Chapter 3 CONVENTIONAL FUELS AND ALTERNATIVE FUELS

Transcription

1 Chapter 3 CONVENTIONAL FUELS AND ALTERNATIVE FUELS Fossil fuels are formed by natural resources such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years. The fossil fuels include coal, petroleum, and natural gas which contain high percentages of carbon. Fossil fuels range from volatile materials with low carbon:hydrogen ratio like methane, liquid petroleum and nonvolatile materials composed of almost pure carbon like anthracite coal. Methane can be found in hydrocarbon fields as alone, associated with oil, or in the form of methane clathrates. It is generally accepted that they formed from the fossilized remains of dead plants and animals by exposure to heat and pressure in the Earth's crust over millions of years. 3.1 Fuel & Oxidizer Chemically, the fuel can be defined as one, which donates electrons. In contrast, an oxidizer can be defined as one, which accepts electrons. This property of elements to accept or donate electrons is known as electronegativity, which dictates whether an element can be classified as fuel or an oxidizer. This word electronegativity is defined as the power of an atom in a molecule to attract electrons towards it. The electronegativity values for various elements are given in Table 3.1 below. Table 3.1 Electronegativity of various elements [25]. Elements Electronegativity O 3.5 N, Cl 3.0 C, S, I 2.5 H, P 2.1 B 2.0 Be, Al 1.5 Mg 1.2

2 28 Oxygen has the second highest electronegativity. Although, it is available abundantly in nature, it is mostly accompanied with nitrogen which, being an inert gas, reduces the actual capability of oxygen. The fuels such as carbon, hydrogen, aluminium, magnesium, etc. have lower electronegativity in comparison to oxygen. In combustion studies, air is a very common oxidizer used most of the times in which the fuel elements such as carbon, hydrogen, aluminium, boron, etc oxidized easily by oxygen. The most common fuels that we use in our day-to-day life are basically hydrocarbons. Of course, hydrogen is emerging as the next generation fuel being the least pollutant fuel. Metals are used as additives in some special fuels particularly in rocket engine Gaseous Fuel and Oxidizer Recently, gaseous fuels are preferred most over liquid and solid fuels as it is easier to control emissions from gaseous fuel operated combustion devices as they do not contain any mineral impurities and are easier to burn for achieving higher efficiency. Besides this, a gas handling system is least expensive to establish and operate among all others forms of fuels. The most common gaseous fuels that we use profusely are natural gas and liquefied petroleum gas. Apart from these two fuels, other gases such as biogas, producer gas, coke oven gas, acetylene, methane, propane and hydrogen are being used as fuels. Table 3.2 summarizes several commonly used gaseous fuels along with their applications. Natural gas is generally found in porous rocks, which is believed to be formed by anaerobic (bacteria-assisted) decomposition of organic matter under heat and pressure, million years ago. It contains mainly methane (CH 4 ) ranging from 75 to 99% by volume and other gases such as CO, CO 2, N 2, C 2 H 6 etc as shown in Table 3.3. It must be kept in mind that natural gases like coal and coke oven gas has regional variations. Recently, liquefied natural gas is being produced by condensing dry natural gas at -102ºC causing cryogenic refrigeration systems.

3 29 Table 3.2 Types of gaseous fuel and oxidizer. Fuel Oxidizer Application 1. Liquefied Petroleum Gas (LPG) Air/O 2 Domestic, burner, furnace. 2. Natural Gas (NG) Air/O 2 IC engines, furnace 3. Producer Gas Air/O 2 IC engine 4. CH 4, C 3 H 8, H 2 Air/O 2 IC engine 5. Biogas Air/O 2 Engine, burner 6. Acetylene Air/O 2 Gas welding, cutting Liquidfied Petroleum Gas mainly consists of propane and butane. The typical composition of LPG is shown in Table 3.3. Propane and butane are generally produced by atmospheric fractional distillation of crude oil. Interestingly, LPG can be stored as liquid in tank at around pressure of 0.8 MPa at normal atmospheric temperature (around 35ºC). But it becomes gas when it is released to ambient pressure (0.1MPa), because butane has a boiling point of -0.5ºC while propane has boiling point of -42.1ºC at ambient pressure. Type CO 2 % Table 3.3 Typical compositions of gaseous fuels [26]. O 2 N 2 CO H 2 CH 4 C 2 H 6 C 3 H 8 C 4 H 10 SG Heating value % % % % % % % % (kj/m 3 ) Gross Net Natural Gas Biogas Propane Butane Coal Gas Coke oven Gas Producer Gas Blast furnace LPG

4 30 Biogas is gaining popularity as a renewable source of energy in Third World countries, which can be easily produced by anaerobic digestion of biomass. In India, the cattle dung is the main constituent through which biogas is produced in a digester. It mainly contains methane and carbon dioxide. Typical compositions of biogas can be found in Table 3.3. It finds application both in direct burning (cooking stove) and power producing devices mainly in IC engines. Producer gas can be generated by gasifying biomass, coal, etc. It finds applications both in direct heating and power producing devices. A typical producer gas is given in Table 3.3. It can be noted that primary fuels are carbon monoxide and hydrogen. The other constituents of this gas are nitrogen (around 40%) and carbon dioxide which make it a low calorific value fuel. However, using oxygen instead of air for gasifying the solid fuel enhances the heating content of this gas. In recent times, hydrogen is contemplated as the future green fuel as it produces least emission. Hydrogen can be easily produced by reforming natural gas, partial oxidation of liquid fuels and gasification of coal or biomass. Acetylene can be produced by hydration of calcium carbide, which is being very much used in gas welding shops in India. Gaseous fuels can be characterized by their composition, density and heating value. The compositions of various gaseous fuels are already shown in Table 3.3. The heating value of a fuel can be defined as the amount of heat released per unit volume when it undergoes oxidation at normal pressure and temperature (0.1 MPa and 298 K). The Higher Heating Value (HHV) corresponds to the heating value of fuel when water is condensed. In contrast, Lower Heating Value (LHV) of a fuel is referred as amount of heat released by burning of 1 kg of fuel assuming that latent heat of vaporization in the reaction products is not recovered. The following expression can relate both these two heating values: LHV HHV ( m / m ) H H 2O fuel v (3.1) Where H v is the latent heat of vaporization of water at K.

5 Liquid Fuel and Oxidizer Liquid fuel is one of the major energy sources, particularly in transport sector. Some of the common and special liquid fuels and oxidizers are listed in Table 3.4, along with their respective applications. Liquid fuels are mainly obtained from the crude oil. Besides this, liquid fuels can also be obtained from biomass, coal tar, sand and oil shale, etc. Basically, typical crude oil is a mixture of alkanes (propane, butane etc.), alkenes, aromatics like benzene, toluene, etc.) and cycloalkanes (napthene) of organic compounds containing C, H, O, N and S elements. Table 3.4 Types of liquid fuels and oxidizers. Fuel Oxidizer Application 1. Gasoline Air SI engine, aircraft piston engine 2. HSD Air CI engine 3. Furnace oil Air Furnaces 4. Kerosene Air Aircraft, gas turbine, ramjet, domestic 5. Alcohols Air IC engine 6. Hydrazine, Unsymmetric Di-Methyl Liquid O 2, Red Ramjet/scramjet, Hydrazine (UDMH), Mono-Methyl Fuming Nitric liquid propellant Hydrazine (MMH), Liquid hydrogen, Acid (RFNA), rocket triethyl amine N 2 O 4 The crude oil from Assam, India contains on an average, 40% paraffin, higher alkanes, 25% napthene and cycloalkanes and 35% aromatics. The ultimate analysis of a crude oil indicates that, on an average, it contains 84% of carbon, up to 0.5% of nitrogen and 0.5% of sulphur.

6 32 Table 3.5 Fuel properties of certain common liquid fuels [27,28]. Fuel type Automotive Gasoline Diesel Fuel Methanol Kerosene Aviation Turbine Fuel Specific gravity Kinematics viscosity At 293 K (m 2 /s) Boiling point range (K) at STP Flash point (K) Auto-ignition Temperature (K) Stoichiometric air/fuel by weight Heat of vaporization (kj/kg) Lower heating value (MJ/kg) Solid Fuel and Oxidizer Wood, coal, charcoal, soft coke, agricultural waste (biomass), animal dung are some of the widely used solid fuels, which are derived basically from fuels available naturally. Biomass contains plant products such as wood, leaves, bark, branches, agricultural residues like rice husk, rice straw, wheat straw, groundnut shell, etc. Biomass as a fuel has been in use since time immemorial. It is well known that coal has been produced from biomass millions years ago due to natural calamities while entrapped under high pressure and temperature conditions. Coals can be mined either from open or deep underground mines. Municipal waste and industrial refuse can also be used as solid fuels. Besides these, several special fuels using polymers are being devised in last fifty years to meet the demand of solid fuels in the chemical rocket engines. Several applications of certain solid fuels are given in Table 3.6.

