EMISSION CHARACTERISTICS OF ALTERNATIVE TRANSPORTATION FUELS

ESL875 TERM PAPERSubmitted by:Vinita Kumari 2011CH70188

Biodiesel:It is produced from transesterification of vegetable oil in which the fatty acid triglycerides are reacted with a suitable alcohol (Methyl, Ethyl, or others) in the presence of a catalyst(KOH, NaOH) under a controlled temperature(60-70 0C) for a given length of time.Combustion:0.04437 A+0.01675 B+0.08432 C+0.1766 D+0.0205 E+9.037 O2+33.98 N26.42 CO2+5.92 H2O+33.98 N2Chemical Composition[6]

Emissions:[1]

Reduced CO emission:A 100% sulphur dioxide reduction is reasonable taking into account that biodiesel, by its vegetal origin, does not contain sulphur. The CO emissions for biodiesel combustion in diesel engines are 40 to 50% lower than those for conventional diesel; this happens due to the presence of oxygen molecules in the biodiesel, mainly in the methyl or ethyl ester, helping to obtain complete combustion. Reduced Particulate Matter:High gas temperatures and high temperatures of the combustion chamber wall contributes to less smoke and particulates. PM emissions among biodiesels could be due to either their chemical composition or their physical properties.The oxygen content of biodiesel is favorable in reducing the PM emission.PM emission decreases with increasing degree of unsaturation. The reduction of smoke is due to the dilution of aromatics, which are soot precursors.PM emission depends on viscosity and surface tension. Fuels with low cetane value undergo prolonged premixed combustion phases that are responsible for less soot formation.

Reduced Hydrocarbons:Since biodiesel is an oxygenated fuel, it promotes combustion and results in the reduction of unburned hydrocarbon emissions. A decrease of unburned hydrocarbons due to complete combustion takes place, because the chains of carbon-hydrogen and oxygen in esters help the formation of CO2 and water unlike to what occurs with diesel fuel.Increased NOX EmissionThe possible reasons are: The shorter ignition delay, caused by biodiesels higher cetane number, because of advanced combustion timing which increases peak pressure and temperature and hence NOX emission. An increase in flame temperature in either premixed or diffusion burn, which is caused by reduction in the concentration of carbonaceous soot a highly effective heat radiator. The double bonds present in biodiesel may cause a higher adiabatic flame temperature, and hence a higher temperature at the flame front and hence increased NOX emission. Unsaturated compounds present in biodiesel may form higher levels of radicals during pyrolysis and combustion. Prompt NO is formed by reaction of radical HC species with nitrogen, ultimately leading to formation of NO.Ethanol:Ethanol refers both to ethyl alcohol and to a blend of ethyl alcohol and gasoline used as a motor vehicle fuel. In the U.S. most ethanol is used in blends of up to 10% ethanol and 90% gasoline (E10 or gasohol) to reduce carbon monoxide emissions and prevent air pollution. E10 is not considered an alternative fuel, and conventional gasoline engines can run on E10.Although motor vehicle gasoline engines can run on E10, only flexible fuel vehicles (FFVs) with specially modified engines can use the more corrosive E85. The main difference between FFVs and conventional gasoline vehicles are the materials used in the fuel management system and modifications to the engine calibration system.FFV engine parts are modified to resist corrosion, and a fuel system sensor in the engine analyzes the fuel mixture and adjusts the fuel injection and ignition accordingly.CO emission:The reduction in CO concentration using blended fuels is due to the fact that ethanol (C2H5OH) has less carbon than gasoline (C8H18). Another significant reason of this reduction is that the oxygen content in the blended fuels increases the oxygen-to-fuel ratio in the fuel-rich regions. The most significant parameter affecting CO concentration is the relative airfuel ratio (). Relative airfuel ratio () approaches 1 as the ethanol content of the blended fuel increases, and consequently combustion becomes complete.CO2 Emission:CO2concentration increases as the ethanol percentage increases. CO2emission depends on relative airfuel ratio and CO emission concentration. As a result of the lean burning associated with increasing ethanol percentages, the CO2emission increased because of the improved combustion.HC Emissions: Ethanol can significantly reduce HC emissions. The concentration of HC emission decreases with the increase of the relative airfuel ratio, the reason for the decrease of HC concentration is similar to that of CO concentration described above.

