Fuels in General

A “fuel” can be described as any material that can undergo exothermic structural change� Often, this change is chemical in nature such as through combustion, which requires an oxidant� Occasionally, the change may occur through exothermic decomposition such as in the case of some propellants� The chemical potential energy of a fuel released through combustion should be viewed as, in principle, much more versatile and valuable than its equivalent in the form of heat energy� This is described later in Chapter 4 when the distinction among heat, work, and energy is discussed�

Much of the widely used fuels are “organic” in nature, which contain the relatively versatile carbon atoms� There are some “inorganic” fuels such as H2, NH3, H2S, and metals that are much less widely available in comparison with organic fuels� They constitute at present only a small fraction of the energy extracted from fuels in general� Moreover, common fuels may be “naturally occurring” in origin, such as petroleum, natural gas, and coal, or “manufactured,” such as alcohols and hydrogen�

Most of the fuels available to us are of “fossil” origin; they were formed primarily from living matter in geological times� Historically, their relatively plentiful reserves, low cost, convenience of use, high energy content, and the lack of environmental concerns during the past made fossil fuels very attractive� They were used extensively over the past decades� However, this situation has been changing in recent years and attention is increasingly being given by all to the conservation and optimum use of these irreplaceable energy resources� They constitute our nonrenewable natural resources legacy�

Fuels can be broadly classified in a variety of ways, such as on the basis of their properties or field of application� Some of the most common classifications depend on whether they are

1� Solid, liquid, or gas (at ambient conditions) 2� Organic or nonorganic 3� Fossil or non-fossil 4� Conventional or alternative

5� Mineral fuels or biofuels 6� Of low heating value, medium heating value, or high heating value 7� Natural or manufactured

Fuels, apart from their availability and cost, can vary widely in properties and suitability for different specific applications� Some of the main desirable features of fuels apart from low cost are discussed in this section� Of course, these features often may not be achieved necessarily in any one fuel, but they dictate the suitability of a fuel for its application in a specific device and location:

• High energy density on mass and/or volume bases • Good combustion characteristics, for example, high burning

rates, low ignition energy requirements, and high combustion temperatures

• Requiring relatively easy and inexpensive refining and processing before becoming available for usage

• High thermal stability and low deposit-forming tendency • Compatibility with hardware and materials in engine, furnace, or

combustor applications • Good characteristics against the risks of fire and explosion during

storage, transport, and application • Low toxicity and exhaust emissions • Easy to transport, handle, and store • May be available with less impurities and in pure form when

required • Ability to mix readily with other fuels when needed • Sufficiently low in sulfur, ash, and nitrogen contents • Good storage, handling, and combustion characteristics at low ambi-

ent temperatures

However, there are also a number of concerns regarding the selection of a fuel for a specific application that requires a satisfactory solution� Such concerns include the following:

• Ensuring continued supplies of the fuel • Maintaining reasonable cost

• Providing the necessary infrastructure for transport, storage, and distribution of the fuel

• Dealing with the continued difficulties encountered in providing reliable forecasting of reserves, demand, and costs

• Ecological concerns and their continued effective controls • Limitations to proper usage due to inadequacy of current technol-

ogy and performance, and limited scientific knowledge • Social and political adverse conditions and factors

Table 2�1 lists the capacity to store energy of a range of common fuels (based on their lower heating value) and devices on mass basis� It illustrates the advantages and superiority of fossil fuels over other fuels� Fossil fuels represent relatively huge sources of energy on mass basis in comparison with, for example, common automotive electric batteries� It can be seen that hydrogen on mass basis has a much higher energy storage capacity than gasoline or diesel fuels� However, on volume basis, whether in the gaseous or liquid state, gasoline and diesel fuels are much superior to hydrogen in terms of energy storage capacity� It also shows liquid gasoline to have almost 200 times the energy of the common lead-acid battery�

Much confusion is caused by the many different units used in the fields of energy and fuels� This mainly arises from the continued reliance of some sectors of industry on empirically derived historical units such as those of the British Imperial System of units� Fortunately, by using the widely accepted and adopted international SI system of units, a much simpler and less confusing unified universal approach to the whole field of units is realized�

For various forms of energy, whether of work or heat types, the “joule” (J) is used throughout, whereas for amounts of substances the “mole” (mol) is used and for mass measurement the “kilogram” (kg) is used� The unit “kelvin” (K) is used for absolute temperature and the unit “Celsius” (°C) is used for temperature (Table 2�2)� Mole (mol) is the amount of substance of a system that contains as many elementary entities as there are atoms in 0�012 kg of carbon 12� The unit mol replaces the previously known unit gram-mole�

Numerous “derived units” can be developed from the base SI units of meter (m), second (s), kilogram (kg), and mole (mol)� Notable among them are pascal (Pa) for pressure, newton (N) for force, watt (W) for power, and joule (J) for energy� Table 2�3 lists the main recognized derived units that have been given special names as part of the SI system�

A great advantage of the SI system is its set of recognized multiples and submultiples that simplify the reporting of various physical quantities� These are shown in Table 2�4�

There are extensive statistics with much detailed information of historical, current, and projected nature becoming increasingly available about the availability, production, and consumption of various fuels and energy resources� Such information originates in great detail from a wide variety of official, commercial, institutional, and private sources� They are made widely available through publications, reports, and the Internet� It is necessary when examining such information to exercise much care because there is a significant amount of data that lack the accuracy expected or on occasion may even be distorted, be out of date, or require further qualification� For example, when considering reserves of fossil fuels, it is essential to distinguish whether the data are related to actual, probable, or possible available resources, as shown schematically in Figure 2�1� Moreover, the quality of the fuel in place needs to be evaluated since, for example, lightweight crude commands much higher prices than heavier crude�

Table 2�5 shows a listing of proved oil reserves and their corresponding consumption by world regions� The dominant position of the Middle East is evident, whereas the decline in the relative ratio of volumes of reserves of production in Europe, especially the United Kingdom, appears quite serious�

Reserves are determined using methods that require elaborate and complex calculations using assumed and tested empirical parameters or increasingly reservoir iterative simulation methods� As a reservoir produces and begins to get depleted, it becomes increasingly easier to predict its remaining reserves� Only those reserves that are deemed possible to produce and are directly correlated to the production performance of a reservoir may be broadly classified into three categories: (1) proved, (2) probable, and (3) possible resources�

Proved reserves are estimated on a specified date and relate to those reserves that are deemed recoverable based on the reservoir’s current condition� Reserves are considered proven when they are supported by actual production figures�

Probable reserves are similar to that of the proved ones, but they need sufficient geological and engineering support to be considered as proved� They are considered to be recoverable�

Possible reserves are less likely to be recovered than probable reserves�

Of course, it is to be remembered that a banker’s idea of the size of proved reserves is likely to be smaller than that of a petroleum engineer, which in turn is likely to be smaller than that of an enthusiastic geologist� For publicly traded companies to protect public interest, there are fairly strict and conservative guidelines as to what constitutes a reserve� It is a requirement by law of most regulatory agencies that exploit the reserves to establish what a reserve is� Consistent sets of classifications are used for resources such as those of crude oil, condensate, natural gas, and natural gas liquids�

Figure 2�2 shows definitions of the different types of oil and gas reserves to be employed officially by the energy industry according to the requirements of the Energy Resources Conservation Board of the Government of the Province of Alberta, Canada�

A number of general observations can be derived from analyzing the enormous amount of information about the statistics of fuels and energy past, present and, possibly, future:

• The global consumption of fossil fuels has been increasing steadily� This is not projected to decrease and may continue to rise in the foreseeable future�

• The consumption of fossil fuels by the chemical industry as raw materials for the manufacture of a wide range of products and for upgrading of some low quality fuels instead of being used merely as a source of fuels is also expected to continue to increase�

• The accumulated known proved reserves of fossil fuels are not expected to increase substantially as their rates of consumption continue to increase�

• New discoveries of fossil fuel reserves when made appear under increasingly more difficult conditions and tend to be much more expensive to discover and exploit such as at greater depths and increasingly at long distances offshore at sea�

• The demand for fuels by some rapidly developing economies such as those of China, Korea, and India has been increasing at a very rapid rate�

• Natural gas consumption has been steadily increasing, driven by stricter emission control requirements, the increased availability of new reserves of gas, and the increased development of liquefied natural gas (LNG) production and transportation facilities� This trend is expected to continue to increase�

• The relative contribution of nuclear power is not expected to increase markedly in the near future and would not curtail the overall consumption of fossil fuels�

• The relative proportion of fuel consumption between the various application sectors globally will in all likelihood remain essentially the same as in recent years�

• The use of alternative sources of energy will increase, but their overall contribution to reducing reliance on fossil fuels will remain relatively minor for quite some time to come�

• The global reserves of coal in comparison with those of oil and gas are huge, but there appear to be some serious and almost insurmountable limitations to expanding the usage of coal, particularly those of environmental origin�

• The efficiency of utilization of fuel resources has been steadily improving in recent years� But the consumption of fuels is more affected by ever-increasing world population and life expectancy and by the improvement in the overall standard of living globally�

• The effectiveness of oil recovery from petroleum reservoirs has been steadily improving, but new discoveries are getting harder to find and more expensive to explore and produce�

Figure 2�3 shows the relative distribution of the annual consumption of energy globally in recent decades� Among coal, oil, natural gas, and electrical power via hydroelectric and nuclear powers, oil is the dominant energy resource followed by coal and natural gas� Although the overall contributions of hydroelectric and nuclear powers show slight increases, they remain relatively small� Natural gas consumption shows significant increases in recent years�

Patterns of distribution of energy consumption and their variation in recent years among different regions of the world are shown in Figure 2�4� The dominant share in this consumption is that of developed nations, particularly the United States and Canada� Moreover, consumption by the United States of oil and gas continues to increase markedly over the years, whereas their production continues to decline� At present, the dependence of the United States on imported oil from different countries shows Canada to be the prime supplier followed by Mexico (Figures  2�5 through 2�8)� Canadian oil is increasingly derived from the huge resources of oil sand deposits in Western Canada� However, the exploitation and processing of these resources is being increasingly considered to be energy and environmentally intrusive�

The current capacity of LNG is increasing substantially, making it a marked and significant supplement to the supplies of conventional natural gas� Table 2�6 shows the extent of LNG imports by countries in 2002� The corresponding figures of LNG exports are shown in Table 2�7� Japan and Korea took the major parts of these exports�

There are locations in some parts of the world mainly due to the lack of gas pipelines, transportation systems, and the absence of facilities for either

its consumption or liquefication, are unfortunately obliged to flare huge quantities of gas� Often, these are produced in conjunction with oil, which in comparison with gas can be exported readily� This continued practice not only wastes a valuable energy resource but also contributes substantially to increased emissions and global warming� This is a global concern that needs to be addressed satisfactorily in the coming years�

The estimated worldwide reserves of coal of different qualities, shown in Figure 2�9, are relatively huge and spread over many parts of the world� The United States appears to have a substantial fraction of coal reserves� However, as will be shown later in Chapter 14, the wide use of coal as an energy resource faces numerous challenges that have the potential to undermine its future exploitation as a fuel�

1� It has been advocated that it is unlikely that new energy systems will be devised to replace the existing dependence on fossil fuels for at least the next two decades or more� Do you agree with this view? Indicate briefly the basis for your opinion�

2� Suggest why worldwide reserves of fossil fuels appear to be continuing to increase while their annual consumption is increasing�

3� What do you consider the main desirable properties of a fuel destined for use in the large-capacity boilers of an electric power station?