CNG Review Part 2
There are four main parameters which determines the emission in an engine.EMISSIONS
Air/Fuel ratio has the greatest influence on emission. The main components of emission are carbon monoxide, carbon dioxide, nitrogen oxide, hydrocarbons.
CO reduces the blood's oxygen level, inducing sluggishness and endangering people with heart and circulating disorders. Smog (due to the reaction of NOx and hydrocarbons ) causes respiratory disorders, particularly in asthmatics, children and older people. [13]
We can consider principal components of emission in an engine powered by gaseous fuels.
Following parameter is used in order to understand emission mechanism.
in the exhaust depends on availability of oxygen. In other word (F/A) ratio is important in determining CO level.
Engines powered by gaseous fuels like propane, natural gas have the same or lower CO concentrations in the exhaust for the same Fuel/Air ratio. Because these fuels have higher hydrogen to carbon ratio and they mix better than liquid fuels providing more homogeneous mixtures and can be operated much leaner.
B) HYDROCARBON EMISSIONS
The mechanisms which cause HC emissions can be definedas follows:
Quenching distance identifies the smallest capable crevice to prevent a flame front from being quenched. According to that, the flame of fuel with low quenching distance can far better penetrate into difficult chamber zones like quenching areas with high surface to volume ratios and reach the unburned mixtures than flames of fuels with high quenching distance. [15] This characteristic of fuel might be important in hydrocarbon emissions.
Smog is a ground level photochemical ozone phenomenon that is a result of emissions, sunshine and moisture in stagnant air basin. The mechanism of smog depends on reactive hydrocarbon emission and NOx . No ozone is formed without NOx. There is a certain ratio of reactive hydrocarbon to NOx that maximizes the ozone formed per unit mass of each of the reactive hydrocarbons. Methane has a low photochemical potential compared to other hydrocarbons.
The paraffins, especially methane in the exhaust stream of a natural gas vehicle are non-reactive hydrocarbons. It means low photochemical potential. The reduction olefins and aromatics provides low reactivity in the formation of ozone. Reactivity index for gasoline is 1 for paraffins, 8 for olefins, 3 for aromatics respectively. Reactivity index for natural gas: 0 for paraffins, 8 for olefins, 3 for aromatics.
Table 5. shows that a reduction in active hydrocarbons elements is 90 % when using natural gas over the use of gasoline.
Natural gas fuel systems, due to being totally enclosed and pressure tight, does not suffer from refueling, evaporative, running losses and emissions from the fuel storage system.
C) NOx EMISSON
NOx are reactive with hydrocarbons resulting smog formation. It is also toxic itself. Although emission of CO and hydrocarbons is related to incomplete combustion, NOx emission mainly depends on combustion temperature and combustion duration. While NO and NO2 occurs together, NO is the predominant one. NO nominally accounts for approximately 90 % of the total NOX . The rate of formation of NO is primarily dependent on the temperature of the burned gases. For instance if the temperature of the burned gas is raised by 100 C the rate of formation NO is increased 10 times. If the temperature is elevated by 300 C the rate of formation increases by 1000 times. [9]
It is obvious that temperature control is the main parameter in the control of NO.
define the temperature of combustion. Exhaust gas acts as a diluter of the inlet charge and this reduces the peak flame temperature. Higher heat capacity of CO2 and H2O provides exhaust gas more effective than same amount of air.
Time is the second factor in the occurrence of NO. The contribution of NO formation is more significant , if the mixture is burned early . Because the early burned mixture has more time to reach equilibrium. Another appearance of time factor is at different engine speeds. At higher engine speeds contribution of NOx is lower due to the less time required to achieve the engine cycle.
It should be noted that gases especially, methane burn cooler than gasoline. The result of this, at stoichometric conditions methane produces lower levels of NOx (flame temperature of fuel) is one of the parameter in the formation of NOx)
The contribution of oxidation of atmospheric nitrogen is primary source in NO formation. The oxidation of fuel nitrogen-components increases NO formation, if the fuel has significant content of nitrogen.
The NO formations from atmospheric nitrogen can be predicted by using Zeldovich mechanisms.
NO concentrations occur in postflame gases as well as flame front gases. However, the flame reaction zone is extremely thin (~0.1 mm) and residence time within this region is short due to occurring of combustion at high pressure. Besides this, burned gases produced early in the combustion process are compressed to a higher temperature than they reached immediately after combustion because of rising of cylinder pressure during most of the combustion process. These facts show that NO concentrations occurred in postflame gasses dominate any flame-front produced NO. It is appropriate to assume that the combustion and NO formation process are decoupled and to approximate the concentrations of O, O2, OH, H, and N2 by their equilibrium values at the local pressure and equilibrium temperature.