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NOX Emission: As the percentage of ethanol in the blends increases NOX emission increases. When the combustion process is closer to stoichiometric, flame temperature increases, therefore, the NOxemission is increased, particularly by the increase of thermal NO.Carbonyl Emissions:Carbonyl emissions(acetaldehyde,formaldehayde,propionaldehyde) increases with increasing ethanol percentage in the ethanol gasoline blend. Carbonyls are formed primarily from the reaction of hydrocarbons with OH radicals. Combustion of ethanol tends to form carbonyl compounds due to its hydroxyl structure. In addition, the combustion of ethanol with two carbons in structure can easily form acetaldehyde which contains two carbons as well. The higher emission of carbonyls can be attributed to the addition of rich oxygen-containing ethanol.

LPG(Liquified Petroleum Gas):LPG is a quite niche alternative fuel that can be used in special spark ignition engines or as an auxiliary fuel in dual fuel compression ignition engines together with diesel oil. LPG is recovered directly from oil and gas fields (WLPGA) in which case no actual refining is needed and also formed as a by-product in crude oil processing either in distillation phase or after-treatment (cracking) processes. The use of LPG in transportation is concentrated in few countries (Korea, Turkey, Russia, and Poland) and it is mainly used in bi-fuel light duty vehicles. LPG forms easily a homogenous mixture with air. This combined with the relatively simple chemical structure of LPG, it burns cleanly and is well-suited for spark-ignition engines. For compression ignition (diesel) engines, LPG is not suitable as the sole fuel. In spark ignition engines, similar compression ratios are typically used with LPG as with gasoline, even though the octane number of LPG (112 for propane, 94 for butane) is higher than that of gasoline. This is due to the fact that the combustion temperature is higher when LPG is used and this lowers the knock limit especially at high engine loads. Exceptions to this are the engines in which LPG is injected in liquid form. In bi-fuel cars, the upper limit for compression ratio is restricted by gasoline. Efficiency of LPG engines is similar to gasoline engines. A higher thermal efficiency and, therefore, improved fuel economy can be obtained from internal combustion engines running on LPG as opposed to unleaded gasoline. This is because LPG has a higher octane number, typically 112 research octane number(RON) for pure propane, which prevents the occurrence of detonation at high engine compression ratio. In dual fuel engines under low loads, when the LPG concentration is lower, the ignition delay of the pilot fuel increases and some of the homogeneously dispersed LPG remains unburned, resulting in poor emission performance. Poor combustion of LPG under low loads because of a dilute LPGair mixture results in high CO and unburned HC emissions. However, at high loads, increased admission of LPG can result in uncontrolled reaction rates near the pilot fuel spray and lead to knock.

Hendriksen(2003),Verbeek 2008)Emission Characteristics:HC emissions have been reported as 40% lower, carbon monoxide (CO) as 60% lower and carbon dioxide (CO2) as substantially reduced, principally due to the high hydrogen/carbon ratio(propane,butane) of LPG when compared to gasoline. It can also be attributed to the better mixing obtained by gaseous fuel dosification and due to the higher cylinder-to-cylinder uniformity achieved. In some cases LPG having a slightly greater tendency to produce CO may be due to its higher combustion chamber surface to volume ratio and thus a proportionally higher charge cooling and flame quenching effect. An increasing proportion of LPG in gasoline promotes faster burning velocity of mixture and hence reduce the combustion duration and subsequently the in-cylinder peak temperature increases.

LPG combustion normally produces higher temperatures due its slightly superior heating value, its higher burning speed and its lack of charge cooling effect (obtained with gasoline by its evaporation).Therefore NOX emission is increased with increasing proportion of LPG in gasoline. Combustion of LPG occurs in a nearly uniform fuel air mixture leading to a reduction in incomplete combustion deposits such as soot on the walls of combustion chamber.[5]

As compression ratio increases, brake thermal efficiency increases. LPG has a higher octane rating and hence the engine can run effectively at relatively high compression ratios without knock. The CO and HC emissions increase as the compression ratio, speed, and load increase.

