel vehicles with developed technology for cleaner diesel fuels, quick responding electronic control units, advanced engine design and effective exhaust after treatment. Nitrogen oxides (NOX) are a very important and interesting family of air polluting chemical compounds. This paper explains why NOXare important air pollutants and how NOX are formed and react in the atmosphere. This paper also discusses the principles on which all NOX control and pollution prevention technologies are based; available NOX technologies for various combustion sources; and performance and cost of NOX technologies.
INTRODUCTION
Theoretical combustion in engines should result CO2 and H2O as exhaust emissions. However, actual combustion will never be theoretical due to several operating and design conditions. Fuel comprises not only hydrocarbons but also traces of sulfur, nitrogen and other chemicals and air comprises several constituents other than O2 and N2. A number of compounds, both gaseous and solid, appear in the engine exhaust when combustion takes place between fuel and air. Heterogeneous nature of air fuel mixture in diesel engines, high combustion pressure and temperatures further complicates the combustion process. Regions of rich and poor mixture ratios along with local stoichiometric composition pockets exist within the combustion chamber. Fuel rich composition produce CO, soot, dark smoke and unburnt HC, on the other hand fuel oxygen rich regions generate NOX at high temperature conditions
. NEED TO CONTROL OXIDES OF NITROGEN
Atmosphere consists of about 79% nitrogen (N2) which dissociates into its atomic state (N) at high temperatures (around 1800 K and above). The single nitrogen atom can form different oxides like N2O, NO, N2O2, NO2, N2O3, N2O4and N2O5. EPA regulates NO2 only because it is the most prevalent form of NOX in the atmosphere (out of seven compounds of NOX family). Combustion primarily results emissions of NOX in the form of NO (about 85%) and are oxidized in the atmosphere to NO2 within short time and hence it is of primary concern. About 15% of nitrogen oxides from diesel engines are emitted as NO2, which is five times more toxic than NO. The other oxides of nitrogen emitted are negligibly small. Apart from an important pollutant, NO2 also reacts in the atmosphere to form ozone (O3) and acid rain. The family of NOX compounds and their properties are listed in Table 1.
. MAJOR SOURCES AND MECHANISM OF FORMATION OF NOX
About 50% of NOx emitted is contributed by automobiles and other mobile sources. The major sources of NOx formation during combustion processes can be explained by three different mechanisms, namely: 1. Thermal NOx, 2. Fuel NOx and 3. Prompt NOx (EPA Technical Bulletin, 1999).
3.1. Themal NOx
The nitrogen and oxygen molar concentrations and the combustion temperature control the concentration of thermal NOx. Combustion at temperatures well below 1600 K forms much smaller concentrations of thermal NOx. The strong triple bond in the nitrogen molecule breaks to its radicals at high temperature (greater than 1800 K) and nitrogen then reacts with excess oxygen to form NOx. Most of NO2 formed during combustion is decomposed quickly. NO can also disintegrate to form N2 and O2, but this rate is very slow. Thus, almost all of the NOx emitted is NO. It is generally accepted that in combustion of near stoichiometric fuel-air mixtures, the principal reactions governing the formation of NO from molecular nitrogen are given below. The first two reactions suggested by Zeldovich and third equation given by Lavoie et al. (1970)explain the formation of thermal NOX (John B. Heywood, 1988).