7 33 Table 3.6 Types of solid fuels and oxidizers. Solid fuel Oxidizer Applications 1. Biomass (wood, sawdust, Air, O 2 Domestic, IC engine etc.) with producer gas 2. Coal, coke Air, O 2 Domestic, IC engine 3. Special fuels Nitro Cellulose (NC), Hydroxyl Terminated Poly-Butadiene (HTPB), Carboxyl Terminated Poly- Butadiene (CTPB) Nitroglycerine, Ammonium percolate, Ammonium nitrate, Nitrogen tetra oxide Solid propellant rocket, hybrid rocket 3.2 Conventional Fuels Most of the road vehicles today operate on Petroleum derived Gasoline and Diesel fuels. Between 30 to 70% of the Petroleum crude throughout the oil refineries is converted to automotive fuels thus controlling the refinery economics. Since the first internal combustion engine and the automobile were developed more than 100 years ago, the properties of engine fuel have been changing. A number of factors have been responsible for evolution of modern fuels. Crude oil prices, progress in refinery processing technology, developments in engine and vehicle technology, vehicle performance and durability requirements, and more recently the environmental regulations besides the geo-political considerations are the important factors that have brought changes in fuel quality. In early days, the main objective of engine designers was to improve power out put and reliability. The one obvious route to improve performance of the spark ignition engine was to increase engine compression ratio. With increase in engine compression ratio however, knocking combustion was encountered in SI engines demanding high octane number fuels to suppress engine knock. In the pursuit to increase knock resistance of Gasoline, tetra-ethyl lead (TEL), an anti-knock additive was discovered in 1921 by Thomas Midgley of General Motors Research Laboratory and was introduced for use in Gasoline on February 1, 1923 [29]. The principal requirements of automotive fuels are given in Table 3.7.

8 34 Table 3.7 Principal Quality Requirements of Automotive fuels. Fuel Quality Combustion Quality High heat of combustion High volumetric energy content Low temperature performance High temperature performance Oxidation Stability Deposit formation control Material compatibility Flow characteristics Relationship with engine and vehicle performance Better ignition and combustion qualities result in better fuel economy and reduction in emissions of pollutants. High octane number for SI engines and cetane number for CI engines are necessary for good combustion. A smaller mass of fuel is to be carried on board of vehicle for the same operation range. A smaller fuel tank and lower vehicle space is necessary, improving vehicle packaging. Liquid fuels being sold on volume basis, it results in better economics for the operators. A significant fraction of fuel should vaporize at low engine temperatures for a better engine cold start and warm-up, good lowtemperature drivability, fuel economy and emissions. For ease of hot starting, reduced vapour lock and evaporative emissions, fuels are blended appropriately to meet the needs of seasonal and geographical variations in ambient temperature. Good low temperature oxidation stability reduces fuel deterioration during storage and deposit formation in the fuel system. Helps in maintaining the engine performance, fuel economy and emissions close to the designed level by keeping the fuel and combustion systems clean. Deposit control additives are low cost products and now widely used for minimizing deposit formation. Material compatibility is essential to prevent corrosion of metallic and deterioration of rubber and elastomeric components of the fuel system. Fuel has to be in fluid condition at low temperature and is important particularly for diesel fuels. Also appropriate diesel fuel viscosity is essential for flow as well as good injection characteristics.

9 35 High octane number fuels could be produced at a low cost with the use of TEL. It led to increase in engine compression ratio to : 1 during late 1950s and 1960s in the USA and Europe. However, since 1970 reduction of engine emissions has become an over-riding requirement for the engine designers and fuel engineers. TEL was widely used in Gasoline until 1975 when the Gasoline vehicles for the first time employed catalytic converters for emission control. Since then, TEL has been gradually phased-out from Gasoline and today Gasoline is almost totally lead-free all over the world. Besides the environmental considerations, there are several other requirements that are to be met by the engine fuels Motor Gasoline Gasoline is a mixture of nearly 400 different types of hydrocarbons [30]. The types of hydrocarbons present in Gasoline are normal-paraffins, iso-paraffins, olefins, aromatics and to a smaller extent cyclo-paraffins. It has hydrogen to carbon ratio varying from 1.7 to 2.0 and is typically characterized by the molecular formula C 8 H 16. The Gasoline is liquid at room temperature with boiling range approximately of º C. The principal properties of Gasoline that are specified in the fuel standards are: Distillation, ºC Reid vapour pressure Specific gravity Research Octane Number (RON) Motor Octane Number (MON) Oxidation stability Gum content, mg/100 ml Lead content, g/l Sulfur, wt % Benzene, vol. %

10 36 Octane Quality High antiknock quality of Gasoline is needed to prevent or minimize knocking combustion in high compression ratio SI engines. Knocking combustion in SI engines can cause engine overheating, loss in efficiency and increase in emissions. Persistent knocking can lead to mechanical damage to engine under high load operation. Earlier lead antiknocks, tetra-ethyl lead and tetra-methyl lead were used to boost octane number of Gasoline. Now however, due to catalyst poisoning and lead being a health hazard by itself, Gasoline world over has become almost lead-free. The lead-free Gasoline is blended with high-octane fuel components like aromatics, iso-parrafins, alcohols and methyl tertiary butyl ether (MTBE) to improve anti-knock quality. The regular grade Gasoline in Europe has a minimum of 91 RON and 82.5 MON. Due to refinery economic reasons, the octane quality of premium unleaded Gasoline however, is now kept at 95 RON and 85 MON as compared to 98 RON and 87 MON for the leaded premium Gasoline earlier [31]. Volatility The volatility of Gasoline is experimentally evaluated by its distillation range and Reid vapour pressure (RVP). RVP measured by American Society for testing and Materials (ASTM) is an important parameter of Gasoline front-end volatility. It has good correlation with the evaporative losses during refueling, from tank vent and carburetor when vehicle is running or during heat soaking after vehicle is stopped. Some other parameter derived from the distillation characteristics and RVP are also used to evaluate drivability performance of Gasoline. For hot weather drivability performance, vapour lock index (VLI) is widely used in Europe and other countries. Oxidation Stability The fuel stays in the storage and transportation systems for several weeks after production before it is consumed in vehicles or engines. During this period, the fuel undergoes slow oxidation under the prevailing ambient conditions. Oxidation stability of Gasoline is a measure of its suitability for long-term storage and its tendency to form deposits in the engine especially the fuel system. The most commonly used methods are induction period and existent gum test.

11 37 To improve oxidation stability of Gasoline anti-oxidants are usually added. The type and amount of antioxidants depends on the Gasoline composition and storage demands. These additives are based on aromatic diamine, alkyl phenol and amino-phenol compounds. Additives called as metal deactivators are also used to nullify the catalytic oxidation effect of some metals like copper present in the Gasoline. Oxygenate Content Various oxygenates, mainly ethers and alcohols are blended in unleaded Gasoline to boost octane number of Gasoline. After the use of lead antiknocks has been banned, use of ethers and alcohols for improving octane quality provides a relatively low cost option to the fuel refiners. The main types of alcohols and ethers that have been used are given in Table 3.8. Table 3.8 Oxygenates used in Gasoline. Alcohols Methanol Ethanol Isopropyl alcohol Isobutyl alcohol Tertiary butyl alcohol (TBA) Ethers Methyl tertiary butyl ether (MTBE) Tertiary amyl methyl ether (TAME) Ethyl tertiary butyl ether (ETBE) The amount of oxygenates added to Gasoline is limited for use in the vehicles designed to operate on the conventional hydrocarbon fuels mainly due to two reasons. One is mixture leaning effect due to presence of oxygen in the fuel and another reason is their adverse effect on the fuel system materials. In addition, the use of oxygenates particularly alcohols increases fuel volatility and may lead to hot weather drivability problems caused by vapour lock. Also, increase in RVP would increase evaporative emissions, resulting in further environmental pollution caused by unburned fuel emissions. Increase in aldehydes emissions with the use of oxygenates containing Gasoline, particularly in the non-catalyst equipped cars is another concern.

12 Diesel Fuels Diesel fuels are mixtures of a large number of hydrocarbons, which generally boil within the temperature range of 150 to 390º C. Earlier, diesel fuels were produced mostly by blending a number of refinery streams from the atmospheric distillation unit. However, to meet the increasing demand of the diesel fuels, products of secondary refinery conversion processes like thermal and catalytic cracking, hydro-cracking etc are also blended in the current diesel fuels. The quality of the final product depends on the characteristics of the crude oil processed and the characteristics of the various streams blended in the product. The key properties of the diesel fuel, which affect engine performance and emissions, are: Ignition quality Distillation (volatility) characteristics Density Viscosity Hydrocarbon composition Sulphur content, and Stability and injection system cleanliness Other significant properties include cold flow characteristics at low ambient temperatures, water and sediment content, carbon residue etc. Ignition Quality Ignition quality of diesel fuel is an expression of the ease of self-ignition of diesel injection spray in the hot compressed air inside the engine cylinder. Cetane number is the most widely used and accepted measure of ignition quality of the diesel fuels. A higher cetane number is being specified now for the diesel fuels as it results in improved cold starting, warm-up, reduced combustion noise, lower fuel consumption and exhaust emissions. Cetane or ignition improvers are used to improve ignition quality of the diesel fuels, which do not naturally meet the specification limits. In premium quality diesel fuels, oil companies use cetane improvers to produce diesel fuels even above the specification limits in certain countries. Most commonly used cetane improving additives

