Alternative Fuels

The term “alternative fuels” refers to fuels that may be used in engine transportation and power generation applications other than the conventional liquid fuels, such as gasoline and diesel fuels� Table 16�1 shows a listing of some of the main properties of methanol and ethanol in comparison to those of a typical gasoline� It includes the following fuel groups:

• Natural wellhead and pipeline gases, methane, liquefied natural gas (LNG), compressed natural gas (CNG), biogas, landfill gas, coal-bed methane gas, and so on

• Liquefied petroleum gas (LPG), propane, butane, and the like • Industrially processed fuel gas mixtures such as coal gas, coke oven

gas, blast furnace gas, and synthesis gas • Hydrogen, both as liquid and gas • A range of liquid fuels, including alcohols, ethers, and bioderived

fuels

Some of the motivating factors for using alternative fuels include:

• Often, there are perceived environmental benefits associated with their use in comparison to conventional liquid fuels�

• In most cases, there are cost advantages in relation to their local applications and reduced associated operational and maintenance costs�

• Diversification of the types of fuels and resources available reduces dependence on only a few types of fuels and their main sources of supply�

• There may be some operational and performance benefits�

• Integration with other fuel and energy requirements such as with cogeneration applications and supplementing conventional fuels may be attractive�

• Alternative fuels may enhance safety, starting, and cold weather operation�

There are also some potential limitations and disadvantages associated with operation using alternative fuels, including the following:

• The portability characteristics of alternative fuels are often much inferior to those of conventional liquid fuels, bringing about increased costs, limitations in operational range, and increases in weight and bulk in vehicular applications, especially with gaseous fuels�

• Equipment and infrastructure modification involve costs and may increase complexity�

• Performance of the combustion devices may be adversely affected depending on how conversion to alternative fuel operation is implemented� So far, often this is done in small numbers and on an ad-hoc basis�

• Using alternative fuels may result in undermining the safe operation of the device�

• Alternative fuels invariably are mixtures of fuels where their composition may vary sufficiently to affect performance adversely if not suitably optimized, which often is the case�

• There are special problems with some fuels, such as those involving alcohols, where there are some toxic, environmental, cold starting, and materials compatibility concerns�

• There is often a need to have two-fuel or more systems onboard available, which adds to the cost and complexity of control�

Any application of alternative fuels should maximize the use of the available energy whether with respect to the direct energy of the fuel or indirectly because of the state of the fuel, such as compression work in CNG, cold in LNG, or liquid hydrogen applications� Moreover, the total energetic costs of producing the alternative fuel, such as the work needed to compress natural gas or liquefy methane or hydrogen, should be considered� Often, this energy expenditure can be quite significant� In any case, the use of alternative fuels in transportation applications increases relatively the energy consumption through having increased mass due to the weight of the fuel tanks�

In principle, the design and associated expected performance of fuel-consuming devices should take into account the substantial differences that could exist between the different possible fuels and their mixtures with the required air for combustion� For example, gaseous fuels such as methane or hydrogen will displace some of the intake air, reducing the total energy that can be released within a device in comparison to liquid fuels unless special remedial measures are taken to compensate for this reduction� In addition, the possible enhancement of volumetric efficiency in liquid fuel applications due to the evaporative cooling of the fuel, thereby increasing the volumetric efficiency and power output, is missing when gaseous fuels are employed�

Great care is needed when comparing the characteristics of the different fuels since substantially different answers and conclusions may be arrived at depending on the basis on which the comparison is made� For example, the stoichiometric air/fuel mass ratio can vary substantially from one fuel to another, and yet a greater variation can be obtained on a volume basis� Similarly, a comparison on the basis of the heating value on a mass basis can produce a different ranking of the fuel for a certain application from that when done on the basis of the energy released per unit volume of the fuel-air mixture�

So far, the conversion of devices to alternative fuel applications has tended not to be made on a sufficiently optimized and strong scientific basis� Consequently, the merits of alternative fuel applications often cannot be assessed sufficiently realistically and accurately� This is of course in contrast to conventional fuel applications that benefited enormously over the years from the support of much research, development, and usage� Accordingly, the discussion of the merits and cost of conversion to alternative fuel operation can be made only on a tentative and qualified basis� The associated costs vary very widely with time, location, fuel availability, infrastructure, tax incentives, and whether design changes are permitted and made to the device or not�

The most common alcohols that are being considered as fuels or as supplements in substantial concentrations to common liquid fuels are those of methyl and ethyl alcohols (CH3OH and C2H5OH)� These are expected to have some attractive features as fuels, but there are a number of serious limitations associated with their usage� These have been considered recently to be sufficiently serious to have brought a virtual halt to the use of pure methanol or on occasion even ethanol as transportation fuels� At present, the use of ethanol as an additive to gasoline in relatively small proportions is increasing and receiving much support from some governments and establishments� However, there is an increased display of opposition by others to its increased use as a replacement or even supplement to conventional gasoline�

Some of the main positive features of alcohols are the following:

• They are manufactured fuels that can be produced from a wide range of fossil and non-fossil renewable fuel resources�

• They are usually liquid at normal ambient conditions and are easy to transport� They do not freeze readily�

• They may be used relatively easily as additives in small proportions to other conventional fuels, especially gasoline�

• They have relatively high octane numbers� • They may produce lower emissions of oxides of nitrogen, carbon

monoxide, and unburned hydrocarbons� • They cool on evaporation and may increase the volumetric efficiency

of engines, leading to relatively higher power output� • They are, in contrast to conventional liquid fuels, pure monocompo-

nent fuels with their properties fully known� This would permit a better prediction and universal control of their performance�

Some of the drawbacks associated with the use of alcohols as fuels are the following:

• They are manufactured and they require energy and capital resources to be produced�

• They have a relatively very low heating value, especially methanol, in comparison to conventional liquid fuels, producing less power and lower efficiencies�

• Methanol is highly toxic, is highly miscible with water, and can contaminate water resources�

• The production of ethanol at present uses many agricultural feed products that are needed for human and animal consumption�

• Evaporative emissions are dangerous� • Flammable atmospheres are always present within the partially

filled fuel tanks� This is not like the case of gasoline, in which the atmosphere within the fuel tanks contains mixtures always too rich to support flame propagation and fire�

• Undesirable poisonous aldehydes are produced and may appear in the exhaust emissions�

• Severe difficulties in cold-weather starting and pure monocomponent fuels will have a single temperature-volatility characteristic�

• There are safety hazards due to the associated nonluminous flames produced, making the detection of accidental inflammation difficult�

• They have poor compatibility to a relatively wide range of materials, including some plastics and rubber�

• Engines require modifications with increased capital costs when consuming alcohol fuels�

• Alcohols have a low cetane number; this reduces the effective value of diesel fuels containing some alcohol�

• They require highly specialized infrastructure for their wide distribution�

• Their manufacture, whether from natural gas, coal, or biomass, consumes much energy, while contributing to greenhouse gas emissions�

Table 16�2 shows a comparison of some properties of alcohols with those for CNG and LNG�

Hydrogen can be viewed as an energy carrier� It is not available anywhere in its free state naturally and requires much energy to be produced� Much of this energy is recovered up on combustion� Hydrogen is widely used by the petrochemical industry, mainly in the manufacture of a wide range of chemicals and for upgrading the quality of gasoline and other fossil fuels� Hydrogen has a very limited use as a fuel at present� The viability of hydrogen as a fuel, of course, is critically dependent on the effective and economic solution of a number of problems associated with its manufacture, since it is not available free in nature and must be manufactured, requiring much energy and capital expenditure�

Much hydrogen is manufactured from fossil fuels, natural gas, oil, and coal, via steam reforming or partial oxidation� Both catalytic and noncatalytic approaches are employed� Much of the bulk of the hydrogen needed by industry is produced in large-capacity units employing natural gas as the fuel� Currently, hydrogen on a thermal basis costs some 2 to 10 times that of methane�

High-purity hydrogen is produced through the electrolysis of water, consuming electrical energy that is generated mainly via the combustion of fossil fuels or hydroelectric and nuclear power� There are very limited prospects for hydrogen production using electrical energy generated using wind energy, solar voltaic energy, or tidal energy� There are also very limited longterm prospects for hydrogen manufacture through bacterial action from solar energy and water, oxidation of metals, or special chemical processes using low-temperature heat� Hydrogen may be stored and transported as compressed gas, liquefied, and metal hydrides, or occasionally adsorbed in special materials such as some alloys of some uncommon metals�

Hydrogen has exceedingly attractive features as an energy resource carrier� For example, as a fuel, the following favorable characteristics can be assigned to hydrogen:

• It is a renewable fuel that can be manufactured from water with the expenditure of energy�

• It is a clean fuel producing on combustion much less exhaust emissions than other fuels with no unburned hydrocarbons, carbon monoxide, oxides of sulfur, or particulates� However, its combustion in air produces oxides of nitrogen�

• Hydrogen has some very attractive combustion characteristics, such as

− Clean combustion with very fast burning rates (Figure 16�1) − Wide flammable mixture range − Low ignition energy but high autoignition temperatures

− High flame temperatures − Highly buoyant and diffusive − High heating value on mass bases − Remains a gas down to extremely low cryogenic temperatures − Catalytically sensitive and responsive − Its exhaust gas produces water and, on condensation, energy

through cogeneration

However, there are at present some serious limitations to its wide use as a fuel, such as

• Requiring much useful energy to be manufactured • Potential safety and materials compatibility problems (Figure 16�2)

• Lack of infrastructure for its distribution • Portability, storage, handling, and transport problems • Low heating value on volume and liquid basis • Exceptionally low temperature as a cryogenic liquid • Extremely difficult to liquefy, requiring much useful work • Low luminosity of its flames

Hydrogen produced via the electrolysis of water can be of very high purity and can be used in applications such as fuel cells, where very-highpurity hydrogen is required� However, electrolysis-produced hydrogen is not environmentally friendly or energy efficient, since there are sizable energetic losses involved with electricity generated from fossil fuels� These limitations contribute to exhaust emissions and greenhouse gas releases� Electrical energy derived from green sources such as solar and wind energy is of limited availability and has its own significant energy losses� Accordingly, the production of hydrogen through electrolysis represents perhaps a mere few percent of the total hydrogen produced� Virtually more than half of all of this hydrogen comes from processes that rely on natural gas as the raw material (Figure 16�3)� Some economics in electrical power can be obtained through carrying the processes whenever possible at higher temperatures�

The long-term vision of the hydrogen economy is for a future where most of the energy required is available cleanly and safely through hydrogen

combustion that is produced from renewable sources� Electrical and transportation needs are satisfied through reliance on hydrogen-fueled cells� Such a vision of the future remains largely a wishful concept, and it remains impossible to predict when it can be implemented, especially because fuel cells operated on hydrogen have not progressed to the level and degree needed� In addition, the wide availability of relatively cheap electrical energy produced primarily from nuclear energy sources has not materialized so far�

Hydrogen industrial pipelines that exist currently are of relatively small diameter in comparison to those transporting natural gas (e�g�, 0�20 m) and do not operate at the very high pressures employed in natural gas pipelines� This is mainly due to having to use steels that are less prone to hydrogen embrittlement while having low strength� The tendency toward steel embrittlement increases with the increase in temperature and when high-strength steels are used� Table 16�3 shows a comparison of some key properties of hydrogen with the corresponding values of methane and the liquid fuel iso-octane�

There are some thermochemical cycles where solar energy is used to break up water into hydrogen and oxygen, such as those involving sulfur and iodine� However, there are at present no practical devices implementing successfully such proposed concepts�

What about liquid hydrogen, LH2? Where does it fit in the utilization of hydrogen as a fuel? Examination of the experience in this respect leads to the conclusion that there are very few prospects for its wide application in the near future� Some of the main factors that are acting as obstacles are the following:

• It is a cryogenic fluid with an extremely low boiling temperature; at atmospheric pressure, its boiling temperature is around only 20 K� It has a very low liquid density (70 g/L), around only 10% of that of gasoline, which would correspond to approximately few times the volume of gasoline for the same energy�

• Much useful mechanical/electric work is needed for liquefaction� In addition, the cold is often considered as a nuisance and usually wasted�

• Expensive and relatively large and heavy fuel tanks are needed for its storage and transport�

• There are numerous safety and dispensing problems associated with its employment�

• There are other problems such as increased ice formation, corrosion, materials compatibility, and storage time limitations�

The inability to carry readily a sufficient amount of natural gas in applications such as onboard vehicles represents a major limitation to employing the readily available and attractive fuel for such common applications� Much progress has been made in recent years in the construction of highpressure, compact, lightweight cylinders for carrying CNG, making it the preferred mode of carrying the gas onboard vehicles (Figure 16�4)� It permits driving distances that are often comparable to LPG or even occasionally conventional liquid fuels�

Many CNG storage tanks are constructed of aluminum or steel liners with fiberglass or carbon fiber over-wraps to increase the strength while minimizing weight� Pressures of 200 and 240 bars are common, which correspond

typically on an energy basis to an equivalent amount of gasoline of around 27% and 33%, respectively� The weight, bulk, and cost of the fuel storage tanks remain a serious limitation to CNG’s wide application, particularly for transport� They constitute typically a significant fraction of the cost of the conversion of vehicles to CNG fueling� However, such limitations are of little significance in the case of buses, where there is usually lots of room over the roof of the bus to locate the fuel tanks and the additional weight can be easily tolerated�

Recent composite cylinders can have masses of only around 20% of the mass of the corresponding equivalent all-steel cylinders� In addition, so far CNG fuel tanks cannot be made of irregular shapes as with gasoline applications to fit into available spaces in the vehicle, which contributes to their relative bulkiness and wastage of trunk-carrying capacity� The typical mass of the CNG cylinder ranges from about 1�0 kg/L water equivalent to as low as around 0�45 kg/L of water equivalent for cylinders made up of composite materials� As long as CNG is used, the cost will include the cost of compression work, which is usually wasted by having to expand the fuel gas down to atmosphere as it enters the intake section of the engine�

There are some operational and materials compatibility problems with the use of CNG arising from the possible presence of lubricating oil droplets and vapors arising from the compression system� Furthermore, the presence of any water vapor in the gas can result in its freezing, especially on the rapid expansion of the high-pressure gas for use� This will lead to early corrosion problems, particularly with the presence in the gas of very small concentrations of hydrogen sulfide� Effective natural gas driers are employed to ensure sufficient removal of any water vapor in the gas before and during its compression and storage� There is also the likelihood of the formation of hydrates, which may contribute to uneven flow of gas� The odorants mercaptans are normally added to CNG for safety and leak detection�

Because of the closed pressurized nature of the CNG fuel system and the avoidance of any fuel leakage, there is no contribution from any evaporative fuel emissions� However, the fuel system on a vehicle faces a number of demanding requirements� The fuel must be safely stored at high pressures and then safely and accurately delivered to the engine� Variations in fuel pressure and temperature, which have a negligible effect in gasoline-fueled engines, present major challenges in the case of CNG vehicles and need to be considered by their control systems� The problem may be compounded by the possible variations in the fuel composition, which can change with location and time�

The use of CNG as a transportation fuel, whether for converted spark ignition or compression ignition engines, is going to continue to increase with the relative economical price of the gas in comparison to liquid fuels and the greater emphasis on clean and efficient combustion, especially in countries where there are indigenous supplies of natural gas and rigorous emissions controls� However, the penetration of CNG in to the transport sector,

especially for automobiles, in all likelihood will remain in the near future, lagging behind those of the conventional liquid variety� Attractive tax incentives do help to encourage the movement toward increased reliance on CNG as a motor fuel�

Most big fleets of marine transport vehicles may have their own gas compression facilities available on site being supplied by the gas grid system usually at pressures higher than those supplied for domestic consumers� The operation of a CNG supply facility could be at a big economic disadvantage if the gas supply to the facility is not at a high pressure, since the cost of compression would increase quite significantly�

1� Hydrogen, which has been often described as the fuel of the future, has some uniquely attractive properties as a fuel for potential applications in a variety of heat-and power-producing devices, yet it is not as widely used as it could be� Very briefly list some of the main limitations holding back its wider application as a fuel�

2� The application of ethanol and methanol as the main fuels for engines or additives to gasoline has been progressing relatively slowly in recent years compared to the extent that was projected enthusiastically decades ago� Briefly outline the reasons behind this trend as far as the transport sector is concerned�

3� Alternative fuels are being advocated increasingly as a potential replacement to conventional liquid fuels in the transport sector� Briefly outline the basis for this trend�

There is a wide range of fuels of non-fossil origin that may be naturally occurring or manufactured and that have the potential to be used as fuels in engine applications� They are described as alternative fuels because they are other than the commonly used conventional liquid fuels, such as gasoline or diesel fuel� These alternative fuels, which may contribute to the increased diversification of fuel and energy resources, have some distinct advantages, including lower cost and less negative environmental impact than conventional fossil fuels� However, they have also specific limitations that vary in severity depending on the nature of the fuel and its intended field of application� Notable examples of alternative fuels include CNG, alcohols, and hydrogen