Figure 5. Flame regions in combustion
As a result, the initial NO formation rate can be calculated by the equation as follow.
As can be seen from the equation above, NO formation is strongly dependent on temperature; for instance if the temperature of the burned gas is raised by 100 C the rate of formation NO is increased 10 times. If the temperature is elevated by 300 C the rate of formation increases by 1000 times. It is also weakly dependent on oxygen concentrations.
Experimental studies have been shown that equation 2 is in good agreement with meauserements of NO in the postflame zone. In combustion zone, NO-formation rates which are measured appear larger than predicted by equation 2. The biggest discrepancies occur in rich-burn application.
With extreme dilution, at NO levels of about 100 ppm or less, NO formation within the flame reaction zone must be considered. The concentrations of radicals such as O, OH, and H can be in excess of equilibrium levels resulting in much higher formation rates within the flame than in the postflame gases. Although postflame approach is still valid, flame-front (prompt) NO formation would no longer be negligible.
Fuel nitrogen is also an important source of NO emission. The extent of NO conversion from fuel nitrogen is strongly dependent on local combustion enviroment and the initial fuel nitrogen content. The NO formation is not effected by the kind of parent fuel molecule. Oxidation of fuel nitrogen to NO is rapid. The method, which is applicable for atmospheric nitrogen, that quench the reaction system to prevent NO formation from fuel nitrogen can not be applied. In another word, formation of NO from fuel nitrogen is weakly dependent on temperature in contrast to NO formation from atmospheric nitrogen which is strongly dependent of temperature. The NO occurrence from fuel is dependent on equivalence ratio. Lean-burn and stoichometric applications result in high NO formation from fuel nitrogen (close to 100 %). Rich mixtures cause relatively low NO formation. Because CH and NH radicals which can be available reduce NO concentrations.
High concentration rate of NO formation from fuel nitrogen in lean-burn application would cause a dilemma. Because lean application is convenient in point of reducing hydrocarbon and carbon monoxide emissions.
Time is the second factor in the occurrence of NO. The contribution of NO formation is more significant , if the mixture is burned early. Because the early burned mixture has more time to reach equilibrium. The characteristic of time can be described by equation below
Characteristic time of NO formation is usually comparable or longer than times characteristics of changes in engine conditions. It means formation process is kinetically controlled. Equilibrium NO concentrations may occur for close-to stoichometric conditions at the maximum temperatures and pressures, since NO is of the same order as typical combustion times (1ms)
The affect of spark timing to the formation of NOx should be explained as follow. Advancing the timing provides combustion occurs earlier in the cycle by increasing the peak cylinder pressure. More fuel is burned before cylinder reaches top dead center; the peak pressure moves closer to top dead center where the cylinder volume is smaller. Higher peak cylinder pressures cause higher peak burned gas temperatures and higher NO formation rates.
Retarding the timing makes cylinder pressure reduced; because more of the fuel burns after top dead center. Another appearance of time factor is at different engine speeds. At higher engine speeds contribution of NOx is lower due to the less time required to achieve the engine cycle.
NOx react with hydrocarbons which results in smog formation. Smog is ground level photochemical ozone formation which is the consequence of the reactions between Nox and hydrocarbons.
Ozone is a strong oxidizer which affects the respiratory system, leading to damage of lung tissues. Chronic exposures to elevated ozone levels are responsible for losses inimmune system functions, accelerated aging and increased susceptibility to other infections. In additiondue to its nature an oxidizer, there are prospects for permanent loss of the alveoli cells.[2]
Figure 6. Life Cycle of Ozone Formation
In first step, NO2 is disassociated NO and an free radical oxygen atom by ultraviolet light. Then, appearing oxygen atom quickly makes a combination with molecular oxygen to form ozone.
M represents any other molecule especially N2 or O2 which absorb the energy of the reaction Without the M body, only oxygen exchange within an oxygen molecule would occur. Although a triple reaction is required, the reaction is cinetically fast. The third reaction completes the cycle.
This reaction also occurs fast. Constant level of each species, NO, NO2, and O3 could be formed when these three reaction are happened. The steady-state ozone formation can be predicted as a function of initial NO2 concentration. Further research has been showed that there should be a mechanism which convert NO to NO2 without consuming O3.
The overall higher production of ozone can be explained by the impact of ractive hydrocarbons. Olefins is the most ractive group because of double bond. Oxygen atom attack olefin and divide it into two parts. Highly reactive free radical appears andcontinuesto get involved in other reactions.
then original reactions occur,
where:
