Alternative Fuels

Alternative Fuels
1-1 Introduction
In the early days of development, the view was widely held that machines, capable of operating efficiently on a wide variety of cheap fuels—solid, liquid, and gaseous. The choice of fuel is dictated on the grounds of cost, availability, and ease of handling. This is true even today, provided one keeps in mind the important and restrictive effects of aircraft engine requirements on the physical and chemical properties of fuel.
Today, the ever rising cost of petroleum fuel is prompting research into developing alternative liquid fuels based on coal, biomass, and another feedstock. Finally, these domestically produced alternative fuels have to be capable of using the available infrastructure of fuel refining, transportation, distribution, and consumption.
The alternative synthetic liquid fuels of major interest will be largely derived from biomass (carbohydrates, algae, and vegetation), coal, oil shale, tar sands, and heavy oil. For longer term, methane and hydrogen are among the candidate fuels now being considered. All these fuels must be compatible with engine and fuel-system requirements and, where appropriate, with air- craft design features and operational procedures. The main impact of trends and developments relative to combustor design will be felt in fuel-nozzle design for multifuel capability and in fuel and air management for minimum soot and gaseous emissions. The future of the alternative fuel industry depends on the following key factors:
¬ Right fuel properties and handling for the engines and infrastructure already in place,
¬ Environmental impact that includes competition with food, water, and land,

¬ CO2 life cycle analysis and carbon footprint issues,

¬ Economics of return on investment, production, and sustainability.

2 Types of Hydrocarbons

Pure hydrocarbon fuels are compounds of two elements only, carbon and hydrogen. They maybe gaseous, liquid, or solid at normal pressure and temperature, depending on the number of carbon atoms and their molecular structure. Those with up to four carbon atoms are gaseous; those with twenty or more are solid, and those in between are liquid. It is usual to classify the hydrocarbons present in petroleum fuel into four main groups: paraffinic, oleffinic, naphthenic, and aromatic. The proportions in which these groups are present largely de ne the character of the fuel.

2.1 Paraffins
Paraffnic oils are found mainly in the United States, North Africa, and Nigeria. They have the general formula CnH2n 2. Thus, the simplest hydrocarbon, methane, is in this class; its molecule can be represented as:


The remaining normal paraf ns are built up from methane as straight chains, e.g.,

Alternative paraffin configurations, or isoparaffins, are in the form of branched chains, such as

Current aviation fuels contain an average of 60% paraffins, depending on the source of the crude oil and the distillation process. In general, paraffins tend to have a higher hydrogen/carbon ratio, lower density, lower freeze point, and high gravimetric calorific value than other types of hydrocarbon fuels. They possess high thermal stability, and their combustion is characterized by freedom from coke deposition and exhaust smoke.
2.2 Olefins
Olefins conform to the general formula CnH2n. They do not normally exist in crude oil, but are produced by conversion processes in the refinery. As ole ns are unsaturated, i.e., their molecules contain less than the maximum possible number of hydrogen atoms, they are very active chemically and readily react with a great many compounds to form resinous gums and rubberlike materials. For this reason, ole ns are very undesirable in gas turbine fuels, and are found only in trace quantities.
Olefin molecules must contain at least two carbon molecules, and the lightest molecule is, therefore, C2H4, ethylene. More complex molecules of the ole n series are obtained by adding CH2, as in


2.3 Naphthenes
Naphthenes, which have the general formula (CH2)n, are saturated hydrocarbons in which the carbon atoms are linked to form rings instead of chains as in the case of paraffins. Naphthenes bear names identical to those of the paraffins that have the same number of carbon atoms, with the addition of the pre x “cyclo,” e.g.,

Naphthenes are major constituents of jet fuel, i.e., about 25–35%. They closely resemble paraffins in their chemical stability, high gravimetric heat of combustion, and low soot-forming tendencies.
2.4 Aromatics
Aromatics are ring compounds containing one or more six-member rings with the equivalent of three double bonds. Although similar in structure to the naphthenes, they contain less hydrogen and, in consequence, their specific energy is appreciably lower. Aromatic compounds in fuel cause swelling of o-ring and this helps seal the high pressure aircraft fuel system. The disadvantages of aromatic compounds include a marked tendency to soot formation and a high hygroscopicity that can lead to precipitation of ice crystals when the fuel is subjected to low temperatures. Aromatics also have a strong solvent action on rubber that can cause trouble in fuel systems and on aircraft fitted with soft-rubber-lined fuel tanks.
The characteristic formula for the aromatics is CnH2n-6. The simplest member is benzene, in which each carbon atom carries only one hydrogen atom:

More complex molecules of the aromatic group are obtained either by replacing one or more of the hydrogen atoms with hydrocarbon groups or by “condensing” one or more rings . Another example is:

3 Production of Liquid Fuels
The production process can be divided into three basic categories:
1. Separation process: Crude oil is separated into its primary fractions, consisting of gasoline, distillate fuels, and fuel oil; these then provide the basic material for the desired range of fuel products. Separation is accomplished by a distillation process that exploits the fact that the various components in crude oil have different boiling points. When a crude oil is heated, the first gases evolved are chiefly methane, ethane, propane, and butane. Next, vapors are released that condense to form light distillates and then gasoline. As boiling proceeds, kerosene emerges, followed by the middle distillates used in gas oil and diesel fuel. Finally, a residue is left that is used in the manufacture of lubricating oils, wax, and bitumen.
2. Upgrading process: These processes improve the quality by using chemical reactions to remove any compounds present in trace quantities. Commonly used upgrading processes are sweetening, hydro treating, and clay treatment.
3. Conversion process: These processes change the molecular structure of the feedstock, usually by “cracking” large molecules into small ones, e.g., catalytic cracking and hydrocracking.
3.1 Additives
Additives are fuel-soluble chemicals in the parts per million concentration range, which are blended with fuel to enhance fuel properties and handling,
¬ Gum Prevention
¬ Corrosion Inhibition/Lubricity Improvers
¬ Anti-Icing
¬ Antistatic–Static Dissipators
¬ Metal Deactivators
¬ Antismoke
4. Classification of Liquid Fuels
Table 1 lists the typical properties of conventional liquid fuels for gas turbines. Light distillates such as gasoline and kerosene are used for aircraft, and heavy distillates are used for industrial gas turbines.
Gasolines (including naphtha) are excellent fuels of high burning quality. However, their low viscosity may result in poor lubricity, while their low ash point and high volatility may require special attention to safety. Aviation gasolines have a typical distillation range of 300–495 K. They include Avgas (UK military), JP-4 (US military), and Jet B (US civil) fuels. The kerosene consist of refined hydrocarbons from the distillation of crude petroleum, or blends of the latter with suitable cracked products. They include the normal aviation kerosene, such as Jet A-1, Jet A, and Navy’s high- ash-point JP-5.
Light to heavy distillates include No. 2-GT gas turbine fuel, No. 2 burner fuel, diesel oil, and marine gas oil. Diesel fuels have additional requirements of cetane number, and the primary fuel in this group is the 2-D diesel fuel.

4.1 Aircraft Gas Turbine Fuels
The fuel specifications for aircraft engines are stricter than those for all other types of gas turbines. The main requirements due to the airframe, engine fuel system, and combustion chamber are listed below.
4.1.1 Airframe
1. Low fire risk. This implies low vapor pressure, low volatility, high ash point, and high conductivity to minimize the buildup of static electricity during fueling.

2. High heat content for maximum range and/or payload, i.e., high calorific value on a weight or volume basis, depending on whether the aircraft is “weight limited” or “volume limited.”

3. High thermal stability, to avoid filter plugging, sticking of control valves, etc.

4. Low vapor pressure, to minimize evaporation losses at high altitude.

5. High specific heat, to provide effective heat absorption on high- speed aircraft.

4.1.2 Engine Fuel System
¬ Pumpability. That is, the fuel must remain liquid and ow freely to the atomizer. This is essentially a requirement for low viscosity.

¬ Freedom from filter clogging by ice or wax crystals. Ice formation is eliminated by the use of additives or by fuel heating. Wax formation is related to gum content and thermal stability.

¬ Freedom from vapor locking. This is achieved by the use of low- vapor-pressure fuels.

¬ High lubricity for minimum pump wear. This is obtained through the presence or addition of highly polar compounds.
4.1.3 Combustion Chamber
¬ Freedom from contaminants that cause blockage of small passages in fuel nozzles.

¬ Good atomization. Atomization quality is most strongly affected by viscosity.

¬ Rapid evaporation. Evaporation rates are dependent on fuel volatility, and on atomization quality that determines the surface area of the atomized fuel. Maximum rates of evaporation are achieved with fuels of low viscosity and high volatility.

¬ Minimum carbon formation, low flame radiation and low coke deposition.

4.2 Aircraft Fuel Specifications
Table 2 shows the typical aviation fuel properties of past and current air- craft fuels. The most widely used aviation fuel specifications are those issued by the U.S. ASTM D1655 and U.S. MIL-DTL-83133E, and the UK Ministry of Defense (MOD) DEF STAN 91-91. A number of other countries have similar agencies (Canadian General Standards Board CGSB-3.22, Russian GOST 10227, and International Air Transport Association (IATA) publication). Finally, the U.S. military maintains specifications of JP-8 fuel widely used by the Air Force and JP-5 used by the Navy. Two kerosene-type commercial jet fuels are widely used today: Jet A-1 (freeze point –51 C) on international flights and Jet A (freeze point –45 C) on almost all domestic flights in the United States.
4.3 Industrial Gas Turbine Fuels
For industrial and marine applications, the choice of fuel may be governed by economy and availability. This usually means residual oils or surplus gas, although sometimes a compromise is made. Since industrial fuels must necessarily be cheap, they are often impure. Also, industrial handling and storage practices are not up to aeronautical standards, and contamination of liquid fuel by water, salt, and sand is commonplace. Thus, to make the heavier fuels suitable for reliable gas turbine use, it is usually necessary to provide the following treatments.
• Washing to remove trace metals, such as sodium, potassium, and inorganic particulate matter

• Inhibition of vanadium in the fuel by the addition of magnesium compounds

• Filtration to remove solid oxides, silicates, and other compounds that could clog fuel pumps, flow dividers, and fuel nozzles.


5. Classification of Gaseous Fuels
By far the most common gaseous fuel for industrial gas turbines is natural gas. However, the diminishing supply of natural gas has led to increased interest in other gaseous fuels, including by-products from industrial processes, low-energy gas from coke or oil, and coke-oven gas. All gaseous fuels are advantageous in terms of high thermal stability and clean (soot- and ash- free) combustion. Table 3 lists the typical properties of common gaseous fuels.
Natural gas consists mainly of methane, along with minor amounts of other gaseous hydrocarbons, such as butane, ethane, and propane. Some natural gases contain up to 15% of nitrogen and carbon dioxide. If the sulfur content is negligibly small, the gas is described as “sweet.” However, if hydrogen sulfide is present in significant amounts, the gas is termed “sour” and must be purified prior to combustion.
Coal gas is produced by the carbonization of bituminous coals in gas retorts or coke ovens. Its composition and heating value vary with the type of coal and the temperature of carbonization. Fuels of high heat content tend to be rich in hydrogen and methane, with a nitrogen content of less than 11%. However, fuels of low heating value may contain as much as 55% nitrogen. The major impurity of concern is sulfur. Sulfur alone is not necessarily harmful, but if trace metal compounds, particularly sodium and potassium, are present along with sulfur, then turbine blade corrosion and erosion can occur.
Producer gas, which is obtained by the partial combustion of coal or coke in air, has a fairly low energy density, between 4.5 and 5.2 MJ/m3. The energy density of blast-furnace gas, which is produced in fairly copious amounts in iron works, is even lower—of the order of 3.78 MJ/m3. For this reason, it is not considered suitable as a gas turbine fuel.