13 39 are nitrates and peroxides like iso-propyl nitrate, cycle-hexyl nitrates, ethyl-hexyl nitrate (EHN) and di-tertiaryl-butyl peroxide. These compounds decompose readily at high compression temperatures in the engine, produce free radicals that accelerate precombustion reactions in the fuel-air mixture and thereby reduce ignition delay. The response of cetane improvers depends on the cetane number and hydrocarbon composition of the base fuel. Distillation Range Higher the volatility of the fuel more readily it vaporizes, mixes with air and burns in the combustion chamber. The low volatility components boiling above 350º C may not burn completely forming engine deposits and producing high exhaust smoke emissions. The mid boiling point often is taken as an overall representation of the fuel volatility and has been observed to affect smoke emissions [32,33]. The volatility is also correlated to the other physio-chemical properties of the fuel like, density, viscosity and ignition quality. Density The specific gravity of diesel fuel varies generally in the range It is interrelated to volatility, cetane number, viscosity and heat of combustion etc. In general, an increase of 10% in density decreases heat of combustion by about 4% and, thus the volumetric energy density of the fuel increases by approximately 6% [32]. The fuel injection pump calibration being on volume basis, the fuel density influences engine power, fuel economy and smoke emissions. Viscosity The viscosity of diesel fuel affects injection characteristics, fuel atomization, drop size distribution, spray cone angle and penetration. An increase in viscosity reduces spray cone angle, increase droplet size and spray penetration. Viscosity of fuel affects fluidity of fuel at low temperatures. High viscosity can reduce fuel flow rates to the injection system resulting in an inadequate fuelling of the engine. Low viscosity on the other hand, results in an increase in leakage of fuel past the pumping elements. If the fuel viscosity is too low, at high temperatures it could result in total fuel leakage. Thus, the fuel viscosity influences metering characteristics of the injection system. Viscosity of fuel is important for lubrication and protection of the injection equipment from wear. Therefore, for a

14 40 given engine application the fuel viscosity range is specifies. Most specification limits kinematic viscosity of diesel fuel in the range of 2 to 5 centistokes. Stability Resistance of fuel to degradation during storage before it is consumed is an important requirement. More cracked products are being blended in diesel fuels to increase its yield from the same crude barrel. Heavy residues from the atmospheric distillation units are catalytically cracked to produce distillate fuels. During long-term storage of diesel fuels particularly those containing thermally and catalytically cracked stocks, high amounts of sediments are formed due to slow oxidation occurring at atmospheric temperature conditions. The distillate products from cracking processes consist of higher amount of olefins and also nitrogen and sulphur containing compounds than those from the atmospheric distillation units. The olefins are more prone to oxidation and the oxidation process is further enhanced by nitrogen containing compounds, such as pyrolles and indoles. The oxidation products in the fuel are polymerized finally to high molecular weight compounds called gums. The gums are of two kinds, soluble gum that remains dissolved in the fuel and the insoluble gum, which gets precipitated out in the fuel. The insoluble gum is also referred as sediments, which causes a number of problems in the engine. The gums cause plugging of fuel filters, the problem being more severe for the paper element micro-filters. Chemical Composition Aromatic content is of great concern as it increases particulate and poly-aromatic hydrocarbons (PAH s) emissions. The diesel fuels specifications in most of the countries now limit aromatic content to 10% maximum. Also limits on the PAH s are being specified. One side effect of reduction in aromatic content is reduction in lubricity characteristics of the diesel fuels resulting in high wear rates of the injection pump elements and injector needles. Detergent additives are used in fuel to solve injector cleanliness problems. Small dosage of additives can keep injectors clean while high dosages of these additives can partially clean the heavily clogged injector holes [34]. Sulphur Content Sulphur containing compounds naturally occur in the fuel and diesel fuels have significantly higher sulphur content than Gasoline. Sulphur on combustion produces

15 41 sulphur dioxide (SO 2 ), most of which is exhausted into atmosphere. A small fraction of SO 2 about 1 to 3% is oxidized in the oxygen rich diesel exhaust to sulphur trioxide (SO 3 ) and to sulphates found in particulate emissions [35]. The sulphur trioxide on combining with water forms sulphuric acid that causes wear of metallic components. It is well known that high sulphur levels of diesel fuel increases wear of piston rings and cylinder liners, the wear rates being higher at lower coolant temperatures. In addition, sulphur increases deposit formation in the combustion chamber and the deposits become harder in presence of sulphur. Sulphuric acid aerosols adsorbed on the foot particles and are emitted as particulate emission. Depending on sulphur content, its contribution can be significantly large to particulate emissions. This is why, sulphur content of diesel fuels is being reduced to very low levels (< 500 ppm and even down to 50 ppm) as more and more stringent emission standards are being implemented [31]. Lubricity The diesel fuel itself provides lubrication of diesel pumping and injection elements. The heavier, high viscosity hydrocarbons and polar compounds are believed to be the lubricating compounds providing natural lubricity to the diesel fuel. The polar compounds get adsorbed on the injection systems surfaces and act as anti-friction layer. Hydro-treating refining processes are used to remove sulphur from the diesel fuels. In the process, polar compounds are also removed. As the sulphur content of diesel fuel decreases, the lubricity of diesel fuel goes down. Low lubricity can result in excessive injection pump wear and in some cases in total mechanical failure. 3.3 Alternative Fuels More and more stringent environmental regulations being enacted in the USA and Europe have lead to research and development activities on clean alternative fuels. Energy security has been another important consideration. A number of liquid and gaseous fuels are among the potential fuel alternatives. Most important among them are, alcohols, ethanol and methanol, natural gas, liquefied Petroleum gas (LPG), hydrogen, ethers like di-methyl ether (DME), vegetable oil esters commonly called as biodiesel, bio gas etc. The US Clean Air Act Amendments (CAAA) besides most of these alternative fuels lists reformulated Gasoline, electricity and any other fuels that permit attaining the legislated emission standards as the clean alternative fuels [31].

16 42 Some of the important properties of different alternative fuels are compared in Table 3.9 with those of typical Gasoline and diesel fuels. Table 3.9 Properties of various fuels for vehicles [39,40]. Property Gasoline Diesel Methanol Ethanol Natural Propane DME RME Hydrogen gas Mol. wt Density (liquefied) (liquid) Lower heating value MJ/kg Heat of at vaporization, 20º C kj/kg Boiling point, ºC Octane number, research Octane number, motor Cetane number > Stoichiometric A/F ratio, mass Vapour flammability limits, (vol. %) Stoichiometric CO 2 emissions, g/mj fuel Adiabatic flame temperature (K) High Petroleum crude prices generated a lot of interest in ethyl alcohol produced from agricultural products during 1980s notably in Brazil. On the other end, during the same period environmental considerations were foremost in the USA for vigorous technological development activities related to methanol. At that time, methanol was considered a more convenient and economically attractive carrier of natural gas across the

17 43 continents for import of cheaper energy available in the form of natural gas. Methanol being liquid it was better suited than natural gas for storage on-board of vehicles. Interest in methanol however, has almost more or less died down it being otherwise highly toxic to living beings and its corrosive nature for many materials used in fuel handling and engine fuel systems. Presently, natural gas and biodiesel have attracted an increasing interest of the governments, vehicle manufacturers and fuel suppliers. Hydrogen is also considered a long-term potential alternative. According to an estimate, the reserves will last for 218 years for coal, 41 years for oil, and 63 years for natural gas, under a business-as-usual scenario [36,37,38] Alcohols Alcohol is made from renewable resources like biomass from locally grown crops and even waste products such as waste paper, grass and tree trimmings etc. Alcohol is an alternative transportation fuel since it has properties, which would allow its use in existing engines with minor hardware modifications. Alcohols have higher octane number than Gasoline. A fuel with a higher octane number can endure higher compression ratios before engine starts knocking, thus giving engine an ability to deliver more power efficiently and economically. Alcohol burns cleaner than regular Gasoline and produces lesser carbon monoxide, hydro carbons and oxides of nitrogen [41,42,43]. Alcohol has higher heat of vaporization, therefore it reduces the peak temperature inside the combustion chamber leading to lower NO x emissions and increased engine power. However, the aldehyde emissions go up significantly. Aldehydes play an important role in formation of photochemical smog. Methanol (CH 3 OH) is a simple compound. It does not contain sulfur or complex organic compounds. The organic emissions (ozone precursors) from methanol combustion will have lower reactivity than Gasoline fuels hence lower ozone forming potential. If pure methanol is used then the emission of benzene and PAH s are very low [42]. Methanol gives higher engine efficiency and is less flammable than Gasoline but the range of the methanol-fuelled vehicle is as much as half less because of lower density and calorific value, so larger fuel tank is required. It has invisible flames and it is explosive in enclosed tanks. The cost of methanol is higher than Gasoline. Methanol is toxic, and has corrosive characteristics, emits ozone creative formaldehyde. Methanol