Because the C.V.of Gasoline is (43MJ/Kg) less compared to the LPG (46.1MJ/Kg).When load increase on the engine the CO,HC and CO2emissions also increase. However, these emissions higher for Gasoline when compared with LPG,because of high hydrogen to carbon ration in LPG(propane +butane ) as compared to Gasoline.[5][5]

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Hydrogen(H2)Hydrogen (H2) is the lightest and simplest gas. This makes it a very clean energy source. Storage of this gaseous fuel for transportation use poses challenges that are currently being researched. The two methods of manufacturing hydrogen fuel currently result in costs of $3 to $4 dollars a gallon and use electricity or natural gas, which typically results in air emissions. A safe hydrogen fuel distribution system needs to be developed to make the quantities necessary for transportation readily available. The ability to create the fuel from a variety of resources and its clean-burning properties make it a desirable alternative fuel.Pure hydrogen and hydrogen mixed with natural gas (hythane) have been used effectively to power automobiles with internal combustion engines. Hydrogen's real potential rests in its future role as fuel for fuel cell vehicles. Hydrogen and oxygen fed into a proton exchange membrane (PEM) fuel cell "stack" produce enough electricity to power an electric automobile, without producing any harmful emissions from the vehicle. However, there are four basic issues regarding hydrogen-fueled engines and vehicles: engine backfire and susceptibility of hydrogen to surface ignition, somewhat reduced engine power, high nitric oxide (NOx) emissions, and the problem of on-board storage of the fuel and safety. Although partial solutions have been found to most of these problems, there still is no general consensus of the best method to finally resolve all of these issues.Emission Characteristics:

It can be observed that the minimum HC emission for the three compression ratios used in testing occurred at an equivalence ratio of about 0.7 and for a compression ratio of 11:1, and further increase of the equivalence ratio beyond this value resulted in a continuous increase of the HC emitted[3] As seen from the figure, the highest HC emission for the three compression ratios used in testing occurred at an equivalence ratio of about 1.2 for a compression ratio of 7:1. At equivalence ratio of 1.0, the percentage reduction in HC emission noticed when the compression ratio was increased from 7 to 11 was around 22.4%.

The HC and CO emissions as seen in the Figures are extremely low as expected in an engine using hydrogen-ethanol as a fuel. This is because the hydrogen fuel has no hydrocarbons, and the fact that HC emissions arise mainly from unburned fuels. The absence of hydrocarbons in hydrogen fuel also keeps the CO emissions very low.[3]Ideally, all of the carbon in the hydrocarbons should be converted to CO2 in complete combustion. Incomplete combustion on the other hand leads to the generation of some CO. The reason for the presence of some CO and HC emissions in the engine fuelled with hydrogen-ethanol is due to combustion of the lubricating oil in the engine. The oil is not intended for combustion, and there are ways of minimizing its ingress into the combustion chamber. The oil can make its way into the combustion chamber past the piston rings, through leakage at the intake valve guide, or through the crankcase ventilation system.[3]As seen from the above figure variation in NOx concentration levels is a function of the equivalence ratio for all compression ratios. It increases for all compression ratios initially with increasing equivalence ratios, reaches a peak value, and decline with increasing equivalence ratio thereafter. These trends can be explained by the fact that NO formation reactions depend upon temperature in the combustion chamber, mixture strength, and available oxygen, and they occur primarily in the post flame gases. The type of the fuel used affects the flame temperatures and the sufficiency of the available oxygen is affected by the stoichiometry, which is in turn a function of the type of fuels used. As the mixture air-fuel ratio gets leaner, the temperature prevalent in the combustion chamber drops thus leading to a weakening of the NOx formation kinetics. As the compression ratio increases, it is observed in figure that the peak NOx emission occurs at equivalence ratios that are leaner. At higher compression ratios, the charge condition at the start of combustion would be more homogeneous and this helps in shifting the peak NOx occurrence points to the leaner side.Compressed natural gas (CNG):The term CNG (Compressed Natural Gas) stands for natural gas which is compressed at a pressure of 200 bar. The use of natural gas as a fuel requires engines which work according to the Otto principle. Therefore, for cars running on natural gas, Otto engines are used, which are optimised for the use of natural gas. The adaptation can be done exclusively for natural gas without the possibility of using gasoline (dedicated vehicles) or for gasoline and natural gas (bivalent vehicles). Switching from one fuel to the other can be done by the driver at the dashboard or automatically if one of the fuels is running out. However, diesel engines are required in natural gas fuelled trucks and buses, which are adapted to the Otto principle exclusively in the monovalent mode.