5.1 Gaseous Fuel Impurities
The problems arising from impurities in low-energy liquid and gaseous fuels are deposition, corrosion, and pollution. Of these, ash deposition has proved to be the most persistent, in some cases causing unacceptable losses in power output after only a few hundred hours of running. In addition to sulfur, which may be present in fuel in concentrations up to 5% by mass, five trace metals are of most concern: calcium, lead, potassium, sodium, and vanadium. If they are present in the fuel in significant amounts, the last four can cause turbine-blade erosion, while all five can cause deposits. The two elements most commonly found in petroleum fuels are sodium and vanadium. Both can only be tolerated in small amounts, owing to their ability to form complex compounds of low melting point that are semi-molten and corrosive at metal temperatures as low as 894 K. Clearly, turbine operation at such a low inlet temperature would severely limit power output and thermal efficiency. This is why limits must be placed on the acceptable levels of trace metals in fuels for modern heavy-duty gas turbines.
6 Alternative Fuels
A fuel that either augments or replaces the conventional fuel on a potentially permanent basis with no adverse effects on engine performance, maintenance, or operational life may be defined as an alternative fuel. Given the current and future strong emphasis on fuel efficiency and emissions, alternative fuels range from highest quality fuels, such as hydrogen and methane, to low-grade liquid and gaseous fuels that remain deficient in many aspects, even after extensive refining. Other fossil fuels that can be processed to produce gaseous and liquid hydrocarbons include tar sands, oil shale, biomass, and coal.
6.1 Pure Compounds
These types of fuels include liquid hydrogen, liquid methane, liquid propane, liquid ammonia, and alcohols/oxygenates.
6.1.1 Hydrogen
From a combustion viewpoint, hydrogen is probably the nearest thing to an ideal fuel. It is characterized by high flame speeds, wide burning limits, easy ignition, and freedom from soot formation. Moreover, liquid hydrogen has a cooling capacity far superior to that of any other fuel. The main draw- backs of hydrogen lie in its very low density and low boiling point, which necessitate the use of large, heavily insulated storage tanks on the aircraft. It is also quite costly to produce. Currently, industrial quantities of hydrogen gas are most economically derived from fossil sources using steam reforming of natural gas (CH4 H2O CO 3H2), partial oxidation of methane
6.1.2 Methane
Liquid methane has a specific energy of about 49 MJ/kg, as compared with about 42.8 MJ/kg for kerosene. Its cooling capacity is not as great as that of hydrogen, but is still very large, owing to the very low temperature (112 K) of the liquefied gas. This low temperature offers considerable cooling potential for supersonic aircraft, as well as the possibility of designing highly cooled turbine blades to permit the use of higher turbine inlet temperatures. Other advantages of methane include good thermal stability and clean combustion.
The main problems arising with methane stem from its low density and low boiling point. Methane requires about 70% more storage space than current kerosene fuels (although significantly less space than hydrogen); this could prove a major problem with aircraft configurations having thin or variable-geometry wings [51,52]. Other problems include the condensation of atmospheric moisture, leading to ice formation on aircraft wings and loss of fuel by boil-off during climb.
6.1.3 Propane
It is clear that the characteristics of propane are similar to those of methane, and wherever methane has potential application,
propane usually also merits consideration. Compared with methane, it has a lower specific energy and a lower cooling capacity. However, its higher boiling point implies easier handling; in particular, it may be stored as a liquid at ambient temperatures by modest pressurization of the fuel tank.
6.1.4 Ammonia
Ammonia has a low heat of combustion and is of interest mainly on account of its great potential as a heat sink. Because of its low heat release, it is unlikely to be used on aircraft as main fuel, but it could find application as a secondary fuel in situations where its high cooling capacity can be exploited to advantage. Like propane, it is storable on the ground as a pressurized liquid at ambient temperatures.
6.1.5 Alcohols
These fuels comprise hydrocarbons that contain one or more oxygen atoms within the molecular structure. There are two types:

As a commercial product, bioethanol generally comprises 98.5% ethanol plus water together with methanol. Both the United States and Brazil are producing bioethanol at less than the cost of gasoline.
Alcohols are not practical as fuels for long-range aircraft, owing to their high oxygen content and correspondingly low calorific value. For example, half the molecular mass of methanol (CH3OH) is comprised of oxygen. The lighter alcohols are considered safer to handle than gasoline because of their higher ash point and the fact that alcohol res can be extinguished with water. They do, however, tend to be corrosive to some metals, and special precautions would be required to avoid this problem.
The main advantages of alcohol (oxygenate) fuels are:
1. Carbon neutrality due to their production from vegetable matter

