Modeling of Properties and SI Engine Pollution of Associated Petroleum Gas in Iraq

Modeling and simulation of process performance has been conducted around the use of the Iraqi associated petroleum gas as fuel for the spark-ignition engine. The study included a statistical evaluation of the effect of each component on the total properties of the gas. Finally, the gas was tested as a possible fuel for the spark-ignition engine from the emission point of view. This was done numerically using commercial software well established and verified for modeling the operation and performance of spark-ignition engines. The study was conducted on Ricardo E6/T variable compression ratio engine. The range of the speed studied was 1000-3000 rpm and was limited to a lean range 0.8-0.95. It was also found that the presence of Methane in higher quantity helped in improving the calorific value (on a mass basis) but at the cost of gas density. The presence of higher carbon value gasses did not help improve the fuel heating value. The highest negative impact on the heating value is CO, H2S, and C2H6 respectively. The study also showed that the associated gas can be used as a fuel after removing sulfur from it.


Introduction:
Petroleum deposits often have a gaseous component, commonly called Associated Petroleum Gas (APG) consisting mainly of methane and other short-chain hydrocarbons.
Usually, APG can be collected from the oil sites from flared up in the special chimney or vented to the atmosphere. In some cases, it is re-injected inside the oil well to equalize the pressure inside the well and to recover its productivity. Recently, a number of scholars has paid more attention to do some works on APG as a potential source of clean energy [1].
In recent developments, Iraq is planning to cease APG flaring completely by 2021. This will take place in phases to drive investment to utilize APG in a proper manner [2].
Flaring is preferable to venting from a global warming point of view. Flaring oxidizes carbon and creates Carbon Dioxide (CO 2 ), but reduces the overall global warming potential of the emissions by destroying methane and other hydrocarbons [3]. It reduces emissions of flammable global warming gases, but also creates other pollutants such as Oxides of Nitrogen (NOx), Carbon Monoxide (CO), and Black Carbon (BC). Some studies [4, and 5] reported a serious negative impact on the environmental, socio-economic environment and heavy degradation of host communities and wildlife.
APG is subject to several treatment processes for the purification of non-hydrocarbon substances, and the separation of unwanted liquid components and isolate secondary products of economic value in order to reach Marketed Gas (MG) for commercial purposes No.27-(6) 2020 Journal of Petroleum Research & Studies (JPRS) E3 and in accordance with standard specifications and conditions both for utilization in thermal and chemical systems as well as for transportation.
Among the properties required by the Iraqi treatment plants for economically profitable MG are [6]: 29.33 to 32.4 kJ/m 3 calorific values, low H 2 S and CO 2 contents to avoid corrosion of the pipes, free from solid particles and moisture content, free from He and N 2 gasses, and, mercury concentration should not exceed 0.1 PPB as it causes Aluminum embrittlement. As far as its utilization as fuel [7] showed that due to the heavy presence of C 5+ hydrocarbon contents, the fuel is prone to heavy detonation and tar and soot formation.
Hence, specialized pretreatments must be applied before using it in gas turbine power generation.
Another attempt to utilize APG as fuel for the Aircraft Gas Turbine engine was conducted by Gur'yanov [8]. They concluded that for successful utilization of the fuel, pretreatment, as well as engine modification and introduction of specific inert gas must be used.
Rajović et al. [9] tested the impact of using APG as fuel in a cogeneration cycle using the life-cycle approach. The results obtained were promising in that the greenhouse gasses were reportedly reduced. Further, they reported a marginal reduction in resource depletion due to the use of APG instead of conventional fuel. A more interesting study by Nithyanandam et al. [10] showed that Benzene can be produced from APG and then used for several other applications.
Three major terms in identifying some challenges by using APG as fuel for power generation defined [11]. Cost, Hydrogen Sulfide and low Wobbe Index. In their study, they developed an optimization technique for the best concentration between wet, natural and AP gases.
In this work, a statistical analysis of the constituents of the gas is considered to investigate their effects on the key properties needed by the market. Then the study will come up with an equation relating the concentrations to those properties. Finally, the performance of this gas compared with other fuels will be tested from an environmental point of view.  First, a test of significance on the effect of fuel constituents on its properties is conducted using the ANOVA test. Then the correlation matrix is established to study the relative effect of each constituent on the fuel properties. Finally, to study the effect of using APG as fuel for the SI engine, a well-verified simulation model is used. The engine used for the study is the Ricardo E6/T variable compression ratio engine. Table (2) below shows the general properties of the engine used. The ignition timing is set at 15 o bTDC which is nearly best for both fuels. The mixture strength used is the lean mixtures (equivalence ratio is varied 0.8 to 0.95) while engine speed was varied from 1000 rpm to 3000 rpm at an increment of 500 rpm.