18 44 poses an environmental hazard in case of spill, as it is totally soluble with water. Ethanol is similar to methanol, but it is considerably cleaner, less toxic and less corrosive. It gives greater engine efficiency. Ethanol is a grain alcohol, and can be produced from agricultural crops e.g. sugar cane, corn etc. Ethanol is more expensive to produce, has lower range, having cold starting problems and requires large harvest of these crops. Higher energy input is required in ethanol production compared to other energy crops and it leads to environmental degradation problems such as soil degradation [24]. Methanol and ethanol can be produced from renewable sources as well as from fossil fuels. Methanol is mainly produced from natural gas. Coal and cellulose containing biomass like wood etc. may also be used to produce methanol. Ethanol is produced almost entirely from the renewable sources from fermentation of carbohydrate containing biomass like sugar, grains, tapioca etc. Neat ethanol (95% ethanol + 5% water) and anhydrous ethanol blended up to 20% in Gasoline have been widely used in Brazil during 1980s. In the USA, use of ethanol initially started in the agricultural surplus states like Nebraska for blending in the reformulated Gasoline as oxygenate. Now, ethanol is the preferred oxygenate replacing methyl tertiary butyl ether (MTBE). The 10% ethanol- Gasoline blends used in the USA are commonly referred as Gasohol. Germany, Sweden, New Zealand and California focused mainly on methanol as an automotive fuel due to its potential near the natural gas field and it being liquid can be more easily handled and transported compared to natural gas. Physical and chemical characteristics of alcohols make alcohols as excellent fuels for the SI engines. The ignition quality of alcohols being poor, these cannot replace diesel fuels directly and a source of ignition is to be provided for their combustion in the diesel engine cylinder. Gasoline at least in 15% v/v amount was added to alcohols to increases visibility of flame from the fire safety considerations. Alcohols are not suitable fuels for compression ignition as they have very poor ignition quality. Cetane number of methanol and ethanol is 5 and 8, respectively. Further, the alcohols are not easily soluble in the diesel fuels. To prepare alcohol-diesel blends high amounts of additives (emulsifiers/solublizers) are required and ignition improvers are to be used to compensate for loss in ignition quality [44,45]. The following three approaches have been largely considered practically feasible for total or part replacement of diesel fuels by alcohols:

19 45 (i) (ii) (iii) Improving ignition quality of alcohols by use of ignition improvers. Use of glow or spark plugs as a positive source of ignition. Dual-fuel operation, using pilot diesel injection as an ignition source for alcohol-air mixtures [46,47]. Widespread use of alcohols as motor fuels so far has not established. Firstly significant cost benefits did not exist and also emission benefits compared to Petroleum fuels have not been attractive. In addition, there are some negative factors and undesirable effects on engines relative to conventional fuels. The main advantages and disadvantages of alcohols with respect to conventional Gasoline and Diesel fuels are summarized in Table 3.10 & Table Table 3.10 Advantages of alcohol motor fuels compared to Gasoline and Diesel. Property/Performance Parameter Compared to Gasoline and Diesel fuels Advantages Octane number Higher A higher engine compression ratio in SI engines can be used resulting in higher thermal efficiency Latent heat of vaporization Higher Lower intake temperature may be Adiabatic temperature flame Lower used to increase charge density and higher volumetric efficiency Potentially lower NO x emissions and lower heat losses Flame luminosity Lower Lower heat losses from combustion PM emissions Lower Due to clean burning characteristics PM emissions are even lower than the Gasoline engines Toxic emissions - Lower benzene and 1,3 butadiene Nature of sources Renewable esp. of ethanol emissions Sources more widespread around the world, hence better energy security. Lower net CO 2 emissions

20 46 Table 3.11 Disadvantages of alcohol motor fuels compared to Gasoline and Diesel. Property/Performance Parameter Compared to Gasoline and Diesel fuels Disadvantages Volumetric energy Much lower Higher volumetric fuel consumption content hence larger fuel storage space and weight Cetane number Much lower Cannot be directly used in compression ignition engines. Needs a source of ignition increasing complexity of engine/fuel system Vapour pressure Lower Poor cold starting and warm up performance, higher unburned fuel emissions during starting/warm up phase CO and NO x Emissions Similar No definite trend is observed, So, no advantage over Petroleum fuels have been noted Aldehyde emission Higher Formaldehyde and acetaldehyde emission are higher Material corrosion/adverse effects Higher Methanol and a lesser degree ethanol are more corrosive to metals, elastomers and plastic components. Needs selection of suitable materials for the fuel system. Engine wear Higher Washes away lubricants film during cold starting, resulting in higher cylinder and piston ring wear Flame Luminosity Almost invisible Neat alcohols present fire safety hazards. Addition of Gasoline or other material required to increase flame luminosity

21 Natural Gas Natural gas has been used now for more than 50 years as fuel for stationary engines for power generation, gas compression and agricultural machinery. Presently, a large number of natural gas vehicles (NGVs) are in operation throughout the world in Argentina, Australia, Canada, Italy, India, New Zealand, countries of former Soviet Union and a number of other Asian and European countries. Natural gas is finding favour as an alternative fuel due to its large-scale availability and potentially high environmental benefits. In the USA, stringent particulate emission standards for the urban buses implemented in the year 1994 and later, provided impetus to the development of natural gas fuelled urban buses. Table 3.12 gives the number of NGVs annual gas consumption and number of natural gas filling station in operation in different regions worldwide for the year 2010 and Table 3.13 represents natural gas vehicle growth since Table 3.12 Worldwide Population of NGVs in 2010 [48]. Country Natural Gas Vehicles Monthly Sales Average N/m 3 Refuelling Stations Iran 2,070, ,000, Argentina 1,901, ,305, Brazil 1,646, ,812, India 1,100,000 NA 596 Italy 676,850 62,030, China 500,000 NA 1652 Colombia 320,036 45,000, Thailand 211,402 95,600, Bangladesh 200,000 91,550, Egypt 139,804 38,000, USA 110, ,000, Russia 100,052 27,710, Germany 85,000 14,600, Korea 28,324 81,680, Sweden 23,125 6,770, Switzerland 9,279 1,320,000 15

22 48 Table 3.13 Natural gas vehicle growth since 2000 [48]. Region Average % NGV growth since 2000 Asia 52.50% Europe 15.40% North America 0.40% South America 25.90% Africa 19.30% Total 29.80% The principal constituent of natural gas is methane (80 to 95% by volume). The balance is composed of small and varying amounts of other hydrocarbons such as ethane, propane, butane and heavier hydrocarbons and non hydrocarbon gases carbon dioxide, nitrogen, water, hydrogen sulphide and other trace gases. Typical composition is given in Table The natural gas before transportation or use is upgraded by removing water, hydrogen sulphide and condensable higher hydrocarbons. It helps in prevention of condensation of these compounds in pipeline and also valuable by-products are obtained. Natural gas, once flared-off as an un-needed byproduct of petroleum production, is now considered a very valuable resource [49]. Table 3.14 Composition of CNG. Constituents % Volume Methane Ethane Propane i-butane n-butane i-pentane n-pentane Carbon dioxide Nitrogen Moisture content 2.0 ppm

23 49 In a nationwide survey across the USA reported in 1992, variations in composition of natural gas could result in variations of 14% in density, 20% in Wobbe index and 25% in stoichiometric air-fuel ratio [50]. The Wobbe index is defined as W=H/ ρ, where H is volumetric heating value and ρ is specific gravity. It has an almost a linear relation with air-fuel equivalence ratio. Reasonably small variations in Wobbe index have little effect on emissions using modern engines with three way catalysts and closed loop feedback control. However, large variations in gas composition can have significant effects on engine performance and emissions, especially if the engine performance and emissions are optimized on a fixed gas composition and engine is not equipped with means of adjusting to other composition can also affect the composition and reactivity of the exhaust HC emissions. Oil wells are primary source and refineries are a secondary source of natural gas where the dissolved gases in the Petroleum crude are released during the refining process, but in lesser volumes. CNG is a safe fuel. Being lighter than air, it diffuses easily into the atmosphere and does not form a sufficiently rich mixture for combustion to take place. In this respect, CNG is superior to other fuel. Storage of propane on vehicles is not as cumbersome as CNG, but the cost of propane is higher than that of CNG. CNG represents a more cost effective emission reduction measure than quite a few other options that have been the subject of serious debate in recent years. International Standards Organization (ISO) provides specifications for the natural gas delivered to the vehicle and not to the pipeline gas [51]. These specifications also include parameters like water content, sulphur content, condensate and free oil that may come from the natural gas compressor. Natural gas liquefies at -161ºC at atmospheric pressure. To use liquefied natural gas (LNG) as automotive fuel cryogenic systems are required. There are other problems too with the use of LNG. Liquid phase in the fuel tank should not become enriched with other higher hydrocarbons during refilling cycles. Leakage of even small amounts of LNG in an enclosed space may form explosive mixtures and risk of fire hazards increases by manifold. Therefore, in most of the NGVs today, natural gas is stored on board in high-pressure cylinders at a pressure of 200 to 300 bars as CNG, storage of natural gas at high pressure on board provides an acceptable range of vehicle operation.