2. Lower carbon content and lower freeze point

3. Higher ash point, latent heat of vaporization and octane rating

4. Reduced combustion particulates, carbon monoxide (CO) and oxides of nitrogen (NOx)

5. Lean mixture operation due to higher flame speed

The main disadvantages are: -
1. Toxicity of methanol

2. Lower specific energy and energy density

3. Highly corrosive, poor lubricity in pumps and injectors

4. Lower vapor pressure impedes cold starting, part load, and transient operation

5. Methanol can produce spark knock, has lower cetane rating

6. Generates aldehyde emissions of ozone pollution

6.2 Supplemental Fuels
These include alternative nonpetroleum fossil fuel sources, such as tars and shale oil, which occur naturally in the earth, but are not readily accessible because of their cohesion with rock or sand. Kerosene-type fuel derived from Canadian Athabasca tar sand was virtually indistinguishable in its combustion characteristics from a high-quality, petroleum-derived JP-5.
When oil shale is subjected to heating, its resinous content decomposes into an oily liquid from which a crude oil (syncrude) may be derived. Subsequent refining to reduce the content of nitrogen, oxygen, and sulfur can yield a product fairly close to that of Jet A, but with high aromatic content. Combustor tests on such fuels have resulted in higher than normal emissions of NOx due to the higher content of fuel-bound nitrogen.

6.3 Slurry Fuels
The slurry fuels of interest for aircraft applications are suspensions of powdered metals, such as beryllium, boron, aluminum, and magnesium, in gasoline or kerosene. They offer the possibility of greater flight range or higher thrust than can be obtained with conventional hydrocarbons.
7 Synthetic Fuels
The term “synthetic” is used to describe fuels derived from nonpetroleum feedstock, such as coal and biomass. Two processes for producing liquid hydrocarbons from coal are: direct coal liquefaction and coal gasification. In the coal liquefaction process, a main objective is to increase hydrogen- carbon ratio. A small increase in this ratio produces a fairly heavy liquid similar to petroleum-based residual fuel oil. The more expensive degree of hydrogenation produces lighter liquid fuel comparable to gasoline. The past evidence on fuels produced by coal liquefaction is somewhat contradictory. A JP5-type fuel refined from crude coal liquids failed to meet the specification requirements in several respects, in spite of intensive hydrogen treatment. The thermal stability was poor, the heat of combustion marginal, the density too high, and the smoke point too low.
7.1 Fuels Produced by Fischer–Tropsch Synthesis of Coal/Biomass
Coal gasification is a two-stage process that involves the production of syngas (CO + H2) via the gasification of coal and the conversion of that syngas to light hydrocarbons via Fischer–Tropsch (FT) synthesis. The FT process is currently being operated commercially by Sasol Corporation of South Africa, producing 40,000 barrels/day of liquid fuel. Inexpensive iron catalysts are used for the FT process. Other FT catalysts are cobalt, nickel, ruthenium, and molybdenum. Modern gasifiers produce syngas with low (0.6–0.7) H2/CO ratio. Iron is known as a good water gas shift catalyst because syngas with (H2/CO) 2 is required to synthesize paraffins.
7.2 Biofuels
Gas turbines are fuel flexible energy converters. Biofuels are potential new- comers. There are 10 qualified biofuels in the European Union [1]. Of these, the five most promising are: biodiesel, bioethanol, bio-methanol, vegetable oils (VO), and bio-dimethylether (bio-DME).
Biofuels have a complex molecular structure, often in the form of carbo- hydrates, i.e., Cm(H2O)n. Figure 10.20 shows the gas chromatograph of the composition of various biofuels. The combustible oils from vegetable matter comprise natural esters of the trihydric alcohol, glycerol (HOCH2CHOHCH2 OH) with the long straight-chain fatty acids (RCOOH), where the hydrocarbon radical R varies from about C15H31 to C17H35.
Vegetable oils and biodiesels have organic salts as contaminants (Na K 10–50 ppm, Ca 5–40 ppm, and viscosity of 20–300 CST). The main attractions of alternative biofuels over conventional petroleum-based fuels are:
¬ Carbon neutrality and lower carbon content.
¬ Higher ash point giving greater fire safety.
¬ Lower sulfur.

¬ Higher cetane number of rapeseed methyl ester (RME) and lower emissions of hydrocarbons and particulates.

The main drawbacks are:
1. Higher viscosity and cold filter plugging point, hence need for gum removal

2. Lower specific energy and energy density, hence higher fuel consumption

3. Higher SIT, hence lower cetane number

4. Greater corrosively with tendencies to carbon deposits and injector coking