The model used is basically an extension of that developed by Ferguson and Benson [12,13] and their relative modifications and improvements. The model is used to simulate engine operation. The main features of the model are stated below: The instantaneous variation in the cylinder volume 'V' it's derivative ' ' with crank angle rotation 'T' taking the bottom dead center (BDC) as a reference as well as are calculated using equations (1) and (2) as below : …… (1) …….. (2) The gas temperature 'T' and pressure 'P' variation with crank angle inside the cylinder during compression stroke are calculated using the following equations (3) and (4): …….. (4) Where "P" is the cylinder pressure (kPa), "T" is the crank angle (deg), "R" is the constant of gas (kJ/kg-K), "V" is the volume of cylinder (m 3 ) between the cylinder head and top of piston, "T" is the temperature of cylinder (K), "Q" is heat transfer (kJ), suffix "ht" means lost to coolant and "cr" means lost to crevice, "CRL" means connecting rod length (m), "S" is stroke length (m), "CR" is compression ratio, "u" is internal energy (kJ/kg-K), "Vs" is the swept volume (m 3 ).
For in-cylinder work and heat transfer, the following equations (5)  Where "W" is the work (kJ), "Tw" is the temperature of cylinder wall (K), "Aw" is the surface area of the cylinder (m 2 ), "hw" is the heat transfer coefficient and "Z" is the angular velocity.
The amount of energy lost through crevice volume , in SI engines is calculated using the semi-empirical expression found in Gatowski et al. [14] as shown below: The sign convention followed in applying this equation is that when is positive, this means that the flow is into the crevice volume and the term in bracket is evaluated at the crevice conditions and Vice Versa.
The Heat Transfer Coefficient (h w ) from gas to walls is calculated by Woschni's (1968) formula [15]: ……….. (8) where; v = 6.18 C m is a gas velocity during scavenging and intake stroke; v = 2.28 C m is a gas velocity during compression stroke; is the gas velocity during the other strokes P a , T a , V a is pressure, temperature, and volume of the cylinder at the beginning of compression (IVC) respectively.
The rate of formation of NO ' ' was calculated using the theory developed by Lavoie et.al. and its modifications using equation (9): The detailed method is given in Winterbone [16].
The formation of carbon monoxide is calculated using the theory developed by [17]. The following relation is used to calculate the concentration of CO: ..…… (10)  To understand the above trend, a statistical correlation study is conducted on the data and the results are presented in Tables (3 and 4). Before starting the statistical analysis, a test of significance for the data is conducted by using the ANOVA test to see whether the results are significant or not.
As shown in Table ( in this example. It tells how much more variability between variables than within variables. The larger ratio is more confident the researcher can be in rejecting the null hypothesis (Ho), which is that all means are equal and there is no treatment effect.    The density equation has R 2 = 0.995 (testing) and 0.991 (validation) with RMSE = 9.443e-11 (testing) and 1.3113e-5 ( In this study, the fuel used e.g. Iraqi APG is a mixture of components that are various in nature and properties. For example, the ignition temperature in air ( o C) for methane is 595, ethane 510, propane is 470, butane is 460, while gasoline has 220. Therefore, the engine Further, the performance of the Iraqi APG is better than Gasoline for low equivalence ratios (0.8 and 0.9) for all speeds. There was a maximum reduction of 20% at low speed and the least reduction was 1.6% at 2000 rpm. At higher speeds and equivalence ratios, the performance was worsened compared with all other fuels. The increment in CO reached 13% at 3000 rpm. This is due to the highest C/H ratio compared to other gaseous fuels.
Hence, a higher number of carbon atoms is burned. Further, the amount of air admitted at higher is reduced with engine speed. Hence, the performance worsened with higher equivalence ratios and engine speeds. Another reason was the dissociation of carbon dioxide with cylinder temperature based on Figure (2) that shows higher or nearly equal exhaust temperature for Iraqi NG compared with other fuels. Another strong and widely accepted theory [20] is called the crevice theory. It states that some amount of the fuel and air gets trapped inside certain places inside the cylinder e.g.
between piston and cylinder, where the flame cannot reach. This portion gets released as the cylinder pressure reduces and is emitted to the atmosphere with the exhaust.
As shown in Figure (3), the amount of HC emissions emitted decreases with engine speed.
This is expected to be as a result of the insufficient time for the HC molecules to be absorbed and then desorbed by oil inside the cylinder. Further, the reduction in the cycle time also reduced the amount entrained by the crevices inside the cylinder, hence, reduces the amount released with exhaust during the exhaust stroke.