24 50 High antiknock quality of natural gas makes it a fuel that is better suited for SI engines. The natural gas engine operation may be broadly classified in the following types: (i) (ii) (iii) Bi-fuel Operation: The conventional Gasoline vehicles are converted to operate either on Gasoline or natural gas, as the operator prefers. Dedicated or Mono-fuel Operation: Vehicle operating only on gas using a positive source of ignition such as spark plug or hot surface ignition like glow plug. Dual-Fuel Operation: When the diesel injection constitutes 10% or less of the total fuel, the diesel injection system particularly the injectors are replaced by the new ones of a different calibration. These engines are known as pilotinjection engines and do not have the dual-fuel flexibility of operation on diesel alone. In a new development of pilot injection engine high-pressure natural gas is directly injected in the combustion chamber and ignited by the pilot diesel spray. As natural gas has a very high antiknock quality, dedicated natural gas engines can be built with much higher compression ratio than the Gasoline engines resulting in significant improvements in fuel efficiency and lower carbon dioxide emissions. This is particularly useful for heavy-duty vehicle application. Lean burn spark or low plugignited, high compression ratio engines can be built to give very low particulate emissions and high-energy efficiency. The stoichiometric SI engines can utilize the three-way catalyst (TWC) emission technology and therefore, it provides the greatest emission reduction potential. Problems with thermal stresses and low power density have favoured the use of lean-burn combustion system over TWC in heavy-duty application Effect of Natural Gas on Emissions With natural gas operation, large reductions in engine-out emissions compared to either Gasoline or diesel fuel operation can be achieved. It could be mentioned that most light-duty SI natural gas engines are stoichiometric similar to their Gasoline-fuelled counterparts. With natural gas, mixture enrichment during cold starting is not required unlike the Gasoline operation. Hence, use of natural gas results in lower unburned fuel emissions during cold staring and warm-up phase.

25 51 CNG buses without after-treatment have high emissions of formaldehyde, which is considered a possible human carcinogen. The formaldehyde emissions can be reduced with an oxidation catalyst but not to the low level of a diesel bus equipped with catalytic regeneration particulate trap (CRT). In addition to emissions benefits, NGV has other differences from the vehicles operating on the conventional Petroleum fuels as below: (i) The natural gas is stored in high-pressure cylinders. It results in weight penalty of the vehicles and for heavy-duty vehicle it may increase weight of the vehicle by 600 to 1000 kg to provide an acceptable range of operation. Low weight cylinders of composite material are available that reduce the cylinder weight by more than half. These cylinders have a liner made of steel, aluminium or non-metal, which is over-wrapped by hoop or fully wound carbon/fiberglass filament in a resin matrix. These composite material cylinders however, are more expensive than the conventional steel cylinders and the cost may be higher by a factor of 3 or 4. (ii) Compared to Petroleum fuels, emissions of carbon dioxide, a green house gas are lower in the dedicated natural gas engines as a higher engine compression ratio can be used. (iii)low emissions of non-methane hydrocarbons from natural gas vehicles result in low photochemical reactivity and ozone forming potential of the exhaust gases. Emissions of air toxics such as benzene and 1-3, butadiene are very low Liquefied Petroleum Gas LPG is commonly known as propane (C 3 H 8 ), a combustible hydrocarbon based fuel. It comes from the refining of crude oil and natural gas. There are currently three grades of propane available, HD5 for ICEs, commercial propane and commercial propane-butane mixture for other uses. The commercial grade of propane for automotive use is known as HD5 and composition is shown in Fig The exact composition of propane varies slightly between different parts of the country and different refineries. Compared to Gasoline the energy content of LPG is 74%.

26 52 Composition of LPG 100 % Amount Propane Propylene Butane Iso-butane Methane Series Constituents Fig 3.1 Composition of LPG. In the USA, LPG contains more than 85% propane while in Europe and Asia, propane constitutes just about half of LPG, the balance being largely the butane. It remains in gaseous state at normal ambient temperatures and pressures (the boiling point of propane and butane at atmospheric pressure is about -45º C and -2º C, respectively). The pressure inside storage tank keeps LPG liquid, and it becomes gas when released from the tank. The liquid form has an energy density 270 times greater than the gaseous form, making it efficient for storage and transportation. The benefits of LPG as a clean burning motor fuel results in practice largely from its ability to change between the liquid and gaseous phase much more readily compared to natural gas. In Europe, LPG motor fuel was first used in 1950s, especially in Italy and the Netherlands who offered tax incentives making it economically more attractive. In 2003, worldwide population of LPG vehicles stood at 9.5 millions consuming annually about 16.5 million tons of LPG [48]. Population of LPG vehicles in some countries in the year 2003 is given in Table South Korea had the largest LPG passenger car fleet that stood at 1.7 million units, ahead of Italy (1.2 million), Poland (1.1 million) and Turkey (1 million). The use of LPG as motor fuel would help diversify the transport energy supply while exploiting local resources that may be present in abundance in several countries.

27 53 Table 3.15 LPG Vehicle Fleet Worldwide in 2003 [48]. Country Number of Motor Vehicles (thousands) Australia 492 Czech Republic 145 France 180 Italy 1,220 Japan 290 Mexico 700 Netherlands 290 Poland 1,100 Russia 550 South Korea 1,723 Turkey 1,000 United States 190 Worldwide 9,416 Consumption of LPG (thousand tons) 1, ,202 1,528 1, , ,740 1, ,445 Most LPG vehicles employ bi-fuel systems for operation either on Gasoline or LPG. It provides flexibility to vehicle operation, which is important as the number of LPG filling stations is usually small. One drawback with a bi-fuel system is that neither fuel can achieve optimum performance. Optimization of engine for LPG operation is possible only for the dedicated gas engines. However, variation in propane/butane ratio in LPG possess a problem as the octane number of the two main constituents, propane (RON is 112) and butane (RON is 94) is quite different. When more stringent emission standards like Euro IV are to be met, the bi-fuel vehicles may require a major technology upgradation. For bi-fuel vehicles like conventional motor fuels, specific technological development will be necessary to ensure compliance with the stringent emission standards. The advantages and disadvantages of LPG as a motor fuel are similar to those for natural gas. The main advantages and disadvantages of LPG compared to Gasoline are given below:

28 54 (i) (ii) (iii) (iv) (v) (vi) (vii) Good cold start and warm-up characteristics due to its gaseous state Higher antiknock quality of LPG provides an opportunity for use of a higher compression ratio and improvement of engine performance and thermal efficiency Emissions are substantially lower compared to Gasoline vehicles. LPG has disadvantage compared to natural gas in respect of non-methane hydro carbon (NMHC) emissions as these consists of higher amounts of reactive olefinic hydrocarbons. The ozone forming potential of LPG with Gasoline, Diesel and CNG is compared in Fig.3.2. LPG has significantly lower smog formation potential compared to Gasoline and Diesel fuels. LPG operation results in negligible PM emissions compared to Diesel. LPG is relatively a low sulphur fuel. Lower energy content results in higher volumetric fuel consumption As the fuel on board is at a higher pressure, additional safety regulations are to be implemented. As LPG is heavier than air, restrictions on vehicle parking in confined space are also to be applied. Ozone Forming Potential, mg/km CNG LPG Gasoline Diesel Fig 3.2 Comparison of ozone forming potential of different fuels for cars, during summer [36]. Introduction of gaseous fuels in the intake manifold decreases the air partial pressure notably compared to Gasoline. This reduction in power is inherent in the structure of gaseous fuels.

29 55 One way to compensate this loss is to use supercharger or turbocharger in order to increase air flow rate. Superchargers and turbochargers provide more power from the engine by compressing the inducted air higher density than ambient. Volumetric efficiency goes up with turbochargers and superchargers along with better bsfc. Turbocharger has a turbine and compressor in a common shaft. Turbine is driven by the exhaust gas. Using of exhaust gas provides the recovery of waste energy which leads to increase in the overall efficiency. An intercooler or aftercooler is applied in order to provide further increase in the combustion air density. Supercharger is operated on the same principle with turbocharger. But the driven of compressor is achieved by engine's crankshaft. Turbo lag which indicates the delay between boost and throttle response. This can be a problem in SI engines. However this is not noticeable in large diesel engines. The drawback with supercharger appears in cruise conditions. Because supercharger can not adjust itself to this condition due to direct connection to crankshaft as easily as turbocharger can. An electric clutch that turn the supercharger on and off and a by-pass application which takes air from the supercharger output and introduce it in the intake are the current methods for the solution. Another approach that has been considered is to enrich the oxygen content of the intake air by using a membrane gas separator. The oxygen enrichment approach is under research at the present time and is not available on purchased vehicle. The second reason which causes power loss is related to the intake manifold air density. The heat of vaporization of Gasoline helps to decrease the temperature of mixture, producing the dense mixtures. Although propane and methane have higher heat of vaporization value, they are already in gaseous state when inducted into the intake manifold and they do not provide this cooling effect. Development of liquid fuel injection systems for LPG engines should provide better performance and efficiency. Beside this liquid fuel injection provides better A/F ratio control. Back-fire is almost eliminated due to introducing less volume of explosive gases in the inlet system. Cooling effect of endothermic expansion of the liquid increases the resistance to pre-ignition and knock. This leads higher compression ratio which means higher power output.

