Mechanisms of Nitrogen Oxides Formation in Engine Emissions, Study notes of Sustainability Management

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Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture4/4_1.htm[6/15/2012 2:56:06 PM] Module 2: Genesis and Mechanism of Formation of Engine Emissions Lecture 4: Mechanisms of Nitrogen Oxides Formation POLLUTANT FORMATION The Lecture Contains: Formation of Nitrogen Oxides Thermal NO Rate Constants for Zeldovich Mechanism Rate of NO Formation NO Formation is a Function of Temperature and [O2] Prompt NO Fuel NO Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture4/4_2.htm[6/15/2012 2:56:06 PM] Module 2: Genesis and Mechanism of Formation of Engine Emissions Lecture 4: Mechanisms of Nitrogen Oxides Formation Formation of Nitrogen Oxides Nitric oxide is the major component of NOx emissions from the internal combustion engines. During combustion, three probable sources of NO formation are: (i) Thermal NO : By oxidation of atmospheric (molecular) nitrogen at high temperatures in the post-flame burned gases. (ii) Prompt NO : Formed at the flame front within the flame reaction zone. (iii) Fuel NO : Oxidation of fuel-bound nitrogen at relatively low temperatures Thermal NO is the dominant source of nitrogen oxides in IC engines. Thermal NO NO is formed in the high temperature burned gases behind the flame front. The rate of formation of NO increases exponentially with the burned gas temperature although, it is slower compared to the overall rate of combustion. Kinetics and Modelling of Thermal NO Formation The following three reactions commonly referred to as the extended Zeldovich mechanism govern the formation of thermal NO (2.1) (2.2) (2.3) k1, k2 and k3 are the reaction rate constants for the forward reactions and k-1, k-2 and k-3 are for the reverse reactions The original Zeldovich mechanism consisted of the first two reactions (2.1) and (2.2) and the third reaction (2.3) was added by Lavoie. The forward part of the first reaction (2.1) is highly endothermic with high activation energy of about 314 kJ /mol and is a rate determining reaction in NO formation. Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture4/4_4.htm[6/15/2012 2:56:06 PM] (2.11) where w = R1/( R2+ R3) Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture4/4_5.htm[6/15/2012 2:56:07 PM] Module 2: Genesis and Mechanism of Formation of Engine Emissions Lecture 4: Mechanisms of Nitrogen Oxides Formation ,mole fraction (2.17) NO Formation is a Function of Temperature and O2 From Eq. 2.11 the initial rate of NO formation when [NO]/[NO]e<< 1; (2.12) The concentration of atomic oxygen at equilibrium, using reaction ½ O2 ↔ O is given by; (2.13) Equilibrium constant K p(O) is (2.14) Using value of k1 from Table 2.2 and, the Eqs. 2.13 and 2.14 the initial rate of NO formation then reduces to (2.15) Temperature being in exponential term in Eq.2.15, it strongly influences NO formation rates. From Eq. 2.15 it follows that the NO formation is maximized under the conditions of high temperature and high oxygen concentrations. These conditions occur at fuel-air equivalence ratios 5-10% leaner than stoichiometric mixture. Typical combustion duration is about 1 to 2 ms in SI engines operating close to stoichiometric conditions at 3000 -5000 rpm. NO formation at peak pressure and temperature conditions may reach close to equilibrium [NO] concentrations as illustrated below; The characteristic time ( ) necessary to reach equilibrium concentration of NO may be approximated as, (2.16) The mole fraction [NO]e can be estimated from the reaction O2 + N2 ↔ 2NO, as Objecti ves_template q|Previous Next [> file://C\/...nts %20and% 20S etting s/iitkranal /My%20Documents/Google%20Talk% 20Received% 20Files/en gine_combustion/lecture4/4_5.htm[6/15/2012 2:56:07 PM] Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture4/4_7.htm[6/15/2012 2:56:07 PM] Module 2: Genesis and Mechanism of Formation of Engine Emissions Lecture 4: Mechanisms of Nitrogen Oxides Formation Example 2.1: Using the Eq. 2.18 based on initial rate of NO formation estimate whether during typical SI engine combustion the kinetically formed NO could reach the level of equilibrium concentrations Solution For the charge that burns early in the cycle the peak burned gas temperatures of 2700 K or higher could be obtained. At full load maximum cylinder pressure is of the order of 30 – 40 atm. Under real engine conditions, the rate of NO formation changes with time as the temperature and pressure change with time during the cycle and also the NO concentration (Eq. 2.11). However, for an approximate analysis let us assume that the average temperature and pressure of the charge elements burnt early are 2700 K and 35 atm At T= 2700 K and P = 35 atm for an early burn charge element For an engine operating at 4500 rpm, it would take 10.3º CA to reach equal to equilibrium NO concentrations. This time period is well within the typical combustion duration being in the range 30- 40º CA For the charge elements burning later in the cycle the temperatures reached may be around 2300 K and pressure may be down to 20 atm. At these conditions, For a late burn element : For a late burn element on the other hand it needs about 4.07 ms i.e., 110º CA which is too long a period in the engine cycle. Due to expansion, the burned gas temperatures would have fallen by then to further low levels of around 1300-1400 K and in the late burn elements the kinetically formed NO would never reach equilibrium concentrations. The NO formation in the late burn elements is frozen at a value higher than that predictted by the equilibirium considerations .This is demonstrated later in this module in Fig. 2.7. Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture4/4_8.htm[6/15/2012 2:56:07 PM] Module 2: Genesis and Mechanism of Formation of Engine Emissions Lecture 4: Mechanisms of Nitrogen Oxides Formation Prompt NO In the flame reaction zone NO may be formed rapidly. The prompt NO is formed in the flame by reaction of intermediate chemical species of CN group with O and OH radicals. The hydrocarbon radicals CH, CH2, C, C2 etc. formed in the flame front react with molecular nitrogen to give intermediate species such as HCN and CN by the reactions (2.19) to (2.21). Large concentrations of HCN near the reaction zone in fuel rich flames have been observed and rapid formation of NO has been seen to be associated with rapid decay of HCN. (2.19) (2.20) (2.21) The contribution of prompt NO in the stoichiometric laminar flames is estimated to be about 5 to 10 percent only. In the engines as the combustion occurs at high pressures, the thickness of flame front is very small (~ 0.1 mm) and the residence time of chemical species in this zone is very short. Moreover, the burned gases produced by the charge elements that burn early during the combustion process are compressed to a much higher temperature than the temperatures attained immediately after combustion. The formation of thermal NO in the burned gases behind the flame front therefore, is much higher compared to any NO formation in the flame front. However, contribution of prompt NO may be significant under lean engine operation or engine operation with high dilution such as use of exhaust gas recirculation. Fuel NO Fuel NO is formed by combustion of fuels with chemically bound nitrogen. The fuel nitrogen produces at first intermediate nitrogen containing compounds and reactive radicals such as HCN, NH3, CN, NH etc. These species are subsequently oxidized to NO. Although petroleum crude may contain about 0.6 % nitrogen but gasoline has negligible nitrogen. Diesel fuels have higher nitrogen content than gasoline, but this too is usually less than 0.1% by mass. The fuel nitrogen therefore, does not make significant contribution to NO formation in automotive engines operating on gasoline, diesel, natural gas and alcohols etc.