Fig. (3) Hydrocarbon variation with an engine speed for different equivalence ratios
Another factor that may help reduce HC emissions is the higher cylinder wall temperatures at higher speeds. This helps prevent flame quenching and causes the thickness of the oil film on the cylinder wall to be reduced (due to a decrease in oil viscosity with temperature) and hence if adsorbed by the oil, the amount of HC adsorbed and consequently desorbed with exhaust products will be less.

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This reduction in lubricating oil thickness at the cylinder wall results in the increase in Henry's constant (which is defined as the ratio of partial pressure of the fuel vapor (P fs ) in the gas phase adjacent to the interface S state to the mole fraction of the fuel dissolved in the oil film (n fL )) [21] and thus, reduces the fuel vapor absorbed.
Higher exhaust temperature is shown in Figure (2) can also be the reason for the reduction in HC as it may help decrease desorption of HC molecules and also improve the oxidation of HC present in the exhaust products at higher engine speeds.
Several researchers [21] attributed this behavior of HC emissions to the existence of cylinder quench layers. They showed that the observed quench distances were of the right order of magnitude to account for HC emissions. Others found that the quenched mass gets mixed and oxidized inside the cylinder due to fuel-burning during expansion and exhaust processes.
Equivalence ratio Φ also promoted the existence and adsorption and desorption of fuel vapor in the oil and inside the crevices if increased to explain the increase of HC levels with equivalence ratio.  NOx molecules in the exhaust are not directly products of the combustion process but, rather, are products of a high-temperature side reaction which involves the disassociation of some of the atmospheric nitrogen molecules, the oxidation of the liberated nitrogen atoms to nitric oxide (NO), and the subsequent conversion of the NO to other nitrogen oxides, such as nitrogen dioxide (NO 2 ).
NOx emissions decrease with engine speed. This is clear for all fuels, the reduction in oxygen concentration and combustion duration are the prime reasons for this behavior. It is also noticed that NOx emissions are maximum for an equivalence ratio of 0.9. As for the associated gas of Iraq, NOx emissions are higher than others, due to a higher calorific value which results in higher cylinder temperature as shown in Figure (2).

Conclusion:
Modeling of Iraqi APG is made which is able to predict the properties to a good degree of accuracy. Statistical analysis shows that the C1 component has a great positive influence on APG calorific value and a negative effect on gas density. CO 2 and H 2 S are the major components that negatively affecting heating value. The study shows that the Iraqi association gas cannot be used without the removal of sulfur from it to reduce the sulfurous-related exhaust emissions.
The main advantage shown by the associated gas is the reduction in CO emissions in the exhaust, comparable levels of NOx as that for gasoline and the lower cylinder pressure and temperature. This means lower levels of thermal and mechanical stresses.