30 56 Higher compression ratio improves thermal efficiency and provides more power that can be produced by the engine. Higher octane rating of propane compared to Gasoline allows higher compression ratio for the engine. Natural gas and propane are generally considered to reduce engine maintenance and wear in SI engines. The most commonly cited benefits are extended oil change intervals, increased spark plug life, and extended engine life. Natural gas and propane both exhibit reduced soot formation over Gasoline. Reduced soot concentration in the engine oil is believed to reduce abrasiveness and chemical degradation of the oil. Gasoline fuelled engines particularly carbureted engines require very rich operation during cold starting and warm up. Some of the excess fuel collects on the cylinder walls, "washing" lubricating oil off walls and contributing to accelerated wear during engine warm up. Gaseous fuels do not interfere with cylinder lubrication. Engines powered by gaseous fuels are generally considered easier to start than Gasoline engines in cold weather. Because gaseous fuel are already vaporized before inducted into engine. However, under very cold temperatures, cold-start difficulty occurs for propane and natural gas. This is probably due to ignition failure caused by very difficult ionization conditions, sluggishness of mechanical components. Hot starting can cause difficulties for gaseous fuelled vehicles, especially in warm weathers. After an engine is shut down, the engine coolant continues to draw heat from the engine, raising its temperature. If the vehicle is restarted within a critical period after shutdown, (long enough for the coolant temperature to rise, but before the entire system cools), the elevated coolant temperature will heat the gas more than normal, lowering its volumetric heating value and density. This would cause mixture enleanment. Gasoline shows very little change over the normal temperature or pressure range. Propane, however, is gas at ambient conditions. Its physical properties depend mainly on the temperature and pressure at which they are being stored. There must be space left in a propane fuel tank. As the temperature rises, the volume of liquid increases significantly. Due to this, propane system has some kind of safety fill stop device to prevent tank fills to not more than 80 to 85%. This provides room for liquid expansion if the temperature increases after the tank is filled. Due to low viscosity of propane and its storage under

31 57 pressure, it may leak through small cracks, pumps, seals and gaskets more readily than Gasoline Biodiesel Use of vegetable oils as diesel engine fuel is almost as old as the diesel engine itself. In a 1912 speech, Rudolf Diesel said, the use of vegetable oils for engine fuels may seem insignificant today, but such oils may become, in the course of time, as important as Petroleum and the coal tar products of the present time [52]. However, due to availability of cheaper Petroleum crude, interest in fuels derived from vegetable oils diminished. The revival of biodiesel production started with farm co-operatives in the 1980s in Austria and in 1991, the first industrial-scale plant started biodiesel production with a capacity in excess of 10,000 m 3 per year. Through 1990s, plants were established in many European countries, including the Czech Republic, France, Germany and Sweden. In 1998, the Austrian Biofuels Institute identified 21 countries with commercial biodiesel projects. In the 1990s, France launched the production of biodiesel obtained from rapeseed oil. The European Directive 2003/30/EC proposed to promote the use of biofuels or other renewable fuels for transport to reach 2% share of the total automotive fuel market by December 31, 2005 and 5.75% by December Of this, biodiesel is expected to constitute the major part [53]. Biodiesel is a renewable fuel that is produced from a variety of edible and nonedible vegetable oils and animal fats. The term biodiesel is commonly used for methyl or ethyl esters of the fatty acids in natural oils and fats that meet the specifications for their use in the CI engines. Straight vegetable oils are not considered as biodiesel although attempts have been made to use these as well in the CI engine. Biodiesel is typically produced by a reaction of vegetable oils or animal fats with an alcohol such as methanol or ethanol in the presence of a catalyst to yield mono-alkyl esters. Glycerin is obtained as a by-product, which is removed. The straight mineral oils have very high viscosity that makes flow of fuel difficult even at room temperatures and presence of glycerin in the vegetable oil causes formation of heavy carbon deposits on the injector nozzle holes.

32 58 A variety of vegetable oils such as those from soybean, rapeseed, sunflower, jatropha carcass, palm, and cottonseed etc. have been widely investigated for production of biodiesel. Rapeseed oil and some other vegetable oils when transformed to their methyl esters have many characteristics such as density, viscosity, energy content, and cetane number close to that of diesel. The more widely used are Rapeseed Methyl Ester (RME) in Europe and Soybean Methyl Esters (SME) in the US. They are collectively known as Fatty Acid Methyl Esters (FAME). Recently non-edible oil produced from jatropha-curcass seeds has gained interest as this plant can be easily grown on wastelands. The vegetable oil esters are practically free of sulphur and have a high cetane number generally in the range 46 to 60 depending upon the feedstock. The cetane number of methyl esters tends to be slightly lower than of ethyl or higher esters [54]. Biodiesel from saturated feed stocks such as animal fat and recycled restaurant cooking fats will generally have a higher cetane number than the esters of oils high in poly-unsaturates such as soybean oil. Due to presence of oxygen, biodiesel have a lower calorific value than the diesel fuels. The emission studies [55] show that the use of biodiesel results in reduction of CO, HC and PM, but slight increase in NO x emissions. Reduction in CO emission could probably be attributed to presence of oxygen in the fuel molecule. Decomposition of biodiesel produces a variety of oxygenated hydrocarbons in addition to hydrocarbons. Response of the standard HC measurement technique, the heated flame ionization detector is different for the methyl esters than HC emission [56] and this could be partly responsible for the difference in HC emissions between the normal diesel fuels and biodiesel. The methyl esters have a lower compressibility, which results in advance of dynamic injection timing with biodiesel compared to diesel. Change in injection timing and differences in cetane number and combustion characteristics and particulate emissions are observed to be significantly lower with biodiesel compare to diesel fuels. Volumetric fuel consumption with biodiesel is higher than diesel due to its lower heating value. An increase of 10-11% in fuel consumption compared to diesel may be expected when comparing their heating values.

33 59 As biodiesel is produced from vegetable oils or animal fats, its use has been promoted as a means for reducing greenhouse gas CO 2 emissions that would otherwise be produced from the combustion of Petroleum-based fuels. The total impact that biodiesel could have on global warming would be a function not just of its combustion products but also of the emissions associated with the full biodiesel production and consumption lifecycle. On an average the carbon content on mass basis of plant-based biodiesel is 77.8% and of animal fat based biodiesel is 76.1% compared to 86.7% for the Petroleum based diesel. Since biodiesel is free from sulfur hence less sulfate emissions and particulate reduction is reported in the exhaust. Due to near absence of sulfur in biodiesel, it helps reduce the problem of acid rain due to transportation fuels [57]. Higher thermal efficiency, lower BSFC and higher exhaust temperatures are reported for all blends of biodiesel compared to mineral diesel [58]. Biodiesel is oxygenated fuel (hence more complete combustion) and causes lesser particulate formation and emission. This is also due to oxygenated nature of biodiesel where more oxygen is available for burning and reducing hydrocarbon emissions in the exhaust [59,60,61]. The biodiesel have a slightly higher carbon content per unit energy (2.068 kg/100mj) than the conventional diesel (2.042 kg/100mj) and thus may be expected to give higher CO 2 emissions on combustion. The measured data however, suggest that the combustion generated CO 2 from biodiesel and conventional diesel are substantially similar [62]. The cost of producing methyl or ethyl esters from edible oils is currently much more expensive than hydrocarbon based diesel fuel. Due to the relatively high costs of vegetable oils (about 1.5 to 2 times the cost of diesel), methyl esters produced from it cannot compete economically with hydrocarbon-based diesel fuels unless granted protection from considerable tax levies applied to the latter. In absence of tax relief, there is a need to explore alternate feedstock for production of biodiesel. The cost of biodiesel can be reduced if we consider non-edible oils and usedfrying oils instead of edible oils. Non-edible oils such as mahua, karanja, babassu,

34 60 jatropha, neem etc., are easily available in many parts of the world, and are cheaper compared to edible oils. Most of these non edible oils are not used to their potential and in fact produced in surplus quantities. Several countries including Netherlands, Germany, Belgium, Austria, USA, Japan and India discard used frying oils. With the mushrooming of fast food centers and restaurants in the world, it is expected that considerable amounts of used-frying oils will be discarded. This oil can be used for making biodiesel, thus helping to reduce the cost of water treatment in the sewerage system and in the recycling of resources [24] GAS-TO-LIQUID (GTL) FUELS: GTL conversion is a broad term for a group of technologies that are used to produce synthetic liquid hydrocarbon fuels from a variety of feed stocks. These fuels have characteristics similar to those of Petroleum fuels and would form a more convenient substitute for them. The synthetic gas, a mixture of carbon monoxide and hydrogen is produced from a variety of feed stocks like coal, natural gas and biomass, and is converted to a mixture of hydrocarbons of different molecular weights and structures. The chemical conversion process was first developed by Petroleum deficient but coal rich Germany during 1920s and is known as Fischer-Tropsch (F-T) process after the name of its inventors. Therefore, GTL diesel is also known as F-T diesel. The basic process consists of two steps. 1. Production of synthesis gas, and 2. F-T synthesis. Synthesis gas is produced by steam reforming of natural gas, coal or biomass or by partial oxidation of hydrocarbons like natural gas. Steam reforming reactions are: CH y H 2 O ( 1 0.5y) H 2 CO (3.2) The value of n depends on the type of feedstock. For example, for typical hydrocarbon feed stocks y = 2.2 to 4 as they have high content of hydrogen and for coal y<<1. The partial oxidation reaction for natural gas to generate synthesis gas proceeds as below; CH O 4H 2CO (3.3)