Emission Characteristics:CO emission:

[4]Carbon monoxide present in the exhaust gas is due to unavailability of oxygen during the combustion process. Poor mixing, local rich regions and incomplete combustion is also the source for CO emissions The carbon monoxide values for diesel are in range of 0.08% to 0.02%and it is getting more while inducting CNG gas. Some amount of CNG gas replacing air in the intake pipe that leads to insufficient of air for proper combustion and fuel becomes rich mixture. This may be the reason for getting more CO emissions while using CNG gas as fuel.

CO2 emission:The carbon dioxide values for diesel are in range of 1.5% to2.00% and it is getting less while inducting CNG gas. Some amount of CNG gas replacing air in the intake pipe that leads to insufficient of air for proper combustion and fuel becomes rich mixture. This may be the reason for getting less CO2emissions while using CNG gas as fuel. Figure shows that the CO2emission values are getting lower for CNG induction of irrespective of induction length and for induction length decreasing the CO2values is decreasing with load compared to other induction lengths.

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Because of non-homogeneity of fuel air mixture some local spots in the combustion chamber will be too lean to combust properly. Other spots may be too rich, without enough oxygen to burn all the fuel. With under mixing some fuel particles in fuel rich zone never react due to lack of oxygen. By induction of CNG at, there was a little replacement of intake air by Unburned hydrocarbons:

[4]CNG which causes low volumetric efficiency and leadstoimproper mixing offuel.[4]NOx emissions are result of attaining very high temperatures in the combustion chamber. In cylinder pressure and fuel air ratio also decides the NOxemission in the exhaust gas. As the induction distance increases away from the engine the NOx emissions are decreasing . The increasing in NOx emissions is due to increase in temperature and in cylinder pressure compared to that of diesel operation

References:[1]California Air Resources Board, National Biodiesel Board and A Comprehensive Analysis of Biodiesel Impacts on Exhaust Emissions, United States Environmental Protection Agency, EPA420-P-02-001, October 2002[2] B. Ghobadiana, T. Tavakolia, D.R. Buttsworthb, T.F. Yusafb, M. Faizollahnejad,Performance and exhaust emissions of a gasoline engine with ethanol blended gasoline fuels using artificial neural network, Applied EnergyVolume 86, Issue 5, May 2009, Pages 630639[3] Syed Yousufuddin K. Venkateswarlu G. R. K Sastry, Effect of Compression Ratio and Equivalence Ratio on the Emission Characteristics of a Hydrogen-Ethanol Fuelled Spark Ignition Engine, International Journal of Advanced Science and TechnologyVol. 40, March, 2012[4] Alpesh K. Panchal Chirag M. Patel Gaurav P. Rathod Tushar M. Patel,Performance and Exhaust Gas Emission of Compressed Natural Gas Fueled Internal Combustion Engine in Dual Fuel Mode, International Journal for Research in Technological Studies| Vol. 1, Issue 6, May 2014[5] Norazlan B. H The Study of Combustion Characteristics for different Compositions of LPG, Faculty of Chemical and Natural Resources Engineering Universiti Malaysia Pahang MAY 2008[6] Christian R. C., Joao A. C. Jose L. S., Biodiesel CO2 emissions: A comparison with the main fuels in the Brazilian market Fuel Processing Technology,Volume 90, Issue 2, February 2009, Pages 204211