35 61 The steam reforming and partial oxidation reactions are endothermic in nature and the energy needed is supplied by the combustion of the feedstock itself with oxygen. Fischer-Tropsch synthesis in generic form is described by the reaction. nco 2nH 2 n( CH 2 ) nh 2O (3.4) (-CH 2 -) is the basic building block of paraffin hydrocarbons. The product is primarily straight chain hydrocarbons with small quantities of isoparaffins and olefins. Therefore, the F-T fuel has a high cetane number and is best suited as fuel for the diesel engines. The F-T synthesis takes place over cobalt based catalyst at temperatures between 180 to 250 C and pressures ranging from 20 to 40 bar. As the catalyst gets poisoned by sulphur the synthesis gas is made sulphur free before F-T synthesis. Commercial plants are in operation in South Africa (Sasol) that uses coal and natural gas, and in Malaysia and Qatar based on natural gas. The properties of GTL fuels depend on the pressure, temperature and the catalyst used for synthesis. The Table 3.16 gives the properties of GTL fuel and the range in which these are generally obtained. The GTL fuel when compared to conventional diesel has, High hydrogen content Similar heat of combustion Lower density and hence, lower energy content per unit volume Higher cetane number Ultra-low sulphur Near zero or very low aromatic content The GTL diesel is composed of hydrocarbons like Petroleum derived diesel fuels. Hence its effect on engine performance would so the trends similar to those obtained with change in properties of the conventional diesel fuels. Use of GTL diesel alone as well in blends with conventional diesel has been investigated on light and heavy duty Euro III and Euro IV diesel engines. Reduction in emissions with 100% GTL fuel are significant particularly the particulate, unburned HC and CO emissions. The soot emissions are low with GTL fuels as these have negligible aromatic content. Combustion with GTL fuels

36 62 results in reduced HC and CO emissions due to higher cetane numbers and lower densities. The NO x emissions are found to reduce slightly or similar to conventional diesel fuels. The specific fuel consumption is also similar although, some studies have reported 2 to 3% improvements in fuel efficiency. However, due to lower densities, the volumetric fuel consumption is nearly 5% higher. Table 3.16 Properties of GTL Diesel Fuels [63,64,65]. Properties Range 20 C Kinematic viscosity at 40 C, mm 2 /s Cetane number Distillation - Initial boiling point, ºC % evaporation point Sulphur, ppm by mass <1 Total aromatics,% mass H/C atomic ratio Lower heat of combustion, MJ/kg Di-methyl Ether (DME) During 1990s interest began to be focused in di-methyl ether (DME) as a potential diesel engine fuel. DME can be produced from dehydration of methanol. Haldor Topsoe developed a process for direct production of DME from synthesis gas [66]. The synthesis gas (CO+H 2 ) can be produced from a variety of raw materials e.g., natural gas, coal, biomass etc. The DME produced from biomass can be categorized as a renewable fuel, while DME produced from natural gas can act as an energy carrier in liquid form which is much easier to transport across continents than the natural gas. DME is the simplest ether and has chemical formula CH 3 -O-CH 3. It has vapour pressure of 5.1 bar at 20º C and can be stored, transported and dispensed like LPG. It is environmentally benign, is not harmful to ozone layer and it readily degrades in troposphere to carbon dioxide and water. DME is considered non-toxic and is not classified as a carcinogen, teratogen or mutagen. It is non-corrosive and burns with

37 63 visible blue flame. Important properties of DME are listed in Table 3.9. Its calorific value is 33% lower than the conventional diesel but it has a high cetane number making it a suitable fuel for CI engines. DME has no carbon-carbon bonds and oxygen constitutes 35% of its weight. These factors contribute to an almost smoke free combustion. Density of DME is about 80% of diesel fuel and calorific value is just about twothird of diesel. Therefore, compared to diesel twice the volume of DME should be injected to get the same engine power. Moreover, it has a high compressibility and, low viscosity and lubricity compared to diesel. Thus, the fuel injection system designed for diesel fuel cannot be used for DME [67]. Gray and Webster studied [68] emissions of a 5.9 liter Cummins engine equipped with oxidation catalyst with DME and diesel fuel. Table 3.17 summarizes the overall average regulated exhaust emissions with the engine operating on DME and diesel fuel. Table 3.17 Emissions Results with DME and Diesel Fuel on a 5.9 liter Cummins engine with Oxidation Catalyst, g/hp-h [69]. Fuel CO CO 2 NO x HC PM DME Diesel Emission operation on DME reduced CO emissions by 43% and PM emissions by 75% compared to diesel. NO x emissions were only slightly lower. However, the HC emissions more than doubled, but most HC emissions were unburned DME that is environmentally benign. Use of exhaust catalysts may be beneficial in reducing unburned DME and methane emissions. DME provides good engine cold starting. DME although is non corrosive to metals but some rubber and elastomer components may not be compatible with it. Therefore, material of seals has to be carefully selected. DME has a poor lubricity requiring use of additives to protect injection equipment against excessive wear. It burns with visible blue flame and the flame luminosity is quite good. This is important from fire safety angle. It being gas at room temperature and atmospheric pressure, precautions to prevent its leakage need to be taken as it could form explosive mixtures with air.

38 Hydrogen Interest in hydrogen as a potential alternative automotive fuel has grown due to need of reducing dependence on fossil fuels and to minimize air pollution. Hydrogen can be produced from a variety of fossil and non-fossil sources. Presently the most economic process to manufacture hydrogen is from hydrocarbon like natural gas or naphtha by steam reforming. Coal gasification is another method. In these processes however, carbon dioxide is also produced. Production of hydrogen by electrolysis of water is used in some industrial plants but it is very expensive due to high consumption of electricity. Use of the solar energy to produce hydrogen by photo-electrolysis is another potential route. Hydrogen is a colorless, odourless and nontoxic gas. It burns with an invisible and smokeless flame. The combustion products of hydrogen consist of mainly water and some nitrogen oxides. The major hurdles in the use of hydrogen as a fuel are lack of production, distribution and storage infrastructure. On board storage of hydrogen is a major challenge. Hydrogen has very low boiling point (-253ºC) and a very low volumetric energy density. The following methods of on-board storage of hydrogen are under consideration and some of them are being used in demonstration vehicles: (i) (ii) (iii) (iv) Compressed H 2 in high-pressure cylinders at MPa: It results in high weight penalty and safety risks. As a metal hydride: Hydrogen can be stored as a metal hydride like irontitanium metal hydride (FeTiH 2 ), magnesium hydride, and magnesium-nickel hydride or adsorbed on carbon. Metal hydrides release hydrogen on heating by a heat source like vehicle exhaust gas. The main problems of hydride storage system are limited storage capacity, contamination of storage materials by the impurities in hydrogen, and high cost. Storage of liquid hydrogen in cryogenic tanks: Liquefaction of hydrogen is highly energy intensive. Energy spent in liquefaction of hydrogen to 20 K is nearly equal to the energy content of the liquid hydrogen. Thermal insulation of the cryogenic tanks at 20 K is also very challenging. Chemical hydrogen carriers: Hydrogen can be stored as a constituent of a chemical compound like methyl-cyclohexanol, sodium boro-hydride (NaBH 4 )

39 65 etc. A catalyst is required to dehydrogenate the chemical compound at high temperature e.g. 500º C for hydrogenous methyl-cyclohexanol [70]. Volumetric energy density of compressed hydrogen is just one-third energy density of natural gas. Liquid hydrogen also has a very low volumetric energy density, which is about one-fourth of Gasoline. The liquid, hydride and compressed hydrogen storage methods are compared in Table 3.18 for storing 5-gallon (19 liter) Gasoline equivalent of energy storage. Hydrogen storage space required is at least 10 to 12 times that for Gasoline. Storage and fuel weight for hydrides is 27 times and for compressed H 2 is 4 to 5 times of Gasoline. Table 3.18 Comparison of hydrogen storage methods [71]. Energy stored, MJ Fuel mass, kg Tank mass, kg Total Fuel System mass, kg Volume, l Gasoline Liquid H 2 Hydride Fe-Ti (1.2%) Compressed H 2 (70MPa) Hydrogen fuel-cell vehicles are expected to have more commercial potential in long run. Though it is believed that significant production volumes for customers will not be available until time frame, automotive manufacturers world over like Toyota, Honda, General Motors, Ford, Chrysler, BMW are going ahead with limited production and field trials of fuel cell powered cars and buses. Hydrogen fuelled ICE vehicles are however, regarded as transition or bridging strategy to stimulate building of hydrogen infrastructure and related hydrogen infrastructure and related technologies. Hydrogen has significantly different combustion characteristics than Gasoline. Octane rating of hydrogen is 106 RON, making it more suitable for SI engines. The laminar flame speed of hydrogen is 3 m/s, about 10 times that of Gasoline and methane. Hydrogen has very wide flammability limits ranging from 5-75% by volume (φ= ), which may lead to pre-ignition and backfiring problems. Its adiabatic flame temperature is higher by about 110º C than for Gasoline (Table 3.9). If inducted along with intake air, the volume of hydrogen is nearly 30% of the stoichiometric mixture, decreasing the

40 66 volumetric efficiency and engine power considerable. Another option is direct injection of liquid hydrogen into the engine cylinder that provides some advantages like cooling of charge, higher volumetric efficiency and no danger of backfiring. Hydrogen on combustion produces water and there are no emissions of carbon containing pollutants such as HC, CO and CO 2 and air toxics benzene, PAH, 1-3 butadiene and aldehydes. Trace amounts of HC, CO, and CO 2 originating from burning of lubricating oil however, may be emitted. NO x is the only pollutant of concern from hydrogen engines. Very low NO x emissions are obtained with extremely lean engine operation (φ<0.05) [70]. Injection of water into intake manifold or exhaust gas recirculation which in this case consists primarily water vapour, can further suppress formation of nitrogen oxides. In addition, water injection provides charge cooling and control of pre-ignition and backfiring in the engines using external mixture preparation. The direct fuel injection in the cylinder mitigates some of the problems faced by the engines with external mixture preparation. Hydrogen fuelled engines produces almost no CO 2 and its global warming potential is insignificant. Considering the total well-to-wheel energy analysis however, when hydrogen is produced from fossil resources hydrogen fuelled vehicles provide no overall reduction in greenhouse gas emissions and in some cases even worse than the vehicles fuelled by the conventional Gasoline and diesel fuels. Also the addition of H 2 to other traditional slower burning fuels with narrow operational mixture range such as those of methane and bio-gases, can accelerate significantly the flame propagation rates, extend greatly the lean operational mixture range while reducing the emissions of CO [72,73] Vehicle Emissions and Air Pollution Since, 1970 more and more stringent vehicle emission regulations have been implemented in the developed countries like USA, Europe and Japan, but still vehicles contribute significantly to the urban air pollution [74]. Combustion of various fossil fuels leads to emission of several pollutants, which are categorized as regulated and unregulated pollutants. Regulated pollutants are ones, whose limits have been prescribed by environmental legislations (such as USEPA, EURO and Bharat norms) whereas there are some pollutants for which no legislative limits have

41 67 been prescribed. These are categorized as unregulated pollutants. Regulated pollutants include NO x, CO, HC, particulate matter (PM) and unregulated pollutants include formaldehyde, benzene, toluene, xylene (BTX), aldehydes, SO 2, CO 2, methane etc., [75,76,77]. These regulated as well as unregulated pollutants contribute to several harmful effects on human health, which are further categorized as short-term and long-term health effects. The short-term health effects are caused by CO, nitrogen oxides, PM, formaldehyde (primarily regulated pollutants) etc., while long-term health effects are caused mainly PAHs, BTX, formaldehyde (primarily unregulated pollutants) etc. CO is fatal in large dosage, aggravates heart disorders, affects central nervous system, and impairs oxygen-carrying capacity of blood by forming carboxy-hemoglobin. Nitrogen oxides cause irritation in respiratory tract. HC cause drowsiness, eye irritation, and coughing [78,79,80]. These pollutants also contribute towards several regional and global environmental effects. Regional environmental effects such as summer smog are because of aldehydes, carbon monoxides, nitrogen oxides etc. Winter smog is because of particulate. Acidification is caused by nitrogen oxides, sulphuric oxides etc. Several global effects like ozone layer depletion, global warming etc. are caused by CO 2, CO, methane, non-methane hydrocarbons, nitrogen oxides etc [81,82]. According to a report of Central Pollution Control Board (CPCB), out of the total pollution load, 65% of CO, 22% of HC and 12% of Nitrogen oxides are due to Gasoline and diesel vehicles [83]. Since CNG fuel is in gaseous form, it does not need to be vaporized. Therefore, no fuel enrichment process is needed during cold starting or in transient conditions. This contributes in the reduction of CO emission. In addition, due to higher H/C ratio, CNG combustion produces 25% less carbon dioxide than Gasoline or diesel at the same engine efficiency [84]. The main problem that all researchers and manufacturers are facing now is the low power output of CNG engine due to loses in volumetric efficiency, low flame speed and absence of fuel evaporation [85].

42 68 One major step towards emission control was introduction of exhaust oxidation catalysts on passenger cars in the US and Japan in since then, high level of advancements in engine technology including death of carburettor in the US in 1990 and its replacement by multi-port fuel injection (MPFI), multi-valves per cylinder, electronically controlled variable valve lift and timing, Gasoline direct injection (GDI) engine have taken place [74]. Exhaust catalytic conversion being one of the mainstay of emission control, it has seen many improvements like three-way catalytic control of HC, CO and NO x simultaneously, electrically heated catalyst for emission reduction during engine reduction during engine start-up, NO x storage catalysts to function under lean engine operation such as in GDI engines etc [74]. The vehicles primarily emit the harmful gases CO, unburned fuel/hydrocarbons also called as volatile organic compounds (VOC) and NO x. Among the mobile source, diesel vehicles are the main contributors to smoke and particulate matter (PM) emissions. Vehicles also emit sulphur di and tri-oxides (SO x ), their amount depending upon the sulphur content of the engine fuel. Some of the vehicle emissions in the atmosphere produce other harmful chemicals, the secondary pollutants. The main secondary pollutants are: oxidants like ozone, nitrogen dioxide (NO 2 ) and total suspended particulates (TSP) including host of other organic compounds like peroxy-acetyl nitrate (PAN) etc. Carbon dioxide is not a pollutant for local environment but it being a green house gas its contribution to global warming is causing an increasing concern. It is estimated that CO 2 is responsible for about 50% of the global greenhouse effect. The adverse morbidity and mortality effect of air pollutants like suspended particulate matter (SPM), respirable suspended particulate matter (RSPM or PM 10 ), sulfur dioxide (SO 2 ), carbon monoxide (CO) and Ozone (O 3 ) are well documented. In the case of Delhi, the situation deteriorated in the 1990s as vehicles growth outpaced population growth and economic development, vehicle had risen to nearly 3.6 million by During the period, Delhi s population increased from 9.5 to 13.8 million and road-length from 22,000 to 25,000 km. The World Bank estimated that a person was dying every 70 minute in Delhi in 1995 from air pollution [86]. The Supreme Court (SC) of India ruled in 1998 that all the public transport should move away from diesel to CNG by 31 st March, 2001 and by 1 st December, 2002 all the buses were converted to CNG.

43 69 The United States, with less than 5% of the world s total population, consumes 25% of the world s current energy production and generates about 25% of the world s carbon emissions [87]. The emission norms and year of implementation for India are shown in Table 3.19 and Table Table 3.19 Emission norms for India for heavy duty vehicles > 3.5 Tones (g/kwh) [88] PRE EURO - 0 EURO - 1 EURO - 2 EURO - 3 EURO - 4 NO x < 5.00 < 3.00 CO HC PM N.A N.A < 0.15 < 0.10 Table 3.20 Emission norms and year of implementation. Norms Year of Implementation 1996 (Euro 0) ( Catalytic Converter Norms) 1998 Bharat Stage I (Euro I) 1999 Bharat Stage II (Euro II) 2000/2001 Bharat Stage III (Euro III) April, 2005 Bharat Stage IV (Euro IV) April, Carbon Monoxide (CO) Carbon monoxide is formed due to deficiency of oxygen during combustion. It is an odorless gas but is highly toxic. On inhalation it is rapidly absorbed by lungs and combines with hemoglobin in the blood forming carboxy-hemoglobin. CO has 200 to 240 times greater affinity than oxygen to combine with hemoglobin [89]. The CO-hemoglobin complex is far more stable than oxy-hemoglobin. This exposure to CO reduces oxygen carrying capacity of the blood to body tissues. The decrease in release of oxygen due to

44 70 CO intoxication damages tissue and cells and adverse effects are higher and more rapid to the brain and nervous system as these have a higher oxygen demand. The toxic effects of CO depend both on the exposure time and concentration as shown in Fig The early signs of CO poisoning are shortness of breath, rapid breathing, headache, dizziness, impaired judgment i.e., confusion and lack of motor coordination. These signs and symptoms results due to reduced supply of oxygen to brain tissues, a condition called hypoxia. Nausea, vomiting and diarrhea may appear later. Exposure to high CO concentrations or for a longer period may lead to cardiac arrest, pulmonary edema, loss of consciousness and eventually to death. If the concentration of CO in the inhaled air is high enough, loss of consciousness and death may occur within a short time. Treatment of CO intoxication includes remove of affected person from exposure to the air having high CO and administration of 100% oxygen to accelerate dissociation of carboxy-hemoglobin to hemoglobin. Hemoglobin then can combine with oxygen and correct the tissue hypoxia [89]. Fig. 3.3 Toxicity of carbon monoxide [90] Nitrogen Oxides Oxides of nitrogen NO and NO 2 are formed during combustion at high temperatures. During combustion in IC engines, the principal oxide of nitrogen formed is